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Raman spectra of gas hydrates : differences and analogies to ice 1h and (gas saturated) water
(2005)
It is generally accepted that Raman spectroscopic investigations of gas hydrates provide vital information regarding the structure of the hydrate, hydrate composition and cage occupancies, but most research is focused on the vibrational spectra of the guest molecules. We show that the shape and position of the Raman signals of the host molecules (H2O) also contain useful additional information. In this study, Raman spectra (200-4000cm(-1)) of (mixed) gas hydrates with variable compositions and different structures are presented. The bands in the O-H stretching region (3000- 3800cm(-1)), the O-H bending region (1600-1700cm(-1)) and the O-O hydrogen bonded stretching region (100-400cm(-1)) are compared with the corresponding bands in Raman spectra of ice Ih and liquid water. The interpretation of the differences and similarities with respect to the crystal structure and possible interactions between guest and host molecules are presented. (c) 2005 Elsevier B.V. All rights reserved
The 26 December 2004 Sumatra-Andaman earthquake of Mw 9.3 triggered a massive tsunami in the Indian Ocean. We here report on observations of the Indian Ocean tsunami at broadband seismic stations located on islands in the area. The tsunami induces long-period (> 1000 s) signals on the horizontal components of the sensor. Frequency-time analysis shows that the long-period signals cannot be due to seismic surface waves, but that it arrives at the expected time of the tsunami. The waveforms are well correlated to tide gauge observations at a location where both observations are available. To explain the signals we favour tilt due to coastal loading but we cannot at the present stage exclude gravitational effects. The density of broadband stations is expected to increase rapidly in the effort of building an earthquake monitoring system. They may unexpectedly become useful tsunami detectors as well
A key question for the development of geothermal plants is the seismic detection and monitoring of fluid injections at several kilometers depth. The detection and monitoring limits are controlled by several parameters, for example, the strength of seismic sources, number of receivers, vertical stacking, and noise conditions. For a known reference reflector at 2.66 km depth at a geothermal site in northern Germany the results of a simple surface seismic experiment were therefore combined with numerical forward modeling for different injection scenarios at 3.8 km depth. The underlying idea is that changes of reflectivity from the injection at 3.8 km must be larger than the variance of the measurements to be observable. Assuming that the injection at 3.8 km depth would produce a subhorizontal disklike target with a fracture porosity of 2% or 5% (the critical porosity) the water injection volume has to be at least 443 and 115 m(3), respectively, to be detectable from the surface. If the injection on the other hand does not create subhorizontal but subvertical pathways or only reduces the seismic velocities via the increased pore pressure in the immediate vicinity of the bore hole, the injection is undetectable from the surface. The most promising approach is therefore to move sources and/or receivers closer to the target, that is, the use of borehole instrumentation
Ambient vibration techniques are promising methods for assessing the subsurface structure, in particular the shear-wave velocity profile (V-s). They are based on the dispersion property of surface waves in layered media. Therefore, the penetration depth is intrinsically linked to the energy content of the sources. For ambient vibrations, the spectral content extends in general to lower frequency when compared to classical artificial sources. Among available methods for processing recorded signals, we focus here on the spatial autocorrelation method. For stationary wavefields, the spatial autocorrelation is mathematically related to the frequency-dependent wave velocity c(omega). This allows the determination of the dispersion curve of traveling surface waves, which, in turn, is linked to the V-s profile. Here, we propose a direct inversion scheme for the observed autocorrelation curves to retrieve, in a single step, the V-s profile. The powerful neighborhood algorithm is used to efficiently search for all solutions in an n- dimensional parameter space. This approach has the advantage of taking into account the existing uncertainty over the measured curves, thus generating all V-s profiles that fit the data within their experimental errors. A preprocessing tool is also developed to estimate the validity of the autocorrelation curves and to reject parts of them if necessary before starting the inversion itself. We present two synthetic cases to test the potential of the method: one with ideal autocorrelation curves and another with autocorrelation curves computed from simulated ambient vibrations. The latter case is more realistic and makes it possible to figure out the problems that may be encountered in real experiments. The V-s profiles are correctly retrieved up to the depth of the first major velocity contrast unless low-velocity zones are accepted. We demonstrate that accepting low-velocity zones in the parameterization has a dramatic influence on the result of the inversion, with a considerable increase in the nonuniqueness of the problem. Finally, a real data set is processed with the same method
The Cretaceous eo-Alpine collisional event in the European Eastern Alps is generally accepted to induce W-NW- directed thrusting both in basement and in sedimentary cover units. This study presents the first evidence of eo-Alpine W-NW directed normal kinematics along the Schneeberg Normal Fault Zone, which separates eo-Alpine high-pressure rocks in a footwall position from pre-Alpine basement rocks in a hanging wall position. New Garnet Sm-Nd data indicate that exhumation of the high-pressure rocks along the normal fault zone started around 95 Ma ago and continued up to low greenschistibrittle conditions at 76 Ma, as indicated by a Rb-Sr age from a low temperature mylonite. The occurrence of pre-Alpine basement rocks both in the hanging wall and the footwall of eo-Alpine high-pressure rocks suggests exhumation by extrusion processes. Despite the displacement or removal of parts of the lower portion of the high-pressure unit by Tertiary strike-slip faults, eo-Alpine top-to-ESE thrusting, as expected for the structurally lower part of an extruding wedge, was found at and below the base of the eo-Alpine high-pressure rocks. A Rb-Sr age of 77 Ma from a greenschist facies mylonite in this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge. The tectonic transport direction within the extruding wedge was E-SE, opposite to the W-NW direction so far reported for the eo-Alpine event in the Eastern Alps. The contemporaneity of opposite tectonic transport directions during continental subduction may be explained by a double-vergent wedge model with a narrow zone of ductile flow, where the high-pressure rocks were exhumed. (c) 2005 Elsevier B.V. All rights reserved
Thermobarometrical and mineral-chemical investigations by electron microprobe and LA-ICP-MS on a sillimanite- bearing pegmatoid from the Reinbolt Hills provide important constraints on the P-T-X-age relations of part of East Antarctica during Pan-African tectonism. U-Th-total Pb ages of monazite imply that the pegmatoid of originally Grenvillan age (zircon U-Pb age of ca. 900 Ma) underwent a major, late Pan-African (Cambrian) regional, granulite-facies metamorphism between 500 and 550 Ma. Most of the monazite formed during this event, as result of apatite metasomatism owing to infiltration of high-grade metamorphic fluids. Apatite-biotite and other mineral thermobarometers define the peak metamorphic temperatures and pressures with 850-950 degrees C and 0.8-1.0 GPa. The F-Cl-OH relations in apatite, and biotite, the chemistry of fluid inclusions and the presence of K-feldspar microveins suggest that the metasomatising fluid was a CO2-bearing, diluted KCl brine. The pegmatoid is the first record of monazite-(Ce) formed from fluorapatite that is rich in U (up to 2.6 Wt% UO2) and possesses Th/U ratios <1 (0.09 on average). These chemical signatures are direct reflection of the U and Th concentration patterns in the parental fluorapatite
Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia
(2005)
Foreland sedimentation in the Subandean Zone of south-central Bolivia spans from the Upper Oligocene to present. It records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin, and thereby contains stratigraphic information on the tectonic evolution of the adjacent Andean fold-thrust belt. Within the Neogene orogenic wedge individual siliciclastic-dominated depositional systems formed ahead of an eastward-propagating deformation regime. We defined, described, and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the Neogene foreland basin in the Subandean Zone and the Chaco Plain. These are combined to interpret depositional settings. The up to 7.5 km-thick Neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture: (1) The basal, Oligocene-Miocene, up to 250 m-thick Petaca Formation consists dominantly of calcrete, reworked conglomeratic pedogenic clasts, and fluvial sandstone and mudstone. This unit is interpreted to represent extensive pedogenesis under an and to semiarid climate with subordinate braided fluvial processes. (2) The overlying, Upper Miocene, up to 350 m thick Yecua Formation records numerous small-scale transgressive-regressive cycles of marginal marine, tidal, and shoreline facies of sandstone, ooid limestones, and varicoloured mudstone. (3) The Upper Miocene, up to 4500 m-thick Tariquia Formation principally consists of sandstone with interbedded sandstone-mudstone couplets representing frequent crevassing in anastomosing streams with an upsection- increasing degree of connectedness. (4) The up to 1500 m-thick Lower Pliocene Guandacay Formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone. (5) The Upper Pliocene, up to 2000 m-thick Emborozu Formation consists predominantly of alluvial-fan-deposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone. The coarsening- and thickening-upward pattern and eastward progradation, coupled with the variable proportions of overbank facies, channel size, and degree of channel abandonment, in the Tariquia, Guandacay, and Emborozu Formations reflect a distal through proximal fluvial megafan environment. This long-lived megafan grew by high sedimentation rates and a north east-through-southeast radial paleoflow pattern on large, coarse-grained sediment lobes. The marked overall upsection change in pattern and depositional styles indicate fluctuations in accommodation space and sediment supply, regulated by basin subsidence, and are attributable to Andean tectonics and climatic controls. (c) 2005 Elsevier B.V. All rights reserved
Fe K-edge X-ray absorption near edge structure (XANES) and Mossbauer spectra were collected on synthetic glasses of basaltic composition and of glasses on the sodium oxide-silica binary to establish a relation between the pre- edge of the XANES at the K-edge and the Fe oxidation state of depolymerised glasses. Charges of sample material were equilibrated at ambient pressure, superliquidus temperatures and oxygen fugacities that were varied over a range of about 15 orders of magnitude. Most experiments were carried out in gas-flow furnaces, either with pure oxygen, air, or different CO/CO2 mixtures. For the most reduced conditions, the samples charges were enclosed together with a pellet of the IQF oxygen buffer in an evacuated silica glass ampoule. Fe3+/Sigma Fe x 100 of the samples determined by Mossbauer spectroscopy range between 0% and 100%. Position and intensity of the pre-edge centroid position vary strongly depending on the Fe oxidation state. The pre-edge centroid position and the Fe oxidation state determined by Mossbauer spectroscopy are nonlinearly related and have been fitted by a quadratic polynomial. Alternatively, the ratio of intensities measured at positions sensitive to Fe2+ and Fe3+, respectively, provides an even more sensitive method. Pre- edge intensities of the sample suite indicate average Fe co-ordination between 4 and 6 for all samples regardless of oxidation state. A potential application of the calibration given here opens the possibility of determining Fe oxidation state in glasses of similar compositions with high spatial resolution by use of a Micro-XANES setup (e.g., glass inclusions in natural minerals). (c) 2004 Elsevier B.V. All rights reserved
Quartz crystals from topaz-zinnwaldite-albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 mum. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn-W mineralization. The combination of special experimental techniques with confocal laser Raman- microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55 wt% H2O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid
Alpine thermal and structural evolution of the highest external crystalline massif : the Mont Blanc
(2005)
The alpine structural evolution of the Mont Blanc, highest point of the Alps (4810 m), and of the surrounding area has been reexamined. The Mont Blanc and the Aiguilles Rouges external crystalline massifs are windows of Variscan basement within the Penninic and Helvetic nappes. New structural, Ar-40/Ar-39, and fission track data combined with a compilation of earlier P-T estimates and geochronological data give constraints on the amount and timing of the Mont Blanc and Aiguilles Rouges massifs exhumation. Alpine exhumation of the Aiguilles Rouges was limited to the thickness of the overlying nappes (similar to 10 km), while rocks now outcropping in the Mont Blanc have been exhumed from 15 to 20 km depth. Uplift of the two massifs started similar to 22 Myr ago, probably above an incipient thrust: the Alpine sole thrust. At similar to 12 Ma, the NE-SW trending Mont Blanc shear zone (MBsz) initiated. It is a major steep reverse fault with a dextral component, whose existence has been overlooked by most authors, that brings the Mont Blanc above the Aiguilles Rouges. Total vertical throw on the MBsz is estimated to be between 4 and 8 km. Fission track data suggest that relative motion between the Aiguilles Rouges and the Mont Blanc stopped similar to 4 Myr ago. Since that time, uplift of the Mont Blanc has mostly taken place along the Mont Blanc back thrust, a steep north dipping fault bounding the southern flank of the range. The "European roof'' is located where the back thrust intersects the MBsz. Uplift of the Mont Blanc and Aiguilles Rouges occurred toward the end of motion on the Helvetic basal decollement (HBD) at the base of the Helvetic nappes but is coeval with the Jura thin-skinned belt. Northwestward thrusting and uplift of the external crystalline massifs above the Alpine sole thrust deformed the overlying Helvetic nappes and formed a backstop, inducing the formation of the Jura arc. In that part of the external Alps, similar to NW-SE shortening with minor dextral NE-SW motions appears to have been continuous from similar to 22 Ma until at least similar to 4 Ma but may be still active today. A sequential history of the alpine structural evolution of the units now outcropping NW of the Pennine thrust is proposed
An important task of seismic hazard assessment consists of estimating the rate of seismic moment release which is correlated to the rate of tectonic deformation and the seismic coupling. However, the estimations of deformation depend on the type of information utilized (e.g. geodetic, geological, seismic) and include large uncertainties. We therefore estimate the deformation rate in the Lower Rhine Embayment (LRE), Germany, using an integrated approach where the uncertainties have been systematically incorporated. On the basis of a new homogeneous earthquake catalogue we initially determine the frequency-magnitude distribution by statistical methods. In particular, we focus on an adequate estimation of the upper bound of the Gutenberg-Richter relation and demonstrate the importance of additional palaeoseis- mological information. The integration of seismological and geological information yields a probability distribution of the upper bound magnitude. Using this distribution together with the distribution of Gutenberg-Richter a and b values, we perform Monte Carlo simulations to derive the seismic moment release as a function of the observation time. The seismic moment release estimated from synthetic earthquake catalogues with short catalogue length is found to systematically underestimate the long-term moment rate which can be analytically determined. The moment release recorded in the LRE over the last 250 yr is found to be in good agreement with the probability distribution resulting from the Monte Carlo simulations. Furthermore, the long-term distribution is within its uncertainties consistent with the moment rate derived by geological measurements, indicating an almost complete seismic coupling in this region. By means of Kostrov's formula, we additionally calculate the full deformation rate tensor using the distribution of known focal mechanisms in LRE. Finally, we use the same approach to calculate the seismic moment and the deformation rate for two subsets of the catalogue corresponding to the east- and west-dipping faults, respectively
Pressure solution in chalk
(2005)
Pressure-solution residues in the Cretaceous to Paleocene chalk from the North Sea area were studied to understand the pressure-solution process and fluid flow. Residue seams reach lengths and thickness of respectively. Seams can more than 800 and 0.15 m (2625 and 0.5 ft), present significant barriers to fluid flow because they have nannodarcy permeability and hold back large pressure differentials. Stylolite amplitudes decrease when they merge, making it difficult to quantify volume loss. Measuring amplitudes allows volume-loss estimates using a new dissolution ratio method. The sum of the thicknesses of all residues is multiplied by the maximum amplitude-to-residue thickness ratio. On average, 30 mm (1.2 in.) of chalk had to be dissolved to produce a 1-mm (0.04-in.)-thick residue. The volume loss of chalk from Flamborough averages about 50% and is 60% in the Machar oil field, United Kingdom Central Graben. The minimum fluid volume required to produce the volume loss in 1 unit volume of preserved chalk on the Machar field is estimated be 8000 unit volumes. This and the stable isotopes of vein carbonates imply that the pressure-solution process involved an open-fluid system. High illite content in mixed layered illite-smectite and high illite crystallinity of clays in the pressure-solution seams in the Machar reservoir indicate high diagenetic maturity, with implied temperatures in the range of 200-360degreesC (392-680degreesF), which is greater than the present-day reservoir temperature of 85degreesC (185degreesF). The anomalously hot temperatures are interpreted to be produced by deep mineralizing fluids transported from the Central Graben. The lack of abundant veining in the chalk and the large fluid volumes involved in the chalk dissolution suggest that this was an open pressure-solution system
Whether variations in the spatial distribution of erosion influence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new Ar-40/Ar-39 white mica and apatite fission- track (AFT) ages that measure the vertical component of exhumation rates along an similar to 120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The Ar-40/Ar-39 data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crystalline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented similar to 80 x 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation defined by the AFT data correlates with a region of high precipitation, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene
The Menderes Massif and the overlying Lycian Nappes occupy an extensive area of SW Turkey where high-pressure- low-temperature metamorphic rocks occur. Precise retrograde P-T paths reflecting the tectonic mechanisms responsible for the exhumation of these high-pressure-low-temperature rocks can be constrained with multi-equilibrium P-T estimates relying on local equilibria. Whereas a simple isothermal decompression is documented for the exhumation of high-pressure parageneses from the southern Menderes Massif, various P-T paths are observed in the overlying Karaova Formation of the Lycian Nappes. In the uppermost levels of this unit, far from the contact with the Menderes Massif, all P-T estimates depict cooling decompression paths. These high-pressure cooling paths are associated with top-to-the-NNE movements related to the Akcakaya shear zone, located at the top of the Karaova Formation. This zone of strain localization is a local intra-nappe contact that was active in the early stages of exhumation of the high-pressure rocks. In contrast, at the base of the Karaova Formation, along the contact with the Menderes Massif, P-T calculations show decompressional heating exhumation paths. These paths are associated with severe deformation characterized by top-to-the-east shearing related to a major shear zone (the Gerit shear zone) that reflects late exhumation of high-pressure parageneses under warmer conditions
The rise and fall of the Classic Maya provides a textbook example of human social evolution. It is therefore significant to discover that the history of theMaya was so closely tied to environmental constraints. If Maya civilization could collapse under the weight of natural climate events, it is of more than academic interest to ponder how modem society will fare in the face of an uncertain climate in the years ahead. An understanding of how ancient cultures responded to climatic changes in the past may thus provide important lessons for humanity in the future
The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications
(2005)
The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende + plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the complex, relict high-pressure mafic granulites are garnet + clinopyroxene-bearing rocks with a secondary development of orthopyroxene around both garnet (kelyphites) and clinopyroxene (coronas). South of the newly defined central, E-W-trending, Zhujiafang shear zone, numerous mafic boudins and less-deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet (full of quartz inclusions) between the magmatic plagioclase and pyroxene domains. Additional orthopyroxene (after magmatic augite) and sodic rims to magmatic plagioclase clearly indicate medium-pressure granulite-facies metamorphism. These findings suggest tectonic juxtaposition in this area of three different structural levels of the same Proterozoic-imprinted crust: high-pressure granulite grade in the northern Hengshan, medium-pressure granulite grade in the southern Hengshan and amphibolite- to greenschist-facies grade in the Wutaishan to the SE. (c) 2004 Elsevier Ltd. All rights reserved
Possible causes to explain platform drowning have been hotly debated by carbonate sedimentologists for more than a decade now. In this paper, we present multiple evidence to explain the drowning of a carbonate megabank that covered most of the modem Northern Nicaragua Rise (NNR) during an interval spanning from late Oligocene to early Miocene by the interaction of several environmental factors. The recovery during ODP Leg 165 of late Oligocene to middle Miocene sedimentary sequences in the sub-seafloor of the modern channels and basin, Pedro Channel and Walton Basin, respectively, that dissect the NNR (Site 1000) and south of the rise in the Colombian Basin (Site 999), combined with information from dredged rock samples, allows us to explore in more detail the timing and possible mechanisms responsible for the drowning of the megabank and its relationship to Miocene climate change. The modern system of isolated banks and shelves dissected by a series of intervening seaways and basins on the NNR has evolved from a continuous, shallow-water carbonate "megabank' that extended from the Honduras/Nicaraguan mainland to the modern island of Jamaica. Available information suggests that this megabank broke apart and partially drowned in the late part of the late Oligocene at around 27 Ma and finally foundered during the late early Miocene around 20 Ma, resulting in limited neritic coral growth in the areas where the modern isolated carbonate banks and shelves are occurring today. Available information also suggests that the southern and central parts of Pedro Channel were already a deep-water area before the major episode of platform drowning, and its formation predates the initiation of the Caribbean Current. However, after the partial drowning of the megabank, the channel has become a major pathway for the Caribbean Current. Stratigraphic units identified in deep-water carbonates sampled at ODP Sites 999 and 1000 help to constrain the environmental setting leading to the drowning of the banks. Changes in lithology and mass accumulation rates of both the carbonate and non-carbonate fraction parallel stable isotope shifts and likely indicate regional changes in climate and circulation during the late Oligocene-middle Miocene interval. Carbonate mass accumulation rates (MARS) at Site 999 suggest increased regional productivity during the early Miocene. Terrigenous MARS at both Sites 999 and 1000 show a general increase from the Burdigalian through the Serravallian. The temporal association among episodes of neritic platform deposition, followed by increased productivity as identified by higher carbonate MARs and positive excursion in carbon isotopes, suggests that oceanographic changes such as local upwelling and nutrification have led to the partial drowning of the NNR "megabank". (c) 2005 Elsevier B.V. All rights reserved
Aftershocks rates seem to follow a power law decay, but the question of the aftershock frequency immediately after an earthquake remains open. We estimate an average aftershock decay rate within one day in southern California by stacking in time different sequences triggered by main shocks ranging in magnitude from 2.5 to 4.5. Then we estimate the time delay before the onset of the power law aftershock decay rate. For the last 20 years, we observe that this time delay suddenly increase after large earthquakes, and slowly decreases at a constant rate during periods of low seismicity. In a band-limited power law model such variations can be explained by different patterns of stress distribution at different stages of the seismic cycle. We conclude that, on regional length scales, the brittle upper crust exhibits a collective behavior reflecting to some extent the proximity of a threshold of fracturing
We investigate the controls on the architecture of coarse-grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half-graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault-controlled subsidence along the basin-bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well-exposed, vertically arranged fluvio-deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo-seaward into distal pro-delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert-delta facies associations as they are traced palaeo-seaward. This transition is characterised by down-transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset-foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault-controlled subsidence during each PU. During episodes of low slip rate, shoal-water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long-term (> 100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault
We report on a receiver function study of the crust and upper mantle within DESERT, a multidisciplinary geophysical project to study the lithosphere across the Dead Sea Transform (DST). A temporary seismic network was operated on both sides of the DST between 2000 April and 2001 June. The depth of the Moho increases smoothly from about 30 to 34-38 km towards the east across the DST, with significant north-south variations east of the DST. These Moho depth estimates from receiver functions are consistent with results from steep-and wide-angle controlled-source techniques. Steep-angle reflections and receiver functions reveal an additional discontinuity in the lower crust, but only east of the DST. This leads to the conclusion that the internal crustal structure east and west of the DST is different. The P to S converted phases from both discontinuities at 410 and 660 km are delayed by 2 s with respect to the IASP91 global reference model. This would indicate that the transition zone is consistent with the global average, but the upper mantle above 410 km is 3-4 per cent slower than the standard earth model
Seismic tomography, imaging of seismic scatterers, and magnetotelluric soundings reveal a sharp lithologic contrast along a similar to 10 km long segment of the Arava Fault (AF), a prominent fault of the southern Dead Sea Transform (DST) in the Middle East. Low seismic velocities and resistivities occur on its western side and higher values east of it, and the boundary between the two units coincides partly with a seismic scattering image. At 1 - 4 km depth the boundary is offset to the east of the AF surface trace, suggesting that at least two fault strands exist, and that slip occurred on multiple strands throughout the margin's history. A westward fault jump, possibly associated with straightening of a fault bend, explains both our observations and the narrow fault zone observed by others
On 26 December 2004, a moment magnitude M-w = 9.3 earthquake occurred along Northern Sumatra, the Nicobar and Andaman islands, resulting in a devastating tsunami in the Indian Ocean region(1). The rapid and accurate estimation of the rupture length and direction of such tsunami-generating earthquakes is crucial for constraining both tsunami wave- height models as well as the seismic moment of the events. Compressional seismic waves generated at the hypocentre of the Sumatra earthquake arrived after about 12 min at the broadband seismic stations of the German Regional Seismic Network (GRSN)(2,3), located approximately 9,000 km from the event. Here we present a modification of a standard array- seismological approach and show that it is possible to track the propagating rupture front of the Sumatra earthquake over a total rupture length of 1,150 km. We estimate the average rupture speed to be 2.3-2.7 km s(-1) and the total duration of rupture to be at least 430 s, and probably between 480 and 500 s.
The Hengshan complex forms part of the central zone of the North China Craton and consists predominantly of ductilely-deformed late Archaean to Palaeoproterozoic high-grade, partly migmatitic, granitoid orthogneisses, intruded by mafic dykes of gabbroic composition. Many highly strained rocks were previously misinterpreted as supracrustal sequences and represent mylonitized granitoids and sheared dykes. Our single zircon dating documents magmatic granitoid emplacement ages between 2.52 Ga and 2.48 Ga, with rare occurrences of 2.7 Ga gneisses, possibly reflecting an older basement. A few granitic gneisses have emplacement ages between 2.35 and 2.1 Ga and show the same structural features as the older rocks, indicating that the main deformation occurred after similar to 2.1 Ga. Intrusion of gabbroic dykes occurred at similar to 1920 Ma, and all Hengshan rocks underwent granulite-facies metamorphism at 1.88-1.85 Ga, followed by retrogression, shearing and uplift. We interpret the Hengshan and adjacent Fuping granitoid gneisses as the lower, plutonic, part of a late Archaean to early Palaeoproterozoic Japan-type magmatic arc, with the upper, volcanic part represented by the nearby Wutai complex. Components of this arc may have evolved at a continental margin as indicated by the 2.7 Ga zircons. Major deformation and HP metamorphism occurred in the late Palaeoproterozoic during the Luliang orogeny when the Eastern and Western blocks of the North China Craton collided to form the Trans-North China orogen. Shear zones in the Hengshan are interpreted as major lower crustal discontinuities post-dating the peak of HP metamorphism, and we suggest that they formed during orogenic collapse and uplift of the Hengshan complex in the late Palaeoproterozoic (< 1.85 Ga)
Numerical thermodynamic modelling of mineral composition and modes for specified pressure-temperature paths reveals the strong influence of fractional garnet crystallisation, as well as water fractionation, on garnet growth histories in high pressure rocks. Disequilibrium element incorporation in garnet due to the development of chemical inhomogeneities around porphyroblasts leads to pronounced episodic growth and may even cause growth interruptions. Discontinuous growth, together with pressure- and temperature-dependent changes in garnet chemistry, cause zonation patterns that are indicative of different degrees of disequilibrium element incorporation. Chemical inhomogeneities in the matrix surrounding garnet porphyroblasts strongly affect garnet growth and lead to compositional discontinuities and steep compositional gradients in the garnet zonation pattern. Further, intergranular diffusion-controlled calcium incorporation can lead to a characteristic rise in grossular and spessartine contents at lower metamorphic conditions. The observation that garnet zonation patterns diagnostic of large and small fractionation effects coexist within the same sample suggests that garnet growth is often controlled by small-scale variations in the bulk rock chemistry. Therefore, the spatial distribution of garnet grains and their zonation patterns, together with numerical growth models of garnet zonation patterns, yield information about the processes limiting garnet growth. These processes include intercrystalline element transport and dissolution of pre-existing grains. Discontinuities in garnet growth induced by limited element supply can mask traces of the thermobarometric history of the rock. Therefore, thermodynamic modelling that considers fractional disequilibrium crystallisation is required to interpret compositional garnet zonation in terms of a quantitative pressure and temperature path of the host rock
Seaward progradation of several kilometers has been documented mostly for leeward margin low-angle carbonate slope systems with a dominant platform top sediment source. However, steep and high-relief margins fronting deep basins can also prograde and as such are somewhat perplexing. Characteristics of two prograding Carboniferous examples provide a model which may apply elsewhere: (1) outcrops in Asturias, northern Spain serve as important analogs for (2) hydrocarbon reservoirs in steep-sided isolated platforms of the North Caspian Basin, Kazakhstan, such as Tengiz. Seismic and well data from Tengiz corroborate outcrop patterns for slope development, showing progradation of up to 5 and more than 10 km, respectively, despite the high-relief (up to 600 m) and steep (similar to 20-35 degrees) nature of these margins. The two examples share a highly productive microbial boundstone factory extending from the platform break down the slope to nearly 300 m (or more) depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. The broad depth range of microbial boundstone increases the potential for production during both lowstands and high stands of sea level and thereby facilitates progradation independent from platform-top- derived sediment. Rapid in situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. What controls the microbial cement boundstone formation remains a debate but its presence is a key factor controlling the progradational geometry of these and possibly other margins. This new model of "slope" shedding has implications for slope readjustment processes and resulting architecture, sequence stratigraphic interpretation, reservoir characterization, and reservoir modeling. Especially the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke, under certain stress regimes, shattering and fracturing thereby generating significant non-matrix permeability. Key considerations are the contrasts with the Bahamian high stand shedding depositional model, slope progradation rates that range from 450 to > 1500 m/My, and net growth rates of in situ boundstone of similar to 1000 m/My, comparable to or higher than accretion rates for metazoan skeletal reef growth. (c) 2005 Published by Elsevier B.V
In this paper we explore the relative control of paleoceanography, eustasy, and water temperature over the evolution of a carbonate slope system deposited on the Marion Plateau (Northeastern Australia). Growth of several carbonate platforms started in the early Miocene on this plateau, and although they occurred in low-latitude subtropical waters they are composed mainly of heterozoan organisms. We investigated an upper to distal slope transect drilled during ODP Leg 194 and located close to the Northern Marion Platform. We reconstructed mass accumulation rates of carbonate as well as the evolution in the ratios of carbon and oxygen stable isotopes. Power spectrum analysis of the carbon isotope record revealed the existence of cycles with main frequencies centered around 409 Kyr and 1800 Kyr. We interpret the 409 Kyr cycle as being paced by changes in the eccentricity of the Earth orbit, and we suggest that the 1800 Kyr cycle could be linked to long-term eustatic changes. Finally, on the basis of the timing of changes in mass accumulation rates of carbonate we infer that the strength and direction of oceanic currents affected sedimentation on the Marion Plateau by shifting depocenters of slope sedimentation, a process probably further modulated by sea-level changes. We argue that the evolution and demise of the heterozoan carbonate systems present on the Marion Plateau were controlled mainly by the evolution of strong benthic currents, and that eustasy and water temperature alone did not account for the drowning of the platforms in the mid Miocene
In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HCl and H2SO4 solutions at temperatures of 300, 600, and 900 degrees C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HCl experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the H2SO4 experiments it ranges from sharp to diffuse. In the 1 N HCl experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the H2SO4 experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCl experiment (600 degrees C and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10-20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N H2SO4 experiment at 900 degrees C and 1,000 MPa indicates a pervasive nano- porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution-reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions
Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last similar to 8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of similar to 61.4 km(3) of material from the basin, leading to a basin-wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasiphysically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins
Rhodoliths (free-living coralline red algae) can thrive under a wide range of temperatures, reduced light, and increased nutrient levels, and often form a distinct so-called rhodalgal lithofacies that is an important component of Cenozoic shallow-water carbonates. Global distributions illustrate that from the late-early to early-late Miocene (Burdigalian-early Tortonian), rhodalgal facies reached peak abundances and commonly replaced coral-reef environments, accompanied by a decline in other carbonate-producing phototrophs. We argue that the dominance of red algae over coral reefs was triggered in the Burdigalian by enhanced trophic resources associated with a global increase in productivity, as evidenced by a long-term shift toward higher carbon isotope values. Rhodalgal lithofacies expanded further in the middle Miocene when strengthened thermal gradients associated with the establishment of the East Antarctic lee Sheet led to enhanced upwelling while climate change generated increased weathering rates, introducing land-derived nutrients into the oceans. Globally cooler temperatures following a climatic optimum in the early-middle Miocene contributed to sustain the dominance of red algae and prevented the recovery of coral reefs. The global shift in nearshore shallow-water carbonate producers to groups tolerant of higher levels of trophic resources provides further evidence for increased nutrient levels during that time interval and shows the sensitivity of shallow-water carbonate facies as indicators of past oceanographic conditions
"Green water"-the water stored in the soil and productively used for plant transpiration-is an important quantity particularly in rainfed agriculture (in contrast to "blue water" available in streams and lakes, on which irrigation relies). This study provides preliminary estimates of contemporary (1961-1990) global green water flows (i.e. plant transpiration), using a well-established, process-based dynamic global vegetation and water balance model, LPJ. Transpiration is analysed with respect to differences between a simulation that accounts for human land cover changes and a simulation under conditions of potential natural vegetation. We found that historic land cover change usually reduced the green water flow to the atmosphere, resulting in a global decrease of similar to 7% in total. To further explore how the biophysical setting influences the green water flow, we analyse the ratio between soil moisture-limited canopy conductance of carbon and water and energy-controlled potential conductance. This plant physiology-based ratio measures the degree to which actual green water flow falls below the potential flow that would occur when the soil is saturated, thus it represents a measure of the water limitation of terrestrial vegetation. We found that plant water limitation is lowest in the wet tropics and at high latitudes, where soil moisture is high enough to meet the atmospheric demand for transpiration. The present results are preliminary, since irrigation is not yet accounted for, and because the model simulations are compromised primarily by the quality of the input data. A more comprehensive and consistent assessment of global green and blue water flows and limitations using an enhanced LPJ model is identified as a prime task for future studies. (c) 2005 Elsevier Ltd. All rights reserved
The uranium deposit at Niederschlema-Alberoda, Germany, contains a rich variety of Bi minerals deposited between the Permian and the Cretaceous; these have been studied for paragenetic relations, composition, and conditions of formation. Particular attention is given to the rare Bi selenides watkinsonite, nevskite, and cuproan bohdanowiczite. Whereas watkinsonite and nevskite only occur intergrown with clausthalite, bohdanowiczite is more widespread and also is associated with Cu selenides. Watkinsonite from this second confirmed locality worldwide has an average composition (Cu1.47Ag0.49)(Sigma 1.96)(Pb1.01Hg0.01 Fe-0.01)(Sigma 1.03)Bi-3.98(Se7.98S0.05)(Sigma 8.03), ideally (Cu,Ag)(2)PbBi4Se8. These findings suggest that the empirical formula of watkinsonite originally proposed for the type specimen from the Otish Mountains uranium deposit in Quebec [CU2+xPb1+xBi4-xSe,S,Te)(8), x approximate to 0.3] requires revision. The composition of nevskite is (Pb0.06Bi0.95)(Sigma 1.01)Se-0.99, on average. Bohdanowiczite from the Cu- selenide assemblage shows extensive substitution of Cu+ for Ag+, expressed by the crystallochemical formula (Ag1.80- 0.94CU0.16-1.05Pb0.00-0.05)(Sigma 1.97-2.07)BiSigma 1.97-2.03SeSigma 3.96-4.04. This observation seems to argue for the natural existence of CU2Bi2Se4, the Se-dominant analogue of emplectite. The Bi selenides were deposited at temperatures of about 100 degrees C, in the Jurassic. The lack of thermodynamic data for all the Bi selenides limits reliable inferences on the fugacities of selenium and sulfur that prevailed during their formation. Other Bi minerals from this locality comprise members of the bismuthinite-aikinite solid-solution series of Permian age and, more importantly, native Bi and Bi sulfides (matildite, bismuthinite, wittichinite), deposited in the Cretaceous
Potential fields are classically represented on the sphere using spherical harmonics. However, this decomposition leads to numerical difficulties when data to be modelled are irregularly distributed or cover a regional zone. To overcome this drawback, we develop a new representation of the magnetic and the gravity fields based on wavelet frames. In this paper, we first describe how to build wavelet frames on the sphere. The chosen frames are based on the Poisson multipole wavelets, which are of special interest for geophysical modelling, since their scaling parameter is linked to the multipole depth (Holschneider et al.). The implementation of wavelet frames results from a discretization of the continuous wavelet transform in space and scale. We also build different frames using two kinds of spherical meshes and various scale sequences. We then validate the mathematical method through simple fits of scalar functions on the sphere, named 'scalar models'. Moreover, we propose magnetic and gravity models, referred to as 'vectorial models', taking into account geophysical constraints. We then discuss the representation of the Earth's magnetic and gravity fields from data regularly or irregularly distributed. Comparisons of the obtained wavelet models with the initial spherical harmonic models point out the advantages of wavelet modelling when the used magnetic or gravity data are sparsely distributed or cover just a very local zone
Basin formation dynamics of the Tertiary Piedmont Basin (TPB) are here investigated by means of cross-section numerical modelling. Previous works hypothesised that basin subsidence occurred due first to extension (Oligocene) and then to subsequent loading due to back-thrusting (Miocene). However, structural evidence shows that the TPB was mainly under contraction from Oligocene until post Pliocene time while extension played a minor role. Furthermore, thermal indicators strongly call for a cold (flexure-induced) mechanism but are strictly inconsistent with a hot (thermally induced) mechanism. Our new modelling shows that the TPB stratigraphic features can be reproduced by flexure of a visco- elastic plate loaded by backthrusts active in the Western Alps in Oligo-Miocene times. Far-field compression contributed to the TPB subsidence and controlled the basin infill geometry by enhancing basin tilting, forebulge uplift and erosion of the southern margin of the basin. These results suggest that the TPB subsidence is the result of a combination of mechanisms including thrust loading and farfield compressional stresses. (c) 2005 Elsevier B.V. All rights reserved
Carpholite+chloritoid+pyrophyllite association occurs widely in the Triassic metaclastic rocks of the Afyon Zone in west-central Turkey. Fe-Mg-carpholite is associated with rare aragonite pseudomorphs and glaucophane in marbles and metabasites, respectively. The Afyon Zone consists stratigraphically of a Pan-African basement and an overlying Mesozoic cover sequence. The Pan-African basement, which shows Barrovian-type amphibolite-facies metamorphism, comprises garnet-mica schists, intruded by sodic amphibole-bearing metagabbros and leucocratic metagranites. It is unconformably overlain by a continuous metasedimentary sequence extending from Triassic to early Palaeocene. This cover sequence begins with metaconglomerates, which pass upwards into phyllites. Fe-Mg-carpholite occurs within this metaclastic sequence as rosette-like crystals in metapelites and fibres in quartz segregations. The metaclastic rocks are succeeded by metamorphosed platform carbonates, grading into Latest Mesozoic metamorphosed pelagic limestones, which in turn progress up to a Late Mesozoic-Early Tertiary olistostrome. This sequence is tectonically overlain by the blueschists of the Tavsanh Zone. Fe-Mg-carpholite-bearing assemblages imply temperatures of about 350 degrees C and minimum pressures of 6-9 kbar, corresponding to burial depths of about 30 km for the Mesozoic passive continental margin sediments and the underlying Pan-African supracrustal metasediments and metaintrusives. The metamorphic rocks of the Afyon Zone are unconformably overlain by Upper Palaeocene-Lower Eocene sedimentary rocks, indicating a Paleocene age for the regional HP/LT metamorphism. This implies continuous younging of HP/LT metamorphism in the Anatolides related to northward subduction of the Anatolide-Tauride platform beneath the Sakarya Zone. From north to south this involved the Tavsanh Zone (Campanian, 80 +/- 5 Ma), the Afyon Zone (Palaeocene?), the Menderes Massif (Middle Eocene) and the Lycian Nappes (Late Cretaceous-Eocene?), all of which were probably derived from the frontal part of the Anatolide-Tauride platform. (c) 2005 Elsevier B.V. All rights reserved
[1] The Puna-Altiplano plateau in South America is a high-elevation, low internal relief landform that is characterized by internal drainage and hyperaridity. Thermochronologic and sedimentologic observations from the Sierra de Calalaste region in the southwestern Puna plateau, Argentina, place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present-day basins and the filling pattern of these basins during late Eocene-Miocene time. Paleocurrent indicators, sedimentary provenance analyses, and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the Sierra de Calalaste range between 24 and 29 Ma. This event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions, which, in the Salar de Antofalla, were established not later than late Miocene. Upper Eocene-Oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift. Provenance data indicate a common similar source located to the west for both the southern Puna and the Altiplano of Bolivia during the late Eocene- Oligocene with sporadic local sources. This suggests the existence of an extensive, longitudinally oriented foreland basin along the central Andes during this time
An analysis of the shear (S) waves recorded during the wide-angle reflection/refraction (WRR) experiment as part of the DESERT project crossing the Dead Sea Transform (DST) reveals average crustal S-wave velocities of 3.3-3.5 km s(-1) beneath the WRR profile. Together with average crustal P-wave velocities of 5.8-6.1 km s(-1) from an already published study this provides average crustal Poisson's ratios of 0.26-0.27 (V-p/V-s = 1.76-1.78) below the profile. The top two layers consisting predominantly of sedimentary rocks have S- wave velocities of 1.8-2.7kms(-1) and Poisson's ratios of 0.25-0.31 (V-p/V-s = 1.73-1.91). Beneath these two layers the seismic basement has average S- wave velocities of around 3.6 km s(-1) east of the DST and about 3.7 km s(-1) west of the DST and Poisson's ratios of 0.24-0.25 (V-p/V-s = 1.71-1.73). The lower crust has an average S-wave velocity of about 3.75 km s(-1) and an average Poisson's ratio of around 0.27 (V-p/V-s = 1.78). No Sn phase refracted through the uppermost mantle was observed. The results provide for the first time information from controlled source data on the crustal S-wave velocity structure for the region west of the DST in Israel and Palestine and agree with earlier results for the region east of the DST in the Jordanian highlands. A shear wave splitting study using SKS waves has found evidence for crustal anisotropy beneath the WRR profile while a receiver function study has found evidence for a lower crustal, high S-wave velocity layer east of the DST below the profile. Although no evidence was found in the S-wave data for either feature, the S-wave data are not incompatible with crustal anisotropy being present as the WRR profile only lies 30 degrees off the proposed symmetry axis of the anisotropy where the difference in the two S-wave velocities is still very small. In the case of the lower crustal, high S-wave velocity layer, if the velocity change at the top of this layer comprises a small first-order discontinuity underlain by a 2 km thick transition zone, instead of just a large first-order discontinuity, then both the receiver function data and the WRR data presented here can be satisfied. Finally, the S-wave velocities and Poisson's ratios which have been derived in this study are typical of continental crust and do not require extensional processes to explain them
The Sierra del Cuera (Cantabrian Mountains, Asturias, Spain) exhibits an exceptionally well-preserved upper Bashkirian-lower Moscovian succession of a high-relief carbonate platform with steep adjacent slope. During the late Bashkirian, the platform margin prograded rapidly basinward, and during the early Moscovian, platform aggradation increased. Fusulinida distributions have been estimated in three platform study windows representative of the Bashkirian and Moscovian lithofacies. Microbial boundstone accumulations at the platform margin and slope contain scarce fusulinoideans (particularly during the Moscovian aggradation, when water depth was relatively greater). Boundstones are characterized by tetrataxids, lasiodiscids, pseudoammodiscids, biseriamminids, endothyrids and palaeotextulariids. Grainstone units with sediments deposited above wave base are characterized by the predominance of Pseudoendothyra, Eostaffella (Bashkirian) or Profusulinella (Moscovian), Bradyina and palaeotextulariids. The open-marine facies, below effective wave-base but intermittently affected by storm currents, is enriched in endothyrids and palaeotextulariids in the Bashkirian part, and in Profusulinella, Eofusulina and Schubertella in the Moscovian part. The Moscovian facies, of lower energy and deeper water depth, contain some microbially precipitated micrite and have scarce Fusulinida, mostly represented by endothyrids, biseriamminids, and fusulinoideans (Eostaffella, Ozawainella and Profusulinella). Lagoonal open- to restricted-marine facies with beresellid algae contain Profusulinella, Schubertella and biseriamminids. Crinoidal packstones were deposited in moderate-energy settings close to the platform margin, below wave-base, and are characterized by Ozawainella, Eostaffella, palaeotextulariids and tetrataxids. These analyses show that fusulinid foraminifera inhabited mostly shallow and open-marine environments. Their distribution was controlled by depth-related variables, water energy and open-marine vs. restricted-marine conditions. Fusulinoideans increase in the Moscovian strata, and show different trends in the upper Bashkirian and lower Moscovian platforms, possibly because of: a) changes in the paleoecology of Fusulinida at the species level from the Bashkirian to the Moscovian; and/or b) variations in the depositional environments and physicochemical characteristics of seawater from the Bashkirian progradational phase to the Moscovian aggradational phase. Pseudoendothyra, Profusulinella, Schubertella, bradyinids and palacotextulariids showed an affinity for high-energy settings. Profusulinella and Schubertella, however, preferred the shallowest facies and also tolerated restricted marine conditions. Eostaffella's distribution largely differs between the Bashkirian and Moscovian parts because it characterizes the high-energy grainstones of the upper Bashkirian and the open-marine, moderate- to low-energy environments of the lower Moscovian. Endothyrids were excluded from lagoonal environments with restricted circulation and abnormal salinity. Biscriamminids preferred open-marine, moderate- to low-energy settings with muddy substrates, but were not li
The effect of water activity on the oxidation and structural state of Fe in a ferro-basaltic melt
(2005)
Experimental investigations have been performed at T = 1200 degrees C, P = 200 MPa and fH(2) corresponding to H2O-MnO-Mn3O4 and H2O-QFM redox buffers to study the effect of H2O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mossbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe3+/Sigma Fe ratio of the glass is directly related to aH(2)O in a H-2-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H2O <-> H-2 + 1/2 O-2). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO(2) and Fe3+/Fe2+ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of similar to 0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + 1/4 O-2 = FeO1.5). The mean centre shifts for Fe2+ and Fe3+ absorption doublets in Mossbauer spectra show little change with increasing Fe3+/Sigma Fe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses. Copyright (c) 2005 Elsevier Ltd
Late Quaternary intensified monsoon phases control landscape evolution in the northwest Himalaya
(2005)
The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sediment records. However, the mechanisms of long-lived (2-5 k.y.) intensified monsoon phases, the related changes in precipitation distribution, and their effect on landscape evolution and sedimentation rates are not yet well understood. The and high-elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal (i.e., strengthened) monsoon seasons. Analogous to present-day rainfall anomalies, enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure. We associate landslide triggering during intensified monsoon phases with enhanced precipitation, discharge, and sediment flux leading to an increase in pore-water pressure, lateral scouring of rivers, and over- steepening of hillslopes, eventually resulting in failure of slopes and exceptionally large mass movements. Here we use lacustrine deposits related to spatially and temporally clustered large landslides (>0.5 km(3)) in the Sutlej Valley region of the northwest Himalaya to calculate sedimentation rates and to infer rainfall patterns during late Pleistocene (29-24 ka) and Holocene (10-4 ka) intensified monsoon phases. Compared to present-day sediment-flux measurements, a fivefold increase in sediment-transport rates recorded by sediments in landslide-dammed lakes characterized these episodes of high climatic variability. These changes thus emphasize the pronounced imprint of millennial-scale climate change on surface processes and landscape evolution
The interplay between topography and Indian summer monsoon circulation profoundly controls precipitation distribution, sediment transport, and river discharge along the Southern Himalayan Mountain Front (SHF). The Higher Himalayas form a major orographic barrier that separates humid sectors to the south and and regions to the north. During the Indian summer monsoon, vortices transport moisture from the Bay of Bengal, swirl along the SHF to the northwest, and cause heavy rainfall when colliding with the mountain front. In the eastern and central parts of the Himalaya, precipitation measurements derived from passive microwave analysis (SSM/I) show a strong gradient, with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen. The end of the monsoonal conveyer belt is near the Sutlej Valley in the NW Himalaya, where precipitation is lower and rainfall maxima move to lower elevations. This region thus comprises a climatic transition zone that is very sensitive to changes in Indian summer monsoon strength. To constrain magnitude, temporal, and spatial distribution of precipitation, we analyzed high-resolution passive microwave data from the last decade and identified an abnormal monsoon year (AMY) in 2002. During the 2002 AMY, violent rainstorms conquered orographic barriers and penetrated far into otherwise and regions in the northwest Himalaya at elevations in excess of 3 km asl. While precipitation in these regions was significantly increased and triggered extensive erosional processes (i.e., debris flows) on sparsely vegetated, steep hillslopes, mean rainfall along the low to medium elevations was not significantly greater in magnitude. This shift may thus play an important role in the overall sediment flux toward the Himalayan foreland. Using extended precipitation and sediment flux records for the last century, we show that these events have a decadal recurrence interval during the present-day monsoon circulation. Hence, episodically occurring AMYs control geomorphic processes primarily in the high-elevation and sectors of the orogen, while annual recurring monsoonal rainfall distribution dominates erosion in the low- to medium- elevation parts along the SHF. (C) 2004 Elsevier B.V. All rights reserved
The up to similar to4 km high southern Patagonian Andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies, and cause one of the most drastic orographic rain shadows on earth. Geologic data imply that this climatic pattern has been established or significantly enhanced during Miocene surface uplift of this Andean segment. We report evidence for important climatic and ecologic changes in the eastern foreland of the Patagonian Andes that appear to be the result of this uplift. To provide constraints on Miocene plant ecosystems and precipitation in the eastern (leeward) foreland of the Patagonian Andes, we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a similar to500 m thick section of the continental Santa Cruz Formation. The age of these deposits was constrained by Ar/Ar dating of intercalated tuffs, which range from similar to22 to 14 Ma. At similar to16.5 Ma, the delta(13)C values increase by similar to3parts per thousand, the delta(18)O values decrease by >2parts per thousand, and the scatter in the oxygen isotope data increases significantly. We interpret these changes as the consequence of >1 km surface uplift in this Andean segment (from the delta(18)O values), and increased aridity to its east (from the delta(13)C values and the increased scatter in the delta(18)O values). Sediments overlying the Santa Cruz Formation are very limited in extent and volume, and dominated by coarse conglomerates related to Pleistocene and older glaciations. It thus seems that, by similar to14 Ma, deposition in the eastern foreland of the Southern Patagonian Andes had essentially ceased as the result of rain shadow formation. (C) 2004 Elsevier B.V. All rights reserved
Composite ground-motion models and logic trees: Methodology, sensitivities, and uncertainties
(2005)
Logic trees have become a popular tool in seismic hazard studies. Commonly, the models corresponding to the end branches of the complete logic tree in a probabalistic seismic hazard analysis (PSHA) are treated separately until the final calculation of the set of hazard curves. This comes at the price that information regarding sensitivities and uncertainties in the ground-motion sections of the logic tree are only obtainable after disaggregation. Furthermore, from this end-branch model perspective even the designers of the logic tree cannot directly tell what ground-motion scenarios most likely would result from their logic trees for a given earthquake at a particular distance, nor how uncertain these scenarios might be or how they would be affected by the choices of the hazard analyst. On the other hand, all this information is already implicitly present in the logic tree. Therefore, with the ground-motion perspective that we propose in the present article, we treat the ground-motion sections of a complete logic tree for seismic hazard as a single composite model representing the complete state-of-knowledge-and-belief of a particular analyst on ground motion in a particular target region. We implement this view by resampling the ground-motion models represented in the ground-motion sections of the logic tree by Monte Carlo simulation (separately for the median values and the sigma values) and then recombining the sets of simulated values in proportion to their logic-tree branch weights. The quantiles of this resampled composite model provide the hazard analyst and the decision maker with a simple, clear, and quantitative representation of the overall physical meaning of the ground-motion section of a logic tree and the accompanying epistemic uncertainty. Quantiles of the composite model also provide an easy way to analyze the sensitivities and uncertainties related to a given logic-tree model. We illustrate this for a composite ground- motion model for central Europe. Further potential fields of applications are seen wherever individual best estimates of ground motion have to be derived from a set of candidate models, for example, for hazard rnaps, sensitivity studies, or for modeling scenario earthquakes
This study concerns the Quantitative Phase Analysis (QPA) of historical bricks coming from the complex of the Great Palace of the Byzantine Emperors in Istanbul. The studied samples are characterised by different chemical compositions (low and high calcium content), variable firing temperatures and different amounts of soluble salts as damage products. In the low-Ca samples, the decrease of the phyllosilicate content (from 23.4 to 6.9 wt%) is associated to the increase of the amorphous fraction (from 24 to 48%). This clear negative correlation between the phyllosilicate content and the amorphous fraction indicates that in low-Ca systems vitrification processes are overwhelming with respect to nucleation and recrystallisation processes. By contrast, high-Ca samples present newly formed Ca(Mg) silicates (diopside from 5.7 to 27.2%; anorthite from 1.4 to 8.7%) and aluminium silicates (gehlenite only in two samples, 6.2 and 7.7%) associated to the decrease of quartz (from 27.7 to 11.5%), phyllosilicate (from 6.5% until complete break down) and amorphous (from 30 to 14%) phase fractions. These findings support the role played by the CaO(MgO) content deriving from carbonates decomposition which reacts with Al2O3 and SiO2 oxides from dehydroxylated clay minerals and quartz grains. The above results have been obtained by X-ray powder diffraction data using the combined Rietveld refinement - internal standard method in order to estimate both the crystalline and the amorphous phase fractions. In addition, the coexistence of two distinct plagioclases in high-Ca samples was modelled as follows: a primary albite, which tends to incorporate Ca during the firing process as demonstrated by the increasing of gamma crystallographic angle, and a newly formed anorthite. Finally, by difference between the X-ray fluorescence data and the chemical compositions inferred by QPA, it proved possible to roughly estimate the residual chemical composition attributable to the amorphous fraction. On the basis of our data, we believe that Rietveld refinement combined with the internal standard method represent a powerful tool to better characterise complex polycrystalline and amorphous mixture as in the case of historical bricks
The PEGASOS project was a major international seismic hazard study, one of the largest ever conducted anywhere in the world, to assess seismic hazard at four nuclear power plant sites in Switzerland. Before the report of this project has become publicly available, a paper attacking both methodology and results has appeared. Since the general scientific readership may have difficulty in assessing this attack in the absence of the report being attacked, we supply a response in the present paper. The bulk of the attack, besides some misconceived arguments about the role of uncertainties in seismic hazard analysis, is carried by some exercises that purport to be validation exercises. In practice, they are no such thing; they are merely independent sets of hazard calculations based on varying assumptions and procedures, often rather questionable, which come up with various different answers which have no particular significance. (C) 2005 Elsevier B.V. All rights reserved
Microseismic data from observatories in Europe, which have been continuously recorded since about 100 years, contain information on the wave-climate in the North Atlantic. They can potentially be used as additional constraints in high-resolution temporal and spatial reconstructions of the storminess and oceanic waveheights in the past. To resolve spatial patterns data from observatories in different regions are needed. While previous recent studies analyzed only few observatory archives and relatively short time ranges, this is a first attempt to process the whole available data archive from different observatories. We correct and compare smoothed microseismic data from different stations and discuss their correlation and possible use for studies of storminess variability. Microseismic amplitudes at four seismic stations in northern Europe show amplitude peaks in 1920 and 1925, a slow decline in amplitudes till the middle of the 1930's followed by a steady increase of amplitudes till about 1990. From 1990 on microseismic amplitudes decrease. We find a good correlation between the average surface wind velocity in the North Atlantic and microseismic amplitudes at inland stations far away from the coast. Coastal stations are more influenced by local swell and are thus potentially useful to recover regional changes in wind and ocean wavefields with time. The study demonstrates that the analysis of microseismic has the potential to assess climate changes during the last 100 years
Annite and Fe-rich siderophyllite constitute the rock-forming micas in the late-Variscan composite granite pluton of Konigshain, Lausitz, Germany. This multiphase pluton is composed of three fractionated, but not chemically specialized monzogranite types, which contain lithophile elements such as Li, Rb, Cs, Sn, and F in average quantities. Abundant miarolitic pegmatites of the NYF family with a broad diversity of rare minerals occur in the apical part of the pluton. These pegmatitic cavities locally contain di- and trioctabedral micas as well as cation-deficient micas. Trioctahedral micas comprise F-rich manganoan lithian siderophyllite to manganoan zinnwaldite, zinnwaldite, and minor lepidolite. The formula [calculated on the basis of 22 anion valencies and 2 (F + OH + Cl)] of the most Mn-rich siderophyllite is (K0.85Rb0.08Na0.04)(0.97)(Al0.99Li0.91Fe0.51Mn0.42Ti0.01Zn0.01)(2.85) (Si3.21Al0.79)(4)O- 10(F1.80OH0.19Cl0.01)(2). This mica constitutes one of the most Mn-rich siderophyllite compositions reported to date. The lithium micas poorer in Mn are distinguished by elevated concentrations of Rb (up to 2.5 wt % Rb2O), CS (UP to 1.2 wt % Cs2O), and F (up to 9.6 wt %). This fluorine content is probably consistent with the maximum possible F occupation of 2 of the (F,OH,Cl)-site. The structural formula of the most Li-rich lepidolite is (K0.83Rb0.07Cs0.03)(0.93) (Li1.62Al1.00Fe0.38)(3.00)(Si3.62Al0.38)(4) O-10(F1.91OH0.09)(2). During hydrothermal alteration, lepidolite and zinnwaldite became partially depleted in K, Li, Rb, Cs, and F and gradually transformed into cation-deficient micas (lithian phengite to illite of phengitic affinity)
The "Bohemians" of New Zealand - an ethnic group? In 1982 a small group of New Zealanders established contacts with the region of origin of their about 200 German-speaking ancestors who had emigrated from Bohemia for economic reasons in the 1860s and 1870s. They had all come from about twenty villages situated west of plzen and founded a rural settlement and participated in the foundation of a second one in New Zealand. Since World War I there had been no further contacts between the emigrants and their descendants on the one hand and their relatives in Bohemia on the other hand. For two reasons new contacts were established after such a long time: (1) the back-to the-roots-movement had spread to New Zealand from the USA, Canada and Australia, and (2) the status of cultural diversity keeps being enhanced in New Zealand since about 1970. These processes also influenced those people in New Zealand who call themselves "Bohemians" because of their ancestors' region of origin. Their total number is estimated at 10, 000 to 15, 000 at present. Up to now hardly any attention was attached to them in New Zealand by academic research and/or the general public. This paper discusses the history and today's situation of the former immigrants' community as well as the New Zealand "Bohemians " in general, raising the questions to what extent they can be defined as an ethnic group now and whether they will retain their status as a specific group in future
Cretaceous magmatism in the Eastern Cordillera of Colombia is related to lithospheric stretching during the late Early-early Late Cretaceous. The small amount of preserved igneous material is represented by small mafic intrusions. This study focuses on three localities, from east to west: Pajarito, Pacho, and Caceres. The investigated igneous bodies are classified as gabbros, pyroxene-hornblende-gabbros, and pyroxene-hornblendites mainly composed of plagioclase, clinopyroxene, and/or amphibole. Although their timing of emplacement and geodynamic position seem similar, significant differences in their geochemical and petrological characteristics rule out simple models of melt genesis. Clinopyroxene and bulk chemistry indicate increasing alkalinity from west to east. Trace element concentrations point to melt sources that range from a slightly enriched mantle in the west to a highly enriched one in the east. In addition, the data reflect a decreasing degree of partial melting from west to east and the decreasing importance of residual garnet in the mantle source. Probable mantle metasomatism in the source region by slab-derived fluids, as displayed by high Ba/Nb and moderate Sr-n/P-n, is clear in the west and very slight to the east. Mantle metasomatism and melt generation probably are processes of different epochs. The lack of large volumes of igneous rocks and the absence of tectonically controlled unconformities in the investigated areas indicate that a mantle plume did not affect the regional tectonics and magmatism. We favor a model of rift-related magmatism in which melt composition is modified from east to west from a highly enriched to a less enriched mantle region, the latter metasomatized by fluids derived from an older subduction phase. (c) 2005 Elsevier Ltd. All rights reserved
Early Ilerdian (Early Eocene, Shallow Benthic Zones 5 and 6) carbonate systems of the Pyrenees shelf were deposited after a time of severe climatic ('Paleocene-Eocene Thermal Maximum, PETM') and phylogenetic ('Larger Foraminifer Turnover') changes. They reflect the radiation of nummulitid, alveolinid, and orbitolitid larger foraminifera after remarkable biotic changes at the end of the Paleocene, and announce their subsequent flourishing in the Middle Eocene. A paleoenvironmental model for tropical carbonate environments of this particular time interval is provided herein. During the Early Ilerdian, the inner and middle ramp deposits from Minerve, Campo and Serraduy revealed the end-member of a tropical carbonate factory with carbonate production dominated by the end-members of biotically (photo-autotrophic skeletal) controlled and biotically induced carbonate precipitation. Inner platform environments are dominated by alveolinids and in part by orbitolitids, middle platform environments are dominated by nummulitids. Corals are present, but they do not form reefs, which is a typical feature for the Eocene. Nummulite shoal complexes, which are well-known from the Middle Eocene are also absent during the studied Early Ilerdian interval, which may reflect the early evolutionary stage of this group
In silicate glasses and melts, water acts according to two main processes. First, it can be dissolved in high temperature/high pressure melts. Second, it constitutes a weathering agent on the glass surface. A number of in-situ x- ray absorption fine structure (XAFS) studies for Fe, Ni, Zr, Th and U show that the more charged cations (Zr, Nb, Mo, Ta, Sn, Th and U) are little affected by the presence of dissolved water in the melt. In contrast, divalent iron and nickel are highly sensitive to the presence of water, which enhance nucleation processes, for example, of phyllosilicates at the angstrom-scale. Such information provides additional constraints on the role of water deep in the Earth, particularly in magmatology. By contrast, the weathering of glass surfaces by water can be studied from a durability perspective. Experimental weathering experiments Of nuclear waste glasses performed in the laboratory show a variety of surface enrichments (carbon, chlorine, alkalis, iron) after exposure to atmospheric fluids and moisture. Mn-, and Fe-surface enrichments of analogous glasses of the XIVth century are related to the formation of Mn and Fe oxy/ hydroxides on the surface. The impact on the glass darkening is considered in terms of urban pollution and mass tourism
Fe in magma : an overview
(2005)
The strong influence of physical conditions during magma formation on Fe equilibria offers a large variety of possibilities to deduce these conditions from Fe-bearing phases and phase assemblages found in magmatic rocks. Conditions of magma genesis and their evolution are of major interest for the understanding of volcanic eruptions. A brief overview on the most common methods used is given together with potential problems and limitations. Fe equilibria are not only sensitive to changes in intensive parameters (especially T and fO(2)) and extensive parameters like composition also have major effects, so that direct application of experimentally calibrated equilibria to natural systems is not always possible. Best estimates for pre-eruptive conditions are certainly achieved by studies that relate field observations directly to experimental observations for the composition of interest using as many constraints as possible (phase stability relations, Fe-Ti oxides, Fe partitioning between phases, Fe oxidation state in glass etc.). Local structural environment of Fe in silicate melts is an important parameter that is needed to understand the relationship between melt transport properties and melt structure. Assignment of Fe co-ordination and its relationship to the oxidation state seems not to be straightforward. In addition, there is considerable evidence that the co- ordination of Fe in glass differs from that in the melt, which has to be taken into account when linking melt structure to physical properties of silicate melts at T and P.
Logic trees are widely used in probabilistic seismic hazard analysis as a tool to capture the epistemic uncertainty associated with the seismogenic sources and the ground-motion prediction models used in estimating the hazard. Combining two or more ground-motion relations within a logic tree will generally require several conversions to be made, because there are several definitions available for both the predicted ground-motion parameters and the explanatory parameters within the predictive ground-motion relations. Procedures for making conversions for each of these factors are presented, using a suite of predictive equations in current use for illustration. The sensitivity of the resulting ground-motion models to these conversions is shown to be pronounced for some of the parameters, especially the measure of source-to-site distance, highlighting the need to take into account any incompatibilities among the selected equations. Procedures are also presented for assigning weights to the branches in the ground-motion section of the logic tree in a transparent fashion, considering both intrinsic merits of the individual equations and their degree of applicability to the particular application
We test the capability of broadband arrays at teleseismic distances to image the spatio-temporal characteristics of the seismic energy release during the Dec 26, 2004 Sumatra earthquake at early observation times. Using a non-plane-wave array location technique previously reported values for rupture length (about 1150 km), duration (about 480 s), and average rupture velocity (2.4-2.7 km/s) are confirmed. Three dominant energy releases are identified: one near the hypocenter, a second at 6 degrees N94 degrees E about 130 s later and a third one after 300 s at 9 degrees N92-93 degrees E. The spatio-temporal distribution of the radiated seismic energy in the source region is calculated from the stacked broadband recordings of two arrays in Germany and Japan and results in rough estimates of the total seismic energy of 0.55.10(18) Nm (GRSN) and 1.53.10(18) Nm (FNET) respectively. Changes in the relative ratio of energy as function of spatio-temporal location indicate a rotation of the focal mechanism during the rupture process
The structure and alterations of subducted oceanic lithosphere ( e. g., thickness and seismic velocity of oceanic crust) can be obtained by analyzing guided seismic waves generated by earthquakes within the slab (Wadati- Benioff zone). In northern Chile prominent secondary phases from intermediate-depth seismicity, observed in the forearc region can be interpreted as guided waves. For the observation of guided waves it is usually required to have stations close to the wave guide, a fact which is not directly given for forearc stations in subduction zone environments. With the help of finite difference simulations we model the decoupling mechanism of guided waves at the contact between the descending oceanic plate and the upper plate crust where the wave guide is opened due to the equalization of seismic velocities. Provided that suited stations are available, this mechanism allows for the use of intermediate depth seismicity to study the shallow subduction zone structure ( <= 100 km depth)
Basement-cored uplift provinces are often characterized by high-angle reverse faulting along preexisting crustal heterogeneities, which may greatly affect the mechanics of deformation and the coupling between erosion and orogenic structure. Herein we construct a coupled deformation-erosion model to understand the mechanics and erosion of mountain belts in which the spatial distribution of deformation is largely influenced by the presence of preexisting high-angle faults. In this case, deformation is accommodated along, and topography is built above, these structures. This topographic loading leads to increasing lithostatic stresses beneath these regions. As a result, active deformation may migrate to frictionally stronger structures in adjacent regions where lithostatic loading is lower. The migration of deformation to such nearby structures depends on the Hubbert-Rubey pore fluid pressure ratio of the crust (lambda), the orientations of the frictionally weaker and stronger preexisting faults (beta(1) and beta(2), respectively), the friction coefficients (mu(b1) and mu(b2)) and Hubbert-Rubey fluid-pressure ratios (lambda(b1) and lambda(b2)) of these faults, and the surface slope of the topography above the frictionally weaker structure (alpha), assuming zero surface slope above the frictionally stronger structure. In general, we found that for a given alpha and beta(1), as mu(b1) increases lambda=lambda(b1)=lambda(b2) increases, and beta(2) decreases, the value of mu(b2) required to force deformation to migrate increases. However, as erosional processes lead to decreasing surface slopes, deformation will be inhibited from migrating to frictionally stronger structures in adjacent regions. Our model results may help to explain some aspects of the deformation observed and the possible correlation between precipitation and the migration of deformation within these tectonic provinces
Stable isotope paleoaltimetry makes use of systematic trends in the distribution and isotopic composition of modern precipitation with climate and topography, and of the potential to estimate the isotopic composition of paleoprecipitation from authigenic (in-situ formed) minerals. To illustrate the usefulness as well as potential limitations of this method, we review (1) processes controlling the isotopic composition of modem precipitation, (2) stable isotope data from modern precipitation across regions of high topography, and (3) stable isotope data from authigenic minerals that have been used to infer paleotopography. From this we conclude that stable isotope studies of authigenic minerals can permit useful inferences on paleotopography, with uncertainties that critically depend on a detailed understanding of local- to global-scale paleoclimate during the time interval of interest
Geological Atlas of Africa
(2005)
This is the first attempt to summarize the geology of Africa by presenting it in an atlas and to synthesize the stratigraphy, tectonics, economic geology, geohazards and geosites of each country and territory of the continent. Furthermore, the digitized geological maps are correlated and harmonized according to the current stratigraphic timetable. The atlas aims to contribute to capacity building in African Earth Sciences and to initiate research and economic opportunities by providing a database of basic geological background information. (Springer)
Subduction factory : 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents
(2005)
Analysis of contemporary and past gully erosion and infilling processes allowed to reconstruct the long-term evolution of a permanent gully system under cropland. An active and a buried gully under cropland were investigated. The recent sediment deposits within the active gully, adjacent to the buried gully, showed that the recent gully was filling in at a mean rate of 6.4 cm a(-1). In the buried gully, several erosion and deposition phases could be identified and the type of deposited sediments revealed a complex infilling history. Charcoal, pottery and brick fragments of different sizes were found at all depths of the gully infilling. Their age indicates that the first gully incised after the midst of the 17th century, most probably in the second half of the 18th century or the early 19th century. Gully morphology and analogy with the processes in the recent gully indicate that the buried gully filled in rapidly. Overall, five cycles of cut and fill occurred in 350 years or less and four cycles even within little less than a few decades, indicating that gully development and infilling under cropland can be very rapid processes. (c) 2005 Elsevier B.V All rights reserved
Lake sediments in 10 Ethiopian, Kenyan, and Tanzanian rift basins suggest that there were three humid periods at 2.7 to 2.5 million years ago (Ma), 1.9 to 1.7 Ma, and 1.1 to 0.9 Ma, superimposed on the longer-term aridification of East Africa. These humid periods correlate with increased aridity in northwest and northeast Africa and with substantial global climate transitions. These episodes could have had important impacts on the speciation and dispersal of mammals and hominins, because a number of key events, such as the origin of the genus Homo and the evolution of the species Homo erectus, took place in this region during that time.
Permian basins
(2005)
In the estimate of dispersion with the help of wavelet analysis considerable emphasis has been put on the extraction of the group velocity using the modulus of the wavelet transform. In this paper we give an asymptotic expression of the full propagator in wavelet space that comprises the phase velocity as well. This operator establishes a relationship between the observed signals at two different stations during wave propagation in a dispersive and attenuating medium. Numerical and experimental examples are presented to show that the method accurately models seismic wave dispersion and attenuation
One of the most difficult issues when dealing with optical water remote-sensing is its acceptance as a useful application for environmental research. This problem is, on the one hand, concerned with the optical complexity and variability of the investigated natural media, and therefore the question arises as to the plausibility of the parameters derived from remote-sensing techniques. Detailed knowledge about the regional bio- and chemico-optical properties is required for such studies, however such information is seldom available for the sites of interest. On the other hand, the primary advantage of remote-sensing information, which is the provision of a spatial overview, may not be exploited fully by the disciplines that would benefit most from such information. It is often seen in a variety of disciplines that scientists have been primarily trained to look at discrete data sets, and therefore have no experience of incorporating information dealing with spatial heterogeneity. In this thesis, the opportunity was made available to assess the potential of Ocean Colour data to provide spatial and seasonal information about the surface waters of Lake Baikal (Siberia). While discrete limnological field data is available, the spatial extension of Lake Baikal is enormous (ca. 600 km), while the field data are limited to selected sites and expedition time windows. Therefore, this remote-sensing investigation aimed to support a multi-disciplinary limnological investigation within the framework of the paleoclimate EU-project ‘High Resolution CONTINENTal Paleoclimate Record in Lake Baikal, Siberia (CONTINENT)’ using spatial and seasonal information from the SeaWiFS satellite (NASA). From this, the SeaWiFS study evolved to become the first efficient bio-optical satellite study of Lake Baikal. During the course of three years, field work including spectral field measurements and water sampling, was carried out at Lake Baikal in Southern Siberia, and at the Mecklenburg and Brandenburg lake districts in Germany. The first step in processing the SeaWiFS satellite data involved adapting the SeaDAS (NASA) atmospheric-correction processing to match as close as possible the specific conditions of Lake Baikal. Next, various Chl-a algorithms were tested on the atmospherically-corrected optimized SeaWiFS data set (years 2001 to 2002), comparing the CONTINENT pigment ground-truth data with the Chl-a concentrations derived from the satellite data. This showed the high performance of the global Chl-a products OC2 and OC4 for the oligotrophic, transparent waters (bio-optical Case 1) of Lake Baikal. However, considerable Chl-a overestimation prevailed in bio-optical Case 2 areas for the case of discharge events. High-organic terrigenous input into Lake Baikal could be traced and information extracted using the SeaWiFS spectral data. Suspended Particulate Matter (SPM) was quantified by the regression of the SeaDAS attenuation coefficient as the optical parameter with SPM field data. Finally, the Chl-a and terrigenous input maps derived from the remote sensing data were used to assist with analyzing the relationships between the various discrete data obtained during the CONTINENT field work. Hence, plausible spatial and seasonal information describing autochthonous and allochthonous material in Lake Baikal could be provided by satellite data.Lake Baikal, with its bio-optical complexity and its different areas of Case 1 and Case 2 waters, is a very interesting case study for Ocean Colour analyses. Proposals for future Ocean Colour studies of Lake Baikal are discussed, including which bio-optical parameters for analytical models still need to be clarified by field investigations.
Subduction zones are regions of intense earthquake activity up to great depth. Sources are located inside the subducting lithosphere and, as a consequence, seismic radiation from subduction zone earthquakes is strongly affected by the interior slab structure. The wave field of these intraslab events observed in the forearc region is profoundly influenced by a seismically slow layer atop the slab surface. This several kilometer thick low-velocity channel (wave guide) causes the entrapment of seismic energy producing strong guided wave phases that appear in P onsets in certain regions of the forearc. Observations at the Chile-Peru subduction zone presented here, as well as observations at several other circum-pacific subduction zones show such signals. Guided wave analysis contributes details of immense value regarding the processes near the slab surface, such as layering of subducted lithosphere, source locations of intraslab seismicity and most of all, range and manner of mineralogical phase transitions. Seismological data stem from intermediate depth events (depth range 70 km - 300 km) recorded in northern Chile near 21 Grad S during the collaborative research initiative " Deformation Processes in the Andes" (SFB 267). A subset of stations - all located within a slab-parallel transect close to 69 Grad W - show low-frequency first arrivals (2 Hz), sometimes followed by a second high-frequency phase. We employ 2-dimensional finite-difference simulations of complete P-SV wave propagation to explore the parameter space of subduction zone wave guides and explain the observations. Key processes underlying the guided wave propagation are studied: Two distinct mechanisms of decoupling of trapped energy from the wave guide are analyzed - a prerequisite to observe the phases at stations located at large distances from the wave guide (up to 100 km). Variations of guided wave effects perpendicular to the strike of the subduction zone are investigated, such as the influence of phases traveling in the fast slab. Further, the merits and limits of guided wave analysis are assessed. Frequency spectra of the guided wave onsets prove to be a robust quantity that captures guided wave characteristics at subduction zones including higher mode excitation. They facilitate the inference of wave guide structure and source positioning: The peak frequency of the guided wave fundamental mode is associated with a certain combination of layer width and velocity contrast. The excitation strength of the guided wave fundamental mode and higher modes is associated with source position and orientation relative to the low-velocity layer. The guided wave signals at the Chile-Peru subduction zone are caused by energy that leaks from the subduction zone wave guide. On the one hand, the bend shape of the slab allows for leakage at a depth of 100 km. On the other, equalization of velocities between the wave guide and the host rocks causes further energy leakage at the contact zone between continental and oceanic crust (70 km depth). Guided waves bearing information on deep slab structure can therefore be recorded at specific regions in the forearc. These regions are determined based on slab geometry, and their locations coincide with the observations. A number of strong constraints on the structure of the Chile-Peru slab are inferred: The deep wave guide for intraslab events is formed by a layer of 2 km average width that remains seismically slow (7 percent velocity reduction compared to surrounding mantle). This low-velocity layer at the top of the Chile-Peru slab is imaged from a depth of 100 km down to at least 160 km. Intermediate depth events causing the observed phases are located inside the layer or directly beneath it in the slab mantle. The layer is interpreted as partially eclogized lower oceanic crust persisting to depth beyond the volcanic arc.
About 24 % of the land surface in the northern hemisphere are underlayed by permafrost in various states. Permafrost aggradation occurs under special environmental conditions with overall low annual precipitation rates and very low mean annual temperatures. Because the general permafrost occurrence is mainly driven by large-scale climatic conditions, the distribution of permafrost deposits can be considered as an important climate indicator. The region with the most extensive continuous permafrost is Siberia. In northeast Siberia, the ice- and organic-rich permafrost deposits of the Ice Complex are widely distributed. These deposits consist mostly of silty to fine-grained sandy sediments that were accumulated during the Late Pleistocene in an extensive plain on the then subaerial Laptev Sea shelf. One important precondition for the Ice Complex sedimentation was, that the Laptev Sea shelf was not glaciated during the Late Pleistocene, resulting in a mostly continuous accumulation of permafrost sediments for at least this period. This shelf landscape became inundated and eroded in large parts by the Holocene marine transgression after the Last Glacial Maximum. Remnants of this landscape are preserved only in the present day coastal areas. Because the Ice Complex deposits contain a wide variety of palaeo-environmental proxies, it is an excellent palaeo-climate archive for the Late Quaternary in the region. Furthermore, the ice-rich Ice Complex deposits are sensible to climatic change, i.e. climate warming. Because of the large-scale climatic changes at the transition from the Pleistocene to the Holocene, the Ice Complex was subject to extensive thermokarst processes since the Early Holocene. Permafrost deposits are not only an environmental indicator, but also an important climate factor. Tundra wetlands, which have developed in environments with aggrading permafrost, are considered a net sink for carbon, as organic matter is stored in peat or is syn-sedimentary frozen with permafrost aggradation. Contrary, the Holocene thermokarst development resulted in permafrost degradation and thus the release of formerly stored organic carbon. Modern tundra wetlands are also considered an important source for the climate-driving gas methane, originating mainly from microbial activity in the seasonal active layer. Most scenarios for future global climate development predict a strong warming trend especially in the Arctic. Consequently, for the understanding of how permafrost deposits will react and contribute to such scenarios, it is necessary to investigate and evaluate ice-rich permafrost deposits like the widespread Ice Complex as climate indicator and climate factor during the Late Quaternary. Such investigations are a pre-condition for the precise modelling of future developments in permafrost distribution and the influence of permafrost degradation on global climate. The focus of this work, which was conducted within the frame of the multi-disciplinary joint German-Russian research projects "Laptev Sea 2000" (1998-2002) and "Dynamics of Permafrost" (2003-2005), was twofold. First, the possibilities of using remote sensing and terrain modelling techniques for the observation of periglacial landscapes in Northeast Siberia in their present state was evaluated and applied to key sites in the Laptev Sea coastal lowlands. The key sites were situated in the eastern Laptev Sea (Bykovsky Peninsula and Khorogor Valley) and the western Laptev Sea (Cape Mamontovy Klyk region). For this task, techniques using CORONA satellite imagery, Landsat-7 satellite imagery, and digital elevation models were developed for the mapping of periglacial structures, which are especially indicative of permafrost degradation. The major goals were to quantify the extent of permafrost degradation structures and their distribution in the investigated key areas, and to establish techniques, which can be used also for the investigation of other regions with thermokarst occurrence. Geographical information systems were employed for the mapping, the spatial analysis, and the enhancement of classification results by rule-based stratification. The results from the key sites show, that thermokarst, and related processes and structures, completely re-shaped the former accumulation plain to a strongly degraded landscape, which is characterised by extensive deep depressions and erosional remnants of the Late Pleistocene surface. As a results of this rapid process, which in large parts happened within a short period during the Early Holocene, the hydrological and sedimentological regime was completely changed on a large scale. These events resulted also in a release of large amounts of organic carbon. Thermokarst is now the major component in the modern periglacial landscapes in terms of spatial extent, but also in its influence on hydrology, sedimentation and the development of vegetation assemblages. Second, the possibilities of using remote sensing and terrain modelling as a supplementary tool for palaeo-environmental reconstructions in the investigated regions were explored. For this task additionally a comprehensive cryolithological field database was developed for the Bykovsky Peninsula and the Khorogor Valley, which contains previously published data from boreholes, outcrops sections, subsurface samples, and subsurface samples, as well as additional own field data. The period covered by this database is mainly the Late Pleistocene and the Holocene, but also the basal deposits of the sedimentary sequence, interpreted as Pliocene to Early Pleistocene, are contained. Remote sensing was applied for the observation of periglacial strucures, which then were successfully related to distinct landscape development stages or time intervals in the investigation area. Terrain modelling was used for providing a general context of the landscape development. Finally, a scheme was developed describing mainly the Late Quaternary landscape evolution in this area. A major finding was the possibility of connecting periglacial surface structures to distinct landscape development stages, and thus use them as additional palaeo-environmental indicator together with other proxies for area-related palaeo-environmental reconstructions. In the landscape evolution scheme, i.e. of the genesis of the Late Pleistocene Ice Complex and the Holocene thermokarst development, some new aspects are presented in terms of sediment source and general sedimentation conditions. This findings apply also for other sites in the Laptev Sea region.
Diagenetic studies of carbonate rocks focused for a long time on photozoan carbonate assemblages deposited in tropical climates. The results of these investigations were taken as models for the diagenetic evolution of many fossil carbonates. Only in recent years the importance of heterozoan carbonates, generally formed out of the tropics or in deeper waters, was realized. Diagenetic studies focusing on this kind of rocks are still scarce, but indicate that the diagenetic evolution of these rocks might be a better model for many fossil carbonate settings ("calcite-sea" carbonates) than the photozoan model used before. This study deals with the determination of the diagenetic pathways and environments in such shallow-water heterozoan carbonate assemblages. Special emphasis is put on the identification of early, near-seafloor diagenetic processes and on the evaluation of the amount of constructive diagenesis in form of cementation in this diagenetic environment. As study area the Central Mediterranean, the Maltese Islands and Sicily, was chosen. Here two sections were logged in Olio-Miocene shallow-water carbonates consisting of different kinds of heterozoan assemblages. The study area is very suitable for the investigation of constructive early diagenetic processes, as the rocks were never deeply buried and burial diagenetic pressure solution and cementation as cause of lithification could be ruled out. Nevertheless, the carbonate rocks are well lithified and form steep cliffs, implying cementation/lithification in another, shallower diagenetic environment. To determine the diagenetic pathways and environments, detailed transmitted light and cathodoluminescence petrography was carried out on thin sections. Furthermore the stable isotope (δ<sup>18O and δ<sup>13C) composition of the bulk rock, single biota and single cement phases was determined, as well as the major and trace element composition of the single cement phases. Petrographically three (Sicily) to four (Maltese Islands) cementation phases, two phases of fabric selective and one of non-fabric selective dissolution, one phase of neomorphism and one of chemical compaction could be distinguished. The stable isotope measurements of the single cement phases pointed to cement precipitation from marine, marine-derived and meteoric waters. The trace element analysis indicated precipitation under reducing conditions, (A) in an open system with low rock-water interaction on the Maltese Islands and (B) in a closed system with high rock-water interaction on Sicily. For the closed systems case, aragonite as cement source could be concluded because its chemical composition was preserved in the newly formed cements. By integrating these results, diagenetic pathways and environments for the investigated locations were established, and the cement source(s) in the different environments were determined. The diagenetic evolution started in the marine environment with the precipitation of fibrous/fibrous-bladed and epitaxial cement I. These cements formed as High Mg Calcite (HMC) directly out of marine waters. The paleoenvironmentally shallowest part of the section on the Maltese Islands was also exposed to meteoric diagenetic fluids. This meteoric influence lead to the dissolution of aragonitic and HMC skeletons, which sourced the cementation by Low Mg Calcitic (LMC) epitaxial cement II in this part of the Maltese section. Entering the burial-marine environment the main part of dissolution, cementation and neomorphism started to take place. The elevated CO2 content in this environment, caused by the decay of organic matter, lead to the dissolution of aragonitic skeletons, which sourced the cementation by LMC epitaxial cement II, bladed and blocky cements. The earlier precipitated HMC cement phases were either partly dissolved (epitaxial cement I) or neomorphosed to LMC (fibrous/fibrous-bladed and epitaxial cement I). In the burial environment weak chemical compaction took place without sourcing significant amounts of cementation. In a last phase the rocks entered the meteoric realm by uplift, which caused non-fabric selective dissolution. This study shows that early diagenetic processes, taking place at or just below the sediment-water-interface, are very important for the mineralogical stabilization of heterozoan carbonate strata. The main amount of constructive diagenesis in form of cementation takes place in this environment, sourced by dissolution of aragonitic and, to a lesser degree, of HMC skeletons. The results of this study imply that the primary amount of aragonitic skeletons in heterozoan carbonate sediments must be carefully assessed, as they are the main early diagenetic cement source. In fossil heterozoan carbonate rocks, aragonitic skeletons might be the cement source even when no relict structures like micritic envelops or biomolds are preserved. In general, the diagenetic evolution of heterozoan carbonate rocks is a good model for the diagenesis of "calcite-sea" time carbonate rocks.
The role of feedback between erosional unloading and tectonics controlling the development of the Himalaya is a matter of current debate. The distribution of precipitation is thought to control surface erosion, which in turn results in tectonic exhumation as an isostatic compensation process. Alternatively, subsurface structures can have significant influence in the evolution of this actively growing orogen. Along the southern Himalayan front new 40Ar/39Ar white mica and apatite fission track (AFT) thermochronologic data provide the opportunity to determine the history of rock-uplift and exhumation paths along an approximately 120-km-wide NE-SW transect spanning the greater Sutlej region of the northwest Himalaya, India. 40Ar/39Ar data indicate, consistent with earlier studies that first the High Himalayan Crystalline, and subsequently the Lesser Himalayan Crystalline nappes were exhumed rapidly during Miocene time, while the deformation front propagated to the south. In contrast, new AFT data delineate synchronous exhumation of an elliptically shaped, NE-SW-oriented ~80 x 40 km region spanning both crystalline nappes during Pliocene-Quaternary time. The AFT ages correlate with elevation, but show within the resolution of the method no spatial relationship to preexisting major tectonic structures, such as the Main Central Thrust or the Southern Tibetan Fault System. Assuming constant exhumation rates and geothermal gradient, the rocks of two age vs. elevation transects were exhumed at ~1.4 ±0.2 and ~1.1 ±0.4 mm/a with an average cooling rate of ~50-60 °C/Ma during Pliocene-Quaternary time. The locus of pronounced exhumation defined by the AFT data coincides with a region of enhanced precipitation, high discharge, and sediment flux rates under present conditions. We therefore hypothesize that the distribution of AFT cooling ages might reflect the efficiency of surface processes and fluvial erosion, and thus demonstrate the influence of erosion in localizing rock-uplift and exhumation along southern Himalayan front, rather than encompassing the entire orogen.Despite a possible feedback between erosion and exhumation along the southern Himalayan front, we observe tectonically driven, crustal exhumation within the arid region behind the orographic barrier of the High Himalaya, which might be related to and driven by internal plateau forces. Several metamorphic-igneous gneiss dome complexes have been exhumed between the High Himalaya to the south and Indus-Tsangpo suture zone to the north since the onset of Indian-Eurasian collision ~50 Ma ago. Although the overall tectonic setting is characterized by convergence the exhumation of these domes is accommodated by extensional fault systems.Along the Indian-Tibetan border the poorly described Leo Pargil metamorphic-igneous gneiss dome (31-34°N/77-78°E) is located within the Tethyan Himalaya. New field mapping, structural, and geochronologic data document that the western flank of the Leo Pargil dome was formed by extension along temporally linked normal fault systems. Motion on a major detachment system, referred to as the Leo Pargil detachment zone (LPDZ) has led to the juxtaposition of low-grade metamorphic, sedimentary rocks in the hanging wall and high-grade metamorphic gneisses in the footwall. However, the distribution of new 40Ar/39Ar white mica data indicate a regional cooling event during middle Miocene time. New apatite fission track (AFT) data demonstrate that subsequently more of the footwall was extruded along the LPDZ in a brittle stage between 10 and 2 Ma with a minimum displacement of ~9 km. Additionally, AFT-data indicate a regional accelerated cooling and exhumation episode starting at ~4 Ma. Thus, tectonic processes can affect the entire orogenic system, while potential feedbacks between erosion and tectonics appear to be limited to the windward sides of an orogenic systems.