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Reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite-facies metapelites and amphibolites namely the Tamil Nadu area, southern India; the Val Strona, area of the Ivrea-Verbano Zone, northern Italy, the Bamble Sector, southern Norway, and the northwestern Austroalpine Otztal Complex. The titanite rims, and hence the stability of titanite (CaTiSiO4O) and Al-OH titanite, i.e. vuaganatite (hypothetical end-member CaAlSiO4OH), are discussed in the light of fH(2)O- and fO(2)-buffered equilibria involving clinopyroxene, amphibole, biotite, ilmenite, magnetite, and quartz in the systems CaO-FeO/Fe2O3-TiO2-SiO2-H2O-O-2 (CFTSH) and CaO-FeO/Fe2O3-Al2O3- SiO2-H2O-O-2 (CFASH) present in each of the examples. Textural evidence suggests that titanite reaction rims on ilmenite in rocks from Tamil Nadu, Val Strona, and the Bamble Sector originated most likely due to hydration reactions such as clinopyroxene + ilmenite +quartz+ H2O = amphibole +titanite and oxidation reactions such as amphibole + ilmenite + O-2 = titanite + magnetite + quartz + H2O during amphibolite-facies metamorphism, or, as in the case of the Otztal Complex, during a subsequent greenschist-facies overprint. Overstepping of these reactions requires fH(2)O and fO(2) to be high for titanite formation, which is also in accordance with equilibria involving Al-OH titanite. This study shows that, in addition to P, T, bulk-rock composition and composition of the coexisting fluid, fO(2) and fH(2)O also play an important role in the formation of Al-bearing titanite during amphibolite- and greenschist-facies metamorphism.
Several previous studies have addressed the diagenetic evolution of heterozoan carbonate assemblages. Generally it is assumed that early diagenetic processes in heterozoan settings are mainly destructive, including abrasion and dissolution on the sea floor. Constructive diagenesis (cementation) is delayed to later stages in the burial environment, with pressure solution of calcitic grains acting as a cement source. This paper presents a study of Oligo- Miocene inner- to outer-ramp heterozoan carbonates from the Central Mediterranean (Maltese Islands and Sicily) indicating that early diagenetic processes are more important than previously assumed. Four to five different cement types, including fibrous, two types of epitaxial, bladed and blocky cement, are distinguished based on transmitted light microscopy. Cathodoluminescence microscopy allowed a differentiation between primary high-Mg calcitic (fibrous and epitaxial cement I) and primary low-Mg calcitic (epitaxial cement II, bladed and blocky) cements. Stable-isotope data indicate cement precipitation from marine, marine-derived, and meteoric waters. Trace-element analyses point to cementation in an open system (Maltese Islands) and a closed system (Sicily). Our investigations show that the majority of constructive diagenetic processes in these rocks occurs rather early in the shallow, marine burial environment, which is transitional between the marine seafloor and the deep-burial diagenctic environment. The main cement source in this environment is assumed to be aragonite. We suggest careful consideration of the importance of aragonitic components in fossil heterozoan settings, which seem to be more abundant than previously assumed and can act as a major early cement source. Due to the low preservation potential of these components, detailed geochemical studies are necessary to detect aragonite as the cement source. Our findings also have implications when considering the reservoir qualities of these rocks, because primary porosity can be occluded early and secondary porosity is not preserved
[ 1] The Eastern Cordillera of Colombia is key to understanding the role of inherited basement anisotropies in the evolution of active noncollisional mountain belts. In particular, the Rio Blanco-Guatiquia region of the Eastern Cordillera is exemplary in displaying a variety of phenomena that document the importance of the orientation, geometry, and segmentation of preorogenic anisotropies. We document the first unambiguous evidence that extensional basement structures played an important role in determining the locus of deformation during contractional reactivation in the Eastern Cordillera. Detailed structural field mapping and analysis of industry seismic reflection profiles have helped to identify the inherited San Juanito, Naranjal, and Servita normal faults and associated transfer faults as important structures that were inverted during the Cenozoic Andean orogeny. Apparently, the more internal faults in the former rift basin were not properly oriented for an efficient reactivation in contraction. However, these faults have a fundamental role as strain risers, as folding is concentrated west of them. In contrast, reactivated normal faults such as the more external Servita fault are responsible for uplifting the eastern flank of the Eastern Cordillera. In addition, these structures are adjacent and intimately linked to the development of thin-skinned faults farther east. In part, the superimposed compression in this prestrained extensional region is compensated by lateral escape. The dominant presence of basement involved buckling and thrusting, and the restricted development of thin-skinned thrusting in this inversion orogen makes the Eastern Cordillera a close analog to the intraplate Atlas Mountains of Morocco and other inverted sectors of the Andean orogen farther south
In low-seismicity regions, such as France or Germany, the estimation of probabilistic seismic hazard must cope with the difficult identification of active faults and with the low amount of seismic data available. Since the probabilistic hazard method was initiated, most studies assume a Poissonian occurrence of earthquakes. Here we propose a method that enables the inclusion of time and space dependences between earthquakes into the probabilistic estimation of hazard. Combining the seismicity model Epidemic Type Aftershocks-Sequence (ETAS) with a Monte Carlo technique, aftershocks are naturally accounted for in the hazard determination. The method is applied to the Pyrenees region in Southern France. The impact on hazard of declustering and of the usual assumption that earthquakes occur according to a Poisson process is quantified, showing that aftershocks contribute on average less than 5 per cent to the probabilistic hazard, with an upper bound around 18 per cent
Leech et al. [Mary L. Leech, S. Singh, A.K. Jain, Simon L. Klemperer and R.M. Manickavasagam, Earth and Planetary Science Letters 234 (2005) 83-97], present 3 clusters of ages for growth stages in zircon from quartzo- feldspathic gneisses hosting coesite-bearing eclogite from the Tso Morari Complex, NW India. These age clusters, from oldest to youngest, are interpreted to represent the age of ultrahigh-pressure metamorphism, a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of Indian crust to have started earlier than previously accepted. However, no petrographic evidence, such as inclusions in the zircons relating to particular metamorphic events, is presented to substantiate the proposed sequence of metamorphic stages. Previously published data from eclogites of the same area indicate that coesite-eclogite is not the first but at least the second eclogite facies stage. In addition, the newly proposed time interval between coesite-eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet-garnet couples in eclogite. Neither the age of ultrahigh-pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data
The estimation of minimum-misfit stochastic models from empirical ground-motion prediction equations
(2006)
In areas of moderate to low seismic activity there is commonly a lack of recorded strong ground motion. As a consequence, the prediction of ground motion expected for hypothetical future earthquakes is often performed by employing empirical models from other regions. In this context, Campbell's hybrid empirical approach (Campbell, 2003, 2004) provides a methodological framework to adapt ground-motion prediction equations to arbitrary target regions by using response spectral host-to-target-region-conversion filters. For this purpose, the empirical ground-motion prediction equation has to be quantified in terms of a stochastic model. The problem we address here is how to do this in a systematic way and how to assess the corresponding uncertainties. For the determination of the model parameters we use a genetic algorithm search. The stochastic model spectra were calculated by using a speed-optimized version of SMSIM (Boore, 2000). For most of the empirical ground-motion models, we obtain sets of stochastic models that match the empirical models within the full magnitude and distance ranges of their generating data sets fairly well. The overall quality of fit and the resulting model parameter sets strongly depend on the particular choice of the distance metric used for the stochastic model. We suggest the use of the hypocentral distance metric for the stochastic Simulation of strong ground motion because it provides the lowest-misfit stochastic models for most empirical equations. This is in agreement with the results of two recent studies of hypocenter locations in finite-source models which indicate that hypocenters are often located close to regions of large slip (Mai et al., 2005; Manighetti et al., 2005). Because essentially all empirical ground-motion prediction equations contain data from different geographical regions, the model parameters corresponding to the lowest-misfit stochastic models cannot necessarily be expected to represent single, physically realizable host regions but to model the generating data sets in an average way. In addition, the differences between the lowest-misfit stochastic models and the empirical ground-motion prediction equation are strongly distance, magnitude, and frequency dependent, which, according to the laws of uncertainty propagation, will increase the variance of the corresponding hybrid empirical model predictions (Scherbaum et al., 2005). As a consequence, the selection of empirical ground-motion models for host-to-target-region conversions requires considerable judgment of the ground-motion analyst
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
We report measurements of radial and azimuthal anisotropy in the tipper mantle beneath southern and central Finland, which we obtained by array analysis of fundamental-mode Rayleigh and Love waves. Azimuthally averaged phase velocities were analysed in the period range 15 to 190 s for Rayleigh waves and 15 to 100 s for Love waves. The azimuthal variation of the Rayleigh wave phase velocities was obtained in the period range 20 to 100s. The limited depth resolution of fundamental-mode surface waves necessitated strong damping constraints in the inversion for anisotropic parameters. We investigated the effects of non-unicity on the final model by experimenting with varying model geometries. The radial anisotropy beneath Finland can be explained by a lithosphere at least 200km thick, predominantly (> 50% by volume) composed of olivine crystals having their a-axes randomly distributed in the horizontal plane. On the contrary, the measured lithospheric azimuthal anisotropy is small. This call be reconciled with body-wave observations made in the area that indicate a complex pattern of rapidly varying anisotropy. Below 200-250km depth, that is below the petrologic lithosphere as revealed by xenolith analyses conducted in the area, the magnitude of the azimuthal anisotropy increases and would be compatible with a mantle containing 15-20% by volume of olivine crystals whose a-axes are coherently aligned in the N-NE direction. The alignment of the a-axes is off the direction of present-day absolute plate motion in either the no-net-rotation or hot-spot reference frame, currently N55-N60. We interpret this mismatch as evidence for a complex convective flow pattern of the mantle beneath the shield, which, by inference, is decoupled from the overlying lithosphere.
In the humid tropics, continuing high deforestation rates are seen alongside an increasing expansion of secondary forests. In order to understand and model the consequences of these dynamic land-use changes for regional water cycles, the response of soil hydraulic properties to forest disturbance and recovery has to be quantified.At a site in the Brazilian Amazonia, we annually monitored soil infiltrability and saturated hydraulic conductivity (K-s) at 12.5, 20 cm, and 50 cm soil depth after manual forest conversion to pasture (year zero to four after pasture establishment), and during secondary succession after pasture abandonment (year zero to seven after pasture abandonment). We evaluated the hydrological consequences of the detected changes by comparing the soil hydraulic properties with site-specific rainfall intensities and hydrometric observations. Within one year after grazing started, infiltrability and K-s at 12.5 and 20 cm depth decreased by up to one order of magnitude to levels which are typical for 20-year-old pasture. In the three subsequent monitoring years, infiltrability and K-s remained stable. Land use did not impact on subsoil permeability. Whereas infiltrability values are large enough to allow all rainwater to infiltrate even after the conversion, the sudden decline of near-surface K-s is of hydrological relevance as perched water tables and overland flow occur more often on pastures than in forests at our study site. After pasture abandonment and during secondary succession, seven years of recovery did not suffice to significantly increase infiltrability and K-s at 12.5 depth although a slight recovery is obvious. At 20 cm soil depth, we detected a positive linear increase within the seven-year time frame but annual means did not differ significantly. Although more than a doubling of infiltrability and K-s is still required to achieve pre-disturbance levels, which will presumably take more than a decade, the observed slight increases of K-s might already decrease the probability of perched water table generation and overland flow development well before complete recovery.
Major earthquakes ( M > 8) have repeatedly ruptured the Nazca-South America plate interface of south-central Chile involving meter scale land-level changes. Earthquake recurrence intervals, however, extending beyond limited historical records are virtually unknown, but would provide crucial data on the tectonic behavior of forearcs. We analyzed the spatiotemporal pattern of Holocene earthquakes on Santa Maria Island (SMI; 37 degrees S), located 20 km off the Chilean coast and approximately 70 km east of the trench. SMI hosts a minimum of 21 uplifted beach berms, of which a subset were dated to calculate a mean uplift rate of 2.3 +/- 0.2 m/ky and a tilting rate of 0.022 +/- 0.002 degrees/ky. The inferred recurrence interval of strandline-forming earthquakes is similar to 180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is similar to 0.4 m per event