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This thesis work describes a new experimental method for the determination of Mode II (shear) fracture toughness, KIIC of rock and compares the outcome to results from Mode I (tensile) fracture toughness, KIC, testing using the International Society of Rock Mechanics Chevron-Bend method.Critical Mode I fracture growth at ambient conditions was studied by carrying out a series of experiments on a sandstone at different loading rates. The mechanical and microstructural data show that time- and loading rate dependent crack growth occurs in the test material at constant energy requirement.The newly developed set-up for determination of the Mode II fracture toughness is called the Punch-Through Shear test. Notches were drilled to the end surfaces of core samples. An axial load punches down the central cylinder introducing a shear load in the remaining rock bridge. To the mantle of the cores a confining pressure may be applied. The application of confining pressure favours the growth of Mode II fractures as large pressures suppress the growth of tensile cracks.Variation of geometrical parameters leads to an optimisation of the PTS- geometry. Increase of normal load on the shear zone increases KIIC bi-linear. High slope is observed at low confining pressures; at pressures above 30 MPa low slope increase is evident. The maximum confining pressure applied is 70 MPa. The evolution of fracturing and its change with confining pressure is described.The existence of Mode II fracture in rock is a matter of debate in the literature. Comparison of the results from Mode I and Mode II testing, mainly regarding the resulting fracture pattern, and correlation analysis of KIC and KIIC to physico-mechanical parameters emphasised the differences between the response of rock to Mode I and Mode II loading. On the microscale, neither the fractures resulting from Mode I the Mode II loading are pure mode fractures. On macroscopic scale, Mode I and Mode II do exist.
We present a wavelet coherence method that is capable of displaying local coherence information between two seismic stations in the sense of a spectrogram. We have analyzed the vertical components of a 20-min-long time series from four stations that were situated in the seismic near field of Stromboli volcano. Typical volcanic seismic signals recorded in the near field of Stromboli volcano consist of continuous volcanic tremor superimposed on frequent Strombolian explosion signals. The tremor exhibits a banded and frequency-stable structure, whereas the broadband explosion signals span two or three frequency decades. We demonstrate that signals related to explosion earthquakes are strongly correlated within the network over 1.5 frequency decades. Using synthetic data, we show how coherent signal portions can be extracted out of noisy data using a coherence-filtering method. A time delay analysis using coherence information results in a coarse source location estimation that lies within the crater region. With the exception of randomly fluctuating coherence peaks, low correlations have been observed in the characteristic bands that are assumed to be generated by continuous tremor. In the low-frequency band that is related to the ocean microseisms (period approximate to 4-8 sec), we observe mostly high correlation that breaks down during the appearance of explosion earthquake signals. Based on further analysis using the inverse wavelet transformation, we propose a model that describes the breakdown phenomenon as a superposition of two independent events
The results of this study clearly identify four key parameters controlling the estimation of probabilistic seismic hazard assessment (PSHA) in France in the framework of the Cornell-McGuire method. Results in terms of peak ground acceleration demonstrate the equally high impact, at all return periods, of the choice of truncation of the predicted ground-motion distribution (at + 2sigma) and of the choice between two different magnitude-intensity correlations. The choice of minimum magnitude (3.5/4.5) on hazard estimates can have an important impact at small return periods (<1000 years), whereas the maximum magnitude (6.5/7.0), on the other hand, is not a key parameter even at large return periods (10,000 years). This hierarchy of impacts is maintained at lower frequencies down to 5 Hz. Below 5 Hz, the choice of the maximum magnitude has a much greater impact, whereas the impact due to the choice of the minimum magnitude disappears. Moreover, variability due to catalog uncertainties is also quantified; these uncertainties that underly all hazard results can engender as high a variability as the controlling parameters. Parameter impacts, calculated at the centers of each source zone, show a linear trend with the seismicity models of the zone, demonstrating the lack of contributions coming from neighboring zones. Indeed, the region of influence that contributes to the PSHA estimate at a given site decreases with increasing return periods. The resulting overall variability in hazard estimates due to input uncertainties is quantified through a logic tree, obtained coefficients of variation vary between 10% and 20%. Until better physical models are obtained, the uncertainty on hazard estimates may be reduced by working on an appropriate magnitude-intensity correlation
The displacement histories of the San Jacinto and southernmost San Andreas fault zones are constrained by offset data with ages in the range of 5 Ma to 5 ka. Apparent discrepancies between long- and short-term average displacement rates can be reconciled with a time-variable rate model. In this model, the displacement rate on the San Andreas decelerated from similar to35 mm/yr at 1.5 Ma to as low as 9 +/- 4 mm/yr by 90 ka. Over this same time period, the rate on the San Jacinto fault zone accelerated from an initial value of zero to a rate of 26 +/- 4 mm/yr. The data also imply that the rate of the San Andreas fault accelerated since ca. 90 ka, from similar to9 mm/yr to the modern rate of 27 +/- 4 mm/yr, whereas the San Jacinto decelerated from 26 +/- 4 mm/yr to the modern rate of 8 +/- 4 mm/yr. The time scale of these changes is significantly longer than the earthquake cycle, but shorter than time scales characteristic of lithospheric-scalle dynamics. The emergence of the San Jacinto fault zone ca. 1.5 Ma coincided with the development of a major restraining bend in the San Andreas fault zone, suggesting that the formation of new subparallell faults could be driven by conditions that inhibit displacement on preexisting faults
Three diatomite beds exposed in the Ol Njorowa Gorge south of Lake Naivasha, Central Kenya Rift, document three major lake-level highstands between 175 and 60 kyr BP. Diatom transfer-function estimates of hydrological and hydrochemical parameters suggest that a deep and large freshwater lake existed during the highstands at 135 and 80 kyr BP. In contrast, a shallower but more expanded freshwater lake existed at 110 kyr BP. The best analog for the most extreme highstand at 135 kyr BP is the highstand during the Early Holocene humid period from 10 to 6 kyr BP. The environmental conditions as reconstructed from diatom assemblages suggest long-lasting episodes of increased humidity during the high lake periods. This contrasts to the modern situation with a relatively shallow Lake Naivasha characterized by rapid water level fluctuations within a few decades. The most likely cause for the variable hydrological conditions since 175 kyr BP is orbitally driven insolation changes on the equator and increased lateral moisture transport from the ocean.
The India-Eurasia continental collision zone provides a spectacular example of active mountain building and climatic forcing. In order to quantify the critically important process of mass removal, I analyzed spatial and temporal precipitation patterns of the oscillating monsoon system and their geomorphic imprints. I processed passive microwave satellite data to derive high-resolution rainfall estimates for the last decade and identified an abnormal monsoon year in 2002. During this year, precipitation migrated far into the Sutlej Valley in the northwestern part of the Himalaya and reached regions behind orographic barriers that are normally arid. There, sediment flux, mean basin denudation rates, and channel-forming processes such as erosion by debris-flows increased significantly. Similarly, during the late Pleistocene and early Holocene, solar forcing increased the strength of the Indian summer monsoon for several millennia and presumably lead to analogous precipitation distribution as were observed during 2002. However, the persistent humid conditions in the steep, high-elevation parts of the Sutlej River resulted in deep-seated landsliding. Landslides were exceptionally large, mainly due to two processes that I infer for this time: At the onset of the intensified monsoon at 9.7 ka BP heavy rainfall and high river discharge removed material stored along the river, and lowered the baselevel. Second, enhanced discharge, sediment flux, and increased pore-water pressures along the hillslopes eventually lead to exceptionally large landslides that have not been observed in other periods. The excess sediments that were removed from the upstream parts of the Sutlej Valley were rapidly deposited in the low-gradient sectors of the lower Sutlej River. Timing of downcutting correlates with centennial-long weaker monsoon periods that were characterized by lower rainfall. I explain this relationship by taking sediment flux and rainfall dynamics into account: High sediment flux derived from the upstream parts of the Sutlej River during strong monsoon phases prevents fluvial incision due to oversaturation the fluvial sediment-transport capacity. In contrast, weaker monsoons result in a lower sediment flux that allows incision in the low-elevation parts of the Sutlej River.
The aim of the study is to record the occurrence of sediment deformation structures in one of the tectonically most active areas on the globe, the Tien Shan range in Central Asia and to examine the significance of the deformations as indicators of palaeoseismicity. Soft-sediment deformation structures in form of balls and pseudo-nodules are exposed in the Issyk-Kul basin, within interfingering beds of shallow lacustrine, beach and fluviatile origin. Additional deformation structures that were encountered are: a complex and chaotic folded structure, giant balls and a "pillar" structure which has not been previously reported, where marl intrudes down into coarse pebbley sand and forms pillar morphology. Liquefaction features and bedforms related to storm and breaking waves were not encountered. Neither was there evidence of turbidites. Seven field criteria for relating soft-sediment deformation to palaeoseismic triggering provide strong evidence for a seismic origin of the deformation structures. Empirical relationships between magnitude and the maximum distance from an epicenter to liquefaction sites make the active epicentral zone north of Lake Issyk- Kul, with its frequent high magnitude events, the most favorable source for the deformation structures. Luminescence dating of the sediments gives a time window of 26 +/- 2.1 to 10.5 +/- 0.7 ka BP, indicating latest Pleistocene seismic activity.(C) 2003 Elsevier B.V. All rights reserved
The Tien Shan is a most active intracontinental mountain-building range with abundant Quaternary fault-related folding. In order to improve our understanding of Quaternary intermontane basin deformation, we investigated the intermontane Issyk-Kul Lake area, an anticline that was up-warped through the piedmont cover, causing partitioning of the alluvial fan veneer. To follow the morphological scenario during the warping process, we relied on surface-exposed and trenched structures and on alluvial fans and bajadas as reference surfaces. We used air photos and satellite images to analyze the spatial -temporal morphological record and determined the age of near surface sediments by luminescence dating. We demonstrate that the up-warped Ak-Teke hills are a thrust-generated subdued anticline with strong morphological asymmetry which results from the coupling of the competing processes of up-warp and erosional feedback. The active creeks across the up-warped anticline indicate that the antecedent drainage system kept pace with the rate of uplift. The rivers which once sourced the piedmont, like the Toru-Aygyr, Kultor and the Dyuresu, became deeply entrenched and gradually transformed the study area into an abandoned morphological surface. The up-warp caused local lateral drainage diversion in front of the northern backlimb and triggered the formation of a dendritic drainage pattern upfan. Luminescence dating suggest that the period of up-warp and antecedent entrenchment started after 157 ka. The morphologically mature study area demonstrates the response of fluvial systems to growing folds on piedmont areas, induced by a propagating frontal fold at a thrust belt edge, following shortening. (C) 2004 Elsevier B.V All rights reserved
We present two case studies that demonstrate how a common evaluation methodology can be used to assess the reliability of regional climate model simulations from different fields of research. In Case I, we focused on the agricultural yield loss risk for maize in Northeastern Brazil during a drought linked to an El-Nino event. In Case II, the present-day regional climatic conditions in Europe for a 10-year period are simulated. To comprehensively evaluate the model results for both kinds of investigations, we developed a general methodology. On its basis, we elaborated and implemented modules to assess the quality of model results using both advanced visualization techniques and statistical algorithms. Besides univariate approaches for individual near-surface parameters, we used multivariate statistics to investigate multiple near-surface parameters of interest together. For the latter case, we defined generalized quality measures to quantify the model's accuracy. Furthermore, we elaborated a diagnosis tool applicable for atmospheric variables to assess the model's accuracy in representing the physical processes above the surface under various aspects. By means of this evaluation approach, it could be demonstrated in Case Study I that the accuracy of the applied regional climate model resides at the same level as that we found for another regional model and a global model. Excessive precipitation during the rainy season in coastal regions could be identified as a major contribution leading to this result. In Case Study II, we also identified the accuracy of the investigated mean characteristics for near- surface temperature and precipitation to be comparable to another regional model. In this case, an artificial modulation of the used initial and boundary data during preprocessing could be identified as the major source of error in the simulation. Altogether, the achieved results for the presented investigations indicate the potential of our methodology to be applied as a common test bed to different fields of research in regional climate modeling
The upper Eocene-lower Oligocene sediments deposited in the eastern part of the Tertiary Piedmont Basin in northern Italy provide a complete record of the unroofing of the Alpine orogenic prism during the early stages of exhumation in the Ligurian sector. From late Priabonian till late Rupelian time, the sediments in the study area were derived from two different sources, one characterised by white micas with Si<6.5 pfu and Permian Ar-40/Ar-39 ages (270 Ma), and the other characterised by white micas with S>7 pfu and Eocene-Oligocene Ar-40/Ar-39 ages (32-50 Ma). The first source is considered to be indicative of low-pressure metamorphic rocks that covered the HP rocks of the Ligurian Alps, and were completely eroded by Chattian time. From this time on, the study area started to record the first input from western Alpine sources characterised by a larger span of ages with a more frequent Eoalpine signal. Thus, sediments deposited in the eastern part of the Tertiary Piedmont Basin contain the only available evidence of rocks belonging to high crustal levels in the Alpine orogenic prism that were not affected by the Alpine overprint. These data also provide time constraints to the poorly dated first conglomerates deposited in this area. Ar-40/Ar-39 geochronology reveals a minimum age of 33 +/- 1.4 Ma for the Pianfolco Conglomerates in the type locality, and of 31.4 +/- 3.5 Ma for the Borbera Conglomerates. (C) 2004 Elsevier B.V. All rights reserved
The remaining carbon stocks in wet tropical forests are currently at risk because of anthropogenic deforestation, but also because of the possibility of release driven by climate change. To identify the relative roles of CO2 increase, changing temperature and rainfall, and deforestation in the future, and the magnitude of their impact on atmospheric CO2 concentrations, we have applied a dynamic global vegetation model, using multiple scenarios of tropical deforestation (extrapolated from two estimates of current rates) and multiple scenarios of changing climate (derived from four independent offline general circulation model simulations). Results show that deforestation will probably produce large losses of carbon, despite the uncertainty about the deforestation rates. Some climate models produce additional large fluxes due to increased drought stress caused by rising temperature and decreasing rainfall. One climate model, however, produces an additional carbon sink. Taken together, our estimates of additional carbon emissions during the twenty-first century, for all climate and deforestation scenarios, range from 101 to 367 Gt C, resulting in CO2 concentration increases above background values between 29 and 129 p.p.m. An evaluation of the method indicates that better estimates of tropical carbon sources and sinks require improved assessments of current and future deforestation, and more consistent precipitation scenarios from climate models. Notwithstanding the uncertainties, continued tropical deforestation will most certainly play a very large role in the build-up of future greenhouse gas concentrations
Paleomagnetic dating of climatic events in Late Quaternary sediments of Lake Baikal (Siberia)
(2004)
Lake Baikal provides an excellent climatic archive for Central Eurasia as global climatic variations are continuously depicted in its sediments. We performed continuous rock magnetic and paleomagnetic analyses on hemipelagic sequences retrieved from 4 underwater highs reaching back 300 ka. The rock magnetic study combined with TEM, XRD, XRF and geochemical analyses evidenced that a magnetite of detrital origin dominates the magnetic signal in glacial sediments whereas interglacial sediments are affected by early diagenesis. HIRM roughly quantifies the hematite and goethite contributions and remains the best proxy for estimating the detrital input in Lake Baikal. Relative paleointensity records of the earth′s magnetic field show a reproducible pattern, which allows for correlation with well-dated reference curves and thus provides an alternative age model for Lake Baikal sediments. Using the paleomagnetic age model we observed that cooling in the Lake Baikal region and cooling of the sea surface water in the North Atlantic, as recorded in planktonic foraminifera δ18 O, are coeval. On the other hand, benthic δ18 O curves record mainly the global ice volume change, which occurs later than the sea surface temperature change. This proves that a dating bias results from an age model based on the correlation of Lake Baikal sedimentary records with benthic δ18 O curves. The compilation of paleomagnetic curves provides a new relative paleointensity curve, “Baikal 200”. With a laser-assisted grain size analysis of the detrital input, three facies types, reflecting different sedimentary dynamics can be distinguished. (1) Glacial periods are characterised by a high clay content mostly due to wind activity and by occurrence of a coarse fraction (sand) transported over the ice by local winds. This fraction gives evidence for aridity in the hinterland. (2) At glacial/interglacial transitions, the quantity of silt increases as the moisture increases, reflecting increased sedimentary dynamics. Wind transport and snow trapping are the dominant process bringing silt to a hemipelagic site (3) During the climatic optimum of the Eemian, the silt size and quantity are minimal due to blanketing of the detrital sources by the vegetal cover.
Geologic context of geodetic data across a Basin and Range normal fault, Crescent Valley, Nevada
(2004)
[1] Geodetic strain and late Quaternary faulting in the Basin and Range province is distributed over a region much wider than historic seismicity, which is localized near the margins of the province. In the relatively aseismic interior, both the magnitude and direction of geodetic strain may be inconsistent with the Holocene faulting record. We document the best example of such a disagreement across the NE striking, similar to55degrees NW dipping Crescent normal fault, where a NW oriented, 70 km geodetic baseline records contemporary shortening of similar to2 mm/yr orthogonal to the fault trace. In contrast, our geomorphic, paleoseismic, and geochronologic analyses of the Crescent fault suggest that a large extensional rupture occurred during the late Holocene epoch. An excavation across the fault at Fourmile Canyon reveals that the most recent event occurred at 2.8 +/- 0.1 ka, with net vertical tectonic displacement of 4.6 +/- 0.4 m at this location, corresponding to the release of similar to3 m of accumulated NW-SE extension. Measured alluvial scarp profiles suggest a minimum rupture length of 30 km along the range front for the event, implying a moment magnitude M-w of at least 6.6. No prior event occurred between similar to2.8 ka and similar to6.4 +/- 0.1 ka, the C-14 calender age of strata near the base of the exposed section. Assuming typical slip rates for Basin and Range faults (similar to0.3 mm/yr), these results imply that up to one third, or similar to1 m, of the extensional strain released in the previous earthquake could have reaccumulated across the fault since similar to2.8 ka. However, the contemporary shortening implies that the fault is unloading due to a transient process, whose duration is limited to between 6 years ( geodetic recording time) and 2.8 ka ( the age of the most recent event). These results emphasize the importance of providing accurate geologic data on the timescale of the earthquake cycle in order to evaluate geodetic measurements
Hessite, Ag2Te, and native tellurium (?) constitute two, previously unknown tellurium species within the complex mineral assemblage at Niederschlema-Alberoda, Erzgebirge, Germany. Hessite is always intimately associated with clausthalite and has a composition close to ideal stoichiometry. The mean empirical formula is (Ag1.98Sb0.01)(1.99)(Te0.96Se0.05)(1.01). Paragenetic relations and thermodynamic data suggest that hessite crystallized in equilibrium with clausthalite, berzelianite, and tiemannite under conditions of almost identical, high fugacitities of Se-2 and Te-2, which very locally were approached in the main selenide stage of Jurassic age. Native tellurium (?) formed as replacement product of hessite. Niederschlema-Alberoda provides the first record of hessite from an uranium deposit worldwide. Hessite and native Te are the first tellurium minerals reported from the Erzgebirge metallogenic province
The Niederschlema-Alberoda uranium deposit, in the Erzgebirge region of Germany, contains an uncommon assemblage of metallic minerals, in particular selenides, sulfides, arsenides, tellurides, and native elements, in addition to uraninite and coffinite. The complex mineralogy resulted from the superposition of several mineralizing events over the time interval from the Permian to the Cretaceous; these events introduced and redeposited a great variety of metallic elements within the hydrothermal uranium deposit (Pb, Ag, Cu, Hg, Tl, Bi, Co, Ni, As, Sb, Se, S, Te). One of the exotic minerals is jolliffeite, an arsenoselenide with end-member composition NiAsSe, so far only known from Lake Athabasca, Saskatchewan, Canada. A single, small, anhedral grain of jolliffeite from Niederschlema-Alberoda is included and partly replaced by sulfurian eskebornite. Associated minerals comprise hematite, Ni-Co-Se-bearing lollingite, clausthalite, tiemannite, mercurian hakite-giraudite solid solutions, sulfurian berzelianite, sulfurian umangite, hessite, Ni-Co-As-bearing pyrite, and Se-rich chalcopyrite. The sulfurian jolliffeite has the empirical formula (Ni0.85Cu0.09Co0.05Fe0.02Ag0.01)Sigma(1.02)As(0.98)(Se0.77S0.23)(Sigma1. 00) and differs from type jolliffeite mainly by substantial substitution of Cu (2.6-3.3 wt.%) for Ni and S (3.2-4.1 wt.%) for Se. Substantial S-for-Se substitution in jolliffeite implies extensive and probably complete miscibility between NiAsSe and its S-dominant analogue, gersdorffite-Pa3 (NiAsS). We suggest that a localized accumulation of Ni and As in the Se-(S)-bearing hydrothermal fluid gave rise to the crystallization of jolliffeite at some rare locations at a late stage of formation of the Jurassic selenide assemblage
In the earliest emplaced granite subintrusion of the multiphase peraluminous Satzung pluton, Erzgebirge, Germany, a mineral aggregate was observed consisting of sekaninaite (X-Fe = 0.74-0.94), Zn-rich hercynite (X-Zn = 0.03- 0.11), tri- and dioctahedral layer silicates of different composition and color, and minor quartz. Geological, textural, and compositional criteria argue that the sekaninaite, hercynite, quartz, and the brown biotite are not primary or secondary granite minerals, but are of metamorphic origin representing a xenolith uptaken from the granite melt near its level of emplacement. The metamorphic origin is supported by the occurrence of this mineral assemblage in metamorphic rocks exposed locally in the Erzgebirge basement. Reaction of the polymineralic metamorphic aggregate with the surrounding melt and subsequent interaction with alkali-, F- and LILE-rich residual fluids account for the widespread decomposition of the sekaninaite and formation of several layer silicates including green biotite, muscovite, berthierine/Fe chlorite, and sericite. The observed enrichment of the relic sekaninaite and its replacement products in elements such as Na, Li, Be, Rb, Cs, and F is result of interaction of the metamorphic fragment with the surrounding melt/fluid, in accordance with the evolved nature of the Satzung magmatic-hydrothermal system