Time constraints for low-angle shear zones in the Central Rhodopes (Bulgaria) and their significance for the exhumation of high-pressure rocks     

Silke Jahn-Awe  Jan Pleuger  Dirk Frei  Neven Georgiev  Nikolaus Froitzheim  Thorsten J. Nagel 《International Journal of Earth Sciences》2012,101(7):1971-2004

In the Central Rhodopes of southern Bulgaria, an eclogite-bearing rock sheet belonging to the Middle Allochthon (Starcevo Unit) is over- and underlain by eclogite-free, amphibolite-facies rock units along low-angle shear zones, the Borovica Shear Zone at the top and the Starcevo-Ardino Shear Zone at the base. The age of these shear zones is determined by U–Pb zircon dating of pre-, syn- and posttectonic magmatic rocks, mostly pegmatite veins, using LA–SF–ICP–MS. Zircons from pre- to syntectonic pegmatites within the Borovica Shear Zone yielded ages of ca. 45–43?Ma, indicating that the shear zone was active at that time, and zircons from a pretectonic pegmatite and a posttectonic granitoid body within the Starcevo-Ardino Shear Zone yielded ages of ca. 45 and ca. 36?Ma, respectively, giving a time frame for the activity of that shear zone which probably rather postdated the activity of the Borovica Shear Zone. By combining the ages with the kinematics of the shear zones and the metamorphic history of the rock units, the following scenario is sketched: Soon after the Starcevo Unit reached peak pressure (eclogite facies), it was exhumed to a mid-crustal level by top-to-the-north-west, extensional unroofing along the Borovica Shear Zone, in a kinematic framework of orogen-parallel extension. Beginning at ca. 40?Ma, the partly exhumed Starcevo Unit was underthrust from the south-west by continental crust of the foreland (Apulia), forming the Lower Allochthon of the Rhodopes, along the Starcevo-Ardino Shear Zone. These results underline the significance of orogen-parallel extension for the exhumation of high-pressure rocks. With respect to regional geology of the Hellenides and the Aegean, it is found that the tectonic architecture of the Rhodopes is essentially of Tertiary age. Cretaceous syn-metamorphic shear zones do exist but are largely restricted to higher levels of the nappe stack (Upper Allochthon). The Rhodopes do not represent an older essentially Mesozoic core of the Hellenides but are formed by the internal, higher-metamorphic portions of the same major nappe systems as occur in the Hellenides.  相似文献   

Biodegradation of forest floor organic matter bound to minerals via different binding mechanisms   总被引：6，自引：0，他引：6  

Robert Mikutta  Christian Mikutta  Thorsten Scheel  Reinhold Jahn 《Geochimica et cosmochimica acta》2007,71(10):2569-2590

Mineral-associated organic matter (OM) represents a large reservoir of organic carbon (OC) in natural environments. The factors controlling the extent of the mineral-mediated OC stabilization, however, are poorly understood. The protection of OM against biodegradation upon sorption to mineral phases is assumed to result from the formation of strong bonds that limit desorption. To test this, we studied the biodegradation of OM bound to goethite (α-FeOOH), pyrophyllite, and vermiculite via specific mechanisms as estimated from OC uptake in different background electrolytes and operationally defined as ‘ligand exchange’, ‘Ca²⁺ bridging’, and ‘van der Waals forces’. Organic matter extracted from an Oa forest floor horizon under Norway spruce (Picea abies (L.) Karst) was reacted with minerals at dissolved OC concentrations of ∼5-130 mg/L at pH 4. Goethite retained up to 30.1 mg OC/g predominantly by ‘ligand exchange’; pyrophyllite sorbed maximally 12.5 mg OC/g, largely via ‘van der Waals forces’ and ‘Ca²⁺ bridging’, while sorption of OM to vermiculite was 7.3 mg OC/g, mainly due to the formation of ‘Ca²⁺ bridges’. Aromatic OM components were selectively sorbed by all minerals (goethite ? phyllosilicates). The sorption of OM was strongly hysteretic with the desorption into 0.01 M NaCl being larger for OM held by ‘Ca²⁺ bridges’ and ‘van der Waals forces’ than by ‘ligand exchange’. Incubation experiments under aerobic conditions (initial pH 4; 90 days) revealed that OM mainly bound to minerals by ‘ligand exchange’ was more resistant against mineralization than OM held by non-columbic interactions (‘van der Waals forces’). Calcium bridges enhanced the stability of sorbed OM, especially for vermiculite, but less than the binding via ‘ligand exchange’. Combined evidence suggests that the extent and rate of mineralization of mineral-associated OM are governed by desorption. The intrinsic stability of sorbed OM as related to the presence of resistant, lignin-derived aromatic components appears less decisive for the sorptive stabilization of OM than the involved binding mechanisms. In a given environment, the type of minerals present and the solution chemistry determine the operating binding mechanisms, thereby the extent of OM sorption and desorption, and thus ultimately the bioavailability of mineral-associated OM.  相似文献   

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from Hf-W in CAIs, metal-rich chondrites, and iron meteorites     

Thorsten Kleine  Klaus Mezger  Erik Scherer 《Geochimica et cosmochimica acta》2005,69(24):5805-5818

The ¹⁸²Hf-¹⁸²W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the ¹⁸²Hf/¹⁸⁰Hf and ε_W at the time of CAI formation to (1.07 ± 0.10) × 10⁻⁴ and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial ε_W values that are similar to or slightly lower than the initial value of CAIs. These low ε_W values may in part reflect ¹⁸²W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of ²⁶Al, which was sufficiently abundant (initial ²⁶Al/²⁷Al >1.4 × 10⁻⁵) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.  相似文献   

Constraints on fault slip rates of the southern California plate boundary from GPS velocity and stress inversions   总被引：2，自引：0，他引：2  

Thorsten W. Becker  Jeanne L. Hardebeck  Greg Anderson 《Geophysical Journal International》2005,160(2):634-650

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History of subduction and back-arc extension in the Central Mediterranean   总被引：11，自引：0，他引：11  

Claudio Faccenna  Thorsten W. Becker  Francesco Pio Lucente  Laurent Jolivet  Federico Rossetti 《Geophysical Journal International》2001,145(3):809-820

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Effect of Iron and Manganese Ions on the Crystal Growth of Calcium Carbonate     

K. Johannsen  T. Alex  H. Burwieck  T. Dorsch  K. Wichmann 《洁净——土壤、空气、水》1997,25(4):202-207

The present study describes examinations of growth rate of calcium carbonate using seed crystals of different sizes in the range of 10 to 50 μm at concentrations in the range of 5 to 50 g·L^?1. The rate constant related to the crystal surface per volume was found to be independend of the crystal size. The effect of temperature on the rate constant was described using the Arrhenius equation. The use of suspension of lime as precipitating agent leads to decrease of the rate constant compared to lime water. This effect can be explained by the dissolution of suspended calcium hydroxide particles. Using the presented conditions (SI₀ = 2.8), no impact of iron and manganese ions was observed.  相似文献   

Lu-Hf garnet geochronology of eclogites from the Balma Unit (Pennine Alps): implications for Alpine paleotectonic reconstructions     

Daniel Herwartz  Carsten Münker  Erik E. Scherer  Thorsten J. Nagel  Jan Pleuger  Nikolaus Froitzheim 《Swiss Journal of Geoscience》2008,101(1):173-189

Three samples of eclogite from the Balma Unit, an ophiolite sheet on top of the Monte Rosa Nappe in the Pennine Alps, were investigated in terms of their P-T evolution, geochemistry, and Lu-Hf geochronology. The paleogeographic origin of this unit is controversial (North Penninic vs. South Penninic). It has been interpreted as a piece of Late Cretaceous oceanic crust, on the basis of ca. 93 Ma U-Pb SHRIMP ages of synmagmatic zircon cores in an eclogite. Trace element and isotope data suggest a mid ocean ridge (MOR) rather than an intraplate or OIB setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 550–600 Cº most likely did not exceed the closure temperature of the Lu-Hf system. Hence, Lu-Hf ages most likely reflect garnet growth in the studied samples. To minimize inclusion effects on age determinations, a selective digestion procedure for garnet was applied, in which zircon and rutile inclusions are not dissolved. The ages obtained for three samples, 42.3 ± 0.6 Ma (MSWD: 0.47), 42 ± 1 Ma (MSWD: 3.0) and 45.5 ± 0.3 Ma (MSWD: 0.33), are younger than all Lu-Hf ages reported so far for South Penninic Units. Metamorphic zircon domains of the 42.3 Ma sample (PIS1) were previously dated by U-Pb SHRIMP at 40.4 ± 0.7 Ma, indicating that the growth of metamorphic zircon post-dated the onset of garnet growth.These new data put important constraints on the paleogeographic reconstruction of the Alps. The MORB character of the rocks, together with their previously published protolith age, imply that oceanic spreading was still taking place in the Late Cretaceous. This supports a North Penninic origin for our samples because plate tectonic models predict Cretaceous spreading in the North Penninic but not in the South Penninic Ocean. If the Balma Unit is indeed North Penninic, the new Lu-Hf data, in combination with published geochronological data, require that two independent subduction zones consumed the South and North Penninic oceans.  相似文献   

Natural organics as fluorescent tracers of river-sea mixing     

Janet E. Dorsch  Terry F. Bidleman   《Estuarine, Coastal and Shelf Science》1982,15(6)

The use of dissolved organic matter fluorescence as a tracer of river-sea mixing was examined in two South Carolina estuaries. Fluorescence declined linearly with seawater dilution in laboratory mixing studies, and also behaved conservatively in an estuary where a single river emptied into a bay. Fluorescence-salinity relationships were also studied in another estuary where a piedmont river (high suspended sediment, low fluorescence) and a coastal plains river (low sediment, high fluorescence) mixed with ocean water. The factor of 2 or greater difference in fluorescence between the two rivers allowed their relative contribution to the estuarine water mass to be distinguished. Petroleum hydrocarbons, measured in estuarine water at 0·7-1·8 μg l⁻¹ concentrations, contributed negligibly to water fluorescence.  相似文献   

Effects of a fully submerged boulder within a boulder array on the mean and turbulent flow fields: Implications to bedload transport   总被引：1，自引：0，他引：1  

Athanasios N. Papanicolaou  Casey M. Kramer  Achilleas G. Tsakiris  Thorsten Stoesser  Sandeep Bomminayuni  Zhuo Chen 《Acta Geophysica》2012,60(6):1502-1546

The objective of this coupled experimental and numerical study is to provide insight into the mean and turbulent flow fields within an array of fully submerged, isolated, immobile boulders. Our study showed that the velocity defect law performed well for describing the mean flow around the boulder within the array. A prerequisite, however, was to accurately estimate the spatial variability of u* around the boulder, which was achieved via the boundary characteristics method. The u* exhibited considerable spatial variability within the array and form roughness was shown to be up to 2 times larger than the skin roughness in the boulder near-wake region. Because the boulders bear a significant amount of the flow shear, the available bed shear stress for entrainment of the mobile sediment, ?? _ols , near the boulders was roughly 50% lower than the ambient ?? _ols . The ?? _ols variability induced by the boulders could lead to a threefold overestimation of the sediment transport rate.  相似文献   

Understanding the time‐varying importance of different uncertainty sources in hydrological modelling using global sensitivity analysis          下载免费PDF全文

Francesca Pianosi  Thorsten Wagener 《水文研究》2016,30(22):3991-4003

Simulations from hydrological models are affected by potentially large uncertainties stemming from various sources, including model parameters and observational uncertainty in the input/output data. Understanding the relative importance of such sources of uncertainty is essential to support model calibration, validation and diagnostic evaluation and to prioritize efforts for uncertainty reduction. It can also support the identification of ‘disinformative data’ whose values are the consequence of measurement errors or inadequate observations. Sensitivity analysis (SA) provides the theoretical framework and the numerical tools to quantify the relative contribution of different sources of uncertainty to the variability of the model outputs. In traditional applications of global SA (GSA), model outputs are aggregations of the full set of a simulated variable. For example, many GSA applications use a performance metric (e.g. the root mean squared error) as model output that aggregates the distances of a simulated time series to available observations. This aggregation of propagated uncertainties prior to GSA may lead to a significant loss of information and may cover up local behaviour that could be of great interest. Time‐varying sensitivity analysis (TVSA), where the aggregation and SA are repeated at different time steps, is a viable option to reduce this loss of information. In this work, we use TVSA to address two questions: (1) Can we distinguish between the relative importance of parameter uncertainty versus data uncertainty in time? (2) Do these influences change in catchments with different characteristics? To our knowledge, the results present one of the first quantitative investigations on the relative importance of parameter and data uncertainty across time. We find that the approach is capable of separating influential periods across data and parameter uncertainties, while also highlighting significant differences between the catchments analysed. Copyright © 2016 The Authors. Hydrological Processes. Published by John Wiley & Sons Ltd.  相似文献