Carbonate deformation during exhumation through the ductile-brittle transition: insights from (U-Th)/He thermochrometry, stable and clumped-isotopes
Abstract
Results of an integrated study combining carbonate clumped-isotope thermometry and (U-Th)/He dating on zircons (ZHe) from the SW Helvetic Nappes in Switzerland are presented. The 37 new ZHe ages indicate partial and possibly complete resetting of detrital zircon in the parautochthonous nappes at about 17 Ma, whereas the overlying Wildhorn Nappe was never buried to depths where temperatures exceeded 200°C. After combining our ages with data from literature, the cooling curve could be determined and exhumation rates estimated, with the latter being ~0.2 km/Ma in the Rawil Depression between 17 and 5 Ma and ~0.8 km/Ma for the time period from 5 Ma to the present. The cooling curve could then be used to assign absolute ages to clumped-isotope temperatures determined from different structures. This stable-isotope study involved the analysis of mylonites, veins of different generations and dimensions, cataclasites, fault mineralizations, and carbonate host rocks from this area, which have been exhumed from low grade metamorphic conditions (Tmax<300°C). Our results are consistent with earlier measurements of O and C stable isotope ratios from the region and furnish independent constraints on both isotopic composition and the temperature of fluids during precipitation and recrystallization. Two trends are observed in the stable isotopes: the first is related to hydrothermal fluids during nappe emplacement and the second to progressive opening of the system during the transition from ductile to brittle deformation. The results reveal a wide range of temperatures associated with deformation from about 230° to 70°C. The temperature recorded in the mylonites (160-180°C) may indicate the lowest temperature under which the mylonite was recrystallized. Fault rocks show progressively lower temperatures with decreasing relative age, in accordance with the structural history established from field observations. From the established pattern of exhumation and cooling ages, deformation and (re-) crystallization ages can be indirectly inferred.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T43F2717C
- Keywords:
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- 8104 TECTONOPHYSICS Continental margins: convergent;
- 3653 MINERALOGY AND PETROLOGY Fluid flow;
- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 8118 TECTONOPHYSICS Dynamics and mechanics of faulting