Program > Browse abstracts by speaker > Raimbourg Hugues

Due to the widespread presence of carbonaceous material (CM) in the sediments that constitute paleo-subduction zones, Raman spectroscopy on carbonaceous material (RSCM) has recently developed as a very powerful geothermometer, based on the irreversible evolution in crystallinity of CM under increasing temperatures.

If the effect of heating on CM crystallinity is well documented, questions remain as to the effect of other parameters, such as strain. This issue is particularly acute in fault zones, where heat and/or strain may localize and where anomalies in the Raman signature of CM have been observed.

To answer these questions, we have carried out in this work series of slow strain-rate deformation experiments on CM-bearing sediments, both in Paterson-type and Griggs-type apparatuses, under variable conditions of temperature (400 to 700°C), pressure (150MPa to 1GPa) and strain-rates (10-4 to 10-7s-1). From the observation of the microstructures in the samples after the experiments, it appeared that strain is systematically heterogeneously distributed. High-strain domains, where grain-size is strongly reduced, consist in many instances (even in high-pressure experiments where no stick-slip were observed) in the juxtaposition of micro-breccia (rounded clasts, high porosity) and ductile layers (elongated clasts, low porosity).

RSCM analyses of experimentally-deformed material showed that CM crystallinity is enhanced in high-strain zones, irrespective of the microstructures and deformation process. The enhanced crystallinity observed is therefore best explained as the result of strain localization.

These deformation experiments confirm independently the effect of strain on CM evolution, which we demonstrated in a separate work on natural fault zones. Altogether, these studies highlight the complexity of the processes that govern the crystallinity of carbonaceous particles, which include both temperature and strain. Therefore, CM signature in fault zones could potentially lead to discriminate creeping fault zones from the ones that rupture seismically and liberate by friction a large amount of heat.