Dating shallow thrusts by quantifying shear heating

Abstract:

The generation of heat during faulting – i.e. the shear heating - has been extensively studied (e.g. Brun and Cobbold, 1980; Scholz, 1980; Leloup et al., 1999; Schmalholz et al., 2009). However, several more traceable mechanisms can generate fault zone heating, including advection, upwelling of hot fluids, and selective enrichment of heat-producing elements. The difficulty in recognizing the thermal effect of mechanical work often leads to an underestimation, or even total neglect, of the contribution of shear heating to the thermal budget. Quantifying how each mechanism contributes to heating is a challenge, which may be addressed by a comprehensive thermal characterization of the fault zone. Furthermore, recognizing the contribution of shear heating is a fundamental pre-requisite for dating the fault motion through thermochronometric techniques. In this talk, I show an approach that integrates field and micro-structural observations, clay mineralogy, fluid inclusion microthermometry, and zircon (U-Th)/He thermochronometry to estimate the temperatures experienced by a fault zone and the surrounding wall rocks. The ZHe thermochronometer has a well-defined He partial retention zone of 130–200°C and a closure temperature (Tc) of ~180°C. Consequently, it is ideally suited to dating large heat-producing faults that were active at shallow depths (<6-7 km) where wall-rock temperature does not exceed Tc. This approach has been addressed to the Penninic Front in the Western Alps. The large dataset provides the necessary constraints to explore the potential causes of heating, and how they are related to fault motion. The thermal history is verified by numerical models that simulate the deformation at variable strain rates and the circulation of hot fluids in the fault damage zone.