Two-dimensional thermal modelling of normal faulting: the Simplon Fault Zone, Central Alps, Switzerland

Abstract

Two-dimensional finite-difference thermal modelling of the major extensional Simplon Fault Zone in the Central Alps establishes that the shape of cooling curves for samples from the footwall and hanging wall at various distances from the detachment are very sensitive to the actual displacement history. If the displacement had occurred within a very limited time span (18-15 Ma), samples from the hanging wall close to the detachment should have significantly increased in temperature, with a peak around 14 Ma. This is not discernible in the cooling curves interpreted from the mineral ages. If the rate of normal fault displacement had been lower but constant from 18 Ma to present, the cooling curves for the footwall should show a correspondingly steady decrease in temperature. The interpreted cooling curves from mineral ages are steep after 18 Ma, becoming shallower in more recent times. This shape can be reproduced by the thermal modelling for a variable rate of fault displacement, initially rapid (total relative displacement 10 mm/a across the fault zone) between 18 and 15 Ma but reducing to a more steady 0.4 mm/a between 15 Ma and 3 Ma. The corresponding exhumation rate of the footwall was 4.6 mm/a from 18 to 15 Ma, and 0.6 mm/a from 15 to 3 Ma, whereas the hanging wall was exhumed at a constant rate of 0.4 mm/a. The difference in cooling curves for samples with increasing horizontal distance into the footwall reflects a distributed ductile shear, with the sample at 20 km distance initially at 25 km depth at 25 Ma, whereas that at 70 m distance was only at 19 km depth. At the same time samples in the hanging wall were at 10 km depth. Total relative displacement parallel to the fault plane was around 36 km. The thermal modelling demonstrates that during rapid fault displacement the “apparent” exhumation rate calculated assuming a constant geothermal gradient is too low, whereas the “apparent” rate calculated for the subsequent period of slower displacement is too high.