2-DIMENSIONAL THERMAL MODELING OF NORMAL FAULTING - THE SIMPLON FAULTZONE, CENTRAL ALPS, SWITZERLAND

Two-dimensional finite-difference thermal modelling of the major exten sional Simplon Fault Zone in the Central Alps establishes that the sha pe of cooling curves for samples from the footwall and hanging wall at various distances from the detachment are very sensitive to the actua l 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, wit h a peak around 14 Ma. This is not discernible in the cooling curves i nterpreted from the mineral ages. If the rate of normal fault displace ment had been lower but constant from 18 Ma to present, the cooling cu rves for the footwall should show a correspondingly steady decrease in temperature. The interpreted cooling curves from mineral ages are ste ep after 18 Ma, becoming shallower in more recent times. This shape ca n be reproduced by the thermal modelling for a variable rate of fault displacement, initially rapid (total relative displacement 10 nun/a ac ross the fault zone) between 18 and 15 Ma but reducing to a more stead y 0.4 mm/a between 15 Ma and 3 Ma. The corresponding exhumation rate o f 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 m m/a. The difference in cooling curves for samples with increasing hori zontal 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 ti me samples in the hanging wall were at 10 km depth. Total relative dis placement parallel to the fault plane was around 36 km. The thermal mo delling demonstrates that during rapid fault displacement the ''appare nt'' exhumation rate calculated assuming a constant geothermal gradien t is too low, whereas the ''apparent'' rate calculated for the subsequ ent period of slower displacement is too high.