Two‐ and three‐dimensional thermal modeling of a low‐angle detachment: Exhumation history of the Simplon Fault Zone, central Alps

Abstract

Two alternative models have been proposed to explain footwall exhumation along major low‐angle detachments: (1) crustal‐scale exhumation along a detachment fault that maintained a low dip angle or (2) exhumation along a high‐angle fault passively rotated by isostatic rebound (“rolling hinge model”). These proposed models were tested against a well‐documented example of a low‐angle detachment fault in the European central Alps, the Simplon Fault Zone (SFZ). An extensive thermochronological data set provides the basis for 2‐ and 3‐D thermokinematic models (Pecube), coupled with a stochastic inversion algorithm (the Neighbourhood Algorithm). Model results establish that the thermochronological pattern is better reproduced by a low‐angle detachment that maintained a 30° dip, rather than by a rolling hinge model. Although a range of histories involving either steady state or variable exhumation rates is possible, the preferred model of highest probability is for a variable rate, with the fault zone initiated at 18.5 ± 2.5 Ma and active until the present day. Footwall exhumation was relatively fast until 14.5 ± 1.5 Ma (∼1.4 mm yr−1). This enhanced SFZ footwall exhumation is similar in timing and kinematics to orogen‐parallel extension reported throughout the Alpine orogen. After 14.5 Ma, SFZ footwall exhumation continued at a reduced rate (∼0.7 mm yr−1) until 4 Ma. The subsequent increase (to ∼1 mm yr−1) reflects enhanced regional erosion rates across both footwall and hanging wall after circa 4 Ma (from 0.35 ± 0.15 mm yr−1 to 0.70 ± 0.15 mm yr−1), probably in response to climate changes during the Pliocene.