Switches in the minimum compressive stress direction induced by overpressure beneath a low-permeability fault zone

Abstract

The Zuccale fault is a gently east-dipping normal fault exposed on Elba. Its displacement of 7—8 km occurred from the mid-Miocene to the early Pliocene and the fault has been exhumed from 3—6 km depth. A complex hydrofracture system exposed in the footwall block consists of three vein sets: two vertical sets trending N—S and E—W and one sub-horizontal. The veins show a crack-and-seal texture and mutually crosscut each other. The regional stress field throughout the period when the Zuccale fault was active was extensional with the minimum principal stress oriented E—W, consistent only with the N—S trending set of vertical hydrofractures. We interpret all three sets of orthogonal fractures as the result of local stress changes due to exhumation, superimposed on tectonic stresses, with build-up of overpressure beneath the low-permeability phyllosilicate-rich fault core and release of overpressure when the fault slipped.