Interplay between stress permutations and overpressure to cause strike-slip faulting during tectonic inversion

Abstract

Field data from an orogenic foreland and an orogenic belt (the Mesozoic rocks of southern England and the Umbria-Marche Apennines of Italy respectively) indicate the following. Firstly, stress evolution during the tectonic cycle, between maximum compressive stress (σ1) being vertical during extension and least compressive stress (σ3) being vertical during contraction, can involve phases when the intermediate compressive stress (σ2) is vertical, promoting strike-slip deformation. Secondly, variations in the relative magnitudes of the stress axes are caused by variations in overburden and tectonic forces. Thirdly, overpressure can develop because of compaction during burial, and, as overburden is reduced during uplift and erosion, the vertical stress (σV) reduces but fluid pressure (Pf) remains approximately constant. Brittle deformation, including transient strike-slip faults, reverse-reactivated normal faults and normal-reactivated thrusts, is preferentially developed in overpressured areas because high Pf promotes faulting.