Tectonic inversion, positive and negative: a role for strike-slip deformations?

Interpretations of stress orientations, fluid pressures (Pf) and deformation chronologies are keys to most structural analyses. Here, we link the changes in the magnitudes of stress axes with related changes in Pf to understand a common phase of strike-slip deformation between periods of regional extension and regional contraction. 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 (s1) being vertical during extension and least compressive stress (s3) being vertical during contraction, can involve phases when the intermediate compressive stress (s2) 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 (sV) 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. These field examples, from different tectonic settings, suggest such behaviour is common in areas that have undergone single as well as multiple inversion histories.