Fault slip rate variability on 104–105yr timescales for the Salsomaggiore blind thrust fault, Northern Apennines, Italy

Abstract

Abstract Documenting fault slip rate variability on intermediate (103–105 yr) timescales is crucial for understanding the discrepancies between short-term (101–102 yr) and long-term (106 yr) patterns of deformation. A major obstacle in bridging this gap has been the inability to document multi-Myr records of fault slip with 104–105 yr resolution. Here we present a 3 Myr long record of thrust fault slip with 40 kyr resolution by inverse modeling of Late Pliocene–Early Pleistocene growth strata exposed on the forelimb of the Salsomaggiore blind thrust anticline in the Northern Apennines, Italy. We augment geological data with seismic reflection and well data to construct structural models of the Salsomaggiore anticline and forelimb growth strata. We show that the deformation of the growth strata was due to slip on both a deep, late-stage, thick-skinned reverse fault and the shallow, thin-skinned Salsomaggiore thrust. We show that the thick-skinned fault slipped at a steady rate of 1.4 (+/− 0.7) mm/yr since its activation between 1.0 and 1.8 Ma, while the shallower thin-skinned Salsomaggiore thrust has slipped at rates that varied between ≤ 0.1 and 6 mm/yr over the last 3 Myr. The Salsomaggiore thrust exhibits a high frequency slip rate variability at 40–500 kyr timescales that is likely related to strain partitioning on connected imbricate thrusts. A major deceleration in Salsomaggiore slip rate between 1.0 and 1.8 Ma is coincident with orogenic wedge thickening due to the initiation of thick-skinned reverse faulting following the clastic infilling of the Po foreland basin during the Middle Pleistocene.