Intrinsically Variable Blind Thrust Faulting

Abstract

We propose that most fault slip rate variability across a range of time and spatial scales is due to intrinsic faulting processes, rather than extrinsic changes in surface loads or stress boundary conditions. This hypothesis is tested by comparing very high geologic resolution slip histories of blind thrust faults from three transects in the Northern Apennines, Italy. We investigated whether these slip histories document synchronous, or independent, behavior of the disconnected, blind thrust faults that core mountain front anticlines bordering the Po foreland. The slip history for these thrusts is reconstructed by applying forward structural modeling to deformed growth strata and fluvial terraces preserved on the limbs of the growing anticlines. We present a new age model using magnetostratigraphy and cyclostratigraphy for a section of growth strata exposed in the Panaro River and supplement this with age models for two other published transects. The blind thrust fault at each transect exhibits variable slip behavior over the past 3 Myr, but for most of that time the variability was both asynchronous and independent of boundary condition changes, such as Plio-Pleistocene sediment accumulation variability. However, a major deceleration in slip rates at all three locations is temporally coincident with the overfilling of the Po foreland beginning in the early Pleistocene. We attribute the deceleration to a switch from shortening on shallowly detached thrusts to shortening on a crustal-scale basement-involved fault. This switch has implications for the time and spatial scales at which extrinsic boundary conditions may contribute to deformation variability.