Hybrid thrust sequences – A new structural perspective

Abstract

Thrust sequences show a range of geometric and kinematic characteristics in cross-sections and in map view. Two types of thrust sequences are commonly observed in thrust systems – forward-breaking and break-back thrust sequences. For nearly half a century, it has been hypothesized that forward-breaking and break-back thrust sequences represent end-members of a continuous spectrum of brittle and ductile structures. Many natural and experimental observations of transitional thrust sequences display both forward-breaking and break-back (hybrid) thrust sequences within the same thrust system. An explanation of hybrid thrust sequence development has neither been provided conceptually nor experimentally. We present geometric and kinematic results of natural observations and numerical experiments that show a transition from forward-breaking to break-back thrust sequences caused by the effects of stratigraphic variations, sedimentary environment, surface processes, critical-wedge effects, progressive deformation with an increase in compressive stress, presence of detachments and their frictional properties, and strain rate. We also discuss numerous geometric, kinematic, chronological and mechanical criteria to identify the characteristic signatures of hybrid thrust sequences. This study provides a template to distinguish different thrust sequences and to characterize the deformation conditions under which hybrid thrust sequences develop in contractional settings. Mechanical and field criteria, displacement patterns imaged on seismic reflection sections across onshore and offshore thrust systems, and the way that thrusts interact in terms of geometry, kinematics and strain, are used to illustrate thrust development.