EPISODIC ACCRETION AND STRAIN PARTITIONING IN A MODEL SAND WEDGE

We model led thrust wedge accretion and deformation partitioning in a passively layered sand, detached and shortened above a smooth rigid decollement. The thrust wedge accretes in piggyback style during forward advance of a rear wall. The passively layered sand accommodates the shortening penetratively, by kinking/ramp folding, and by imbrication. In single layers, the style of compression changes with time and/or stratigraphic position within the sand prism. Initial penetrative layer shortening, above a slipped zone of decollement is succeeded by formation of monoclinal or conjugate shear bands at temporary terminations of decollement. The shear bands subsequently narrow down and lock as thrust ramps. The model sand wedge accrets episodically rather than steadily. Episodicity is controlled by the stick/slip mode of decollement propagation, and by volume loss and compaction of the wedge material in response to convergence. This results in formation of a wedge with a convex-upwards cross-sectional topography.