Evolution of the concept and structure of a MUSHWAD

Abstract

A mushwad forms as a result of bulk ductile deformation and large-scale tectonic thickening of depositionally thick, weak, décollement-host strata. A mushwad has floor and roof thrusts like a conventional brittle duplex; but the internal structure of a mushwad, which includes disharmonic small-scale folds and faults, differs from the large-scale imbricate horses in a conventional duplex. The concept of mushwad structure has evolved as more subsurface data from deep drill holes and seismic reflection profiles have become available. The formation of a mushwad requires an anomalously thick shale-dominated succession at the level of the décollement and a structural buttress (generally a basement fault) to deflect the décollement upward in a frontal ramp. Tectonic thickening of the mushwad elevates and distorts the roof, producing a complex antiformal structure that differs from conventional frontal thrust ramps.