The geometry of drag zones adjacent to salt diapirs

Abstract

Drag zones are highly strained regions developed adjacent to the flanks of salt diapirs, and are produced when the sedimentary overburden is folded or rotated into steeply dipping attitudes sub-parallel to the diapiric walls. This case study focuses on a diapiric province on Cape Breton Island, Nova Scotia, where five Visean-age salt diapirs penetrate Upper Carboniferous (Namurian–Stephanian) conglomerates, sandstones, shales and coal seams. 2D marine seismic coverage extends over several diapirs thus allowing the drag zones studied onshore to be located relative to the adjacent diapir. The width of diapiric drag zones within the case studies varies from 70 m up to 500 m, with narrow drag zones reflecting the low mean competence of shales and siltstones which may have been shallowly buried and poorly lithified at the onset of diapirism. Broader drag zones are dissected by two sets of extensional fractures together with major faults, suggesting that the overburden was semi-lithified and displayed greater flexural rigidity. Overburden displaying high competency contrasts results in strain localization and partitioning, with pervasive granulation seams and minor faults developed in sandstones and bedding-parallel shears in coal and shale horizons. Drag zones are segmented by steeply outward-dipping faults associated with decametric, asymmetric drag profiles. These faults facilitate the upward movement of material and effectively expand the diapiric process into adjacent overburden via a newly termed process of lateral diapiric accretion.