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 m
arine seismic coverage extends over several diapirs thus allowing the drag
zones studied onshore to be located relative to the adjacent diapir. The wi
dth of diapiric drag zones within the case studies varies from 70 m up to 5
00 m, with narrow drag zones reflecting the low mean competence of shales a
nd siltstones which may have been shallowly buried and poorly lithified at
the onset of diapirism. Broader drag zones are dissected by two sets of ext
ensional fractures together with major faults, suggesting that the overburd
en was semi-lithified and displayed greater flexural rigidity. Overburden d
isplaying high competency contrasts results in strain localization and part
itioning, with pervasive granulation seams and minor faults developed in sa
ndstones 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 overburd
en via a newly termed process of lateral diapiric accretion.