Diapir Geometry as a Control Upon Drag Zone Profile and Deformation Style

John P. Brown, Ian Davison, Ian G. Alsop, and Martin R. Gibling

Diapir drag profiles of seventeen off-shore (seismic coverage) and four coastal exposures in Nova Scotia were examined. All diapiric structures rise from the Visean Windsor salt to pierce the Inverness Formation (Westphalian C-D) which is remarkably homogeneous, effectively normalising the lithological control upon drag zone deformation. This allows other factors which may affect deformation within diapir drag zones to be considered. Drag Zone profiles are related to the geometry of the salt/sediment contact:

1. Steep (planar) (78°) to slightly overhanging (12°) diapir margins. A wide (~2000m), but low angle (<20°) drag zone shows a small (<120 m) inner drag zone with a high dip angle (30°-50°) adjacent to the salt/sediment contact.

2. Steep (irregular) to gently dipping (58°) (irregular) diapir margins. A smooth 'dip fan' shows a parabolic profile which extends outwards ~3000m where maximum dip ranges from 35°-56° at the salt/sediment contact.

3. Gently dipping (58°) and 'shouldered' diapir margins (bottle shaped). An extremely steeply dipping (65°-74°) inner drag zone above the protruding shoulder gives way to a parabolic 'dip fan'.

Excellent on-shore outcrops mirror the above division of drag zone profiles. Above diapir shoulders sediments dip at ~74° and show intense deformation (pervasive granulation seams, extensional faults and conjugate joint sets). At steep (planar) diapir salt/sediment contacts deformation (tight, overturned syncline) is restricted to a 20m zone after which stratal dip is reduced to ~22° with no pervasive deformation. From the above data, a strong correlation between diapir geometry and drag zone deformation is revealed.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California