Mark G. Rowan¹, Frank J. Peel², Bruno C. Vendeville³

(1) Rowan Structural Consulting, Boulder, CO
(2) BHP Petroleum (Americas) Inc
(3) Bureau of Economic Geology, University of Texas at Austin

ABSTRACT: Gravity-Driven Foldbelts on Passive Margins

Many passive margins have deepwater, contractional foldbelts formed above salt or overpressured shale. Margin failure, accommodated by proximal extension and distal shortening, is caused by some combination of gravity gliding above a basinward-dipping detachment and gravity spreading of a sedimentary wedge with a seaward-dipping bathymetric surface. Continued deformation is driven primarily by shelf and upper slope deposition, which maintains the bathymetric slope and the resulting gravity potential, and by increased basinward tilting. Deformation is retarded or halted by distal thickening of the overburden caused by the folding itself or by lower slope and abyssal sedimentation. Net shortening is significantly less than in collisional/ accretionary foldbelts because the driving forces are weaker than those induced by global tectonic motions.

Structural styles are varied but largely dependent on the nature of the décollement layer, not the driving forces. Foldbelts detached on overpressured shale typically comprise basinward-vergent thrust imbricates and associated folds because of the relative strength and frictional behavior of the plastic shale. Deformation does not occur until there is sufficient overburden and high fluid pressures. In contrast, salt is a viscous material with essentially no strength, leading to symmetrical detachment folds and early deformation beneath only a thin overburden. Moreover, the surface slope can be reduced by proximal subsidence into salt and distal inflation of salt, and much of the shortening can be accommodated by lateral squeezing of diapirs and salt massifs.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado