ABSTRACT: The Rise and Fall of Diapirs During Thin-Skinned Extension

M. P. A. Jackson, B. C. Vendeville

Physical models comprising brittle, layered sand on viscous, less dense silicone polymer ("salt") elucidate the rise, widening, and subsequent sagging of diapiric walls during extension driven by gravity gliding and spreading down a 2° slope. Faulting of prekinematic overburden (deposited before deformation) controlled the location and style of diapiric piercement. Grabens deepened faster than half grabens and were more likely to be diapirically intruded (probability 0.9 vs. 0.5). Below grabens, symmetric salt rollers formed with a pair of cuspate crests in the footwalls of each flanking fault. These rollers evolved into symmetric diapirs with a central cuspate crest. Regional faults dipping basinward formed half-grabens underlain by asymmetric rollers, each with a s ngle sharp crest. These rollers rose by rotating the footwall and indenting the hanging wall. Indentation folded the fault surface and severed a wedge from the rest of the hanging wall. The resulting asymmetric diapir had a single crestal cusp on its basinward flank.

During stretching, fault blocks of prekinematic strata initially tilted landward. With further extension the diapir widened; tilted blocks partly grounded on basement as the viscous layer became depleted. Further diapiric withdrawal caused blocks to rotate basinward rather than landward. Insufficient salt remained for further rise, so the crests of the widening diapirs began to sag and were indented by overlying grabens, forming multiple crestal cusps. This inversion of the diapir, coupled with change in rotation direction of the flanking blocks, triggered the formation of counterregional (landward-dipping) growth faults above the diapirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990