Is the Coulomb Wedge Model Applicable to Passive Margin Deformation?

Abstract

The Coulomb Wedge model (Davis et al., 1983) simply and elegantly describes the mechanics of thrust wedges driven by orogenic processes. But the elegance of the Davis model hides some mathematical errors that limit its quantitative use. Davis modelled a “critical taper” at the point of compressional failure, like a snowplough driving a snow wedge. A steeper taper can be pushed without failure: steeper still, it fails in extension. Compressional belts in passive margin settings are driven by gravitational collapse. We show how the existing Coulomb Wedge model can be adapted to describe the mechanics of a passive margin linked system. The contractional toe of the system is a conventional Davis wedge, but the driving snowplough is replaced by the gravity collapse of the back of the wedge, at critical taper for extensional failure. This illustrates many important differences between the two settings. Systems driven by crustal convergence may involve basement; passive margin wedges should be thin-skinned. The Davis wedge is rate controlled; a passive margin wedge is limited by the supply of gravitational potential energy. When a Davis wedge encounters an obstacle, the whole wedge deforms and stress increases until the obstacle is breached. In contrast, a passive margin wedge hitting the same obstacle may freeze permanently. A Davis wedge may develop self-similarly indefinitely. A passive margin wedge changes shape as it moves, losing motive force, and unless refuelled by sedimentation, it will slow or stop. Consequently, the timing of movement is commonly linked to depositional episodes. Mixed mode behaviour is common: a wedge driven by crustal convergence may also involve gravitational spreading.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015