Joel H. Le Calvez¹, Bruno C. Vendeville¹, Patrick Walsh², Randall A. Marrett¹

(1) University of Texas at Austin, Austin, TX
(2) Texaco, Houston, TX

ABSTRACT: Processes Leading to Formation of Arcuate Grabens and Faults Above Salt

Two sets of experiments were designed to understand the origin of map-view curvature of young grabens in Canyonlands National Park, Utah. The grabens formed by thin-skinned extension of brittle sedimentary cover above an evaporitic formation that pinches out to the southwest. Extension was caused by gravitational spreading after the Colorado river locally incised the entire brittle section. Erosion removed the lateral buttress, thus triggered spreading by (1) lowering the horizontal stress and (2) providing accommodation space into which the section could spread gravitationally. One set of experiments tested the hypothesis that river-valley curvature caused the curved graben geometry. Results indicate that these conditions form two families of faults (both strike-parallel and strike-perpendicular to the river valley), which are absent at a regional scale in the field example. In contrast, another set of experiments tested the hypothesis that graben curvature is due to heterogeneous shear stress within the brittle layer, between the central part of the graben system and its lateral edges that are pinned. In the central part of the experimental models, the brittle cover readily extends above the viscous layer and spreads toward the simulated river valley. Where evaporites are absent, the brittle cover is pinned to its substratum. Where erosion by the simulated river canyon has not reached the viscous layer, differential stress is insufficient to initiate spreading and extension. Shear between the central domain and its fixed lateral boundaries forces stress trajectories to be curved and leads to formation of arcuate fault traces.

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