AN UPTHROWN VALLEY? UNCONVENTIONAL FAULTING IN SEVIER VALLEY, UT

Extensional terranes worldwide are characterized by basin-range physiography in which hanging-wall blocks subside to form elongate valleys and footwall blocks are upthrown to form ranges. However, the relation is reversed in Sevier Valley in the central Utah Transition Zone. Sevier Valley lies on the upthrown, topographically low footwall of the valley-forming normal fault, here referred to as the Salina detachment, and the hanging wall block forms the topographically high Wasatch and Sevier plateaus. The reversal of the typical relationship between structure and physiography of an extended terrane reflects effects of the evaporitic, mechanically weak Arapien Shale in the footwall.

The standard half-graben extensional fault model assumes a rigid footwall block that limits fault-related deformation to the hanging wall (e.g., rollover anticline). A mechanically weak footwall turns this assumption on its head. The Arapien Shale buoyantly rebounds in response to unroofing by the Salina detachment in a manner reminiscent of a salt dome to form a structural geometry that resembles the "isostatic rolling hinge" fault model proposed to explain metamorphic core complexes. Thus, rather than forming a deep basin on the hanging wall adjacent to a steep fault-bounded footwall high, footwall rebound irons out the exhumed footwall fault surface to form the low-standing low-relief area of Sevier Valley.

This structural interpretation has several geomorphic implications. Drainage and sediment-dispersal patterns in the standard model are reversed: sediment transport is dominantly from the hanging wall to the footwall. Footwall topographic relief is subdued compared to more typical normal-fault footwall ranges (e.g., the Wasatch Range). Counterintuitively, surface-rupturing earthquakes should cause localized uplift on the valley side of the fault relative to the hanging-wall range; thus, rather than accentuating gradients of drainages entering the basin, surface rupture should locally decrease gradients at the fault to cause aggradation and ponding of sediment at canyon mouths.