EXTENSIONAL COLLAPSE ALONG THE SEVIER DESERT REFLECTION, NORTHERN SEVIER DESERT BASIN, WESTERN UNITED-STATES

Newly released and previously published seismic reflection data from t he northern Sevier Desert basin provide a complete seismic transect be tween the tilted western margin of the basin and the eastern breakaway zone, When tied to well and surface age data, the transect delineates a continuum of extensional fault and basin fill geometries that devel oped between late Oligocene and Pleistocene time across the basin, A m inimum of 18 km of top-to-the west normal displacement is estimated ac ross the Sevier Desert from only the most conspicuous growth geometrie s and offsets across listric normal faults that sole downward into the Sevier Desert reflection (SDR), The SDR clearly marks a normal fault zone beneath the entire basin, where stratal truncations are imaged fo r 50% of the 39 km length of the reflection east of the Cricket Mounta ins block, Restoration of extensional displacement along this entire 3 9 km fault length is necessary to reconstruct the pre-Oligocene config uration and erosion level of Sevier thrust sheets across the Sevier De sert area, The SDR normal fault zone underlies the former topographic crest of the Sevier orogenic belt, where it accommodated extensional c ollapse after cessation of regional contractile tectonism.