Seismic reflection profiles (approximately 550 km), gravity data, and well log information from west central Utah, have been interpreted to reveal an asymmetric west-dipping Cenozoic basin beneath the Sevier Desert. The basin is underlain by a 10-degrees-12-degrees westward dipping, slightly undulatory, detachment surface, the Sevier Desert detachment. Finite difference synthetic seismograms were used to constrain the geometry of the interpreted reflectors in the basin. A two-dimensional, one-layer gravity inversion of three gravity profiles in the basin, using a 0.35 g/cm3 density contrast, corroborated the structure determined from the seismic interpretations. Physical properties and lithological data from eight wells were interpreted to acquire velocity-depth and density-depth functions and to obtain stratigraphic control. A minimum width of the Sevier Desert detachment of 80 km to 130 km and an area of 5600 km2 to 9100 km2 was estimated primarily from the reflection data. On the basis of geological and geometrical information, we estimated a minimum post-middle Pliocene extension of 2.5 km to 3.5 km at a rate of 0.6 mm/yr to 0.8 mm/yr, corresponding to a strain rate of 2.7 10(-16) s-1 to 3.5 10(-16) s-1. Up to 4.0 +/- 0.6 km of predominantly post Eocene lacustrine and fluvial sediments overlie the Sevier Desert detachment including a well-defined reflection that is interpreted as a middle Pliocene 4.2 m.y. basaltic flow that is present throughout the central and western part of the basin. The geometry of the middle Pliocene reflector, a variable approximately 300 to approximately 1500 m post-middle Pliocene offset of a approximately 40-degrees eastward dipping western basin-bounding fault, and up to approximately 6 km east and west stepping of the generally north-trending western basin-bounding fault, suggests a complex late Tertiary deformational history. The varying deformational pattern may be a response to a changing geometry of the underlying detachment surface.
