COMBINED EXTENSIONAL-DETACHMENT AND SALT TECTONICS FORM AN ATYPICAL EXTENSIONAL BASIN, SEVIER VALLEY, UTAH

The geologic structure of Sevier Valley in central Utah is atypical of extensional basins. Bounding ranges on the east and west are underlain by Tertiary strata and Jurassic rocks underlie low hills on the eastern side of the valley. The map pattern thus suggests that the valley is a horst rather than a graben! We interpret the valley to have formed along a normal fault but with an unconventional geometry owing to effects of weak and buoyant evaporitic mudstone in the footwall block.

In eastern Sevier Valley, steeply tilted fault blocks of Tertiary strata sit upon a gently inclined, five- to ten-meter thick zone of red clay-rich rock that grades downward into the Jurassic Arapien Shale. This red zone persists at the upper contact of the Arapien Shale for more than 50 km along strike. Faults cutting overlying Tertiary strata terminate downward at the red zone and require the contact to be tectonic. Diehl et al. (1998) interpreted the contact as a pressure-solution surface rather than a fault, but close examination reveals the zone to be fault breccia and gouge that contain slickensides and top-to-the-east S-C fabrics in gypsum beds and mudstone. We therefore interpret the contact to be a top-to-the-east low-angle normal fault that emplaced Tertiary strata upon Jurassic.

The outcrop area of Arapien Shale is bounded on the east by an east-dipping normal fault against Tertiary clastic strata that are folded into the west-facing Wasatch monocline. We interpret the low-angle fault to root into this fault and the Wasatch monocline to be a hanging-wall rollover anticline. The low-angle fault segment probably formed by rolling-hinge abandonment of the shallow part of the fault, enhanced by the buoyancy and mechanical weakness of evaporitic Arapien Shale in the footwall. Quasidiapiric ascent of the unroofed Arapien Shale resulted in an anomalous geometry in which the trace of the basin-forming fault is on the hanging-wall side of the valley and the valley floor is underlain by footwall rocks.