Abstract

The Red Hills are along the eastern edge of the Basin and Range province, directly west of the Colorado Plateau, southwest Utah. A succession of Oligocene and Miocene volcanic and volcaniclastic rocks about 610 m (2,000 ft) thick has been tectonically detached at a shallow crustal level from underlying Tertiary rocks along a low-angle shear zone. Evidence for the proposed detached plate and low-angle shear zone is: (1) a pulverized zone, interpreted as fault gouge, that separates an upper (allochthonous) plate from an underlying lower (autochthonous) plate; (2) attitudes in upper plate rocks that are sleeper than those in lower plate rocks and that locally dip into lower plate rocks; (3) intense faulting and fragmentation of upper plate rocks; (4) high- and low-angle faults that appear to be restricted to the upper plate; (5) tectonic omission of strata along the shear zone; (6) numerous megabreccia blocks composed of chaotically mixed, upper plate rock types lying atop the upper plate; these blocks are interpreted as gravity-slide blocks shed during decoupling and breakage of the upper plate.

The stratigraphy of the Red Hills is the same as that in the Markagunt Plateau, about 15 km (9 mi) east of the Red Hills. Evidence for the same low-angle shear zone also exists in the Markagunt Plateau. Thus, the shear zone in the Red Hills may be of regional extent. The age of the shear zone is probably less than 22.5 (age of the youngest rocks in the upper plate). The shear zone apparently formed through tilting of the upper plate. Tilting may have resulted from crustal extension and associated block faulting, or from emplacement of magma at shallow depths.