Abstract

Geologic mapping of structures along the western edge of the Markagunt Plateau, southwestern Utah, in the transition zone between the Basin and Range and the Colorado Plateau provinces, reveals anastomosing high-angle faults and previously unrecognized Tertiary low-angle faults and thrust faults. Most of these structures were formed in the Miocene, but some have remained active into the Pleistocene.

One of the oldest Tertiary structures is the Red Hills low-angle shear zone, interpreted as a shallow structure that decoupled an upper plate, composed of a Oligocene-Miocene ash-flow tuff and volcaniclastic rock succession, from a lower plate, composed of Tertiary sedimentary rocks. Timing of deformation along the shear zone is between 22.5 to 20 Ma. Other low-angle faults are younger, some formed during uplift of the Markagunt Plateau, while others may be related to local magmatic intrusion.

Tertiary thrust faults along the western front of the Markagunt Plateau place an upper plate of Upper (?) Cretaceous Iron Springs Formation and Paleocene to Oligocene sedimentary and Oligocene volcanic rocks over a lower plate consisting locally of Oligocene and Miocene volcanic rocks but mostly of Paleocene-Eocene Claron Formation and older rocks. The youngest exposed rock unit in the lower plate is the 22-22.5 Ma Harmony Hills Tuff. These faults may be true thrust faults related to strike-slip faulting along the plateau margin or megaslide blocks overlying a monoclinal fold formed during uplift of the plateau. The thrusts are consistently associated with a monocline, which we interpret as a continuation of the Parowan monocline exposed at Cedar City.

North- to northeast-trending high-angle faults bound a horst-and-graben system and cross cut both the Red Hills shear zone and the thrust faults. The faults were formed later than about 20 Ma, an interpretation based on displacement of the 20-Ma Iron Peak laccolith and associated dikes. Some of these faults may have remained active into the Pleistocene. Faults bounding horsts and grabens in the central part of this system anastomose along strike and merge toward the southern end of the map area. Within grabens, debris eroded from horst walls has formed lobe-shaped landslide deposits. Pleistocene basaltic cinder cones are localized along graben-bounding faults. Gravity-slide blocks are common throughout the plateau in megabreccia deposits of Miocene age that resulted from shearing along the Red Hills shear zone; other megabreccia deposits were formed as the result of intrusion and volcanism.