TWO-STAGE EXHUMATION OF MID-UPPER CRUSTAL ROCKS IN THE LONE PEAK SALIENT OF THE WASATCH FAULT, UTAH

The Lone Peak salient, first described by Gilbert (1928), is the largest footwall promontory of the Wasatch fault. The physiography of the salient is accentuated by the Traverse spur, a low relief WSW-ENE oriented range that extends 8-13 km west of the Wasatch range-front. The salient is cored by the Oligocene Little Cottonwood stock, whereas the Traverse spur is a dismembered structural block derived from the Charleston-Nebo thrust sheet comprised of Pennsylvanian-Permian Oquirrh Group overlain by late Eocene volcanic and sedimentary rocks. Exposures of the fault in the salient reveal that it dips 25-45^o to the west, southwest and south in a radial pattern away from the rounded apex of the salient. Topographic profiles of the salient conform to the fault dip and are suggestive that the salient is a large-scale structural corrugation that formed as a result of footwall flexure (Spencer and Ohara, 2008).

Contemporary models of the Lone Peak salient propose that the Wasatch normal fault has sustained more than 11 km of vertical offset during the Neogene (e.g., Parry and Bruhn, 1987). These results have been portrayed as being typical of the entire fault system, but the genesis of the Lone Peak salient is unique and ultimately an expression of Precambrian and Late Cretaceous-middle Eocene structural inheritance. Furthermore, the effects of an earlier episode of late Paleogene crustal extension, of significant magnitude, have been under appreciated.

Our investigation used ⁴⁰Ar/³⁹Ar K-feldspar and biotite data, combined with hornblende thermobarometry and fluid inclusion data to evaluate the exhumation history of the southern and western margin of the salient. U/Pb zircon dating shows that the Little Cottonwood stock is the same age on its east and west margins, ca. 30 Ma. These results reveal that there is a step-wise history of extensional exhumation starting with an initial period from 36-20 Ma, followed by an eight million year hiatus before the final culminating period from 12-0 Ma. The magnitude of total exhumation is immense. Rocks as deep as 13-18 km have been tectonically unearthed along the western margin of the salient. These two episodes of extensional exhumation coincide with late Paleogene extensional collapse of the Cordillera followed by Basin and Range normal faulting.