Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low-temperature thermochronology data

The Wasatch Mountains are often cited as an example of normal fault growth and footwall flexure. They represent a tilted footwall at the edge of the Basin and Range extensional province, a major rift basin. Thus understanding the detailed spatial and elevation changes in coupled thermochronometer data, and how these changes can be interpreted, may aid in the analysis of thermochronometer data from other extensional regions around the world. We present a dense data set from the Cottonwood Intrusive Belt (CIB) of the Wasatch that includes apatite fission track (AFT), zircon fission track (ZFT), and apatite (U-Th)/He ages. ZFT, AFT, and apatite (U-Th)/He ages are 10, 5, and 3 Ma, respectively, adjacent to the Wasatch fault. AFT and (U-Th)/He ages increase slightly with distance east of the fault until about 15-20 km, where a more abrupt increase in these ages occurs at or near the Silver Fork-Superior fault zone. ZFT and AFT ages are concordant with 31-38Ma pluton emplacement ages on the eastern side of range. Modeling of the data leads to the following interpretation: (1) Early cooling and similar to3-4 km of exhumation for the middle and eastern parts of the range occurred in the late Oligocene-middle Miocene. (2) Beginning at 10-12 Ma, the locus of exhumation shifted westward toward the present range front, where the rocks cooled from >200degreesC in the last 10-12 Myr. Our data and interpretations are consistent with a model in which the locus of faulting and exhumation shifted opposite the direction of tilt, similar to that predicted by rolling-hinge extensional models. However, this westward shift and rapid Miocene to recent exhumation may be a local effect superimposed on lower fault displacement and exhumation rates elsewhere along the Wasatch.