EXHUMATION OF THE WASATCH MOUNTAIN FRONT AND IMPLICATIONS FOR WASATCH FAULT AND FOOTWALL SEGMENTATION
New apatite fission-track ages from along the Wasatch front (same separates as He age samples) range from ~5 to 30 Ma. Whereas the He ages are generally consistent along the range front, fission-track ages change by as much as 3.5 times across segment boundaries. FT ages increase from 56 Ma to 14 Ma between the Weber and northern Salt Lake segments and increase from 45 Ma to 8 Ma between the southern Salt Lake and Provo segments. Farther south, FT ages increase from 8 to 30 Ma at the Provo to Nephi segment boundary. The fission-track age variations are interpreted to represent differences in range front exhumation rates during the Miocene. These differences in exhumation rates may have lead to the early (Miocene) formation of most of the mountain front steps and salients. For the last ~5 Ma, most of the Wasatch front has been exhuming at the same average rate (0.2-0.4 mm/yr) and the steps and salients have persisted during that time. The steps and roughness in the exhuming footwall form nonconservative rupture barriers that probably have controlled the locations of the late Quaternary surface faults.