EXHUMATION OF THE WASATCH MOUNTAIN FRONT AND IMPLICATIONS FOR WASATCH FAULT AND FOOTWALL SEGMENTATION

The Wasatch fault is separated into ten segments based on its surface fault trace and topographic salients in the footwall Wasatch Mountains. The topographic salients and apparent changes in depth of exposure of footwall rocks at the segment boundaries suggest these boundaries partition long-term exhumation between the segments. Common elevation apatite (U-Th)/He ages from footwall sites along the base of the Wasatch Mountains for the five medial segments (~200 km) have overlapping uncertainties and average 5.3±1.2 Ma. These ages indicate exhumation rates of 0.2–0.4 mm/yr since the Pliocene for most of the Wasatch Range front. The exception is the southern Salt Lake segment where ~2 Ma He ages suggest average exhumation rates of 0.6–1.0 mm/yr. This partitioning has been accommodated along the E-W-striking Deer Creek fault system.

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 5–6 Ma to 14 Ma between the Weber and northern Salt Lake segments and increase from 4–5 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.