EXHUMATION OF THE WASATCH MOUNTAIN FRONT – THE YOUNGEST PHASE OF DEFORMATION ALONG THE WASATCH LINE

The youngest phase of deformation along the Wasatch Line is associated with continental extension. Much of this extension is accommodated along the Wasatch fault, which currently is the major structural feature along the Wasatch Line and divides the Basin and Range and Colorado Plateau-Rocky Mountains geologic provinces. The 370 km-long Wasatch fault zone is separated into 10 segments with present-day boundaries that include bedrock salients, en echelon fault steps, oblique intersections of fault traces, and long gaps in the fault zones. Steps and salients are spatially coincident with geophysical anomalies and commonly juxtapose rocks from different structural levels. Segments between the boundaries commonly display differences in drainage basin properties and range-crest elevations. 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.

Apatite fission-track and (U-Th)/He ages are used to assess exhumation of the five medial segments of Wasatch Mountain front in order to constrain timing, rates, and magnitudes of late Cenozoic exhumation. Range front He ages average 5.3 ±1.2 Ma with overlapping uncertainties and 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. In contrast, same-sample fission-track ages range from ~5 to 30 Ma. Whereas the He ages generally are consistent along the range front, fission-track ages change by as much as 3.5 times across segment boundaries. The changes in ages across segment boundaries prior to 5 Ma and the relative consistency in ages since 5 Ma suggest that most of the range-front segmentation was acquired prior to the Pliocene, perhaps coincident with major unroofing elsewhere in the Great Basin during the middle Miocene. The segment boundaries, once formed, act as long-lived rupture barriers that control the locations of the late Quaternary surface faults.