DOMINO STYLE FAULT BLOCK ROTATION IN THE COTTONWOOD CREEK AREA OF THE SOUTHERN PORTNEUF RANGE, SOUTHEASTERN IDAHO

     New geologic mapping and kinematic analysis was completed in the southern Portneuf Range of southeast Idaho to characterize the Valley Fault, a large-offset normal fault interpreted as the breakaway for the regionally extensive Bannock detachment system. The Valley Fault separates Miocene Salt Lake Fm from underlying Neoproterozoic to Cambrian Brigham Group and Cambrian rocks. The fault underlies several klippen within the range, contrary to previous mapping that interpreted a depositional contact between Miocene and Brigham Group strata. Three-point problems indicate the Valley Fault strikes NNW and dips 10-15^o WSW, in stark contrast to previous work which interpreted the fault as steeply W-dipping.  Hanging wall strata, consisting of conglomerate with rare tuff, are undated but lithologically correlative to the late Miocene upper Salt Lake Fm. These rocks strike NNW and dip 11^o ENE adjacent to the fault and steepen progressively downsection to ~60° ENE. Footwall strata strike NNW and dip 45±15° ENE. Four fault sets cut the footwall. Chronologically they include 1) a localized, gently E-dipping thrust fault that places Camelback Quartzite on Twin Knobs Limestone; 2) a subhorizontal fault with top-to-the-west normal offset of ~600m; 3) the Valley Fault; and 4) NE-striking normal faults with up to 740m of offset. Top-to-the-west offset across the Valley Fault, using the Tertiary unconformity as a marker, is 5800 m.

     The initial dip of the Valley Fault is interpreted to have been steeper than its present 10-15° dip. If footwall strata were subhorizontal prior to extension, as indicated by reconstruction of the Tertiary unconformity (Rodgers & Janecke, 1992), then the Valley Fault had an initial dip of 60±15° WSW based on measured bedding-to-fault angles. Hanging wall bedding-to-fault angles diminish upsection from ~70° to 20°.  Combined with the modern low dip of the Valley Fault, these data are interpreted to indicate the Salt Lake Fm accumulated adjacent to the active Valley Fault while the fault as well as footwall and hanging wall strata tilted NE through time. The Valley Fault's planar to mildly listric shape, the progressive domino-style tilting, and the moderate amount of total slip suggest the Valley Fault is a typical Basin-Range normal fault rather than a low angle breakaway fault for the Bannock detachment system.