CHOOSING THE FAVORITE SPORT OF FRENCHMAN MOUNTAIN, SURFING OR RAFTING

As the Frenchman Mountain structural block (FM) moved 65+ km WSW from the Colorado Plateau, it experienced near-zero vertical deformation, requiring either surfboard-style transport on the frontal slope of a detachment-related rolling hinge or passive rafting on a west-flowing undermass; the first an upper-plate-active model of regional elongation and the second a lower-plate-active model of localized elongation. Recent studies of basins bordering FM (Boulder basin (BB) on the east, Nellis basin (NB) on the north, and Las Vegas basin (LVB) on the west) provide data that aid in choosing between these highly contrasting tectonic models as they apply to the final stages of motion of FM. Compared with NB and LVB, BB has weak gravity expression, indicating that any underlying detachment fault must be shallow. But geophysical data argue strongly against projecting a shallow detachment westward beneath FM and LVB. In BB, steep dip and strike slip of faults, normal drag of bedding at faults, presence of an E-W structural ridge formed in contraction in the middle of the basin, lack of footwall-derived detritus at faults, and N- or S-directed sediment provenance patterns are all inconsistent with large-magnitude east-west extension, regardless of the depth of an inferred detachment. The supradetachment basin model calls for breakaway-style downward-flattening faults at the east margins of BB and LVB, but such faults have not been identified despite detailed mapping of the east margin of BB and analysis of multifaceted geophysical data at the east margin of LVB. Lithostratigraphic relations in NB and westernmost BB suggest that FM essentially completed its journey (by ca. 8.5 Ma) before it was A) tilted to form a highland source of detritus or B) tectonically inserted southward into a pre-existing basin environment as a high-standing east-tilted source of detritus. Although difficult to reconcile with a detachment fault model, any or all of these relations can be readily incorporated into a localized rafting model in which FM and its adjacent basins moved on a high-velocity current of middle crust fed by south-directed regional flow that was backstopped against, and flowed parallel to, the north margin of the northern Colorado River extensional corridor.