Chapter 5: Sedimentologic and stratigraphic constraints on the Neogene translation and rotation of the Frenchman Mountain structural block, Clark County, Nevada
Stephen M. Rowland, Joseph R. Parolini, Edward Eschner, Alonzo J. McAllister, Jonathan A. Rice, 1990. "Chapter 5: Sedimentologic and stratigraphic constraints on the Neogene translation and rotation of the Frenchman Mountain structural block, Clark County, Nevada", Basin and Range Extensional Tectonics Near the Latitude of Las Vegas, Nevada, Brian P. Wernicke
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The Frenchman Mountain structural block lies near the intersection of a right-lateral strike-slip fault, a left-lateral strike-slip fault, and a regionally significant low-angle normal fault. It has commonly been presumed that this block was translated several tens of kilometers northwestward or southwestward during Basin and Range extension, but the details of this translation have not been rigorously examined. Al-though it is not yet possible to reconstruct the detailed histories of the individual faults that were involved in the translation of the Frenchman Mountain block, the determination of the net translational and rotational history of this block can be used to evaluate the relative importance of various types of faulting during extension of the Lake Mead region.
The Frenchman Mountain block contains a thick section of Paleozoic, Mesozoic, and Miocene strata, including the syntectonic Miocene Horse Spring Formation. Comparisons of clast compositions in the breccias of the Thumb Member of the Horse Spring Formation with various areas of exposed Precambrian basement indicate that the Gold Butte granite complex, 65 km to the east, is the only viable presently exposed source area for these breccias.
Sedimentology of the Thumb Member indicates that this unit was deposited in proximal and medial alluvial fan settings. Channel orientation and facies relations indicate a transport direction of N60W ± 30°. Two-meter clasts and the presence of distinctly channelized, matrix-supported breccia indicate that these sediments were deposited no farther than 5 km from their source. Large blocks up to 100 m long and 20 m thick in southern Rainbow Gardens were translated no farther than a few hundred meters. We conclude that the pre-extension position of the Frenchman Mountain block was probably on the western or northwestern margin of the Gold Butte granite complex, directly adjacent to the Gold Butte fault. This reconstruction requires that a fault exists on the southern boundary of Rainbow Gardens roughly beneath Las Vegas Wash.
Pennsylvanian eolian cross-bed orientations at Frenchman Mountain are identical to those in the Gold Butte area. This suggests that no significant tectonic rotation accompanied the westward translation of the Frenchman Mountain block.
As an independent test of Frenchman Mountain’s preextension position we have compared the thicknesses and facies of Cambrian and Devonian strata in the Frenchman Mountain block with those in the eastern Lake Mead region. These Paleozoic data indicate a preextension position in the northeastern part of the Lake Mead area, thus supporting the interpretation that the block originally lay adjacent to the Gold Butte granite complex.
Considering all available Paleozoic and Miocene sedimentologic and stratigraphic data, and assuming that the Gold Butte granite complex itself experienced about 10 km of westward translation, our restoration vector for the Frenchman Mountain block relative to the Colorado Plateau has a magnitude of 80 ± 8 km and a bearing of N80E + 5°.
The documentation of 80 ± 8 km of translation and associated stratal tilting, with no significant tectonic rotation, indicates that detachment faulting has been the dominant Neogene deformational process in the Lake Mead region.