Abstract 0.0 Exit to eastbound Interstate Highway 40 toward Flagstaff from Andy Devine Avenue in Kingman. 0.1 Behind and to the left in the foothills of the Cerbat Mountains is the Kingman Feldspar mine, a 1700 Ma zoned pegmatite (Heinrich, 1960; Wasserburg and Lanphere, 1965) that intrudes Early Proterozoic granitic rocks. Minerals of principal economic interest are microcline and allanite. 0.9 Prominent strata at the top of the Grand Wash Cliffs (fig. 26) in the far distance at 9:00 are Cambrian through Mississippian limestone and associated sedimentary rocks. Rocks beneath the Paleozoic section are mainly Early Proterozoic foliated to gneissic granite to granodiorite. Most of the Proterozoic rocks of the Grand Wash Cliffs are unmapped; only the Garnet Mountain area (Blacet, 1975; Theodore and others, 1982) has been mapped in any detail. 2.6 To the left and north is Hualapai Valley, which drains to the north into the playa of Red Lake at the northern end of the Cerbat Mountains. One of the thickest accumulations of halite in Arizona (minimum of 4,000 ft; possibly as thick as 10,000 ft) underlies Red Lake and the Hualapai Valley south of Red Lake (Peirce, 1973; 1976). 3.8 Crossing Rattlesnake Wash. Hills to our right are composed of mildly flow foliated, 1400 Ma coarse-grained porphyritic granite of Rattlesnake Hill 1400 (Kesler, 1976). 5.1 Outcrops on the right that weather into large bouldery piles are the granite of Rattlesnake Hill (fig. 24). 6.2 Exit 59, DW Ranch Road, provides access into the northern Hualapai Mountains. Roadcuts

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.