Field studies in the Whipple Mountains, southeastern California, and in the Buckskin and Rawhide Mountains, western Arizona, have defined the existence of an 01igocene(?) to middle Miocene gravity slide complex that is at least 100 km across in the direction of its transport (N50° ± 10°E). The regionally developed complex is underlain by a subhorizontal detachment fault, named the Whipple detachment fault in western areas and the Rawhide detachment fault in eastern areas. The fault, which was warped and domed after its formation, separates a lower-plate assemblage of Precambrian to Mesozoic or lower Cenozoic igneous and metamorphic rocks and their deeper, mylonitic equivalents from an allochthonous, lithologically varied upper plate.Most lower-plate crystalline rocks were subjected to regional Late Cretaceous and / or early Tertiary mylonitization and metamorphism. The abrupt (3-to 30-m-wide) upper limit of mylonitization, the Whipple "mylonitic front," is a mappable zone of high strain and, presumably, high thermal gradient. In parts of the Whipple Mountains, mylonitization was accompanied by the intrusion of subhorizontal sheets or sills of adamellite to tonalite up to a few hundreds of metres thick, although elsewhere thick sections of mylonitic rocks are devoid of such sills. The sills include both peraluminous and metaluminous varieties and are compositionally distinct relative to plutons in the overlying upper plate, being richer in Al, Mg, Ca, Na, and Sr and depleted in K and Rb. The compositions of most of 79 on July 18, 2015 memoirs.gsapubs.org Downloaded from
INTRODUCTION Detachment Faults, Regional SettingNearly two decades ago, Peter Misch (1960) described an extensive terrane in the border area between northern Nevada and Utah that he thought was underlain by a low-angle regional fault (Fig. 1). He named this fault "Snake Range decollement" and the area in which it occurs "Northeastern Nevada structural province." The decollement typically separates complexly faulted, unmetamorphosed, upper-plate rocks from stratigraphically older, lower-plate crystalline complexes, the latter now known to include metamorphic rocks of both Mesozoic and Tertiary age. Misch noted that some mountain ranges in the structural province were fault blocks of basin-range type; others were "elongate-domal uplifts" with only subordinate block faulting. In recent years "decollement" structures on July 18, 2015 memoirs.gsapubs.org Downloaded from S = Snake Range; F = Funeral Range; G = Garlock fault; LV = Las Vegas; E = Eldorado Mountains; W = Whipple Mountains; P = Phoenix; T = Tucson.and domal ranges of the type recognized by Misch have also been discovered in more southerly areas of the Cordillera (Fig. 1), in the Death Valley area of California (Keene Wonder fault and Funeral Range; Reynolds, 1974) and, more extensively, in southernmost Nevada, southeastern California, and Arizona-the area discussed here.Many of the low-angle faults in the Nevada-Utah region studied by Misch are in areas that lack on July 18, 2015 memoirs.gsa...
