Mylonitization and detachment faulting in the Whipple-Buckskin-Rawhide Mountains terrane, southeastern California and western Arizona

Abstract

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...

Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

“…1). First recognized as such and described by Davis et al (1980), it is representative of the style of core complex developed in the Colorado River extensional corridor and adjacent areas in Arizona (Spencer and Reynolds, 1991). Together with the Buckskin, Rawhide, and possibly other nearby core complexes it may be part of a continuous larger extensional structure (Davis and Lister, 1988).…”

Strain localization on an oceanic detachment fault system, Atlantis Massif, 30°N, Mid‐Atlantic Ridge

“…The characteristic morphology of metamorphic core complexes is recognizable in nearly all occurrences, and includes a broad, elliptical dome of middle-to lower-crustal rock with long dimension between $10 km to 25 km (the footwall of the low-angle fault system). The dome is surrounded by rotated, block faulted ridges of upper-crustal rock that form the hanging wall [Crittenden et al, 1980;Davis et al, 1980]. In continental settings, the domed footwall is elevated up to 1000 m above the surrounding hanging wall, and is characterized by smooth, extension-parallel corrugations with wavelengths ranging from several hundred meters to tens of kilometers and amplitude ranging from tens to hundreds of meters [John, 1987].…”

“…[5] In this paper we test the geological viability of oceanic core complex hypothesis by examining the evolution and spatial relations of deformation textures in rocks exposed at an active inside corner high, and compare them to those of a continental metamorphic core complex [Crittenden et al, 1980;Davis et al, 1980Davis et al, , 1986John, 1987;Spencer and Reynolds, 1989] (Figure 3). These features in continental settings are formed as the footwall is denuded from high-temperature/highpressure conditions in the middle to lower crust to surface temperature and pressure and include the following: (1) penetrative, gently dipping ductile deformation fabrics at structural levels up to 1000 m beneath the main detachment surface, (2) localized, ductile deformation fabrics with subhorizontal mylonitic foliation and extensionparallel lineations that crosscut penetrative fabrics within a mylonitic front up to several hundred meters beneath the detachment, (3) brittle, cataclastic deformation fabrics associated with intense low-temperature alteration that overprint all ductile fabrics and are localized into a zone between several meters and several hundred meters thick beneath the detachment fault; fabrics are dominantly subhorizontal, parallel to the inferred fault(s) and subparallel to oblique to mylonitic fabrics, and Idealized schematic cross sections depicting the development of a metamorphic core complex.…”

Geologic map of the east-central Meadow Valley Mountains, and implications for reconstruction of the Mormon Peak detachment, Nevada

“…4). Studies of imbricate normal fault blocks suggest that the mean tilt direction tends to parallel slickenlines and other transport indicators (e.g., Anderson, 1971;Davis et al, 1980;Davis and Hardy, 1981). Thus, the tilt direction of bedding is often used as a proxy for maximum elongation direction in extensional allochthons, and for the transport direction on underlying detachments, assuming bedding was subhorizontal at the onset of extension.…”