Mylonitization and detachment faulting in the Whipple-Buckskin-Rawhide Mountains terrane, southeastern California and western Arizona
Gregory A. Davis, J. Lawford Anderson, Eric G. Frost, Terry J. Shackelford, 1980. "Mylonitization and detachment faulting in the Whipple-Buckskin-Rawhide Mountains terrane, southeastern California and western Arizona", Cordilleran Metamorphic Core Complexes, Max D. Crittenden, Jr., Peter J. Coney, George H. Davis
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Field studies in the Whipple Mountains, southeastern California, and in the Buckskin and Rawhide Mountains, western Arizona, have defined the existence of an O1igocene(?) 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 the minerals in the mylonitized sills and their country rock gneisses did not reequilibrate during metamorphism. However, reequilibrated phases do occur in the ultrafine-grained mylonitic matrix and in tension gashes developed perpendicularly to mylonitic lineation. As a result of incomplete reequilibration, bimodal compositional ranges exist for plagioclase, epidote, celadonitic muscovite, and biotite. The minimum depth for intrusion and mylonitization is estimated to be 9.6 km from consideration of the interaction of compositionally corrected curves of muscovite stability and the adamellite solidus. Metamorphic mineral assemblages and feldspar thermometry indicate that mylonitization occurred from solidus temperatures of the plutonic sills down to middle greenschist grade.
Allochthonous (upper-plate) units in the detachment complex include Precambrian to Mesozoic crystalline rocks, Paleozoic and Mesozoic metasedimentary rocks, Mesozoic metavolcanic rocks, and Tertiary volcanic and sedimentary rocks. The oldest Tertiary rocks are debris flows (some containing mylonitic rocks), fanglomerates, lacustrine sediments, and volcanic rocks, all of the Oligocene(?) to lower Miocene Gene Canyon Formation. Red beds and volcanic rocks of the Copper Basin Formation overlie Gene Canyon rocks unconformably and are tilted less steeply than the older Tertiary rocks along northeast-dipping listric normal faults that occur widely within the upper plate. In the Whipple Wash area of the eastern Whipple Mountains, volcanic rocks of the Copper Basin Formation sit unconformably on brecciated lower-plate mylonitic rocks in a channel cut ~70 m below the Whipple detachment fault. These volcanic rocks were themselves involved in renewed detachment faulting along that fault. Collectively, these stratigraphic-structural relations indicate that detachment faulting occurred during Tertiary sedimentation over a significant period of time, and was therefore of growth-fault rather than catastrophic nature. Upper Miocene valley-fill sediments and alkali basalts unconformably overlie upper-plate structures and tilted strata, thus providing an upper age limit for the detachment faulting.
Northeastward movement of the thin (<5 km) upper plate is believed to have occurred under the influence of gravity, although the Whipple-Rawhide detachment fault could not have originally dipped more than a few degrees. The head, or breakaway zone, of the crustal slide is apparently defined by northeast-dipping normal faults in the Mopah Range, just west of the Whipple Mountains. Central areas of the slide complex in the vicinity of the Colorado River (Whipple and Buckskin Mountains) are characterized by extreme distension of the detached slab along northwest-striking, northeast-dipping, listric normal faults. There is telescoping of allochthonous units in distal, or toe, portions of the slide complex in the Rawhide and Artillery Mountains of western Arizona, where thrust faulting of older rocks over rocks as young as middle Miocene is common. Northeastward displacements of allochthonous units in excess of several tens of kilometres are indicated by field relations in the Buckskin and Rawhide Mountains.