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Metamorphic “klippen” in the Diamond Mountains, Nevada, and the implications for Mesozoic shortening and extension
Abstract
The western flank of the Diamond Mountains in central Nevada exposes a small belt of metamorphosed strata previously inferred to be klippen belonging to the Roberts Mountains allochthon. The Roberts Mountains allochthon is an assemblage of deep marine, early Paleozoic strata that was thrust eastward over the continental shelf during the late Paleozoic Antler orogeny, and the klippen in the Diamond Mountains would represent the easternmost exposures of the allochthon at this latitude. The occurrence of fragments of the allochthon in the Diamond Mountains is enigmatic because cross cutting relationships indicate that the fault beneath the klippen postdates the Antler orogeny and, therefore, the fault can not be the Roberts Mountains thrust. This relation suggests complex telescoping of the Roberts Mountains allochthon post-dating the Antler orogeny. However, reevaluation of the metamorphic rocks indicates that 1) they are probably Permian in age and post-date the Antler orogeny and 2) they geometrically do not define klippen but rather form part of the hanging wall of the regionally extensive north-trending Phillipsburg fault. The Phillipsburg fault dips about 45°west and extends for at least 40 km along the west flank of the Diamond Mountains. Overall, hanging wall rocks dip steeply to the west, are probably overturned, and contain a partitioned high-angle west-dipping cleavage. Silurian to Permian strata are exposed in the footwall and define a north-trending overturned anticline-syncline pair that locally contains axial planar cleavage similar in orientation to cleavage in the hanging wall of the fault. The Phillipsburg fault has been inferred to be a contractional fault by some workers and an extensional feature by others. Retrodeformation of strata in the hanging wall suggest a normal sense of slip for the Phillipsburg fault and that hanging wall strata represent an offset segment of the steeply west-dipping limb of the overturned Mesozoic (?) anticline. The Phillipsburg fault appears to be partially responsible for exhumation and relative uplift of strata in the Diamond Mountains and consequently is probably a Cenozoic normal fault. In summary, the metamorphic strata previously interpreted as klippen belonging to the Roberts Mountains allochthon and the fault that bounds them do not bear any relation to the Antler orogeny. Moreover, the Roberts Mountains allochthon was not structurally translated into the Diamond Mountains.
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Reflexion seismic data in Railroad, Diamond, Mary's River and Goshute Valleys provide information on their structural development that cannot be deduced solely from outcrop and well data. These valleys contain Tertiary sediments that, in dip section, define an asymmetrical basin bounded along the eastern flank by a major listric normal fault with about 3.0-4.5 km of displacement. The west flank is defined by a gentle east-dipping ramp. Seismically the trace of the listric fault is interpreted to dip westward and sole into the Palaeozoic section exploiting regionally recognized Mesozoic decollement surfaces. The Tertiary depocentre, adjacent to this fault, shifted from west to east with continued slippage through time, the greatest movement occurring in the Miocene and post-Miocene. In the strike direction, the valleys are separated into at least two sub-basins by an east-west structurally high axis. The axis is postulated to be the result of a tear fault associated with movement along the listric normal fault. Tertiary stratigraphy varies between valleys and between sub-basins in a given valley. All the valleys contain Miocene and younger rocks; however, not all sub-basins contain the pre-Miocene section, suggesting a complex scheme of structural development.
A review of selected subsurface data (mostly seismic-reflection data) from the Great Basin indicates that the basins form in one of 3 ways. Historic, Holocene, or latest Pleistocene surface ruptures are associated with each type of basin. Indentification of the type of normal faulting associated with each basin, as well as with the province boundaries, is critical to accurate assessment of both earthquake hazards and resource potential. As basins mature, they grow larger by one or more of 4 sedimentary or structural processes and as they grow broader or coalesce, the deformation style becomes less complex. Evolutionary trends also reflect a broadening of the spacing between the faults that play an active role in basin development. The possibility exists that, in some areas, extension on widely spaced, steep, deeply penetrating faults is a late-stage process that evolved from earlier large-magnitude extension on shallow listric faults, rotated planar faults, and/or detachment faults that developed near sites of strong thermal (magmatic) disturbance. -from Authors