Abstract

Radiolarian biostratigraphy and detailed geologic mapping have been used to resolve the complex structure and stratigraphy of part of the Golconda allochthon in the Independence Mountains, Nevada. Here, the Schoonover sequence is latest Devonian to Early Permian in age, spanning the time interval from the emplacement of the Roberts Mountains allochthon onto the shelf in the earliest Mississippian Antler orogeny to the inception of the Sonoma orogeny. The history of the Schoonover basin is tied to that of the adjacent shelf in Nevada and provides important insights into the upper Paleozoic paleogeographic framework of the continental margin.

In the Schoonover sequence, latest Devonian (Famennian) chert overlies basaltic and andesitic greenstone and is in turn overlain by Early Mississippian (Kinderhookian) chert interbedded with tuff and volcaniclastic rocks derived from an island arc. The Kinderhookian volcaniclastic rocks grade upward, with no obvious depositional break, into siliciclastic turbidites and pebbly mudstones that contain debris derived primarily from erosion of the Antler orogenic belt, although in some beds volcanic rock fragments and greenschist-grade metasedimentary clasts are also common. The age of underlying (Kinderhookian) and interbedded and overlying (Osagian to Meramecian) chert sequences indicates that the siliciclastic turbidites are synchronous with the early deposits of the autochthonous foredeep basin of the Antler orogenic belt. The dual or composite source terranes represented by the Schoonover siliciclastic rocks place the basin between an arc and the Antler orogen on the edge of the shelf.

Meramecian-age basalt flows in the Schoonover are coeval with subsidence and basaltic volcanism in autochthonous shelf sequences in northern Nevada, suggesting that a rifting event may have occasioned the end of Antler-age compression in Nevada. The onset of limestone turbidite deposition in the Schoonover corresponds to the reestablishment of carbonate shelf conditions on the continental margin in latest Mississippian–earliest Pennsylvanian time.

The Late Devonian–earliest Mississippian part of the Schoonover depositional history has not yet been documented elsewhere in the Golconda allochthon, but Late Mississippian to Permian rocks in the Schoonover sequence are analogous to those of the Havallah sequence, suggesting deposition in the same basin.

Thrust faults in the Schoonover repeat the Late Devonian to Permian section, detaching the basinal sequence from its depositional basement. Thrust faults extend along strike for at least 10 km, but thrust plates are <1 km thick, reflecting the originally thin sequence involved in thrusting. Fold and fault data indicate that thrust plates formed and were emplaced due to northwest-southeast shortening and southeast-directed thrusting. Deformation within the allochthon postdates deposition of Early Permian strata, and the emplacement of the allochthon postdates deposition of latest Permian autochthonous strata and predates intrusion of Jurassic plutons.

The stratigraphic relations documented in the Schoonover sequence are compatible with a back-arc thrusting model for the formation of both the Roberts Mountains and Golconda allochthons, but they are more difficult to reconcile with models that interpret the allochthons as accretionary prisms developed in front of farther-traveled arcs that collided with a passive margin.

In a broader context, the stratigraphy of upper Paleozoic allochthonous rocks in Nevada records short-lived episodes of crustal shortening along the continental margin, separated by longer episodes of extensional and/or transcurrent tectonics. This data suggest that the western United States was a southwest Pacific–style active margin at least as far back as the Devonian.

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