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Dating Of Late Permian Deformation In Eastern California


Contractional deformations of Late Permian age have been previously interpreted in the Sierra Nevada-Death Valley region and documented in the El Paso Mountains of eastern California. Deformation involving thrusting in the former region, previously considered Late Permian? in age, has been termed the Sierra Nevada-Death Valley thrust system (SNDVTS). Here we present the results of analyzed zircon populations from these two regions that refine the timing of the deformations and suggest a possible tectonic link between the SNDVTS and Late Permian deformation in the El Paso Mountains. In the El Paso Mountains a previously undated, undeformed pluton penetrating deformed eugeoclinal rocks has been dated as 257.39 ¬±0.1 Ma by Schmitz. The youngest eugeoclinal rocks contain latest Leonardian conodonts and are capped by an arkose from which Riggs and Cecil have recovered ∼267 Ma detrital zircons. These rocks are overlain by deformed andesitic rocks previously dated at ∼262 Ma. This deformation, therefore, apparently occurred around 260 Ma, an age in agreement with that of the Weiss pluton in the El Paso Mountains which has previously been dated as ∼260 Ma and interpreted as syndeformational. A sample from the Mount Pinchot pendant farther north in the central Sierra Nevada was analyzed by Paterson. It contains detrital zircons with a strong peak at 252-263 Ma. To the east, in Owens Valley, Riggs obtained detrital zircon ages of ∼263 Ma from the Reward Conglomerate, which is overlain conformably by a Wuchapingian (∼260 Ma) fossil bed. Deposition of both units preceded the SNDVTS. Riggs also recovered ∼257 Ma zircons from the unconformably overlying upper part of the Conglomerate Mesa Formation, which postdated the SNDVTS. Together, these samples and those from the El Paso Mountains suggest major deformational and magmatic events overlapping at about 260 Ma. A possible paleogeographic reconstruction places the El Paso Mountains west of the Mount Pinchot pendant and Owens Valley in Late Permian time, and our data indicate that plutons and/or volcanoes in the El Paso Mountains terrane could have been the source of detrital zircons in both of the other areas. West-vergent deformation in the El Paso Mountains and east-vergent deformation in the SNDVTS could have taken place simultaneously as the oceanic El Paso Mountains terrane collided with the continental block to the east along a convergent segment of the Late Permian continental margin.