Large-magnitude Permian shortening and continental-margin tectonics in the southern Cordillera

Abstract

The Last Chance allochthon, with displacement estimated to be 55 to 105 km and lateral extent possibly exceeding 300 km, was inferred to be Middle Triassic and thus kinematically distinct from a belt of Permian-Triassic deformation in the western Mojave Desert. Correlations of structures forming the Death Valley thrust belt, which includes the Last Chance allochthon, several subjacent structures, and their inferred lateral equivalents, suggest that prior to Tertiary extension, individual structures were laterally continuous for distances exceeding 150 km along strike, from the Nevada Test Site area southwest to the Darwin Plateau. Their inferred Mesozoic age and the presence of an Early Triassic overlap sequence on the Darwin Plateau precluded projection of these structures farther south, in spite of long-recognized strain compatibility problems associated with large displacement on thrusts that apparently terminated there without suitable accommodation structures, such as tear faults or lateral ramps. These problems may be resolved if the Last Chance allochthon is Permian, consistent with structural correlations, new isotopic data from post-tectonic stocks, and stratigraphic evidence. A large Permian turbidite basin near the Darwin Plateau can be interpreted as a marine foredeep resulting from emplacement of the 5-km-thick allochthon, rather than a local trough caused by transcurrent faulting during inferred Permian-Triassic continental truncation. I propose that anomalous eugeoclinal rocks in the western Mojave Desert have been emplaced against cratonic strata there by the Last Chance allochthon, which foreshortened an originally narrow continental margin.