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We recognize two important paleogeographic links between the Klamath-Sierran segment and Mojave-Sonoran segment of the early Mesozoic arc in the southwest Cordillera: (1) both segments of the arc occupied an extensional graben-depression in Late Triassic to early Middle Jurassic time, and (2) the graben-depression acted as a long-lived (>40 m.y.) trap for Early and Middle Jurassic craton-derived quartz arenites deposited within both segments of the arc. The Early Mesozoic arc formed at a high angle to the truncated continental margin across cratonal, miogeoclinal, and eugeoclinal settings inherited from Paleozoic time. Intra-arc extension in the cratonal setting (Mojave-Sonoran Deserts) resulted in accumulation and preservation of thick terrestrial volcanic and sedimentary sequences. Intra-arc extension in the miogeoclinal setting (southern and central Sierra Nevada) resulted in deposition of very thick shallow marine and lesser deep marine successions. Intra-arc subsidence is also documented from deep marine to shallow marine successions in the eugeoclinal setting (northern Sierra and adjacent northwest Nevada).

The Klamath-Sierran segment and Mojave-Sonoran segment of the arc evolved dissimilarly in late Middle Jurassic to Late Jurassic time. Contractional deformation documented in the Klamath-Sierran segment was not nearly as widespread or intense in the central Mojave Desert, and probably did not affect the southern Mojave to Sonoran Desert areas at all. Extensional tectonics, instead, continued to dominate the Mojave-Sonoran segment of the arc, probably due to continental rifting associated with the opening of the Gulf of Mexico.

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