DEVELOPMENT OF THE LAST CHANCE-DEATH VALLEY THRUST SYSTEM: AN EARLY PERMIAN TRANSPRESSIONAL FOLD-THRUST BELT ALONG THE SOUTHWEST MARGIN OF LAURENTIA

The Permian margin of southwest Laurentia experienced a transition crucial for the development of the North American Cordillera throughout the rest of the Phanerozoic. Inception of the California-Coahuila transform during the Pennsylvanian provided the necessary instability to allow induced subduction initiation during the middle Permian. The evolution to a dominantly convergent margin is recorded by the early Permian Last Chance-Death Valley thrust system, which developed as transpression was progressively partitioned between strike-slip and contractional components. Subduction initiated along an outboard strand of California-Coahuila transform coincident with the latter stage of fold-thrust belt development.

We conducted new geologic mapping and structural analysis in the Last Chance Range of Death Valley National Park to elucidate the relative timing, magnitude and kinematics of thrust belt development. Two periods of contraction are preserved in the Last Chance Range: early eastward emplacement of the Last Chance allochthon and overprinting by top-to-ENE faulting and folding. Whereas the overall structure of the Last Chance-Death Valley thrust system differs from a typical retroarc fold-thrust belt, each stage of fold-thrust belt development can be explained in the context of maintaining critical wedge taper. Emplacement of the Last Chance allochthon onto the Mississippian Rest Spring Shale allowed large-magnitude transport on a low-friction basal décollement. Out-of-sequence thrusting, observed in the northwest Last Chance Range, initiated to increase wedge taper and accommodate continued contraction. By this stage, transpression was fully partitioned, and the trend of contractional structures were subparallel to the California-Coahuila transform. Deformation then propagated forward with development of the Marble Canyon and Lemoigne thrusts. Contraction in the Last Chance-Death Valley belt ceased prior to the initiation of subduction-related magmatism at ca. 260-250 Ma. Understanding the Last Chance-Death Valley thrust system in this context benefits our knowledge of upper-plate strain during induced subduction initiation, and the tectonic evolution of the Mesozoic North American subduction system.