TIMING, KINEMATICS, AND BASIN EVOLUTION OF THE SYNCONTRACTIONAL BIRD SPRINGS CONGLOMERATE, SOUTHEREN SEVIER FOLD AND THRUST BELT, NEVADA: IMPLICATIONS FOR MID-CRETACEOUS PLATE DYNAMICS

Despite decades of focused research on the Sevier fold and thrust belt (SFTB), our understanding has not kept pace with advances in understanding the north-central SFTB of WY, UT, and ID. Through integrated structural and stratigraphic analyses, 1: 4,000 geologic mapping, detrital zircon geochronology (DZ), and aerial photogrammetry of the Bird Springs Conglomerate (BSC; named and described herein), we elucidate the timing, kinematics, and associated basin evolution of the Bird Springs thrust (BST) in southern Nevada, the leading edge of the SFTB at this latitude. Based on cross-cutting relationships, fanning of dips, and the internal stratigraphy, we interpret the BSC to represent a small (~2500 m²) sedimentary growth syncline deposited on the actively deforming footwall of the BST. The basin is dominated by braided channel deposits and massive, structure-less sandstones, and can be delineated into three discrete tectono-stratigraphic packages. DZ analysis of sandstones from each of these three successive packages yielded maximum depositional ages of 101.4±0.4 Ma, 101.3±0.4 Ma, and 100.9±0.3 Ma, respectively. The syncline is an open, asymmetric fold (interlimb angle ~90⁰) plunging ~07⁰ towards 228, with a vergence direction of ~138. An outcrop-scale thrust with opposite vergence cuts through the syncline core, repeating the NW limb. The orientation of this thrust is ~053, 56⁰ SE. These vergence directions are corroborated by mesoscopic structural analyses from the BST footwall and BSC. The BST increases stratigraphic separation to the south, suggesting a southward increasing displacement gradient. This study has major implications for plate margin processes operating on intraplate tectonics during the mid-Cretaceous. Our evolutionary model for the BSC as an incipient braid-channel dominated alluvial fan requires rapid thrusting because channel-filling and subsequent lateral migration (subsidence/deposition) is ultimately, and quickly, terminated by frontal thrust advance (burial). Given our interpretations and the regional context, we conclude that a major plate reorganization caused Cordilleran-scale synchronicity of rapid frontal Sevier shortening ca. ~100 Ma.