101 MA SHORTENING ALONG FRONTAL THRUSTS WITHIN THE SOUTHERN SEVIER FOLD-THRUST BELT: BIRD SPRING RANGE, SOUTHERN NEVADA

In southern Nevada, the Bird Spring thrust (BST) carried Cambrian-Triassic passive margin and continental strata eastward and onto earlier layer-parallel thrust faults and folds rooted within Permian-Triassic red-beds during retroarc shortening in the mid-Cretaceous. We document the sedimentary and structural evolution of early motion along this thrust system via mapping, stratigraphy and sedimentology, and detrital zircon geochronology of a thin (2-25 meters), intermittently exposed 101 Ma synorogenic conglomerate and sandstone sequence (Kbs) preserved below emergent portions of the BST. Unconformities within the Kbs sequence, which we highlight via the internal structural relations of a mesoscale growth-syncline and the spatial and stratigraphic distribution of three unconformity-bound petrofacies, developed during fold-growth and early layer parallel shortening (ELPS) of the BST footwall, particularly adjacent to, and in front of, frontal and/or oblique ramps. The lowest petrofacies, Kbsa, which we interpret to be pre-folding, has a maximum depositional age (MDA) of 101.4 Ma; Kbsb, the middle petrofacies, is bracketed by an overlying MDA of 101.3 Ma in the lower portions Kbsc, the uppermost petrofacies. Strata within the upper portions of Kbsc have an MDA of 100.9 Ma. In general, the growth-syncline is a SE-verging, open, asymmetric fold and is capped unconformably (>50 degrees angularity) by a sub-horizontal succession of the youngest petrofacies, Kbsc. In summary, ELPS along the BST, or the leading edge of the southern Sevier fold-thrust belt, commenced ca. 101 Ma, adding to a robust signature of increasing rates of retroarc shortening throughout the Cordillera at that time.