GEOCHEMISTRY AND GEOCHRONOLOGY OF NEOGENE BASALTS ALONG THE COALDALE FAULT, CENTRAL WALKER LANE BELT, NEVADA

Neogene to Quaternary basaltic volcanism dominates a large portion of the geology in the central Walker Lane belt of western Nevada. Structurally the area is characterized by a regional-scale releasing bend termed the Mina Deflection that links the northern and southern transcurrent segments of the Walker Lane Belt. Along the southern edge of the Mina deflection in the vicinity of the Coaldale fault, these basalt flows unconformably overlie Oligocene to Miocene rhyolites and clastic synextensional sediments and tend to form the stratigraphic capstones. Several of these Neogene basalt flows exhibit left-laterally offsets of up to 2 km across the Coaldale fault. Most basalt exposures consist of multiple cooling units and reach a thickness of >100 m in the Volcanic Hills area to the southeast of the Coaldale fault. In this study, we investigated the geochemistry and the structural setting of Neogene basalts along the east-west trending left-lateral Coaldale fault in order to evaluate spatial and temporal variations of Neogene basaltic volcanism. Neogene to Quaternary basalt samples were collected from both sides of the Coaldale fault east of Montgomery Pass over an east-west distance of ~10 km for geochronology and geochemistry. In addition, we also conducted detailed mapping and sampling of a series of new road cuts along U.S. Highway 6 east of Montgomery Pass that expose a tilted sequence of multiple basalt flows with massive columnar jointing and well-preserved flow tops. ICP-MS analysis of major and trace element compositions and TIMS isotope geochemical data from seventeen different basalt samples were used to constrain the mantle source geochemistry and to estimate the depth range and percentage of decompression melting that characterize the generation of these basalts (e.g., Wang et al. 2001). ⁴⁰Ar/³⁹Ar geochronology of selected basalts samples was used to establish a stratigraphic framework for the different eruptive units in order to constrain the temporal variations in basalt geochemistry and to evaluate the implications for the tectonic evolution of the area. The temporal variations in depth and percentage of melting are of particular interest since basaltic volcanism temporally spans the onset of right-lateral shearing and the formation of localized pull-apart basins within the Mina Deflection.