IDENTIFICATION OF A DISTINCT TERRANE IN THE CORDILLERA THAT IS OFFSET ALONG THE EARLY CRETACEOUS DEXTRAL MSNI FAULT SYSTEM

Mapping along the Nevada-southeast Oregon portion of the MSNI fault (this portion previously called the western Nevada shear zone; Wyld and Wright, 2001) has identified several lithotectonic assemblages that are clearly out of place with respect to geologic provinces to the east. These include (in Oregon) the Middle Jurassic continental arc assemblage of the Pueblo Mountains, and (in Nevada) Permian volcanogenic rocks at Black Rock Point, Permian and early Mesozoic carbonate, clastics and bimodal volcanogenic rocks in the Granite Range, and an assemblage of argillite, quartzite and carbonate in the Fox Range that is at least in part Late Triassic or younger. Quartzites from the Fox and Granite Ranges have a detrital zircon age signature identical to that of Jurassic erg sandstones of the Colorado plateau. These disparate assemblages were exposed by Cenozoic Basin and Range extension as isolated fragments flanked to the east by the completely unrelated Black Rock and basinal terranes, and to the west by extensive Cenozoic cover. In previous studies, we have attempted to identify reasonable matches elsewhere in the Cordillera for the individual lithotectonic assemblages west of the MSNI. Here, we attempt to identify a single source region for the entire package. This source must contain: Middle Jurassic, subaerial, rhyolitic to andesitic volcanogenic strata; early Mesozoic shallow(?) marine argillite, limestone, erg-derived quartz sandstone, and local volcanogenic rocks; and upper Paleozoic shallow marine carbonate and volcanogenic strata. Results are still preliminary, but the best option for a source area appears to be at about the latitude of Mono Lake, CA where Middle Jurassic continental arc volcanics are found to the west (Inyo Mountains, eastern Sierra), and Triassic to Early Jurassic sedimentary successions, including limestone, argillite, and quartz sandstone, are found to the east (Luning, Sunrise, Dunlap Formations of western Nevada). Paleozoic basement is generally not exposed in the inferred source area, so rocks of that age are harder to match, but the nearby Permian Black Dyke Formation (Nevada) bears similarity to the Permian rocks seen at Black Rock Point. Based on these correlations, total offset along this portion of the Early Cretaceous MSNI fault is ~350-400 km.