EARLY CRETACEOUS, MARGIN-PARALLEL, DEXTRAL FAULTING AND TERRANE TRANSLATION IN THE U. S. CORDILLERA

Wyld and Wright (2001) presented evidence that a major dextral strike-slip fault of Early Cretaceous age cuts through NW Nevada and SE Oregon, and likely connects south to the Mojave-Snow Lake fault (Sierra Nevada) and north to the western Idaho shear zone. Here, we summarize new data on the Nevada-Oregon segment of this boundary, which we now call the MSNI (Mojave-Snow, Nevada, Idaho) fault.

Although overprinted by younger plutons, tectonism, or cover strata, the MSNI fault can be recognized by juxtaposition of completely unrelated terranes. In the Nevada-Oregon sector, new mapping allows us to trace the fault from the Pueblo Mountains (Oregon) south to Black Rock Point (Nevada), where it jogs west toward Gerlach along a presumed Cenozoic tear fault, and then extends south between the Fox and Selenite Ranges towards Pyramid Lake. Analysis by Christe and Wyld (2004) suggests that the fault then continues south to the east side of Lake Tahoe, beyond which it projects directly into the Mojave-Snow Lake fault. In places, the boundary splays into multiple strands separating narrow fault slivers. Rocks west of the fault in Oregon-Nevada include Middle Jurassic continental arc assemblages, plus sedimentary successions containing quartzite with a detrital zircon signature identical to that of Colorado Plateau Jurassic erg deposits whose sands were dispersed in the Early-Middle Jurassic into the continental arc of the southwest Cordillera. The Nevada-Oregon units must have originated in this region, up to 400 km or more south of their present location. Timing of displacement is bracketed between late Middle Jurassic (youngest displaced rocks) and 112 Ma (oldest cross-cutting plutons).

Additional, probably related, faults appear to exist west of the MSNI. For example, the geology of NW Nevada-SE Oregon bears no relation to that of the Klamath Mountains, and a buried fault must separate these provinces. Along with evidence presented by Wright (2004) that an Early Cretaceous dextral fault extends along the west side of the Sierra-Klamath provinces, we argue that the MSNI fault is part of a larger dextral system that slivered and translated parts of the U.S. Cordillera, much like the Canadian Cordillera was dismembered in the Late Cretaceous-Tertiary, and the U.S. Cordillera is being dismembered today.