Tracking the Late Mesozoic Strike-Slip System of the Western U.S. Cordillera

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

Large-scale strike-slip faulting is increasingly recognized as an important process in the development of orogenic belts, including the North American Cordillera. These faults can accommodate substantial displacement of terranes and shuffle their relative positions in complex ways, resulting in a collage of terranes whose relationship is difficult to decipher. Regional strike-slip faulting in orogenic belts can also yield crucial insight into ancient plate interactions. In the North American Cordillera, pre-Cenozoic strike-slip faulting is believed to have played an important structural role in terrane transport and assembly, but the specifics of proposed strike-slip episodes are mired in uncertainty and controversy. Key questions involve the location of strike-slip boundaries, and the amount, sense, and timing of displacement along them. These uncertainties must be resolved before any reliable paleogeographic and paleotectonic reconstructions of the pre-Cenozoic Cordillera can be generated. This project is focusing on the western Nevada shear zone system (WNS), a recently identified major tectonic boundary in southeast Oregon and northwest Nevada. Data obtained so far along the WNS suggest that it accommodated several 100 km dextral displacement of outboard terranes in the late Mesozoic, and that it likely connects up with two other important late Mesozoic tectonic boundaries, the Salmon River suture of western Idaho and the Mojave-Snow Lake fault of the central and southern Sierra Nevada in California. This project is testing these conclusions more rigorously and extensively by a variety of new studies. If initial hypotheses concerning the boundary are correct, it will have far-reaching implications for paleogeographic and paleotectonic reconstructions and interpretations of the North American Cordillera. Specific research being done in this project is as follows. (1) Detailed mapping, structural, and geochronologic studies of the WNS are being extended south into the Gerlach area of western Nevada, which will expand knowledge of the boundary to a strike distance of over 200 km. (2) Distribution, origin, and significance of microterranes along the WNS are being determined from the Gerlach area to southeast Oregon. These stratigraphically and structurally distinct lithotectonic assemblages may be fault slices dragged northward along anastomosing strands of the WNS strike-slip system. (3) Proposed terranes linkages across the strike-slip system are being tested to determine whether the inferred sense and amount of displacement is valid, and whether the inferred continuity between the WNS, Mojave-Snow Lake fault, and Salmon River suture can be more rigorously supported. (4) Timing of strike-slip displacement and deformation along the WNS are being determined by a variety of geochronologic studies.