Spatial variability in the structure of the Roberts Mountains allochthon, western Nevada

Abstract

Structural variability in rocks of the upper plate of the Roberts Mountains thrust indicates a complex history of thrust emplacement during the mid-Paleozoic Antler orogeny. The Roberts Mountains allochthon consists predominantly of a highly deformed assemblage of structurally imbricated lower continental-slope and continental-rise sedimentary and volcanic rocks of early Paleozoic age. It has been interpreted variously as a back-arc thrust and as a grounded accretionary prism that overrode the upper Precambrian and lower Paleozoic passive margin of western North America. Current understanding of structural and stratigraphic relations supports the second interpretation. The trace of the Roberts Mountains thrust trends south-southwesterly across Nevada from near the Idaho border to the vicinity of Tonopah (38°N lat.), where it is interpreted to swing westerly and extend to the Sierra Nevada. Subparallel to the thrust trace is the trend of the western boundary of the late Precambrian passive continental margin as deduced from lower Paleozoic facies patterns and the 87Sr/86Sr = 0.706 line. The westerly deflection of these and younger trends has been attributed by some workers to crustal deformation of late Mesozoic and/or Cenozoic age, and by others to an original bend in the continental margin. Results presented here support the latter interpretation. Structures in the northern and central segments of the Roberts Mountains thrust (characterized by south-southwest trace) consist of a single phase of folds and indicate easterly transport. The inferred southern segment of the thrust, with a westerly trend, exhibits two and locally three phases of folds interpreted as being genetically related to emplacement of the thrust. First-phase folds in the south are correlated with folds in the central and northern segments of the thrust and, after the effects of superimposed structures are geometrically removed, also yield an easterly transport direction. Cross folds in the southern part of the allochthon and their absence to the north indicate different structural histories for the regions. Structures in the allochthon are interpreted as having developed during the formation of an accretionary prism. Differences in structural, history are related to differential rates of slip on the allochthon's basal thrust during transport and emplacement. Development of cross folds in the south is thought to be due to a local reduction in the rate of displacement and corresponding internal shortening of the upper-plate rocks caused by resistance to subduction of a west-facing promontory of continental crust.