Abstract Rocks in the major roof pendants of the eastern Sierra Nevada have been mapped in various degrees of detail to better understand their stratigraphy, internal structure, and geologic history, and their relationships to other rock assemblages in the region. Ten formations ranging in age from Middle(?) Cambrian to Middle(?) Permian are recognized in these pendants, which along with other minor pendants, constitute a tectonostratigraphic unit called the Morrison block . Rocks of the Morrison block were first deformed by north-northwest-trending thrust faults and footwall syn-clines involving strata as young as Early or Middle Permian. We designate this event, which correlates with a similar pre-middle Early Triassic event recognized in rocks near Tinemaha Reservoir, the Morrison orogeny. Structures produced during this orogeny include a probable cryptic thrust fault separating rocks assigned to the Morrison block from those in the Big Pine Creek pendant, which may belong to the White-Inyo block, and the Nevahbe thrust, which separates lower from upper Paleozoic rocks in the eastern part of the Mount Morrison pendant and may separate the Pine Creek and Bishop Creek pendants. In the Mount Morrison pendant structures produced during the Morrison orogeny apparently were later refolded twice prior to sinistral displacement on the Laurel-Convict fault, which cross-cuts older structures and is intruded by a pre-latest Late Triassic dike . Other thrust faults in the eastern Sierra Nevada include the Golconda thrust of early Middle Triassic age and the Lundy Canyon thrust of Late Triassic age. The Golconda thrust system apparently overprints the Roberts Mountains thrust and separates rocks of the Morrison block from those of the Golconda and Roberts Mountains allochthons in the Saddlebag Lake pendant, and perhaps from those of the Roberts Mountains allochthon in the Northern Ritter Range and Log Cabin Mine pendants . After thrust-faulting, but prior to intrusion of the Late Triassic Wheeler Crest Granodiorite, dextral movement on the Tinemaha fault displaced Paleozoic facies and structural belts in the Sierra Nevada northward, producing most of the present complicated paleogeographic patterns apparent in the region. Other less important structures, such as the Laurel-Convict fault, have further complicated the geology of the Morrison block .

ABSTRACT Basal Triassic strata in the Saddlebag Lake pendant in the eastern Sierra Nevada, California, preserve a record of magmatism and sedimentation that marks the maturation of the early Mesozoic Cordilleran arc at this latitude. Facies analysis, geochemistry, and U-Pb geochronology provide the basis for a model that shows ways in which the types of volcanism and the geochemistry of magmatism reflect the presence and evolution of continental crust in the upper plate of a young subduction zone. Facies analysis of the breccia of Frog Lakes, an andesitic dome complex within the Saddlebag Lake pendant, demonstrates a subaqueous environment of deposition that is inferred to be characteristic of the interbedded tuff of Greenstone Lake and underlying rhyolitic tuff of Saddlebag Lake and conglomerate of Cooney Lake. This subaqueous volcanism and sedimentation suggest that subduction drag on the continental upper plate had greater effect than the buoyancy imparted by the felsic crust. Geochemistry of ignimbrites and dome deposits together with U-Pb isotopic and geochemical analyses of zircon from ignimbrites and the conglomerate of Cooney Lake and from similar-age strata from the Mount Morrison pendant to the south demonstrate the nature of upper-plate continental crust in the period between ca. 250 and ca. 220 Ma. Breccia clasts are medium-K tholeiitic arc basalt, basaltic andesite, and andesite, and they broadly overlap in bulk composition with underlying mafic hypabyssal intrusions. Minor- and trace-element geochemistry of tuff and detrital zircon grains indicates that Triassic silicic melts were relatively cool and fractionated, consistent with late zircon saturation in Zr-poor melts. We infer that the eruption and welding of thick, rhyolitic ignimbrites are, to first order, unique to magmas derived from subduction under continental crust.