Geology of the Mount Pinchot quadrangle, southern Sierra Nevada, California

The Mount Pinchot quadrangle, California, includes the main crest and eastern scarp of a part of the southern Sierra Nevada and a segment of the west side of Owens Valley. The maximum relief is nearly 10,000 feet, and more than one-half of the quadrangle lies higher than 10,000 feet above sea level. The Sierra Nevada here is underlain chiefly by a mosaic of grantic intrusives of Cretaceous age which encloses small masses of pre-Cretaceous metamorphic rocks and hybrid mafic rocks. The pre-Cretaceous rocks include two series; an older series of metasedimentary rocks of Paleozoic ( ? ) age, and a younger series of predominantly metavolcanic rocks of Mesozoic age. The Paleozoic(?) metasedimentary rocks are present as nearly vertical tabular septa of schist and hornfels derived from shale, limestone, and sandstone. Jurassic and Triassic metavolcanic rocks are predominantly from rhyolite tuff, but metaandesite, metadacite, and metabasalt are also present. The largest roof pendant of metavolcanic rocks represents a vertical homoclinal section more than 7,00!' feet thick with the tops of the beds to the southwest. Both metasedimentary and meta volcanic rocks ·are of the hornblende hornfels grade of metamorphism. Twenty-seven separate granitic intrusive masses were mapped. Of these, 9 are older than a large swarm of northwest-trending mafic dikes which completely crosses the quadrangle, and 18 are younger. In composition the plutons range from alaskitic quartz monzonite to dark-colored granodiorite; the average composition is close to the boundary between quartz monzonite and granodiorite. The plutons are believed to have made room for themselves chiefly by forcible intrusion, but stoping and assimilation of wallrock were locally operative. Six of the intrusive masses contain porphyritic potassium feldspar. The composition of the porphyritic plutons is restricted to an intermediate composition, close to the average composition of a:ll the granitic rocks. Phenocrysts apparently grew after initial emplacement of the mass, but before final intrusion of the remobilized core of zoned plutons. Mafic inclusions are present in most of the granitic rocks except the alaskites. Mafic inclusions ·are more abundant and larger in the more mafic plutons and in the thinner plutons. At least five of the plutons are zoned from a silicic core to a calcic margin. In some plutons the silicic core has remobilized and intruded its margins. The main uplift of the range occurred in late Tertiary ( ?) and Pleistocene time along a zigzag fault zone that follows closely the base of the range. Normal faults downdropped on the west are common in benches along the range front we8t of the principal fault zone and are related to warping. Late Cenozoic basalt cinder cones •and lava flows occur in the eastern part of the quadrangle at the foot of the Sierra Nevada. Olivine basalts were extruded