Stratigraphic and sedimentologic evidence of a submarine island-arc volcano in the lower Mesozoic Peñon Blanco and Jasper Point Formations, Mariposa County, California

Mesozoic transpression, transtension, subduction and metallogenesis in northern and central California

“…Minor turbiditic, lithic metasandstone occurs in the basal section; graded beds are typically >1.5 m thick. Rare paleocurrent indicators suggest consistent SE transport directions, subparallel to the modern strike (Bogen, 1985). Penetratively deformed metaandesite and metabasaltic andesite flows are interlayered with the Mariposa metaclastic rocks.…”

“…The mafic metavolcanic rocks consist of lower actinolite-pumpellyite-facies mineral assemblages . Bogen (1985) reported that the cherts contain latest Triassic to earliest Jurassic microfossils. New mapping demonstrates that the cherts are only preserved locally overlying the mafic lavas.…”

“…3. Previous workers interpreted this package of rocks, informally termed the Pen˜on Blanco arc, as either a polygenetic ophiolite complex built in a transtensional setting along the continental margin (Saleeby, 1981), or an oceanic island arc (Bogen, 1985).…”

“…The Pen˜on Blanco Formation consists of 700 m of basaltic metatuff, up to 3.5 km of massive to bedded augite-and plagioclase-porphyritic metavolcanic rocks, 1-3.5 km of tuff breccia containing xenocrysts of augite + plagioclase and clasts of hornblende-bearing metavolcanic and metavolcaniclastic rocks (Saleeby, 1982;Bogen, 1985;Sharp, 1988). Metamorphic grade is generally lower than in the Jasper Point rocks.…”

“…3 Geological map of the Pen˜on Blanco area and the Mariposa Formation in the southern part of the Jura-Triassic arc belt and adjacent units. Field relationships were compiled from Morgan (1976) and Bogen (1985) and new geological mapping by . The Jura-Triassic arc belt is bounded on the SW by the Bear Mountain fault zone and on the NE by the Melones fault system.…”