JURASSIC MAGMATISM IN THE CENTRAL MOJAVE DESERT - IMPLICATIONS FOR ARC PALEOGEOGRAPHY AND PRESERVATION OF CONTINENTAL VOLCANIC SEQUENCES

Exposures of Jurassic magmatic rocks in the west-central Mojave Desert provide insight into the changing character of volcanism throughout J urassic time and the paleogeography of the continental arc. Middle Jur assic explosive volcanism (Lower Sidewinder volcanic series) resulted in collapse of multiple calderas. This was followed by north-south ext ension broadly coeval with batholith emplacement. Late Jurassic effusi ve volcanism (Upper Sidewinder volcanic series) appears to reflect tra nstension over a larger region during intrusion of the Independence di ke swarm. The Lower Sidewinder volcanic series consists of a subaerial , nested caldera complex with an aggregate thickness of >4 km. The fir st caldera formed during the eruption of crystal-poor rhyolite ignimbr ite. Outflow and intracaldera facies are intercalated with quartzose s andstone of probable cratonal provenance. The second caldera formed du ring the eruption of crystal-rich rhyolite to dacite ignimbrite. This ignimbrite exhibits complex mineralogical zoning and contains two unit s of probable collapse-related mesobreccia within the 1,400-m-thick in tracaldera sequence. A third caldera, nested within the second, is fil led with crystal-rich biotite-dacite ignimbrite and tuff breccia. A fo urth caldera, largely coincident with the second, formed during erupti on of lithic- and pumice-lapilli dacite ignimbrite. Eruption and colla pse appear to have been multistage, as several units of reworked tuff and fallout tuff occur within the 1,750-m-thick intracaldera sequence, and caldera collapse breccias occur at the base and near the middle o f the sequence. The top of this sequence contains reworked tuffs and e piclastic rocks, suggesting that the fourth caldera provided a posteru ptive depocenter for accumulation of sediment. Normal faulting, tiltin g, and erosion followed explosive volcanism and was broadly contempora neous with intrusion of Middle Jurassic porphyritic quartz monzonite p lutons. Plutons and tilted Lower Sidewinder volcanic series are intrud ed and unconformably overlain by latest Jurassic volcanic rocks (Upper Sidewinder volcanic series). The Upper Sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lava s, hypabyssal intrusions, and dikes, including the 148 Ma Independence dike swarm. Regional northeast-southwest extension is suggested by th e presence of the northwest-striking dike swarm and by bimodal volcani sm. The absence of debris-flow deposits and epiclastic rocks suggests an intra-arc region of low relief. Although local caldera-related subs idence was the principal control on accumulation of Lower Sidewinder v olcanic rocks, preservation was likely enhanced by Middle Jurassic ext ension. Late Jurassic extension was of broader regional extent but les ser magnitude because it did not create basins or cause significant ti lting of strata. The geometry, timing, and regional setting of Middle and Late Jurassic extension and magmatism suggest a sinistral oblique subduction regime for the Jurassic arc of the southern U.S. Cordillera .