Abstract

Exposures of Jurassic magmatic rocks in the west-central Mojave Desert provide insight into the changing character of volcanism throughout Jurassic time and the paleogeography of the continental arc. Middle Jurassic explosive volcanism (Lower Sidewinder volcanic series) resulted in collapse of multiple calderas. This was followed by north-south extension broadly coeval with batholith emplacement. Late Jurassic effusive volcanism (Upper Sidewinder volcanic series) appears to reflect transtension over a larger region during intrusion of the Independence dike swarm.

The Lower Sidewinder volcanic series consists of a subaerial, nested caldera complex with an aggregate thickness of >4 km. The first caldera formed during the eruption of crystal-poor rhyolite ignimbrite. Outflow and intracaldera facies are intercalated with quartzose sandstone of probable cratonal provenance. The second caldera formed during the eruption of crystal- rich rhyolite to dacite ignimbrite. This ignimbrite exhibits complex mineralogical zoning and contains two units of probable collapse-related mesobreccia within the 1,400-m-thick intracaldera sequence. A third caldera, nested within the second, is filled with crystal-rich biotite-dacite ignimbrite and tuff breccia. A fourth caldera, largely coincident with the second, formed during eruption of lithic- and pumice-lapilli dacite ignimbrite. Eruption and collapse 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 of the sequence. The top of this sequence contains reworked tuffs and epiclastic rocks, suggesting that the fourth caldera provided a posteruptive depocenter for accumulation of sediment.

Normal faulting, tilting, and erosion followed explosive volcanism and was broadly contemporaneous with intrusion of Middle Jurassic porphyritic quartz monzonite plutons. Plutons and tilted Lower Sidewinder volcanic series are intruded and unconformably over-lain by latest Jurassic volcanic rocks (Upper Sidewinder volcanic series). The Upper Sidewinder volcanic series is characterized by alkalic basalt to basaltic andesite and rhyolite lavas, hypabyssal intrusions, and dikes, including the 148 Ma Independence dike swarm. Regional northeast-southwest extension is suggested by the presence of the northwest-striking dike swarm and by bimodal volcanism. The absence of debris-flow deposits and epiclastic rocks suggests an intra-arc region of low relief.

Although local caldera-related subsidence was the principal control on accumulation of Lower Sidewinder volcanic rocks, preservation was likely enhanced by Middle Jurassic extension. Late Jurassic extension was of broader regional extent but lesser magnitude because it did not create basins or cause significant tilting 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.

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