The early Mesozoic continental arc of Arizona, California, and western Nevada is here interpreted to have occupied a continuous graben depression, more than 1000 km long, similar to the modern extensional are of Central America. High calculated rates of subsidence, the presence of syndepositional normal faults and fault-talus breccias, the abundance of calderas and high-level intrusions, and interstratification of cale-alkalic and alkalic volcanic rocks provide support for this model. The arc graben depression acted as a long-lived (more than 40 m.y.) trap for craton-derived quartz sands, which were funneled along the length and across the width of the depression.

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