The geology and geochemistry of volcanic rocks in the Lava Mountains, California: Implications for Miocene development of the Garlock Fault.

摘要

The Lava Mountains are located in the Mojave Desert, southern California along the Garlock Fault, a major continental strike-slip fault that separates the southwestern Basin and Range to the north from the Mojave block to the south. Chemical similar dacitic lava and pyroclastic flows erupted from the 9-km diameter Western Lava Mountains volcano. Dacites with limited chemical variability reflect interaction of mantle derived basalt with both metabasalt and graywacke of the Mesozoic Rand Schist and Cretaceous “Atolia-type” granitoids. Geophysical data suggests a low angle contact between these lithologies at a depth of about 20 km. This boundary may have served as a density barrier to rising magma. Magma trapped at this barrier could not ascend until it evolved by fractional crystallization and assimilation to a density less than that of the host rock. This density was achieved as soon as SiO₂ evolved to values greater than about 63 wt.%; the resulting magma corresponds to the composition of Lava Mountains dacite. Both geochemistry and stratigraphy were used to develop correlations of volcanic units across the Garlock Fault between the Lava Mountains to the south and the El Paso Mountains to the north of the fault. This work demonstrated a total of 32–40 km movement of the fault since 10.3 Ma, which indicates a displacement rate of 3.1–3.8 mm/yr.