A TRANSECT THROUGH A TILTED MAGMATIC ARC: BEDROCK GEOLOGY OF THE INDIAN COVE QUADRANGLE, JOSHUA TREE NATIONAL PARK, CALIFORNIA

Joshua Tree National Park, in southern California 200 km east of Los Angeles, preserves a cross-section from 0 to ~30 km depth through a tilted crustal block of a Mesozoic arc and its Proterozoic host rocks. We observe a transition from large granite plutons that are roughly equidimensional in map view and small (<1 km²) hornblende gabbro plutons in the upper crust, to thin, tabular, compositionally heterogeneous mid-crustal (10-20 km depth) intrusive bodies of a wide range of compositions, to intermediate-composition plutons complexly intruding migmatitic host gneisses at the lowest exposed section. The existence and exposure of such a mid-crustal, magmatic "sheeted complex" raises many questions regarding its role in magma transport and differentiation, crustal rheology, heat flow, and many other issues.

We are actively mapping the bedrock geology of western Joshua Tree National Park at a minimum scale of 1:24,000 in a comprehensive effort to understand the sheeted complex, its magmatic architecture, and the processes of creating coeval granitoid and gabbroid plutons in the upper crust through such a chemically diverse horizon.

The second quadrangle of our mapping program, the Indian Cove 7.5-minute quadrangle, is uniquely located in the transition zone from sheeted complex to upper crustal plutons. Two years of USGS-supported (EDMAP) mapping has revealed, among other discoveries, additional structural complexity within the sheeted complex. We now divide this region into three zones: thick (>10 m) granite sheets, thin (<5 m) heterogeneous sheets, and thin (<5 m) homogeneous granite sheets. In the northwest corner of the quadrangle, we find a possible detachment fault in the upper sheeted complex that may or may not be related to sheeted complex development.

Additionally, host gneisses found in the southern Indian Cove quadrangle may be an uplifted block of relict kyanite-bearing, migmatitic gneisses that preserves the conditions of the lower crust not exposed elsewhere in Joshua Tree National Park. The small gabbroic plutons, previously assumed to be a suite of Jurassic intrusions, are found to also be Proterozoic amphibolites and Late Cretaceous intrusions with complex intrusion histories. The upper crustal granites may represent large reservoirs of magma during periods of voluminous magma production.