EDMAP-SUPPORTED MAPPING OF A TILTED ARC CRUSTAL SECTION IN JOSHUA TREE NATIONAL PARK, CALIFORNIA

Mapping of arc crustal sections (i.e. Kohistan or Sierra Nevada) has revealed a complex pattern of magmatic and metamorphic host rock beneath volcanic arcs. The western edge of Joshua Tree National Park (200 km east of Los Angeles) exposes a cross-section from 0 to ~30 km depth through a tilted crustal block of a Mesozoic continental arc and its Proterozoic host rocks. We observe a transition from granitic upper crustal plutons that are roughly equidimensional in map view, to thin, tabular, compositionally heterogeneous mid-crustal (10-20 km depth) intrusive bodies, and to intermediate-composition plutons in migmatitic host gneisses beneath. 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 and its magmatic architecture. The Keys View 7.5-minute quadrangle was the first preliminary map completed, and work has continued to the adjacent quadrangle to the north, the Indian Cove quadrangle. After two years of USGS-supported (EDMAP) mapping in the three-year project, a preliminary map of this quadrangle is nearly complete.

So far mapping efforts have revealed additional complexity in 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 Indian Cove quadrangle, we find a possible detachment fault in the upper sheeted complex that may or may not be related to sheeted complex development. Beneath the sheeted complex lies a migmatized zone, which could represent a melt source or melt transport zone for the sheeted complex. These observations set the stage for detailed study of the formation and significance of sheeted complexes in arcs.