MIOCENE STRUCTURAL EVOLUTION OF THE BUCKSKIN DETACHMENT FAULT ZONE, WEST-CENTRAL ARIZONA: INSIGHTS FROM GEOLOGIC MAPPING IN THE SOUTHERN LINCOLN RANCH BASIN

The Buckskin Mountains, in west-central Arizona underwent large-magnitude crustal extension during the early to middle Miocene. To understand the structural and stratigraphic development of this highly deformed region, we mapped a 13 km² area in the eastern Buckskin Mountains at 1:10,000-scale as part of a USGS EdMap project. This area exposes the Buckskin detachment, a major low-angle normal fault which juxtaposes brittley-extended rocks in the hanging wall against mylonitic rocks in the footwall. The hanging wall is primarily composed of intensely-fractured Proterozioic granitoid overlain by synextensional sandstone, conglomerate, and limestone. The mylonitic footwall consists of crystalline gneisses and overlying carbonate-rich metasedimentary rocks that were exhumed by tens of kilometers of top-to-the-NE directed slip along the Buckskin detachment fault. Our detailed mapping and structural analysis show that the footwall mylonites record top-to-the-NE-directed extension consistent with the slip direction on the brittle Buckskin detachment fault. The mylonites also document shortening with foliations locally folded into upright, NE-SW trending anticlines and synclines paralleling the extension direction. The carbonate-rich mylonites are present along most of the detachment, where they absorbed strain at temperatures below quartz plasticity, as they have undergone little cataclasis compared to quartz-rich rocks along the detachment. The upper plate is highly extended by moderately- to gently-dipping normal faults, and the youngest synextensional strata have locally been tilted ~40-70° SW by listric normal faults. Some kilometer-scale NE-dipping faults with an apparent reverse sense of slip cut the detachment fault, indicating a change in deformation style after extension ceased. Our 1:10,000-scale geologic map and cross sections document the stratigraphy and structure of this highly deformed area that is actively being explored for rare earth element deposits.