2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 208-41
Presentation Time: 7:00 PM

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


BIRD, Emily, Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Drive, Mail Stop 5F2, Fairfax, VA 22030, SINGLETON, John, Department of Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Drive, Fairfax, VA 22030 and HATFIELD, Marques E., Atmospheric, Oceanic, and Earth Sciences, George Mason University, 4400 University Dr, Fairfax, VA 22030, ejenkin4@gmu.edu

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.