TERTIARY EXTENSION AND FAULT BLOCK ROTATION IN THE TRANSITION ZONE, CEDAR MOUNTAINS AREA, ARIZONA

New geologic mapping from the Cedar Mountains - Bloody Basin - Cooks Mesa area in the central Arizona Transition Zone presents an enigmatic geologic relationship that has previously gone unstudied. Here, fault blocks of Proterozoic basement overlain by Tertiary conglomerates and tuffaceous-lacustrine deposits and capped by a thick accumulation of basalt flows (previously dated in reconnaissance fashion at 14.3 Ma) are rotated ~20^o W/WSW along east-dipping, high-angle normal faults. The faults dominantly trend N and NW and likely become listric at depth. Half-graben type basins formed during rotation are filled with a syn- to post-extensional basin-fill conglomerate, and are locally overlain by undated, flat-lying, and post-extensional basalts. The extended area is essentially a corridor, bounded to the west by a master fault along the front of Cooks Mesa, a flat-lying expression of the same general stratigraphy, and to the east by the Verde River and beyond it, the Mazatzal Range. Southward, the extensional corridor continues parallel to the Verde River at least until Kentuck Mountain, where it encounters the Basin and Range boundary. It also continues from the mapping area northward across the Bloody Basin.

This setting defies both spatial and temporal boundaries with respect to the general understanding of the tectonic and physiographic development of Arizona. Fault block rotation is considered to be characteristic of the mid-Tertiary orogeny (~38 to 17 Ma), the effects of which are restricted to the modern Basin and Range province. The Basin and Range disturbance followed, beginning ~15 Ma in Arizona, with graben subsidence along high-angle normal faults without rotation. Extension in the Cedar Mountains area is typical of the mid-Tertiary orogeny; however, it’s location within the Transition Zone and 14.3 Ma tilted basalts violate the spatial and temporal boundaries as defined above. New geologic mapping at 1:24000 carefully documents this scenario, examines the geometries of these structures, and provides a better understanding of this anomalous area. In addition, new dates from this research will bracket the timing of the tilting through sampling of the highest tilted flow and lowest non-tilted flow.