Miocene strata in the northwestern Buckskin Mountains of western Arizona are exposed in the upper plate of the Buckskin detachment fault, which formed as a result of regional extension. Miocene deposits in the upper plate include, in ascending order: basal megabreccia, lower and upper Tunnel Peak sedimentary units with an intervening mafic volcanic flow, a megabreccia mosaic unit, and lower and upper Prospect sedimentary units, also with an intervening mafic volcanic flow unit. The Miocene sedimentary rocks reveal an up-section decrease in dip below the megabreccia mosaic deposit. These deposits accumulated during half-graben formation produced by synthetic, down-on-the-northeast normal faulting, and have been tilted to the southwest. A depositional model for early sedimentation is that of alluvial fans deposited from side canyons into a northwest-trending basin, and deposition of sand and gravel in a braided stream that flowed to the northwest along the axis of the basin. Later sedimentation took place in alluvial fans that were derived from a domal uplift to the southeast. These deposits are tilted to the southwest but do not preserve fanning dips.
Cross-section reconstructions support a model which involves tilting of early northeast-dipping normal faults in the hanging wall of younger northeast-dipping faults. This tilting produced southwest-dipping faults that were originally northeast-dipping. Overturned bedding in the lowest sedimentary units can also be explained by tilting associated with two episodes of normal faulting.
A distinctive megabreccia mosaic unit occurs in the study area and in other basins in the region (e.g. Artillery Mountains). Emplacement of this megabreccia unit occurred at approximately 21 to 20 Ma and is believed to be related to uplift and denudation of upper-plate rocks produced by doming of the lower plate. Uplift of the lower plate also produced a secondary breakaway that resulted in a stranded, inactive area of the upper plate (Spencer and Reynolds, 1989a). The study area was located within this inactive region. New data shows that faults in the study area became inactive at the time of megabreccia emplacement, consistent with the above model.
Regional correlations indicate that the Buckskin Mountains and the Whipple Mountains experienced extension at approximately the same time. However, secondary breakaway in the Buckskin Mountains occurred approximately 6 million years before it was initiated in the Whipple Mountains.