TECTONIC EXTENSION IN THE NORTHERN PLOMOSA MOUNTAINS, WESTERN ARIZONA, AND RECONSTRUCTION OF A MESOZOIC THRUST ZONE

The northern Plomosa Mountains in the Colorado River extensional corridor are southwest of the large Harcuvar core complex. Structural geology in the range is dominated by the northeast-dipping Plomosa detachment fault that separates footwall crystalline rocks, including Orocopia schist, from a southwest-tilted fault-block array of granitic and metamorphic rocks overlain by Oligocene to Miocene sedimentary and volcanic rocks. The footwall is mylonitic, with lineations trending SW-NE and top-northeast shear indicators.

Hanging-wall strata consist of a three-part sequence: (1) Basal conglomerate, arkosic sandstone, and limestone, (2) mafic lava flows with interbedded rock-avalanche breccias derived from granitic and metamorphic rocks, and (3) dacitic lava in southern areas and conglomerate and sandstone in central areas. Sequences (1) and (2) are similar across all tilt blocks, but sequence (3) is not and is interpreted to reflect the activity of normal faults that broke the landscape into multiple basins after deposition of underlying strata. In two fault blocks, conglomerate at the top of tilted sections contains mylonitic clasts probably derived from the detachment-fault footwall.

The change from basin genesis, and the creation of relief sufficient to mobilize rock-avalanche breccias, to multiple independent basins, is interpreted to indicate a geodynamic transition that we explain with critical-taper theory. Our interpretation is that upper-plate breakup occurred when the surface slope of the upper plate was tilted sufficiently toward the breakaway, and the dip of two underlying normal faults decreased, to the point of wedge instability and internal extension.

A COCORP seismic-reflection profile across Cactus Plain northeast of the northern Plomosa Mountains revealed a northeast-dipping reflector interpreted here as the Plomosa detachment fault, and numerous southwest-dipping reflectors that project up dip into the layered gneisses of the southern Buckskin Mountains. Restoration of extension on the Plomosa and Buckskin detachment faults places carbonate and quartzite tectonites in the southern Buckskin Mountains at Battleship Peak downdip from thrust faults and tectonites in northern Plomosa Mountains. Tectonites in the two areas are plausibly part of the same thrust zone.