The structural geology of the Foy Ridge area, Twin Buttes, Arizona

Abstract

Foy Ridge is located about twenty-six miles south-southwest of Tucson, Arizona, and about a mile north of the Twin Buttes mining district. The sediments of the Roy Ridge area range in age from Cambrian to Recent. Separation of the Paleozoic section into mappable units has been complicated by post-Permian metamorphism. This metamorphism has destroyed any fossils that were probably once present, although a few remains of a Devonian Cladopora reef have been recognized. The metamorphism has caused a recrystallization of the limestones and an alteration of the shales to hornfels with stringers of epidote. The result is a sequence of altered Paleozoic rocks closely resembling one another. Lateral compression acting in a northeast-southwest direction played the major role in the tectonic history of the Foy Ridge area. Foy Ridge is the inverted limb of a large fold overturned to the northeast. As the fold was being produced thrust faults and wrench faults formed in the limbs. These first faults were cut off by the Foy Ridge fault that formed in the southwest limb of a northwest plunging anticline. This anticline was thrust forward (southwest) and upward until it was pressed against the inverted limb (Foy Ridge) and the adjacent fold. A disharmonic fold probably formed in the normal limb of the major fold of which Foy Ridge is a part. Thrust faults in the disharmonic fold repeated the lower Paleozoic section on the southwest side of Foy Ridge. As compression continued the last major thrust of the area was produced. This was the Bolsa Overthrust, which probably originated in the core of the disharmonic fold. It cut across the inverted limb of the major fold and may have also cut across the crest of the northwest plunging anticline northeast of the Foy Ridge fault. Intrusive activity followed or accompanied the overthrusting and folding of the area. This was closely followed by post-intrusion normal faulting. The Twin Buttes, two prominent hills immediately east of the thesis area, are a southeastern extension of the northeastern limb of the northwest plunging anticline that formed the hanging wall of the Foy Ridge fault. The "breached anticline" of the Twin Buttes mining district is probably related to the normal limb of the fold that formed Foy Ridge. The synclinal area between these two folds is covered by alluvium.