Empire Mts, Pima Co., Arizona, USA

‡Ref.: Schrader, F.C. & J.M. Hill (1915), Mineral deposits of the Santa Rita and Patagonia Mountains, Arizona, USGS Bull. 582: 141.

Stone, R.W. (1920), Gypsum deposits of the United States, USGS Bull. 697: 52.

Wilson, E.D., et al (1951), Arizona zinc and lead deposits, part II, Arizona Bureau of Mines Bull. 158: 49-50.

Galbraith, F.W. & D.J. Brennan (1959), Minerals of Arizona: 60, 94.

Anthony, J.W., et al (1995), Mineralogy of Arizona, 3rd. ed.: 111, 242, 377.

The Empire Mountains extend between Davidson Canyon and Cienega Creek northeastward from the Santa Rita Mountains, of which they are an outlier, and attain a maximum altitude of about 5,360 feet, or about 500 feet above the surrounding surface. They are between 7 and 8 miles long and 4 miles wide. They are characterized by a bold western escarpment facing Davidson Canyon, and a broad eastern pediment.

Cretaceous and older rocks were intruded by quartz monzonite and granodiorite which, in the west-central portion of the range, crops out over an area some 3 miles long from north to south by 1 to 2 miles wide and is termed the Sycamore stock. Its age, tentatively, may be considered as Laramide (late Cretaceous-early Tertiary). Dikes of rhyolite porphyry, aplite, syenite, andesite, and basalt intrude various formations of the sedimentary series, and some of them cut the Sycamore stock.

After deposition of the Cretaceous beds, this region underwent intense deformation.

In general, the beds lie in a broad dome surrounding the Sycamore stock. Superimposed on the dome were numerous southeastward-pitching folds. Bedding-plane faults are common within them.

The dominant structural feature according to Galbraith is a low-angle fault which dips to the southeast. On this fault the entire Paleozoic mass has moved over the Upper Cretaceous sedimentaries.

Large tear faults accompanied, and smaller normal faults followed, the overthrusting. Intrusion was later than the overthrusting and possibly concurrent with the normal faulting.

Minor thrust faults are present in the main overthrust mass. The tear faults strike northwestward, dip steeply, and have effected important horizontal displacements. One of the largest, the Andrada fault, lies immediately north of the Total Wreck Mine. As shown by Alberding, the beds on the northeast side of this fault appear to have been moved 8,000 feet northwestward relative to those on the southwest side.

Steeply dipping normal faults strike northwest, north, and east-west. Low-angle faults of reverse character strike northeastward.