Arizona Geological Society

Jon Spencer, Arizona Geological Survey

Restoration of tectonic extension in the greater Tucson area and implications for original distribution of porphyry copper deposits

Abstract

Research to improve understanding of the structural geology and evolution of southeastern Arizona is partly motivated by a desire to better understand the original distribution of porphyry copper deposits.  This in turn could assist exploration efforts.

Late Cenozoic normal faulting and extension in an approximately east-west direction produced long, sinuous north-south trending basins, including Oro Valley – Tucson – Green Valley and McClellan Valley – Avra Valley – Altar Valley.  Most bounding normal faults are inactive and buried, and geophysical methods are typically necessary to determine approximate basin structure.  Total extension is difficult to estimate because of cover, but is probably on the order of a few kilometers in each basin.

Five areas of extension provide a basis for conceptual reconstruction of Oligo-Miocene extension in the greater Tucson area, as follows: (1) In the Sierrita Mountains, restoration of 11 km of north-directed displacement above the San Xavier fault is required to restore the Pima-Mission ore body to a position near the Twin Buttes ore body, as proposed many years ago (Cooper, 1960, USGS Bulletin 1112), and an additional 5 km of extension is necessary to restore the San Xavier North deposit to near Twin Buttes.  Although details of matching deformed and altered Paleozoic and Mesozoic strata associated with the ore bodies are insufficiently understood to specify an exact restoration, the structural geology of the San Xavier fault and the stratigraphy of its hanging wall clearly indicate that it is a north-directed low-angle normal fault with many kilometers of displacement. (2) A Quaternary normal fault at the foot of the northern Santa Rita Mountains appears to be the modern surface expression of the pre-Quaternary low-angle normal fault that displaced mineralized rocks in the Helvetia area from a position 3 km to the east near the range crest and north of the Rosemont ore body. (3) Total displacement on the Catalina detachment fault necessary to have uncovered the Rincon Mountains footwall is at least 40 km, which is the extension-parallel extent of exhumed footwall crystalline rocks and metasedimentary tectonites.  (4) Restoration of 30 to 40 km displacement on the Catalina detachment fault places the western Santa Catalina Mountains below the 72 Ma Amole granite in the northern Tucson Mountains.  However, there are no intrusive equivalents to the Amole granite in the Catalina Mountains, as the Leatherwood quartz diorite and granite of Alamo Canyon are both 68-69 Ma, indicating that extension between the Tucson and Santa Catalina Mountains is less than about 20 km for combined middle and late Tertiary faulting. (5) The Guild Wash – Suizo – Durham Hills – Star Flat – Cloudburst detachment system north of the Tortolita and Santa Catalina Mountains must have accommodated ~40 km of extension if it displaced the depositional base of tilted middle Tertiary strata north of the Tortolita Mountains from near the Galiuro Range front in the northern San Pedro Valley.  This large amount of offset must be represented farther south by extension between the Santa Catalina and Tucson Mountains, as there are simply no other good options, but that conflicts with maximum 20 km extension between the Tucson and Santa Catalina Mountains (#4 above).  Reduced offset on the Guild Wash – Suizo – Durham Hills – Star Flat system is possible if the Cloudburst detachment fault projects southward beneath rather than over the San Manuel – Kalamazoo ore body and the Santa Catalina Mountains, and so is a structurally lower, imbricate fault.  Restoration of 8 to 15 km of extension on the Cloudburst detachment fault with this geometry places the San Manuel ore body within a few kilometers of the Bunker Hill porphyry copper mineral district in the Galiuro Mountains.