STRUCTURAL RECONSTRUCTION OF MID-TERTIARY NORMAL FAULTS AND MYLONITIC SHEAR ZONES, TORTOLITA MOUNTAINS AND VICINITY, NORTHWESTERN END OF THE CATALINA METAMORPHIC CORE COMPLEX, ARIZONA: TESTING MODELS OF CORDILLERAN METAMORPHIC CORE COMPLEXES

A highly extended domain that covers more than one dozen 7.5’-quadrangles around the NW end of the Catalina core complex includes the Tortolita and Suizo Mtns, Durham Hills, Black Mtn, and Black Hills. The area is bound on either side by relatively unextended rocks of the Tucson Mtns and Galiuro Mtns and is suited to testing models of core complexes because it contains numerous geologic markers needed for robust reconstructions.

Normal faults cut across Laramide reverse faults, bound half grabens, and had steep (~65°) initial dips. Faults and associated shear zones can be assigned to sets according to current orientation. If smaller faults that dip northerly or southerly are excluded, then there are four major sets, at least three of which initially dipped westward and one eastward. Crosscutting relationships indicate that these sets constitute at least four distinct temporal generations. Faults of the earliest generation (1) originally dipped steeply westward but now dip gently eastward; these include Cloudburst and Star Flat faults and Carpas Wash shear zone. Faults of generation 2 originally dipped steeply eastward but now dip eastward at moderate angles and include Durham Hills and Suizo Mtns shear zones and Owl Head Buttes and Indian Springs faults. Faults of generation 3 initially dipped steeply westward and now dip moderately westward; they include Guild Wash shear zone and San Manuel and Willow Springs faults. Faults of generation 4 initially dipped steeply westward and still do; these include Pirate, Black Mtn, and Cowhead Tank faults.

Distributions of faults of different sets variably overlap in map view; the net result is that rocks dip eastward in the eastern and central portions of the area and westward in the western portion. Certain older faults vary in depth of exposure from brittle faults to mylonitic shear zones over strike distances of ~10 km. The reconstruction indicates that even the biggest faults and shear zones have displacements of only 2 to 5 km. At least four of the faults in the region previously have been widely termed “detachment” faults, and these belong to generations 1, 2 and 3. There is no evidence there were ever any master faults at depth, be they low-angle normal faults or flat décollements, when any of these four sets of faults formed. A new model for the origin of core complexes is required.