MULTIPLE GENERATIONS OF TERTIARY NORMAL FAULTS AND RELATED TILTING NEAR HACKBERRY WASH, TORTILLA MOUNTAINS, PINAL COUNTY, ARIZONA

The Tortilla Mountains are located within the Laramide arc of southwestern North America, which was built on Paleozoic and Proterozoic carbonate-clastic strata and underlying Proterozoic crystalline basement. Both the oldest overlying Tertiary rocks (~26 Ma) and underlying strata dip ~90° east. Recent mapping and structural interpretations of the pre-Tertiary geology provide a new model of how this portion of the arc was tilted on end during Tertiary extension.

We recognize three sets of Tertiary normal faults, and the fault with the most displacement in each set has <3 km of displacement. Faults in each set initiated as high-angle (~60°), west-dipping normal faults that rotated as displacement progressed and then passively rotated concurrent with movement on younger, crosscutting faults. Faults of the first (oldest) set of normal faults rotated through horizontal and now dip at gentle to moderate angles to the east. Faults of the second set dip gently westward, and faults of the third (youngest) set dip moderately westward. Rocks in the hanging wall of a fault of the first set, dismembered by movement on crosscutting younger faults, are well exposed as extensional klippen.

The internal geometry of the Tortilla Mountains is more complex than a single, tilted crustal panel. A stepwise structural reconstruction suggests that extensional deformation can be modeled as three, sequentially developed, partially superimposed, east-tilted half grabens. Each half-graben was associated with one of the three sets of normal faults. This interpretation of the Tertiary structure has implications for understanding the distribution of Laramide deformation, arc magmatism, and associated hydrothermal activity and for conducting mineral exploration and development in the region.