Laramide structures that are exposed along much of the Laramide porphyry copper province of southeastern Arizona have been dismembered and tilted by superimposed Cenozoic normal faults, such that the overall style of shortening (e.g., thin-skinned, low-angle thrusts versus basement-cored, moderate-angle reverse faults) is unclear pending a compelling reconstruction of superimposed extension. This study integrates new geologic mapping in the Romero Wash–Tecolote Ranch area with earlier work and utilizes palinspastic reconstructions of structures related to extension and shortening to determine the geometry of structures in this segment of the Laramide orogenic belt and their relationship to local products of magmatism and hydrothermal alteration.
Geologic mapping indicates that multiple generations of Cenozoic normal faults can be identified through crosscutting relationships. Near both ends of the study area, the oldest synextensional strata dip vertically, indicating that the area has been tilted 90° eastward during extension. Structural reconstructions of normal faults reveal two reverse faults within the area, the Romero Wash fault and the Tecolote fault. Once restored, both faults originally dipped in opposite directions at moderate angles and demonstrate clear evidence for associated fault-propagation folds. As a result of 90°E tilting by Cenozoic normal faulting, the Romero Wash fault became an overturned reverse fault, whereas the Tecolote fault became an apparent normal fault. Constraints from the surface geology and forward models indicate that the Tecolote fault has ∼2.8 km of displacement on a fault that had an initial dip of 55°E, whereas the Romero Wash fault has ∼0.75 km of displacement on a fault that had an initial dip of 58°W. Considering their relative displacements and orientations, the Romero Wash fault is interpreted to be a backthrust to the Tecolote fault. These results are consistent with the interpretation that Laramide shortening in southeastern Arizona was dominated by basement-cored uplifts. Based on reconstruction of a cross section, the amount of Laramide shortening (e = Δl/li) in the study area was 1.8 km or 20%, and the total amount of subsequent Cenozoic extension (e = Δl/li) was 14.6 km or 200%.
Two stocks of granodiorite, with associated porphyry dikes and related hydrothermal alteration, occur within the study area. Sericitic and propylitic alteration and sparse quartz veins are present without observed potassic alteration. A barren porphyry dike (65.9 ± 0.7 Ma, U-Pb zircon) cuts the Romero Wash fault, limiting the upper age of the fault. Similar dikes nearby are sericitically altered, suggesting that alteration postdates shortening, similar to interpretations reached at the nearby porphyry copper systems of Ray, Resolution, and Kelvin-Riverside.
- Received 19 June 2016.
- Revision received 7 December 2016.
- Accepted 27 January 2017.
- © Geological Society of America