13-8 Late Cretaceous to Paleogene regional thrust faulting in the hinterland of the southern U.S. Cordilleran orogenic wedge: evidence from the Sierrita Mountains, Arizona

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

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Competing tectonic models for southern Arizona propose that Late Cretaceous to Paleogene contractional deformation is either part of the Laramide orogen and Laramide foreland belt or the Mexican orogen and a Cordilleran orogenic wedge.  New geologic mapping and structural analysis from the western Sierrita Mountains reveal a major low-angle thrust fault that places Paleozoic carbonate strata structurally above metamorphosed Mesozoic volcaniclastic rocks. In the northern map area, strata and the thrust fault were tilted southwest by late Cenozoic normal faulting and are locally overturned. In the southern map area, the thrust forms a pseudo-klippe and dips < 15°.  Deformation fabrics in the footwall and mylonites overprinted by brittle faulting in the hanging wall indicate top-north, reverse-sense shear.  Cross-cutting relationships, new zircon U-Pb geochronology, and new Ar/Ar and (U-Th)/He thermochronology suggest the thrust was active between 73 and ca. 50 Ma and accumulated > 15 km of horizontal shortening.  Thrust faults and reverse-sense shear zones with similar structural relationships and kinematics are located along strike in neighboring mountain ranges and are interpreted to be part of the greater Baboquivari thrust.  Ductile fabrics and metamorphism imply the thrust fault system represents more deeply exhumed structural levels compared to southeast Arizona and is consistent with contractional deformation in the interior, or hinterland, of a Cordilleran orogenic wedge.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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