Progressive deformation and superposed fabrics related to Cretaceous crustal underthrusting in western Arizona, U.S.A.

In the Maria fold and thrust belt, a newly recognized E-trending Cretaceous orogenic belt in the southwestern United States, ductile thrusts, large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust. Style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold-thrust belt in adjacent segments of the Cordillera. The net effect of polyphase deformation in the Maria belt was underthrusting of Paleozoic and Mesozoic metasedimentary rocks under the Proterozoic crystalline basement of North America. The structure of the Maria belt is illustrated by the Granite Wash Mountains in west-central Arizona, where at least four non-coaxial deformation events (D₁-D₄) occurred during the Cretaceous. SSE-facing D₁ folds are associated with S-directed thrusts and a low-grade slaty cleavage. D₁ structures are truncated by the gently-dipping Hercules thrust zone (D₂), a regional SW-vergent shear zone that placed Proterozoic and Jurassic crystalline rocks over upturned Paleozoic and Mesozoic supracrustal rocks. Exposures across the footwall margin of the Hercules thrust zone show the progressive development of folds, cleavage and metamorphism related to thrusting. D₃ and D₄ structures include open folds and spaced cleavages that refold or transect D₁ and D₂ folds. The D₂ Hercules thrust zone and a D₃ shear zone are discordantly crosscut by late Cretaceous plutons.