Detailed geologic mapping and structural investigation of the footwall of the northern Snake Range (NSR) metamorphic core complex reveal km‐scale Late Cretaceous fold and thrust relationships of the northern Snake Range Fold and Thrust System (NSRFTS) that are broadly coeval with the deep burial and amphibolite facies metamorphism of Eocambrian strata. The O'Neill Peak Recumbent Syncline (OPRS) ‐ an NNW‐trending, non‐cylindrical, eastward‐opening, inclined to recumbent syncline and its overlying anticlinal closure, the O'Neill Peak Anticline, affect the entire Middle to Upper Cambrian stratigraphy, effectively doubling its structural thickness from ∼3.5 to ∼8 km. West of the OPRS, pervasive isoclinal folding in lower Cambrian schistose units and local thrust duplication of lower and middle Cambrian units of the Eightmile Thrust System record layer‐parallel shear and shortening. Detailed cross sections are used to create an interpretive pre‐extensional reconstruction of the NSR that integrates the NSRFTS with associated supracrustal shortening structures now exposed in the Confusion Range to the east. We conclude that the NSRFTS and overlying structures accommodated ≥10 km of horizontal shortening, resulting in ≥7 km of structural burial of the deepest structural levels in the NSR footwall, thus explaining the localized kyanite‐grade metamorphism and elevated thermobarometric P‐T estimates. This burial was apparently quite localized as evidenced by decreasing metamorphism to the north and south. The highest magnitude of subsequent extensional exhumation of the NSR footwall appears to coincides with the area of deepest burial, suggesting that Mesozoic crustal thickening influenced both the magnitudes and geometry of subsequent Cenozoic extensional exhumation.

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Metamorphic core complexes in the western North American Cordillera are commonly interpreted as the result of a single phase of large-magnitude extension during the middle to late Cenozoic. We present evidence that mylonitic shear zones inthe Harcuvar and Buckskin-Rawhide core complexes in west-central Arizona also accommodated an earlier phase of extension during the Late Cretaceous to early Paleocene. Microstructural data indicate substantial top-NE mylonitization occurred at amphibolitefacies, and 40 Ar/ 39 Ar thermochronology documents post-tectonic footwall cooling to <500°C by the Paleocene to mid-Eocene. Amphibolite-facies mylonites are spatially associated with voluminous and variably deformed footwall leucogranites that were emplaced from ca. 74-64 Ma, and a late kinematic ca. 63 Ma dike indicates this phase of mylonitization had waned by the early Paleogene. Reconstruction of the footwall architecture indicates that this latest Cretaceous -early Paleocene deformation occurred within a NE-dipping extensional shear zone. The leucogranites were likely the result of crustal melting due to orogenic thickening, implying a model whereby crustal heating triggered gravitational collapse of overthickened crust. Other tectonic processes, such as the Laramide underplating of Orocopia Schist or mantle delamination, may have also contributed to this episode of orogenic extension. Miocene large-magnitude extension was superimposed on this older shear zone and had similar kinematics, suggesting that the location and geometry of Miocene extension was strongly influenced by tectonic inheritance.We speculate that other Cordilleran core complexes also experienced a more complex and polyphase extensional history than previously recognized, but in many cases the evidence may be obscured by later Miocene overprinting.

Metamorphic core complexes in the western North American Cordillera are commonly interpreted as the result of a single phase of large‐magnitude extension during the middle to late Cenozoic. We present evidence that mylonitic shear zones in the Harcuvar and Buckskin‐Rawhide core complexes in west‐central Arizona also accommodated an earlier phase of extension during the Late Cretaceous to early Paleocene. Microstructural data indicate substantial top‐NE mylonitization occurred at amphibolite‐facies, and 40Ar/39Ar thermochronology documents post‐tectonic footwall cooling to <500°C by the Paleocene to mid‐Eocene. Amphibolite‐facies mylonites are spatially associated with voluminous and variably deformed footwall leucogranites that were emplaced from ca. 74–64 Ma, and a late kinematic ca. 63 Ma dike indicates this phase of mylonitization had waned by the early Paleocene. Reconstruction of the footwall architecture indicates that this latest Cretaceous—early Paleocene deformation occurred within a NE‐dipping extensional shear zone. The leucogranites were likely the result of crustal melting due to orogenic thickening, consistent with a model whereby crustal heating triggered gravitational collapse of overthickened crust. Other tectonic processes, such as the Laramide underplating of Orocopia Schist or mantle delamination, may have also contributed to this episode of orogenic extension. Miocene large‐magnitude extension was superimposed on this older shear zone and had similar kinematics, suggesting that the location and geometry of Miocene extension was strongly influenced by tectonic inheritance. We speculate that other Cordilleran core complexes also experienced a more complex and polyphase extensional history than previously recognized, but in many cases the evidence may be obscured by later Miocene overprinting.