Mesozoic tectonics of the Maria fold and thrust belt and McCoy basin : an examination of polyphase deformation and synorogenic response
Here we summarize all current knowledge about the Maria Fold and Thrust Belt and McCoy Basin tectonic provinces of southeastern California. We also present new geologic mapping, structural analysis from macroscopic to microscopic scale, U-Pb zircon ages and Ar-Ar hornblende and biotite ages from key areas in the Maria Fold and Thrust Belt and McCoy Basin to resolve kinematics and timing of polyphase deformation events related to the Mesozoic Cordilleran Orogeny in southeastern California. The Maria Fold and Thrust Belt (MFTB) is an east-west trending belt of amphibolite grade metamorphic rocks characterized by largely south-vergent folds and ductile shear zones that place Jurassic and Proterozoic crystalline rocks over Paleozoic and Mesozoic sedimentary and volcanic rocks correlative to strata observed on the Colorado Plateau and in southeastern Arizona. Rocks in the MFTB have undergone polyphase ductile deformation and high grade metamorphism. The McCoy Basin trends subparallel with the MFTB and is defined by exposures of Jurassic-Cretaceous McCoy Mountains Formation, a > 7 km thick siliciclastic wedge of sandstone, siltstone and conglomerate. In contrast with the MFTB, rocks in the McCoy Basin have undergone primarily brittle deformation and have undergone low grade regional metamorphism. Important questions remain regarding the tectonic evolution of these two tectonic provinces, the relationship of these two provinces to each other and their relationship to the Cordillera at large. Resolving kinematics and timing of polyphase deformation in the MFTB and assessing synorogenic response in the McCoy Basin using established and new methods of structural geology and geochronology will resolve these questions and illuminate fundamental geologic processes related to orogenesis. This dissertation is divided into three chapters, with the goal of assessing kinematics and timing of deformation in the MFTB and McCoy Basin. A general encompassing hypothesis for investigations is that sedimentation and later deformation of rocks in the McCoy Basin can be directly linked to Mesozoic polyphase deformation in the MFTB. Structural analysis of key areas shows that there are three deformation events in the MFTB-McCoy Basin region. D1 is characterized by initially subrecumbent isoclinal folds and shear zones and a north-dipping foliation designated S1. D2 is characterized by mesoscopic and macroscopic southwest-facing isoclinal folds that refold S1 and shear zones that imbricate and severely attenuate Paleozoic and Mesozoic strata (to less than 1% of original stratigraphic thickness). Analysis of kinematic indicators including elongate mineral lineation, stretched pebbles and concretions and microstructural textures indicates that D1 and D2 represent two stages of a single progressive deformation event formed by top-to-the-southeast-directed reverse and dextral shear. A diorite deformed by D1/D2 yields an U-Pb zircon age of 86.3 ± 1.3 Ma, indicating that D1/D2 represents Late Cretaceous deformation. Published detrital zircon ages indicate that sedimentation in the McCoy Basin was coeval with middle crustal deformation in the MFTB. Kinematics of D1/D2 suggests that this event is related to the Sevier Orogeny and timing of deformation is consistent with other regional studies. The tectonic setting of the McCoy Basin is interpreted to have evolved from a broad Late Jurassic-Early Cretaceous rift valley to a Late Cretaceous retroarc foreland basin. The geometry of the early rift basin placed important structural controls on the geometric and spatial configurations of the Late Cretaceous MFTB and McCoy Basin. D3 is characterized by northeast-vergent folds and shear zones and is coeval with emplacement of Late Cretaceous (79-67 Ma) granites and pegmatites, which crosscut D1/D2 fabrics. Strain field analysis of dikes and quartz veins and kinematic analysis of the Granite Mountains metamorphic core complex and the geometry of D3 folds and shear zones indicates that D3 formed as a result of northeast-directed synconvergent extension. D3 is also coeval with peak metamorphism in the region, which is confirmed by Ar-Ar ages of hornblende of ~70 Ma. At this time, the MFTB was emplaced over the McCoy Basin along the south-vergent Maria Frontal Thrust. The kinematics and timing of D3 deformation suggest that it is related to the Laramide Orogeny and supports the hypothesis that the change from Sevier to Laramide tectonism in the region is marked by a reorientation of the principal stress field and a change from compression to extension in the Sevier hinterland. The main phase of orogenic activity ended by ~55 Ma, based on biotite Ar-Ar ages, which is consistent with regional observations. Compilation of previously published and unpublished mapping as well as other types of geologic data was an integral part of this investigation. Regional maps were compiled from a single digital archive that may be readily shared and distributed to the scientific community at large.
structural geology, Mesozoic tectonics, southern Cordillera, Maria Fold and Thrust Belt, middle crustal deformation, Sevier Fold and Thrust Belt, Laramide, Cretaceous deformation, Big Maria Mountains, McCoy Mountains, Palen Pass, McCoy Basin, southeastern California, Cretaceous plutonism
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