STRUCTURAL EVOLUTION OF THE BIG MARIA SYNCLINE, SE CALIFORNIA: A KINEMATIC ANALYSIS OF DUCTILE DEFORMATION AND EXTREME ATTENUATION IN THE MARIA FOLD AND THRUST BELT

The Big Maria syncline in the Big Maria Mountains (BMM) contains metasedimentary rocks correlative to the classic Paleozoic cratonal section of the Colorado Plateau. These rocks have been overturned and locally attenuated to less than 1% of original thickness. This study examined the structural geology of the Big Maria syncline and the stratigraphy of the Paleozoic units, in order to constrain the timing and style of deformation. Results include a 1:24,000-scale geologic map of the syncline and vicinity with cross sections, structural analysis of the syncline and other features, and an examination of Paleozoic stratigraphy. This study confirmed strong correlations of the Paleozoic metasedimentary rocks in the BMM with Paleozoic rocks of the Grand Canyon. Mesozoic metasedimentary rocks overlying the Paleozoic rocks confirm that the BMM were part of the stable craton until at least the middle Jurassic. Attenuation for the overturned Paleozoic units in the Big Maria syncline varies between 3.8-11.1% of original thickness, with an average attenuation of 8%. Structural analysis in this study suggests at least two deformation events are recorded. The first deformation event produced large, east-west trending, south-vergent folds, including the Big Maria syncline. The second deformation event refolded these earlier folds around northwest-trending axes. Preliminary analysis of kinematic indicators suggests a top-to-the-southeast sense of shear, but these fabrics may have been refolded from an original top-to-the-south orientation. In the BMM all units, including the Jurassic granitic rocks, but not the Cretaceous leucogranite dikes, are affected by all episodes of deformation. Timing of deformation in the BMM is broadly constrained between ~160 Ma, the age of the Jurassic plutonic rocks, and ~79 Ma, the age of the leucogranite dikes. Results from this study suggest that deformation was more likely related to Cretaceous underthrusting rather than Jurassic plutonic events as interpreted by some workers. Placing events in their proper tectonic context will clarify the Mesozoic tectonics for this important part of the Cordillera.