STRUCTURAL EVOLUTION OF THE BIG MARIA SYNCLINE, SE CALIFORNIA

The Big Maria syncline is a kilometer-scale, roughly east-west trending nappe fold in the Big Maria Mountains in SE California, the axial plane of which strikes roughly east-west, dips approximately 45° to the north, and plunges west-northwest. It folds Proterozoic, Paleozoic, and Mesozoic metamorphic rocks and consists of an upright, relatively unattenuated southern limb, and an overturned, extremely attenuated northern limb. The nature and timing of the formation of this structure have been controversial, and several models for its formation have been proposed. This study investigates the kinematics of this structure and discusses constraints on the timing of deformation. Two deformation events, D1 and D2, are recognized in the Big Maria syncline. D1 folds Proterozoic, Paleozoic and Mesozoic rocks into tight-to-isoclinal folds with a pervasive axial-plane cleavage, S1, which is subparallel to original bedding and to the axial trace of the Big Maria syncline. A stretching lineation contained within S1 plunges to the west-northwest, and shear fabrics in deformed rocks indicate a top-to-the-southeast sense of shear. Shear strains were highest on the overturned limb, but both limbs show the same shear sense. S1 is crosscut by Cretaceous leucogranite dikes. D2 refolds original bedding, S1, and the Cretaceous dikes around upright-to-slightly-overturned folds with northwest-trending axes. These folds contain an axial-plane cleavage (S2) that dips steeply SW and that is expressed in micaceous rocks and that crosscuts the Cretaceous dikes. Observations from this study suggest that the Big Maria syncline formed as the result of top-to-the-east-southeast shearing, which is oblique (reverse and dextral). This sense of shear suggests D1 may be related to the Sevier orogenic event. Timing of D1 is broadly constrained between 160-75 Ma. D2 was likely the result of top-to-the-northeast shearing and may be related to the Mule Mountains thrust event. Timing of D2 is constrained between 75-65 Ma. These deformation events are analogous to deformation events observed elsewhere in the Maria Fold and Thrust Belt. Refined understanding of these deformation events is important in better constraining the Mesozoic tectonic history of this important part of the Cordillera.