STRUCTURAL EVOLUTION OF THE EAST CENTRAL BIG MARIA MOUNTAINS, MARIA FOLD AND THRUST BELT, SOUTHEASTERN CALIFORNIA

The Big Maria Mountains of southeastern California, one of the largest ranges of the Maria Fold and Thrust Belt (MFTB), record progressive noncoaxial, contractional Mesozoic deformation in an intracratonal setting. Detailed structural mapping and kinematic analysis in a portion of the overturned limb of the regional recumbent syncline, which is in contact with a Jurassic pluton, led to the identification of refolded folds and sheath folds and the refinement of existing models. The kinematics of shearing which formed early structures (F2 folds) are opposite that of most major structures in the MFTB.

Folding phases were separated into distinct time relative groups and are observed on outcrop to map scales. All phases fold a pronounced, pre-existing foliation, S1; metamorphic porphyroblasts indicative of upper greenschist to lower amphibolite facies metamorphism overgrow S1 prior to the observed folds. F2a isoclinal folds and F2b sheath folds formed during one progressive deformation based on style, orientation and lack of cross cutting relationships. Rarely an axial planar foliation is defined by aligned metamorphic minerals. Sense of shear when refolding is removed is apparently top-to-the-northeast. F3 tight folds have axial planes oblique to previous axial surfaces and S1 foliation and refold both isoclinal and sheath folds. F3 show both north and south vergence. Open, upright to overturned F4 folds are associated with top-to-the-north thrusts and formed later, most likely as a result of Late Cretaceous to early Tertiary deformation. The Jurassic pluton is affected by all phases of deformation including S1. The orientation of the contact between the pluton and the country rocks requires that the country rocks were intruded by the pluton and then subsequently deformed.

The data from this study allows previous models for the Big Maria Mountains to be refined by showing that refolding has occurred and that the Jurassic pluton was emplaced prior to deformation. Shearing that formed the F2 folds appears to be top-to-the-northeast, which is opposite of most regional folds in the MFTB. The Plomosa Mts structures have similar vergence, however, so the fold kinematics within the BMM is not completely anomalous.