ENIGMATIC EAST-WEST FABRICS IN THE TUOLUMNE BATHOLITH; IS THERE A TECTONIC SIGNIFICANCE?

The main NW-SE striking structures of the Sierra Nevada are well known, but our mapping has revealed that E-W striking magmatic fabrics are present in nearly all phases of the Tuolumne Batholith (TB), spanning an age range of 93 Ma to 85 Ma. This magmatic fabric is ubiquitous in most units, but is locally secondary to a NW-SE magmatic foliation. To gain a better understanding of the origin and significance of the E-W fabric, we have looked for structures with similar orientations in the surrounding host rock. New structural data is presented here from the May Lake and Benson Lake pendants, which lie along the western margin of the TB. Several E-W striking structures are preserved in the May Lake pendant. The largest of these structures is a set of gentle to open upright kink folds with a 100-110 striking axial planar cleavage. Elsewhere a closely spaced, E-W striking fracture cleavage occurs. Other structures associated with N-S shortening include sets of centimeter to meter scale conjugate thrust faults and box folds with a roughly E-W bisector. These structures overprint isoclinally folded strata and axial planar cleavage, which is shown to be syn- the 102 Ma El Capitan intrusion based on the magmatic folding of late El Capitan dikes. Since the adjacent Glen Aulin (GA) Granodiorite (93 Ma) shows only local NW-SE sub-solidus deformation, these structures are bracketed between 102 Ma and the time of solidification of the GA.

Similar structures occur in the Benson Lake Pendant. This set of structures associated with N-S shortening exhibits an average strike of 060-070. These structures include the axial planes of upright gentle folds of an older foliation, and a closely spaced fracture cleavage. Sporadic thrust faults also indicate N-S shortening. Finally, locally developed crenulations with E-W axial planes are observed in the Saddlebag Lake pendant on the eastern margin of the TB.

All of these structures appear to be spatially and temporally correlated. If they are tectonically related, then there must be a N-S oriented shortening event recorded in the TB which may be related to conjugate kink structures elsewhere in the Sierra discussed by Paterson (1989). We hypothesize that these structures are related to an increment of Late Cretaceous shortening, possibly due to elastic relaxation or strain partitioning during transpression.