Structural gradients and brittle‐ductile behavior during high‐temperature solid‐state deformation in Lake Edison granodiorite (Sierra Nevada, California)

In the Bear Creek area (Sierra Nevada, California), the high temperature post-magmatic deformation structures of the Lake Edison granodiorite (Kle; 88 Ma old), developed during pluton cooling, include steeply dipping orthogneiss foliations, joints (fractures with no shear offset) and plastic shear zones. A weak orthogneiss foliation and SC fabric are present and intensifies towards the contact with the younger Mono Creek granite (Kmc; 86 Ma) within the dextral Rosy Finch Shear Zone (RFSZ)(Tikoff and de Saint Blanquat, 1997), a major kilometer-wide deformation zone crosscutting the pluton contacts. Joints form a dominant (or exclusive) steeply dipping set oriented E-W to ENEWSW over most of the pluton. Plastic shear zones exploited the precursory joints (Fig. 1a) and leucocratic dykes (Fig. 1b), which is recognized to be a general process of shear zone nucleation (Pennacchioni and Mancktelow, 2007). Segmentation of joints and of overprinting shear zones resulted in high displacement gradients towards the shear zone tips that are accommodated by development of foliated domains and wing cracks, filled with quartz, at the opposite contractional and extensional sides, respectively. Quartz veins also discontinuously decorate the shear zone planes and are almost invariably sheared and recrystallized. The synkinematic assemblage in granodiorite mylonites along the shear zones, and the lattice preferred orientation of recrystallized quartz (showing a strong Y maximum of c-axis) indicate deformation temperatures ≥ 500°C. The high temperature structures, where in turn exploited at lower temperature (about 300°C) by cataclastic faults associated with localized precipitation of greenschist facies mineral assemblages (quartz, chlorite, epidote). This deformation sequence is almost identical to that recognized in the Adamello pluton (Southern Alps, Italy)(Pennacchioni et al., 2006). The distribution of the structures across the Kle pluton is heterogeneous. We present detailed surface maps representative of the structural assemblage at different distances to the contact with the Kmc, along the natural transect of the Bear Creek East Fork valley, that crosscut the RFSZ. The structural profile shows the appearence of a conjugate set of shear zones and joints, discontinuolusly decorated by leucochratic dykes, in the granodiorites, well within the RFSZ and close to the contact with the Kmc (Fig. 2). The two sets strike E-W to ENE-WSW (left-handed shear zones and joints corresponding to the set dominant over most of the pluton) and NNE-SSW (right-handed shear zones and subparallel joints). The two sets of shear zones and joints show mutual overprinting relationships (though the E-W set more commonly overprints the other set), suggesting that their development was coeval. The overall structural association is consistent with a regional shortening direction oriented about NW-SE, bisecting the acute angle of the conjugate set and orthogonal to the orthogneiss foliation within the RFSZ. This interpretation simplifies the reconstruction of the stress field during deformation. In fact, the assumption that joints developed orthogonal to σ3 would imply complex and unrealistic rotations of the stress field to account for conjugate arrangement of joints and their subsequent shear reactivation. References Pennacchioni, G., Di Toro, G., Brack, P., Menegon, L., Villa, I.M., 2006. Brittle-ductile-brittle deformation during cooling of tonalite (Adamello,Southern Italian Alps). Tectonophysics 427, 171-197. Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780. Tikoff, B., Saint Blanquat, M., 1997. Transpressional shearing and strike-slip partitioning in the Late Cretaceous Sierra Nevada magmatic arc, California. Tectonics 16, 442-459.