PRIMARY P-FRACTURE ARRAYS IN THE CATHEDRAL PEAK GRANODIORITE, SIERRA NEVADA BATHOLITH, CALIFORNIA

It is often considered that fracturing in granitic rocks occurs exclusively by opening mode fractures. We present evidence for the formation of primary P-fracture arrays, on the eastern margin of the Cathedral Peak granodiorite unit of the Tuolumne Intrusive Suite in the Sierra Nevada Batholith. These P-fracture arrays contain en echelon pull-apart openings connected by fracture segments, indicating dextral motion. This shear sense is corroborated by offset of igneous layering, with the amount of offset corresponding to the length of the pull-apart openings. The P-fracture zones strike NE and are sub-vertical. While multiple zones were observed and are oriented in approximately the same direction, each P-fracture array appears isolated and there is no clear arrangement of one array to another. Precipitation of chlorite and epidote within pull-apart openings and shear fractures indicate solute-rich hydrothermal activity coincident with fracture formation. Macroscopic bleaching zones surrounding the fractures may provide information about sources of hydrothermal mineralization. The bleaching occurs centimeters from the main fracture and 10s of centimeters from the margin of the pull apart zone, and work is underway to determine the composition and distribution of mineralization within the bleach zone, within the host rock, and within the pull-apart vein fill.

Minor tensional fracturing is observed outside this zone, which is most pronounced in the large K-feldspar megacrysts. The infinitesimal strain (interpreted as least compressive stress) direction from these Mode I fractures are oriented at a low angle to the P-fracture array. We interpret the tension fractures and P-fracture arrays form simultaneously. If so, this suggests that the P-fracture arrays form during transtensional deformation, with an oblique divergence angle of ~55. Our data suggests that shear fracturing of crystalline rock is possible at high levels of fluid pressure, in orientations that indicate highly oblique divergence.