P-FRACTURE DEVELOPMENT IN A MAGMATIC SYSTEM, TUOLUMNE INTRUSIVE SUITE, CALIFORNIA, USA: AN EXAMPLE OF HYBRID EXTENSIONAL-SHEAR FRACTURE?

We studied conjugate P-fracture arrays – which occurred in the presence of magmatic fluids – in the eastern edge of the Cathedral Peak granodiorite, associated with the Cascade Lake shear zone (CLSZ). The Cascade Lake shear zone (CLSZ) is a subvertical, NNW trending, dextral transpressional shear zone. It deforms both wall rock and the rocks of the Tuolumne Intrusive Suite, including the Cathedral Peak granodiorite. Within the Cathedral Peak granodiorite, P-fracture arrays are visible in en echelon rhombs filled by epidote-rich material. Rhombs are connected by thinner epidote-rich shear zones. Arrays are often surrounded by a bleached zone up to 10s of cm wide, and the zones of bleaching increases in width around the rhombs. This observation suggests that large volumes of fluid were present during fracture formation and epidote precipitation. Both sinistral and dextral P-fracture arrays occur. Sinistral shear zones are oriented ~047 and generally dip steeply to the NW. Dextral shear zones are oriented ~019 and dip steeply. Where faces of fracture surfaces are present, lineation is subhorizontal. Additional distributed fracturing is also found in these rocks, especially in more competent features such as K-feldspar megacrysts. The angle between P-fractures is about 28^o, which indicates a maximum horizontal shortening direction at 14^o from the average fracture array orientation. This orientation puts the shortening direction at ~033, which is consistent with the orientation of opening mode fractures in the K-spar megacrysts. An angle of 28^o between the conjugate P-fracture arrays is very different from the 60^o expected between conjugate fractures produced by Mohr-Coulomb failure. We suggest that these arrays formed from “hybrid”, or extensional-shear, failure during transtension facilitated by significant pore fluid pressure.