The structural and metamorphic history of the Oakhurst roof pendant, Mariposa and Madera counties, California

The Oakhurst roof pendant, near Oakhurst, California, consists of three pre-Cretaceous clastic metasedimentary rock units, surrounded and intruded by rocks of the Sierra Nevada batholith. Two zones of probable sheared granitic and ultramafic rocks trend northwesterly and are approximately aligned with the Foothills fault system farther north. The dominant mesocopic feature of the pendant is a northwesterly striking foliation which forms a downward converging fan. Earlier-formed hornblende lineations have been transposed into this foliation plane, as shown by Schmidt-net projections. Rocks comprising the three unites show three textural stages: 1) Early amphiboles and diopside overgrown by later static amphibole pophyroblasts. Hornblende lineations in one of these units show a great-circle Schmidt-net distribution. 2) A superimposed, well-developed, cataclastic s-plane (the mesoscopic foliation). 3) Late, static growth of poikiloblastic micas and diopside, straight well-crystallized micas, near polygonal quartz and feldspar, epidote and biotite after amphibole and diopside, and chlorite after biotite. The sheared granitic and ultramafic rock bodies display the same prominent foliation and static and retrograde metamorphic textures as do rocks of the three major units. These data are interpreted as reflecting three dominant post-sedimentary structural (D) and thermal (M) events: 1) M1/D1 consists of episodes of synkinematic metamorphism, which resulted in an early foliation, S1, and of static metamorphism, M1. M1 is of the epidote-amphibolite facies. 2) D2resulted in the cataclastic development of the prominent fanning foliation, S2; the fan is indicative of an antiformal structure. this antiform and foliation developed as a consequence of subvertical movement, as indicated by the geometry of transposition of hornblende lineations into S2. The sheared granitic and ultramafic rock bodies were emplaced along two shear zones during D2. These zones could be the roots of thrust sheets and possibly and extension of the Foothills fault system, which has been mapped as terminating 20 miles to the northwest. 3) M2 was a static metamorphic event associated with batholithic intrusion. Locally, this even was characterized initially by high temperatures, but retrograde conditions generally prevailed. M2 was dominantly of the albite-epidote-hornfels facies, but locally of the hornblende-hornfels facies. Radiogenic dates of batholithic rocks near the pendant date M2 as Late Jurassic-Middle Cretaceous. Therefore, M1/D1 and D2 are pre-Late Jurassic. M1/D1 may be a southerly expression of the Late Permian-Early Triassic Sonoma Orogeny. D2 apparently reflects the deformation associated with the classical Nevadan Orogeny, and M2 corresponds to late or post-orogenic granitic intrusion of the Sierra Nevada batholith