The Structural and Metamorphic History of the Oakhurst Roof Pendant, Mariposa and Madera Counties, California
Russell, Lee R
MetadataShow full item record
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 mesoscopic 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 units 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 ^-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 prominant 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) M^/D- consists of episodes of synkinematic metamorphism, which resulted in an early foliation, _S, , and of static metamorphism, M . M is of the epidoteamphibolite facies. (2) D resulted in the cataclastic development of the prominent fanning foliation, S_^; 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 S^^. The "sheared granitic" and "ultramafic" rock bodies were emplaced along two shear zones during D. These zones could be the roots of thrust sheets and possibly an extension of the Foothills fault system, which has been mapped as terminating 20 miles to the northwest. (3) M was a static metamorphic event associated with batholithic intrusion. Locally, this event was characterized initially by high temperatures, but retrograde conditions generally prevailed. M^ was dominantly of the albiteepidote- hornfels facies, but locally of the hornblendehornfels facies. Radiogenic dates of batholithic rocks near the pendant date M as Late Jurassic-Middle Cretaceous Therefore, M /D and D are pre-Late Jurassic. M /D may be a southerly expression of the Late Permian-Early Triassic Sonoma Orogeny. D apparently reflects the deformation associated with the classical Nevadan Orogeny, and M corresponds to late or postorogenic granitic intrusion of the Sierra Nevada batholith.