Abstract
Foliation, broken bedding, and two “shear zones” in pre-Cretaceous clastic metasedimentary rocks in a roof pendant west of Oakhurst, California, are on trend with the western Sierra Nevada metamorphic belt to the northwest. In the pendant, foliations define a westward-verging, upward-diverging fan, and lineations have orientations that plot along a great circle, or a small circle of large diameter, with a maximum at the intersection of the circle and the synoptic foliation. Rock microtextures are characterized by porphyroblast clasts, especially of hornblende, in a matrix of aligned cataclastic fragments and some superimposed static mineral growth.
Three major events are interpreted. The first, M1, is characterized by probable synkinematic metamorphism of the epidote-amphibolite facies. The second, here correlated with the classical Nevadan orogeny, involved the cataclastic development of the prominent, penetrative, northwest-striking foliation surface, S1, and the incomplete transposition of M1 hornblende lineations (a kinematic axis: N50°E, 64°NE). During this event, bodies of ultramafic and granodioritic rocks were emplaced and deformed along the “shear zones.” Finally, contact metamorphism, M2, chiefly of the albite-epidote-hornfels facies, resulted from the intrusion of batholithic rocks between approximately 98 and 136 m.y. ago.
Regional implications include the following: (1) the shear zones may be southerly remnants of the Foothills fault system, probably the Melones fault zone; (2) M1 could represent a moderately deep expression of the Sonoma or Antler orogenies; and (3) the kinematics of S1 favor an interpretation of primarily vertical displacement for the Nevadan deformation and, hence, lend possible support to most subduction models.
