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.

First Page Preview

First page PDF preview
You do not currently have access to this article.