Regional, structural and strain analyses of terranes in the Western Metamorphic Belt, Central Sierra Nevada, California

Abstract

A comparison of total strains in 225 samples from the Western Metamorphic Belt, Sierra Nevada, shows that strain types and intensities strongly depend on rock type. Greywackes and clast-supported conglomerates are least strained, whereas more intense strains occur in lapilli tuffs, pebbly mudstones and slates. Large belts of volcanic rock record plane strain whereas nearby slate and greywacke sequences record flattening-type strain suggesting that volume losses of 45–65 % occurred in slates and 0–26% in greywackes. Lapilli tuffs record less than 11% volume loss. The differences in volume loss and strain from one rock type to another require that discontinuities in total strain occur at most lithologic contacts. Independently of the lithologic control, regional structural patterns and strain fields support the delineation of three terranes (Foothills, Merced River and Northern Sierra) and two distinct domains (western and eastern Foothills terranes) at these latitudes within the Western Metamorphic Belt. These terranes, each bordered by large ductile shear zones, differ in average strain intensity and orientation but not in strain type. In the Foothills terrane strain intensities decrease from moderate values in the western Foothills terrane to low values in the eastern Foothills terrane. Strain intensities in the study area are highest in the Melones and Bear Mountains fault zones and in terranes east of the Melones Fault. Structures associated with regional deformation in each terrane do not extend far across terrane boundaries; multiple generations of regionally developed structures are, therefore, rare in this part of the Western Metamorphic Belt. Outside of fault zones, we see one regionally developed cleavage in the Foothills terrane, a single but older cleavage in the Merced River terrane and two still older cleavages in the Northern Sierra terrane. However, younger deformations may have reactivated and rotated older structures in adjacent terranes. The timing and kinematics of the deformation in the Foothills terrane suggest that faulting and rigid-body rotations preceded pervasive shortening, continued fault movement and rotation of faults to steep dips. If orientations of ductile structures reflect plate motions, structures in the Merced River terrane suggest Jurassic-Triassic orthogonal NNE convergence, whereas structures in the Foothills terrane support late Jurassic orthogonal NE convergence changing to oblique ENE convergence about 150 Ma ago.