Strains and structures associated with a terrane bounding stretching fault: the Melones fault zone, central Sierra Nevada, California

Abstract

An examination of the structures, total strains, and rock types across the Melones fault zone indicates that in the central Sierra Nevada this fault separates and locally imbricates two distinct terranes. East of the fault, a widespread early Jurassic cleavage displays west-northwest to northwest strikes and is associated with moderate to high strain intensities. These structures form in Jura-Triassic metasedimentary rocks in part derived from a continental source. A weak northwest striking cleavage and low strain intensities occur west of the fault in lower Jurassic volcanic rocks and upper Jurassic slates and sandstones. The fault zone itself consists of steep east-dipping ductile-brittle shear zones or stretching faults along which the early Jurassic structures are transposed parallel to the Jura-Cretaceous and fault-related structures. Large flattening type strains are common in the fault zone and may be the result of heterogeneous strains developed when the shear zones bend around resistant blocks or by volume losses. Total strains in the eastern hanging wall reflect the superposition of fault-related on older strains; therefore, stretching lineations in this hanging wall do not reflect directions of fault motion. Stretching lineations in the footwall and fault zone, kinematic indicators in the fault zone, and a small eastward jump in metamorphic grade suggest an east-over-west or reverse sense of movement; no evidence for significant strike-slip or oblique-slip motion was found. Removal of footwall strains indicates that the Jura-Cretaceous shortening across the fault was responsible for both the transposition of older structures east of the fault and the rotation of the fault to its present steep dips. Thus, the Melones fault is a large ductile-brittle shear zone along which the eastern hanging wall (Merced River Terrane) moved to the southwest over the western footwall (eastern Foothills Terrane).