Tectonic character of the Melones Fault zone, western Sierra Nevada, California

The Melones Fault zone is one of the major tectonic features of the western Sierra Nevada, where it is delineated by aligned alpinetype serpentinite bodies. It has been interpreted as a strike-slip fault, reverse fault, and high-angle root zone of westerly directed thrusts. More recently the Melones has been portrayed as a zone of suturing between collided plates. Hence, a full understanding of the zone has become increasingly important. This work summarizes the characteristics of the Melones Fault zone and its relationship to adjacent terranes and to the Sierra region as a whole. Lithologic, mesoscopic structural, microtextural, and gravity data were employed in the examination of the fault zone at Coulterville, Jackson, Georgetown,Foresthill, and along the North Yuba River, California.

Serpentinite, slate, and metasiltstone comprise the sheared matrix of the fault zone. Floating within this matrix are tectonic blocks of diverse lithologic types, which include clinopyroxenite, orthopyroxenite, websterite, hornblende gabbro, diorite, serpentinite, hornblendite, uvarovitic/chromitic rocks, andesitic-basaltic metavolcanic rocks, marble, bedded chert, slate, and metasiltstone. East of the fault zone, lithologies are of the Paleozoic Shoo Fly Formatlon and Calaveras assemblage; the former includes quartz sandstone, quartzofeldspathic sandstone, and slate, whereas the latter is comprised of metasiltstone, graywacke, slate, and chert. West of the Melones, the lithologic assemblage is largely of Mesozoic (pre-Late Jurassic) andesitic-basaltic metavolcanic rocks flanked to the east by a sequence of graywacke, tuffaceous slate, metasiltstone, radiolarian chert, volcanic ash, tuff, and tectonic slices of serpentinite and metavolcanic rocks.

One conspicuous deformation is recorded in rocks of the fault zone and adjacent areas. This deformation produced steep foliations oriented parallel to axial planes of steeply plunging folds within and east of the fault zone as well as subhorizontal folds west of the zone. East of the Melones lineations are largely steep, whereas west of the zone lineations are chiefly subhorizontal. Slickensides are steep, approximately parallel to fold axes, within fault-zone serpentinites. In zones of tight folding on both sides of the Melones, a foliation parallel to bedding is present; however, it is absent where folds are open or nonexistent. At places, a crenulation cleavage cuts the axial-plane foliation. Mineral assemblages associated with this deformation indicate greenschist facies metamorphism. Deformation is consplcuouely stronger wlthln the Helones Fault zone and results in widespread cataclastic fabrics that typically mask textures of recrystallization. Greenschist, and local lawsonite blueschist facies recrystallization affected fault-zone rocks at places. This regional structuralmetamorphic event is considered to be the Nevadan Orogeny, primarily because Mesozoic as well as Paleozoic rocks are involved. A subvertical kinematic picture is inferred on the basis of steep lineations on cleavage surfaces.

Limited textural evidence suggests pre-Nevadan deformation and greenschist facies metamorphism in Paleozoic rocks east of the fault zone. In addition, mesoscopic structural geometry (steeply plunging folds and lines of intersection of cleavage and layering) supports steep pre-Nevadan attitudes in these rocks. Static metamorphism of the albite-epidote hornfels facies affected most rocks of the region. Recrystallization is more advanced in rocks of the fault zone, perhaps due to increased ease of fluid migration. Gravity surveys conducted across the fault zone yield positive residual Bouguer anomalies associated with the zone, which permit computer models that portray subvertical sheets of ultramafic rock extending to a depth of 6 kilometers.

These data are compatible with a regional model in which the Melones Fault zone is a tectonic mélange generated during the partial collision of a Paleozoic continental-margin sequence with a Mesozoic island arc-marginal basin system during the Nevadan Orogeny. Ophiolitic rocks of the fault zone are fragments of marginal-basin crust and upper mantle emplaced during subduction and suturing.