STRUCTURE OF FAULT ZONES AND DEFORMED TONALITES IN THE SMARTVILLE COMPLEX, NORTHERN SIERRA NEVADA

The 165Ma to 155Ma Smartville Complex is a rifted volcanic-plutonic arc sequence of oceanic origin, bounded by major northwest-trending fault zones, located west of accretionary terranes of the northern Sierra Nevada. Deformation of the Smartville occurred during the Late Jurassic Nevadan Orogeny, mainly within steep, east-dipping shear zones. Mesoscopic and microscopic structures and fabrics reveal that relative motion was dominantly west-down, dip-slip, during regional shortening. Orientations of quartz c-axes, gathered from a 5-axis universal stage, describe the general strain field of deformed Smartville rocks and estimate the direction of tectonic transport. Steep ENE-dipping foliations and subvertical stretching lineations, from within several brittle shear zones, formed during coaxial flattening. Ductile fabrics from metatonalitic gneiss have similar orientations, and may have been formed during dextral transpression within a ductile shear zone. Nearly orthogonal extension was responsible for deformation within a rare foliated metatonalitic dike, which may represent one of several extensional pulses within the Smartville arc. Direction of tectonic transport was consistently east/west (present coordinates).

It is likely that the Smartville Complex formed in situ, along the accreted North American margin, post-dating obduction of older oceanic arcs to the east. Fabrics observed in the Smartville do not record the accretionary event, but instead record later shortening, with a west-down, reverse sense of motion. The Franciscan subduction zone to the west, whose initiation was closely contemporaneous with Smartville magmatism, may have been responsible for both Smartville genesis and deformation shortly thereafter, by orchestrating major changes in relative plate motions during the Mid to Late Jurassic.