TEHAMA-COLUSA SERPENTINITE MELANGE – FRANCISCAN, COAST RANGE OPHIOLITE OR BOTH?

There are two principal hypotheses for the origin of serpentinite mélange: (1) formation on the seafloor in a fracture zone/transform fault setting, and (2) formation within a subduction zone with mixing of rocks derived from both the upper and lower plates. The first hypothesis requires that the sheared serpentinite matrix be derived from hydrated abyssal peridotites and that the block assemblage consists exclusively of oceanic rocks. The second hypothesis implies that the sheared serpentinite matrix is derived from hydrated refractory peridotites with supra-subduction zone affinities, and that the block assemblage includes rocks derived from both the upper plate and the lower plate.

The Tehama-Colusa serpentinite mélange underlies the Coast Range ophiolite in northern California and separates it from high P/T metamorphic rocks of the Franciscan complex. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with fore-arc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory fore-arc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts, chert, and rare blueschist and high-P amphibolite.

It has been shown that peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large-offset transform fault/fracture zone that evolved into subduction zone mélange complex. Mélange matrix formed by the hydration and volume expansion of refractory fore-arc peridotite, followed by subsequent shear deformation. Mélange blocks were formed largely by the breakup of upper plate crust and lithosphere, with minor off-scraping and incorporation of lower plate crust. We propose that the methods discussed here can be applied to serpentinite matrix mélange world-wide in order to understand better tectonic evolution of the orogens in which they occur.