EXTENSIONAL EXHUMATION IN FRANCISCAN EASTERN BELT BLUESCHISTS, NORTHERN COAST RANGES, CALIFORNIA

The mechanism of exhumation of high-P rocks in accretionary complexes is still debated, and the chaotic nature of their internal structure hinders interpretation in terms of tectonic processes. Excellent riverbed exposures of the relatively coherent Eastern Belt of the Franciscan Complex provide evidence that helps resolve these problems. We distinguish three main sets of structures in blueschist-facies Late Jurassic to Early Cretaceous greywacke-shale sequences in the Yolla Bolly and Pickett Peak Terranes.

1) Early highly disruptive deformation appears to be confined to strata-bound units (“broken formation”), and shows textures characteristic of soft-sediment deformation. This is likely to represent mass-wasting processes on the inner trench wall, but may include some deformation of underconsolidated subducted sediment.

2) Predominantly E-vergent folds, thrusts, and pressure-solution cleavage, with associated intensely folded sheet veins parallel to bedding and shear surfaces, likely represents deformation related to subduction and underplating. The volume of vein material is commensurate with the volume loss (up to 50%) previously documented in cleaved greywacke sandstones, and suggests source-to-sink distances were of the order of tens to hundreds of meters.

3) Conjugate sets of normal-sense shear zones and faults intensively disrupt earlier structures, and produce large vertical shortening strains. Shear zones formed continuously as the rocks were exhumed from the ductile regime (with pressure-solution and associated veining) into the brittle regime. No overall sense or direction of shear is developed, suggesting roughly coaxial flattening strain. Superposition of these extensional shears on previous broken formation produces characteristic mélange textures.

The widely developed and pervasive normal-sense shears provide a mechanism for thinning the accretionary wedge and exhuming these high-pressure rocks. Underplating of Late Cretaceous and Early Tertiary rocks was coeval with this process, contributing to an overall circulation of material through the rear part of the wedge.