UNROOFING FRANCISCAN BLUESCHISTS

Within the conterminous United States, blueschist facies are widely distributed along the active part of the transform plate boundary in the California Coast Ranges. And, where subduction has been continuous within the Cascade forearc since at least the early Tertiary, exposed blueschist facies rocks are rare and Tertiary blueschists are unknown. The blueschist facies rocks of the Franciscan Complex are Mesozoic in age ~ 170 to 80 Ma in age; however, the regional blueschist terranes were largely exhumed during the Tertiary between 55 and 30 Ma and there is no evidence to indicate they were unroofed and exposed as a regional belt concurrent with subduction.

The tectonic significance of regional belts of blueschist facies rocks is different from that of the isolated tectonic blocks of high-grade blueschist, garnet amphibolite, and eclogite. Blueschist and eclogites that occur as blocks are more likely to have experienced a multi-process tectonic unroofing and reworking history. For example, blueschist bearing-serpentinite diapirs have been documented in the Great Valley and Marianna forearc. Likewise, blocks transported by serpentinite diapires intrude forearc and accretion complexes, then have been redeposited by olistostromes and mass flow deposits. Blueschist and eclogitic blocks are also found in association with the basal tectonized zone underlying arc-related assemblages of the hanging-wall of the subduction zone.

Curiously, blueschists are not exposed where there has been the most continuously active subduction. It seems that while blueschist and eclogite facies rocks form in subduction zones, their exposure as regionally extensive belts may primarily be controlled by perturbations in subduction. The timing of their exhumation represents a change in the plate boundary resulting from collision of oceanic islands, island arcs, or continental crust; or a change to highly oblique convergence. So to summarize: Blueschist facies rocks are not common along active, unobstructed subduction margins. They are common in active collision zones, obliquely convergent margins, seamount or otherwise obstructed subduction, & active transform plate boundaries. Blueschist facies rocks have not been documented in active subduction complexes where normal faulting and extensional collapse of actively growing accretionary wedges are imaged.