UNROOFING REGIONAL BLUESCHISTS

Blueschists and eclogites are sparse in forearc regions above active subduction zones. However, they are commonly found exhumed in collision zones and transform plate boundaries. For example, along the west central edge of the N. American Plate, blueschists have been unroofed and are widely distributed along the active part of the transform plate boundary in the California Coast Ranges. In contrast, subduction has been continuous within the Cascade forearc since the early Tertiary, at least; however, exposed blueschist facies rocks are few and Tertiary blueschists are unknown. A similar situation persists along the active margin of the S. American Plate where subduction has persisted throughout the Late Mesozoic and Cenozoic, yet blueschist facies rock of that age are nearly absent. Curiously, they are found in the Caribbean and nearby in irregular plate boundary conditions. In the Indonesian region where subduction processes have been frequently interrupted by complex plate interactions, collisions and reversal of subduction polarity, blueschists are common in the forearc. Isolated tectonic blocks of high-grade blueschist and eclogite have a different tectonic significance than regional belts. Blueschist and eclogitic blocks are more likely to have experienced a multi-process tectonic unroofing and reworking. Blueschist-bearing serpentinite diapirs have been observed in the Marianna forearc. Likewise, blocks transported by serpentinite diapirs that intrude forearc complexes have also been redeposited by olistostromes and mass flow deposits. High grade blocks are also found in nappes along basal tectonized serpentinite zones underlying arc-related assemblages of the hanging-wall of the subduction zone. Curiously, blueschists are not commonly exposed where there has been the most continuously active subduction. Thus, while blueschists and eclogites form in subduction zones, their exposure as regionally extensive belts seems primarily controlled by perturbations in subduction. They mainly represent a change in the plate boundary conditions resulting from a collision of oceanic islands, island arcs, or continental crust, or a change to highly oblique convergence and transform boundary.