PACIFIC RIM ACCRETION, PAST AND UNDERWAY—THE IMPORTANCE OF FOREARC CRUSTAL UNDERPLATING (Invited Presentation)

INTRODUCTION: Accretionary complexes of late Paleozoic to early Tertiary age are exposed discontinuously along lengthy sectors of the Pacific rim from southern Chile northward via Alaska, the Koryak Highlands of NE Russia, to Japan. Most of these complexes display evidence of low temperature, high pressures alteration, commonly to blueschist facies. As long understood, the implication of varying degree of high P/T metamorphism is that the bulk of exposed accretionary complexes are exposed forearc crustal underplates (depths of 10 km or >) and not the accretionary offscrapings of trench bordering frontal prisms.

ACCRETION AT MODERN PACIFIC SUBDUTION ZONES: Most modern Pacific-rim SZs are characterized by coastal exposures of a lithified, rock framework (basement) of Mesozoic to early Tertiary age. Basement is typically an accretionary complex (e.g., Kodiak-Chugach FMs of Alaska, the Sanbagawa-Shimanto FMs of Japan), or an igneous massif of an accreted oceanic body or in-situ-formed arc. The basement rock framework extends seaward to near the trench floor, terminating behind a 10-50-km-wide frontal prism of late Cenozoic accreted trench sediment. Volumetrically, these outer forearc fronting prism are small with respect to that of the older, landward complexes of forearc basement. However, modern, state-of-the-art seismic reflection profiles of Pacific forearcs reveal the coastal region existence of volumetrically large masses of subducted, crustally underplated material. Known examples include coastal southern Chile, Cascadia (perhaps the best), southern Alaska, the Aleutian Islands, southern Japan, and North Island, New Zealand.

SUMMARIZING CONCLUSION: Modern, deep crustal reflection imagery of Pacific subduction zones support the inference drawn from exposed high P/T accretionary complexes that the characteristic tectonic setting for their accumulation was as a forearc crustal underplate. Seismic imaging documents that this process is currently underway at many Pacific-rim subduction zones.