Abstract

Elongate fault-bounded blocks of an Early Jurassic subduction complex are preserved along a major tectonic boundary in southern Alaska, the Border Ranges fault system (BRFS). The high-P/low-T metamorphic rocks of the subduction complex are juxtaposed along one strand of the BRFS with an approximately coeval pluton and its associated thermal aureole. New U-Pb and Rb-Sr isotopic dates from the subduction complex are 204 ± 8 Ma and 195 ± 10 Ma (2 sigma errors), respectively. The similarity of these dates to previously reported K-Ar dates supports petrographic and petrologic observations that the subduction complex did not experience any metamorphism related to the intrusion of the now adjacent pluton. Petrographic and petrologic data also show that the pluton did not experience a high-P/low-T metamorphism. The pluton and the metavolcanics that it intrudes are interpreted as part of a Late Triassic to Early Jurassic primitive island-arc complex. If the subduction complex is related to this coeval island arc, then the forearc and back part of the accretionary prism have been tectonically eroded, either by subduction erosion, strike-slip faulting, or a combination of these processes. If, however, the subduction complex was juxtaposed with the island arc during strike-slip faulting that accompanied and postdated the Cretaceous accretion of terranes of southern Alaska with North America, then the subduction complex could be related to an Early Jurassic island arc exposed on Vancouver Island, where it is part of Wrangellia. The new isotopic ages and petrologic data indicate that the BRFS probably began its history as a major fault zone as early as the Middle Jurassic, prior to the well-documented Cretaceous megathrust history.

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