PROVENANCE AND THERMAL EVOLUTION OF FLYSCH OF THE CHUGACH-PRINCE WILLIAM TERRANE, PRINCE WILLIAM SOUND, ALASKA

The Chugach-Prince William (CPW) composite terrane contains Upper Cretaceous-Eocene flysch well-exposed for over 2000 km along the southern Alaska margin and it is inferred to be one of the thickest accretionary complexes in the world. Flysch of the CPW was accreted to the continental margin soon after deposition and then it experienced a regionally extensive thermal metamorphism driven by near-trench plutonism of the diachronous Sanak-Baranof (SB) plutons between 62 and 50 Ma. In the Prince William Sound area, the rocks were again intruded by near-trench plutons of the Eshamy Suite at ~37-40 Ma. New and recently published crystallization ages of detrital zircon (>2500 single grain ages) demonstrate the coherency of the flysch in the CPW terrane from the inboard McHugh Complex, the Valdez Group, and the outboard Orca Group. The crystallization ages of detrital zircon record the progressive evolution of an Upper Cretaceous to Paleocene volcano-plutonic arc complex (~90 to 50 Ma) built on an older basement dominated by Lower Cretaceous to Triassic rocks (~140-200 Ma). Our work establishes clear stratigraphic packages in the Orca Group: it was deposited before, during, and after plutonism of the ~54 Ma plutons of the SB belt and the ~38 Ma Eshamy plutons. The youngest rocks of the Orca Group on Montague Island (most outboard) are essentially unmetamorphosed and are Upper Eocene or younger. Double-dated zircon from this young unit indicates exhumation cooling of the volcano-plutonic complex and Cretaceous cooling of the basement complex. The Coast Mountains orogen is a likely candidate source because it has a similar intrusive and thermal history as revealed in the sediments, however these data do not bear on where deposition occurred along the continental margin. The post-accretionary thermal evolution is revealed by 26 new zircon fission track samples that show a complex pattern of thermal resetting dominated by cooling following intrusive events at c. 50 Ma and c. 38 Ma. The outermost rocks of the Orca Group also reveal cooling ages between c. 25 and 32 Ma that we infer to be related to the onset of collision of the Yakutat terrane.