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The Chinook Trough is a pronounced deep located in the central North Pacific Ocean that spans approximately 1500 km in a northeast-southwest direction. Geophysical and geomorphological considerations suggest that this feature is the southern lithospheric scar marking initiation of Late Cretaceous north-south rifting within the ancient Farallon Plate. If this hypothesis is correct, then, by analogy to other active and passive mid-ocean rift zones, Late Cretaceous sediment deposited immediately south of the Chinook Trough should contain significant amounts of hydrothermal material deposited in association with the intraplate rifting event. Because the proposed tectonic origin for the Chinook Trough places seafloor south of the eastern end of the trough beneath the Cretaceous calcite compensation depth (CCD), expected hydrothermal components deposited during the Late Cretaceous at these locations also should lack carbonate dilution. Ocean Drilling Program Sites 885 and 886 were drilled approximately 60 km south of the northeast section of the Chinook Trough. High-resolution chemical and mineralogical analyses demonstrate that sediment deposited after 75-81 Ma at these sites contains an extensive record of hydrothermal deposition without associated carbonate. Various proxy indicators suggest that the flux of hydrothermal material decreases in a roughly exponentially pattern upcore and reaches background levels in the early Paleogene. Lowermost sediment is similar in composition to that of Neogene and Holocene distal hydrothermal sediment deposited along the East Pacific Rise and is characterized as follows: (1) a primary mineralogy of poorly crystalline goethite, amorphous Fe-Mn oxyhydroxides, and apatite; (2) bulk sediment Fe concentrations between 23.5% and 32.5%; (3) bulk concentrations of As, Mo, Sb, and Zn that are 1-2 orders of magnitude higher than "normal" biogenic-free pelagic sediment; and (4) a pronounced bulk Ce-anomaly. These results are consistent with the hypothesis that the Chinook Trough represents onset of north-south Late Cretaceous rifting, and they provide a detailed record of hydrothermal deposition emanating from an intraplate spreading center.

Chinook Trough Rifting and Hydrothermal Deposition at Sites 885 and 886