Implications for crustal accretion at fast spreading ridges from observations in Jurassic oceanic crust in the western Pacific

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Abstract
Downhole logging data and basement stratigraphy interpretations are used to determine the spreading environment and crustal accretion history of the ocean basement cored at ODP Hole 801C located in the Jurassic Magnetic Quiet Zone of the western Pacific. High‐resolution microresistivity data obtained with the Formation MicroScanner are used to measure the dip of the extrusive layers and indicate a 10°–30° increase in dip down the drill hole with lava flow contacts dipping back toward the original ridge axis. This structural pattern and the high proportion of massive flows relative to pillow units are consistent with prevailing crustal accretion models proposed for faster spreading ridges (e.g., >60 km/m.y., full‐rate). A detailed analysis of the age data, basement lithology, related geochemistry, and structural attitudes suggest the shallowest 100 m of the drilled section (e.g., Sequences I–III) were emplaced just off the ridge (Sequence III) or significantly farther off‐axis up to 5–15 m.y. later (Sequences I and II). The remainder of the drilled section (e.g., Sequences IV–VIII) has geochemical, lithological and physical trends that are assumed to be representative of crust created at faster spreading ridges. The pattern of dipping lava flow contacts from this deeper section of the drill hole suggests lava flows emanating from the ridge axis are limited to 1–2 km off‐axis. Our results suggest ocean crust drilled at Hole 801C was created at faster spreading rates; however, caution should be used when incorporating Sequences I–III into geochemical reference sections for faster spreading ridges.

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