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

Abstract

[1] 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°i ncrease 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.

Deep‐tow magnetic anomaly study of the Pacific Jurassic Quiet Zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior

“…Four polarity changes are found within the upper, 150‐m, off axis lava sequence while two polarity changes are found within the lower lava sequence, which was presumably accreted at or near the spreading axis. Formation microscanner (FMS) log data show significant dips in the lava sequences within the hole [ Pockalny and Larson , 2003]. If the tilted lava sequence boundaries indicate that magnetization boundaries are similarly tilted, then changes in magnetic polarity can occur within a vertical hole [ Tivey et al , 2005].…”

Nitrogen concentration and δ15N of altered oceanic crust obtained on ODP Legs 129 and 185: Insights into alteration-related nitrogen enrichment and the nitrogen subduction budget

“…However, more recent studies (Koppers et al, 2003;Pockalny and Larson, 2003) reinvestigated the basalt sections at Site 801 and divided them into an earlier-emplaced ''lower series'' (Sequences IV-VIII; >168 Ma) and a latererupted ''upper series'' (Sequences I-IV; $160 Ma), which is defined by the Lower Hydrothermal deposits (Sequence V) within Sequence IV. Those studies confirmed that there is a 10-Myr gap between the ''lower series'' and the ''upper series''.…”

Paleomagnetic constraints on deformation of superfast-spread oceanic crust exposed at Pito Deep Rift

“…Constraints on magmatic accretion, mechanical deformation, and hydrothermal alteration processes are determined from the internal geologic structure of ophiolites [Moores and Vine, 1971;Penrose Conference Participants, 1972;Cann, 1974;Casey et al, 1981;Varga and Moores, 1985;Nicolas, 1989], marine seismic studies [Christeson et al, 1992;Detrick et al, 1993;Kent et al, 1994;Hallenborg et al, 2003], and limited deep crustal drilling [Alt et al, 1993;Pockalny and Larson, 2003;Tivey et al, 2005;Wilson et al, 2006]. Additional constraints come from studies of dike intrusion events [Delaney et al, 1998;Perfit and Chadwick, 1998], and eruptions along modern spreading centers [Soule et al, 2009, and references therein].…”