2026-10-15T10:28:02Z https://oai.datacite.org/oai

doi:10.1594/pangaea.714870 2024-07-20T20:13:40Z PANGAEA PANGAEA.REPOSITORY

Age determination of the lower oceanic crust at ODP Hole 176-735B John, Barbara E Foster, David A Murphy, John M Cheadle, Michael J Baines, A Graham Fanning, C Mark Copeland, Peter PANGAEA 2004 Issued: 2004 doi:10.1016/j.epsl.2004.02.014 Drilling/drill rig Leg176 Joides Resolution Ocean Drilling Program (ODP) The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (~850°C) drilled in Hole 735B is estimated at 11.93F0.14 Ma, based on U-Pb analyses of zircon by ion probe. This crystallization age is older than the 'crustal age' from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949-11.935 Ma). 40Ar/39Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42 +/- 0.21 Ma), implying cooling rates of >800°C/m.y. over the first 500,00 years to temperatures below ~330-400°C. Fission-track ages on zircon (mean 9.35 +/- 1.2 Ma) and apatite reveal less rapid cooling to <110°C by ~7 Ma, some 4-5 m.y. off axis.Comprehensive thermochronometric data from the structurally intact block of gabbro between ~700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range ~ 900-330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T<=280°C) are not consistent with these predictions and record anomalously high temperatures for crust >700 m below sea floor at 8-10 Ma (i.e. 2-4 m.y. off axis). We offer two hypotheses for this thermal anomaly:(i) Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from ~19.5 to 7.5 Ma. (ii) Late detachment faulting, which led to significant crustal denudation (2.5-3 km removed), further from the ridge axis than conventionally thought. Supplement to: John, Barbara E; Foster, David A; Murphy, John M; Cheadle, Michael J; Baines, A Graham; Fanning, C Mark; Copeland, Peter (2004): Determining the cooling history of in situ lower oceanic crust-Atlantis Bank, SW Indian Ridge. Earth and Planetary Science Letters, 222(1), 145-160 en Collection Supplementary Publication Series of Datasets application/zip Creative Commons Attribution 3.0 Unported https://creativecommons.org/licenses/by/3.0/legalcode