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Structural and thermal evolution of an infant subduction shear zone: Insights from sub-ophiolite metamorphic rocks recovered from Oman Drilling Project Site BT-1B
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  • Alissa Jeanne Kotowski,
  • Mark Cloos,
  • Daniel Fritz Stockli,
  • Eytan Bos Orent
Alissa Jeanne Kotowski
McGill University

Corresponding Author:alissa.kotowski@mcgill.ca

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Mark Cloos
University of Texas at Austin
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Daniel Fritz Stockli
University of Texas at Austin
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Eytan Bos Orent
University of Arizona
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The thermal structure of the subduction interface changes drastically within the first few million years following subduction initiation (i.e. subduction infancy), resulting in changing metamorphic conditions and degree of mechanical coupling. Metamorphic soles beneath ophiolites record snapshots of subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high-temperature (HT) and lower low-temperature (LT) units. This apparent inverted metamorphic gradient has been attributed to metamorphism under different Pressure-Temperature (P-T) conditions along the interface. However, peak P-T and timing of LT sole subduction are poorly constrained. Samples from Oman Drilling Project core BT-1B (104 m of metamorphic rocks) reveal that the LT sole subducted to similar peak P as the HT sole, but experienced ~300˚C lower peak T. Prograde fabrics in meta-sedimentary and meta-mafic rocks record Si-in-phengite values and amphibole chemistries consistent with peak P-T of ~8-12 kbar and ~450-550˚C in the epidote-amphibolite facies. Retrograde fabrics record a transition from near pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U-Pb ages (two samples) constrain timing of peak LT sole subduction to 95.7 ± 1.1 Ma, which may post-date the HT sole by ~6-8 Myr. In light of previous HT sole thermobarometry and geochronology, these new results support a model of protracted subduction and accretion while the infant subduction zone cooled at rates of ~100˚C/Myr for ~1-5 Myr. Temporal overlap of LT sole metamorphism and ophiolite crust formation suggests that underthrusting and cooling may lead to interface weakening, facilitating upper plate extension and forearc spreading.
Dec 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 12. 10.1029/2021JB021702