Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub-Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT-1B

Abstract

Subduction interface thermal structure changes drastically within the first few million years of underthrusting (i.e., subduction infancy). Metamorphic soles beneath ophiolites record snapshots of dynamic conditions and mechanical coupling during subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high-temperature (HT) and lower low-temperature (LT) units. This inverted metamorphic gradient has been attributed to evolving metamorphic Pressure-Temperature (P-T) conditions during infancy; however, peak P-T and timing of LT sole subduction are poorly constrained. Oman Drilling Project core BT-1B sampled the base of the ophiolite in a location lacking the HT sole. Metasedimentary and meta-mafic samples collected from 104 m of core reveal that the LT sole subducted to similar peak P as HT rocks preserved elsewhere in Oman, but experienced ∼300°C lower peak T. Prograde fabrics record Si-in-phengite and amphibole chemistries consistent with peak P-T of ∼7–10 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 (n = 2) constrain timing of peak LT sole subduction to ∼91 Ma (post-dating initial HT sole subduction by ∼12–13 Myr) and dynamic retrogression through ∼90 Ma. Combined with existing geo/thermo-chronology, our results support a model of protracted subduction and accretion while the infant subduction zone experienced multi-phase, slow-fast-slow cooling. Temporal overlap of HT sole cooling (rehydration?) and ophiolite formation suggests that cooling may lead to interface weakening, facilitating upper-plate extension and spreading. The LT sole formed in a rapidly-refrigerating forearc after ophiolite formation and may reflect the transition to self-sustaining subduction. © 2021. The Authors.