| Abstrakt: |
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. Plain Language Summary: Subduction zones continuously renew and recycle the surface of the Earth, influence global water and geochemical cycling, and generate a driving force for Plate Tectonics that is unique to our Earth. However, the mechanisms and tectonic conditions that allow subduction to initiate in the first place are poorly understood. Studies of metamorphic rocks from ancient subduction zones can provide insight into how pressure, temperature, metamorphic rock types, and water content change within tectonic plate boundaries. Using rocks from Oman that formed in an infant subduction zone, this study applies metamorphic petrology (quantitative measurements of mineral chemistry) and geochronology (timing of mineral growth) to constrain the changes in temperature through time spanning the first ∼10–15 million years of the subduction zone's lifetime. We synthesize new and pre‐existing data that together suggest the infant subduction zone experienced a multi‐phase cooling history, characterized by slow‐fast‐slow cooling. The first slow‐fast transition may represent a point in time where the infant subduction zone "turned on," that is, the transition from slow, forced convergence of two tectonic plates, to a faster‐moving, self‐sustaining, and plate‐boundary scale subduction zone. Key Points: The low‐temperature metamorphic sole beneath the Samail Ophiolite records subduction and return flow before juxtaposition with the ophioliteHigh sampling density of the 104 m‐thick core reveals it comprises three subunits that record continuous and stepwise changes in peak P‐TLow‐T sole rocks reached similar peak P as the high‐T sole but colder peak T (7–10 kbar, 500°C) and witnessed rapid interface refrigeration [ABSTRACT FROM AUTHOR] |
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