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.

中文翻译：

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婴儿俯冲剪切带的结构和热演化：从阿曼钻探项目现场 BT-1B 回收的亚蛇绿岩变质岩的见解

俯冲作用的最初几百万年（即俯冲初期），俯冲界面热结构发生巨大变化）。蛇绿岩下的变质鞋底记录了俯冲婴儿期动态条件和机械耦合的快照。在 Samail 蛇绿岩（阿曼）之下，鞋底由结构上较高的高温 (HT) 和较低的低温 (LT) 单元组成。这种倒置的变质梯度归因于婴儿期不断变化的变质压力-温度（PT）条件。然而，峰值 PT 和 LT 单一俯冲的时间限制很差。阿曼钻井项目核心 BT-1B 在缺乏 HT 底的位置对蛇绿岩底部进行了取样。从 104 m 岩心采集的变质沉积和变质镁铁质样品表明，LT 底部俯冲到与阿曼其他地方保存的 HT 岩石相似的峰值 P，但经历了约 300°C 的较低峰值 T。Prograde 织物记录的 Si-in-phengite 和闪石化学性质与绿帘石-角闪岩相中 7-10 kbar 的峰值 PT 和 450-550°C 一致。在绿片岩相条件下，逆行织物记录了从近乎普遍的韧性到局部脆性应变的转变。钛矿 U-Pb 年龄 (n  = 2）将峰值 LT 唯一俯冲的时间限制在 ~91 Ma（约会后初始 HT 唯一俯冲 ~12-13 Myr）和动态倒退到 ~90 Ma。结合现有的地球/热年代学，我们的结果支持了一个长期俯冲和吸积模型，而婴儿俯冲带经历了多阶段、慢-快-慢冷却。HT 单一冷却（再水化？）和蛇绿岩形成的时间重叠表明冷却可能导致界面弱化，促进上板延伸和扩散。LT 鞋底在蛇绿岩形成后快速冷却的前弧中形成，可能反映了向自我维持俯冲的过渡。