Abstract Stratigraphic and structural constraints on the initiation and early evolution of the Cascadia convergent margin, following accretion of the igneous Siletzia terrane at 50–45 Ma, remain elusive. This study applies a novel approach based on the combination of Nd, Sr, C and O isotope analyses of the oldest-known methane-seep carbonates (Humptulips Formation, middle Eocene, 42.5–40.5 Ma) of Cascadia to provide insight into its early hydrogeological regime and incipient subduction history. Both anomalously high eNd and low 87Sr/86Sr signatures of the studied carbonates attest to interactions between the seeping fluids and mafic, igneous components of the forearc basement. Moderately negative δ13Ccarbonate values imply a predominantly thermogenic origin of hydrocarbons at three out of four studied seeps, with likely an important contribution of biogenic methane at a site that was closer to the ancient shoreline. When combined with structural constraints, the recorded signals point to discharge of fluids originating mostly from deep portions of the young subduction wedge underthrust beneath the accreted Siletzia terrane, and their channelled ascent through its thick volcanic succession. These results document the presence of a fluid generation and migration system typical of active convergent margins prior to maturation of typical, calc-alkaline arc magmatism in the Cascades at 40 Ma. The strongly pronounced involvement of the exotic, 143Nd-enriched and 87Sr-depleted fluids reflects the distinctive structural architecture of the newly reorganised subduction zone. Of particular importance was the combination of a relatively thin sedimentary cover of the infant forearc, its extensional tectonics, and the proximity of the young trench to the volcanic backstop that the deep, subducting plate-derived fluids must have interacted with before their seepage at the seafloor.

中文翻译：

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中始新世甲烷渗漏碳酸盐的 Nd、Sr 和稳定同位素组成揭示的 Cascadia 会聚边缘的流体排出系统和构造结构

摘要 在 50-45 Ma 的火成岩 Siletzia 地体增生之后，Cascadia 收敛边缘的起始和早期演化的地层和结构限制仍然难以捉摸。本研究应用了一种基于 Nd、Sr、C 和 O 同位素分析组合的新方法，对 Cascadia 已知最古老的甲烷渗出碳酸盐（Humptulips 组，中始新世，42.5-40.5 Ma）提供洞察力，以深入了解其早期水文地质制度和初期俯冲历史。所研究碳酸盐的异常高 eNd 和低 87Sr/86Sr 特征都证明了渗出流体与弧前基底的镁铁质、火成岩成分之间的相互作用。中度负的 δ13Ccarbonate 值意味着在四分之三的研究渗漏处主要是碳氢化合物的热成因，在更靠近古代海岸线的地点，生物甲烷可能有重要贡献。当与结构限制相结合时，记录的信号表明流体的排放主要来自于增生的 Siletzia 地体下方的年轻俯冲楔的深部，并通过其厚厚的火山序列引导上升。这些结果证明，在 40 Ma 的 Cascades 中典型的钙碱性弧岩浆作用成熟之前，存在典型的活动会聚边缘的流体生成和迁移系统。外来的、富含 143Nd 和缺乏 87Sr 的流体的强烈参与反映了新重组的俯冲带独特的结构结构。