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 εNd 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.
Published Version
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