Two-stage exhumation of high-pressure rocks by corner flow and mud volcanism within an active subduction zone – A case study from serpentinite mud volcanoes along the Mariana convergent margin

During International Ocean Discovery Program (IODP) Expedition 366 cores were recovered from three serpentinite mud volcanoes containing clasts that originate from the subduction-channel along the Philippine Sea Plate – Pacific Plate boundary. The drilled and sampled mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoru) are located at distances of 55 to 72 km from the Mariana Trench. Mafic rock clasts, embedded within a serpentinite mud matrix, from the flanks and summits of both Asùt Tesoru and Fantangisña Seamounts were analyzed for reconstruction of their metamorphic and deformational overprint in order to reveal the tectono-metamorphic conditions at the metamorphic peak within the subduction channel and the subsequent low-grade overprint during exhumation. Several seamounts comprise clasts of lower plate metabasites with different metamorphic overprint (from low-grade sub-greenschist facies to lower blueschist facies). The metabasites are also associated with clasts of fossiliferous carbonates and cherts with different degrees of metamorphic and deformational overprint, that also originated from the Pacific lower Plate. This implies that these rocks were exhumed from different depths within the subduction channel before being regurgitated within a serpentinite mud matrix. The blueschist facies metamorphic rocks, being affected by metamorphic pressures in the range of 11 to 13.8 kbar at minimum, were very likely exhumed from greater depth within the subduction channel before being captured by uprising, localized serpentinite mudflows, indicating evidence that corner flow is actually taking place along the Mariana convergent margin, and that the high-pressure rocks were exhumed by corner flow in an active subduction zone. Final exhumation, however, is related to the embedding of the rocks within a serpentinite mud matrix and the buoyant ascent of serpentinite mudflows along forearc fracture zones extending from the plate boundary to the upper plate sea floor.