Deep Seismic Structure Across the Southernmost Mariana Trench: Implications for Arc Rifting and Plate Hydration.

The southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central‐north Mariana and Izu‐Bonin margins. This paper presents a new P wave velocity model of the crust and uppermost mantle structure based on a 349‐km‐long profile of wide‐angle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0 ± 0.2 km/s at the base of the crust and low values of only 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5–7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift that is undergoing active rifting. The observed significant velocity reduction in a near‐trench crustal zone of ~20–30 km in the subducting plate is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults. Plain Language Summary: The southernmost Mariana margin is a convergence margin that lacks a mature island arc. The trench reaches the deepest point on Earth at the Challenger Deep, the origin of which is still little known. We acquired and analyzed new active source wide‐angle reflection/refraction seismic data along a profile across the Challenge Deep, obtaining a new model of crustal and uppermost mantle velocity structure of both the subducting Pacific and overriding Philippine Sea plates. Significant velocity reduction is observed in the crust of the subducting plate, especially in a 20‐ to 30‐km‐wide zone near the trench axis, which is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. A zone of slight velocity reduction is imaged beneath the tectonically active Southwest Mariana Rift. Significant velocity reduction is also observed in the uppermost mantle of both subducting and overriding plates and is interpreted as a result of mantle serpentinization. Key Points: We conducted an active‐source seismic experiment across the Challenger Deep, revealing a new velocity model of the southernmost Mariana marginReduction in crustal velocity was observed in the subducting plate, especially in a near‐trench zone, possibly due to porosity changes and fluids; Relatively low upper mantle velocities were interpreted as mantle serpentinizationA low‐velocity zone of ~15‐ to 20‐km‐wide was imaged beneath the Southwest Mariana Rift, interpreted as a weak zone of initial rifting [ABSTRACT FROM AUTHOR]