A new deep seismic structure across the southernmost Mariana Trench: Implications for arc magmatism, initial arc rifting and plate hydration

A new model of crustal velocity structure across the Challenger Deep and Mariana arc system is presented based on active source wide-angle reflection and refraction seismic profiling in the southernmost Mariana Trench. We find that the subducting Pacific plate has an average crustal thickness of 6.0 km with a P-wave velocity of 7.0±0.2 km/s. The uppermost mantle of the subducting Pacific plate is characterized by extremely low velocities (∼7.3 km/s). The overriding Philippine Sea plate shows a maximum crustal thickness of approximately 18 km beneath the current Mariana arc, including a relatively thin (2-4 km) middle crust with a Vp of 6.0-6.5 km/s, and tapers toward flanks. The underlying lower crust with a Vp of 6.5-7.4 km/s consists of two layers with a Vp of 6.5-7.0 km/s in the upper part and a Vp of 7.0-7.4 km/s in the lower part. In the current Mariana arc, the absence of conduit-like features and earthquakes in the crust suggests moderate or dwindling magmatism in the southernmost Mariana. A marked velocity reduction through the crust is imaged between the West Mariana Ridge (rigid) and the Mariana arc (partial melting). We propose that the reduction was the result of a weak zone between the rigid West Mariana Ridge and partial melting Mariana arc blocks subjected to compressive stresses and created a fracture zone with significantly low Vp. The area has undergone much less opening and is in a tectonic rifting stage. Approaching the trench, the velocity reduction is interpreted in terms of bending-related faulting and hydration in the crust and uppermost mantle. In the outer forearc, the velocity reduction increases slightly, which may also be related to sediment input and an increase in water content.

Key points:

- A velocity model of the southernmost Mariana margin across the Challenger Deep and Mariana arc is presented.

- The SWMR shows a low-velocity zone and the initial rupture of the SWMR may be ascribed to compressive stresses.

- The velocity reduction approaching the trench may be related to bending-related faulting and fault-controlled hydration in the crust and uppermost mantle.