Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism

We present the first detailed 2D seismic tomographic image of the trench‐outer rise, fore‐ and back‐arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo‐Australian plate where ∼80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending‐related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ∼7.3 km/s, ∼10% lower than typical for mantle peridotite (∼30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc‐type igneous basement with uppermost and lowermost crustal velocities of ∼3.5 and ∼7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0–5.5 km/s) suggests that the outer fore‐arc is probably fluid‐saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore‐arc. Extension in the outer fore‐arc is evidenced by (1) trenchward‐dipping normal faults and (2) the presence of a giant scarp (∼2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore‐arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ∼7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.