Enhanced and Asymmetric Melting Beneath the Southern Mariana Back-Arc Spreading Center Under the Influence of Pacific Plate Subduction

The back-arc spreading at the southern Mariana is categorized as slow, but surface morphological and geophysical features of the spreading centers suggest that the spreading process is associated with enhanced melting in the upper mantle, due to water being derived from the subducted Pacific slab. A marine magnetotelluric experiment was performed along a transect across a segment at 13 degrees N to reveal the key processes of melting, dehydration, and dynamics in the upper mantle, and their relationships to the surface characteristics. Our inversion model of electrical resistivity shows (a) a conductive body at 10-20 km depth beneath the spreading axis, and (b) another conductive area expanding asymmetrically under and around the conductive body. Away from the spreading center, there is (c) a resistive area thickening up to similar to 40 km on the remnant arc side, and (d) another resistive area with a constant thickness of similar to 150 km on the trench side. Implications of these model features are (a) a melt body beneath the spreading axis; (b) a hydrous mantle above the subducted slab and asymmetric passive decompression melting in the mantle wedge; (c) a residual mantle off from the spreading axis; and (d) a cold mantle wedge tip and the subducted Pacific mantle. The structure markedly contrasts with that in the central Mariana Trough at 18 degrees N, suggesting that the horizontal distance between the location of the spreading center and the root of the buoyant upwelling above the subducted slab is a key parameter that controls the mantle dynamics beneath the back-arc spreading.