Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore-arc mantle

Three-dimensional tomographic images of the subducting Philippine Sea slab and the fore-arc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in V-P, V-S, and V-P/V-S. Analysis of the trade-off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data. The regularization parameters were also chosen to ensure suppression of artificial V-P/V-S anomalies. The subducting slab is characterized by high V-P, high V-S, and intermediate to low V-P/V-S. Notable in the mantle wedge is the high V-P/V-S anomalies that abut the surface of the subducting slab at depths of 30-80 km. The previously identified positive V-P/V-S channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert V-S and V-P/V-S to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200 degrees C-400 degrees C relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches similar to 15%, or 2% water content, at 50 km depth in the fore-arc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt-eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea Plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.