| Summary: | 博士 === 臺灣大學 === 地質科學研究所 === 98 === The termination of the Ryukyu trench against Eurasia and the oblique subduction of the Philippine Sea Plate create a subduction-collision complex offshore Taiwan, which has not previously been elucidated in detail. We combine traveltime data from the seismic networks in Taiwan and Japan to better illuminate how the subducting Ryukyu slab deforms in this subduction-collision zone. More than 5000 events recorded by both networks were relocated with the double-difference method using an optimal regional 1-D velocity model. The offshore seismicity indicates that the double seismic zone, with a gap of 15-20 km, exists in the subducting slab in the depth range of 40-80 km. Focal mechanisms suggest that the double seismic zone is caused by east-west compression resulting from oblique convergence.
The improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the Ryukyu trench in the depth range 50-100 km. The anomalous curvature, together with the focal mechanisms, suggests that the slab folds against the Eurasian lithosphere and that this deformation cannot be fully elastic. We model this deformation mode as the developing instability of a viscoelastic Maxwell layer embedded in a viscous medium. The characteristic wavelength of the instability, i.e., ~250 km, is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along-strike elastic membrane strain as small as 0.01, or more than 3 orders of magnitude higher if 5% elastic strain is allowed.
Three dimensional tomographic images of the subducting Philippine Sea slab and the forearc 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 VP, VS, and VP/VS. Analysis of the tradeoff between the data variance-reduction and model-variance helps to determine the appropriate strictness for regularization to avoid either over- or under-interpretation of data. The regularization parameters were also chosen to ensure suppression of artificial VP/VS anomalies. The subducting slab is characterized by high VP, high VS, and intermediate to low VP/VS. Notable in the mantle wedge is the high VP/VS anomalies that abut the surface of the subducting slab at depths of 30-80 km. The previously identified positive VP/VS channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study.
We convert VS and VP/VS to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200-400oC relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches ~15%, or 2% water content, at 50 km depth in the forearc 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.
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