Pull-down basin in the central part of Japan due to subduction-induced mantle flow |
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| Authors: | Takeshi Kudo Koshun Yamaoka |
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| Institution: | a Tono Geoscience Center, Japan Nuclear Cycle Development Institute, 1-63, Yamanouchi, Akeyo, Mizunami, 509-6132, Japan;b Research Center for Seismology and Volcanology, Graduate School of Environmental Studies, Nagoya University, Chikusa, Nagoya, 464-8602, Japan |
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| Abstract: | The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone.A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes. |
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| Keywords: | Lake Biwa Philippine Sea Slab Gravity anomaly Isostasy Mantle flow Numerical simulation |
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