11:00 〜 11:15
[SCG45-02] Marine geophysical observations and geodynamic modeling study in the vicinity of Hawaii-Emperor Bend area to constrain the viscosity structure of the oceanic asthenosphere
★Invited Papers
キーワード：海底マグネトテルリック観測、海底地震観測、ダイナミクスモデリング、ハワイ-天皇海山列屈曲、海洋アセノスフェア、粘性率
In a plate tectonics, lithosphere can move laterally because of existence of underlying soft asthenosphere. The estimates for the softness (viscosity) of the asthenosphere differ by an order of magnitude despite more than 30 years of research. We launched a Japan-Germany collaborative research project to constrain the viscosity structure of the oceanic asthenosphere through passive electromagnetic (EM) and seismological observations on the seafloor and geodynamic modeling studies. Our study stands on a new viewpoint focusing on interaction between mantle upwelling (plume) and lithosphere and bending geometry of Hawaii-Emperor volcanic chain that was produced by Hawaiian plume.
A plume material moves upward in the mantle. Once it reaches to the base of the lithosphere, it is dragged laterally according to the lithospheric motion. The velocity of the plume material is slower than the lithosphere and the velocity difference is dependent on the viscosity contrast between the lithosphere and asthenosphere. Because the Emperor volcanic chain is largely oblique to the current Pacific plate motion, which is parallel to the Hawaiian volcanic chain, the plume material would locate in the west of the Emperor volcanic chain depending on the viscosity structure (Fig. 1). Since the plume material should be hotter and may contain more volatiles such as H2O and CO2 than the ambient mantle material, we expect that the plume material can be image as an anomalous structure in terms of electrical conductivity and seismic velocity from EM and seismic observations.
We have carried out geodynamic simulations that reconstruct mantle flow in the central Pacific area since 100 Ma, including the Hawaii plume –a hot upwelling from deep in the mantle, the plate motion change at 47 Ma, and global mantle flow. The simulations show that a hot anomaly representing the plume material distributes in the asthenosphere beneath east of the Emperor chain and its horizontal location is dependent on the assumed viscosity profile, which does or does not have low viscosity layer in the asthenosphere (Fig. 1). The two models predict weak or strong lithosphere-asthenosphere coupling scenarios, respectively. We further examined if the hot anomalies can be imaged as anomalies in electrical conductivity or seismic velocity and if the two scenarios can be distinguishable from the images. We synthesized magnetotelluric (MT) responses and seismic shear wave phase velocities at 50 sites over the western edge of the anomalies and inverted them. The results encourage us to put the marine EM and seismological observations into practice.
Figure 1. (a) Bathymetric map of the study area and planned observation array (red circles). Orange and yellow shades indicate the high temperature anomalies at 220 km depth predicted by the geodynamic modeling assuming the two different viscosity structure models shown in (b).
A plume material moves upward in the mantle. Once it reaches to the base of the lithosphere, it is dragged laterally according to the lithospheric motion. The velocity of the plume material is slower than the lithosphere and the velocity difference is dependent on the viscosity contrast between the lithosphere and asthenosphere. Because the Emperor volcanic chain is largely oblique to the current Pacific plate motion, which is parallel to the Hawaiian volcanic chain, the plume material would locate in the west of the Emperor volcanic chain depending on the viscosity structure (Fig. 1). Since the plume material should be hotter and may contain more volatiles such as H2O and CO2 than the ambient mantle material, we expect that the plume material can be image as an anomalous structure in terms of electrical conductivity and seismic velocity from EM and seismic observations.
We have carried out geodynamic simulations that reconstruct mantle flow in the central Pacific area since 100 Ma, including the Hawaii plume –a hot upwelling from deep in the mantle, the plate motion change at 47 Ma, and global mantle flow. The simulations show that a hot anomaly representing the plume material distributes in the asthenosphere beneath east of the Emperor chain and its horizontal location is dependent on the assumed viscosity profile, which does or does not have low viscosity layer in the asthenosphere (Fig. 1). The two models predict weak or strong lithosphere-asthenosphere coupling scenarios, respectively. We further examined if the hot anomalies can be imaged as anomalies in electrical conductivity or seismic velocity and if the two scenarios can be distinguishable from the images. We synthesized magnetotelluric (MT) responses and seismic shear wave phase velocities at 50 sites over the western edge of the anomalies and inverted them. The results encourage us to put the marine EM and seismological observations into practice.
Figure 1. (a) Bathymetric map of the study area and planned observation array (red circles). Orange and yellow shades indicate the high temperature anomalies at 220 km depth predicted by the geodynamic modeling assuming the two different viscosity structure models shown in (b).