14:45 〜 15:00
[SIT19-05] Constraining the Upper Mantle Seismic Anisotropy beneath the Oldest Pacific Seafloor from Shear-wave Splitting
キーワード：seismic anisotropy, shear-wave splitting, Western Pacific Plate
Seismic anisotropy in the upper mantle is a globally observable phenomenon that provides insights into current and fossil geodynamic processes. Measuring teleseismic shear-wave splitting (SWS) is an effective tool for detecting the seismic anisotropy, represented by fast polarization directions (FPDs) and delay times (DTs). This study investigated upper-mantle anisotropy beneath the oldest region of the western Pacific Plate (160–180 Ma), known as the Pacific Triangle, situated around 900 km east of the Mariana Trench, by utilizing the SWS. The 143 teleseismic core phases (XKS and ScS) and direct S phases recorded by 11 broadband ocean-bottom seismometers from the Oldest-1 array during 2018–2019 were processed at 0.05–0.125 Hz to estimate the two parameters. Our results are compared with the P-wave velocity structure by Kang et al. (2023) at a depth range of 95–350 km; the findings from both complementary studies allow for a first-order understanding of the mantle flow in the oldest Pacific.
With respect to both the N73°W absolute plate motion (APM) and the N40°W paleo-spreading direction, a high degree of scatter in FPDs is observed at most stations, indicating the presence of a complex mantle structure. The variability in the FPDs likely reflects a complex interaction between asthenospheric mantle flow and lithospheric imprints from past deformation, including multiple reheating events and changes in mantle flow directions. In particular, our relatively consistent NNW-SSE FPDs in the western and northwestern regions differ from the two orientations of azimuthal anisotropy observed at depths from the oceanic Moho to 50 km, which exhibit WNW-ESE and NE-SW directions in the western and eastern regions with respect to the Magellan Seamount Trail (MST), respectively (Kawano et al., 2023). This suggests that our SWS results for the western and northwestern regions mainly represent deeper mantle anisotropy. The observed FPDs might be primarily influenced by the subduction-driven mantle flow due to their close proximity to the trench. On the other hand, in the central region, our FPDs exhibit rotational patterns that coincide with a high-velocity anomaly at observed at depths ranging from 95 km to 185 km. This rotational behavior might indicate asthenospheric mantle flow bypassing the high-velocity roots. Alongside the FPD results, a higher number of null (non-split) measurements is observed, particularly at two stations located on the MST and adjacent to the ancient triple junction. These null results correspond to pronounced slow-velocity zones at depths of 100–300 km, suggesting possible mantle upwelling in these areas.
With respect to both the N73°W absolute plate motion (APM) and the N40°W paleo-spreading direction, a high degree of scatter in FPDs is observed at most stations, indicating the presence of a complex mantle structure. The variability in the FPDs likely reflects a complex interaction between asthenospheric mantle flow and lithospheric imprints from past deformation, including multiple reheating events and changes in mantle flow directions. In particular, our relatively consistent NNW-SSE FPDs in the western and northwestern regions differ from the two orientations of azimuthal anisotropy observed at depths from the oceanic Moho to 50 km, which exhibit WNW-ESE and NE-SW directions in the western and eastern regions with respect to the Magellan Seamount Trail (MST), respectively (Kawano et al., 2023). This suggests that our SWS results for the western and northwestern regions mainly represent deeper mantle anisotropy. The observed FPDs might be primarily influenced by the subduction-driven mantle flow due to their close proximity to the trench. On the other hand, in the central region, our FPDs exhibit rotational patterns that coincide with a high-velocity anomaly at observed at depths ranging from 95 km to 185 km. This rotational behavior might indicate asthenospheric mantle flow bypassing the high-velocity roots. Alongside the FPD results, a higher number of null (non-split) measurements is observed, particularly at two stations located on the MST and adjacent to the ancient triple junction. These null results correspond to pronounced slow-velocity zones at depths of 100–300 km, suggesting possible mantle upwelling in these areas.