Crustal and upper mantle structure beneath Abaga area in Inner Mongolia revealed by Rayleigh-wave phase velocity tomography

There are strong interactions between the crust and the mantle in the Abaga area. To study the structure of the crust and upper mantle in that area, the fundamental-mode Rayleigh-wave phase velocity dispersion along 3,331 inter-station paths were estimated by the continuous wavelet transformation method. The dispersion data were used to construct the first 2-D phase velocity maps for waves with periods of 12–80 s and a horizontal resolution of 0.5°×0.5° by applying a linear inversion. The results show that the short-period phase velocity distributions (12–20 s) are affected by exposed Cenozoic volcanoes and the thickness of the sedimentary layer. Phase velocity maps for waves with periods of 30–40 s indicate that the direction of the Solonker suture zone is in good agreement with the extension of the high-speed anomalous connection in the study area. It is concluded that the Solonker suture zone extends throughout the entire lithosphere. For periods of 30–80 s, stable low-velocity anomalies were observed near the Holocene volcanoes in the northeast and south of the study area, suggesting that the low-velocity anomaly is related to volcanic activity. Moreover, the two low-velocity anomalies may be connected, suggesting that the Darigan Volcano may have the same thermal source as the Honggeertu Volcano and even the Datong Volcano. And the thermal source of the two low-velocity anomalies is speculated to be upwelling of hot mantle material. In addition, the velocities in the crust and upper mantle are lower than those from the AK135 model and are close to the velocity for the destroyed North China Craton, and very similar to that for the East African Rift Valley. This may indicate that the lithospheric structure in the Abaga area is active.