Sn velocity tomography beneath the Himalayan collision zone and surrounding regions
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 11 June 2012
Accepted: 25 December 2012
Published: 23 August 2013
We present a tomographic Sn velocity model of the uppermost mantle beneath the Himalayan collision zone and surrounding regions. A total of 43,905 Sn phases are used in the investigation. The average Sn velocity in the study area is approximately 4.6 km/s, and the velocity perturbations reach 0.2 km/s. The Sn velocity distribution is consistent with Pn tomography results obtained previously. High velocities are found under the Indian plate, the Tarim Basin, and the Sichuan Basin, whereas low Sn velocities are found beneath the Myanmar region, the Hindu-Kush region, and the Lhasa block and western Qiangtang block. These results support the idea that the lithosphere of the Indian plate is subducted into the mantle and causes the upwelling of hot material. The east-west variability of the Sn velocity beneath the Indian plate and southern Tibet indicates that the underthrusting of the Indian continental lithosphere may be in a piecewise manner. The differences between the thermal structure of the crust and upper mantle in southern Tibet suggest that this region may be represented by a tectonic model of hot crust and cold mantle, supporting the idea that crustal material flow may occur in this region.
The Sn velocity at the uppermost mantle is significantly influenced by the temperature, pressure, and material composition. The effect of temperature change is more pronounced on the S wave velocity than on the P wave velocity. In high-temperature regions where partial melting is present, the S wave velocity decreases more significantly than the P wave velocity (Nolet and Ziehuis, 1994; Goes et al., 2000). Therefore, the Sn velocity distribution can provide more evidence of the thermal situation and plate movement characteristics of the study region. Previous studies using waveform records found Sn wave propagation attenuation under the Tibetan plateau (Ni and Barazangi, 1983; Rapine et al., 1997). However, these previous Sn wave studies were influenced by the amount of data, and there are fewer high-resolution inversion results. In the current work, we present an Sn velocity tomographic model using a large number of Sn travel-time data obtained from the International Seismological Centre, the China Earthquake Data Center, and the Annual Bulletin of Chinese Earthquakes. This Sn velocity model provides more insight into the plate tectonic features of the study region.
2. Data and Method
3. Checkerboard Test
4. Results and Discussion
The inversion result shows significant Sn velocity variations around the Himalayan collision zone, the eastern and western Himalayan syntaxis, and surrounding regions. Low Sn velocities are found in the Myanmar region. Considering the high surface heat flow in this region, we infer that these low-velocity anomalies are associated with the high temperatures or partial melting at the uppermost mantle (Hearn and Ni, 1994; Hu et al., 2001). The hot material backarc upwelling may come from the lithosphere of the Indian plate which subducted into the upper mantle or deeper part. A similar situation is found in the Hindu-Kush region, in which the Indian plate subducts to the mantle of this area from the southeast (Zang et al., 1992). The low Sn velocity in this area provides more evidence of the subduction.
The collision between the Indian and Eurasian plates in the Himalayan region is widely accepted, but there are still many different opinions regarding the mechanism of the collision and the subduction. In the inversion result, the northward high-velocity area of the Indian plate roughly reaches the northern part of the IYS, and there are lowSn-velocity zones in the Lhasa and Qiangtang blocks. The low-velocity anomaly in the western Qiangtang block was also obtained by previous studies, but the velocity distribution of the Lhasa block is different (Hearn et al., 2004; Pei et al., 2007). The ray coverage of the dataset of the previous study is not good and the resolution is low. Better ray path coverage and spatial resolution suggest that our result is plausible. The INDEPTH-III study proposed that the Indian lithosphere stretches northward, and that the lower lithosphere subducts into the mantle under 32°N by a larger angle (Zhao et al., 2004). The numerical calculation results of the two-dimensional thermal structure of the lithosphere show that the temperature of the Moho in this area is high, and that the lithosphere reaches the temperature of partial melting (Zhou, 2000). The INDEPTH-IV study interpreted either a piece of Lhasa Terrane or remnant oceanic slab un-derthrust below northern Tibet (Yue et al., 2012). Our inversion results support the idea that the uppermost mantle of the Lhasa and western Qiangtang blocks has high temperatures or partial melting, which causes the low Sn velocity. This phenomenon may be related to the hot material up-welling caused by the subduction of the Indian lithosphere. This result can explain the serious attenuation of the S wave in the Qiangtang block (Ni and Barazangi, 1983; Rapine et al., 1997). It is also consistent with L g attenuation, low P wave velocity, low Q value, low resistivity, and other geophysical observations in this region (McNamara et al., 1995; Wittlinger et al., 1996; Fan and Lay, 2002; Unsworth et al., 2005). The east-west variability of the Sn velocity beneath southern Tibet indicates that the underthrusted Indian continental lithosphere is not a homogeneous body but rather in a piecewise manner. This conjecture is consistent with those of other researchers (Lü et al., 2011; Liang et al., 2012). A high Sn velocity is found in the Tarim and Sichuan Basins, which have a cold lithosphere and stable tectonics. A significantly low Sn velocity is found at the Songpan-Ganzi orogenic belt. This finding is consistent with the results of P-wave inversion in previous studies (Liang and Song, 2006; Liang et al., 2011). At the edge of the present study area, the inversion resolution is limited by sparse ray coverage and the results are not discussed.
A comparison of the Sn velocity distribution with previously-obtained Pn tomography results (Lü et al., 2011) reveals that the high- and low-velocity anomaly areas of the Sn and Pn waves are consistent. Tectonically stable areas such as the Tarim Basin, the Sichuan Basin, and the Indian plate, have high Pn and Sn velocities. The Hindu-Kush and Myanmar regions, as well as the Songpan-Ganzi orogenic belt, have low Pn and Sn velocities. Given that two independent data sets of the inversion of the Pn and Sn travel-time data yield similar results which coincide with each other, the credibility of the inversion results is confirmed. In regions with low Pn velocities, a low Sn velocity is more obvious, showing that the shear wave velocity is more sensitive to the influence of temperature and pressure conditions. The Sn model of the China region obtained by Pei et al. (2007) showed low velocity anomalies at Yunnan, SGFB, and the western Qiangtang block, and a high velocity anomaly at Sichuan Basin which are similar to our model. The velocity anomaly at the Lhasa block, and the high velocity of India and low velocity of the Hindu-Kush region of our model, were not obtained by the previous study.
There is only minimal heat flow data observed in the Tibetan Plateau region. In the compilation of heat flow data in the China continental area (Wang and Huang, 1990), the measured value of the terrestrial heat flow within the vicinity of the Tibet 90°E area is larger than the average value in China. However, the Sn velocity of this area is high. We infer that this area can be represented with a tectonic model of a hot crust and a cold mantle. The surface heat flow may not come from the mantle but from the crust. The difference between the thermal tectonics of the crust and upper mantle suggests a potential decoupling between the crust and upper mantle. It also supports the idea that crustal material flow may occur in this region (Bai et al., 2010; Zhang et al., 2010b).
We imaged Sn velocity variations beneath the Himalayan collision zone and the surrounding regions using seismic travel-time data. The average Sn velocity in the study area is 4.6 km/s. The maximum velocity perturbation is approximately 0.2 km/s. High-velocity structures are found under the Indian plate, Tarim Basin, and Sichuan Basin, whereas low Sn velocities are found beneath the Myanmar region, the Hindu Kush region, and the Lhasa block and Qiang-tang block. A comparison of the Sn velocity distribution with previously obtained Pn tomography results reveals that the high- and low-velocity anomaly areas of the Sn and Pn waves are consistent. The low Sn velocity anomalies are more obvious than the Pn ones. Our results support the idea that the lithosphere of the Indian plate subducted into the mantle and caused hot material upwelling. The low-velocity anomalies are due to the high temperature or partial melting at the uppermost mantle. The high Sn velocity anomalies of the Indian Plate and the low anomalies in the Tibetan Plateau are discontinuous in the east-west direction, indicating that the Indian plate probably subducts in a piecewise manner. The difference between the thermal tectonics of the crust and upper mantle in southern Tibet supports the idea that crustal material flow may occur in this region. The Sn velocity tomography result provides a seismological basis for the study of plate tectonics and geo-dynamic processes in this region.
We thank Professor Thomas Hearn for providing the original tomographic codes. We appreciate the insightful suggestions by two reviewers and the associate editor. All figures were prepared using the Generic Mapping Tolls (Wessel and Smith, 1998). This study was supported by the China Postdoctoral Science Foundation 2012M510043 and 2013T60166, the National Nature Science Foundation of China (Grant Nos. 41074031, 40940021, 41174036), the China Earthquake Program 200808078 and the CAS/SAFEA International Partnership Program for Creative Research Teams (Grant No. KZZD-EW-TZ-05).
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