STRUCTURE OF THE CRUST AND UPPER MANTLE PATTERN AND VELOCITY DISTRIBUTIONAL CHARACTERISTICS IN THE NORTHERN HIMALAYAN MOUNTAIN REGION

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1 J. Phys. Earth, 33, , 1985 STRUCTURE OF THE CRUST AND UPPER MANTLE PATTERN AND VELOCITY DISTRIBUTIONAL CHARACTERISTICS IN THE NORTHERN HIMALAYAN MOUNTAIN REGION Ji-wen TENG,* Shao-bai XIONG,* Zhou-xun YIN,* Zhong-xin XU,** Xiang-jing WANG,** and De-yuan LU*** *Institute of Geophysics, Academia Sinica, Beijing, China **Changchun Geological Institute of China, Beijing, China ***Geological Academia Sinica of Geological Ministry of China, Beijing, China (Received October 19, 1983; Revised December 18, 1984) In order to study the layered structure and characteristics of the velocity distributions in the crust and upper mantle of the northern part of the Himalayas, we have made detonations in Puma Lake, Peiku Lake, and the Dinggye region. Four seismic record sections were obtained along a 475 km long profile in a nearly E-W direction from Puma Lake to Peiku Lake. According to data processing and inversion, 6 groups, t1, t2, t3, t4, t5, t6 of reflected phases through the crust and upper mantle in the area are obtained. They show different kinematic and dynamic properties. The results of data analysis are as follows: 1. The crust is multilayered and there exists a low velocity layer in the crust. The thickness of the low velocity layer is a few kilometers, and with the layer velocity km/s. This indicates that the cause of geothermal distribution and its activity in the Xizang plateau is due to the high temperature in the crustal medium and the existence of melting or partial melting matter in the crust. 2. Structure and velocity of thick crust are horizontally inhomogeneous. The crustal thickness from the north of the Himalayas is km and its velocity, km/s. Crustal deformation is very strong in the Tethys Himalaya region. 3. On the basis of results from ray tracing, theoretical seismogram, and phases of reflection waves, a preliminary model of crust and upper mantle in the northern part of Himalayas is put forward. The extremely thick crust was caused by the results of the collision of the Indian plate against the Eurasian plate, and during the process of continuous pressing the horizontal shortening took place on a large scale in the crust. 1. Introduction The Himalayan ranges and Xizang plateau are regions of great interest to geologists and geophysicists at home and abroad. A great number of people think that this region is the key to clarifying the history of tectonic movement.

2 158 J. W. TENG et al. Comprehensive studies of various geological and geophysical data have shown that the formation of Xizang plateau is due to the collision and compression between the Indian plate and the Eurasian plate which led to the short-contraction of the crust and a large scale south wad push of overlying strata (TENG et al., 1980; INSTITUTE OF GEOPHYSICS, ACADEMIA SINICA, 1981). In 1977, the scientific survey team of Qinghai-Xizang plateau from the Academy of Science used explosion seismic methods for the first time to study the crustal structure from Damxung to Yadong and obtained very good results (INSTITUTE OF GEOPHYSICS, ACADEMIA SINICA, 1981). In order to further clarify the crustal structure in the Himalayan region, a second artificial explosion seismic study was made by French and Chinese seismologists at a region from Peiku Lake in the west to Puma Lake in the east and from Rigaze southward to Nyalam region of southern Tibet Xizang during September and October of The geological Tectonic Background of the Observed Regions Based on the division of China's continental tectonic map (HUANG and REN, 1980) the region of the south Yarlung Zangbo River belongs to the Himalayan folding system which is part of the Mesozoic and Cenozoic geosynclinal zone of southern Tethys. As shown in Fig. 1, Peiku Lake-Dinggye-Puma Lake profile which is on the main profile is located at the north zone of high Himalayan tectonic zone. From the tectonic section (Fig. 2) crossing the Himalayan ranges we can see that it is right at the north wing of large Himalayan inverted anticlinorium, Fig. 1. Map showing the tectonic regions and suture zones within the Qinghai-Xizang plateau.

3 The Crust and Upper Mantle Pattern and Velocity 159 Fig. 2. The geological tectonic section of Himalayas. Fig. 3. Locations of shotpoints and stations at northern region of Himalayan Mountains and a set of marine deposit strata at upper Palaeozoic-Mesozoic period had developed from down below to the upper part (CHANG and ZHENG, 1973). From the history of geological development it can be seen that this region began to fold during the early Himalaya period and the episode of Himalayan movement at midterm of Miocene was a most violently tectonic movement which led to the formation of tectonic on a large scale. From the plate tectonic it can be seen that the main profile was located within the transitional belt of the collision and compression between Indian plate and Eurasian plate (TENG et al., 1980). The geological structure is very complicated at the north part of the supplementary profile (Fig. 3) which cuts across the north Himalayan geosyclinal folding zone and passes through the transitional belt. There are numerous rifts in this region and the great longitudinal inverse

4 160J. W. TENG et al. Table 1. Data for each artificial seismic explosion at Xizang of 1981.

5 The Crust and Upper Mantle Pattern and Velocity161 thrust faults along the strikes of tectonic line are mostly boundary lines of the tectonic zone: the great deep rift zone of the Yarlung Zangbo River separates the Himalayan folding system from the Lhasa folding system. Moreover, a great many lateral faults are distributed there: Yadong rift zone, Gamba-Kuma fault, Dinggue-Majia fault, the old Dingri fault, as well as Selong-Nyalam fault and so on. They usually control the magma activity and hydrothermal activity of the Table 2. Number of each profile and observed point. Table 3. The receiving section, average interval of receiver point, and average distance of sectorial profile for each shot. When the profile is at west of explosion point, distance is taken as negative value.

6 162 J. W. TENG et al. region (WEI et al., 1981; TONG et al., 1981). Since they are almost vertical to the main profile, seismic wave propagation must be affected. 3. Field Experiments and Observational System Eight dynamite charges were exploded at Peiku Lake, Dinggye, and Puma Lake from September 18 to October 10 of Three components of ground motion were recorded with analog magnetic recorder at 30 stations for each explosion, but we analyze only the vertical component recording in this paper. The data connected explosions are shown in Table 1. Figure 3 is a detailed distributional map of shot points and observational points. The observed points of different profile use different marks. The code name of each profile and observed point is shown in Table 2. The receiving section of each shot point is illustrated in Table 3. Fig. 4. Seismogram section of explosion at Puma Lake (from Puma Lake to Dinggye) (PPM). Fig. 5. Seismogram section of Dinggye-Puma Lake profile (PPDE).

7 The Crust and Upper Mantle Pattern and Velocity Seismic Phase and Effect of Wave Field After A/D conversion the analogue magnetic tape recordings of field stations become digitalized data, then through time correction, digital filtering, amplitude normalization, and other processing they were compiled recording section. Figures 4-7 are the four recording sections obtained from the major profile PP. PPM and PPG represent the recording sections of explosions at Puma Lake and Peiku Lake, respectively, while PPDE and PPDW respectively represent the recording Fig. 6. Seismogram section of Dinggye-Peiku Lake profile (PPDW). Fig. 7. Seismogram section of explosion at Peiku Lake profile (from Peiku Lake to Dinggye) (PPG).

8 164 J. W. TENG et al. sections received at east part and west part of PP profile when explosion took place at Dinggye. The main wave phases can be divided into 6 groups which may be denoted by 4, t2, t3, t4, t5, and 4. Among them, t2, t3, t4, and 4 can be reliably compared at all 4 sections with large range of tracing and good continuity, especially after critical point very strong energy can arrive. The basis for identification of reflection wave is its kinematic and dynamic features. t,: The reflection wave for the basement of sedimentary layer. 4: Although on all of the 4 sections the energy of ty reflection wave is not very strong, it is still very easy to identify. t3: The most important reflection wave group on the upper crust and started from about km, it can be traced to nearly 150 km and 230 km on the section PPDW and PPG profiles; on the east section PPM and PPDE profiles it can be traced to 170 km, even to 190 km. Especially on the PPM and PPDE profile even at short distance of less then 100 km, 4 also has prominent energy to arrive. This shows that it came from a very strong reflection interface. t4: A very distinct reflection from the mid-part of the crust with very strong energy, but usually it is less than the energy of t3 and can be traced over km. At great distances it usually appeared as the first strong signal, thus the correlation is reliable. On the PPG profile the correlation quality is not so good due to the great initial disturbance at the range of km. The correlation for t4 has some difficulty owing to the others. t5 : The energy of t5 is quite weak, and it came from reflection of a certain reflection interface at lower crust. t6 : A reflection wave from the transitional zone or boundary of the crustmantle, that is PM which is the most prominent wave group at deep layers. t6 is characterized as strong, and has a low frequency and usually clear critical point (at km nearby). At the nearby critical point the very distinct initial signal can sometimes be seen (for example recordings at km and 230 km on PPM profile). Figure 8A shows several typical t6 reflection wave groups and Fig. 8B their frequency spectral density curves. It can be seen that their main energy all concentrates on the range of 3-5 Hz, and the energy attenuates sharply along the direction of high frequency. That is to say the Moho reflection wave is almost a single frequency signal with very low frequency and long time of continuity. On all of 4 sections this seismic phase-correlation is reliable with the range of tracing from about 150 km to over 310 km. The refraction wave Pn of the top part of upper mantle is not very clear, and is only diplayed on the recording of PPM section.

9 The Crust and Upper Mantle Pattern and Velocity 165 Fig. 8. A. Several typical Moho reflection wave group. The numbers 768, 708, 489, and 668 on the right represent the codes of station and PP is the name of the major profile. B. The frequency spectral density curves map of several typical Moho reflection wave groups.

10 166 J. W. TENG et al. 5. Basic Characteristics of the Earth's Crust and Upper Mantle Structure and Velocity Distribution 5.1 Surface structure of the crust The surface velocity calculated from direct path wave and t, reflection wave is about 5.6 km/s. The thickness of the surface becomes thinner from east to west gradually, at the east part is 4-6 km, at west Dinggye less than 3 km, but at nearby Peikii Lake it tends to thicken. This layer represents a sedimentary covering layer near the surface. From this it can be seen that the velocity of the sedimentary layer at this region is quite high. Below the sedimentary layer there is a layer with thickness of several km and velocity of 6.0 km/s. The basement boundary is near the depth of km. From the features of velocity it can be seen that this layer is a weakly weathering zone of the top granite layer. On the crustal surface either velocity or thickness has great lateral differences with great relief of interface. This shows that in the geological development history this region has experienced strong action of recent tectonic movement. 5.2 Upper crustal structure and crustal low velocity The velocity of t3 reflection layer is quite stable, about km/s. But the depth of lower boundary of this layer still varies largely, it is 23 km at the east end of the PPM profile, only 17 km on the PPDE profile, and only about 20 km west of Dinggye. Fig. 9. The velocity curves map of t3 reflection wave on profile PPDE.

11 The Crust and Upper Mantle Pattern and Velocity167 To invert the velocity and the depth of the reflector from the travel times of reflected waves, we deploy a method which is based on construct of the velocitydepth map. The details of this method have been discussed by MICHEL and HIRN (1980). Figure 9 represents the velocity depth curves map of t3 reflection wave on PPDE. From the displayed result of the diagram we suggest a low velocity layer about the interface, since there is a distinct "shadow region" between two groups of curves. Fig. 10. Ray tracing map of profile PPDE (Dinggye-Puma Lake profile). Fig. 11. Theoretical seismogram of profile PPDE.

12 168 J. W. TENG et al. Taking PPDE profile as an example, Figs. 10 and 11 are the results obtained from the calculation of ray tracing method and theoretical seismogram; the fitted low velocity layer thickness is 3 km and its velocity 5.7 km/s. Of course it is difficult to accurately determine the structure of low velocity layer (thickness and velocity). What is important is to determine whether there is a low velocity layer or not. The fact that at the crust of the Himalayan region there exists low velocity layer may imply that at the crust there exists melting and partial melting matter. 5.3 Structure of the midpart and lower part of the earth's crust Figure 12 shows t4 reflector is an approximately horizontal interface: at the east Dinggye (PPDE) its depth is 30 km. The velocity of the layer is quite stable, 6.3 km/s. At the east end of the profile (PPM) the interface is quite deep and can extend to 38 km; at the west end of PPDE and PPG profiles it is 33 km. Figure 13 is the velocity-depth map of t5 reflection wave on PPDE, which shows here is also a nearly horizontal interface. So probably below tg reflection surface at the crust, velocity is between km/s. From t5 to to reflection wave groups, the difference of depths of two reflectors amounts to km, but we did not found any other reflection wave of this thick part of the crust. It shows that the lower crust of km thickness is somewhat more homogeneous than the upper crust. Figure 14 is the velocity-depth curve map of t6 reflection wave. It can be Fig. 12. t4 reflection wave velocity-depth curve map of profile PPDE.

13 The Crust and Upper Mantle Pattern and Velocity 169 Fig. 13. Velocity-depth curve map of t5 reflection wave. Fig. 14. Velocity-depth curve map of t6 reflection wave.

14 170 J. W. TENG et al. Fig. 15. The crustal structure models of major profile. seen that t6 surface is not a very good horizontal interface, but it is certain that the depth is about km, and the crustal average velocity is 6.2 km/s or so. Figure 14 shows that the average velocity of lower crust is about 6.4 km/s. As previously mentioned, however, according to t4 and t5 interpretation, the lower crustal velocity should be km/s. From this we infer that at the lower crust there may exist a great velocity inverted region. The travel times and the theoretical seismograms of t6 reflection wave are calculated with such a model fit the observations well (Figs. 10, 11). 5,4 Basic model of crustal structure and velocity distribution Figure 15 is the crustal structure model of each section on the major profile. Looking at the whole profile, the crustal total thickness does not vary greatly from east to west. But it can be seen that at two parts of the major profile the depth of every interface has a remarkable change, with Gangba the approximate Gangba limit. The reflection interfaces of t2, t3, t4, and t6 at the east are deeper than those at the west. Surface geological study results have proved that at this region exists Gamba-Kuma (about 25 km to north from Gamba) cross fault. From the evidence of deep structure it can be suggest that this cross fault probably cuts through the whole enormously thick crust. The above mentioned results show that along the section the crust is immensely thick and it is laterally inhomogeneous. The crust is composed of high and low velocity mediums with thickness of km average velocity of km/s. Inside the crust there exist a low velocity layer, 3-5 km thick, with a Iayer velocity of km/s. At the bottom of the crust, immediately above Moho discontinuity, there may also exist a low velocity layer, 8-10 km thick, with a velocity of km/s.

15 The Crust and Upper Mantle Pattern and Velocity171 The field work presented in this paper has all been done by members of the artifical seismic Seep sounding team from China and France. Processing data, computing work, analysis, and interpretation were done in Beijing and Paris. REFERENCES CHANG Cheng-fa and Hsi-lan ZHENG, Some tectonics features of the Mt. Jolmo-Langma area, southern Tibet, China, Sci. Sin., 16 (2), , HUANG Ji-qing and Ji-shun REN, The Tectonic and Evolution, Scientific Publishing House, Beijing, INSTITUTE OF GEOPHYSIC, ACADEMIA SINICA, Explosion seismic study for velocity distribution and structure of the crust and upper mantle from Damxung to Yadong of Xizang plateau, Acta Geophys. Sin., 24 (2), , MICHEL, B. and A. HIRN, Velocity-depth Estimation from wide angle seismic reflection arrivals, Ann. Geophys., 36 (1), , TENG Ji-wen, Shao-zhou WANG, Zhen-xing YAO, Zhen-wu Xu, Zhi-wen ZHU, Bing-ping YANG, and Wen-hu ZHOU, Characteristics of the geophysical field and plate tectonics of the Qinghai- Xizang Plateau and its neighbouring regions, Acta Geophys. Sin., 23 (3), , TONG Wei, Ming-tao ZHANG, Zhi-fei ZHANG, Zhi-jin LIAO, Mao-zheng You, Mei-xing Zxu, Guoying Guo, and Shi-bin Lm, Geothermals beneath Xizang (Tibetan) Plateau, Scientific Publishing House, Beijing, WEI Si-yu, Ji-wen TENG, Bing-ping YANG, and Zhong-yi Hu, Characteristics of geothermal and geophysical field of Xizang plateau, northwestern, Seismol. J., 3 (4), 17-25, 1981.

Specific gravity field and deep crustal structure of the Himalayas east structural knot

49 4 2006 7 CHINESE JOURNAL OF GEOPHYSICS Vol. 49, No. 4 Jul., 2006,,.., 2006, 49 (4) :1045 1052 Teng J W, Wang Q S, Wang GJ, et al. Specific gravity field and deep crustal structure of the Himalayas east