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Crustal structure of seismic velocity in southern Tibet and east-westward escape of the crustal material. ——An example by wide-angle seismic profile from ...
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Science in China Ser. D Earth Sciences 2004 Vol.47 No.6 500—506

Crustal structure of seismic velocity in southern Tibet and east-westward escape of the crustal material ——An example by wide-angle seismic profile from Peigu Tso to Pumoyong Tso ZHANG Zhongjie1, TENG Jiwen1, LI Yingkang2, S. Klemperer3 & YANG Liqiang1 1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100101, China; 2. Institute of 562, Chinese Academy of Geological Sciences, Ministry of Land and Resources, Yanjiao 110101, China; 3. Department of Geophysics, Stanford University, Stanford, CA94305-2215, USA Correspondence should be addressed to Zhang Zhongjie (email: [email protected])

Received October 10, 2003

Abstract The reflecting events from Moho and other interfaces within the crust are recognized from the wavefield characteristics of P- and S-wave for the 480km long wide-angle seismic profile between Peigu Tso and Pumoyong Tso. Then, seismic crustal structures of P- and S-wave velocities and Poisson ratio under the nearly east-west profile in southern Tibet are interpreted by fitting the observed traveltimes with the calculated ones by forward modelling. Our interpreting results demonstrate that the crustal thickness varies remarkably in the east-west direction, showing a pattern that the crust could be divided into three parts bounded by the west of Dingri and the east of Dinggyê, respectively, where the depth of Moho is about 71km for the western part, about 76km for the middle and about 74km for the eastern. There is one lower velocity layer (LVL) at the bottom of the upper crust with depth of 20—30 km. One of the distinct features is that the thickness of LVL abruptly thins from 24km on the west to 6km on the east. The other is that the velocity variation in the crust along east-west direction for both P- and S-wave displays a feature as quasi-periodic variation. The lower velocity (compared to the average value for the continent of the globe) in the lower crust and three sets of north-southward active normal faults are probably attributed to the coupling process of material delamination in the lower crust, crustal thicking and east-westward escape of the crustal material accompanied with the continental collision between India and Eurasia Plate. Keywords: southern Tibet, crustal structure, lower velocity layer, delamination, east-west escape. DOI: 10.1360/03yd0518

The Tibetan plateau, which results from continuous collision between India continent and southern Eurasia continent, is cross-cut by no less than three — major east-west sutures[1 6]. Yarlung Zangbo suture, marked by one 1500km long ophiolite belt appearing at the southernmost Tibet, separates the Tethyan Himalaya to the south from the Lhasa block to the north. Copyright by Science in China Press 2004

In order to understand the deep structure of Tibetan crust and the uplifting process of Tibetan Plateau, one wide-angle seismic profile was acquired on the southern side of Yarlung Zangbo River by Sino-French Programme in 1981[3,4]. The sketchy structure of P-wave velocity and the crustal thickness were obtained with interpretation of P-wave data, but the fine crustal

Crustal structure of seismic velocity & escape of crustal material

structure of seismic velocity has not been obtained yet, and S-wave data were not used in the previous interpretation. It is worthy to note that the Tibetan Plateau and Himalayas mountain system, as the typical region of continent-continent collision, are results from the collision between India continent and Eurasia continent. But in recent several million years, the driving force causing crust thicking can no longer make the thicking visible as before, and the east-west extension taken on the Tibetan crust becomes more signifycant for study. So, in addition to strengthening the study of the collision between India plate and Eurasia plate and the uplift process of Qinghai-Tibet Plateau, it is necessary to study east-west extension in Tibetan crust. Obviously, the wide-angle seismic profile from Peigu Tso to Pumoyong Tso has a significant scientific meaning to the study on crust thicking and west-eastextension in southern Tibet. According to the reasons mentioned above, the deep process and its dynamical meanings are discussed with reprocessing and interpreting the P-wave, especially the new information of S-wave data, aiming to obtain the fine crustal structure of the southern Tibet and its feature of east-west variation. The research region belongs to the Himalayas fold system of Himalayas tectonics zone, where some sea facies strata from Paleozoic to Mesozoic are well developed. It is tectonically active and there are several sets of north-south slip faults, such as Dangxiong-Duoqing Tso fault, Shenzha-Dinggyê fault and Dangruoyong Tso-Nielamu fault. The acquisition of the east-west wide-angle seismic profile crossing the north wing of inverted anticlinorium of Himalayas can provide some important deep information to study the crustal structure of southern Tibet and deep dynamic process and a data base to further deepen our understanding about the problems mentioned above. In this paper, we briefly introduce the wide-angle seismic profile in the southern Tibet firstly, then summarize the crustal structure of seismic velocity and Poisson ratio, and discuss the inhomogeneity as layering and blocking of the crust, the delamination in lower crust and the east-west escape feature of the crustal material, and then present some new recogni-

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tion and basic conclusions. 1

Wide-angle seismic profile

The wide-angle seismic profile stretches from Peigu Tso to Pumoyong Tso in crystalline Himalaya of southern Tibet, with a total length of about 480km. Explosive sources were triggered at Dinggyê, Peigu Tso and Pumoyong Tso (fig. 1). In this study, the original modulus data are digitized with the sample length of 80 s and sample interval of 0.01s. To enhance the signal-to-noise ratio (S/N), seismic sections are filtered with the band-pass of 1—8 Hz and 1—6 Hz for P- and S-wave, respectively. Fig. 2 (a) and (b) shows the reduced seismic profiles of P- and S-wave with reduction velocities of 6.00 km/s and 3.5 km/s. 6 sets of P- and S-wave events are recognized by analyzing geometric and dynamical feature of events in P- and S-wave sections, where the events Pg and Sg are the compressional and shear wave events from crystallized basement. The events Pm and Sm are the reflecting P- and S-wave events from the Moho. The events P3—P6 and S3—S6 are the reflecting P- and S-wave events from some other interfaces within the crust. 2 Seismic velocity and Poisson ratio structure of the crust in southern Tibet After the recognition of the reflecting events in the crust, we used the forward modelling technique of 2D ray tracing scheme adapted to inhomogeneous medium and the fluctuant interfaces to fit the observed reflecting events in the crust, obtained the crustal structures of seismic velocity for P- and S-wave, and then calculated the Poisson ratio with P- and S-wave velocities. The seismic crustal structure is shown in fig. 3(a) for P-wave velocity and fig. 3(b) for S wave velocity, from which we can see that (1) Crustal thickness thickens from 71 km in the west of Peigu Tso to 74 km in the east of Pumoyong Tso. The thickest one at the middle section hits 76 km. The undulation of the lower crustal interface is very abrupt. Generally, Moho in southern Tibet is featured by “shallower in the west and deeper in the east”.

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Science in China Ser. D Earth Sciences

Fig. 1. Sketch map showing locations of the wide-angle seismic profile.

(2) The thickness of the sediment layer thins from 5 km in the west to about 1 km in the east with seismic velocity range of 5.2—5.4 km/s for P-wave and 2.6—3.0 km/s for S-wave, respectively. At the bottom of the upper crust, there is one low velocity layer (LVL) with an obvious variation of depth and thickness along the east-west profile. The depth range of the LVL is 22—44 km in the west and 25—31 km in the east and, its thickness thins from 20 km in the west to about 6 km in the east with seismic velocity range of 5.7—5.9 km/s for P-wave and 3.4—3.5 km/s for S-wave.

From the spatial pattern of Poisson ratio under the wide-angle seismic profile in southern Tibet (fig. 3(c)), we can see that

(3) The thickness of the lower crust varies within 30—40 km with the thickest one being about 40km under the middle segment of the profile. The seismic velocity range in the lower crust is 6.3—6.6 km/s for P-wave and 3.7—4.0 km/s for S-wave.

(3) The range of the Poisson ratio is 0.21—0.23 in the lower crust and there also exists the blocking feature under the east-west seismic profile. The Poisson ratio under the middle section is a little lower than its neighboring areas.

(1) The Poisson ratio for the material in the upper crust shows the blocking feature along the east-west profile, the blocking boundaries to be located in the west of Dingri, and east of Dinggyê and the region near Kangma. From shallow to deep, the Poisson ratio decreases from 0.26—0.28 to about 0.25. (2) The Poisson ratio is about 0.21—0.24 for the lower velocity layer (LVL).

Crustal structure of seismic velocity & escape of crustal material

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Fig. 2. The wide-angle seismic sections with explosive source shot at Peigu Tso. (a) P-wave profile, where the travel times are reduced with reduction velocity of 6.00 km/s, (b) S-wave profile, where the travel times are reduced with reduction velocity of 3.5 km/s.

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Discussion

3.1 East-west variation of seismic wave velocity and crustal structure From the structures of P- and S-wave velocities and Poisson ratio, we can see that the inner structure of the crust under the profile shows an obvious feature of layering and blocking with their boundaries corresponding to the active faults by geological surface mapping. At the same time, along the east-west profile, P- and S-wave velocities show the quasi-periodic variation of “high—low—high” or “low—high—low”. Under the west segment between Peigu Tso and Dingri, P- and S-wave velocities for upper crust vary abruptly. With the increase of depth, P-wave velocity for upper crust increases from 5.2 km/s at the surface to 6.26km/s at the top of LVL. Along the horizontal direction, the variation range of seismic velocity for the lower crust is 6.30—6.80 km/s for P-wave and

3.74—4.0 km/s for S-wave, respectively. For the segment from the west of Dingri to the east of Dinggyê, P-wave velocity for the upper crust increases from 5.36 km/s at the surface to 6.26—6.28 km/s at the top of LVL with increasing depth. In the horizontal direction, the range of seismic velocity for the lower crust is 6.4—6.57 km/s for P-wave and 3.73—4.00 km/s for S-wave. For the segment from the east of Dinggyê to Pumoyong Tso, the velocity range for the upper crust is 6.0—6.28 km/s for P-wave and 3.4—3.66 km/s for S-wave, and 6.38—6.6 km/s for P-wave and 3.4—3.94 km/s for S-wave in the lower crust. 3.2 Delamination in the lower crust under southern Tibet The previous research demonstrates that there are two sorts of delamination in the lower crust, namely, Paleo-delamination and the present undergoingdelamination. The major features of the undergoing-

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Fig. 3. Crustal structure in southern Tibet. (a) P-wave velocity structure, (b) S-wave velocity structure, (c) Poisson ratio structure.

Fig. 4. Sketch map to illustrate the delamination in the crust under southern Tibet and the east-westward escape of its material. DDF, Dangxiong-Duoqing active tectonic zone; SDF, Shenzha-Dinggyê active tectonic zone; DNF, Dangruoyong Tso-Nielamu active tectonic zone.

delamination are that there is no earthquake region at the top of the upper mantle, P-wave velocity and quality factor (Q value) are low; but at even deeper depth

the earthquake source distributes densely, and P-wave velocity and quality factor (Q value) are high. On the other hand, the major features of paleo-delamination

Crustal structure of seismic velocity & escape of crustal material

are that the wave velocity in the lower crust is low because of the delamination of the high density material in the lower crust. From seismic crustal structure, we can see that there is the evidence of delamination in the lower crust in southern Tibet (fig. 3). Summarizing all the evidences, such as the existence of dense earthquake source zone(