| Citation: | Zhixin Zhao, Jiren Xu. Compressive Tectonics around Tibetan Plateau Edges. Journal of Earth Science, 2009, 20(2): 477-483. doi: 10.1007/s12583-009-0039-7
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Various earthquake fault types, mechanism solutions, stress field, and other geophysical data were analyzed for study on the crust movement in the Tibetan plateau and its tectonic implications. The results show that numbers of thrust fault and strike-slip fault type earthquakes with strong compressive stress near NNE-SSW direction occurred in the edges around the plateau except the eastern boundary. Some normal faulting type earthquakes concentrate in the Central Tibetan plateau. The strikes of fault planes of thrust and strike-slip faulting earthquakes are almost in the E-W direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. This implies that the dislocation slip vectors of the thrust and strike-slip faulting type events have quite great components in the N-S direction. The compression motion mainly probably plays the tectonic active regime around the plateau edges. The compressive stress in N-S or NE-SW directions predominates earthquake occurrence in the thrust and strike-slip faulting event region around the plateau. The compressive motion around the Tibetan plateau edge is attributable to the northward motion of the Indian subcontinent plate. The northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins.
The tectonics study on the converging zone is an interesting project (Xu and Yoshiteru, 2002; Avouac and Tapponnier, 1993).The compressive tectonic force due to the collision along the Himalayas between the Indian and Eurasian plates results in the orogenic motions in the Himalayas, Tibetan plateau Altyn, and Qilian Mountains.Such violent crusta movements and frequent strong earthquakes extend to the Tianshan Mountain (Qin et al., 2002; Xu, 2001) The compressive tectonic forces affect crustal motions in West China and those up to the Mongolian regions (Brown et al., 2002; van der Voo et al., 1997).Such motion and deformation make the Tibetan plateau and its surrounding areas become one of the most active regions in terms of tectonics and seismicity in the world (Wang et al., 2003).The synchronous tempora variations of the seismic activities in the Tibetan plateau region and its vicinities are related to the compressional collision between the Indian and Eurasian plates (Zhao et al., 1990, 1988).Many significant results of tectonics and seismicity related to the collision from the continental crust of the Indian plate and subducts underneath Eurasian plate along the southern front of the Himalayas in this region have been reported (Fu et al., 2000; Zeng et al., 2000).Besides the compressive tectonic motions, some extensional geologic tectonic motions in the high Tibetan plateau have been reported (Xu et al., 1988).
In this study, the focal mechanisms for studying tectonic characteristics in and around the plateau are analyzed.The lithosphere motion around the Tibetan plateau edge and tectonic implications are mainly investigated based on the analyses of the types of earthquake occurrences, the stress fields, and other geophysical data.
The characteristics of crust and lithosphere motions are analyzed based on the investigations for fault motions of events in and around the plateau.Earthquakes are divided into three types, i.e., normal, thrust, and strike-slip faulting events depending on the plunge angles of compressive stress P-and extensive stress T-axes (Xu and Zhao, 2009).Focal mechanism solutions of 924 earthquakes with magnitude equal to or greater than Ms 4.5 in the Tibetan plateau and its vicinities during 1933–2003 have been analyzed and used in this study.The 217 solutions of events were determined by authors referring to data of the Chinese Seismic Net and the Bulletin of the Internationa Seismological Center (ISC).The others are selected from the Centroid Moment Tensor (CMT) solutions determined by Harvard University and from United States Geological Survey (USGS).The majority of events used in this analysis occurred mainly in the crust.
In order to help one to understand the presen analysis, Fig. 1 shows the tectonic scheme of the Tibetan plateau and surrounding regions as a background map, which includes the topographic information, elevation, some faults, and names.The elevation there implies the outline of the plateau.
In order to investigate tectonic motion characteristics in the Tibetan plateau and its surroundings, it is necessary to analyze the fault types of earthquakes The occurrence style of earthquake reflects undoubtedly stress regime in a region.Earthquakes can be understood as rock brittle ruptures in the crust and lithosphere generally.Studies on the faulting type of earthquake may help one understand the characteristics of lithosphere motion.Therefore, Fig. 2 shows the distributions of thrust, strike-slip, and normal faulting events that are divided depending on the plunge angles of compressive P-and extensional T-axes.It can be seen in Fig. 2 that many events with thrust and strike-slip faulting concentrate obviously around the Tibetan plateau.The thrust faulting events occurred mainly along the west part of the Himalaya front to the Kashmir region.A number of strike-slip faulting events occurred along the East Himalaya front besides some thrust faulting type events.
The discontinuous zone of strike-slip and thrust faults exists in the Altyn and Qilian Mountains in the north edge of the Tibetan plateau.A lot of large strike-slip faulting events occurred along the Karakorum and Altyn Mountains.Whereas in the Qilian Mountain, there were many thrust faulting events that occurred besides the strike-slip faulting events.The seismic activity with normal faulting events disappears on the northern boundary of the plateau, where the style of seismic activity differs from that in the high-altitude region on the plateau.The thrust faulting events are clearly predominant in the Tianshan Mountain.The focal mechanism solutions mainly show strike-slip faulting type events on the south segment of north-south seismic belt (NSSB), the east boundary of the plateau.It is valuably noticed that several thrust faulting type events occurred in the middle region (about 31ºN) of the north-southern seismic belt besides the strike-slip faulting type events in Fig. 2.Many reverse faulting events occurred in the low surrounding regions of the Tibetan plateau besides the strike-slip faulting events.The seismic activity in the central high Tibetan plateau where many normal faulting events concentrate is quite different.
The stress field in and around the Tibetan plateau is investigated by analyzing distributions of P-axes of focal mechanism solutions for analyses of the style of earthquake occurrence regime.Figure 3 shows horizontal projections of P-axes in about the last 70 years It can be seen from the figure that the P-axes of events lie mainly in the N-S to NE-SW directions in the plateau and its surrounding regions, with a few exceptional events.In the Himalaya fronts, the P-axes are almost in the NNE-SSW direction and are perpendicular to the Himalaya arc.The horizontal projections of P-axes are averagely greater than those of T-axes.I shows that the tectonic movements in the Himalaya front region are characterized by the compressive stress in the NNE-SSW direction.
In the region north of Himalayas and in the Central Tibetan plateau, the horizontal projections of many P-axes as a whole become little although P-axes basically do not change their orientations comparing with those in the south Himalayas.P-axes are mainly in the direction of N-S to NE-SW in the northern borders of the Tibetan plateau, Karakorum, Altyn, and Qilian Mountains.Horizontal projections of P-axes are great in the Qilian Mountain region.The strong compressive stress may act on the region north of the Qilian Mountain.The horizontal projections of P-axes are great and lie nearly in N-S directions in Tianshan Mountain region.Horizontal projections of T-axes are obviously little there.
In the southern segment (south of 33ºN) of the north-south seismic belt of China (along about100º–104ºE), the stress field distribution is one of the most complicated fields in the seismic zones in China, as shown in Fig. 3.The stress field relationships to its vicinity blocks are illegibility on seismotectonic view (Zhao et al., 1990).It seems not to exit a single predomination in the seismic zone of the south segment of the NSSB based on the result from temporal variations of seismic activity.The seismogenic stress field in the NSSB is quite different from other seismic zones in East Asia (Zhao et al., 1987a, b).The horizontal projections of P-axes distribution are very complicated in the southern segment of the NSSB as shown in Fig. 2.Horizontal projections of P-axes are moderate in the eastern boundary.Horizontal projections of P-axes lie almost in NNE-SSW direction in the southern segment.Xu and Oike (1995) analyzed in detail the earthquake generating stress field on tectonics along the NSSB seismic zone by dividing subregions.The results are shown in Fig. 3.It can be seen in Fig. 3 that in the western part of the southern segment NSSB east of the Tibetan plateau, the orientations are changed to near NE-SW directions along the NSSB from the south end to the middle segment about latitude 32°N in Fig. 3.The P-axes are nearly E-W direction in the regions around the 32°N in the NSSB.
As a whole, the results reveal that the average P-axes orientation lies along about NE-SW direction in the western subregions of the southern segment of the NSSB.The orientation of P-axes coincides with that in the Tibetan plateau.The average P-axes orientation, however, lies along about NW-SE direction in the eastern side of the southern segment of the NSSB Such orientation coincides with that in the region from Taiwan to the Yangtze block.Figure 4 implies that the Tibetan plate moving northeastward encounters the Yangtze block moving northwest at the southern segment of the NSSB.It may provide some new ideas for study on the seismogenic stress field, e.g., the 2008Wenchuan M8.0 (30.969ºN, 103.186ºE) large even was generated under such tectonic stress field condition.The orientation of P-axes of the Wenchuan M8.0earthquake lies in the 302ºN, which coincides with the average orientation of P-axes in the source region in Fig. 4.
The north-south crust shortening motions are also confirmable by analyzing GPS data.Figure 5 shows the horizontal displacement vector rate surveyed by GPS in the Tibetan plateau and its surroundings (Yang et al., 2002).It can be seen that great displacement vectors toward the north or NNE directions distribute along the Himalaya Mountains and the southern edge of the Tibetan plateau.The vectors in the central plateau are slightly less than those along the Himalayas, and they change the orientations into the NE direction.Large movement vectors mixed with some small ones in the N-S, NNW-SSE, and NNE-SSW directions lie complicatedly in the regions northwest of Kunlun.The displacement vectors lie in the NE-SW direction in the north Tianshan region.In the north boundary region of the plateau, the displacement vectors near north direction along the west Altyn are greater than those with NE-SW direction along the east Altyn Mountain.The displacements in the NEE direction along the Qilian Mountain are also less than those in the Tibetan plateau.As a whole, the displacement vectors along the southern boundary are much greater than those along the northern boundary of the plateau on the average, as shown in Fig. 5.It suggests that the Tibetan plateau is undergoing a shortening along near the N-S direction.
The displacement vectors lie nearly in the N-S direction in the West Tibetan plateau and the West Kunlun.The northeastward horizontal components of displacement vectors are revealed in the Central Tibetan plateau.The horizontal components of displacements lie identically in near the east-west direction on the eastern boundary of the Tibetan plateau and in the southern segment of the NSSB.The horizontal displacements in the southern part of NSSB have somewhat southward components.The above analyses based on GPS results at the surface imply that there exists the northward crust movement in the southern, western, and northern edges around the plateau.The east-west crustal movement exists in the eastern edge of the plateau, the northern-southern seismic belt.For 2008, the Wenchuan M8.0 event in China (30.969ºN, 103.186ºE) occurred in the middle region of the NSSB.The azimuth of the main compressive stress axis for the Wenchuan M8.0 event however, P-axes lie in the 302ºN.It is different from the displacement vectors from GPS in Fig. 5.
The characteristics and mechanism of earthquake motion types in and around the Tibetan plateau were analyzed once (Klemperer, 2006; Wu et al., 2003; Blisniuk et al., 2001).The activities of thrust and strike-slip faulting type earthquakes existing along the edges around the plateau altitudes were investigated based on distribution characteristics of stress field GPS data in this article.The compressive stress predominates earthquake activity in the edges around the plateau.The shortening deformation and movemen near the N-S direction are also revealed there.The compressive motion becomes obviously an important tectonic active regime in the plateau edges.
The tectonic stresses due to the collision cause a wide distribution of P-axes aligns near NNE-SSW or N-S direction in West China (Xu et al., 2005; Molnar et al., 1993; Molnar and Tapponier, 1975).The strong compressive stress due to the northward movement of the Indian plate also causes the Tibetan plateau to rise gradually (Yeats and Lillie, 1991).A large number of thrust and strike-slip faulting type events occurred in the front of Himalaya Mountains, as shown in Fig. 2Many strike-slip fault events occurred in the Karakorum and Altyn Mountains.The thrust faulting type events occurred frequently in regions of the Qilian and Tianshan Mountains (Harrison et al., 1992).These results show that strong compressive stresses exist in the southern, western, and northern boundaries of the plateau.Moreover, the GPS data show that the northward annual slip rates in the northern margin of the plateau, for example, Altyn and Qilian Mountains, are one tenth of those in the Himalaya front (Zhang et al., 2004; Ralf et al., 2002; Bendick et al., 2000).The recent analysis from GPS shows that northward horizontal displacement vectors along the Himalayas are much greater than those in the northern margins of the plateau (Shen et al., 2001) too.The results imply that the northward motion of the Tibetan plateau shortened in the N-S direction encounters probably strong obstructions at the western and northern margins (Chen et al., 2000).In the eastern margin of the Tibetan plateau, the southern part of NSSB, however, events are mainly the strike-slip faulting type.Some thrust events also occurred in the middle and northern segments of the NSSB.
The shortening motions and the mechanisms in and around the Tibetan plateau and its east vicinity were analyzed by employing the data of more faulting earthquakes, GPS data in this article.It can be concluded as follows.
(1) Tectonic stresses from the relative movement between the Indo-Australian and Eurasian plates cause the orientations of the strong compressive stress axes (P-axes) to align near the NNE-SSW direction in the Tibetan plateau and its surroundings.The compressive stress from the Indian plate northward shortens the plateau crust in the north-south direction.
(2) A large numbers of thrust faulting and/or strike-slip faulting type events occurred in low edges around the Tibetan plateau.In the eastern margin of the Tibetan plateau, the southern part of NSSB, events are mainly the strike-slip faulting type.Some thrust events also occurred in the middle and northern segments of the NSSB.
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Zhixin Zhao, Jiren Xu. Compressive Tectonics around Tibetan Plateau Edges. Journal of Earth Science, 2009, 20(2): 477-483. doi: 10.1007/s12583-009-0039-7
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