Surface velocity fields and seismicity

Since about 50 Ma, continued convergence between India and Eurasia at a slowed rate has caused India to penetrate some 2000 km into Asia (Dewey et al., 1989;

Johnson, 2002). This motion created a zone of active deformation that stretches ~3000 km north of the Himalayan mountain chain (Fig. 10.13). Global Positioning System (GPS) measurements show that India is moving to the northeast at a rate of some 35-38 mm a-1 relative to Siberia (Larson et al., 1999; Chen et al., 2000; Shen et al., 2000; Wang et al., 2001). This rate is considerably slower than the long-term rates of 45-50 mm a-1 estimated from global plate motion models (DeMets et al., 1994), which is typical of the short-term inter-seismic strain rates measured using geodetic data (e.g. Section 8.5).

The geodetic data suggest that deformation within the Tibetan Plateau and its margins absorbs more than 90% of the relative motion between the India and Eurasia plates, with most centered on a 50-km-wide region of southern Tibet (Wang et al., 2001). Internal shortening of the plateau accounts for more than one-third of the total convergence. An additional component of shortening is accommodated north of the Tibetan Plateau in Pamir, Tien Shan, Qilian Shan, and elsewhere, although the rates are not well known in these areas.

South of the Kunlun Fault (Fig. 10.13), the surface velocity field shows that the Tibetan Plateau is extruding eastward relative to both India and Asia (Fig. 10.16). This motion, where slices of crust move laterally out of the way of colliding plates by slip on strike-slip faults, is termed lateral escape. The movement also involves the rotation of material around a curved belt in Myanmar called the eastern Himalayan syntaxis. The term syn-taxis refers to the abrupt changes in trend that occur on either side of the Himalaya in Myanmar and Pakistan where mountain ranges strike at nearly right angles to the trend of the Himalaya. East of the plateau, North China and South China are moving to the east-southeast at rates of 2-8 mm a-1 and 6-11 mm a-1 relative to stable Eurasia, respectively.

A GPS velocity profile across the Tibetan Plateau (Fig. 10.16a) is mostly linear parallel to the predicted direction of the India-Eurasia collision (N21°E), except for a high gradient across the Himalaya at the southern end ofthe plateau (Wang et al., 2001). This mostly linear trend suggests that the shortening across the plateau is broadly distributed; otherwise significant deviations across individual fault zones would be expected. However, this generally continuous style of deformation appears to be restricted mostly to the plateau itself. Rigid block-like motion appears to characterize regions to the north and northeast of the plateau, including the

Taiim Basin and the North and South China blocks. These observations, and geologic data, suggest that the northward growth of the orogen was not a smooth, continuous process, but occurred in an irregular series of steps. In a direction orthogonal (N111°E) to the convergence direction, horizontal motion increases steadily northward from the Himalaya across the Tibetan Plateau (Fig. 10.16b), reflecting the eastward motion of the latter with respect to both India and Eurasia. At its northern margin velocities decrease rapidly as a result of left-lateral strike-slip motion on the Kunlun and other faults (Wang et al., 2001). The Longmen Shan (Fig. 10.13) moves eastward with the South China block (Burchfiel, 2004).

Earthquake focal mechanism solutions, compiled for the period 1976-2000 by Liu & Yang (2003), reveal the style of active faulting in the Himalayan-Tibetan orogen (Fig. 10.17). Zones of concentrated thrust faulting occur along both the northern, southern, and eastern margins of the Tibetan Plateau. Within the Himalaya, thrust faulting is prevalent. South of the Himalaya (Fig. 10.18), intra-plate earthquakes and other geophysical evidence indicate that the Indian plate flexes and slides beneath the Himalaya, where it lurches northward during large earthquakes (Bilham et al., 2001). The overall pattern of the deformation is similar to that which occurs at ocean-continent convergence zones where an oceanic plate flexes downward into a subduction zone. North of the Himalaya, normal faulting and east-west extension dominate southern and central Tibet. Strike-slip faulting dominates a region some 1500 km wide north of the Himalaya and extending eastward into Indo-China. Farthest from the mountain chain is a region of crustal extension and normal faulting extending from the Baikal Rift of Siberia to the northern China Sea. Active strike-slip faulting also occurs in the western Himalayan syntaxis and eastern Himalayan syntaxis in Pakistan and in Myanmar, respectively. South of the syntaxis in Pakistan, movement along north-striking faults is dominantly sinistral; south of the one in Myanmar it is mostly dextral. These opposite senses of motion on either side of India are compatible with the northward penetration of India into southern Asia.

These observations indicate that convergence between India and Eurasia is accommodated by combinations of shortening, east-west extension, strike-slip faulting, lateral escape, and clockwise rotations. In addition, uplift of the high elevations of the Tibetan Plateau by Miocene time (Blisniuk et al., 2001; Kirby et al., 2002)

Distance along direction N21°E

Figure 10.16 (a) GPS velocity field relative to stable Siberia (image provided by Y. Yang and M. Liu and modified from Liu & Yang, 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union), combining data from Chen et al. (2000), Larson et al. (1999), and Wang et al. (2001). Error ellipses are 95% confidence. Velocity scale is shown in lower left corner. (b) GPS velocity profile across the Tibetan Plateau in the direction N2I°E (after Wang et al., 2001, Science 294, 574-7, with permission from the AAAS). Black diamonds represent the component of velocity perpendicular to the profile, light gray diamonds represent the component of the velocity parallel to the profile.

Distance along direction N21°E

Figure 10.16 (a) GPS velocity field relative to stable Siberia (image provided by Y. Yang and M. Liu and modified from Liu & Yang, 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union), combining data from Chen et al. (2000), Larson et al. (1999), and Wang et al. (2001). Error ellipses are 95% confidence. Velocity scale is shown in lower left corner. (b) GPS velocity profile across the Tibetan Plateau in the direction N2I°E (after Wang et al., 2001, Science 294, 574-7, with permission from the AAAS). Black diamonds represent the component of velocity perpendicular to the profile, light gray diamonds represent the component of the velocity parallel to the profile.

40oN

35oN

30oN

25oN

40oN

35oN

30oN

25oN

70oE 75oE 80oE 85oE 90oE 95oE 100oE 105oE 110oE

Figure 10.17 Earthquake focal mechanism solutions showing the predominant east-west crustal extension in the Tibetan Plateau (image provided by Y. Yang and M. Liu and modified from Liu & Yang, 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Data are events with magnitude >5.5 and depth <33 km from the Harvard catalogue (1976-2000).

70oE 75oE 80oE 85oE 90oE 95oE 100oE 105oE 110oE

Figure 10.17 Earthquake focal mechanism solutions showing the predominant east-west crustal extension in the Tibetan Plateau (image provided by Y. Yang and M. Liu and modified from Liu & Yang, 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Data are events with magnitude >5.5 and depth <33 km from the Harvard catalogue (1976-2000).

indicates that significant vertical uplift occurred after India collided with Asia. Currently, the Himalaya are uplifting rapidly at rates between 0.5 and 4 mm a-1 and experience very high rates of erosion along their southern flank (Hodges et al., 2001).

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