Page 327 - Global Tectonics

310 CHAPTER 10

Johnson, 2002). This motion created a zone of active Tarim Basin and the North and South China blocks.
deformation that stretches ∼3000 km north of the These observations, and geologic data, suggest that the
Himalayan mountain chain (Fig. 10.13). Global Posi- northward growth of the orogen was not a smooth,
tioning System (GPS) measurements show that India is continuous process, but occurred in an irregular series
−1
moving to the northeast at a rate of some 35–38 mm a of steps. In a direction orthogonal (N111°E) to the con-
relative to Siberia (Larson et al., 1999; Chen et al., 2000; vergence direction, horizontal motion increases steadily
Shen et al., 2000; Wang et al., 2001). This rate is consid- northward from the Himalaya across the Tibetan
−1

erably slower than the long-term rates of 45–50 mm a Plateau (Fig. 10.16b), reflecting the eastward motion of
estimated from global plate motion models (DeMets the latter with respect to both India and Eurasia. At its
et al., 1994), which is typical of the short-term inter- northern margin velocities decrease rapidly as a result
seismic strain rates measured using geodetic data (e.g. of left-lateral strike-slip motion on the Kunlun and
Section 8.5). other faults (Wang et al., 2001). The Longmen Shan
The geodetic data suggest that deformation within (Fig. 10.13) moves eastward with the South China block
the Tibetan Plateau and its margins absorbs more than (Burchfi el, 2004).
90% of the relative motion between the India and Earthquake focal mechanism solutions, compiled
Eurasia plates, with most centered on a 50-km-wide for the period 1976–2000 by Liu & Yang (2003), reveal
region of southern Tibet (Wang et al., 2001). Internal the style of active faulting in the Himalayan–Tibetan
shortening of the plateau accounts for more than one- orogen (Fig. 10.17). Zones of concentrated thrust fault-
third of the total convergence. An additional compo- ing occur along both the northern, southern, and
nent of shortening is accommodated north of the eastern margins of the Tibetan Plateau. Within the
Tibetan Plateau in Pamir, Tien Shan, Qilian Shan, and Himalaya, thrust faulting is prevalent. South of the
elsewhere, although the rates are not well known in Himalaya (Fig. 10.18), intra-plate earthquakes and other
these areas. geophysical evidence indicate that the Indian plate

South of the Kunlun Fault (Fig. 10.13), the surface flexes and slides beneath the Himalaya, where it lurches
velocity field shows that the Tibetan Plateau is extrud- northward during large earthquakes (Bilham et al.,

ing eastward relative to both India and Asia (Fig. 10.16). 2001). The overall pattern of the deformation is similar
This motion, where slices of crust move laterally out of to that which occurs at ocean–continent convergence

the way of colliding plates by slip on strike-slip faults, zones where an oceanic plate flexes downward into a
is termed lateral escape. The movement also involves the subduction zone. North of the Himalaya, normal fault-
rotation of material around a curved belt in Myanmar ing and east–west extension dominate southern and
called the eastern Himalayan syntaxis. The term syn- central Tibet. Strike-slip faulting dominates a region
taxis refers to the abrupt changes in trend that occur on some 1500 km wide north of the Himalaya and extend-
either side of the Himalaya in Myanmar and Pakistan ing eastward into Indo-China. Farthest from the moun-
where mountain ranges strike at nearly right angles to tain chain is a region of crustal extension and normal
the trend of the Himalaya. East of the plateau, North faulting extending from the Baikal Rift of Siberia to the
China and South China are moving to the east-south- northern China Sea. Active strike-slip faulting also
−1
−1
east at rates of 2–8 mm a and 6–11 mm a relative to occurs in the western Himalayan syntaxis and eastern
stable Eurasia, respectively. Himalayan syntaxis in Pakistan and in Myanmar, respec-
A GPS velocity profile across the Tibetan Plateau tively. South of the syntaxis in Pakistan, movement

(Fig. 10.16a) is mostly linear parallel to the predicted along north-striking faults is dominantly sinistral; south
direction of the India–Eurasia collision (N21°E), except of the one in Myanmar it is mostly dextral. These oppo-
for a high gradient across the Himalaya at the southern site senses of motion on either side of India are compat-
end of the plateau (Wang et al., 2001). This mostly linear ible with the northward penetration of India into
trend suggests that the shortening across the plateau is southern Asia.
broadly distributed; otherwise signifi cant deviations These observations indicate that convergence
across individual fault zones would be expected. between India and Eurasia is accommodated by combi-
However, this generally continuous style of deforma- nations of shortening, east–west extension, strike-slip
tion appears to be restricted mostly to the plateau itself. faulting, lateral escape, and clockwise rotations. In addi-
Rigid block-like motion appears to characterize regions tion, uplift of the high elevations of the Tibetan Plateau
to the north and northeast of the plateau, including the by Miocene time (Blisniuk et al., 2001; Kirby et al., 2002)