Page 336 - Global Tectonics

OROGENIC BELTS 319

sequences failed and were scraped off the Although the role of these various processes
downgoing plate, forming the Himalayan remains uncertain, it seems likely that a
fold and thrust belt. combination of mechanisms accommodates
shortening beneath Tibet.
2 Continental underthrusting. The underthrusting,
or subduction, of continental lithosphere 3 Indentation, lateral escape, and gravitational
beneath another continental plate is one of the collapse. A comparison between the total
most important mechanisms that amount of convergence between India and
accommodates convergence in zones of Eurasia since they collided and estimates of the
continental collision. The rheology of the two total amount of shortening accommodated by
plates and the degree of mechanical coupling fold-thrust belts in the orogen has yielded a

between them control shortening and the shortening deficit ranging anywhere from
evolution of stresses within the overriding 500 km to over 1200 km (Dewey et al., 1989;

plate. In the Himalayan–Tibetan orogen, the Johnson, 2002). This deficit has led to
underthrusting of Indian continental numerous attempts to explain how the
lithosphere drives intra-plate shortening at the convergence not accounted for by folding and
leading edge of the Indian plate and in Tibet, thrusting has been accommodated. A leading
and, possibly, also farther north into Asia. The hypothesis involves the indentation of India
resultant shortening has generated crust that is into Asia and the lateral escape of eastern
up to 70–80 km thick (Section 10.4.5) and has Tibet (Section 10.4.3).
contributed to the uplift and growth of the Indentation is the process by which a rigid
Tibetan Plateau. Like its counterpart in the block presses into and deforms a softer block
central Andes (Section 10.2.4), the plateau is during convergence. The theory of
associated with high crustal temperatures and indentation originally was developed by
widespread intra-crustal melting that have mechanical engineers to predict the

weakened the crust sufficiently to allow it to configuration of lines of maximum shear
fl ow. This process has decoupled the Tibetan stress, or slip lines, in deforming plastic
crust from the underlying convergent motions materials. In geologic applications, the slip
and has altered the dynamics of the orogen. lines correspond to dextral and sinistral
Although geophysical observations show that strike-slip faults whose pattern is controlled
Indian lithosphere is underthrust to at least a by the shape of the indenter and by lateral
point beneath central Tibet, interpretations constraints placed on the plastic medium
differ on how this process is accommodated (Tapponnier & Molnar, 1976; Tapponnier
(Dewey et al., 1989; Yin & Harrison, 2000; et al., 1982).
Johnson, 2002). The main problem is that the In one pioneering application, Tapponnier et al.
underthrusting requires the removal or (1982) explored the effects of indentation as a
displacement of Asian lithosphere from rigid 50-mm-wide block (India) penetrates
under Tibet (Section 10.4.5). Several into a softer block (Asia) made of laminated
mechanisms may alleviate this problem, plasticine. Figure 10.22 shows two
including the downturning of Indian mantle evolutionary sequences where the plasticine

lithosphere beneath the Bangong–Nujiang is either bilaterally confined at the two edges
suture (Figs 10.21, Plate 9.4(bottom) parallel to the motion of the indenter (Fig.
(between pp. 244 and 245), the convective 10.22a–c) or unilaterally confined at only one

removal or delamination of the lithospheric of these edges (Fig. 10.22d–f). The bilaterally

mantle beneath Tibet (England & confined case produces a symmetric pattern
Houseman, 1988; Molnar et al., 1993), the of slip lines ahead of a “dead triangle” that
southward subduction of Asian mantle rapidly welds to the indenter. The
(Willett & Beaumont, 1994), and the removal penetration proceeds by the creation of
of Asian mantle by strike-slip faulting during numerous, short-lived, dextral and sinistral
the lateral escape of Tibet (Section 10.4.3). faults near the triangle’s apex. The unilateral