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wide-angle-reflection/refraction and gravity data across the plate boundary
clearly delineate the two plates (Figs. 10.12(c) and (d)). The density and seismic-
velocity models indicate that the crust beneath the fore-arc basin is continental,
not oceanic. Figure 10.12(d) shows the wide-angle seismic data from a strike line
recorded by one of the ocean-bottom seismometers along the profile and synthetic
seismic data that best fit it. The P-waves that have travelled in the overriding
plate are labelled P
up
P; those that travelled in the top of the subducting plate are
labelled P
toc
.
10.2.3 Continent-continent collisions
Because continental lithosphere is not dense enough to be subducted as a whole
into the mantle, the collision of two continents results in a complex process of
thrusting and deformation, involving a reduction and finally a cessation of relative
motion. Other plates reorganize to take up the motion elsewhere. Two classic
examples of young mountain ranges formed from such continental collisions
are the Himalayas, which are the result of the collision of the Indian plate with
Eurasia, and the Alps, which are a result of the northward motion of the African
plate towards Eurasia.
The Himalayas
Body- and surface-wave studies of earthquake data indicate that the crust beneath
the Himalayas and the Tibetan Plateau is over 70 km thick. This is in contrast to the
40 km-thick crust of the Indian shield. India has a typical shield S-wave velocity
structure with a thick, high-velocity lithosphere overlying the asthenosphere.
However, the lithospheric structure beneath Tibet is complex, and indicates that
the Indian plate is underthrusting not all, but only part of, the Tibetan Plateau
and that Tibet is not a typical shield region (Fig. 10.13).
The major tectonic blocks of the Himalayas and the Tibetan Plateau and their
sutures are shown in Fig. 10.14(a). Figure 10.14(b) shows one attempt to explain
the overall evolution of the region. This evolution has been much more complex
than just a simple collision of India with Eurasia. The reconstruction starts at
140 Ma with the Kunlun and Qiangtang blocks already sutured to Eurasia and the
Lhasa block moving northwards as an oceanic plate is subducted. By 100 Ma, the
Lhasa block was attached to Eurasia and may have undergone internal thrusting
and intrusion while subduction moved to its southern margin. Shortening by
up to 60% seems to have taken place in the northern Lhasa block. Continuing
subduction beneath the southern margin of the Lhasa block meant that the Indian
continent moved northwards (at about 10 cm yr
−
1
; see Section 3.3.3) until, by
∼
40 Ma, the continental collision occurred, at which time the rate of convergence
of India and Eurasia suddenly dropped to 5 cm yr
−
1
. Initially thrusting took
place along the Yarlung-Zangbo (or Indus-Tsangpo) suture. (Zangbo, Zangpo,
Tsangbo and Tsangopo are all used to transliterate the same Chinese word.) Thus
