Page 325 - Global Tectonics

308 CHAPTER 10

−1
40°N ity of India’s northward drift from over 100 mm a to
−1
about 50 mm a or less. This latter time may mark the

final stage of true continent–continent collision (Yin &
Harrison, 2000).
Geologic observations in Tibet and China add
30°N
important details to the sequence of events leading up
to the India–Eurasia collision. The geology indicates
that the main collision between India and Eurasia was
preceded by the collision of several microcontinents,

20°N flysch complexes, and island arcs during Paleozoic and
Mesozoic time. The collision and accretion of these
terranes is marked by a series of suture zones (Fig.
10 Ma 10.13), some of which preserve ophiolites and blocks of
10°N high-pressure metamorphic rocks (Section 9.9). Some
of these sutures expose relics of ultra-high-pressure
(UHP) minerals such as coesite and microdiamond,
commonly as inclusions in unreactive phases of zircon
0° 38 Ma and garnet. The presence of these minerals, and the
high pressures (2.5–4.0 GPa) under which they form,
can reflect situations where a section of continental

55 Ma
crust enters the subduction zone and descends to depths
10°S of 60–140 km before decoupling from the downgoing
plate (Ernst, 2003; Harley, 2004). The mechanisms by
which UHP and other high pressure metamorphic
rocks are exhumed to the surface may involve contrac-
20°S tional, extensional and/or strike-slip deformation
accompanying the evolution of the plate boundary
zone. Hacker et al. (2004) describe processes associated
Ma
71 Myr with the exhumation of UHP terranes in South
71

30°S China.
The Songpan–Ganzi terrane exposes thick Triassic
flysch sequences that rest on top of Paleozoic marine

sediments belonging to the passive margin of North
China. These sequences were deposited, uplifted, and
40°S deformed during the Triassic collision between the
North and South China blocks, forming the Ayimaqin–
Kunlun–Mutztagh suture (Yin & Harrison, 2000). By
the end of the Triassic (Fig. 10.15a), the Lhasa and
50°E 60°E 70°E 80°E 90°E 100°E
Qiangtang terranes had rifted from Gondwana and
Figure 10.14 Northward drift of India with respect to began their journey toward Eurasia (Fig. 10.15b). The
Asia from 71 Ma to the present, determined from Qiangtang terrane collided with the Songpan–Ganzi by
magnetic lineations in the Indian and Atlantic oceans 140 Ma, forming the Jinsha suture. Continued conver-
(redrawn from Molnar & Tapponnier, 1975, Science 189, gence brought the Lhasa terrane into juxtaposition with
419–26, with permission from the AAAS). Qiangtang and eventually welded the two fragments
together, forming the Bangong–Nujiang suture. The
formation of a new subduction zone beneath Lhasa
have occurred as early as 70 Ma. These relationships (Fig. 10.15c) created an Andean-type orogen (Fig.
suggest that the initial collision may have begun as early 10.15d) and eventually resulted in the collision between
as the Late Cretaceous. In general, most authors agree India and Eurasia (Fig. 10.16e), forming the Indus–
that all Tethyan oceanic lithosphere had disappeared by Zangbo suture. Continued convergence (Fig. 10.15f)
45 Ma, and at ∼36 Ma there was a decrease in the veloc- resulted in intra-plate shortening and uplift, and is asso-