Page 323 - Global Tectonics

306 CHAPTER 10

strong (Fig. 10.12e), the foreland displays a simple shear The Himalayan–Tibetan orogen (Fig. 10.13) is one
thin-skinned mode of deformation. Underthrusting of of the best places to study a large-scale continent–
the shield is accompanied by the eastward propagation continent collision that followed the closure of a major
of the thin-skinned thrust belt above a shallow décolle- ocean basin and formed an orogenic plateau. The active
ment at 8–14 km depth and drives deformation in the tectonics, diverse structure, and relatively well-known
lower crust beneath the plateau. This style conforms plate boundary history of this belt allow many tectonic
well to observations east of the Altiplano and north of relationships to be measured directly and provide
23°S. It also simulates the conditions of the Himalayan important constraints on the driving mechanisms of
fold-thrust belt south of the Tibetan Plateau (Section deformation and the manner in which deformation is
10.4.4). In the case where the Paleozoic sediments are accommodated (Yin & Harrison, 2000). In addition,
weak and the foreland warm and weak (Fig. 10.12c), the immense size and high elevations of this orogen
deformation in the foreland is thick-skinned with a deep illustrate how mountain building and global climate
décollement at ∼25 km depth (Fig. 10.12f). This latter are interrelated. These interactions form important
style conforms well to observations east of the Puna elements of orogenesis in most, if not all, tectonic
and south of 23°S and results because the foreland is settings.
weak enough to deform by buckling. This section provides a discussion of four main
These observations and experiments illustrate that aspects of the Himalayan–Tibetan orogen: (i) the rela-
variations in lithospheric strength and rheology play an tive motion of Indian and Eurasia and their tectonic
important role in controlling the tectonic evolution of history prior to collision; (ii) the nature of post-colli-
compressional basins and fold-thrust belts. These effects sional convergent deformation as revealed by seismicity
are prominent at scales ranging from individual thrust and geodetic data; (iii) the geologic history of the Hima-
sheets to the entire lithosphere. laya and the Tibetan Plateau; and (iv) the deep structure
of the orogen. Section 10.4.6 provides a discussion of
the main factors controlling the mechanical evolution
of the orogen.
10.4 CONTINENT–
CONTINENT 10.4.2 Relative plate motions

COLLISION and collisional history

The Himalayan–Tibetan orogen was created mainly by
the collision between India and Eurasia over the past
10.4.1 Introduction 70–50 Myr (Yin & Harrison, 2000). The orogen is part
of the greater Himalayan–Alpine system, which extends
Collisional mountain ranges form some of the most from the Mediterranean Sea in the west to the Sumatra
spectacular and dominant features on the surface of the arc of Indonesia in the east over a distance of >7000 km.
Earth. Examples include the Himalayan–Tibetan This composite belt has evolved since the Paleozoic as
orogen, the Appalachians, the Caledonides, the Euro- the Tethyan oceans (e.g. Fig. 11.27) closed between two
pean Alps, the Urals (Section 11.5.5), the Southern Alps great converging landmasses: Laurasia in the north and
of New Zealand (Sections 8.3.3, 8.6.3), and many of the Gondwana in the south (Sengör & Natal’in, 1996).
S
Proterozoic orogens (e.g. Section 11.4.3). The anatomy Tethys may have been only a few hundred kilometers
of these belts is highly diverse, in part due to differences wide in the west but opened to the east to form an
in the size, shape, and mechanical strength of the col- ocean that was at least several thousands of kilometers
liding plates, and the effects of different precollisional wide.
tectonic histories. In addition, continental collision can The India–Eurasia collision was brought about by
range from being highly oblique, such as occurs on the the rifting of India from Africa and East Antarctica
South Island of New Zealand, to nearly orthogonal. during the Mesozoic (Section 11.5.5) and by its migra-
These differences greatly influence the mechanisms of tion northward as the intervening oceanic lithosphere

collisional orogenesis (Section 10.4.6). was subducted beneath the Eurasian Plate. Magnetic