Page 343 - Global Tectonics

326 CHAPTER 10

to the bottom of the model. Unlike many rigid back-stop at the northern end of the
other models, this experiment also model. To obtain the observed east–west
incorporates lateral variations in rheology extension and the high elevations of Tibet,
using three crustal blocks, including a stiff the Tibetan crust must be very weak. The
Indian plate, a weak Tibetan Plateau, and an model suggests that the force balance evolves
intermediate-strength Asian continent north through time as the crust deforms and
of the plateau. In addition, six layers of thickens. When the plateau is 50% lower
material with different effective viscosities than its present elevation of nearly 5 km,
represent vertical variations in the rheology strike-slip and reverse faulting dominate the
of these blocks (Fig. 10.26b). plateau region. Significant crustal extension

Within this framework, Liu & Yang (2003) occurs when the plateau reaches 75% of its
considered that combinations of the present elevation. The model also suggests

following forces contribute to the present that although far field extensional forces may
state of stresses in the Himalaya and Tibetan enhance the collapse of the plateau they are
Plateau: (i) a horizontal compressive force not required. Basal shear also enhances the
resulting from the collision of India with extensional regime in the Himalaya and
Asia; (ii) buoyancy forces resulting from southern Tibet while increasing shortening
isostatically compensated topography; (iii) in northern Tibet. This latter effect results
basal shear on the Eurasia plate as India because basal shear relieves the compressive
slides beneath Tibet; and (iv) horizontal (indentation) stresses that balance the
forces originating from the pull of buoyancy forces driving extension at the
subduction zones located south and east of southern edge of Tibet. This leads to a

Asia. The stress field is constrained by GPS decrease in compressive stress in the upper
data, earthquake focal mechanisms, crust, which enhances extension. North of
topography, and other observations. Figure the Indus–Zangbo suture, the basal shear
10.26c and d show the predicted stresses in adds to the horizontal compression, resulting
the upper crust (at 10 km depth) using the in increased shortening.
velocity boundary conditions based on

geodetic data: a uniform convergence rate of 4 Lower crustal flow and ductile extrusion. The
−1
44 mm a toward N20°E at the Himalayan simple numerical and analogue experiments
−1
front (V 1 ), 7 mm a to the east on the east of indentation described above illustrate the
−1
side (V 2 ), and 10 mm a to the southeast on sensitivity of deformation in collisional belts to
the southeast side of the model (V 3 ). A local boundary conditions and variations in
−1
velocity of 20 mm a to the north (V N ) lithospheric rheology. A particularly interesting
occurs at the western side of the model and group of numerical experiments has explored
decreases to zero on the eastern side. Higher the effects of weak, fl owing middle and lower
convergent rates lead to enhanced crust on the dynamics of continental collision.
mechanical coupling between the Eurasian This condition of weak crust is in good
and Indian plates, although this effect can be agreement with geologic and geophysical
offset by a Main Boundary Thrust Zone that observations indicating that the middle crust
is mechanically weak. beneath Tibet is hot, fl uid-rich, and/or partially
The model results suggest that the surface molten (Section 10.4.5).

velocity field and regime of deformation in Royden (1996) and Ellis et al. (1998) showed that

the orogen (Section 10.4.3) refl ect a a vertical stratification of the lithosphere into
mechanical balance between gravitational strong and weak layers infl uences the degree
buoyancy, the indenting Indian plate, and the of strain localization during convergence.
specific geometry and the boundary Where the lower crust is relatively strong

conditions of the plateau. Crustal thickening and resists fl ow, the crust tends to couple to
and topographic uplift are enhanced by the the underlying mantle during shortening and
presence of the Tarim Basin, which acts as a results in a relatively narrow zone of