Page 344 - Global Tectonics

OROGENIC BELTS 327

localized strain at the surface. This effect Zealand (Section 8.6.3). Strain-softening
may explain the relatively narrow width, feedbacks also have contributed signifi cantly
triangular shape, and lack of a high orogenic to the tectonic evolution of the Himalayan
plateau in the Eastern Alps and the Southern fold and thrust belt and southern Tibet
Alps of New Zealand. By contrast, where the where Indian lithosphere is underthrust to
lower crust is relatively weak and fl ows easily, the north beneath Eurasia. Hodges (2000)
the crust decouples from the mantle and summarized nine principal geologic and
results in diffuse strain. This latter effect may tectonic features of this relatively narrow
apply to Tibet and the central Andes where zone that require explanation in any
low viscosity zones have developed in the quantitative model of the orogen. These
deep crust during crustal thickening and features include (Fig. 10.20c): (i) rapid

wide, steep-sided plateaux have formed above erosion of the southern flank of the
the weak zones (Sections 10.2.4, 10.4.5). Himalaya; (ii) shortening on the Main
A vertical decoupling of the lithosphere as a Central Thrust (MCT) system and thrust

result of ductile flow in a weak lower crust faults to the south; (iii) extension on the
is well illustrated along the northern and South Tibetan Detachment (STD) system;
eastern margins of Tibet. In these regions (iv) high-grade metamorphism and crustal
balanced cross-sections show that thrust melting in the Greater Himalaya; (v) crustal
faults sole out into décollement surfaces in melting in the middle crust beneath Tibet;
the middle crust (Yin & Harrison, 2000). A (vi) juxtaposition of contrasting lithologies
comparison of geodetic data (Fig. 10.16a) across the MCT; (vii) an inverted
and geologic observations has indicated that metamorphic sequence where high-grade
the lateral motion of the crust in the rocks are thrust over the Lesser Himalaya
Longmen Shan region of eastern Tibet is along the MCT; (viii) the position of the
mostly accommodated by lower crustal fl ow Indus–Zangbo suture; and (ix) normal faults
with little faulting occurring at the surface accommodating north–south extension in

(Burchfiel, 2004). Northwest of the Sichuan the southern Tibetan Plateau.
Basin topography is anomalously high To determine how enhanced erosion coupled
compared to the rest of Tibet. Clark & with continental underthrusting may explain
Royden (2000) and Clark et al. (2005) these principle features, Beaumont et al.
explained these relationships as a result of (2001, 2004) constructed thermomechanical
dynamic pressure resulting from the lateral models involving combinations of two

flow of a partially molten lower crust as it related processes. The first process is a

encounters the strong crust and upper channel flow of ductile middle to lower crust.
mantle of the Sichuan Basin. At the western Channel fl ow involves the lateral movement

margin of the basin, the flowing lower crust of partially molten crust in a narrow zone
diverts northeastward along a rheologically bounded above and below by shear zones.
weak crustal corridor that coincides with the These authors used this type of fl ow to
Paleozoic–Mesozoic Qinling suture. The explain the progressive growth of the
response of the upper crust to this fl ow Tibetan Plateau. The second process is the
may include dynamic uplift and strike-slip ductile extrusion of high-grade metamorphic
faulting, resulting in the anomalously high rocks between coeval normal-sense and
topography of eastern Tibet compared to its thrust-sense shear zones. This latter process
central and southern sectors. is used to explain the exhumation of the
The strain-softening effects of continental Greater Himalaya rocks along the southern
underthrusting coupled with enhanced fl ank of the mountain range. In the models
surface erosion also can result in strain these two processes are linked through the
localization that alters the dynamics of effects of surface denudation (i.e. the
orogenesis. An excellent example of this removal of surface material) that is focused
process occurs in the Southern Alps of New along the southern edge of a plateau and the