Page 338 - Global Tectonics

OROGENIC BELTS 321

(1982) explains the general pattern and of orogens after convergence stops where, in
distribution of strike-slip faulting in eastern many areas, it has been linked to the
Tibet and southeast Asia, it has been less formation of extensional metamorphic core
successful at explaining other aspects of the complexes (Section 7.3).
deformation. One problem is that it predicts To account for these effects, investigators have
lateral displacements of hundreds to a simulated the deformation of Asia using a
thousand kilometers on the large strike-slip viscous sheet that deforms in response to
faults within Tibet. However, estimates of both the edge forces arising from continental
the magnitude of slip on major strike-slip collision and the internal forces generated by
faults, so far, have failed to confi rm the differences in crustal thickness (England &
extremely large magnitudes of displacement. Houseman, 1989; Robl & Stüwe, 2005a).
The Altyn Tagh Fault, for example, may only Rather than modeling displacements on
have 200 km of left-lateral slip and the individual faults, these continuum models
Xianshuihe Fault about 50 km (Yin & simulate deformation as a zone of distributed
Harrison, 2000). These observations suggest fl ow between two colliding plates. Most
that while lateral escape is occurring, it may predict that a zone of shortening and
occur on a smaller scale than originally thickening crust grows in front of and, with
predicted. the appropriate boundary conditions, to the
Another problem with the application shown in side of an advancing indenter. The results
Fig. 10.22 is that it does not predict, or take suggest that the zone of deformation directly
into account, the effects of variations in related to the India–Eurasia collision is much
crustal thickness during deformation. In smaller than that predicted by the plasticine
addition, the region of east–west extension models and that other regions of
and normal faulting in Tibet has no analogue deformation in Southeast Asia are unrelated
in the model. One possible explanation for to the local and tectonic forces arising from
the extension is that it results from the collision. Instead, deformation in these
gravitational buoyancy forces associated with latter regions may result from regional

the great thickness and high elevations of the tectonic stress fields related to the plate
plateau. In this view, an excess in gravitational boundaries located south and east of Asia.
potential energy enhanced by the presence of Continuum models of indentation, in general,
a buoyant crustal root and the possible have been successful at explaining the
removal of mantle lithosphere by convective asymmetry of deformation in Asia, including
erosion or delamination (see also Section the lateral escape of eastern Tibet. They also
10.2.5) drives the gravitational collapse of the are well suited for examining the effects of
overthickened crust (Dewey, 1988; England & variations in lithospheric strength and
Houseman, 1989). Lateral gradients in rheology on the style of deformation
gravitational potential energy may help the observed in India and Asia. Robl & Stüwe
plateau spread out and move laterally toward (2005a, 2005b), for example, explored the
the eastern lowlands where it interacts with effects of variations in the shape,
other lithospheric elements. Whereas other convergence angle, and rheology of a
origins for this extension also have been continental indenter on both lateral and
proposed, quantitative considerations of these vertical strain patterns in Asia during lateral
forces suggest that the evolution of the escape. These authors investigated the
plateau depends as much upon buoyancy sensitivity of a deforming viscous sheet to
forces and local boundary conditions as it indentation involving combinations of these
does on indentation or stresses arising at the parameters. An especially interesting aspect
edges of the Indian and Eurasian plates of their application is the investigation of
(Royden, 1996; Liu & Yang, 2003). The how buoyancy forces arising from crustal
gravitational collapse of over-thickened thickening are balanced by edge forces from
continental crust also explains the evolution indentation.