Page 340 - Global Tectonics

OROGENIC BELTS 323

(a) (c)
x 10 km n 3 n 3
3 3 x 10 km
8 t 40 Myr 8 t 40 Myr
α 45
α 45
η 1000 η 2
6 6

4 4

2 2

2 4 6 8 x 10 km 2 4 6 8 x 10 km
3
3
x 10 km (b) (d)
3 km n 3 3 x 10 km km n 3
8 40 50 60 70 t 40 Myr 8 40 50 60 70 t 40 Myr
α 45 α 45
η 1000 η 2
6 40 6
40
50
>100 60
4 4
60 50
60
2 2
>100 >100

2 4 6 8 x 10 km 2 4 6 8 x 10 km
3
3

Figure 10.24 Finite element model showing the influence of viscosity contrast on the evolution of oblique indenters

(image provided by J. Robl and modified from Robl & Stüwe, 2005a, by permission of the American Geophysical Union.
Copyright © 2005 American Geophysical Union). In both model runs, the geometry was identical, and n = 3 and a = 45°.
Bold dotted line is the outline of the indenter. (a,b) Viscosity contrast h = 1000. (c), (d) Viscosity contrast h = 2. (a) and (c)

show finite element mesh after 40 Myr, (b) and (d) show corresponding diagrams contoured for crustal thickness.

conditions, lateral escape of the crust the thickening tends to be either highly
increases with indenter angle for relatively localized or inhibited by the high strength
strong indenter rheologies and simulates of the material. As the strength of Asia
the patterns of displacement observed in decreases, the magnitude and distribution
eastern Tibet. of crustal thickening increase and
In situations where Asian lithosphere is gravitational buoyancy forces become
especially viscous and strong, lateral escape increasingly important. The numerical
results mainly from horizontal compression simulations of Robl & Stüwe (2005a) and
as blocks move out of the way of the rigid others (Liu & Yang, 2003) suggest that
indenter. In these cases, buoyancy forces buoyancy forces developing in weak thick
arising from crustal thickening contribute crust such as that in Tibet enhance the
little to the horizontal velocity fi eld because rate of lateral escape.