Himalayas and Andes

Once convergence has thickened the crust over a broad region, the available plate forces to overcome spreading forces, rather than forces within individual thrust sheets, determines the evolution of an orogen. The orogen can grow wider by thickening crust over a broader region and convergence between the major plates of the orogen can continue as major strike-slip ``indentor'' faults allow parts of the colliding plates to move laterally from the collision zone without widening it.

Tibet and the Himalaya are the widest active terrestrial orogen. The interior of Tibet is a low-relief plateau which is about 5 km above sealevel [ Fielding et al., 1994]. There is little erosion within the plateau because it is in the rain shadow of the Himalayas and because it is internally drained. Fielding et al. [1994] conclude that crustal thickening at deep levels is responsible for the high elevation of Tibet because there is little evidence for recent shallow thrusting within the plateau. In fact, the region is extending east-west at shallow levels along north-south grabens. The north-south convergence associated with plate motions occurs within the Himalayas and on indentor faults.

The highest peaks on Earth, the high Himalayas on the southern margin of the plateau, are climatically controlled [ Isacks, 1992; Fielding et al., 1994; Masek et al., 1994; Montgomery, 1994]. The average elevation of the high Himalayas is the same as that of the plateau. However, glacial erosion has produced deep valleys. Uplift associated with isostatic compensation has maintained the average elevation around 5 km uplifting the less eroded peaks above 8 km. At the present, rapid erosion is outpacing tectonics such that the southern edge of the plateau is retreating northward.

North-south variation of the topography associated with foreland thrusts on the east side of the Andes provides another example of the effect of climate on a wide orogen [ Isacks, 1992; Masek et al., 1994]. The Altiplano is a dry plateau similar to Tibet. The northern margin of the plateau, the Andes north of 18S, receives intense precipitation. Like the Himalayas, it has a line of high peaks with an average elevation similar to that of the plateau. South of S the thrust belt is arid. The margin of the plateau lacks incised valleys and high peaks.