Some simple physical aspects of the support, structure, and evolution of mountain belts

Abstract

We review separately aspects of two types of forces that resist mountain building and therefore that affect the support, deep structure, and evolution of mountain ranges, using observations from the large-scale tectonics of Asia and the Andes to illustrate them. The first such force might be termed mechanical strength. In its simplest description, the lithosphère is flexed as an elastic plate under the weight of a mountain range thrust on top of the lithosphère. The gross shapes of foredeep basins and gravity anomalies over them show that the simple analyses, in terms of elastic plates over inviscid fluids, are reasonable first approximations. At the same time, the weights of some mountain ranges are inadequate to depress lithospheric plates to the depths of neighboring foredeep basins, and the weights of others would create deeper foredeep basins than observed if additional forces were not present. Thus, the strength of the lithosphère alone does not support all ranges. The strength of the lithosphère, however, does affect the geometry of major, deeply rooted thrust faults, which seem to behave as crustal-scale ramp-overthrust faults. Seismic activity seems to be concentrated on the steeper, deeper sections, whereas slip on the flatter planes parallel to the underthrusting basement seems to occur aseismically. The second force that resists mountain building is gravity; the forces that drive two plates together and that cause crustal thickening must do work against gravity. M…