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

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 lithosphere is flexed as an elastic plate under the weight of a mountain range thrust on top of the lithosphere. 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 lithosphere alone does not support all ranges. The strength of the lithosphere, 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. More gravitational potential energy is stored in a column of mass that includes a high range and thick crustal root than is in a column that is in isostatic equilibrium with the mountain belt but with thinner crust. Because of the increasing amount of work that must be done against gravity acting on an increasingly higher range, the range should reach a maximum mean elevation related to the force at which the plates are pushed together. In this sense, the mean elevations of high plateaus serve as crude pressure gauges for the average compressive stresses pushing on the margins of the plateaus. When the maximum elevations are reached, crustal shortening need not cease; convergence can continue as the range builds outward, growing laterally into a high plateau. Moreover, changes, possibly small ones, in the driving forces can lead to situations in which the crests of high ranges or high plateaus can undergo crustal extension while crustal shortening continues on the flanks of the range. From the simple analogy with the pressure gauge, we can use the simultaneous occurrence of crustal extension at high altitudes with crustal shortening on the flanks of the range to place crude limits on the average strength of the lithosphere at mountain ranges. Finally, we discuss aspects of the tectonic evolution of western North America that appear to be analogous with aspects of the active tectonics of the Himalaya, the Tibetan plateau, and the Tien Shan in Asia and the Andes.