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自然中的山脉作为局部构造特征可以在很长的时间内(大于10Ma)生长并维持。可是,如果根据现今的岩石层流变学知识,忽略地表过程,任何陆内山脉,当其宽度超过可以由岩石层的强度所支持的程度时,将会在几十百万年内塌陷。例如,考虑由石英为主体的地壳流变性,使其地壳根侧向扩展的结果,一条初始地形起伏为3km高、300～400km宽的山脉在大约15Ma内将被消减一半。我们认为地表过程事实上能够阻止这种“地下塌陷”。物质在高地形处的运移和在前沿处的沉积阻止了地壳根的扩展,能够最终驱动物质向造山脉带的净流入。我们进行了一套数值实验以证实这一假设。一个具有脆性-弹性-塑性流变性、并有初始的山脉载荷的岩石层截面受到水平缩短和地表过程的作用。如果侵蚀较强,物质的迁移将比山脉下的地壳增厚的补充进行得更快,地形被迅速地光滑掉。例如,当侵蚀系数k=10~3m~2/a(侵蚀率的量级为每年零点几个毫米)时,一个3km高、300～400km宽的山脉形态在大约15Ma内其高度将减半。这种情形可以称为“侵蚀塌陷”。如果侵蚀不足够活跃,地壳根将侧向扩展出而发生“地下塌陷”。在处于两者中间的第三种情形中,侵蚀造成的物质运移被均衡回跳和下地壳中的向内流动动态地补偿,因而山脉可以生长。在这种“山脉生长”情形中,山脉向一个特征性的均衡形状演化,后者主要依赖于侵蚀规律。侵蚀率可以较高(如0.5～0.9mm/a),接近于构造抬升率(如0.7～1.1mm/a),而几倍于地形抬升率(如0.15～0.2mm/a)。这些实验表明地表过程有利于局部地壳缩短并参与陆内山脉生长,因此对大陆岩石层长期变形的解释和模拟必须考虑地表过程的作用。相反地,当从地貌学过程方面对大尺度的地形特征进行解释和模拟时,则必须考虑岩石层的力学响应。
Mountains in nature as local tectonic features can grow and sustain for a long period of time (greater than 10 Ma). However, given the current knowledge of lithosphere rheology, which neglects the surface processes, any intracontinental range will collapse within tens of millions of years when its width exceeds the level that it can support by the strength of the rock formation. For example, considering the crustal rheology dominated by quartz, spreading its crustal roots laterally, an undulating 3km high mountain range with a width of 300-400km will be reduced by half in about 15Ma. We think that the surface processes in fact can stop this kind of “underground collapse.” The transport of matter at high terrain and the deposition at the front prevents the expansion of the crust roots and can eventually drive the net inflow of matter to the orogenic belt. We conducted a numerical experiment to confirm this hypothesis. A section of lithosphere with brittle-elastic-plastic rheology and initial mountain load is subject to horizontal shortening and surface processes. If the erosion is stronger, the migration of matter will proceed faster than the thickening replenishment of the crust beneath the mountains, and the terrain is quickly smoothed out. For example, a 3 km high, 300-400 km wide mountain pattern will halve its height in about 15 Ma when erosion factor k = 10-3 m -2 / a (erosion rate on the order of millimeters a year). This situation can be called “erosion collapse.” If the erosion is not active enough, the crust roots will expand sideways to “collapse underground.” In the third case, between the two, the material movement caused by erosion is dynamically compensated for by a balanced rebound and inward flow in the lower crust so that the mountains can grow. In this “mountain growth” scenario, the mountains evolve to a characteristic equilibrium shape, the latter mainly relying on the erosion pattern. The erosion rate can be high (eg 0.5-0.9 mm / a), close to the tectonic uplift rate (eg 0.7-1.1 mm / a) and several times the topographic uplift rate (eg 0.15-0.2 mm / a). These experiments show that the surface processes are conducive to local crustal shortening and participate in the growth of intra-continental mountains. Therefore, the interpretation and simulation of the long-term deformation of continental lithosphere must consider the role of surface processes. Conversely, when interpreting and simulating large-scale topographic features from geomorphological processes, the mechanical response of the lithosphere must be considered.