Would you care to elaborate on how forces for mountain building have assumed to have changed over the past several hundred million years?
"About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.
The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year -- a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher? Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up, and such stretching would result in some subsidence due to gravity."
"We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geologic time? The oceans hold one of the key pieces to the puzzle. Because the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field, scientists, knowing the approximate duration of the reversal, can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
Evidence of past rates of plate movement also can be obtained from geologic mapping studies. If a rock formation of known age -- with distinctive composition, structure, or fossils -- mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion. This simple but effective technique has been used to determine the rates of plate motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries, such as the San Andreas Fault."
The concept of the Earth's crust in gravitational balance or equilibrium is called isostasy.
Consider low density crust floating on a denser mantle that flows.
When weight is applied to the crust, it subsides. When weight is removed, it rebounds (uplifts).
What are some ways in which weight could be applied to (or removed from) the crust? The continental glaciers which once covered much of the northern part of North America were on the order of several thousand meters thick (judging by the thicknesses of modern glaciers). (See diagrams in glacier chapter.) A 3000 meter thick ice sheet is about 2 miles thick. If we consider an ice sheet a mile or two thick, we realize that the added weight would have caused the Earth's crust to subside (or downwarp). The continental glaciers melted about 10,000 years ago. In this amount of time, the land has been steadily uplifting due to the removal of the weight. In the Hudson Bay region of Canada, as much as 330 m (about 1000 ft) of uplift has occurred. This is a rate of uplift of roughly 3 - 4 cm/year."
"Subduction zones are not totally efficient in removing the subducting plate. Some fraction of the plate gets left behind as accretionary complexes that accumulate at the leading edge of the overriding plate (Figure 22.2.1). In some cases, this accretion might be episodic, involving the collision of large lithospheric blocks, called tectonostratigraphic terranes. More commonly, only the sedimentary cover of the downgoing plate is accreted, while the underlying crust and mantle lithosphere are fully subducted. The thickness of this sedimentary cover varies considerably, from hundreds of meters at oceanic subduction zones, like the Mariana system, to as much as 7 km at oceancontinent subduction zones, such as the Makran margin of southwest Pakistan.""