Continental Collision - Orogeny and Collapse

Orogeny and Collapse

An orogeny is underway when mountains begin to grow in the collision zone. There are other modes of mountain formation and orogeny but certainly continental collision is one of the most important. Rainfall and snowfall increase on the mountains as these rise, perhaps at a rate of a few millimeters per year (at a growth rate of 1 mm/year, a 5,000 m tall mountain can form in 5 million years, a time period that is less than 10% of the life of a typical collision zone). River systems form, and glaciers may grow on the highest peaks. Erosion accelerates as the mountains rise, and great volumes of sediment are shed into the rivers, which carry sediment away from the mountains to be deposited in sedimentary basins in the surrounding lowlands. Crustal rocks are thrust faulted over the sediments and the mountain belt broadens as it rises in height. A crustal root also develops, as required by isostasy; mountains can be high if underlain by thicker crust. Crustal thickening may happen as a result of crustal shortening or when one crust overthrusts the other. Thickening is accompanied by heating, so the crust becomes weaker as it thickens. The lower crust begins to flow and collapse under the growing mountain mass, forming rifts near the crest of the mountain range. The lower crust may partially melt, forming anatectic granites which then rise into the overlying units, forming granite intrusions. Crustal thickening provides one of two negative feedbacks on mountain growth in collision zones, the other being erosion. The popular notion that erosion is responsible for destroying mountains is only half correct - viscous flow of weak lower mantle also reduces relief with time, especially once the collision is complete and the two continents are completely sutured. Convergence between the continents continues because the crust is still being pulled down by oceanic lithosphere sinking in the subduction zone to either side of the collision as well as beneath the impinging continent.

The pace of mountain building associated with the collision is measured by radiometric dating of igneous rocks or units that have been metamorphosed during the collision and by examining the record of sediments shed from the rising mountains into the surrounding basins. The pace of ancient convergence can be determined with paleomagnetic measurements, while the present rate of convergence can be measured with GPS.