Stage H - Continent Collision


 
 Continent-Continent Collision Mountain Building

By Stage H the remnant ocean basin separating East- and West continents has closed and they have collided to form a Continent-Continent Collision Orogeny. This mountain building has many of the same elements as the Island Arc-Continent Collision: a hinterland, foreland, suture zone, foreland basin, and a towering mountain range, most likely Himalayan size (Detailed Cross Section).
     
One major difference between this collision orogeny and the Stage F arc-continent collision is that because the hinterland began as a DCM with a thick wedge of sediments, it is these DCM rocks that are being thrust toward the foreland. In the arc-continent collision scenario, it is pieces of ocean lithosphere (ophiolite suite) and the volcanic arc that are thrust toward the foreland. 
     
The hinterland overrides the edge of West continent its weight pushes the arc deep into the earth, resulting in Barrovian metamorphism of the arc rocks. This is probably not the first time these rocks have undergone Barrovian metamorphism, since during the arc's formation much of its deeper portions were metamorphosed by the invasion of batholiths.

Sediments
     
The sediments that erode and fill the foreland basin would be different in composition from those eroded from an island arc, even if they are deposited in very similar depositional environments. The hinterland rocks consist of large volumes of DCM sedimentary rocks undergoing a second (or third, or fourth) cycle of weathering and erosion. They are quartz rich, as shown in the QFL (blue field). Also, because the source land is complex, the diversity of lithic fragments is great, including sedimentary, metamorphic and igneous rock fragments. Also, feldspar is present due to the weathering and erosion of metamorphic schists and gneisses (most likely sodium plagioclase), and eventually exposed batholiths (sodium plagioclase and orthoclase).
     
All this is in contrast to the sediments filling the foreland basin of Stage F. Because the hinterland in Stage F was a volcanic arc, the sediments entering the foreland basin were much more volcanic-lithic rich and more quartz poor (QFL, Green Field), in contrast to the much more quartz rich sediments filling the continent-continent collision foreland basin (QFL Blue Field).

Foreland Basin
     
Foreland Basins develop rapidly relative to geologic time. Just before the collision, the foreland is tectonically stable with deposition of Quartz rich sandstones and Limestones (Stage G or Detail Of DCM). Collision occurs and within a few million years the foreland basin subsides hundreds and then thousands of feet (Series Of Stages). The shape of the basin is usually asymmetrical with the deepest portion closest to the mountain, shallowing toward the foreland continent (Detail, Stage II).
     
The deposits may be up to two miles thick and the speed with which the basin subsided is often times preserved in the rock record. Typically the bottom sequence of rocks are those indicative of tectonic stability (quartz and limestone), however, black shales, which are indicative of deep ocean deposition, are juxtaposed on top of the stable rocks. Deposition and subsidence of the basin occur simultaneously but as overthrusting of the hinterland stops, subsidence stops, giving sediment the chance to catch up and fill in the basin (Detail Stage III, and Detail, Stage IV). The rock units commonly situated on top of the deep water black Shales are turbidity currents and submarine fan deposits as the basin initially fills and as the water shallows, deposits of shelf environments are seen.
     
Thick wedges of terrestrial sediments build out toward the coastline near the mountain. These begin with alluvial fan and braided river deposits, which eventually give way to meandering rivers that work their way down to the coast. The rivers dump sediment into the shoreline region building land where there was once water. This building of the shoreline out across the basin is called Progradation, or a prograding shoreline. In time the Shoreline will prograde all the way across the basin, filling it in completely, while the terrestrial sediments will pile up another couple of thousand feet (Detail, Stage V).
     
At the end of this stage, the mountain is mostly gone, eroded down to low hills, and most of its rock has been transferred to the foreland basin. Over the next few million years even these low hills will disappear and the land will be reduced to a peneplain (Wilson Stage I). The surface topography would be flat, however, the geology beneath the surface would be complex.
 
Contributed by Lynn Fichter  
Monday, September 29, 2014
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