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Stage F
Island Arc-Continent Collision Mountain Building

Westcontinent and the volcanic island have now converged and collided, creating a large mountain, and the remnant ocean basin is reduced to a suture zone. Eastcontinent has also come onto the cross section, but it is still far away. Collision mountain building is of two basic kinds: (1) Island arc-Continent collision, and (2) Continent-Continent collision. The island arc-continent collision is described here, the continent-continent collision later.
     Observe the geometry in the Stage F cross section. Because the subduction zone dips east, the island arc has attempted to slide up over the edge of the former divergent continental margin. We can generalize this: in every collision orogeny one plate is going to ride up onto the edge of the other. The overriding plate is called a hinterland. The overridden plate is called a foreland.
      It does not matter what is on the edge of the plate (volcanic arc, hot spot volcano, continent), or which way the subduction zone dips, the overriding piece is always the hinterland, the overridden piece always the foreland.

Suture Zone:
      During the collision the first part of the volcanic arc to be affected is the trench melange. The melange has been accumulating for a long time as it was scraped from the descending oceanic crust, and now it is thrust up over the hinterland along a major thrust fault where it is smeared out and sheared even more. In the end the melange belt will go from being a hundred or more kilometers wide to maybe only 10 kilometers wide, or maybe even a single thrust fault plane. This narrow zone of ground up, smeared out rock is the suture zone and it is the boundary zone which separates the two blocks which have collided and are "sutured" together. It is also all that remains of an ocean basin that may have been thousands of kilometers wide.

Hinterland mountain:
      The volcanic island arc may have been a few kilometers high before the collision but now it is dramatically thrust up even higher into snow capped mountain peaks. Along the way very large thrust faults dipping back toward the hinterland carry rock toward the foreland. Behind the major mountain peaks some volcanic activity may continue from the last magmas rising from the subduction zone. It is the last gasp, however, because with the collision subduction stops, volcanic activity stops, mountain building stops, and the only thing remaining is for the mountain to erode.

      Several things happen in the foreland. The first is that the ancient thick wedge of DCM sediments accumulated on Westcontinent gets compressed, folded into anticlines and synclines, and thrust faulted toward the foreland. Second, the DCM sediments closest to the island arc are depressed down into the earth by the overriding arc, where they are Barrovian metamorphosed forming marble, quartzite, slate, and phyllite. Deeper rocks may metamorphose all the way to amphibolite or granulite facies.
     Third, inland from the mountain a foreland basin rapidly subsides into a deepwater basin which fills with a thick clastic wedge of sediments. Foreland basin clastic wedges are common in the geologic record, although their individual features vary depending on local circumstances.
     One of the things we are interested in is the composition of these sediments filling the foreland basin. Because an island arc has formed the hinterland mountain the sediments eroded from it are dominantly lithic in composition (volcanic and plutonic igneous as well as metamorphic rock fragments), with varying amounts of sodic plagioclase feldspar from the intermediate igneous rocks . However, since some of the parent rocks likely include Westcontinent DCM sedimentary rocks which have already been through one cycle of weathering and and erosion, they will generally be more quartz rich than those from a pure arc (QFL diagram, sediment is evolving along path of red arrow).
     The foreland basin depositional environments the sediments are deposited in typically begin with black deep water shales. But the large volume of sediment eroding from the mountain will quickly (geologically) fill the basin in. Depositional environments typically begin with submarine fans which shallow upward to shelf environments, and then eventually terrestrial deposits (meandering and braided rivers.) Inland toward the craton the foreland basin shallows and the clastic wedge thins and becomes finer grained until it merges with sediments being deposited on the craton. (Observe that there are two different kinds of sedimentary wedges in the Wilson cycle. The first are the DCM wedges which begin thin on the craton and thicken toward the ocean basin. The second are the foreland basin clastic wedges, which begin thick next to the mountains and thin toward the craton.

Denouement of the Mountain Range:
     In time, the hinterland mountains will erode to sea level (a peneplain). But by that time the hinterland (that is, the island arc) is permanently sutured to the Westcontinent (Stage G cross section, left side). Westcontinent is now larger because of the island arc-continent collision, but this was possible only because subduction and fractionation created the intermediate and felsic batholiths which compose the core of the volcanic arc, and which have now become part of a larger, sutured continental crust.

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Last Update: 9/05/00

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