Stage F - Island Collision

Island Arc-Continent Collision Mountain Building

When the West continent and the volcanic island converge and collide, a large mountain is created and the remnant ocean basin is reduced to a suture zone. East continent has moved closer but is still separated from West continent. There are two types of collision mountain building: (1) island arc-continent collision, and (2) continent-continent collision. 
Because the subduction zone dips east (in the above cross-section), the island arc will slide up and over the edge of the former divergent continental margin. The overriding plate is called a hinterland and the overridden plate is called a foreland.

~Suture Zone~
Upon collision, the first part of the volcanic arc to be affected is the trench melange. The melange is scraped from the descending oceanic crust and accumulates over time, and is then is thrust up and over the hinterland along a major thrust fault where it is smeared out and sheared even more. The melange belt can be reduced from a width of a hundred or more kilometers to perhaps 10 kilometers, or even just a single thrust fault plane. This narrow zone of ground up, sheared rock is the suture zone and it is the boundary zone which separates the two blocks that 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 that existed prior to the collision is 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 of the magma rising from the subduction zone. When collision stops, subduction and volcanic activity stop; therefore, mountain building comes to a halt and erosion processes begin.

The ancient thick wedge of DCM sediments accumulated on West continent gets compressed, folded into anticlines and synclines, and thrust-faulted toward the foreland. 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. 
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. Because the hinterland mountain is typically formed from an island arc, the sediments eroded from it are dominantly lithic in composition (volcanic and plutonic igneous with metamorphic rock fragments), with varying amounts of sodic plagioclase feldspar from the Intermediate igneous rocks. However, since some of the parent rocks likely include West continent DCM sedimentary rocks that 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).
In the foreland basin, deep-water black shales are typically deposited first, but the large volume of sediment eroding from the mountain will quickly (geologically) fill the basin. 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. There are two different kinds of sedimentary wedges in the Wilson Cycle. The first are the DCM wedges, which are thin on the Craton and thicken toward the ocean basin. The second are the foreland basin clastic wedges, which are 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 West continent (Stage G Cross Section, left side). West continent 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 that compose the core of the volcanic arc, and which have now become part of a larger, sutured continental crust. 

Contributed by Lynn Fichter 

Friday, August 24, 2018
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