Subduction Zones and Orogeny

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

What is Orogeny?

Orogeny is the variety of processes that occur during mountain-building, including:

Distinctive Patterns of Deposition: Shallow-water sedimentary rocks on the inner side of the mountain belt, thick deep-water sedimentary rocks in the heart of the mountain belt. Thick accumulations of sandstone and conglomerate accumulate late in the history of the mountain range as it erodes.
Deformation: Folding and thrust-faulting
Metamorphism: Greenschist and amphibolite metamorphism in the core of the range, blueschist metamorphism along its outer edge.
Intrusions: Granitic batholiths are usually associated with orogeny.
Volcanic Activity: Along the crest of the mountain range there is typically a chain of andesite volcanoes.
Oceanic Trenches: Along the outer edge of most currently active mountain belts is a narrow, deep oceanic trench.
Seismic Activity: Shallow great earthquakes along the inner wall of the trench, then deeper earthquakes along a planar zone dipping beneath the mountain range, reaching depths of up to 700 kilometers.

Why Mountains Form

Mountains form at subduction zones. Shown below is a true-scale cross-section of the Andean subduction zone in northern Chile (roughly 25 S).

The vertical change of 15 kilometers in only a few hundred kilometers distance is the largest elevation change on Earth in such a short distance. Plates consist not only of the crust, but about 150 kilometers of the underlying mantle as well. Collectively the crust and associated mantle are termed the lithosphere. Oceanic crust is typically 5 kilometers thick. The continental crust thickens from its normal 40 kilometers to 70 beneath the high Andes. When the descending slab reaches a depth of about 100 kilometers, it begins to melt, causing, directly or indirectly, many of the events associated with mountain-building.

Why Mountains are High

Mountains are high because orogeny shortens and thickens the crust, and isostasy causes the thicker crust to rise. Some of the processes are shown above:

Even uniform materials, when compressed from one direction, tend to expand in the direction of least resistance.
Layered rocks shorten by folding, but the stack of layers also becomes thicker.
Thrust-faulting thickens the crust by stacking slices of crust atop one another.
Intrusions add volume to the crust.
A great deal of magma never invades the crust but accumulates at its base, a process called underplating.
Where the crust is heated, thermal expansion causes the rocks to become lighter and more buoyant.

Types of Subduction Zones

Ocean-Ocean

Both plates are oceanic crust
Because both plates are basaltic, most volcanism is basaltic
Initially results in an island chain, but long-continued subduction and accumulation of volcanic rocks and sediments can create a substantial land mass.
Examples:
- Present day: Aleutians, Marianas, Lesser Antilles
- Past: Isthmus of Panama, Cuba and Greater Antilles?

Ocean-Continent

Descending plate is oceanic crust, overriding plate is a continent.
The most "typical" orogenic belt
Initial volcanism is basaltic, but increased melting and mixing of magma with crustal rocks results in andesite and finally rhyolite volcanism.
Examples:
- Present day: Andes, Indonesia, Japan
- Past: California Coast Ranges

Continent-Terrane

Often continent-ocean subduction will bring a continent into contact with an island arc or small continental fragment.
This happens to most orogenic belts eventually.
Result is many fault-bounded regions sharply different from adjacent rocks.
Examples:
- Present day: Eastern "tail" of New Guinea.
- Past: Western North America, China

Continent-Continent

Sometimes continent-ocean subduction will bring a continent into contact with another continent.
Result is an orogenic belt in the middle of a continent.
The two sides of the orogenic belt are separated by a suture marked by blueschist metamorphism, melange, ophiolites and major faults. The rocks on either side of the suture may show quite different histories.
Examples:
- Present day: Alps, Himalaya, Persian Gulf
- Past: Appalachians, Urals

Anatomy of an Orogenic Belt

Shown here is a simple continent-ocean orogenic belt. We can divide an orogenic belt into parallel zones defined by their deformation, lithology, or metamorphism. These zones may approximately coincide with each other but somewhat overlap, so it's necessary to have distinct names for them.

Structural Zones

The Accretionary Prism

Sediment eroded from the orogenic belt accumulates in the trench and is intensely deformed as the plates converge. Like the wedge of earth ahead of a bulldozer, the sediment thickens until it is capable of resisting further deformation.

The Igneous Arc

When the descending plate reaches about 100 kilometers depth, it begins to melt. Magma invades the crust, creating batholiths and a volcanic mountain chain. The intrusions also produce metamorphism, and by making the crust more ductile, make it easier to deform. This is the belt of greatest deformation, metamorphism and igneous activity.

The Foreland

Here, metamorphism is mild but compression of the crust results in folding and thrust-faulting. Often this deformation is "thin-skinned", meaning that rock layers near the surface become detached from deeper layers much the way a carpet wrinkles when a piece of furniture is pushed over it.

This process is called decollement. Usually the layer where separation occurs is made up of weak rocks like salt, gypsum, or shale.

The Craton

This is the stable interior of the continent. It may be thinly mantled with sedimentary rocks or have large areas of ancient igneous and metamorphic rocks.

Lithologic Zones

The Eugeocline

The rocks of the accretionary prism and much of the igneous arc consist of great thicknesses of immature deep water sediment. Often these rocks show evidence of deposition in an unstable setting, such as evidence of turbidity flows or submarine landslides. This sort of deposit is known as flysch. Such rocks are typical of a continental rise or trench setting. Accompanying these rocks are often submarine volcanic rocks, pillow lavas.

The Miogeocline

The rocks of the foreland are typically shallow-water sedimentary rocks typical of a continental shelf, which become thinner toward the interior of the continent. Igneous rocks are uncommon.

As the mountain belt rises, great thicknesses of sandstone and conglomerate are deposited on its flanks and frequently bury much of the foreland. These rocks are typically shallow-water or terrestrial, often red in color, and are called molasse deposits.

The Platform

The stable interior of the continent will often be covered with thin layers of shallow-water or terrestrial sedimentary rocks. This thinly-mantled region, part of the craton, is the platform

The Shield

Areas where ancient crystalline rocks are exposed over wide areas are called shields. Every continent has at least one shield. The shield and adjacent platform together make up the craton. Driving from Green Bay to Wausau takes you from a platform into a shield.

Metamorphic Zones

One of the best indicators of former subduction is the presence of paired metamorphic belts, a belt of typical Greenschist and Amphibolite metamorphism flanked by a belt of Blueschist metamorphism.

Greenschist-Amphibolite Metamorphism

The rising magma from the descending plate heats the crust, resulting in greenschist and amphibolite metamorphism in the igneous arc. At very high temperatures, rocks become very dehydrated; even muscovite mica breaks down to potassium feldspar and amphibole to pyroxene. This sort of metamorphism, called granulite metamorphism, occurs deep in the crust just about everywhere simply due to the normal geothermal gradient. At 25 degrees per kilometer, the temperature at the base of the crust, 40 kilometers deep, is 1000 degrees C. Of course, unusually intense heating can cause it to occur at shallower levels.

Blueschist Metamorphism

At high pressures but low temperatures, rocks are metamorphosed to blueschist grade. The reason temperatures are abnormally low is that the descending slab is still cool and helps keep adjacent rocks cool as well.

Normally sodium is the most predictable major element; it occurs just about exclusively in plagioclase. At high pressure and low temperature, though, albite feldspar breaks down and forms the pyroxene jadeite and amphiboles like glaucophane and aegerine. The amphiboles are bluish, hence the term "blueschist"

It's a bit puzzling that there are very few blueschist rocks older than Mesozoic. Possibly older mountain belts have been eroded to depths where temperatures were too high for blueschist metamorphism. Or perhaps, in most orogenic belts these rocks eventually get heated to greenschist grade, and we only see the places where it hasn't happened yet. Some people have suggested that the geothermal gradient was higher in the past, meaning the deep earth was too hot for blueschist metamorphism.

Eclogite Metamorphism

At about 100 kilometers depth, pyroxene, olivine and plagioclase recrystallize to a denser form to produce sodium-bearing pyroxene and garnet. The result is one of the most beautiful of rocks, eclogite, a mass of light green pyroxene enclosing pink garnets.

Note that the boundary of eclogite metamorphism rises upward within the descending slab. This happens because the rocks are relatively cool. High temperatures inhibit the recrystallization of rocks to denser forms because high temperatures cause materials to expand. Thus eclogite metamorphism occurs at shallower depths in the descending slab. The slab in that area is denser than the surrounding mantle, and its greater density assists it in sinking. This mechanism is called slab pull and is one of the driving mechanisms of plate tectonics.

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Created 25 November 1998, Last Update 11 January 2020