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3.1.1.b Interactions among Components (Cycles-Hydrological, ENSO,NAO)
Wilson Cycle(Opening and Closing of Ocean Basins)
In 1963, J Tuzo Wilson developed a concept crucial to the plate-tectonics theory. He suggested that volcanic island chains may have formed due to the movement of a plate over a stationary hotspot in the mantle.
The Wilson Cycle model follows a series of cross sections. It begins with a hypothetical tectonically quiet continent divided into nine arbitrary stages. that do not exist naturally. The earth’s formation is an ongoing series of related processes that lead from one to the next, along one of three types of plate boundaries:
- divergent: plates move apart and new crust is created,
- convergent: plates move together and crust is destroyed,
- transform: plates slide past one another
Figure 1. Types of b
Stage A A Stable Continental Craton
The first or opening half of the Wilson Cycle starts with a tectonically stable continental craton bordered by ocean basins. The continent is floating in perfect isostatic equilibrium (it will not rise or sink) and there is no earthquake or volcanic activity.
The continent is composed of relatively light weight felsic igneous rock such as granite which enables the continent, in isostatic equilibrium, to float. Its surface is a few hundred meters above sea level. On the surface is a blanket of sandstone, resulting from erosion and sorting. Limestone is also found if the climate is warm, but most clays or shales have been wind blown or washed off the continent into the surrounding ocean basins.
The ocean basins are composed of mafic igneous rocks such as baslt and gabbro. These are heavier rocks so they isostatically float on the underlying mantle a little over 75 km below sea level. Continents and oceans are the natural divisions on the earth.
Stage B Hot Spot and Rifting
A disturbance from deep in the mantle occurs. A plume of hot mafic magma, rises toward the surface, ponds at the base of the continent, and creates a hot spot. Its heat warms the continental crust causing it to expand and swell into a dome 3-4 km high and about a thousand km in diameter. As this swells it thins and stretches until the brittle upper surface cracks along along a series of 3 axial rift valleys radiating away from the center of the hot spot. These form a triple junction with the rift valleys radiating from the center. Rifting begins the splitting of the continent into two pieces of the continent into two pieces, west and east, although they are still connected at this stage. Mafic hot spot volcanoes in the rift fractionally melt the lower continental crust which thenis sent back to the surface to create large volcanoes.
The axial rifts valleys are block-fault (D Type of Faults) graben bordered by horst mountains on either side.
The edges of the tall horsts that border the axial graben are the continental terraces or hinge zones. The tall axial graben contains numerous smaller horsts and graben that rotate as they subside and trap small basins between the down faulted-block and the wall behind the fault. The axial valley floor at first is subareal but later sinks and, except for the lakes which are trapped depressions created when the graben floors drop, is invaded by the sea creating a narrow sub aqueous marine basin.
After the sea invades the rift, alluvial fan deltas develop. Turbidity currents (underwater avalanches) transport thousand of meters of eroded sediment from the horst mountains and smoothed out deep grabens and fill in the basin centre. Shelf and near-shore deposition takes over and sands now dominate. Abundanat cross beds and ripples indicate shallow water processes.
Stage C Creation of New Oceanic Crust: Early Divergent Margin
Sometimes a string of hot spots join together to create convection cells creating a rifting system that creates a new ocean basin.
A great surge of mafic volcanic activity along one side of the axial rift splits the crust in two along one side of the axial rift so that one side has the entire axial rift and the other doesn’t. The volcanic activity, concentrated at the rifting site, continues and fills in the gap between the two pieces of the original continent with volcanic igneous rock and causes them to drift further apart. Within a few million years the two continents can be separated by thousands of kilometers. Because all this new igneous rock is mafic and ultramafic it remains sub aqueous, and floats about 5 km below sea level.
Now there are two plates and a new divergent plate boundary between them. The new ocean basin begins to form the edge of the continent, cools and subsides below sea level and as the continental edge subsides, the sea begins to transgress, or migrate across the edges and deposits layers of sandstone that forms a beach on the Divergent Continental Margin (DCM. Off shore is a shallow shelf deposition composed of shale from a clastic source on the continent, or of limestone or carbonate if the continent is stable and the climate is warm.
Stage D Full Divergent Margin
Westcontinent and the new ocean basin with its rifting center or mid oceanic ridge remain. Heat, still rising to the surface from the convection cells concentrated at the rifting site, causes the ocean to widen. The newly formed DCM moves away from the heat source, and cools. Cool crust is denser than warm crust so as the DCM cools it sinks, rapidly at first, but ever more slowly with time. This is the process of thermal decay. A great wedge of sediment derived from the erosion is deposited on the DCM, consisting mostly of shallow-water marine deposits. In about 5-10 million years the tall horsts sink below the sea and in another 100 millioin years, the DCM cools completely and stabilizes about 14 kilometers below sea level.
Stage E Creating a Convergent Boundary: Volcanic Island Arc Mountain Building
The second or closing half of the Wilson Cycle occurs after divergence stops. A new plate boundary is created, and the two continents begin to move back toward each other. Convergence begins when oceanic crust breaks up or decouples at some place and descends into the mantle along a subduction zone. It is always oceanic crust which decouples and subducts; continental crust is too light. Subduction zones can also along the edge of a continent and is the Cordilleran type. Both kinds of subduction cause volcanic mountain building. At a subduction zone, cold oceanic crust is dragged down into a trench in the mantle 1-2 km below the normal ocean floor. As it slides into the mantle, it heats up. At about 120 km deep rock begins to melt and fractionate, dividing into two fractions each of a different composition, and forms magma. The magma, hot and of low density, rises toward the surface, forms batholiths, breaks onto the ocean floor as lava and builds a volcano which eventually rises high enough to form an island.
The location of the volcano is called the volcanic front – in three dimensions it is a string of volcanoes all rising above the subduction zone. The area on the trench side of the volcanic front is the forarc, and the area on the back side of the volcanic front is thebackarc. A new Convergent Continental Margin (CCM) or boundary is created along the zone of subduction. The ongoing subduction and melting builds a volcano perhaps 7-8 km off the ocean floor, and has a center or mobile core.
As soon as the volcano breaks the surface weathering/erosion processes attack it and form sediments. Sediments on the back arc side just spill onto the ocean floor and stay there undisturbed. On the forearc side, however, the sediments pour into the trench like turbidity currents and are scraped off the into a melange deposit, a chaotic mixture of folded, sheared, and faulted metamorphosed blocks of rock formed in a subduction zone, or they are partially subducted and metamorphosed. If the climate is right, limestone reefs grow around the island.
The ocean basin to the west of the volcanic arc is trapped between the DCM and the subduction zone. As subduction continues, the more oceanic crust is subducted and destroyed and the ocean basin between the two becomes smaller and smaller. This remnant ocean disappears altogether when the continent and volcano collide. Subduction zones always create remnant ocean basins and no ocean basin can survive too long. The oldest ocean basins we know of are only around 200 million years old compared to the 4 billion year age of the earth. The continental crust is too light to subduct so it tends to remain around just about forever, excluding weathering and erosion.
Stage F Island Arc-Continent Collision Mountain Building
The continent and the volcanic island have now converged and collided, creating a large mountain, and the remnant ocean basin is reduced to a suture zone. The other continent has come back a little but it is still far away. Collision mountain building is of two basic kinds Island Arc-Continent and Continent-Continent.
The subduction zone dips down ahead of the volcanic front and, as they collide, the Island Arc slides up over (overrides) the edge of the former DCM. In every collision orogeny, one plate overrides the edge of the other. The overriding plate is called a hinterland. The overridden plate is called a foreland.
During the collision the first part of the volcanic arc to be affected is the trench melange that has accumulated over a long time. Now it is thrust up over the hinterland along a major thrust fault where it is smeared out and sheared even more. This zone of ground up, smeared out rock is the suture zone and it is not only the boundary where two blocks have collided and melded together but also all that remains of an ocean basin.
In the Hinterland, the volcanic island arc, only a few kilometers high before the collision, is now thrusted up into high, snow capped mountain peaks. Along the way very large thrust faults dipping back toward the hinterland carry rock toward the foreland. With the collision subduction stops, volcanic activity stops, mountain building stops, and the only thing remaining is for the mountain to erode.
In the foreland, several things happen. The ancient thick wedge of DCM sediments accumulated on the continent is 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 metamorphose forming marble, quartzite, slate, and phyllite. And inland from the mountain a foreland basin rapidly subsides into a deepwater basin which fills with a thick clastic wedge of sediments.
In time, the hinterland mountains will erode to sea level (a peneplain). But by that time the hinterland – island arc is permanently sutured to the continent which is now larger because of the island arc-continent collision.
The two continents are still being driven together by the beginning of another subduction zone. It can begin anywhere within the ocean basin and form another island arc, and it could dip in any direction. Decoupling occurs under the edge of the smaller continent, forming a Cordilleran type of mountain.
Inland from the volcanic front, backarc spreading occurs. Heat rising from above the subduction zone creates a small convection cell which stretches the continental crust so that normal faults develop into deep graben. The graben fills with a complexity of deposits including coarse clastic sediments from braided rivers and volcanic rock rising from the subduction zone.
The processes of trench formation, subduction and fractional melting of the oceanic crust, melange deposition, and metamorphism are the same here as for an island arc oregeny but this tectonic activity is occurring along the other side of the old DCM which, like all rifted margins, has accumulated a thick wedge of DCM sedimentary rocks. Thus, the rising magmas now inject into the thick wedge of continental margin sediments and heating them.
The remnant ocean basin separating the two 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, and of immense size. This is a mirror image; the hinterland is on the left not right.
One major difference between this collision orogeny and the 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 it is pieces of ocean lithosphere and the volcanic arc that are thrust toward the foreland.
Another difference is that the hinterland is overriding the eastern side of the volcanic arc that collided with larger continent before. The hinterland uses its weight to shove the arc deep into the earth, resulting in metamorphism of the arc rocks.
The sediments eroding from this mountain and filling the foreland basin would also be different in composition from those eroding 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 cycle of weathering and erosion.
Foreland Basins develop very rapidly geologically. Just before the collision the foreland is tectonically stable. Then the collision occurs and within a few million years the foreland basin subsides hundreds and then thousands of meters. The shape of the basin is usually asymmetrical with the deepest portion closest to the mountain and shallowing toward the foreland continent with a lot of subsidence, and a lot of sediment all the way to the top, and beyond. But as the water shallows upward the turbidity currents give way to shelf environments. Thick wedges of terrestrial sediments build out toward the coastline. 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.
Now the mountain is mostly gone, eroded down to low hills, most of its rock transferred to the foreland basin. And over the next few million years even these low hills will disappear and the land will be reduced to a peneplain.
Stage I Stable Continental Craton
The cycle now comes to an end. The original continental craton which was rifted into two pieces is now back together, and stabilized once more. However, that this new continent is quite complex compared with the first craton, and that the basement rocks exposed at the surface are very diversified. There is a volcanic arc trapped between them and there are now two foreland basin clastic wedges. There are two suture zones of melange and a host of different igneous and metamorphic rocks.