Wilson cycle

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The Cyclical Opening and Closing of Ocean Basins

No rock is accidental. No idea in geology is more profound than this; it runs from the center to the whole of geology and influences every subdiscipline of the field. Genuine understanding of the science of geology begins with one’s ability to understand and explain why no rock is accidental.

 

     Tectonics is concerned with deformation in the earth and the forces which produce deformation. Plate tectonics is the theory that the earth’s lithosphere (outer rigid shell) is composed of several dozen “plates”, or pieces, which float on a ductile mantle, like slabs of ice on a pond. In plate tectonic theory earth history, at its simplest, is one of plates rifting into pieces diverging apart and new ocean basins being born, followed by motion reversal, convergence back together, plate collision, and mountain building. This cycle of opening and closing ocean basins is the Wilson Cycle.

     Plate tectonics is one of the great unifying theories in geology. Virtually every part of the earth’s crust, and every kind of rock and every kind of geology can be related to the plate tectonic conditions which existed at the time they formed. Nothing in geology makes sense except in terms of plate tectonic theory.

     One of the most important messages of modern understanding of plate tectonics and the Wilson cycle is that beginning with a parent igneous rock of mafic/ultramafic composition all the other rocks now on the earth can be generated. The most important message of the plate tectonic rock cycle is that each and every rock forms only under a specific set of tectonic conditions. 

 

Most geologic activity occurs at the three kinds of plate boundaries:

(1) divergent boundaries where plates are moving apart and new crust is being created,

 

(2) convergent boundaries where plates are moving together and crust is being destroyed, and

 

(3) transform boundaries where plates slide past one another.

 

     Very interesting geology occurs along transform boundaries,

as all the faulting along the San Andreas fault system in California attests to, but this model does not include transform boundaries.

 

    We have two models summarizing earth evolutionary processes.

(1)   The Wilson Cycle, explored below, and . . . 

 

(2)   The Tectonic Rock Cycle, a more theoretically abstact model of how rocks and the earth evolve.

     The following Wilson Cycle model follows the series of cross sections constituting the Wilson cycle. It begins with a hypothetical geologically (tectonically) quiet continent. The model is divided into nine stages, but the stages are arbitrary and do not exist naturally. The earth is an ongoing series of processes so it is much more important to understand the processes, how they are related, and how one process leads naturally to the next process.

     Also note that this Wilson Cycle is a simple, ideal model. The earth has many continents, which migrate across its spherical surface in very complex ways. Just about any scenario you can think of, and any exception you can imagine is quite possible – and has probably happened during some point in the earth’s history. 

 

Stage A

A Stable Continental Craton

 

 

Imagine a very simple situation – a tectonically stable continental craton bordered by ocean basins all around. The continent is eroded down nearly to sea level everywhere (a peneplain); it is dead flat from edge to edge and corner to corner and there is no tectonic activity anywhere. On the surface is a blanket of mature quartz sandstone (QFL, yellow field), the result of millions of years of weathering and erosion and sorting. Limestones are probably also well developed, if the climate is warm, but most shales (clays) have been wind blown or washed off the continent into the surrounding ocean basins.

      The continent is in perfect isostatic equilibrium; by itself it will not rise or sink. Nothing exciting is happening; no earthquakes or volcanic activity – unrelenting boredom, perhaps for tens or hundreds of millions of years.

 

     Continents are composed of relatively light weight felsic igneous rock (granites, granodiorites, etc.). Light enough that when eroded to a peneplain, and “floating” in isostatic equilibrium, it’s surface is a few hundred feet above sea level. Thus, granite gives us the dry land we live on.

     Ocean basins are composed of mafic igneous rocks (basalt and gabbro), and because these are relatively heavy rocks they isostatically “float” on the underlying mantle a little over 5 miles below sea level. Continents and oceans are thus natural divisions on the earth, not only because they are composed of very different rocks, but also because one lies naturally above sea level, and the other naturally far below sea level. 

    

Stage B

Hot Spot and Rifting

 

 

Into the peaceful stable continent of Stage A comes a disturbance. From deep in the mantle a plume of hot mafic or ultramafic magma, rises toward the surface and ponds at the base of the continent creating a hot spot. Heat from the hot spot warms the continental crust causing it to expand and swell into a dome 3-4 kilometers high and about a thousand kilometers in diameter. As the dome swells it thins and stretches like pulled taffy (or silly putty) until the brittle upper surface cracks along a series of three rift valleys radiating away from the center of the hot spot. These form a triple junction. Details of early rifting. Ideally the three rift valleys radiate from the center at 120o, but often the triple junction is not symmetrical and arms may diverge at odd angles. Rifting is splitting the original continent into two pieces, west and east, although they are still connected at this stage.

     Mafic volcanism is normal and appears as intrusive sills, or vent volcanos and/or flood basalts from fissure volcanos rising along feeder dikes. The volcanics may be mostly volcaniclastic, or lava flows of vesicular and columnar-jointed basalt. Subaqueous pillow basalts are not unusual in later stages.

      Mafic (hot spot) volcanoes are common and appear as vent volcanos and/or flood basalts from fissure volcanos in the rift. Commonly the intense heat of the hot spot will fractionally melt the lower continental crust composed of granodiorites or plagiogranites. The results are alkali granitic magmas that rise to emplace as batholiths, frequently sending conduits to the surface to create large felsic volcanoes. The simultaneous formation of these two very different rock types (one from the bottom and one from the top of Bowen’s Reaction Series) is called a bimodal distribution.

 

Active Rifting

     Axial rifts are typically tens of kilometers across, and the elevation from the rift floor to the mountain crests on either side are as much as 4-5 km. Structurally, rift valleys are block-fault graben bordered by horst mountains on either side (see Hot Spot/Thermal Doming cross section). The edges of the major horsts bordering the axial graben are the continental terraces (also called hinge zones) (see Foundering of Rift Valley/Marine Invasion cross section).

      The major axial graben contains numerous smaller horsts and graben. The normal faults are listric type. The fault surfaces are curved so that the graben blocks rotate as they subside, trapping small basins between the down faulted-block and the wall behind the fault. It is also typical for numerous, smaller lateral graben to form for several hundred kilometers on either side of the axial graben. Initially the axial valley floor is subareal, that is above water (except for lakes), but in time the axial graben subsides and the sea invades creating a narrow marine basin (making it subaqueous).

     A diversity of sedimentary rocks are deposited in the graben, mostly in short system environments where facies changes are very rapid. The horst mountain highlands are composed of felsic and high grade metamorphic continental basement which erode rapidly to coarse, subareal arkosic breccias and conglomerates (fanglomerates) (red field on the QFL diagram). All around the basin edges, at the base of the fault scarps, these accumulate in steep-faced alluvial fans. Away from the alluvial fans, toward the basin axis, the fans give way to braided rivers and then often lakes. See reconstructions for the Rifting of Pangaea.

     The lakes are trapped depressions created when the graven floors drop and pond the water. Many of the lakes are very deep and, based on modern rift lakes, may be extremely alkaline with salt crusts floating on the surface. In the lake bottoms black, organic rich, anoxic clays accumulate because there is no circulation or oxygen in the deep water.

     After the sea invades the rift, fan deltas develop. Here alluvial fans still form next to the mountains but now turbidity currents rather than braided rivers transport sediment toward the basin center. The basin center is still frequently deep and anoxic, and thinly laminated black clays and silts are deposited. Thousands of meters of sediment may accumulate during this stage. 

     In a geologically short time (~ 10 million years) the basin finishes filling. As the former great relief of the horst mountains and deep graben smooth out, shelf and near-shore deposition takes over. Sands now dominate and abundant cross beds and ripples indicate shallow water processes. By this time the Early Divergent Margin stage is beginning (Foundering of Rift Valley).

 

 

Stage C

Creation of New Oceanic Crust: Early Divergent Margin

A hot spot may form, be active for a while, and then just die. But sometimes a string of hot spots joint together to create convection cells. These turn the hot spot into a rifting system poised to create a new ocean basin (the four layers that compose oceanic lithosphere are the ophiolite suite).

      The process of ocean basin formation begins with a great surge of mafic volcanic activity along one side of the axial rift. Axial rifts do not usually split in two, down the middle, but separate along one side or the other (detailed drawing). In this model the activity is on the east (right) side of the rift and so the axial rift will remain with the western continent. At first the magma is injected as a large number of basaltic dikes into the now thinned and stretched granitic continental crust. So many dikes form in fact that it is finally hard to decide looking at them what the original rocks were, granite invaded by basalt, or basalt invaded by granite. This mixture of continental granite and injected basalt is called transition crust (principally because the speed of seismic (earthquake) waves traveling through it is transitional between the slower granite and the faster basalt.) 

     The mafic volcanic activity is concentrated at the rifting site, but is not confined there. Feeder dikes cut through the crust at many places, sometimes hundreds of miles to the sides of the axial rift. This magma may emplace as sills or laccoliths, or may surge to the surface to form fissure volcanos and lava flows.

     As the volcanic activity continues, the two pieces of the original continent begin to drift apart and the gap between them fills with mafic igneous rock. Surge after surge of magma rises from the convection cells in the mantle into the continuously spreading gap as the continents move farther and farther 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 in composition (basalt/gabbro near the surface and dunite/peridotite at depth), and high in density, it “floats” about 5 km below sea level. These layers of rock that form the oceanic lithosphere are the ophiolite suite. 

     The final result is that beginning with only one tectonic plate in Stage A, rifting has created a new divergent plate boundary and two plates, one on the west (containing Westcontinent) and one on the east (containing Eastcontinent).

 

Sedimentary Record

     As the new ocean basin begins to form the edge of the continent cools and subsides below sea level. And as the continental edge subsides below sea level the sea begins to transgress, or migrate across, the edge of Westcontinent (and Eastcontinent too). This is the beginning of deposition of Divergent Continental Margin (DCM) sediments, which will become much more prominent in the next few stages (see Early Divergent Margin). But initially, as the sea begins its transgression a layer of quartz sandstone is laid down as a beach deposit by the transgressing sea. Off shore from it is shallow shelf deposition. Its composition may be dominantly shale if there is a clastic source on the continent. But if the continent is stable, as Westcontinent is, and the climate is warm, then carbonate (limestone) deposition will dominate. 

 

Stage D

Full Divergent Margin

Eastcontinent has now drifted off the eastern side of the cross section, and only Westcontinent and the new ocean basin with its rifting center (mid oceanic ridge) remain. Heat rising to the surface from the convection cells remains concentrated at the rifting site in the center of the new ocean basin, so as the ocean basin widens the newly formed continental margin (now called a divergent continental margin [DCM], or a passive continental margin because it is geologically passive) moves away from the heat source, and cools. Cool crust is denser than warm crust and as the DCM cools it sinks, rapidly at first, but ever more slowly with time (a process called thermal decay). Thus, in about 5-10 million years the horsts which once were 3-5 kilometers above sea level sink below the waves. Ultimately it will take about 110 millioin years for the DCM to cool completely and stabilize (detailed cross section), at which point it will be about 14 kilometers below sea level (Stage E). 

     Meanwhile a great wedge of sediment is deposited on the DCM, expanding and thickening from a feather edge on the continent side toward the ocean basin. These sediments are derived from the eroding continent in the case of clastics, and by chemical and biological activity in the case of carbonates. It consists mostly of shallow-water marine deposits because subsidence and deposition go on at about the same rate.

      When next to a stable craton, the wedge of sedimentary rocks is dominated by mature sandstone, limestones, and dolomites, but if the continent has some tectonic activity many kinds of less mature sedimentary rocks are possible, such as along the east coast of North America today where sublithic sandstones and shales are common. The Virginia coastal region today is a modern DCM, at this point stabilized since the rifting which opened the Atlantic ocean occurred nearly 250 million years ago. 

 

Stage E

Creating a Convergent Boundary:

Volcanic Island Arc Mountain Building

 

 

Divergence, and the creation of new oceanic lithosphere, can go on for tens or hundreds of millions of years. At some point, however, divergence stops and the two continents begin to move back toward each other, initiating the second, closing, half of the Wilson Cycle. This is convergence and a new plate boundary must be created for it. Convergence begins when oceanic crust decouples, that is, breaks at some place and begins to descend into the mantle along a subduction zone. 

     It is always oceanic crust which decouples and descends into a subduction zone; continental crust is too light to subduct. Subduction zones can form anywhere in the ocean basin. In the Stage E cross section subduction is dipping east, but it could have been west, or any direction. For example, in this detail subduction is toward the west. 

     There are just two kinds of locations for subduction zones, however, one within an ocean basin (Island Arc type), the other along the edge of a continent (Cordilleran type). Both kinds of subduction cause volcanic mountain building and they are extremely important. Things are heating up now compared to the boredom of Stage A. The island arc type is described below; the Cordilleran type in Stage G. 

     At a subduction zone oceanic crust dives into the mantle. When oceanic crust subducts it sets in motion a chain of processes which creates several new structural features, and generates a wide range of new kinds of rocks (detail) each reviewed separately below.

 

Structural Features

     At the site of subduction, part of the oceanic crust is dragged down into a trench 1-2 km below the normal ocean floor which is about 5 km deep. The subducting oceanic crust begins its descent cold but heats up as it slides into the mantle. At about 120 km deep rock begins melting to form 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 forearc, and the area on the back side of the volcanic front is the backarc. A new convergent boundary has been created along the zone of subduction. The ongoing subduction and magma generation eventually builds a volcano perhaps 7-8 km off the ocean floor, and its center (mobile core) is made of many batholiths. All of this has set in motion several more processes.

 

Fractional Melting and the Creation of New Igneous Rocks: 

      The mantle rock above the subducting plate selectively melts, and fractionates (or see Igneous Rock Evolution). In fractional melting an igneous rock of one composition is divided into two fractions each of a different composition.

      The original rock descending into the subduction zone is the oceanic lithosphere (ophiolite suite) composed of cold basalt and gabbro of the oceanic crust, and peridotite of the upper mantle (detail). As it descends into the mantle it gradually heats because of the geothermal gradient and friction of subduction. But the descending slab also carries a lot of sea water with it and at about 120 km down the water and heat lead to fractional melting of the mantle material just above the subducting slab. As heating progresses only the lower temperature phases (lower on Bowen’s Reaction Series) in the rock melt to produce magmas of intermediate composition. And since these are fluid and hot they rise up through the crust to eventually emplace and solidify as intermediate rocks (e.g. diorites, granodiorites, etc). The second fraction is the unmelted residue with a composition more mafic/ultramafic than the original rock. That is, its composition is higher in Bowen’s Reaction Series than the original rock. 

     If time and conditions allow, the fractionation process can continue and the intermediate magma fractionate into felsic magma (typically plagiogranites), leaving behind a magma more mafic than the original intermediate starting rock. Thus, beginning with one (mafic) igneous rock many new igneous rocks can be generated, including ultramafic, intermediate, and felsic (model). Or, felsic continental crust is created from the fractional melting of mafic oceanic crust.

     In our subduction zone, the ultramafic residue, being very dense, stays in the mantle, while the hot, less dense, melt rises to the surface where it forms first intermediate and later felsic batholithic magma chambers. From the chamber the magma reaches the surface as lava and forms explosive composite volcanoes, which are dominated by andesite, although it can evolve from mafic, to intermediate, to felsic as the magma fractionates. Hydrothermal metamorphism also occurs when hot lava spills out onto the ocean floor and reacts with cold sea water to form pillow basalts (detail).

 

Sedimentary Processes:

     As soon as the volcano breaks the surface weathering/erosion processes attack it and form lithic rich sediments (detail) (becoming more feldspar rich as erosion exposes batholiths, or as rhyolites and andesites with feldspar phenocrysts weather) that wash into the sea on all sides. Sediments on the backarc side just spill onto the ocean floor as turbidity currents and stay there undisturbed. On the forearc side, however, the sediments pour into the trench as turbidity currents (underwater avalanches). A trench is like the mouth of a conveyor belt and sediments do not stay there long. Instead they are scraped off the subducting oceanic crust into a melange deposit, or they are partially subducted and metamorphosed. A melange is a chaotic mixture of folded, sheared, faulted, and blueschist metamorphosed blocks of rock formed in a subduction zone. It is also normal, if the climate is right, for reefs to grow around the island. These limestones typically interbed with the coarse-grained lithic breccias and conglomerates eroding from the volcano, and the volcanic sands on the beach. During a volcanic eruption, then, lavas and pyroclastics may interbed with limestones to form a very unusual association of rocks.

 

Paired Metamorphism: 

      Two major kinds of metamorphism are common in a volcanic arc forming a Paired Metamorphic Belt. The first is Barrovian metamorphism (low to high temperature, and medium pressure) formed inside the volcano by heat from the batholiths, accompanied by intense folding and shearing. Because the batholiths are invading mafic oceanic crust these rocks are converted into greenschist (chlorite and epidote rich), amphibolite (amphibole rich), and granulite (pyroxene rich) facies rocks as we get closer to the batholiths and deeper in the crust. Also earlier, now crystallized, intermediate and felsic batholiths may be converted into gneisses and migmatites. 

      The second metamorphism is high pressure-low temperature Blueschist metamorphism formed in the melange of the trench. It is high pressure because this is a convergent boundary and the trench sediments are being rapidly subducted between two plates. The low temperature is because cool surface rocks are rapidly subducted and do not have time to heat up. These belts of Barrovian and blueschist metamorphism form a Paired Metamorphic Belt, which is always the result of subduction.

     Other kinds of metamorphism are also associated with the volcanic arc. At depth along the subduction zone the ultramafic layers of the ophiolite suite undergo eclogite metamorphism, and contact and hydrothermal metamorphism would be common along the volcanic pipes and dikes coming off the batholiths (detail).

     Ancient and modern volcanic island arcs are very common. Modern examples are Japan, the Aleutian Islands of Alaska, and the Malaysian archipelago including the islands of Java, Borneo, and Sumatra. Ancient examples are not as obvious because they eventually collide with another island arc or a continent and are hidden, but that is Step F in the model.

 

Remnant Oceans: 

      Now, step back and look at the whole of Cross Section E. Notice that the ocean basin to the west of the volcanic arc is trapped between the divergent continental margin and the subduction zone. Clearly, if subduction continues the ocean basin between the two will become smaller and smaller until the Westcontinent and the volcano collide. Also the more the continent and volcanic arc move together the more oceanic crust is subducted and destroyed. These ocean basins which will soon disappear in a subduction zone are called remnant oceans. 

     The fact that subduction zones always create remnant ocean basins means that no ocean basin can survive long in geologic history (see these examples). In fact, the oldest ocean basins we know of are only around 200 million years old (compared to the 4 billion year age of the earth). In contrast, continental crust, because it is too light to subduct, tends to remain around just about forever, excluding weathering and erosion.) Many parts of the continents are three to four billion years old. 

 

 

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.

 

Foreland:

      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. 

 

 

Stage G

Cordilleran Mountain Building

 The subduction zone under the island arc is now dead, and the mountain on the edge of Westcontinent peneplaned, but Eastcontinent and Westcontinent are still being driven together by forces outside the cross section. Therefore, another subduction zone has to begin. It could begin anywhere within the ocean basin and form another island arc, and it could dip in any direction. But in this model, decoupling occurs dipping east under the edge of the Eastcontinent, forming a Cordilleran (volcanic arc) type of mountain building.

     The processes of trench formation, subduction and fractional melting of the oceanic crust, melange deposition, and Blueschist metamorphism are the same here as for an island arc orogeny. Observe, however, that all this tectonic activity is occurring along an old divergent continental margin which, like all rifted margins (see in Stage C), has accumulated a thick wedge of DCM sedimentary rocks. Thus, the rising intermediate to felsic batholithic magmas now inject into the thick wedge of continental margin sediments heating them to very high grade Barrovian metamorphism (amphibolite to granulite facies). If the sediments are limestones and quartz sandstones the metamorphic rocks will be marbles and quartzites. Less mature sandstones and shales will form slates, phyllites, schists, and gneisses. It is also quite likely that the basement batholiths under the divergent continental margin will be metamorphosed into gneisses and migmatites.

     Along with the metamorphism, the old divergent continental wedge of sediments and invading batholits plus superposed volcanoes are uplifted along major thrust faults until they form towering mountains. The Andes in South America and the Cascades in Washington, Oregon, and northern California are mountains of this type.

     Inland from the volcanic front, in the backarc region, 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. Superficially this may seem like an axial rift but it forms under very different conditions and processes. 

     The graben fills with a great complex of deposits including coarse clastic sediments in alluvial fan and braided rivers and intermediate to felsic volcanics rising from the subduction zone. Because the source land composition is so variable (divergent margin rocks, suture zone rocks, metamorphics, volcanics, and, when erosion is deep enough, felsic and intermediate batholithic rocks) the sediments eroded from it are rich in quartz and (many kinds of) lithics, plus lesser amounts of feldspar (sodic plagioclase and orthoclase – QFL diagram, blue field).

     The volcanics in the backarc basin begin mafic (basalt, scoria, etc.), but slowly turn into intermediate (andesite), and finally felsic (rhyolite) rocks. In the latter stages granite dikes or stocks (small batholiths) may invade the now mostly filled graben. 

 

 

Stage H

Continent-Continent Collision Mountain Building

By Stage H the remnant ocean basin separating East- and Westcontinents 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; observe this is a mirror image; the hinterland is on the left not right).

      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 (observe that the Eastcontinent DCM of Stage F has been invaded by batholiths in Stage G; after metamorphism the DCM rocks are not symbolized in the drawing.) In the arc-continent collision it is pieces of ocean lithosphere (ophiolite suite) and the volcanic arc that are tbrust toward the foreland. But with the Westcontinent DCM rocks, we would expect ramp and flat thrust faulting to be common, stacking up the sedimentary pile to great thicknesses, as well as large nappe structures.

     Also observe that the hinterland is overriding not the edge of Westcontinent, but the eastern side of the volcanic arc that collided with Westcontinent in Stage F. But as a result the hinterland is using its weight to shove the arc deep into the earth, resulting in Barrovian metamorphism of the arc rocks. But this is probably not the first time these rocks have been Barrovian metamorphosed, since during the arc’s formation much of its deeper portions were metamorphosed by the invation of batholits.

      [Note, by the way, that in the detailed cross section it is the DCM of a hinterland continent that is being overridden, not a volcanic arc, and it is this sedimentary wedge that would be depressed into the earth and metamorphosed. Many variations are possible on the theme.]

 

Sediments:

      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 (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 Na plagioclase), and eventually exposed batholiths (Na 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 are common in the geologic record since much of the earth’s history is of volcanic arcs and continents colliding in endless Wilson cycles. So a brief examination of their nature.

      Foreland Basins develop very rapidly geologically. Just before the collision the foreland is tectonically stable with quartz rich sandstones and limestones being deposited (Stage G or detail of DCM). Then the 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 and shallowing toward the foreland continent (detail, Stage II).

     It is not unusual for the total sedimentary thickness in the basin to be two miles thick. A lot of subsidence, and a lot of sediment. The speed of subsidence can be seen in the rock record. The rocks before the collision are often quartz rich sandstones and limestones, both indicative of tectonic stability. But then right on top of them will be black shales deposited in water hundreds of feet deep. The sediments start to fill in the basin, but for a time basin subsidence and deposition are racing with each other. But as the hinterland overthrust grinds to a halt the subsidence slows and then stops. Now the sediment has a chance to catch up and fill in the basin (detail Stage III, and detail, Stage IV).

     And fill it in it does, all the way to the top, and beyond. Typically after the deep water black shales come avalanches (turbidity currents) of sediment building submarine fans out onto the basin floor. These may reach several thousand feet thickness, and are largely responsible for filling in most of the basin. But as the water shallows upward the turbidity currents give way to shelf environments. 

     Meanwhile closer to the mountain, 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 another couple of thousand feet (detail, Stage V).

     By this time 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 (Wilson Stage I). If you could walk across this land it would look flat and featureless, but underneath lies a lot of historical record. To the east the eroded roots of the mountains exposing their batholith and metamorphic rocks, and to the west a thick wedge of foreland basin sediments, but now all buried in the subsurface.

 

 

Stage I

Stable Continental Craton

 

 

The cycle which began in Stage A now comes to an end. The original continental craton of Stage A which was rifted into two pieces in Stage C is now back together, and stabilized once more.

     Note, however, that this new continent is quite complex compared with the Stage A craton, and that the basement rocks exposed at the surface are very diversified. In the enlarged and detailed drawing you can see that in addition to the original Westcontinent and Eastcontinent blocks there is a volcanic arc trapped between then, and there are now two foreland basin clastic wedges (probably filled with quite different sediments since one was eroded from a volcanic arc and one from a cordilleran mountain). There are two suture zones of melange and a host of different igneous and metamorphic rocks. Nonetheless, when everything is finally weathered to completion and the continent is eroded to a peneplain the simple ideal model for sedimentary rocks will be in force and this continental craton will be dominated by a veneer of quartz sand (QFL diagram, yellow field) and limestones. Shales may also be present at first, but with enough time, these are eventually washed off the continental edge into the surrounding oceans.

 

     In Stage A we began with an ideal continent, assuming it was homogeneous in structure and composition. In light of the Wilson cycle history you have just reviewed, it should be clear that the original continent was not homogeneous. Over, and over, and over, since the first crust solidified, the processes of subduction have been making new continental crust. Collisions have been welding them together, and rifting has been fragmenting them.

     It is the work of geologists to read great events in the rocks of the earth’s crust, but it is also something like a flea trying to understand the great dog it is living on. Many geologists spend their time walking the earth, looking at the rocks at their feet, trying to understand the ancient meanings they have. Endlessly fascinating, endlessly frustrating, and immensely satisfying when we glimpse a little of the greatness of it.

 

    The Wilson Cycle is a relatively simple model of how the earth works and evolves. But is there an even simpler or more theoretically abstract model of how the earth works? A single model that incorporates everything in the Wilson Cycle?

 

This is How ends the Wilson cycle