Basins and Domes in Cratons

Basins and Domes in Cratons

kacimo samy   11:09 ص

Basins and Domes in Cratons 

North America’s craton consists of a shield, where Precambrian rock is exposed, and a platform, where Paleozoic sedimentary rock covers the Precambrian.

A craton consists of crust that has not been affected by orogeny for at least about the last 1 billion years. As a result, cratons have cooled substantially, and therefore have become relatively strong and stable. Geologists divide cratons into two provinces: shields, in which Precambrian metamorphic and igneous rocks crop out at the ground surface, and cratonic platforms, where a relatively thin layer of Phanerozoic sediment covers the Precambrian rocks (figure above).

Domes and basins of the North American cratonic platform.

In shield areas, we find widespread exposures of intensively deformed metamorphic rocks with abundant examples of flow folds and tectonic foliation. That’s because the crust making the cratons was deformed during a succession of orogenies in the Precambrian. These orogens are so old that 

erosion has worn away the original topography, in the process exhuming deep crustal rocks. 

In the cratonic platform, the pattern of contacts between stratigraphic formations defines regional domes and basins. These are broad areas that gradually sank or rose, respectively, over geologic time (figure above a, b). For example, in Missouri, strata arch across a broad dome, the Ozark Dome, whose diameter is 300 km. Individual sedimentary layers thin toward the top of the dome, because less sediment accumulated on the dome than in adjacent basins. Erosion during more recent geologic history has produced the characteristic bull’s-eye pattern of a dome, with the oldest rocks (Precambrian granite) exposed near the centre. In the Illinois Basin, strata warp downward into a huge bowl that is also about 300 km across. Strata get thicker toward the basin centre, indicating that the floor of the basin was subsiding as sediment was accumulating there was more room for sediment to accumulate where the basin subsided the most. The basin also has a bull’s-eye shape, but here the youngest strata are exposed in the centre. Geologists refer to the broad vertical movements that generate huge, but gentle, mid-continent domes and basins as epeirogeny.

Credits: Stephen Marshak (Essentials of Geology)

Mountain Topography

Mountain Topography

kacimo samy   11:01 ص

Mountain Topography 

Leonardo da Vinci, the Renaissance artist and scientist, enjoyed walking in the mountains, sketching ledges and examining the rocks he found there. In the process, he discovered marine shells (fossils) in limestone beds cropping out a kilometre above sea level, and he suggested that the rock containing the fossils had risen from below sea level up to its present elevation. Modern geologists agree with Leonardo, and they now refer to processes causing the surface of the Earth to move vertically from a lower to a higher elevation as uplift. In this section, we look at why uplift occurs, how erosion carves rugged landscapes out of uplifted crust, and why Earth’s mountains can’t get much higher than Mt. Everest.

Why Are Mountains High? 

What processes can cause the surface of the Earth to rise? There are many because, as we have seen, mountain building happens in numerous different geologic settings. The lithosphere, which consists of relatively rigid crust and lithospheric mantle, “floats” on the softer asthenospheric mantle below. As a consequence, the elevation of the top surface of the lithosphere, over a broad region, represents a balance between buoyancy force pushing lithosphere up, and gravitational force pulling the lithosphere down. Geologists refer to the condition that exists when this balance has been achieved as isostasy, or isostatic equilibrium. Put another way, isostasy exists where the elevation of the Earth’s surface reflects the level at which the lithosphere naturally floats. (Note that because asthenosphere flows  only very slowly, and because lithosphere is strong enough to hold up loads, isostasy does not exist everywhere).

To picture the relation between isostasy and mountains, imagine placing a block of wood into a bathtub full of water. If the block is less dense than water, it floats, with part of the block remaining above the water surface, and most of the block submerged below. Now, place a denser block of the same thickness next to the first block. The top of the denser block sits lower than that of the less-dense block of the same thickness. Similarly, the top surface of a thicker block sits higher than the top surface of a thinner block of the same density. If you were to add another block of wood on top of one that is already floating, the lower block would sink to adjust for the addition, so as to maintain isostatic equilibrium.

From our bathtub experiment, we can deduce that any phenomenon that changes the thickness and/or density of a floating block will affect the elevation of the block’s surface above the water surface. Since the lithosphere floats on the asthenosphere, the elevation of the lithosphere’s surface depends on the thickness and density of the lithosphere. So to answer the question of why mountain belts can rise, we must identify geologic processes that can change the thickness and/or density of layers in the lithosphere. Let’s consider some examples of how these charges take place.

Crustal shortening and thickening

The concept of isostacy as applied to the collisional mountain ranges.

During collisional orogeny or during certain types of convergent-margin orogeny, horizontal compression causes the crust to shorten horizontally and thicken vertically. In fact, the folding, faulting, and plastic flow that take place during such events can almost double the crust’s thickness. For example, the crust beneath the tallest range, the Himalayas, is 70 km thick, whereas crust beneath the plains of the central United States is 35 km thick (figure above a). To isostatically compensate for the thickening of the crust (the geologic equivalent to adding another block of low-density wood to the top of a floating block), the base of the crust and underlying lithospheric mantle subside (figure above b). Indeed, since the Himalayas are about 8 km high, most of the thickened crust extends downward beneath the range just as most of an ice cube lies under water. This downward protrusion of crust is called a crustal root. We can illustrate this relationship in a bathtub model by lining up a row of floating blocks of different thickness if all the blocks have the same density, the thicker blocks rise farther above water and protrude deeper below the surface (figure above c).

Adding igneous rock to the crust

When lava and/or pyroclastic debris is deposited onto the surface, a volcano grows and may become a mountain. Growth of mountains associated with igneous activity may also occur because intrusions at depth may add material to the crust and, therefore, thicken it. 

Removal of lithospheric mantle

Uplift, due to delamination of the lithosphere root, may happen after collision.

The weight of the lithospheric mantle (composed of very dense rock) pulls the lithosphere down, just as heavy ballast makes a ship settle deeper into the water. Removal of some or all of the lithospheric mantle from the base of a plate, therefore, causes the surface of the remaining lithosphere to rise to maintain isostasy, even if the thickness of the crustal component remains unchanged (figure above a, b). Such removal, a process known as delamination, resembles removal of ballast from the hold of a ship as the weight of the ballast disappears, the deck of the ship rises.

Thinning and heating the lithosphere

In rifts, the lithosphere undergoes stretching and thinning. As a result, relatively less-dense asthenosphere rises beneath the rift, and the remaining lithosphere heats up. Replacing dense lithospheric mantle with less-dense hot asthenosphere, and heating the remaining, overlying lithosphere (thereby causing rocks to expand so their density increases) results in uplift of the rift and its borders.

What Goes Up Must Come Down 

Manifestations of erosion in mountain ranges. When land rises, water and ice start cutting into it.

When the land surface rises significantly, for whatever reason, it doesn't remain a smooth welt or bulge on the Earth’s surface. As soon as a difference in elevation between one location and an adjacent one develops, gravity begins to drive a variety of erosive processes. For example, as slopes steepen, landslides of various types cause rock and debris to tumble from higher to lower elevations; when rain falls, streams form and sculpt valleys and canyons; and if it remains cold enough, glaciers grow and flow, carving peaks and deepening valleys. The net result of all these processes is to grind away elevated areas and produce the jagged landscapes that we associate with mountain terrains (figure above a, b). It’s important to keep in mind that uplift and erosion happen simultaneously in active mountain belts, so for the elevation of a range to increase over time, the rate of uplift must exceed the rate of erosion.

The highest point on Earth, the peak of Mt. Everest, lies 8.85 km above sea level can our planet’s mountain ranges get significantly higher? Probably not. Mountains as high as Olympus Mons on Mars, which rises 27 km above the plain at its base, couldn't form on Earth because of the relatively high geothermal gradient (the rate of increase in temperature with depth) in Earth’s crust. Due to the gradient, quartz-rich crustal rocks at mid-crustal depths (15–30 km) become so warm and weak that they can flow ductilely. When this flow begins, overlying mountains above begin to collapse under their own weight, and spread laterally like soft cheese that has been left out in the summer sun. Geologists call this process orogenic collapse. During orogenic collapse, the upper crust breaks and a system of normal faults develop, to accommodate the horizontal stretching.

The simultaneous activity of uplift, erosion, and organic collapse ultimately brings rock that was metamorphosed at great depth up to the surface of the Earth. This process of revealing deeper rocks by removal of the overlying crust is called unroofing or exhumation.

Credits: Steophen Marshak (Essentials of Geology)

Mountain Building

Mountain Building

kacimo samy   9:02 ص

Mountain Building

Before plate tectonics theory became established, geologists were just plain confused about how mountains formed. In the context of the new theory, however, the many processes driving mountain building became clear: mountains form primarily in response to convergent-boundary deformation, continental collisions, and rifting. Since collision zones, rifts, and plate boundaries are linear, mountain belts are linear. Below, we look at these different settings and the types of mountains and geologic structures that develop in each one.

Mountains Related to Subduction 

Characteristics of convergent-margin orogens.

At some convergent plate boundaries, compressional stresses develop, and these cause crustal shortening and uplift in the overriding plate. Such shortening may produce a fold-thrust belt, in which a thrust system develops (figure above a). As a consequence of this faulting, thrust slices (sheets of rock above a thrust fault) push up and over their neighbours, and rocks within thrust slices bend and become folded. The thrust faults merge with a sub-horizontal fault, called a detachment, at depth. The Andes orogen of western South America displays the rugged topography that can develop in compressional convergent-margin orogens (figure above b).

If subduction continues over a long time, offshore volcanic arcs, oceanic plateaus, and micro-continents may drift into the convergent margin (see figure above a). Such crustal blocks are too buoyant to subduct and sink back into the mantle, so instead they collide with the overriding plate and “suture” (attach) to the edge of the overriding plate. Geologists refer to this process as accretion; the buoyant crustal block is called an exotic terrane when it is offshore, and an accreted terrane once it has attached to the overriding plate. Once accretion occurs, the convergent plate boundary may jump to the seaward side of the accreted terrane, so that subduction can continue. The process of accretion can add substantial new crust to a convergent-margin orogen. For example, the western half of the North American Cordillera, a region that is up to 500 km wide, consists of accreted terranes (figure above c). Orogens that grew laterally by the attachment of exotic terranes have come to be known as accretionary orogens. 

Mountains Related to Continental Collision 

Once the oceanic lithosphere between two continents completely subducts, the continents themselves collide with each other. Continental collision results in the formation of large mountain ranges such as the present-day Himalayas or the Alps and the Paleozoic Appalachian Mountains. The final stage in the growth of the Appalachians happened when Africa and North America collided.

Characteristics of collisional orogens.

During collision, intense compression generates fold-thrust belts on the margins of the orogen (figure above a–c). In the interior of the orogen, where one continent overrides the edge of the other, high-grade metamorphism occurs, accompanied by formation of passive-flow folds and tectonic foliation. During this process, the crust below the orogen thickens to as much as twice its normal thickness. During such crustal thickening, rocks squeeze upward in the hanging walls of large thrust faults. 

Mountains Related to Continental Rifting 

Rift-related mountains.

Continental rifts are places where continents are splitting in two. During rifting, stretching causes normal faulting in the brittle crust (figure above a). Movement on the normal faults drops down blocks of crust, producing deep, sediment-filled basins separated by narrow, elongate mountain ranges that contain tilted rocks. These ranges are sometimes called fault-block mountains. Stretching thins the lithosphere, allowing hot asthenosphere to rise and undergo decompression melting. This process produces magmas that rise to form volcanoes within the rift. Today, the East African Rift clearly shows the configuration of rift-related mountains and volcanoes. And in North America, rifting yielded the broad Basin and Range Province of Utah, Nevada, and Arizona (figure above b).

Forming Rocks in and Near Mountains 

Various rocks form during orogeny.

The process of orogeny establishes geologic conditions appropriate for the formation of a great variety of rocks. We’ll consider examples from all three rock categories (figure above):

• Igneous activity during orogeny: In convergent plate boundaries, melting takes place in the mantle above the subducting plate. In rifts, stretching and thinning of lithosphere causes decompression melting of the underlying mantle. And during continental collision, melting may take place where deep portions of the crust undergo heating. All of these melting regimes produce magma, which rises and freezes to form igneous rocks in the overlying mountains. 
• Sedimentation during orogeny: Weathering and erosion in mountain belts generate vast quantities of sediment. This sediment tumbles down slopes and gets carried away by glaciers or streams that transport it to low areas where it accumulates in alluvial fans or deltas. In some locations, the weight of mountain belts pushes down the surface of the lithosphere, thereby producing a deep sedimentary basin at the border of the range. 
• Metamorphism during orogeny: Contact metamorphic aureoles form adjacent to igneous intrusions in orogens. And regional metamorphism occurs where mountain building thrusts one part of the crust over another; when this happens, rock of the footwall ends up at great depth and thus can be subjected to high temperature and pressure. Because deformation accompanies this process, the resulting metamorphic rocks contain tectonic foliation. 

Measuring Mountain Building  in Progress 

GPS measurements of shortening in the Andes. The lines indicate the velocity of the red dots relative to the interior of South America. The line at the yellow dot indicates relative plate motion.

Not all mountains are just “old monuments,” as John Muir mused. The rumblings of earthquakes and the eruptions of volcanoes attest to present-day, continuing movements in some ranges. Geologists can measure the rates of these movements through field studies and satellite technology. For example, geologists can determine where coastal areas have been rising relative to the sea level by locating ancient beaches that now lie high above the water. And they can tell where the land surface has risen relative to a river by identifying places where a river has recently carved a new valley. In addition, geologists now use the global positioning system (GPS) to measure rates of uplift and horizontal shortening in orogens. Though standard hand-held GPS devices provide locations with accuracies of only about +-2 m, research-quality GPS systems can specify locations to within +-2 mm. By comparing the position of a location within an orogen to a location outside an orogen over a time period of a few years, it is possible to detect crustal motion. Thus, we can “see” the Andes shorten horizontally at a rate of a couple of centimetres per year (figure above), and we can “watch” as mountains along this convergent boundary rise by a couple of millimetres per year.

Credits: Stephen Marshak (Essentials of Geology)

How do mountains build?

How do mountains build?

kacimo samy   12:07 ص

Plate moves around the Earth's surface which exerts powerful lateral forces on rocks. The response of the crust to those forces gives rise to deformation on a large scale, particularly along plate boundaries. For example, dozens or hundreds of large-scale faults can form in zones of plate convergence, resulting in a broad and high mountain belt. The geologic processes that can result in mountain building.

• Crustal shortening/thickening in response to convergence of a subducting plate.
• Continental collision.
• Uplift of sediments accreted by subduction.
• Volcanism.
• "Corner" accretion/uplift due to along-trench rafting of terrain.
• Heating or cooling of lithosphere by the underlying mantle (and hence uplift or subsidence).
• Crustal extension.

Geometrically, if two plates are colliding, they can respond as follows:

• One plate can subduct into the mantle.
• One or both plates can undergo shortening and hence uplift and crustal thickening.
• One or both plates can undergo lateral extrusion (i.e. part of the plate escapes the collision zone by extruding sideways).

Let's consider the first two cases.
Suppose an oceanic plate converges with a continental plate. The oceanic plate, being more dense, subducts into the mantle. If all convergence (100%) is accommodated by subduction, the overlying continent will remain undeformed and should undergo no net uplift (although topography will undoubtedly develop as arc volcanoes appear). Suppose that not all convergence is accommodated by subduction and a few percent of the convergence instead causes the upper plate to shorten. How much uplift is implied?

For 100 millimetres per year of convergence (a typical plate velocity), if 5% is accommodated by long-term shortening of the overlying plate, then the overlying plate will shorten at a rate of roughly 5 millimetres per year.

Geometrically, the 5 mm/yr of horizontal shortening translates into vertical motion as shown above, with the only dependence being on the dip of the fault along which the continent is shortening. For low angle faults (10 degrees), uplift will occur at rates of about 1 mm/yr (1 kilometre per million years). Over 5 million years, this amounts to uplift of 5 kilometres or 3 miles (15,900 feet). Over the same period, the two plates will have converged some 5 million years x 100 kilometres per million years or 500 km. The total uplift then is only a small percentage, 1%, of the total horizontal motion.

Continent-continent collisions are not usually accompanied by subduction because both plates are too buoyant to be thrust deeply into the mantle. The amount of crustal thickening and uplift in such a collision can thus be much greater than for an ocean-continent collision. For example, over the past 40 million years, continental India has driven northward into continental Eurasia across the Himalayas mountain belt at a rate of about 40-50 millimetres per year. If you work it out, this implies that the two plates have somehow shortened by more than 1500 kilometres (about 900 miles) across the Himalayas. Since subduction is not occurring, the shortening has been accommodated by mountain building and lateral escape.

Factors that complicate calculations of total uplift

Plate convergence of hundreds or even a thousand kilometres or more over millions of years might be accompanied by only several kilometres of uplift. This uplift, while spectacular to the eye, is merely a small part of the displacement "budget", which is largely dominated by horizontal motion.

Many important aspects of mountain building are ignored and thus cannot predict total topography given total horizontal motion.

Why is this the case?. It is because of the following:

• Fault dips are rarely well known and faults are often curved, with their dips increasing toward the surface.
• Erosion/mass wasting removes material from the upper reaches of uplifting regions, sometimes nearly as quickly as the region is going up!
• Vertical uplift must fight the downward pull of gravity. In extreme cases, no amount of horizontal convergence is capable of causing further uplift.
• Crustal uplift often coincides with subsidence through a process called isostatic adjustment. This is analogous to climbing into a boat, which sinks lower into the water once your weight is added to that of the boat. Your net height above the water surface is equal to your height on land minus the amount the boat sinks. Continents similarly sink into the underlying mantle when substantial topographic loads are added to their surface.
• Lateral escape of crust sometimes accommodates a significant fraction of plate convergence, leaving less available for inducing vertical uplift.
• Uplift can occur simultaneously or sequentially along many faults that exist in a broad zone of deformation between two converging plates. Relating the total horizontal convergence to the total uplift caused by the convergence then requires measurements across many faults.