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A passive margin is the transition between oceanic and continental crust, which is not an active plate margin. It is constructed by sedimentation above an ancient rift. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic ridge. The transition between the continental and oceanic crust that is created by the rift is known as a passive margin.

Global distribution

Passive margins are found at every ocean and continent boundary that does not have a subduction zone. This includes the entire coasts of Africa, Greenland, India and Australia. They are also found on the east coast of North America and South America, the coastline to west of Europe and most of the oceanic continental boundary of Antarctica. The East Asian coastline contains some passive margins, but their size is much smaller than the previously mentioned margins.

Key Components

Active vs. Passive Margins

As stated previously, a passive margin is the transition between oceanic and continental crust that is not an active plate margin. Active margins are found on the leading edge of the continent where subduction occurs. They are marked by uplift and volcanic mountain belts on the continental plate, and by island-arc chains from the oceanic plate to the over-riding continental plate. Central-southern California, the subduction zones of the Pacific Rim, and the older mountains of the Alps and the Himalayas are all examples of active margins.

Bathymetry

Although there are many kinds of passive margins, the bathymetries of most margins have similar features. Typically they consist of a continental shelf, continental slope continental rise and Abyssal plain. The bathymetries of these features are defined completely by the underlying faulted continental crust and the sedimentation above it. Volcanic passive margins bathymetries may vary, as other than sedimentation and continental crust, igneous intrusions and lava flow are also defining factors.

Cross-section

The main features of Passive margins lie underneath the external characters. In passive margins the transition between the continental and oceanic crust is not abrupt, there is an eventual transition. The subsided continental crust is marked by normal faulting that dips seaward. The faulted crust slowly transitions into oceanic crust and subsides more due to the mass of sediment that collects above it. The lithosphere beneath passive margins is known as transitional lithosphere. The thickness of the lithosphere thins isostatically with the transition to oceanic crust. Volcanic passive margins contain some most of the features of rifted passive margins. But they also are marked by numerous amounts of dykes and igneous intrusions within the subsided continental crust. There are typically a lot of dykes formed along the seaward dipping normal faults. Igneous intrusions within the crust cause lava flow along the top the subsided continental crust and form seaward dipping reflectors.

Subsidence mechanisms

Isostasy

Isostasy is a very important factor in the formation of passive margins. In the initial stage of formation of the passive margin the continental crust is not in isostatic equilibrium. The cause of this in-equilibrium is the stretching and thinning of the crust due to plate movement (plate tectonics). This causes the lower crust and lithosphere to be raised closer to the surface bringing the mantle closer up with it. This process continues until a mid oceanic ridge is formed and the thinning and stretching stops. The lithosphere below the thinned and faulted continental oceanic transition now slowly begins to thicken and returns to isostatic equilibrium. The sedimentation above the faulted continental crust increases its mass and therefore thickens the lithosphere much more also driving the mantle down deeper.

Heat flow

The heat flow at passive margins changes significantly over the period of its formation. This change is because of omnipresent movement of plates due to plate tectonics. The first introduction of heat flow into the continental margin is seen when the mantle is raised closer to the surface. Convection within the mantle keeps a constant heat flow within the rift. The very thin lithosphere beneath the rift causes the upwelling mantle to melt by decompression. The magma vents out the rift forming seafloor and ocean basins. When the passive margin is formed and the rift moves away as a mid-oceanic ridge there is a long period of cooling. In the case of volcanic passive margins even after ridge moves away, igneous intrusions and dykes keep the passive margin volcanically active and the heat flow constant.

Types of Passive Margins

The initial rifting defines the original shape of the margin, but the way sedimentation occurs can change and reshape the passive margin. There are basically three different ways to break down the types of passive margins: the geometry of the formation (rifted and sheared), sediment supply (normal and starved), and types of construction (volcanic and non-volcanic).

Geometry of Passive Margins

Rifted Margin

Traditional, normal formation of passive margin, with large-scale blocks of crust tilted towards the continent. Faulting tends to be listric: thinning as the base approaches the oceanic crust, and very thin in comparison with sheared margins.

Sheared Margin

Sheared margins are highly complex and tend to be rather narrow. Sheared margins differ from rifted passive margins in structural style and thermal evolution during continental breakup. As the sea floor spreading axis moves along the margin, thermal uplift produces a ridge. This ridge traps sediments, thus allowing for thick sequences to accumulate. These types of passive margins are associated with volcanism, though do not have to have much.

Construction of Passive Margins

Non-Volcanic Rifted Margin

Non-volcanic margins are formed with little to no mantle melt, thus have little extrusive activity due to asthenosphere conductive cooling. They are typically wide margins, and are associated with adiabatic decompression, where pressure only factor in temperature change. The long periods of stretching prior to separation are associated with large mounts of thinning.

Volcanic Rifted Margin

Volcanic margins are associated with significant amounts of mantle melting, with volcanism occurring either immediately prior to, or during, the process of continental breakup. Volcanic margins evolve in a combination of ways: extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion are the main causes. Volcanic margins are typically characterized by a thinning lithosphere, stratified igneous crust, and a high P-wave velocity in the lower crust.

Sedimentation

One of the last ways of classifying margins is by the type of sedimentation. As one of the last processes in passive margin formation, there is normally sedimentation still occurring, even on the oldest passive margins today. Depending on how the rifting occurred, when, how and what type of sediment varies.

Normal/Constructional

Constructional margins are the “classic” way sediments occur with passive margins. The main characteristic is thick layers of sediment that gradually decrease as distance increases from the shore. The Atlantic coast of the eastern United States is an excellent example of this, with ranges of thickness from 1000 meters to 200 meters of sediment from after the active phase of rifting.

Starved

Sediment-starved margins normally occur in narrow areas, as there is substantially less sedimentation than constructional margins. This is due because there can be no major accumulation of sediment, when nothing is arriving. This is the main difference between the two, and an illustration of this is in the western Mediterranean Sea.

Shelf Margin Dams

The last type of sedimentation reshaping of the passive margin is when there is some block that stops the sediment of a constructional margin and creates a type of shelf be. When there is folding or uplift, dams can be created which trap sediments and prevent them from gradually declining.

Another type of dam is diapirs: the uprising of older, less dense rocks into younger rocks, which are above the original location. This creates a dome structure which can trap sediments and create a steep continental shelf. Salt domes are the very common, and the east Texan coast has this type of margin dam.

One of the last types of dams is the limestone formations left by coral growth. These create a type of dam that does not rely on tectonics, though the underlying foundation may have originally been some other type of structural dam. Florida’s reefs trapped sediment off the coast, using this sort of dam.

Formation

There are many facts and theories involved with the formation of passive margins. But there are three main stages in the formation of passive margins.

1. In the first stage a continental rift is established due to stretching and thinning of the crust and lithosphere by plate movement. This is the beginning of the continental crust subsidence.
2. The second stage is the most important as the rifting cause the formation of an oceanic basin. The subsiding continental crust undergoes normal faulting as open marine conditions are established. Crust and lithosphere stretching and thinning are still taking place in this stage. Volcanic passive margins also have igneous intrusions and dykes forms during this stage.
3. The last stage in formation happens only when crustal stretching has subsided. Drainage starts flowing towards the passive margin causing sediment to be piled up over it. The seafloor keeps spreading as a mid-oceanic ridge is formed. The lithosphere begins to thicken as isostatic equilibrium sets in.

Economic Significance

Oil and gas hold a very important place in our lives. Plastics, medicines, fiber, clothing, energy and gasoline all come from these resources. Oil and gas are basic elements for humankind to survive because most products are powered by oil and gas. There is no substitute for it. It is at passive margins where most deposits of oil and gas are found.

Here, favorable conditions were created from buried organic matter. Early continental rifting conditions led to the preservation of organic matter. Later development continues and the initial organic matter becomes overlain by marine sediments. Areas with restricted sea water circulation coupled with warm weather conditions will create evaporate deposits such as salt domes and basins. Crude oil will form from these deposits. These are the localities in which petroleum resources are most profitable and productive. Productive fields are found around the globe. To name a few are the Gulf of Mexico, Eastern Canada, and shallow areas in Western Australia.

Every day a very well designed, global system moves 60,000,000 barrels of oil from producers to consumers. A barrel consists of 42 gallons of oil, with a ton being about 7.2 barrels worth of oil. The possession of oil deposits can be a determining factor between a rich and a poor country.

References

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• Morelock, Jack (2004). "Margin Sructure". Geological Oceanography. Retrieved 2007-12-02.

• Curray, J. R. (1980). "The Ipod Programme on Passive Continental Margins". Philosophical Transactions of the Royal Society of London. A 294 (1409): 17–33.

• "Diapir". Encyclopædia Britannica Online. Encyclopædia Britannica. 2007.

• "Petroleum". Encyclopædia Britannica Online. Encyclopædia Britannica. 2007.

• "UNIL: Subsidence Curves". Université de Lausanne: Institute of Geology and Palaeontology. Retrieved 2007-12-02.

• Bird, Dale (February 2001). "Shear Margins". The Leading Edge. 20 (2). Society of Exploration Geophysicists: 150–159. doi:10.1190/1.1438894.{{cite journal}}: CS1 maint: date and year (link)

• Continental Margins Committee, Ocean Studies Board, National Research Council, ed. (1989). Margins: A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (PDF). The National Academies Press. Retrieved 2 December 2007.{{cite book}}: CS1 maint: multiple names: editors list (link)

• Geoffroy, Laurent (October 2005). "Volcanic Passive Margins" (PDF). C. R. Geoscience 337 (in French and English). Elsevier SAS. Retrieved 2007-12-02.{{cite web}}: CS1 maint: date and year (link) CS1 maint: unrecognized language (link)

• R. A. Scrutton, ed. (1982). Dynamics of Passive Margins. USA: American Geophysical Union.