Passive margin

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Rifting-to-spreading transition Rifting to Spreading Transition.jpg
Rifting-to-spreading transition
Passive continental margin Passive Contiental Margin.jpg
Passive continental margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Continental margin Zone of the ocean floor that separates the thin oceanic crust from thick continental crust

The continental margin is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin is the shallow water area found in proximity to continent. The continental margin consists of three different features: the continental rise, the continental slope, and the continental shelf. Continental margins constitute about 28% of the oceanic area.[1]

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. In geology, sedimentation is often used as the opposite of erosion, i.e., the terminal end of sediment transport. In that sense, it includes the termination of transport by saltation or true bedload transport. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. In estuarine environments, settling can be influenced by the presence or absence of vegetation. Trees such as mangroves are crucial to the attenuation of waves or currents, promoting the settlement of suspended particles.

Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.

Contents

Global distribution

Map showing the distribution of Earth's passive margins (yellow swaths). Globald.png
Map showing the distribution of Earth's passive margins (yellow swaths).

Passive margins are found at every ocean and continent boundary that is not marked by a strike-slip fault or a subduction zone. Passive margins define the region around the Atlantic Ocean, Arctic Ocean, and western Indian Ocean, and define the entire coasts of Africa, Greenland, India and Australia. They are also found on the east coast of North America and South America, in western Europe and most of Antarctica. East Asia also contains some passive margins.

Atlantic Ocean Ocean between Europe, Africa and the Americas

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers. It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

Arctic Ocean The smallest and shallowest of the worlds five major oceans, located in the north polar regions

The Arctic Ocean is the smallest and shallowest of the world's five major oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean. It is also seen as the northernmost part of the all-encompassing World Ocean.

Indian Ocean The ocean between Africa, Asia, Australia and Antarctica (or the Southern Ocean)

The Indian Ocean is the third largest of the world's oceanic divisions, covering 70,560,000 km2 (27,240,000 sq mi). It is bounded by Asia on the north, on the west by Africa, on the east by Australia, and on the south by the Southern Ocean or, depending on definition, by Antarctica.

Key components

Active vs. passive margins

This refers to whether a crustal boundary between oceanic lithosphere and continental lithosphere is a plate boundary or not. Active margins are found on the edge of a continent where subduction occurs. These are often marked by uplift and volcanic mountain belts on the continental plate. Less often there is a strike-slip fault, as defines the southern coastline of W. Africa. Most of the eastern Indian Ocean and nearly all of the Pacific Ocean margin are examples of active margins. While a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.

Convergent boundary Region of active deformation between colliding lithospheric plates

Convergent boundaries are areas on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Tectonic uplift The portion of the total geologic uplift of the mean earth surface that is not attributable to an isostatic response to unloading

Tectonic uplift is the portion of the total geologic uplift of the mean Earth surface that is not attributable to an isostatic response to unloading. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thickening, changes in the density distribution of the crust and underlying mantle, and flexural support due to the bending of rigid lithosphere.

Volcanic belt A large volcanically active region

A volcanic belt is a large volcanically active region. Other terms are used for smaller areas of activity, such as volcanic fields. Volcanic belts are found above zones of unusually high temperature (700-1400 °C) where magma is created by partial melting of solid material in the Earth's crust and upper mantle. These areas usually form along tectonic plate boundaries at depths of 10–50 km. For example, volcanoes in Mexico and western North America are mostly in volcanic belts, such as the Trans-Mexican Volcanic Belt that extends 900 km from west to east across central-southern Mexico and the Northern Cordilleran Volcanic Province in western Canada.

Morphology

Bathymetric profile across a typical passive margin. Note that vertical scale is greatly exaggerated relative to the horizontal scale. Bathmetry.png
Bathymetric profile across a typical passive margin. Note that vertical scale is greatly exaggerated relative to the horizontal scale.

Passive margins consist of both onshore coastal plain and offshore continental shelf-slope-rise triads. Coastal plains are often dominated by fluvial processes, while the continental shelf is dominated by deltaic and longshore current processes. The great rivers (Amazon. Orinoco, Congo, Nile, Ganges, Yellow, Yangtze, and Mackenzie rivers) drain across passive margins. Extensive estuaries are common on mature passive margins. Although there are many kinds of passive margins, the morphologies of most passive margins are remarkably similar. Typically they consist of a continental shelf, continental slope, continental rise, and abyssal plain. The morphological expression of these features are largely defined by the underlying transitional crust and the sedimentation above it. Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes generally have a similar morphology. In addition, the shelf break seems to mark the maximum Neogene lowstand, defined by the glacial maxima. The outer continental shelf and slope may be cut by great submarine canyons, which mark the offshore continuation of rivers.

A coastal plain is flat, low-lying land adjacent to a sea coast. One of the largest coastal plains is located in southeastern United States. The Gulf Coastal Plain of North America extends northwards from the Gulf of Mexico along the Lower Mississippi River to the Ohio River, which is a distance of about 981 miles (1,579 km).

Continental shelf A portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea

A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea. Much of the shelves were exposed during glacial periods and interglacial periods.

Amazon River longest river in South America

The Amazon River in South America is the largest river by discharge volume of water in the world, and by some definitions it is the longest.

At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as the fjords of Norway and Greenland.

Norway constitutional monarchy in Northern Europe

Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe whose territory comprises the western and northernmost portion of the Scandinavian Peninsula; the remote island of Jan Mayen and the archipelago of Svalbard are also part of the Kingdom of Norway. The Antarctic Peter I Island and the sub-Antarctic Bouvet Island are dependent territories and thus not considered part of the kingdom. Norway also lays claim to a section of Antarctica known as Queen Maud Land.

Greenland Autonomous country within the Kingdom of Denmark

Greenland is an autonomous constituent country of the Kingdom of Denmark between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for more than a millennium. The majority of its residents are Inuit, whose ancestors began migrating from the Canadian mainland in the 13th century, gradually settling across the island.

Cross-section

Transitional crust composed of stretched and faulted continental crust. Note: vertical scale is greatly exaggerated relative to horizontal scale. PMfinal.png
Transitional crust composed of stretched and faulted continental crust. Note: vertical scale is greatly exaggerated relative to horizontal scale.
Cross-section through transitional crust of a passive margin. Transitional crust as a largely volcanic construct. Note: vertical scale is greatly exaggerated relative to horizontal scale. Volcanic passive margin.svg
Cross-section through transitional crust of a passive margin. Transitional crust as a largely volcanic construct. Note: vertical scale is greatly exaggerated relative to horizontal scale.

The main features of passive margins lie underneath the external characters. Beneath passive margins the transition between the continental and oceanic crust is a broad transition known as transitional crust. The subsided continental crust is marked by normal faults that dip seaward. The faulted crust transitions into oceanic crust and may be deeply buried due to thermal subsidence and the mass of sediment that collects above it. The lithosphere beneath passive margins is known as transitional lithosphere. The lithosphere thins seaward as it transitions seaward to oceanic crust. Different kinds of transitional crust form, depending on how fast rifting occurs and how hot the underlying mantle was at the time of rifting. Volcanic passive margins represent one endmember transitional crust type, the other endmember (amagmatic) type is the rifted passive margin. Volcanic passive margins also are marked by numerous dykes and igneous intrusions within the subsided continental crust. There are typically a lot of dykes formed perpendicular to the seaward-dipping lava flows and sills. Igneous intrusions within the crust cause lava flows along the top of the subsided continental crust and form seaward-dipping reflectors.

In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes more dense than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a sedimentary basin.

Lithosphere The rigid, outermost shell of a terrestrial-type planet or natural satellite that is defined by its rigid mechanical properties

A lithosphere is the rigid, outermost shell of a terrestrial-type planet, or natural satellite, that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.

Subsidence mechanisms

Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation and is due to subsidence of especially the transitional crust. Subsidence is ultimately caused by gravitational equilibrium that is established between the crustal tracts, known as isostasy. Isostasy controls the uplift of the rift flank and the subsequent subsidence of the evolving passive margin and is mostly reflected by changes in heat flow. Heat flow at passive margins changes significantly over its lifespan, high at the beginning and decreasing with age. In the initial stage, the continental crust and lithosphere is stretched and thinned due to plate movement (plate tectonics) and associated igneous activity. The very thin lithosphere beneath the rift allows the upwelling mantle to melt by decompression. Lithospheric thinning also allows the asthenosphere to rise closer to the surface, heating the overlying lithosphere by conduction and advection of heat by intrusive dykes. Heating reduces the density of the lithosphere and elevates the lower crust and lithosphere. In addition, mantle plumes may heat the lithosphere and cause prodigious igneous activity. Once a mid-oceanic ridge forms and seafoor spreading begins, the original site of rifting is separated into conjugate passive margins (for example, the eastern US and NW African margins were parts of the same rift in early Mesozoic time and are now conjugate margins) and migrates away from the zone of mantle upwelling and heating and cooling begins. The mantle lithosphere below the thinned and faulted continental oceanic transition cools, thickens, increases in density and thus begins to subside. The accumulation of sediments above the subsiding transitional crust and lithosphere further depresses the transitional crust.

Classification

There are four different perspectives needed to classify passive margins:

  1. map-view formation geometry (rifted, sheared, and transtensional),
  2. nature of transitional crust (volcanic and non-volcanic),
  3. whether the transitional crust represents a continuous change from normal continental to normal oceanic crust or this includes isolated rifts and stranded continental blocks (simple and complex), and
  4. sedimentation (carbonate-dominated, clastic-dominated, or sediment starved).

The first describes the relationship between rift orientation and plate motion, the second describes the nature of transitional crust, and the third describes post-rift sedimentation. All three perspectives need to be considered in describing a passive margin. In fact, passive margins are extremely long, and vary along their length in rift geometry, nature of transitional crust, and sediment supply; it is more appropriate to subdivide individual passive margins into segments on this basis and apply the threefold classification to each segment.

Geometry of passive margins

Rifted margin

This is the typical way that passive margins form, as separated continental tracts move perpendicular to the coastline. This is how the Central Atlantic opened, beginning in Jurassic time. Faulting tends to be listric: normal faults that flatten with depth.

Sheared margin

Sheared margins form where continental breakup was associated with strike-slip faulting. A good example of this type of margin is found on the south-facing coast of west Africa. Sheared margins are highly complex and tend to be rather narrow. They also differ from rifted passive margins in structural style and thermal evolution during continental breakup. As the seafloor 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 less volcanic.

Transtensional margin

This type of passive margin develops where rifting is oblique to the coastline, as is now occurring in the Gulf of California.

Nature of transitional crust

Transitional crust, separating true oceanic and continental crusts, is the foundation of any passive margin. This forms during the rifting stage and consists of two endmembers: Volcanic and Non-Volcanic. This classification scheme only applies to rifted and transtensional margin; transitional crust of sheared margins is very poorly known.

Non-volcanic rifted margin

Non-volcanic margins are formed when extension is accompanied by little mantle melting and volcanism. Non-volcanic transitional crust consists of stretched and thinned continental crust. Non-volcanic margins are typically characterized by continentward-dipping seismic reflectors (rotated crustal blocks and associated sediments) and low P-wave velocities (<7.0 km/s) in the lower part of the transitional crust.

Volcanic rifted margin

Volcanic margins form part of large igneous provinces, which are characterised by massive emplacements of mafic extrusives and intrusive rocks over very short time periods. Volcanic margins form when rifting is accompanied by significant mantle melting, with volcanism occurring before and/or during continental breakup. The transitional crust of volcanic margins is composed of basaltic igneous rocks, including lava flows, sills, dykes, and gabbro.

Volcanic margins are usually distinguished from non-volcanic (or magma-poor) margins (e.g. the Iberian margin, Newfoundland margin) which do not contain large amounts of extrusive and/or intrusive rocks and may exhibit crustal features such as unroofed, serpentinized mantle . Volcanic margins are known to differ from magma-poor margins in a number of ways:

  • a transitional crust composed of basaltic igneous rocks, including lava flows, sills, dykes, and gabbros.
  • a huge volume of basalt flows, typically expressed as seaward-dipping reflector sequences (SDRS) rotated during the early stages of crustal accretion (breakup stage),
  • The presence of numerous sill/dyke and vent complexes intruding into the adjacent basin,
  • the lack of significant passive-margin subsidence during and after breakup, and
  • the presence of a lower crust with anomalously high seismic P-wave velocities (Vp=7.1-7.8 km/s) – referred to as lower crustal bodies (LCBs) in the geologic literature.

The high velocities (Vp > 7 km) and large thicknesses of the LCBs are evidence that supports the case for plume-fed accretion (mafic thickening) underplating the crust during continental breakup. LCBs are located along the continent-ocean transition but can sometimes extend beneath the continental part of the rifted margin (as observed in the mid-Norwegian margin for example). In the continental domain, there are still open discussion on their real nature, chronology, geodynamic and petroleum implications. [1]

Examples of volcanic margins:

  • The Yemen margin
  • The East Australian margin
  • The West Indian margin
  • The Hatton-Rockal margin
  • The U.S East Coast
  • The mid-Norwegian margin
  • The Brazilian margins
  • The Namibian margin
  • The East Greenland margin
  • The West Greenland margin

Examples of non-volcanic margins:

  • The Newfoundland Margin
  • The Iberian Margin
  • The Margins of the Labrador Sea (Labrador and Southwest Greenland)

Heterogeneity of transitional crust

Simple transitional crust

Passive margins of this type show a simple progression through the transitional crust, from normal continental to normal oceanic crusts. The passive margin offshore Texas is a good example.

Complex transitional crust

This type of transitional crust is characterized by abandoned rifts and continental blocks, such as the Blake Plateau, Grand Banks, or Bahama Islands offshore eastern Florida.

Sedimentation

A fourth way to classify passive margins is according to the nature of sedimentation of the mature passive margin. Sedimentation continues throughout the life of a passive margin. Sedimentation changes rapidly and progressively during the initial stages of passive margin formation because rifting begins on land, becoming marine as the rift opens and a true passive margin is established. Consequently, the sedimentation history of a passive margin begins with fluvial, lacustrine, or other subaerial deposits, evolving with time depending on how the rifting occurred and how, when, and by what type of sediment it varies.

Constructional

Constructional margins are the "classic" mode of passive margin sedimentation. Normal sedimentation results from the transport and deposition of sand, silt, and clay by rivers via deltas and redistribution of these sediments by longshore currents. The nature of sediments can change remarkably along a passive margin, due to interactions between carbonate sediment production, clastic input from rivers, and alongshore transport. Where clastic sediment inputs are small, biogenic sedimentation can dominate especially nearshore sedimentation. The Gulf of Mexico passive margin along the southern United States is an excellent example of this, with muddy and sandy coastal environments down current (west) from the Mississippi River Delta and beaches of carbonate sand to the east. The thick layers of sediment gradually thin with increasing distance offshore, depending on subsidence of the passive margin and the efficacy of offshore transport mechanisms such as turbidity currents and submarine channels.

Development of the shelf edge and its migration through time is critical to the development of a passive margin. The location of the shelf edge break reflects complex interaction between sedimentation, sealevel, and the presence of sediment dams. Coral reefs serve as bulwarks that allow sediment to accumulate between them and the shore, cutting off sediment supply to deeper water. Another type of sediment dam results from the presence of salt domes, as are common along the Texas and Louisiana passive margin.

Starved

Sediment-starved margins produce narrow continental shelves and passive margins. This is especially common in arid regions, where there is little transport of sediment by rivers or redistribution by longshore currents. The Red Sea is a good example of a sediment-starved passive margin.

Formation

Formation of passive margins.png

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. Drainage is generally away from the rift at this stage.
  2. The second stage leads to the formation of an oceanic basin, similar to the modern Red Sea. The subsiding continental crust undergoes normal faulting as transitional marine conditions are established. Areas with restricted sea water circulation coupled with arid climate create evaporite deposits. Crust and lithosphere stretching and thinning are still taking place in this stage. Volcanic passive margins also have igneous intrusions and dykes during this stage.
  3. The last stage in formation happens only when crustal stretching ceases and the transitional crust and lithosphere subsides as a result of cooling and thickening (thermal subsidence). Drainage starts flowing towards the passive margin causing sediment to accumulate over it.

Economic significance

Passive margins are important exploration targets for petroleum. Mann et al. (2001) classified 592 giant oil fields into six basin and tectonic-setting categories, and noted that continental passive margins account for 31% of giants. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world's giant oil fields. Basins associated with collision zones and subduction zones are where most of the remaining giant oil fields are found.

Passive margins are petroleum storehouses because these are associated with favorable conditions for accumulation and maturation of organic matter. Early continental rifting conditions led to the development of anoxic basins, large sediment and organic flux, and the preservation of organic matter that led to oil and gas deposits. Crude oil will form from these deposits. These are the localities in which petroleum resources are most profitable and productive. Productive fields are found in passive margins around the globe, including the Gulf of Mexico, western Scandinavia, and Western Australia.

Law of the Sea

International discussions about who controls the resources of passive margins are the focus of Law of the Sea negotiations. Continental shelves are important parts of national exclusive economic zones, important for seafloor mineral deposits (including oil and gas) and fisheries.

See also

Related Research Articles

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Sedimentary basin Regions of long-term subsidence creating space for infilling by sediments

Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime, the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.

Niger Delta Basin (geology)

The Niger Delta Basin, also referred to as the Niger Delta province, is an extensional rift basin located in the Niger Delta and the Gulf of Guinea on the passive continental margin near the western coast of Nigeria with suspected or proven access to Cameroon, Equatorial Guinea and São Tomé and Príncipe. This basin is very complex, and it carries high economic value as it contains a very productive petroleum system. The Niger delta basin is one of the largest subaerial basins in Africa. It has a subaerial area of about 75,000 km2, a total area of 300,000 km2, and a sediment fill of 500,000 km3. The sediment fill has a depth between 9–12 km. It is composed of several different geologic formations that indicate how this basin could have formed, as well as the regional and large scale tectonics of the area. The Niger Delta Basin is an extensional basin surrounded by many other basins in the area that all formed from similar processes. The Niger Delta Basin lies in the south westernmost part of a larger tectonic structure, the Benue Trough. The other side of the basin is bounded by the Cameroon Volcanic Line and the transform passive continental margin.

Large igneous province Huge regional accumulation of igneous rocks

A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive and extrusive, arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about the causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.

Geology of Australia

The geology of Australia includes virtually all known rock types and from all geological time periods spanning over 3.8 billion years of the Earth's history. Australia is a continent situated on the Indo-Australian Plate.

Red Sea Rift

The Red Sea Rift is a spreading center between two tectonic plates, the African Plate and the Arabian Plate. It extends from the Dead Sea Transform fault system, and ends at an intersection with the Aden Ridge and the East African Rift, forming the Afar Triple Junction in the Afar Depression of the Horn of Africa.

Porcupine Seabight A deep-water oceanic basin on the continental margin of the northeastern Atlantic

The Porcupine Seabight is a deep-water oceanic basin located on the continental margin in the northeastern portion of the Atlantic Ocean. It can be found in the southwestern offshore portion of Ireland and is part of a series of interconnected basins linked to a failed rift structure associated with the opening of the Northern Atlantic Ocean. The basin extends in a North-South direction and was formed during numerous subsidence and rifting periods between the Late Carboniferous and Late Cretaceous. It is bordered by the

Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

Volcanic passive margins (VPM) and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic passive margins occurs before and/or during the rifting process depending on the cause of rifting. There are two accepted models for VPM formation: hotspots/mantle plumes and slab pull. Both result in large, quick lava flows over a relatively short period of geologic time. VPM's progress further as cooling and subsidence begins as the margins give way to formation of normal oceanic crust from the widening rifts.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

Opening of the North Atlantic Ocean

The opening of the North Atlantic Ocean is a geological event that occurred over millions of years, during which the supercontinent Pangea broke up. As modern-day Europe and North America separated during the final breakup of Pangea in the early Cenozoic Era, they formed the North Atlantic Ocean. Geologists believe the breakup occurred either due to primary processes of the Iceland plume or secondary processes of lithospheric extension from plate tectonics.

The South China Sea Basin is one of the largest marginal basins in Asia. South China Sea is located to the east of Vietnam, west of Philippines and the Luzon Strait, and north of Borneo. Tectonically, it is surrounded by the Indochina Block on the west, Philippines Sea plate on the east, Yangtze Block to the north. A subduction boundary exists between the Philippines Sea Plate and the Asian Plate. The formation of the South China Sea Basin was closely related with the collision between the Indian Plate and Eurasian Plates. The collision thickened the continental crust and changed the elevation of the topography from the Himalayan orogenic zone to the South China Sea, especially around the Tibetan Plateau. The location of the South China Sea makes it a product of several tectonic events. All the plates around the South China Sea Basin underwent clockwise rotation, subduction and experienced an extrusion process from the early Cenozoic to the Late Miocene.

Offshore Indus Basin

The offshore Indus Basin is one of the two basins in offshore Pakistan, the other one being the offshore Makran Basin. The Murray Ridge separates the two basins. The offshore Indus basin is approximately 120 to 140 kilometers wide and has an areal extent of ~20,000 square km.

Exmouth Plateau

The Exmouth Plateau is an elongate northeast striking extensional passive margin located in the Indian Ocean roughly 3,000 meters offshore from western and northwestern Western Australia.

Gulf of Mexico basin

The formation of the Gulf of Mexico, an oceanic rift basin located between North America and the Yucatan Block, was preceded by the breakup of the Supercontinent Pangaea in the Late-Triassic, weakening the lithosphere. Rifting between the North and South American plates continued in the Early-Jurassic, approximately 160 million years ago, and formation of the Gulf of Mexico, including subsidence due to crustal thinning, was complete by 140 Ma. Stratigraphy of the basin, which can be split into several regions, includes sediments deposited from the Jurassic through the Holocene, currently totaling a thickness between 15 and 20 kilometers.

Angola Basin

The Angola Basin is located along the West African South Atlantic Margin which extends from Cameroon to Angola. It is characterized as a passive margin that began spreading in the south and then continued upwards throughout the basin. This basin formed during the initial breakup of the supercontinent Pangaea during the early Cretaceous, creating the Atlantic Ocean and causing the formation of the Angola, Cape, and Argentine basins. It is often separated into two units: the Lower Congo Basin, which lies in the northern region and the Kwanza Basin which is in the southern part of the Angola margin. The Angola Basin is famous for its "Aptian Salt Basins," a thick layer of evaporites that has influenced topography of the basin since its deposition and acts as an important petroleum reservoir.

The Tyrrhenian Basin is a sedimentary basin located in the western Mediterranean Sea under the Tyrrhenian Sea. It covers a 231,000 km² area that is bounded by Sardinia to the west, Corsica to the northwest, Sicily to the southeast, and peninsular Italy to the northeast. The Tyrrhenian basin displays an irregular seafloor marked by several seamounts and two distinct sub-basins - the Vavilov and Marsili basins. The Vavilov deep plain contains the deepest point of the Tyrrhenian basin at approximately 3785 meters. The basin trends roughly northwest-southeast with the spreading axis trending northeast-southwest.

Kutai Basin

The Kutai sedimentary basin extends from the central highlands of Borneo, across the eastern coast of the island and into the Makassar Strait. With an area of 60,000 km2, and depths up to 15 km, the Kutai is the largest and deepest Tertiary age basin in Indonesia. Plate tectonic evolution in the Indonesian region of SE Asia has produced a diverse array of basins in the Cenozoic. The Kutai is an extensional basin in a general foreland setting. Its geologic evolution begins in the mid Eocene and involves phases of extension and rifting, thermal sag, and isostatic subsidence. Rapid, high volume, sedimentation related to uplift and inversion began in the Early Miocene. The different stages of Kutai basin evolution can be roughly correlated to regional and local tectonic events. It is also likely that regional climate, namely the onset of the equatorial ever wet monsoon in early Miocene, has affected the geologic evolution of Borneo and the Kutai basin through the present day. Basin fill is ongoing in the lower Kutai basin, as the modern Mahakam River delta progrades east across the continental shelf of Borneo.

References

  1. Norwegian volcanic margin Archived June 22, 2012, at the Wayback Machine