Passive margin
Encyclopedia
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
, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic ridge
and the locus of extension moves away from the continent-ocean boundary
. The transition between the continental and oceanic crust that was originally created by rifting is known as a passive margin.
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.
or not. Active margins
are found on the leading edge of a continent where subduction occurs. These are often marked by uplift
and volcanic mountain belts
on the continental plate, and by island-arc chains on the oceanic plate. Less often there is a strike-slip fault, as is defining 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 crusts are called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all very active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.
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 canyon
s, which mark the offshore continuation of rivers.
At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as the fjords of Norway
and Greenland
.
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 they 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 the subsided continental crust and form seaward-dipping reflectors.
. 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.
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.
time. Faulting tends to be listric: normal faults that flatten with depth.
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.
.
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.
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 basalt
ic igneous rock
s, 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:
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.
Example of volcanic margins:
is a good example.
s and continental blocks, such as the Blake Plateau, Grand Banks
, or Bahama Islands offshore eastern Florida.
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 to Depending on how the rifting occurred, when, how and what type of sediment varies.
and deposition
of sand, silt, and clay by river
s 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 current
s 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 dome
s, as are common along the Texas
and Louisiana
passive margin.
There are three main stages in the formation of passive margins:
, also known as oil and gas, which holds a very important place in our lives. Plastic
s, medicine
s, synthetic fiber
and clothing, and of course fuel for transportation: gasoline
, diesel oil, and kerosene
all come increasingly from deposits buried beneath passive margins. A large proportion of oil and gas are found at passive margins. Mann et al. (2001) examined 592 giant oil fields (which contain proved reserves >500 million barrels of oil or >3 trillion cubic feet of natural gas) and are estimated to contain ~65% of oil. 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
.
negotiations. Continental shelves are important parts of national exclusive economic zone
s, important for seafloor mineral deposits (including oil and gas) and fisheries.
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
and continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
which is not an active plate margin
Continental margin
The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
. It is constructed by sedimentation
Sedimentation
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...
above an ancient rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
, now marked by transitional crust. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-oceanic ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
and the locus of extension moves away from the continent-ocean boundary
Continent-ocean boundary
The Continent-ocean boundary or continent-ocean transition is the boundary between continental crust and oceanic crust on a passive margin. The identification of continent-ocean boundaries is important in the definition of plate boundaries at the time of break-up when trying to reconstruct the...
. The transition between the continental and oceanic crust that was originally created by rifting is known as a passive margin.
Global distribution
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...
, and western Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
, and define the entire coasts of Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located 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...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. They are also found on the east coast of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, in western Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and most of Antarctica. East Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
also contains some passive margins.
Active vs. passive margins
This refers to whether a crustal boundary between tracts oceanic and continental crusts are boundaries of platesTectonic Plates
Tectonic Plates is a 1992 independent Canadian film directed by Peter Mettler. Mettler also wrote the screenplay based on the play by Robert Lepage. The film stars Marie Gignac, Céline Bonnier and Robert Lepage.-Plot summary:...
or not. Active margins
Volcanic passive margin
Volcanic passive margins 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...
are found on the leading edge of a continent where subduction occurs. These are often marked by uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
and volcanic mountain belts
Volcanic belt
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 where magma is created by partial melting of solid material in the Earth's crust and upper mantle....
on the continental plate, and by island-arc chains on the oceanic plate. Less often there is a strike-slip fault, as is defining the southern coastline of W. Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. Most of the eastern Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
and nearly all of the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
margin are examples of active margins. While a weld between oceanic and continental crusts are called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all very active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.
Morphology
Coastal plain
A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...
and offshore continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
-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
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...
. Orinoco
Orinoco
The Orinoco is one of the longest rivers in South America at . Its drainage basin, sometimes called the Orinoquia, covers , with 76.3% of it in Venezuela and the remainder in Colombia...
, Congo
Congo River
The Congo River is a river in Africa, and is the deepest river in the world, with measured depths in excess of . It is the second largest river in the world by volume of water discharged, though it has only one-fifth the volume of the world's largest river, the Amazon...
, Nile
Nile
The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...
, Ganges, Yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
, Yangtze, and Mackenzie
Mackenzie River
The Mackenzie River is the largest river system in Canada. It flows through a vast, isolated region of forest and tundra entirely within the country's Northwest Territories, although its many tributaries reach into four other Canadian provinces and territories...
rivers) drain across passive margins. Extensive estuaries
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
are common on mature passive margins. Although there are many kinds of passive margins, the morphologies
Shape
The shape of an object located in some space is a geometrical description of the part of that space occupied by the object, as determined by its external boundary – abstracting from location and orientation in space, size, and other properties such as colour, content, and material...
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
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
lowstand, defined by the glacial maxima. The outer continental shelf and slope may be cut by great submarine canyon
Submarine canyon
A submarine canyon is a steep-sided valley on the sea floor of the continental slope. Many submarine canyons are found as extensions to large rivers; however there are some that have no such association. Canyons cutting the continental slopes have been found at depths greater than 2 km below sea...
s, which mark the offshore continuation of rivers.
At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as the fjords of Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
and Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located 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...
.
Cross-section
Thermal subsidence
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...
and the mass of sediment that collects above it. The lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's 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 they also are marked by numerous dykes and igneous intrusions
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
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 the subsided continental crust and form seaward-dipping reflectors.
Subsidence mechanisms
Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation space 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 isostasyIsostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
. 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
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
) 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
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
to rise closer to the surface, heating the overlying lithosphere by conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
and advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
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
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
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 of passive margins
There are four different perspectives needed to classify passive margins:- map-view formation geometry (rifted, sheared, and transtensional),
- nature of transitional crust (volcanic and non-volcanic),
- 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
- 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.
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 JurassicJurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
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 spreadingSeafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
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 CaliforniaGulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
.
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 marginsNon-Volcanic Passive Margins
Non-volcanic passive margins constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins . Transitional crust welds continental crust to oceanic crust along the lines of continental break-up...
are formed when extension is accompanied by little mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
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 marginsVolcanic passive margin
Volcanic passive margins 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...
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 basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s, including lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
, dykes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
, and gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
.
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 basaltBasaltBasalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic igneous rockIgneous rockIgneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s, including lavaLavaLava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows, sillsSill (geology)In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
, dykesDike (geology)A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
, and gabbroGabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
s. - 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.
Example 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
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 TexasTexas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
is a good example.
Complex transitional crust
This type of transitional crust is characterized by abandoned riftRift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
s and continental blocks, such as the Blake Plateau, Grand Banks
Grand Banks
The Grand Banks of Newfoundland are a group of underwater plateaus southeast of Newfoundland on the North American continental shelf. These areas are relatively shallow, ranging from in depth. The cold Labrador Current mixes with the warm waters of the Gulf Stream here.The mixing of these waters...
, or Bahama Islands offshore eastern Florida.
Sedimentation
A fourth way to classify passive margins is according to the nature of sedimentationSedimentation
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...
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 to Depending on how the rifting occurred, when, how and what type of sediment varies.
Constructional
Constructional margins are the “classic” mode of passive margin sedimentation. Normal sedimentation results from the transportSediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
and deposition
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
of sand, silt, and clay by river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
s via deltas
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
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
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
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
Mississippi River Delta
The Mississippi River Delta is the modern area of land built up by alluvium deposited by the Mississippi River as it slows down and enters the Gulf of Mexico...
and beaches of carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
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 current
Turbidity current
A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows...
s 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 dome
Salt dome
A salt dome is a type of structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata, forming a diapir....
s, as are common along the Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and Louisiana
Louisiana
Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...
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
- 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.
- The second stage leads to the formation of an oceanic basin, similar to the modern Red SeaRed SeaThe Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
. 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. Salt has low density so this later may migrate upwards as salt domes. 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. - 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 reservoirs of petroleumPetroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, also known as oil and gas, which holds a very important place in our lives. Plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s, medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
s, synthetic fiber
Synthetic fiber
Synthetic fibers are the result of extensive research by scientists to improve on naturally occurring animal and plant fibers. In general, synthetic fibers are created by forcing, usually through extrusion, fiber forming materials through holes into the air, forming a thread...
and clothing, and of course fuel for transportation: gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
, diesel oil, and kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
all come increasingly from deposits buried beneath passive margins. A large proportion of oil and gas are found at passive margins. Mann et al. (2001) examined 592 giant oil fields (which contain proved reserves >500 million barrels of oil or >3 trillion cubic feet of natural gas) and are estimated to contain ~65% of oil. 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
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
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
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
, western Scandinavia
Scandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
, and Western Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
.
Law of the Sea
International discussions about who controls the resources of passive margins are the focus of Law of the SeaLaw of the sea
Law of the sea may refer to:* United Nations Convention on the Law of the Sea* Admiralty law* The Custom of the Sea...
negotiations. Continental shelves are important parts of national exclusive economic zone
Exclusive Economic Zone
Under the law of the sea, an exclusive economic zone is a seazone over which a state has special rights over the exploration and use of marine resources, including production of energy from water and wind. It stretches from the seaward edge of the state's territorial sea out to 200 nautical...
s, important for seafloor mineral deposits (including oil and gas) and fisheries.
See also
- Convergent boundaryConvergent boundaryIn plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
- Divergent boundaryDivergent boundaryIn plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...
- Plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
- Continental shelfContinental shelfThe continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...