What Is A Passive Margin

Transition between oceanic and continental lithosphere that is not an agile plate margin

Rifting-to-spreading transition

Passive continental margin

A passive margin is the transition between oceanic and continental lithosphere that is non an active plate margin. A passive margin forms past sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-bounding main ridge and the locus of extension moves away from the continent-body of water purlieus. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Global distribution [edit]

Map showing the distribution of Earth's passive margins.

Passive margins are found at every ocean and continent boundary that is not marked by a strike-slip error or a subduction zone. Passive margins define the region effectually the Chill Body of water, Atlantic Sea, and western Indian Ocean, and define the entire coasts of Africa, Commonwealth of australia, Greenland, and the Indian Subcontinent. They are likewise constitute on the e coast of North America and South America, in Western Europe and most of Antarctica. Northeast Asia also contains some passive margins.

Key components [edit]

Active vs. passive margins [edit]

The stardom betwixt active and passive margins refers to whether a crustal purlieus between oceanic lithosphere and continental lithosphere is a plate boundary. Active margins are constitute 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 error, as defines the southern coastline of Westward Africa. Most of the eastern Indian Sea and almost all of the Pacific Ocean margin are examples of active margins. While a weld betwixt 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 agile processes on passive margins. Passive margins are but passive in that they are not active plate boundaries.

Morphology [edit]

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 electric current processes. The keen rivers (Amazon. Orinoco, Congo, Nile, Ganges, Yellow, Yangtze, and Mackenzie rivers) drain beyond 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 deep-sea apparently. The morphological expression of these features are largely divers by the underlying transitional crust and the sedimentation above it. Passive margins divers by a large fluvial sediment upkeep and those dominated by coral and other biogenous processes generally have a similar morphology. In addition, the shelf break seems to marker 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.

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

Cantankerous-section [edit]

Transitional crust composed of stretched and faulted continental chaff. Note: vertical calibration is greatly exaggerated relative to horizontal scale.

Cantankerous-section through transitional crust of a passive margin. Transitional crust as a largely volcanic construct. Annotation: vertical calibration is profoundly exaggerated relative to horizontal calibration.

The principal features of passive margins prevarication underneath the external characters. Beneath passive margins the transition betwixt the continental and oceanic chaff is a broad transition known equally transitional chaff. The subsided continental crust is marked by normal faults that dip seaward. The faulted chaff transitions into oceanic crust and may be deeply buried due to thermal subsidence and the mass of sediment that collects above it. The lithosphere below passive margins is known as transitional lithosphere. The lithosphere thins seaward every bit 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 chaff blazon, the other endmember (amagmatic) type is the rifted passive margin. Volcanic passive margins as well are marked past 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.

Subsidence mechanisms [edit]

Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation and is due to subsidence of specially the transitional crust. Subsidence is ultimately acquired past gravitational equilibrium that is established between the crustal tracts, known equally 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 oestrus flow. Heat flow at passive margins changes significantly over its lifespan, high at the showtime and decreasing with age. In the initial phase, 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 cook past decompression. Lithospheric thinning also allows the asthenosphere to ascension 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 add-on, mantle plumes may heat the lithosphere and cause prodigious igneous activity. One time a mid-oceanic ridge forms and seafloor spreading begins, the original site of rifting is separated into conjugate passive margins (for case, 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 in a higher place the subsiding transitional crust and lithosphere further depresses the transitional chaff.

Classification [edit]

There are four different perspectives needed to allocate passive margins:

map-view germination 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 chaff or this includes isolated rifts and stranded continental blocks (unproblematic and complex), and
sedimentation (carbonate-dominated, clastic-dominated, or sediment starved).

The starting time describes the relationship between rift orientation and plate motion, the second describes the nature of transitional crust, and the third describes postal service-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; information technology is more than advisable to subdivide individual passive margins into segments on this footing and utilize the threefold classification to each segment.

Geometry of passive margins [edit]

Rifted margin [edit]

This is the typical manner that passive margins grade, every bit 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 [edit]

Sheared margins class where continental breakdown was associated with strike-slip faulting. A good example of this type of margin is institute on the due south-facing declension of west Africa. Sheared margins are highly circuitous and tend to be rather narrow. They besides differ from rifted passive margins in structural style and thermal evolution during continental breakdown. As the seafloor spreading centrality moves forth 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 [edit]

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

Nature of transitional crust [edit]

Transitional crust, separating truthful oceanic and continental crusts, is the foundation of any passive margin. This forms during the rifting phase and consists of two endmembers: volcanic and non-volcanic. This nomenclature scheme simply applies to rifted and transtensional margin; transitional crust of sheared margins is very poorly known.

Not-volcanic rifted margin [edit]

Not-volcanic margins are formed when extension is accompanied by trivial mantle melting and volcanism. Non-volcanic transitional crust consists of stretched and thinned continental chaff. Not-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 role of the transitional crust.

Volcanic rifted margin [edit]

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 past meaning 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 exercise not contain large amounts of extrusive and/or intrusive rocks and may exhibit crustal features such every bit unroofed, serpentinized drape. Volcanic margins are known to differ from magma-poor margins in a number of ways:

A transitional crust equanimous 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
The presence of a lower crust with anomalously high seismic P-wave velocities (V_p=7.i-7.8 km/s) – referred to as lower crustal bodies (LCBs) in the geologic literature

The high velocities (V_p > seven km) and large thicknesses of the LCBs are testify that supports the case for plume-fed accretion (mafic thickening) underplating the crust during continental breakup. LCBs are located along the continent-sea transition but can sometimes extend beneath the continental function of the rifted margin (as observed in the mid-Norwegian margin for example). In the continental domain, in that location are all the same open give-and-take 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.South. East Coast
The mid-Norwegian margin
The Brazilian margins
The Namibian margin
The Due east Greenland margin
The West Greenland margin

Examples of not-volcanic margins:

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

Heterogeneity of transitional crust [edit]

Simple transitional crust [edit]

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

Complex transitional crust [edit]

This type of transitional chaff is characterized past abandoned rifts and continental blocks, such equally the Blake Plateau, K Banks, or Bahamas offshore eastern Florida.

Sedimentation [edit]

A fourth way to allocate 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 equally 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 [edit]

Constructional margins are the "classic" manner of passive margin sedimentation. Normal sedimentation results from the transport and degradation of sand, silt, and clay by rivers via deltas and redistribution of these sediments by longshore currents. The nature of sediments tin 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 minor, biogenic sedimentation can dominate especially nearshore sedimentation. The Gulf of Mexico passive margin along the southern United States is an fantabulous example of this, with muddy and sandy coastal environments down current (west) from the Mississippi River Delta and beaches of carbonate sand to the due east. The thick layers of sediment gradually thin with increasing altitude 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 border interruption reflects complex interaction between sedimentation, sealevel, and the presence of sediment dams. Coral reefs serve every bit bulwarks that allow sediment to accumulate between them and the shore, cutting off sediment supply to deeper water. Some other blazon of sediment dam results from the presence of common salt domes, as are mutual along the Texas and Louisiana passive margin.

Starved [edit]

Sediment-starved margins produce narrow continental shelves and passive margins. This is peculiarly common in barren regions, where at that place is fiddling send of sediment by rivers or redistribution by longshore currents. The Cherry Sea is a good example of a sediment-starved passive margin.

Formation [edit]

There are three main stages in the germination of passive margins:

In the first phase a continental rift is established due to stretching and thinning of the crust and lithosphere by plate move. This is the beginning of the continental crust subsidence. Drainage is generally abroad from the rift at this stage.
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 h2o apportionment coupled with arid climate create evaporite deposits. Crust and lithosphere stretching and thinning are still taking place in this phase. Volcanic passive margins also have igneous intrusions and dykes during this stage.
The last stage in formation happens but when crustal stretching ceases and the transitional chaff and lithosphere subsides as a effect of cooling and thickening (thermal subsidence). Drainage starts flowing towards the passive margin causing sediment to accumulate over information technology.

Economic significance [edit]

Passive margins are of import exploration targets for petroleum. Mann et al. (2001) classified 592 behemothic oil fields into half dozen basin and tectonic-setting categories, and noted that continental passive margins business relationship for 31% of giants. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world'due south behemothic oil fields. Basins associated with collision zones and subduction zones are where well-nigh of the remaining giant oil fields are establish.

Passive margins are petroleum storehouses considering 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 affair that led to oil and gas deposits. Crude oil will form from these deposits. These are the localities in which petroleum resources are near profitable and productive. Productive fields are found in passive margins around the globe, including the Gulf of United mexican states, western Scandinavia, and Western Australia.

Police force of the Sea [edit]

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

References [edit]

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

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