Passive margin

Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary.

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 consist of both onshore coastal plain and offshore continental shelf-slope-rise triads.

Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes generally have a similar morphology.

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.

Volcanic passive margins also are marked by numerous dykes and igneous intrusions within the subsided continental crust.

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.

The mantle lithosphere below the thinned and faulted continental oceanic transition cools, thickens, increases in density and thus begins to subside.

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.

This is the typical way that passive margins form, as separated continental tracts move perpendicular to the coastline.

They also differ from rifted passive margins in structural style and thermal evolution during continental breakup.

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 margins form part of large igneous provinces, which are characterised by massive emplacements of mafic extrusives and intrusive rocks over very short time periods.

The transitional crust of volcanic margins is composed of basaltic igneous rocks, including lava flows, sills, dykes, and gabbro.

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.

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.

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.

Another type of sediment dam results from the presence of salt domes, as are common along the Texas and Louisiana passive margin.

Passive margins are petroleum storehouses because these are associated with favorable conditions for accumulation and maturation of organic matter.

International discussions about who controls the resources of passive margins are the focus of law of the sea negotiations.