Passive continental margin: what is it?

“Continental margin” is the term used to define the transition zone between a continental crust and an oceanic crust. There are two types of continental margins: the active margin and the passive margin.

Unlike the active margin, the passive margin does not represent a limit of lithospheric plate. It’s about a gradual transition between two crusts of different nature but which, once completed, no longer gives rise to tectonic or magmatic activity. Hence the term “passive”. It is therefore not the scene of a subduction zone, as is the case of the active margin. The passive margin is thus the witness par excellence of continental opening and the formation of a new ocean.

Very variable morphologies testifying to the diversity of the processes at work

The architecture varies greatly from one margin to another, even from one segment to another, and testifies to the diversity of the processes implemented during their development. Generally speaking, a passive margin includes a Continental shelf where the sediment limestone accumulate to sometimes reach considerable thicknesses of several kilometres, a continental slope characterized by a very steep slope and a glaze which marks the foot of the slope.

If the plateau is located at a shallow depth, usually between 200 and 400 meters, the embankment marks a sudden deepening: in a few kilometers you thus pass to the depth of the abyssal plainabout 4,000 meters deep.

The detailed morphology as well as the extension of the plateau and the slope are very variable and are linked in particular to the game of flaws normal during the expansion episode that led to the opening of the new ocean. The presence of underwater volcanoes or castings of lavas also participates in the singularity of each passive margin.

Indeed, volcanism is often a key element in the opening of the ocean and will have a more or less severe impact on the morphology of the margins being formed. This magmatic component also makes it possible to differentiate between two types of passive margins: margins rich in magma and magma-poor margins.

The margins marking the outline of the Atlantic are all passive margins and show the complexity and variability of the processes that took place during the opening of this ocean.

Magma-rich margins (volcanic margins)

The magma-rich margins are characterized by a strong magmatic and more particularly volcanic component. The continental opening will here be associated with significant volcanic outpourings, in connection with the rise of theasthenosphere which is located under the thinned and stretched zone in the continental crust. Lava flows can be superimposed over thicknesses of several kilometers, completely covering the stretched and faulted continental crust. The magma will also accumulate under the crust to form an “under-plated” body. The magma-rich margins thus show very significant crustal thicknesses, due to the accumulation of igneous rocks above and below the continental crust.

The Greenland and Norwegian margins are typically magma-rich passive margins.

Magma-poor margins

The margins poor in magma are characterized on the contrary by a small quantity, even a virtual absence of magmatic material. Their morphology is thus mainly defined by the faults which cut the continental crust and thin it down to zero thickness. This very thin area is called TOC (Ocean-Continent Transition) and can extend over a hundred kilometres. It is characterized by tilted blocks which can, in certain cases, continue by a zone of coat exhumed.

The presence of exhumed mantle is indicative of the inability of the extensional system to generate magma. This configuration can have various causes, in particular a speed very weak extension, which limits the ascent adiabatic of the asthenosphere, or the presence of a anomaly thermal in the coat (coat too cold and therefore difficult to melt). The continental crust being stretched to its maximum and the mantle finding itself unable to melt, the space created by the divergence of the plates is filled by a process of exhumation of the subcontinental mantle, which rises along major tectonic faults. The ocean floor is then composed of peridotites serpentinized (altered by seawater).

The Iberian and Newfoundland margins are typical examples of magma-poor margins with an exhumed mantle domain.

A heavily sedimented environment

In both cases, the evolution of the margins ends with the setting up of a ocean ridge and the creation of new oceanic crust. During their development, but also afterwards, the passive margins are the site of important sedimentary deposits coming in particular from the emerged lands of the continent which they border. These sediments will come to cover the base of the margin.

As they settle, they have the ability to seal levels containing matter organic matter deposited when the margin was at the beginning of its opening and when the system in extension then resembled small closed basins, of the lagoon type. This sedimentation will allow the maturation of organic matter, which can then evolve into oil and gas. The stability of the passive margins promotes this maturation. This explains why many oil fields are located off the Atlantic coast (offshore).

The East African rift, the current example of a forming margin

There is currently a natural laboratory that allows scientists to study the evolution of a continental rift and the formation of new passive margins. This is the case of east african rift, which sees the Horn of Africa (Somali plate) separating from the rest of the African continent. This rift was initiated 22 to 25 million years ago and is still at a relatively young stage, the establishment of the new ridge not expected to occur before 10 million years. However, the study of this region allows us to better understand the processes at work during the separation continental and the formation of the margins.