Passive margin

In Geology, a margin passivates or continental terrace passivates corresponds to the zone of transition between the continental Croûte and the oceanic Croûte (flat abyssal) when there is no Subduction. The margin corresponds then to the one of the two shoulders of the Rift which initiated the oceanic opening (the other shoulder of the rift is on the other side of the ocean).

Characteristics

A passive margin is made of three parts:
  • the platform. It is very soft inclined: one finds uneven approximately 200 m with to 400 Mr.
  • the slope. Its slope is more brutal: 400 m in 4000 Mr.
  • the glacis, whose uneven one is approximately 100 m into 3000 Mr.

A passive margin is characterized by thermal Subsidence (depression related to the cooling of the oceanic crust) and normal faults, which functioned during the phase of rifting and which stopped as soon as the oceanisation started. These faults are typically decakilometric and listric (slightly upwards curved; their Pendage becomes less extremely in-depth). One can also observe formations such as the rocked blocks or the normal faults which appear during the formation of the rift.

Three types of sediments deposited on a passive margin can be distinguished:

  • the sediments pre-rift (or ante-rift), deposited before the phase of rifting, in calm tectonic context. They are interdependent of their base.
  • the sediments syn-rift, deposited during the phase of rifting (but before the oceanisation). The listric normal faults causing a rotation of the blocks, the sedimentary deposits are in range, with many onlaps . These sediments, because of gradual increase in the slope of deposit, frequently present slumps (nontectonic fold occurring in a sediment unconsolidated).
  • the sediments post-rift, deposited after the phase of rift, during the oceanic opening itself, are in horizontal parallel layers. The Subsidence can lead to very great thicknesses of sediments (up to 10 km).

Examples of passive margin

  • American, European and African coasts of the Atlantic ocean .
  • coasts of the Corsica and Provence.

See too

Internal bonds

  • Marge activates

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