Subcontinental lithospheric mantle

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core to crust
, the lithosphere comprising the crust and lithospheric mantle (detail not to scale)

The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of

continental lithosphere
.

The modern understanding of the Earth's

continental plates, acts as a brittle solid whereas the asthenosphere is hotter and weaker due to mantle convection. The boundary between these two layers is rheologically based and is not necessarily a strict function of depth. Specifically, oceanic lithosphere (lithosphere underneath the oceanic plates) and subcontinental lithosphere, is defined as a mechanical boundary layer that heats via conduction and the asthenosphere is a convecting adiabatic layer. In contrast to oceanic lithosphere, which experiences quicker rates of recycling, subcontinental lithosphere is chemically distinct, cold, and older. This translated into the differences between the SCLM and the oceanic lithospheric mantle
.

There are two different types of subcontinental lithosphere that formed at different times in Earth's history: Archean and Phanerozoic subcontinental mantle.

Archean subcontinental mantle

Archean lithosphere is strongly depleted in fertile melt indicators such as CaO and Al2O3. This depletion in major-elements should then be consequence of the Archean lithosphere's formation.

Sierra Nevada arc.[2]
Though there is evidence for the preservation of the Archean lithosphere, there is controversy over the preservation of the Archean mantle, for which the Archean lithosphere would have been derived.

The formation of the Archean SCLM is enigmatic. One early theory that

TTG crust, then the removal of basaltic melt and the enrichment of the mantle wedge with felsic melts could explain the formation of the depleted Archean subcontinental lithosphere. For more information, see Archean subduction
.

Phanerozoic subcontinental mantle

The mechanism of arc subduction is well understood to be the location where new continental crust is formed and is presumably also the site of subcontinental mantle genesis. Firstly, hydrated oceanic crust slabs begin subducting which releases fluids (subduction zone metamorphism) to the mantle wedge above. Continued subduction of the slab leads to further hydration of the mantle which causes partial melting in the mantle wedge. It is expected then that the modern subcontinental mantle is a former, melt-depleted mantle wedge. If the connection between continental crust and the subcontinental lithospheric mantle does not exist, and rather a different Earth process formed both reservoirs, then it further complicates the mechanisms for how the Archean subcontinental mantle formed.

References

  1. PMID 12460473
    .
  2. PMID 10988067
    .
  3. doi:10.2113/107.1-2.107
    .
  4. ISSN 1943-2682
    .
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