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Volcanic passive margins: another way to break up continents.

Geoffroy L, Burov EB, Werner P - Sci Rep (2015)

Bottom Line: Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere.Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain.Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

View Article: PubMed Central - PubMed

Affiliation: Université de Bretagne Occidentale, Brest, 29238 Brest.

ABSTRACT
Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

No MeSH data available.


Related in: MedlinePlus

Evolution with time of basement elevation (see Methods).Note, with time, the deepening and widening of the SDR-related flexure, the flexural shoulder uplift and the long-term buoyancy of the C-Block (see Figs 3 and 5). Author: E.B. Image created from modelling results using Adobe Photoshop CS6.
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f4: Evolution with time of basement elevation (see Methods).Note, with time, the deepening and widening of the SDR-related flexure, the flexural shoulder uplift and the long-term buoyancy of the C-Block (see Figs 3 and 5). Author: E.B. Image created from modelling results using Adobe Photoshop CS6.

Mentions: Both weak lithospheric mantle and initially strong lower crust, are sine qua non preconditions for the development of early conjugate CDFs. The footwall of the two opposite CDFs forms a central rigid continental block (C-Block in Fig. 3b,c), which becomes isolated in less than 1 Ma after the onset of extension (Extended Data Fig. 4). The development of crustal-scale conjugate detachments dipping outward with respect to the C-block is primarily due to the thermally-driven weakening of the mantle lithosphere, which partly flows outward and upward along the bottom of the C-block hardened by the initial “underplating” (Fig. 3b,c; Fig. 4 in Extended Data). This outward mantle flow is partly decoupled from the continental crust. This flow mechanically erodes laterally outward the lowermost parts of the C-block which, from ~2 Ma, becomes restricted to the rigid upper crust (Fig. 3c; Fig. 3 in Extended Data). Although we did not model anatectic processes erosion of the C-Block is probably enhanced by the partial melting of its lower and middle crust33. This, lateral flow creates a bulge of mixed rigid (mafic) and ductile (felsic) crust at the edges of the C-block (Fig. 3b,c). This bulge then localizes the detachments controlling the formation of SDRs. The magma, ascending from the asthenospheric mantle, also flows laterally outward and upward along the bottom of the C-block and is trapped within the detachment faults (Fig. 3b,c). From the outset, the magma-assisted concave-upward CDFs are purely crustal structures which develop in response to gravity collapse of the thick and denser non-deformed continental crust in relation to the thin and buoyant central part of the model (C-block) (Figs 3c and 4). While our model does not aim to reproduce all the small-scale characteristics of conjugate VPMs, it provides new insights on the origin of the inner SDRs which are coeval with crustal necking at VPMs. The seaward blocks observed at VPMs (e.g. Figs 1 and 2b in Extended Data), as well as the outer-SDRs, probably result from the progressive shredding of the C-Block with time (Fig. 4). Thus, more or less dissected micro-continents would be individualized within the oceanic domain, being restricted to the upper crust (Fig. 4). Although microcontinents may have different origins34, there is increasing evidence of such buoyant continental blocks being “lost” within oceans when continental breakup occurs in hot-mantle environments35 such as notably in the South Atlantic, offshore from the studied margins2136.


Volcanic passive margins: another way to break up continents.

Geoffroy L, Burov EB, Werner P - Sci Rep (2015)

Evolution with time of basement elevation (see Methods).Note, with time, the deepening and widening of the SDR-related flexure, the flexural shoulder uplift and the long-term buoyancy of the C-Block (see Figs 3 and 5). Author: E.B. Image created from modelling results using Adobe Photoshop CS6.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595843&req=5

f4: Evolution with time of basement elevation (see Methods).Note, with time, the deepening and widening of the SDR-related flexure, the flexural shoulder uplift and the long-term buoyancy of the C-Block (see Figs 3 and 5). Author: E.B. Image created from modelling results using Adobe Photoshop CS6.
Mentions: Both weak lithospheric mantle and initially strong lower crust, are sine qua non preconditions for the development of early conjugate CDFs. The footwall of the two opposite CDFs forms a central rigid continental block (C-Block in Fig. 3b,c), which becomes isolated in less than 1 Ma after the onset of extension (Extended Data Fig. 4). The development of crustal-scale conjugate detachments dipping outward with respect to the C-block is primarily due to the thermally-driven weakening of the mantle lithosphere, which partly flows outward and upward along the bottom of the C-block hardened by the initial “underplating” (Fig. 3b,c; Fig. 4 in Extended Data). This outward mantle flow is partly decoupled from the continental crust. This flow mechanically erodes laterally outward the lowermost parts of the C-block which, from ~2 Ma, becomes restricted to the rigid upper crust (Fig. 3c; Fig. 3 in Extended Data). Although we did not model anatectic processes erosion of the C-Block is probably enhanced by the partial melting of its lower and middle crust33. This, lateral flow creates a bulge of mixed rigid (mafic) and ductile (felsic) crust at the edges of the C-block (Fig. 3b,c). This bulge then localizes the detachments controlling the formation of SDRs. The magma, ascending from the asthenospheric mantle, also flows laterally outward and upward along the bottom of the C-block and is trapped within the detachment faults (Fig. 3b,c). From the outset, the magma-assisted concave-upward CDFs are purely crustal structures which develop in response to gravity collapse of the thick and denser non-deformed continental crust in relation to the thin and buoyant central part of the model (C-block) (Figs 3c and 4). While our model does not aim to reproduce all the small-scale characteristics of conjugate VPMs, it provides new insights on the origin of the inner SDRs which are coeval with crustal necking at VPMs. The seaward blocks observed at VPMs (e.g. Figs 1 and 2b in Extended Data), as well as the outer-SDRs, probably result from the progressive shredding of the C-Block with time (Fig. 4). Thus, more or less dissected micro-continents would be individualized within the oceanic domain, being restricted to the upper crust (Fig. 4). Although microcontinents may have different origins34, there is increasing evidence of such buoyant continental blocks being “lost” within oceans when continental breakup occurs in hot-mantle environments35 such as notably in the South Atlantic, offshore from the studied margins2136.

Bottom Line: Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere.Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain.Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

View Article: PubMed Central - PubMed

Affiliation: Université de Bretagne Occidentale, Brest, 29238 Brest.

ABSTRACT
Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

No MeSH data available.


Related in: MedlinePlus