The initiation of segmented buoyancy-driven melting during continental breakup

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Abstract

Melting of the mantle during continental breakup leads to magmatic intrusion and volcanism, yet our understanding of the location and dominant mechanisms of melt generation in rifting environments is impeded by a paucity of direct observations of mantle melting. It is unclear when during the rifting process the segmented nature of magma supply typical of seafloor spreading initiates. Here, we use Rayleigh-wave tomography to construct a high-resolution absolute three-dimensional shear-wave velocity model of the upper 250 km beneath the Afar triple junction, imaging the mantle response during progressive continental breakup. Our model suggests melt production is highest and melting depths deepest early during continental breakup. Elevated melt production during continental rifting is likely due to localized thinning and melt focusing when the rift is narrow. In addition, we interpret segmented zones of melt supply beneath the rift, suggesting that buoyancy-driven active upwelling of the mantle initiates early during continental rifting.

Abstract

Our understanding of melt production during early continental breakup remains poorly constrained. Using the Afar triple junction as an example, Gallacher et al. generate a 3D velocity model suggesting that melt production is highest during early continental breakup due to localised thinning of the crust.

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Evolution of magma-poor continental margins from rifting to seafloor spreading.

R. WHITMARSH, G Manatschal, T A Minshull (2001)

The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid-state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward.

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Magma-assisted rifting in Ethiopia.

M D Bastow, Bradley Kendall, C Ebinger … (2005)

The rifting of continents and evolution of ocean basins is a fundamental component of plate tectonics, yet the process of continental break-up remains controversial. Plate driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. However, Buck has proposed that lithospheric heating by mantle upwelling and related magma production could promote lithospheric rupture at much lower stresses. Such models of mechanical versus magma-assisted extension can be tested, because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The Northern Ethiopian Rift is an ideal place to test break-up models because it formed in cratonic lithosphere with minor far-field plate stresses. Here we present evidence of seismic anisotropy in the upper mantle of this rift zone using observations of shear-wave splitting. Our observations, together with recent geological data, indicate a strong component of melt-induced anisotropy with only minor crustal stretching, supporting the magma-assisted rifting model in this area of initially cold, thick continental lithosphere.

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Lower-crustal intrusion on the North Atlantic continental margin

C. J. Parkin, Z. Lunnon, N. Hurst … (2008)

When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean.

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Journal ID (nlm-ta): Nat Commun

Journal ID (iso-abbrev): Nat Commun

Title: Nature Communications

Publisher: Nature Publishing Group

ISSN (Electronic): 2041-1723

Publication date (Electronic): 18 October 2016

Publication date Collection: 2016

Volume: 7

Electronic Location Identifier: 13110

Affiliations

[1 ]Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton , Southampton SO14 3ZH, UK

[2 ]Dipartimento di Scienze della Terra, Università degli Studi di Firenze , Florence 50121, Italy

[3 ]School of Earth and Environment, University of Leeds , Leeds LS2 9JT, UK

[4 ]Sorbonne Universités, UPMC - ISTEP - CNRS UMR7193 , Paris 75252, France

[5 ]Department of Earth and Planetary Sciences, Birkbeck, University of London , London WC1E 7HX, UK

[6 ]School of Earth Sciences, University of Bristol , Bristol BS8 1RJ, UK

[7 ]Institute of Geophysics, Space and Astronomy, Addis Ababa University , Addis Ababa, Ethiopia

[8 ]Eritrea Institute of Technology , Asmara, Eritrea

[9 ]Seismological and Volcanological Observatory Center , Dhamar, Yemen

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[a ] Ryan.Gallacher@ 123456soton.ac.uk

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Publisher Item ID: ncomms13110

DOI: 10.1038/ncomms13110

PMC ID: 5071842

PubMed ID: 27752044

SO-VID: a6715205-db6b-4550-bde0-cf5670cc1904

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The initiation of segmented buoyancy-driven melting during continental breakup

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African Journal of Empirical Research

Most cited references 7

Evolution of magma-poor continental margins from rifting to seafloor spreading.

Magma-assisted rifting in Ethiopia.

Lower-crustal intrusion on the North Atlantic continental margin

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