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Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation

Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation
Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation
The Afar region in East Africa is a key locality for studying continental break-up. Within Afar proper, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension.

Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar elegantly reproduces the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes more subsidence, and higher strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension.

These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions occurred, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.
Continental break-up, Crustal flexure, Magma-rich passive margin, Marginal graben, Passive margin, Rifting
0040-1951
Zwaan, Frank
25329ca2-014b-4cb9-9ef3-3b8a4259c675
Corti, Giacomo
dce88b12-5b7a-43b1-8a58-5bd1bc13634c
Keir, Derek
5616f81f-bf1b-4678-a167-3160b5647c65
Sani, Federico
6c59b4c4-ecca-4ecc-ba2c-66efa5e5e3df
Zwaan, Frank
25329ca2-014b-4cb9-9ef3-3b8a4259c675
Corti, Giacomo
dce88b12-5b7a-43b1-8a58-5bd1bc13634c
Keir, Derek
5616f81f-bf1b-4678-a167-3160b5647c65
Sani, Federico
6c59b4c4-ecca-4ecc-ba2c-66efa5e5e3df

Zwaan, Frank, Corti, Giacomo, Keir, Derek and Sani, Federico (2020) Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation. Tectonophysics, 796, [228595]. (doi:10.1016/j.tecto.2020.228595).

Record type: Article

Abstract

The Afar region in East Africa is a key locality for studying continental break-up. Within Afar proper, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension.

Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar elegantly reproduces the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes more subsidence, and higher strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension.

These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions occurred, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.

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Accepted/In Press date: 18 August 2020
e-pub ahead of print date: 24 August 2020
Keywords: Continental break-up, Crustal flexure, Magma-rich passive margin, Marginal graben, Passive margin, Rifting

Identifiers

Local EPrints ID: 443890
URI: http://eprints.soton.ac.uk/id/eprint/443890
ISSN: 0040-1951
PURE UUID: 4d6a9669-5fd3-4afd-955c-3c1b738eaf45
ORCID for Derek Keir: ORCID iD orcid.org/0000-0001-8787-8446

Catalogue record

Date deposited: 16 Sep 2020 16:34
Last modified: 26 Nov 2021 02:57

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Contributors

Author: Frank Zwaan
Author: Giacomo Corti
Author: Derek Keir ORCID iD
Author: Federico Sani

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