Éditeur(s) : HAL CCSD
Résumé : International audience
The strength of the lithospheric mantle controls the mode of lithosphere deformation: the presence or absence ofa high strength brittle mantle respectively leads to localized or distributed rifting. However, first order geophysicaldata question the existence of such a brittle mantle. Here we use 2-D finite-element large strain modelling toquantify the impact of a ductile localizing mantle – instead of brittle – in triggering continental rifting. As a novelty,the mantle rheology considers the effect of grain boundary sliding during strain-induced grain size reduction,which may promote a significant strength drop and subsequent strain localization at low mantle temperature (<700-800°C). Our results reveal that such ductile localizing mantle implies varying modes of continental rifting thatmainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust. Amedium to strong lower crust implies coupling between the upper crust and ductile localizing mantle, yielding tonarrow continental rifting. In contrast, a weak lower crust implies decoupling between the upper crust and ductilelocalizing mantle, giving rise to a switch from distributed faulting at incipient strain to localized faulting at largestrain. Ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting,although a critical amount of weakening is required. Such ductile localizing mantle provides a relevant geologicaland mechanical alternative to the brittle mantle. It moreover provides a wider variety of modes of upper crustalfaulting that are commonly observed in nature.
EGU General Assembly 2014
Vienne, Austria

insu-01565161
https://hal-insu.archives-ouvertes.fr/insu-01565161

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Much debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain
ISSN: 0040-1951

insu-00949213
https://hal-insu.archives-ouvertes.fr/insu-00949213
https://hal-insu.archives-ouvertes.fr/insu-00949213/document
https://hal-insu.archives-ouvertes.fr/insu-00949213/file/Gueydan_et_al_2014.pdf
DOI : 10.1016/j.tecto.2014.02.005

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Ronda peridotite supplies one of the best objects to document subcontinental mantle deformation, but its internal deformation and exhumation mechanisms remain controversial. Here we provide new structural data and numerical results that constrain the Oligocene-Miocene deformation history of the Ronda massif. We first describe a mantle shear zone in the northern massif that developed subcrustal strain localization during decompression and related partial melting. The deformation regime of this mantle shear zone evolved from penetrative NE-SW stretching to sinistral shear highlighted by discrete shear bands. We then show structural observations that document a viscous deformation in the southern massif occurring prior to the thrust-assisted emplacement of the peridotite during large decompression. Finally, we performed numerical investigations that quantify a high temperature of 980°C for the basal peridotite lens at the time of its crustal emplacement. Our numerical results constrain the timing of ductile deformation of the peridotite just before 22 Ma, probably between 30 and 22 Ma. Altogether, these features led us to conclude that the deformation and exhumation of the Ronda peridotite results from lithosphere thinning subsequently inverted in the course of the Oligocene-Miocene. Among available models, our findings support the hypothesis of peridotite exhumation by the inversion of a thinned back-arc continental lithosphere during westward slab rollback through the Alboran region.
ISSN: 0278-7407

insu-00924180
https://hal-insu.archives-ouvertes.fr/insu-00924180
https://hal-insu.archives-ouvertes.fr/insu-00924180/document
https://hal-insu.archives-ouvertes.fr/insu-00924180/file/tect20062.pdf
DOI : 10.1002/tect.20062

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Analogue and numerical models have shown that the strength of the lithospheric mantle controls the mode of lithosphere deformation. In extension, the presence or absence of a high strength brittle mantle respectively leads to localized or distributed rifting. However, first order geophysical data question the existence of such a brittle mantle. Here we use 2-D finite-element large strain modelling to quantify the impact of a ductile localizing mantle - instead of brittle - in triggering continental rifting. As a novelty, the mantle rheology considers the effect of grain boundary sliding during strain-induced grain size reduction, which may promote a significant strength drop and subsequent strain localization at low mantle temperature (< 700-800 °C). Our results reveal that such ductile localizing mantle implies varying modes of continental rifting that mainly depend on both the amount of weakening in the ductile mantle and the strength of the lower ductile crust. A medium to strong lower crust implies coupling between the upper crust and ductile localizing mantle, yielding to narrow continental rifting. In contrast, a weak lower crust implies decoupling between the upper crust and ductile localizing mantle, giving rise to a switch from distributed faulting at incipient strain to localized faulting at large strain. Ductile strain localization in the lithospheric mantle is therefore sufficient to trigger continental rifting, although a critical amount of weakening is required. Such ductile localizing mantle provides a relevant geological and mechanical alternative to the brittle mantle. It moreover provides a wider variety of modes of upper crustal faulting that are commonly observed in nature.
ISSN: 0040-1951

Droits : info:eu-repo/semantics/OpenAccess
insu-00924254
https://hal-insu.archives-ouvertes.fr/insu-00924254
https://hal-insu.archives-ouvertes.fr/insu-00924254/document
https://hal-insu.archives-ouvertes.fr/insu-00924254/file/Gueydan_precigout_2014_Tectonophysics.pdf
DOI : 10.1016/j.tecto.2013.11.029