Sémardi de Flora Solon : Electrical Conductivity Structure Across Parnaiba basin, NE Brazil

Broadband and long period magnetotelluric (MT) data spanning from 0.001 s to 50,000 s were acquired in Parnaíba cratonic basin along one EW profile of approximately 1430 km long. The investigation depth varies from a couple of hundred meters to approximately 100 km, probing the subsurface resistivity structures of the Precambrian crust. Analysis of broadband data and 3D inversion of long period data revealed the presence of different lithospheric blocks bounded by major electrical discontinuities. These lithospheric blocks constitute: a mainly resistive crust and upper mantle along the western part of the study area, comprising Amazonian craton and western border of Parnaíba basin; a central block characterized by broad highly conductive anomaly extending to upper mantle; and the eastern block characterized by two resistive verticalized anomalies intercalated with a conductive zone. The bulk conductivity increase in the crust beneath the central part of the Parnaíba basin is unexpected for a cratonic basement. Our results support that this is due to the impregnation of the lithosphere by conducting minerals related to tectonic events involving either the Brazilian orogenic processes or to dispersed magmatic residues associated with the continuous igneous intrusions that occurred during the Triassic and Cretaceous.

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Séminaire de Stéphane Molliex, Laboratoire Géosciences Océans, CNRS, UBO, UBS

Les nucléides cosmogéniques: un outil pour quantifier l’érosion quaternaire dans l’approche «source to sink».

14h30, Salle de séminaire LGO

Séminaire de Julien Alleon, Dept. of Earth, Atmospheric, and Planetary Science, MIT

Préservation chimique des plus anciennes traces de vie sur Terre

 

Séminaire de Kenni PEDERSEN (Univ. Aarhus, Danemark)

Models of plate tectonic processes and their interaction with the convecting mantle: magmatism, vertical movements and thermo-chemical structure

During this talk, I will present different results from 2D high-resolution numerical modeling of lithosphere/mantle-scale processes involving plate tectonics and convection in the Earth’s mantle at different scale. The talk will be subdivided into three themes that all employ this modeling technique.

1. Thermal subsidence of extensional basins in the presence of sublithospheric convection. Here, I present models of lithosphere extension above an asthenosphere mantle that is convecting with sufficient vigor to explain e.g. surface heat flow of old (‘steady state’) oceanic lithosphere. The models indicate that thermal subsidence in the presence of such convection can differ substantially from the well-known McKenzie model that only implicitly accounts for convective heat flow. For example, thermal subsidence can become strongly protracted, because the thermal boundary layer may become advected upwards during thinning and therefore transport more heat through the cooling lithosphere than in the case, where the thermal boundary layer is assumed at a constant depth (e.g. the McKenzie model). Furthermore, the presence of sublithospheric convection beneath a sedimentary basin also results in dynamic vertical movements in the order of 10-100 m at a time scale of 2-20 Myr. Such movements are a possible cause of stratigraphic cycles that are otherwise commonly attributed to glacial eustasy.

2. LIP formation as a consequence of rift-induced mantle overturn. Here, I present modeling results that provide yet another mechanism for anomalous magmatism such as that associated with LIP formation in the embryonic North Atlantic during the Paleocene/Eocene as well the protracted high degree of activity that followed and continues in Iceland today. Using thermomechanical modeling we show that the following line of events are physically plausible: Rifting along an old Caledonian suture with a mafic lower crust partly in eclogite facies may lead to rapid delamination of the mantle lithosphere. This again leads to a rapid, but not particularly voluminous phase of magmatism. The detached mantle lithosphere rapidly sinks into the lower mantle and induces a return flow due off lower mantle material into the upper mantle. If the lower mantle has elevated potential temperature relative to the upper mantle, the return flow is amplified by thermal buoyancy, and a partial overturn of the mantle is initiated. Within 6 Myr from onset of lithosphere delamination, hot lower mantle material rises to the base of the rifts and starts melting. This melt phase leads to fast production of large volumes of melt consistent with North atlantic LIP formation at 55 Ma. After this, lower mantle upwelling and melt productivity decreases, but still continues in the following time until the present, consistent with the long-lived melt anomaly of the North Atlantic and present day Iceland.

3. Syn-convergent extension and UHP exhumation in the D’Entrecasteaux Islands of the Woodlark Basin. In this part of the talk, I will briefly present a thermomechanical model for the exhumation of Ultra-High-Pressure (UHP) rocks in the young core complexes of the D’Entrecasteaux Islands that are situated just west of the oceanic Woodlark Basin. Plate kinematic constraints of the latter indicate that UHP exhumation was associated with North/South extension of at least 100 km since 4 Ma. This had led some authors to propose a mechanism of reverse subduction (eduction), where the Northern margin of the Australian continental plate was first subducted and then exhumed by normal motion along the former subduction plane. In the other hand, structural fabrics of the exhumed UHP units imply diapiric exhumation which however is at odds with the amount of extension required from plate kinematics. The model I here present shows that exhumation by reverse subduction led to ductile extrusion and diapir-like fabric, even though exhumation was largely driven by >100 km extension. The model further shows that despite continued convergence between the Australian and the Pacific plates, coeval extension due to the opening of the Woodlark Basin could have been accommodated by subduction further North in the New Britain trench, as subduction of the Australian margin was reversed.

References

Petersen, K., Armitage, J., Nielsen, S. & Thybo, H. Mantle temperature as a control on the time scale of thermal evolution of extensional basins. Earth and Planetary Science Letters 409, 61-70, (2015).

Petersen, K. D., Nielsen, S. B., Clausen, O. R., Stephenson, R. & Gerya, T. Small-Scale Mantle Convection Produces Stratigraphic Sequences in Sedimentary Basins. Science 329, 827-830, (2010).

Petersen, K. D., Schiffer, C. & Nagel, T. LIP formation and protracted lower mantle upwelling induced by rifting and delamination. Scientific Reports 8, 16578, (2018).

Petersen, K. D. & Buck, W. R. Eduction, extension, and exhumation of ultrahigh‐pressure rocks in metamorphic core complexes due to subduction initiation. Geochemistry, Geophysics, Geosystems 16, 2564-2581, (2015).

Soutenance HDR de Christine Authemayou

DYNAMISME DES MARQUEURS GEOMORPHOLOGIQUES DES DEFORMATIONS
ET GEODYNAMIQUE DES MARGES ACTIVES

EN CONTEXTE DE POINT TRIPLE ET DE CONVERGENCE OBLIQUE

Résumé

Cette habilitation à Diriger des Recherche fait le point sur mes activités dans les domaines de la tectonique active, de la géomorphologie et de la structurale. Ces  activités ont pour objectifs principaux de mieux appréhender la dynamique des marges actives dans des contextes de point triple de plaques tectoniques et/ou de convergence oblique. Afin de quantifier les mouvements tectoniques et de déterminer les modes d’accommodation des déformations, j’ai utilisé de nombreux marqueurs de la déformation, principalement géomorphologiques. Cependant, ces marqueurs (alluviaux et/ou marins) résultent souvent de l’interaction des différents processus internes et externes qui complexifient l’interprétation de leur formation et de leur évolution. Dans ce mémoire, plusieurs marqueurs morphologiques sont présentés. Leur formation et leur évolution au cours du temps est décrite et leur pertinence pour la quantification des déformations est évaluée/critiquée. Ces études soulignent l’importance de l’érosion continentale dans la préservation et la datation des marqueurs terrestres, et du rôle de l’érosion marine dans la formation de marqueurs de type “terrasse corallienne” et “plateforme littorale rocheuse sous marine”. Les résultats signalent aussi la nécessité de garder l’approche naturaliste en complément d’analyses morphométriques quantitatives numériques. Les études de paléosismologie ont quand à elle montré la pertinence d’étudier les failles normales d’extrado pour caractériser l’activité sismique d’un pli/faille et la nécessité de réunir plusieurs conditions  pour permettre à un séisme de magnitude modérée d’être enregistré en surface dans un contexte intra-plaque. Enfin, les implications géodynamiques de ces études ont amené (1) à proposer des modèles permettant de maintenir la stabilité d’un point triple de type Subduction/Subduction/Transformante (Pull-up, Zipper modèles), (2) à révéler l’influence des variations spatiales du couplage inter-plaque pour promouvoir cette stabilité dans le cas où la limite transformante se situe dans la plaque supérieure, ou au contraire de nuire à cette stabilité dans le cas où la limite transformante se situe dans la plaque inférieure, et enfin, (3) à témoigner de  l’importance des processus profonds dans le mode d’accommodation des déformations dans un contexte de convergence oblique.

Composition du Jury

Laurent Jolivet (Pr, UPMC Paris)
Laurence Audin (IRD, ISTerre, Grenoble)
Joseph Martinod (Pr, ISTerre, Chambéry)
Serge Lallemand (Pr, GM, Montpellier)
Carole Petit (Pr, Géoazur, Nice)
Olivier Bellier (Pr, CEREGE, Aix-Marseille)
Laurent Geoffroy (Pr, LGO, Brest)