Géodynamique des grands systèmes transformants. Exemples de la dorsale Sud-Est Indienne

Soutenance de thèse de Rim JBARA

Le jury sera composé de :
Mme Mathilde CANNAT (Directrice de recherche,  Institut de Physique du Globe de Paris, Rapporteure)
Mme Sylvie LEROY (Directrice de recherche, Institut des Sciences de la Terre de Paris, Rapporteure)
M. Jean-Yves ROYER (Directeur de recherche, Geo-Ocean Brest, Rapporteur)
Mme Valérie CHAVAGNAC (Directrice de recherche, Géosciences Environnement Toulouse, Examinatrice)
M. Walter ROEST (Chargé de recherche, Geo-Ocean Brest, Examinateur)
M. Daniel SAUTTER  (Directeur de recherche, Institut Terre et Environnement Strasbourg, Examinateur)
Mme Anne BRIAIS (Chargée de recherche, Geo-Ocean Brest, Directrice de thèse)
M. Georges CEULENEER (Directeur de recherche, Géosciences Environnement Toulouse, Directeur de thèse)
Mme Marcia Maia (Directrice de recherche, Geo-Ocean Brest, Co-encadrante)
M. Etienne Ruellan (Directeur de recherche, Géosciences Environnement Toulouse, Co-encadrant)

Résumé: Transform Faults (TFs) are major tectonic features which accommodate lateral motion between tectonic plates. Associated to their off-axis traces, the fracture zones, they constitute a natural archive of the change in plate motion. Thus, investigating large transform systems is a great tool to study the evolution of oceanic basins. In this thesis, a synoptic approach is developed using different geophysical datasets to investigate large transform faults and fracture zones that span the Southern Ocean between 139°E and 155°E along the Southeast Indian Ridge. The first case study focuses on the right-stepping George V TF (139°E). A detailed morphological and geophysical study is carried out based on high-resolution bathymetric and backscatter data acquired during STORM cruise (South Tasmania Ocean Ridge and Mantle), and gravity anomaly and seismicity analyses. The fault exhibits a complex structure and history. It is a multiple TF which evolved recently from two into five transform segments with four intra-transform ridge segments (ITRS). The westernmost ITRS1 appeared at 6 Ma as a response to a clockwise change in the relative motion between Australia and Antarctica. The ITRS3 likely formed around 11 Ma due to a small change in plate motion. The easternmost TF (TF E) appears to result from the conversion of a large, westward propagating overlapping spreading center into a stable transform fault, about 10 My ago. Analysis of the morphology and mantle Bouguer anomalies reveal that the magma supply to the ITRSs decreases from west to east. The ITRS1 displays a shallow topography and shows a complex interaction between the tectonic processes and magma supply. Since 6Ma, it has propagated to the west, inducing the segmentation along the transform fault, forming a restraining stepover. This resulted in the uplift of a 2000 m-high push-up massif. ITRS1 and 2 also display oblique volcanic ridges where fresh picritic basalts have been dredged. I interpret these observations to imply a regional extension in the Australian plate in the George V TF system, which might have favored the upwelling of mantle under and the magma extraction in the TF. Further east of the George V TF, I investigated the evolution of the Tasman TF which exhibits multi-fault segments. The associated fracture zones span the Southern Ocean to the continental margins. I suggest that this segmentation is inherited from the early change in plate motion during the breakup between Australia and Antarctica. The Tasman fault segments are associated with different tectonic structures resulting from local and regional stress. A transverse ridge is formed along one of the fault segments (Tasman TF D) around 6 Ma ago. Along the western segment, I observe a massif similar in shape to the one formed along the George V. I infer a similar mechanism responsible for the formation of the Tasman massif. Another particularity of the Tasman TF, is the formation of the Macquarie micro-plate around 6 Ma. I compare FZs trend and the synthetic flowlines using the Macquarie Antarctic rotation poles. The obtained results reveal that the three eastern ITRSs belong to the Macquarie-Antarctic plate boundary. My results show that those TFs are not magma starved like many mid-ocean ridge segments close to large transform faults. Besides that, a remarkable similarity between both TF systems is that the ITRSs become shallower from east to west. These observations may be related to a mantle anomaly beneath the easternmost SEIR, and their behavior suggests a flow of the asthenosphere to the west.