Déchirure continentale et segmentation du Golfe d'Aden Oriental en contexte de rifting oblique

Abstract : The Gulf of Aden stretches between the Somalia and the Arabia plates. It constitutes an interesting area to study the development of the passive margins. Its rifting originated ca. 35 Ma ago and the spreading began at least 17.6 Ma ago in the studied area. Moreover the Gulf opening direction is strongly oblique to its mean trend. This characteristic is studied in this thesis. On the northeastern margin, the Encens cruise (R/V l'Atalante, 2006) undertook the acquisition of new multichannel seismics data (360 traces), especially on the first-order segment between Alula-Fartak and Socotra fracture zones. A second-order segmentation is observed. It shows different structures and morphologies according to the segment, especially concerning the ocean-continent transition (OCT). From the studied segments, it appears that the western Ashawq-Salalah segment is characterized by an important post-rift magmatic event, whereas the eastern Mirbat segment shows a highly tectonised OCT basement high. The tectono-stratigraphic evolution of the Ashawq-Salalah segment was precisely studied (pre-stack depth migration processing and offshore-onshore correlation of the sedimentary processes). First of all, the margin was structured by syn-rift horsts and grabens. Then a strain localisation appeared on the distal margin. At the start of the OCT formation, a local or regional uplift induced a landslide atop of the more distal horst. Crustal deformation was then located in the OCT, where continental breakup finally occurred. OCT could be constituted of serpentinised underlying mantle. It was finally intruded by magmatic material during post-rift time. Besides, the analysis of the sedimentary cover of the OCT basement high shows that it was subjected to a post-rift surrection related with a volcanic activity (volcano, flows and sills). This evolution may be compared to the latest evolution models of passive margins. The conjugated margin (southeastern Gulf of Aden) shows the same segmentation that the northeastern one (d'Acremont et al., 2005). The microstructural analysis of Socotra Island allows the comparison of the margins and constrains the oblique rifting. As on the northern margin, normal faults directions are classified in three faults families: N110°E normal to the extension, N70°E parallel to the gulf direction and the intermediate N90°E. The stress fields deduced from the micro-structural data inversion are in agreement with the direction of extension indicated by the three major fault directions. Chronologies observed in the field show rotations of the direction of extension from N20°E to N160°E, and reciprocally. This suggests that stress fields alternate during the rifting. Analogue modelling of the oblique rifting allowed to better understand the development of the Gulf of Aden. Horsts and grabens organisation follows an " en-échelons " pattern, with sigmoid shapes, between lateral velocity discontinuities of the model. The three fault families linked to obliquity are observed : N110°E, N90°E and N70°E. Distribution of faults directions during extension shows a N90°E and N110°E faults dominance at the beginning of the extension and the later development of N70°E faults (with or without an oblique-to-extension heterogeneity in the model initial setup). Finally N110°E faults are anew created. Thus, the stress fields chronologies observed on Socotra island can be explained. Bellahsen et al. (2006) conceptual model is applicable to the first stages of the evolution : inherited N110°E basin reactivation and new en-echelon faults formation have occurred according to the plates direction of extension (N20-25°E) since 35 Ma. Later, as the lithosphere carried on thinning along the Gulf direction (N70-75°E), local stresses due to lateral thicknesses variations cause the formation of N70°E faults and reactivation of the N110°E faults. N20°E then N160°E extension chronology observed on the margins is thus reproduced. We can propose a third stage: once the rift thinning is sufficient, local stresses only apply on the rift borders or on major horsts. Anywhere else, N110°E faults are created and N70°E faults are obliquely reactivated. The second extension chronology N160°E then N20°E of micro-structural analysis is also reproduced. Horsts may be subjected to important clockwise rotation inducing left-lateral shear zones. Those may initiate the numerous transfer zones leading to the strong segmentation of the Gulf of Aden.
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Submitted on : Tuesday, October 18, 2011 - 1:51:24 PM
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  • HAL Id : tel-00633368, version 1


Julia Autin. Déchirure continentale et segmentation du Golfe d'Aden Oriental en contexte de rifting oblique. Géologie appliquée. Université Pierre et Marie Curie - Paris VI, 2008. Français. ⟨tel-00633368⟩



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