The role of structural inheritance in oblique rifting: insights from analogue models and application to the Gulf of Aden

Description

The geometry and kinematics of rifts are strongly controlled by pre-existing structures that may be present in both the crust and the mantle lithosphere. In the Gulf of Aden, the Tertiary oblique rift developed through inherited Mesozoic extensional basins that trend orthogonal to the direction of Oligo-Miocene divergence. Such inheritance may produce lateral variations in crustal thickness and thus in rheology of the crust and mantle lithosphere. How can such variations influence the present-day geometry of oblique rifts? May they locally overcome the impact of the oblique rheological weaknesses that in certain cases control the overall trend of the rift system? Moreover, we observe that major fracture zones systematically crosscut the inherited basins: may such inheritance influence the localization of major fracture zones by shifting the initial spreading centers? The brittle-ductile multilayer analogue models presented in this contribution reproduce oblique rifts. We tested the effect of an initial oblique weakness in the lithospheric mantle (i.e. structural or thermal inheritance) by introducing an oblique rheological weakness, which is alternatively initially imposed or dismissed. We tested the effect of the orthogonal Mesozoic inheritance by adding an elongated thicker brittle mantle, orthogonal to the direction of extension, in two models. These models mainly show en-échelon patterns with orthogonal faults and few rift-parallel faults (parallel to the rift obliquity), suggesting that the inherited orthogonal discontinuity is more influential than the oblique weakness in the lithospheric mantle. These results suggest that the presence in the lithosphere of an inherited basin could constitute a barrier to the deformation and sufficiently offset the spreading centers to lead to independent rift systems separated by major fracture faults. Moreover, the genetic nature of the obliquity (boundaries-driven or inherited) could control the length of the transform offset between two spreading centers and thus, the length of the future transform margin (continental domain affected by a transform fault).

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