AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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Exploration Challenges and Perspectives of Hyper-Extended Rift Systems: A Northern Red Sea Viewpoint

Abstract

Successful exploration of ultra-deep offshore rifted margin settings is facing major challenges, as structural and sedimentary records of distal rifted margin, located outboard of crustal necking zones, cannot be understood by simple extrapolation of proximal rifted margin observations. The syn-rift record of distal margins differs in terms of accommodation space, depositional environments and diagenetic histories in comparison with the record from proximal margins, which traditionally provide the bulk of the constraints on early syn-rift evolution. These differences are mainly due to differences in timing, geometric modes, subsidence history, fluid and thermal evolution during progressive hyper-extension of continental lithosphere leading to break-up. In order to make basin modeling of hyper-extended rifts and margins predictive requires calibrated input data, which remain scarce of often confidential. In this study, we use the Gulf of Suez (GS) and Northern Red Sea (NRS) as a laboratory as a major part of the rifting to break-up history is accessible in outcrop. We constrain a new forward kinematic model (Rifter software) to reproduce the NRS evolution until break-up stage, considering both tectono-stratigraphic and thermochronometric data. The first stage of our model shows a diffuse phase of rifting that started at ~23 Ma with little associated syn-rift magmatism. At about 20-15 Ma, rapid suyn-rift subsidence and narrowing of the rift signals the onset of a margin-orthogonal hyper-extension, leading to the formation of crustal necking zones and hyper-extended crust. Both the NRS and GS record concurrent crustal thinning at this stage as well as local lower-crustal exhumation locally as supported by Zabargad and Brothers islands records. These data document significant exhumation of the necking zones that is reproduced in the modeling and should be reflected by arrival of clastic detritus in the distal margin. At 15-14 Ma, the entire crust mechanically coupled with the underlying mantle as shown by abrupt rift narrowing and heating as reflected by thermochronometric and fluid inclusions data and the occurrence of exhumed mantle rocks at Zabargad. Middle Miocene (16-11 Ma) evaporite deposition appears to have been contemporaneous with mantle exhumation within a thermal gradient exceeding 100°C/km. The “mantle coupling event” appears to have been driven by a kinematic reorganization and the onset of margin-oblique divergence, resulting in the onset of deformation along the Aqaba transform fault and abandonment of active rifting in the GS. This observation suggests that obliquity of rifting in respect of underlying mantle dynamic is transient and controlled by the changing rheology of inherited continental lithosphere under extension. At 8-6 Ma, a post-exhumation thermal overprint is recorded in Zabargad island, which is associated with intense hydrothermal activity likely related to incipient oceanic seafloor spreading in the southern and central Red Sea, as recorded by latest Miocene rutile U-Pb and (U-Th)/He ages. We ascribe this record as the northward propagation of lithospheric break-up from south to northern Red Sea and the early dynamic of a segmented mid-oceanic ridge. Hence, the NRS records the transitions from short-lived episode of diffuse rifting to necking and hyperextension during orthogonal rifting as well as the transition to the exhumation phase with the onset of oblique extension and the eventual incipient ocean-breakup, starting in the latest Miocene.