Evolution of a volcanic-rich continental rift system Southern Red Sea rift

The Red Sea offers a world-class example of how structural geometries and continental rift evolution influence syn-rift basin architectures and sediment accumulation through time. Two problems are of current interest in rift studies, a) whether and how active rift structures may exploit inherited tectonic elements and b) how volcanic activity may vary in time and space in the evolution of a rift and how it may influence basin architecture. The syn-rift of the Red Sea can be divided into three zones from south to north; the south represents a volcanic-rich continental rift, the center a transitional zone, and the north a non-volcanic continental rift.
Two study areas were selected to improve our understanding of these rift components, tectonics, sedimentation and volcanism. The Hamd-Jizl Basin (inferred to be of Late Oligocene-Early Miocene age) represents a non-volcanic basin setting and although outcrop is poor, reveals some depositional components of the syn-rift Qattar Formation. It is used to study what tectonic fabrics are present in the Neoproterozoic basement footwall of the basin-bounding Jizl Fault and how these relate to the fabrics and geometry of the Jizl Fault itself. The second study area is the central-southern Red Sea in the Qunfudah region, where the Baid Formation is exposed as a part of the syn-rift Jizan Group. This reveals an alluvial, lacustrine and volcaniclastic sedimentary succession with basalt lavas. Outcrop is limited, but some insights are gained regarding the nature of this transitional syn-rift basin fill and the overall structural context of this Qunfudah study area is characterised within the Red Sea Rift central transitional zone.
Fieldwork included structural mapping, detailed fracture analysis, sedimentary logging and sampling. Subsequent analysis included petrological microscope work, SEM analysis and major element geochemistry of basalts. 40Ar/39Ar dating of basaltic samples show that the Baid Formation in the central Qunfudah rift segment was deposited in the Early Miocene at ~22Ma with volcaniclastic-rich sediments in a freshwater or saline lake setting.
The Hamd-Jizl Basins consist of two early syn-rift half-grabens. The upper part of the Qattar Formation is characterised as a continental basin fill without volcanic input, with proximal alluvial fans and evidence of evaporitic paleosol development. The Jizl Fault
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bounding the northern half-graben has the same NW-SE strike as the Neoproterozoic Najd fault system in its footwall. However, this study shows that structural inheritance was not by reactivation of pre-existing faults but by bedding foliations in the metasedimentary basement rocks being exploited as joint surfaces during the development of the Jizl Fault and its damage zone. These brittle fabrics are pervasive back into the Jizl Fault footwall, so modifying simple models of strain being limited to the fault core and fault damage zone. This model may be more widely applicable to other rift margins where they show a coincidence of strike with pre-rift fabrics.
New mapping shows no evidence of a fault contact between the basement and the syn-rift sediments in the Qunfudah area. South-westward dips and a dip fan in the Baid Formation are likely due to syn-depositional rotation onto NE-dipping normal faults on the southwest side of the Red Sea Rift. This implies the Qunfudah section of the Red Sea Rift is a previously unrecognised SW-dipping rift segment, with a differing dip polarity from the NE-dipping rift segments previously described in the northern and north-central Red Sea.
The Baid Formation includes mixed volcaniclastic, lacustrine and alluvial environments and intercalated basalt lavas. The distal ashfall deposits indicate that explosive eruptions from more evolved sources were occurring at this time, but outside the immediate study area. The southern Red Sea where the volcanic-rich Jizan Group is preserved is inferred to be the likely source area. The new 40Ar/39Ar dating of the Baid Formation basalts (~22.2-21.9 Ma) corresponds in age to marine evaporites and carbonates in the northern Red Sea Rift. This indicates that marine transgression along the rift axis had not reached the southern Red Sea by this time in the Early Miocene and confirms models of initial marine transgression into the Red Sea Rift being from the Mediterranean Sea. At this time there was a gradient from the more elevated volcanic-rich rift in the south, through a lower elevation but still continental mixed volcanic and fluvio-lacustrine depocentre in the Qunfudah transitional rift segment to the marine depocentres in the northern Red Sea and Gulf of Suez.