Post-amalgamation basins of the NE Arabian shield and implications for Neoproterozoic III tectonism in the northern East African orogen

The development and structure of Neoproterozoic III volcanosedimentary basins in the northeastern part of the Arabian shield record periods of uplift, erosion, extension, subsidence, compression, and strike-slip faulting that postdated the completion of terrane amalgamation in the northern East African orogen (EAO) and predated the initiation of a passive margin on the northern flank of the Gondwana supercontinent. The oldest basins, dating from <670 to >650 Ma, include as much as 8000 m sandstone, conglomerate, bimodal volcanic rocks, and limestone, and were deposited in a large area (approximately 72,000 km2) of locally fault-controlled subsidence (Murdama basin) and in narrow grabens (Bani Ghayy basins). The regional unconformity at the base of the basins bevels the newly amalgamated terranes and exposes greenschist-, and locally amphibolite-, and granulite-grade metamorphic rocks, indicating possibly >10 km uplift and erosion in parts of the region prior to deposition. The basins were closed and inverted by folding at about 650 Ma and unconformably overlain by rocks of the Jurdhawiyah group and Hibshi formation, which were deposited between 640 and 620 Ma in fault-controlled basins following a further phase of uplift and erosion. The Jurdhawiyah and Hibshi basins closed and inverted during subsequent north–south shortening and northand south-vergent reverse faulting. A final phase of Neoproterozoic III basin formation resulted in deposition of the Jibalah group (580–570 Ma) in small, isolated, pull-apart basins caused by strikeand dip-slip movements on faults of the Najd fault system. The Murdama and Bani Ghayy basins contain shallow-marine clastic and carbonate deposits indicating that seaways penetrated the core of the EAO soon after terrane amalgamation, which implies that much of the region was at a low elevation soon after terrane amalgamation and orogeny. This limits the extent to which gravity-driven extensional collapse may have been a factor in the Neoproterozoic III tectonic evolution of the region, unlike other parts of the EAO where orogenic extensional collapse was significant. The Jurdhawiyah, Hibshi, and Jibalah basins were probably isolated fault-controlled lakes. The northerly trend of Murdama and Bani Ghayy folds implies bulk east–west shortening, which is consistent with the model of east–west Gondwana convergence conventionally applied to the EAO. East–west convergence conceivably accounts, additionally, for the creation of the Jurdhawiyah and Hibshi basins as a result of concomitant northward extension or tectonic escape. It would also account for the simultaneous creation of the Murdama and Bani Ghayy basins, themselves, linking the two as foreland basins at subsided and extended parts of a newly amalgamated crust in the center of the study area that was downflexed by the overthrusting of an ophiolite complex and other terranes from the east. © 2003 Elsevier Science B.V. All rights reserved.