The deposition of the Jibalah Group in pull-apart basins of the Najd fault system as a final stage of the consolidation of Gondwanaland

The Late Proterozoic Najd fault system strikes northwesterly for 10,000 km across the East African orogen, yet its history of motion and tectonic significance are debated. The Halaban-Zarghat fault is the easternmost of the major Najd faults in the Arabian Shield. Several sedimentary basins of the Precambrian Jibalah Group are bounded by the Halaban-Zarghat and other Najd faults, particularly along right-steps in the fault trace. Among the largest of the basins is the Jifn, which contains a typical Jibalah Group sequence of conglomerate, sandstone, shale, volcanic rocks, and limestone. The geometry and sedimentary facies of the basin are consistent with a dextral pull-apart basin formed at a releasing bend along the Halaban-Zarghat fault. Analysis of structural data from around and within the Jifn basin, the position of other pull-apart basins, and the location of high-grade mylonitic gneiss belts along the Halaban-Zarghat fault, are all consistent with early dextral movement along the fault. Offsets of distinctive older rock units and transection of the Jifn basin by sinistral faults, however, show that the latest and most significant sense of offset on the Halaban-Zarghat and other Najd faults was sinistral.; A U/Pb zircon date of 624.9 +/- 4.2 Ma from rhyolitic basement of the Jifn basin gives a lower age limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat fault. This age is interpreted as the time when East and West Gondwana initially collided and accreted terranes caught between the two continents attempted to escape toward an oceanic free face to the north. A 576.6 +/- 5.3 Ma U/Pb zircon date from a felsite dike that intrudes the Jibalah Group gives an upper age limit for movement along the Halaban-Zarghat fault. This may mark the time that collision and escape tectonics ended, or the time that displacements were transferred to other Najd faults in more interior parts of the East African orogen. This ∼50 Ma time interval overlaps with the time of opening of the Iapetus ocean, leading to the suggestion of a tectonic link.