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The East African Oragen |
1 Department of Earth and Atmospheric Sciences, St Louis University, St Louis, MO, 63103, USA
2 Department of Structural Geology and Remote Sensing, King Abdul Aziz University, Jeddah, Saudi Arabia
The Neoproterozoic Najd Fault System extends for 2000km across the East African Orogen, yet its history of motion and tectonic significance are widely debated. The Halaban-Zarghat Fault is the northeastern-most of the major NW-striking Najd faults in the Arabian Shield. Several sedimentary basins of the Neoproterozoic Jibalah Group are bounded by strands of the Halaban-Zarghat Fault and other Najd faults, particularly along right steps in the fault trace. Among the largest of the basins is the Jifn. The geometry of the Jifn Basin and the sedimentary facies of Jibalah Group indicate that it is a dextral pull-apart basin between strands of the Halaban-Zarghat Fault. A zone of high-grade mylonitic gneiss is located along a left step in the fault zone and may be a deeply eroded pop-up structure related to dextral transpression. Analysis of structural data from around and within the Jifn Basin, the position of other pull-apart basins and high-grade mylonite zones along the Halaban-Zarghat Fault are all consistent with early dextral movement along the Halaban-Zarghat 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 Fault 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 limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat Fault. This age is interpreted as the earliest age for the collision of East and West Gondwana. A 621 ± 7 Ma pluton is offset 10 km dextrally along the Halaban-Zarghat Fault, showing that dextral motions continued for some time past 621 Ma, before switching to sinistral motions, and accreted terranes caught between the two continents were forced toward an oceanic-free face to the north. A 576.6 ± 5.3 Ma U-Pb zircon date from an undeformed felsite dyke that intrudes the Jibalah Group gives an upper time limit for movement along the Halaban-Zarghat Fault. This may mark the time that collision and escape tectonics ended, or it may reflect the time that displacements were transferred to other Najd faults in more interior parts of the East African Orogen.
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