Neoproterozoic tectonics of the Arabian-Nubian Shield

Abstract

The Neoproterozoic tectonic development of the Arabian-Nubian Shield (ANS) can be divided in three parts: 1) the oceanic stage; 2) the arc-accretion stage; 3) the extensional stage. Three key-areas in the Arabian-Nubian Shield, namely the Bi'r Umq Complex, The Tabalah and Tarj Complex and the Wadi Kid Complex, were studied in the framework of this research with the aim to investigate each of these three stages in detail. The Bi'r Umq Complex is part of the Bi'r Umq-Nakasib Suture that trends SE-NW. The ophiolite of the Bi'r Umq complex was formed at 830 Ma. D1 structures resulted from SE-vergent thrusting on the Bi'r Umq Shear Zone (BUSZ). The D1-deformation phase was a result of NW-SE compression. The second phase, D2, was marked by dextral strike-slip in the BUSZ. This phase resulted from WNW-ESE compression. The third phase, D3, involved sinistral strike-slip shear reversal on the BUSZ and resulted from NNE-SSW compression. Deformation in the Bi'r Umq Complex (BUC) occurred during obduction of the Bi'r Umq ophiolite and ended at 760 Ma. The changes of movement on the shear zones of the BUC were related to a change in the direction of motion of the subducting plate. The structural history in the region is associated with the closure of an oceanic basin by subduction and is relict of the “off-shore amalgamation” of a number of island-arcs in the Arabian shield. The Tabalah and Wadi Tarj Complex in the central part of the Asir Terrane in Saudi Arabia, displays typical intra-terrane features. The Tabalah and Wadi Tarj areas contain gabbros and quartz-diorites, which were intruded in an island arc. These rocks were deformed during D1 and D2. The D1-phase was characterized by thrusting. This phase was dated at 779 Ma and resulted from E-W to WNW-ESE compression. The D2-phase was characterized by dextral strike slip that resulted from NNE-SSW compression. This event was dated at 765 Ma. The deformation phases and late intrusions in the Tabalah and Wadi Tarj area are related to the “off-shore amalgamation” along NE-SW trending sutures. In the Wadi Kid Complex in the Sinai, Egypt, a sequence of sub-horizontal amphibolite HT/LP grade schists, was interpreted as low-angle normal shear zones which displayed top-to-the-NW movement and was associated with upper-crustal extension. The activity on the low-angle shear zone was dated at ~595 Ma. These geological features indicate that the Wadi Kid Complex represents a core complex. Published data show that the extension as observed in the Wadi Kid Complex, was pre-dated by a phase of arc-accretion at the N-S trending active continental margins of East- and West Gondwanaland at 700-650 Ma. During this phase, island-arcs and superterranes, accreted upon continental margins. In summary, the arc-accretion at the continental margins led to lithospheric thickening. When convergence slowed down, thermal re-equilibration led to a decrease of strength in the lithosphere. This led to the gravitational collapse of the thickened lithosphere and extension. The Neoproterozoic development of the ANS is similar to the Mesozoic development of the Cordillera in western North America.