Half a Century of Oman Ophiolite Studies: SSZ or MOR, the Arc Disposal Problem

Abstract:

The Samail Ophiolite, one of the largest and best exposed ophiolite complexes, is a Tethyan ophiolite obducted in the Late Cretaceous onto the formerly passive Arabian platform as Arabia began its most recent >1000 km northward migration towards a Miocene collision with Eurasia. The Oman Mountains, northeastern Arabian Peninsula have yet to collide with Eurasia; present uplift and form of the mountains also date to the Miocene. In addition to the scientific scrutiny of the ophiolite complex, the geologic constraints on the timing and emplacement of the ophiolite are abundant with no consensus on the obduction mechanism or its original tectonic setting. The crustal thickness of the ophiolite is comparable to thicknesses observed for “normal” mid-ocean ridges. Largely on the basis of structural and paleomagnetic arguments, some workers have attributed its origin to Pacific-type fast spreading ridges and complex micro plate geometries. Indeed the lower pillow lava sequences and much of the gabbroic crust have isotope and geochemical signatures consistent with a MORB source. However, because of the geochemistry of the upper pillow lavas, the ophiolite is most often characterized as a supra-subduction zone (SSZ) ophiolite, i.e. it sits in the hanging wall of some large tectonic structure for part of its history. In the absence of a preserved arc, the SSZ designation has little explanatory power only being a declaration of allochthony or about chemical properties of the mantle source. That associated continental shelf and oceanic crustal sections have suffered either clockwise or counterclockwise PT time trajectories requires some type of nascent subduction and hanging wall thrust transport of the young ridge crest. The widespread Late Cretaceous obduction of Tethyan oceanic crust and mantle over thousands of kilometers strike length is a problem for SSZ models (arc, forearc, back arc etc.) because arc initiation results in thick crust on short time scales, none of which is preserved. Similarly, fast spreading models require the subduction of >10⁶ km² without the initiation and preservation of a magmatic arc. Some Late Cretaceous major plate boundary reorganization that resulted in the current plate motion regime with NNE convergence between Afro-Arabia and Eurasia will be part of any final solution to the Oman problem.