Vol 52, No 3 (2018)

New geochronological and isotopic geochemical data are given, which make it possible to recognize two types of granitic rocks on the eastern Chukchi Peninsula. Early Cretaceous Tkachen and Dolina granitic plutons with zircon ages (U–Pb SIMS) of 119–122 and 131–136 Ma are related to the first type. They cut through Devonian–Lower Carboniferous basement rocks and are overlain by the Aptian–Albian Etelkuyum Formation. Basal units of the latter contain fragments of granitic rocks. Late Cretaceous Provideniya and Rumilet granitic plutons, which contain zircons with ages of 94 and 85 Ma (U–Pb SIMS), respectively, belong to the second type. They cut through volcanic–sedimentary rocks of the Etelkuyum and Leurvaam formations pertaining to the Okhotsk–Chukotka Volcanic Belt. In petrographic and geochemical features, the Early Cretaceous granitic rocks of the Tkachen Pluton are commensurable with I-type granites, while Late Cretaceous granite of the Rumilet Pluton is comparable to A₂-type granite. The Sr–Nd isotopic data provide evidence that from the Early Cretaceous Tkachen and Dolina plutons to the Late Cretaceous Provideniya and Rumilet plutons, the degree of crustal assimilation of suprasubduction mantle-derived melts increases up to partial melting of heterogeneous continental crust enriched in rubidium. An unconformity and various degrees of secondary alteration of volcanic–sedimentary rocks have been established in the Okhotsk–Chukotka Volcanic Belt, and this was apparently caused by transition of the tectonic setting from suprasubduction to a transform margin with local extension.

The Mizil gneiss dome is an elliptical structure consisting of an amphibolite-facies volcanosedimentary mantle and a gneissic granite core. This dome is located at the northern tip of the Ar Rayn terrane only a few kilometers from the eastern edge of the Arabian shield. Previous investigations have shown the intrusive core to be an adakitic diapir with a U–Pb zircon age of 689 ± 10 Ma; this age is 50–80 Ma years older than other granites in this terrane. Vorticity analysis was carried out on samples from the intrusive core and volcanosedimentary cover; the Passchier and Rigid Grain Net (RGN) methods were used to obtain the kinematic vorticity number (W_k) and the mean kinematic vorticity number (W_m). The W_k and W_m values show a marked increase towards the south; such a pattern indicates a N-S movement of the core pluton thus creating an inclined diapir tilted to the south. Analogue experiments simulating the flow of magma diapirs rising form a subducted slab through the mantle wedge have shown that supra-subduction zone oblique diapirs are produced close to the trench and are elongated normal to the convergence direction as is the case in the Mizil pluton. This effect was found to increase with increasing slab dip due to enhanced drag along the upper surface of the subducted lithospheric plate. Spontaneous subduction which is often associated with rollback resulting in back-arc extension and steep dipping slabs is thought to have occurred in the Mozambique Ocean by 700 Ma. The Mizil pluton is coeval with the back-arc Urd ophiolite from the adjacent Dawadimi terrane, and could therefore have been produced by incipient subduction of a relatively cold slab as observed in many Pacific margin adakites. The tectonic evolution of the eastern shield, as deduced from the Mizil dome and other data from Ar Rayn and neighboring terranes, begins with the subduction of >100 My-old lithosphere beneath the Afif terrane resulting in back-arc spreading and the splitting of the Ar Rayn arc from the Afif microplate, with the concomitant production of a small volume of adakite melt. Other arc terrane(s) docked east of Ar Rayn with the westward-directed subduction still going but a lower angles and greater depth due to trench jump; this phase produced the more prevalent non-adakitic group-1 granites. A major collisional orogeny affected the entire eastern shield between 620–600 Ma and sutured the eastern shield terranes with northern Gondwana.

The present study is an attempt to construct a morpho-structural model for the Korek anticline that is located within the High folded zone of the Zagros Fold and Thrust Belt in the Northern Iraq-Kurdistan region. A detail geological map constructed for this structure, the stratigraphic succession that is exposed in the area was deposited from the middle Jurassic up to upper Cretaceous, and consists of nine formations. The most important morphological landscape and landforms in the studied area are due to endogenic and exogenic processes. Accordingly, the area is divided into four main units, which are units of structural, denudational, fluvial and solution origins. Each unit characterized by its own morphological feature differs from one another. According to structural point of view the field data are analyzed from two point of the viewing, one from map view and secondly from cross sections. The geometrical properties from map view of the Korek Anticline indicated that this fold is an asymmetrical double plunging, open to gentle, non-cylindrically curviplanar fold. The Korek Anticline becomes sharper with the depth, but broader and more open upward, this is one of the most characteristic features of the parallel fold (Class 1B). The presence of numerous reverse faults and strike-slip faults which affect the architecture of the Korek Anticline indicates that this Anticline formed by a Fault-propagation fold mechanism. Detailed balanced and retro-deformable cross sections along this anticline indicate that the shortening percentage can be differentiated in space and times, while the depth to detachment gives values that iscoincident with the Ora Shale Formation of Paleozoic age. A morpho-structural model was constructed for this structure and it consists of two main stages and five substages were responsible for the formation of the Korek structure. The first main stage belongs to structural construction, which consists of three sub-stages, while the main second stage is belonging to morphological destruction, which consist also two sub-stages. The effect of these stages and sub-stages makes the anticline to take the nowadays architects shape.