Results from isotope dating of the Greater Caucasus crystalline basement in the Elbrus subzone of the Main Caucasus Range inside of the HT gneiss-migmatite area of the Gondaray Metamorphic Complex is discussed herein. The measurements of the zircons isotope composition were performed on the ion microprobe SHRIMP-II at the CIR VSEGEI (St. Petersburg). All zircon crystals from the gneiss sample N 526 show chemical zoning and an old clastogenic core. Almost all U-Pb isotope dating points toile on the concordant line of the concordia diagram and show a wide age range from 320–1000 Ма, partially obtained from clastogenic grains of the zircon from the initial pelitic sediments. The youngest ages (320 Ма) belong to regenerating zones of the zircon grains recrystallized during stage of the anatexis and migmatization. The other part of the age range 540–1000 Ма belongs to detrital zircons from different magmatic sources that existed during accumulation of the proto-metamorphic sediments. Several clastogenic zircon grains show a Cambrian age, which is an evidence for the Early Paleozoic age of the metamorphic protolith. Traditionally the age of the Caucasus crystalline basement was suggested to be Precambrian. The ages of rim zones of the recrystallized zircons (320 Ма) have a direct correlation with postmetamorphic granite ages of the Greater Caucasus. It is shown by termochronological modeling that cooling of the Gondaray Metamorphic Complex during a retrogressive stage, from the temperature of migmatite crystallization (650 о С) to the moment of biotite K-Ar isotope system closure temperature (350 о С), was relatively fast (rate of cooling 8–10 о С/Ma) at subisobaric conditions and during a time range about 30–40 Ма.
We report new observations in the eastern Black Sea-Caucasus region that allow reconstructing the evolution of the Neotethys in the Cretaceous. At that time, the Neotethys oceanic plate was subducting northward below the continental Eurasia plate. Based on the analysis of the obducted ophiolites that crop out throughout Lesser Caucasus and East Anatolides, we show that a spreading center (AESA basin) existed within the Neotethys, between Middle Jurassic and Early Cretaceous. Later, the spreading center was carried into the subduction with the Neotethys plate. We argue that the subduction of the spreading center opened a slab window that allowed asthenospheric material to move upward, in effect thermally and mechanically weakening the otherwise strong Eurasia upper plate. The local weakness zone favored the opening of the Black Sea back-arc basins. Later, in the Late Cretaceous, the AESA basin obducted onto the Taurides–Anatolides–South Armenia Microplate (TASAM), which then collided with Eurasia along a single suture zone (AESA suture).
The tectonics of the eastern passive margin of the North Atlantic are reexamined. The Scandinavian North Atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the Scandinavian Caledonides. Combined information from structural geology, potential field data, regional geology, basin development, and geomorphology made it possible to propose a new definition of the passive margin. The rift shoulder is formed by linked faults defining the present rift flank. These faults are the extensional structures located furthest to the east onshore Norway/Sweden that can be linked to the rifting that led to the formation of the North Atlantic. This innermost boundary fault system (IBF) is formed by a set of normal west dipping crustal faults. It extends over a distance exceeding 2000 km from the North Sea, across the Caledonian mountain belt to the Barents Sea. The passive margin width, between the continent‐ocean boundary and the IBF, ranges from 550 km in the south to over 700 km in mid‐Norway to 165 km north of Lofoten. Rifting and faulting on the IBF started in Permo‐Carboniferous, and a succession of rift phases eventually culminated in continental breakup and the formation of the North Atlantic. Basin development between Permian and Cretaceous was toward the future breakaway fault, but faulting was also active on land from Mesozoic to Present. Onshore‐offshore crustal‐scale cross sections show the geometry of the passive margin, the dip orientation of the major faults and the changes in crustal thickness.
New reconstructions are presented for the Cretaceous–Early Tertiary North Atlantic using a combination of palaeomagnetic, hotspot and magnetic anomaly data. We utilize these reconstructions in an analysis of previously described misfits between the North Atlantic Plate elements at successive intervals during this time period. We are able to achieve reasonable overlap between the hotspot and palaeomagnetic reconstructions between 40 and 95 Ma and thus are able to support the idea that the Indo–Atlantic hotspots are relatively stationary. Small, but systematic discrepancies for this time interval can readily be modelled with a long-term, octopole non-dipole field contribution (G3 = g30/g10 = 0.08). However, hotspot and palaeomagnetic reconstructions for the Early Cretaceous North Atlantic show substantial differences that cannot be explained by constant, non-dipole fields and we favour an explanation for these discrepancies in terms of true polar wander (TPW) triggered by mantle instabilities between 125 and 95 Ma; this constitutes the only identifiable event of significant TPW since the Early Cretaceous. Taken in the context of available geochronological and geological data and seismic tomography from the region, the 95–40 Ma reconstructions and their time-consequent geological products are interpreted in terms of specific conditions of mantle-crust coupling and global plate motions/tectonic activity. Highlights from these reconstructions show uniform NE movement of the coupled North American, Greenland and Eurasian plates from 95 to 80 Ma; a marked cusp in the paths for all three elements at 80 Ma where the three plates simultaneously change direction and follow a uniform NW-directed motion until c. 20 Ma when Eurasia diverges NE, away from the still-NW-moving Greenland and North American elements. Positioning of the Iceland plume beneath the spreading-ridge at 20 Ma may have increased upwelling below the ridge, increased the ridge-push, and caused a NE shift in the absolute direction of Eurasia.
Plain Language Summary The southern slope of the Greater Caucasus mountains is the site of a former rift basin. In order to explain shortening deficits, plate deceleration, and the ~5 Ma reorganization of the Arabia‐Eurasia collision zone Cowgill et al. (2016) proposed that this basin closed ~5 Myrs ago. Within the western Greater Caucasus, at least, careful examination of sedimentological, provenance, and seismic data, however, supports an earlier ~35 Ma basin closure age. Basin closure cannot therefore be the driving mechanism for the ~5 Ma deceleration of the Arabian plate and reorganization of the Arabia‐Eurasia collision zone.
The paper presents the results of the U-Th-Pb isotope system study of the accessory zircon from the basalts of the Ghoithsk volcanic region (GVO) of the Western Caucasus. The sample for isotope dating was taken from basalt porphyrites of the Chataltapa volcanic complex, in the Tuapse River basin. It was shown by using of the ion microprobe isotope dating method of the zircon, that the effusion of basalts of the Chataltapa volcanic complex of the GVO occurred in the Jurassic at the boundary of the Aalenian and Bajocian age (169 Ma), during the tectonic transformation of the Greater Caucasus riftogenic basin. A low Th/U ratio was obtained for zircons from basalts, which is more typical for rocks of acidic composition. This fact indirectly confirms that the evolution of the Jurassic rift magmatism in the Caucasus occurred due to the assimilation of the continental type crust.
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