Part Of Nappe Research Articles

Formation conditions of Upper Eocene olistostromes and retro-overthrusts at the southern slope of the Greater Caucasus

The paper considers age, formation conditions, and tectonic setting of Upper Eocene olistostromes of the southern slope of the Greater Caucasus. The formation of olistostromes resulted from the contribution of coarse-clastic material to the Late Eocene basin, which was related to the erosion of thrusted sheets of the Racha–Vandam cordillera of the Gagra–Java zone of the southern slope of the Greater Caucasus and concomitant multiple catastrophic landslide processes. In the Early Pliocene (Rodanian folding phase), Upper Eocene olistostromes along with nappes of the flysch zone were thrust to the south. In the pre- Quaternary (Valakh) folding phase, due to intense shortening, olistostromes in the frontal part of nappes were squeezed, displaced to the north, and thrust with the formation of retro-overthrust, fragments of which remain as isolated blocks (klippen) inside the flysch zone.

Emplacement of hot Lesser Himalayan nappes from 15 to 10 Ma in the Jumla-Surkhet region, western Nepal, and their thermal imprint on the underlying Early Miocene fluvial Dumri Formation

A multichronological study of the weakly metamorphosed Early Miocene fluvial Dumri Formation and the overlying Kuncha and Lesser Himalayan Crystalline nappes has helped to clarify the timing and heat source of the metamorphism, as well as the history of emplacement and cooling of the nappes. U–Pb ages of detrital zircons from the Dumri Formation show peaks at ca 1 Ga and 500–600 Ma, and fission-track dating indicates a thermal imprint on the formation at 11–10 Ma. In the Naudanda Quartzite, overlying the Kuncha Formation, reset fission-track detrital zircon ages are 9.5 Ma, but the U–Pb ages are older than 1.7 Ga. The differences in the U–Pb ages for detrital zircons from the Dumri and Kuncha Formations indicate that the Kuncha nappe was never exposed at the surface during the Early Miocene when the Dumri Formation was deposited. Two-mica garnet schists of the Main Central Thrust (MCT) zone have fission-track detrital zircon ages of 7.8 Ma and 40Ar–39Ar muscovite ages of 19 Ma, though the zircons have U–Pb ages older than 600 Ma. Our interpretation is that the heat for the Dumri Formation metamorphism came from the overlying nappes. The timing of the thermal imprint on the Dumri Formation indicates that the metamorphic nappe reached its present position before ca 10 Ma. The Parajul Khola granite in the frontal zone of the Kuncha nappe yields U–Pb zircon ages of 1.89 Ga, and it underwent an Early Miocene thermal event, cooling below 240°C at 14.7 Ma and below 100°C at 10.3 Ma. These thermochronological data suggest that the frontal part of the Kuncha nappe was exposed at the surface around 15–14 Ma and cooled immediately, but inner parts of the nappe were still hot during its emplacement.

Origin and composition of fluid responsible for metasomatic processes in shear zones of the Bolshie Keivy tectonic nappe, baltic shield: Carbon isotope composition of graphite

The formation of the observed structure of the Keivy terrain is related to the nappe formation during the Paleoproterozoic collisional processes at the southwestern boundary of the Kola fragment of the Archean Craton. The Bolshie Keivy tectonic nappe with a length of >160 km is the upper part of the tec� tonostratigraphic section of the Keivy terrain (1-3) and composed of plates and lenses of blastomylonitic, predominantly highaluminum metamorphic and metasomatic rocks of the socalled Keivy Series (4). It contacts by shear zones with rocks of the lower Leb� yazhinskii nappe mainly composed of biotite and gar� net-biotite gneisses, and, to a lesser extent, biotite- amphibole gneiss and amphibolite gneisses and amphibolites (interpreted as metavolcanites) with numerous bodies of alkaline metasomatites. Plates and lenses of rocks in the considered nappe are sepa� rated from each other by shear zones and often zones of silicification with variable thicknesses. There is a clear connection between repeated metasomatism and narrow fluidtransporting shear zones, elongated for many tens of kilometers. Among the main varieties of metasomatic rocks in the Bolshie Keivy nappe are kyaniterich rocks with variable con� tents of quartz, plagioclase, staurolite, biotite and muscovite, containing kyanite-quartz veins, as well as the areas of plagioclasites and plagioclaserich crystal� line schists interpreted as protolith relics. They are mainly concentrated in the shear zones of the lower and upper parts of nappe, at the contact with metavol� canites of the Lebyazhinskii nappe. These metasoma� tites were the objects of isotope-geochemical investi� gations aimed at solution of the problem of the origin and composition of fluid responsible for metasomatic processes in the shear zones of the Bolshie Keivy tec� tonic nappe. Based on the analysis of parageneses and thermo� dynamic calculations, we determined the physico� chemical conditions of the formation of metasoma� tites and host metamorphic rocks. The calculated val� ues of the water activity in dense hydrous-saline fluid vary from 0.4-0.6 in aluminiferous gneiss to 0.6-0.8 in metasomatites. Petrogenetic diagrams of the stabil� ity of parageneses in P-T coordinates were plotted for the most typical rocks of the KFMASH system for dif� ferent water activities. As a result, the general P-T trend (Fig. 1) was obtained for the Bolshie Keivy nappe reflecting the evolution of metamorphic and metaso� matic petrogenesis in the anticlockwise direction, first by the line of isobaric cooling (6.3-7.2 kbar and 640- 490°C) and, then, isothermal decompression (up to 5-4 kbar and ~500°C). Analysis of the P-T evolution is limited by the area of stability of staurolite paragene� ses. The more lowtemperature alterations in the con� sidered zones were not reflected in considered estima� tions. The age of metasomatites (1762-1720 Ma) is determined by local methods (SHRIMP II) (2). The area of isothermal decompression is confirmed by the results of fluid inclusion study (Table 1). More detailed analysis of these results will be made in a spe� cial paper. In this study we mention only two peculiar� ities of the fluid. The first of them implies that in the period of fluid capture and the formation of primary and refilled (primary-secondary) inclusions, as well as during the later processes (secondary inclusions), fluid was heterogeneous consisting of at least phases: pure CO 2 and solutions of calcium and sodium chlorides with the maximal salinity of >40 wt %. The second fea� ture, the most important for us, implies that in addi� tion to the abovementioned phases, most of the stud� ied samples contained inclusions of methane and

Thermodynamic regime of Svecofennian (1.9 Ga) metamorphism of the Umba nappe of the Lapland collisional orogen

Thermodynamic regime of culmination phase of high-grade metamorphism of the Umba nappe (Lapland allochthon) was studied, and peak metamorphic monazite was dated. Based on calculation of end member reactions, the metamorphic assemblages of aluminous gneisses from the upper and lower parts of the nappe are close to equilibrium. The metamorphic conditions of the rocks are estimated to be about 800°C and 7 kbar in its upper part and 9 kbar in its lower part. The formation of orthopyroxene-sillimanite aggregates points to increasing pressure and temperature at the prograde stage of PT path, whereas cordierite rims mark the onset of decompression and cooling. The pressure difference of 2–2.5 kbar identified by thermobarometric methods between aluminous gneisses from the upper and lower parts of the Umba nappe corresponds to a depth gradient about 7.5 km, which agrees with approximate thickness of the Umba nappe. The study of the eclogitelike rocks developed after the rocks of the Paleoproterozoic Kolvitsa gabbronorite massif made it possible to trace the P-T evolution of metamorphsim: the temperature peak of granulite stage (11 kbar, 860°C) was followed by pressure increase (up to 14 kbar and more), and then decompressional cooling due to the exhumation of the Por’ya Guba nappe together with the Kolvitsa Massif. The U-Pb monazite age of 1904 ± 3 Ma obtained for aluminous gneisses from the upper part of the Umba nappe corresponds within error to the timing of metamorphic events determined from metamorphic zircon in the anorthosites of the Kolvitsa Massif (1907 ± 2 Ma) and zircon from aluminous gneisses in the melange zone (1906 ± 3 Ma). These isotopic data confirm the conclusion of simultaneous high-pressure granulite metamorphism in the upper and lower portions of the Umba nappe.

Potassium‐argon dates from the Arltunga Nappe complex, northern territory

Abstract Twenty‐four mineral separates from the Arunta Complex, four from the metamorphosed Heavitree Quartzite (White Range Quartzite), and one whole rock sample of metamorphosed Bitter Springs Formation, all from the western part of the White Range Nappe of the Arltunga Nappe Complex, and two samples from the autochthonous basement west of the nappe have been dated by the K‐Ar method. The samples from the basement rocks form two groups. Those in the southern or frontal part of the nappe are of Middle Proterozoic (Carpentarian) age (1660–1368 m.y.), determined on hornblende, biotite, and muscovite. In the northern or rear part of the nappe, all but one of the muscovite samples and two biotites are of Middle Silurian to Early Carboniferous age (431–345 m.y.); the remainder of the biotite dates range from 1775 to 548 m.y. (including the two samples from the autochthon), and two hornblendes gave dates of 1639 and 2132 m.y. respectively. All the muscovite samples from the Heavitree Quartzite, and the whole r...

Donnees et hypotheses relatives aux nappes du Tell algerien

Abstract The structure of the Tell ranges of Algeria can be explained by various tectonic schemes based on the observed facts. It is known that Glangeaud's zone I was subjected to slicing in the Nummulitic and uplift in the lower Miocene; these processes undoubtedly contributed toward formation of nappe C and at least part of nappe B. Sliding of the nappes probably took place along a surface produced by pre-Miocene planation.