Pre-Tertiary felsic magmatism of the Nepal Himalaya: recycling of continental crust

At least seven different groups of felsic magmatic rocks have been observed in the Lesser and Higher Himalayan units of Nepal. Six of them are pre-Himalayan. The Ulleri Lower Proterozoic augen gneiss extends along most of the length of the Lesser Himalaya of Nepal and represents a Precambrian felsic volcanism or plutono-volcanism, mainly recycling continental crustal material; this volcanism has contributed sediment to the lower group of formations of the Lesser Himalaya. The Ampipal alkaline gneiss is a small elongated body appearing as a window at the base of the Lesser Himalayan formations of central Nepal; it originated as a Precambrian nepheline syenite pluton, contaminated by lower continental crust. The “Lesser Himalayan” granitic belt is well represented in Nepal by nine large granitic plutons; these Cambro-Ordovician peraluminous, generally porphyritic, granites, only occur in the crystaline nappes; they were probably produced by large-scale melting of the continental crust at the northern tip of the Indian craton, during a general episode of thinning of Gondwana continent with heating and mantle injection of the crust. The Formation III augen gneisses of the Higher Himalaya, such as the augen gneiss of the Higher Himalayan crystalline nappes (Gosainkund) are coeval to the “Lesser Himalayan” granites, and their more metamorphic (lower amphibolite grade) equivalents. Limited outcrops of Cretaceous trachytic volcanism lie along the southern limb of the Lesser Himalaya and are coeval with spilitic volcanism in the Higher Himalayan sedimentary series. This volcanism foreshadows the general uplift of the Indian margin before the Himalayan collision. The predominance of felsic over basic magmatism in the 2.5 Ga-long evolution of the Himalayan domain constitutes an unique example of recycling of continental material with very limited addition of juvenile mantle products.     

Indian attitude towards nature

To understand the Indian mind and culture one needs to explore the attitude that the Indian people have adopted towards nature. The Indian view is both the cause and the effect of man's involvement in and with nature. Geographical conditions have been responsible for shaping the Indian mind and life, but Indian thought has also given a distinct perspective on nature. This dual character of man's relation to nature has imparted a uniqueness to the Indian attitude towards nature.     

Rheology of Indian continental crust and upper mantle

A rheological model of the Indian shield has been constructed using the thermal structure derived from available surface heat flow and heat generation data and the flow properties of characteristic minerals and rocks like quartz, diabase and olivine which respectively represent the upper crust, lower crust and upper mantle. Lateral variations in the thicknesses of the brittle and ductile crust and of the brittle upper mantle have thus been obtained for different tectonic environments. Implications of these results to interpretation of the seismic structure of the Indian shield have been pointed out.     

Late quaternary deformation and palaeoseismicity: Insight into geomorphotectonic evolution of Kachchh, western Indian Subcontinent

Kachchh in western Indian Shield, according to the Bureau of Indian Standard (IS:1893:2002), falls in Seismic Zone V. This is intriguing considering that the region is far away from active Plate margin. Apart from the recent incidences of earthquakes, there are several pre-historic/archaeological records of earthquakes in the region. Beyond these, the geological evidence of earth-movements (causing earthquakes) is provided by the occurrence of several’ active’ faults, which are considered geological markers of palaeoseismicity. There are records of innumerable incidences of faulting in the region in not so distant geological past. Study of fault features especially the scarp faces marking abrupt change in physical relief proves that the different levels of topography in the entire terrain are fault-bound features. Studies also confirm that the topographic difference between the high and ’sunken’ features have formed due to uplift and relative down-sagging during the geomorphotectonic evolution of the terrain. Features that make the region unique are: (i) restriction of fault-related deformation zone to a narrow strip between the southern margin of Thar Desert and the south coast line of the Kachchh Peninsula; (ii) overall sub-horizontality of bedding and other topographic and planation surfaces over the entire region; (iii) evidence of fault-controlled geomorphology indicating vertical movement along fault planes; (iv) evidence constraining the time of geomorphological evolution of the terrain only during the Late Quaternary, making it the youngest neotectonically evolved terrain in the Precambrian Indian Shield.     

Preservation of a Paleoproterozoic rifted margin in the Himalaya:Insight from the Ulleri-Phaplu-Melung orthogneiss

Orthogneiss within the Paleoproterozoic strata of Lesser Himalayan sequence across the Himalaya has been variably linked to development in a continental arc setting, Indian basement, or a continental rift.New whole rock and trace element geochemical data and U/Pb zircon geochronology indicate that the granitoid protoliths to these rocks were derived from upper crustal sources in the Paleoproterozoic and have within-plate, A-type affinities. This is consistent with their generation in a rifted margin and is compatible with paleogeographic reconstructions that indicate an open boundary for present-day northern India in the Paleoproterozoic.     

Thermal aspects of sea-floor spreading and the nature of the oceanic crust

By means of a plate-tectonics model we have computed radial temperature profiles for various spreading velocities for the region bounded by the bottom of oceanic layer two, the top of the lowvelocity zone, the centre of the ridge, and a vertical plane at 1000 km away from the ridge centre. The model we have used differs from previous models in that certain petrological aspects of basalt formation, partial melting, and latent heat effects have been taken into account. Oceanic heat flow was calculated from the ridge crest to 1000 km away. The thermal relationships in this region seem to rule out a gabbroic or amphibolitic third layer in the oceanic crust.     

Long wavelength magnetic anomalies from the lithosphere: Indian shield and Himalaya

A few long-range airborne magnetic profiles flown at an altitude of 7.5 km a.s.l. across the Indian shield are analysed and interpreted in terms of magnetization in the lower crust. The wavelengths of the crustal anomalies are in the range of 51–255 km and this is used to separate them from signals originating at shallow depths. Spectral analysis of these profiles provided a maximum depth of 34–41 km for the long-wavelength anomalies and 9–10 km for the shallow sources identified as Mohorovic̆ić discontinuity and the basement respectively. The magnetic “high” recorded in satellite observations over the Indian shield is interpreted as due to a bulge of 3–4 km in the Moho under the Godovari graben, with a magnetization of 200 nT in the direction of the Earth's present-day magnetic field. Similarly the magnetic lows observed over the Himalaya are interpreted in terms of thickening of the granitic part of the crust from 18 to 23.5 km with a magnetization contrast of 200 nT in the direction of the Earth's present-day magnetic field.     

The crystalline axis of the Himalaya: The Indian shield and continental drift

The Great Himalayan Range and the Central Crystalline Axis are two different units. The Axis came into existence much before the Himalayan orogeny and formed a barrier between the Tethys and the Himalayan geosynclines. Observations indicate that its existence is not a consequence of continental drift. Geological records show that the Indian shield was not involved in the continental drift to the extent postulated in the drift hypothesis.     

Some thougths on the evolution of Himalaya and the Northern limit of the Indian Shield

Based on the sedimentological studies and the distribution of fossil-bearing horizons in the Himalaya a new model for the evolution of Himalaya is visualized. It is proposed that the Lesser and Central Himalaya constitute an integral part of the Peninsular Indian shield. The unfossiliferous orthoquartzite-shale-carbonate sequences of Lesser and Central Himalaya represent Precambrian sediments, deposited in a shallow tidal sea. Within the Lesser Himalaya there are thin detached marine fossiliferous horizons of three ages, viz. Carboniferous-Permian, Jurassic-Cretaceous and Eocene, located in a narrow E-W running zone (Dogadda-Subathu Zone), representing deposits of three transgressions in this zone.This zone is analogous to the Narbada Valley zone of the Peninsular India, where rocks of Permian, Cretaceous and Eocene ages are present. This model properly explains the unfossiliferous nature of the Lesser Himalayan sediments and demands the northern limit of Indian Plate north of Himalaya.

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Zusammenfassung Aufgrand sedimentologischer Untersuchungen und der Verteilung fossilführender Schichten im Himalaya wird ein neues Modell der Himalayaentwicklung dargestellt. Es wird angenommen, daß der Außen- und Zentralhimalaya fester Bestandteil des indischen Schildes waren. Die fossilleeren Abfolgen von Orthoquarziten, Tonen und Kalken des Außen- und Zentralhimalaya bestehen aus präkambrischen Sedimenten, die ehemals in einem Flachmeer unter Gezeiteneinfluß abgelagert wurden. Im Bereich des Außenhimalaya finden sich geringmächtige Einschaltungen mit mariner Fauna, die dem Permokarbon, Jura/Kreide und Eozän zuzuordnen sind. Sie liegen innerhalb einer schmalen E/W-verlaufenden Zone (Dogadda-Subathu Zone) und entsprechen 3 Transgressionen in diesem Gebiet.Diese Zone ist dem Narbada Valley-Bereich der indischen Halbinsel vergleichbar, wo ebenfalls Ablagerungen aus Perm, Kreide und Eozän angetroffen werden.Durch dieses Modell läßt sich die Art der fossilleeren Sedimente des Außenhimalaya erklären. Es führt zu der Annahme, daß die Nordgrenze der indischen Platte nördlich des Himalaya zu suchen ist.

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Résumé Un nouveau modèle de l'évolution de l'Himalaya est présenté, basé sur des études sédimentologiques et sur la répartition des horizons fossilifères, suivant lequel l'Himalaya Central et Externe ferait intégralement partie du boncliez péninsulaire Indien.Les séquences non fossilifères d'orthoquartzites-schistes-carbonates de l'Himalaya Central et Externe représentent des sédiments précambriens déposés dans un environnement marin peu profond soumis aux marées. Dans la région de l'Himalaya Externe, on observe de minces intercalations fossiliferes marines correspondant à trois phases de transgression, au Permo-Carbonifére, au Jurassique-Crétacé et à l'Eocène. Ces horizons sont localisés dans une zone étroite, de direction E-W, la zone de Dogadda-Subathu.Cette zone est comparable à la Valleé de Narbada en Inde péninsulaire, qui comporte également des dépôts permiens, crétacés et éocènes. Le modèle propose permet d'expliquer la nature non fossilifère de la plupart des sédiments de l'Himalaya Externe et conduit á rechercher la limite Nord de la plaque indienne au Nord de l'Himalaya.

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Shallowing of mafic crust and seismic instability in the high velocity Indian Shield

Indian shield has been frequented by number of large and moderate magnitude damaging earthquakes since historical times, including the recent disastrous ones like Latur Mw 6.3 in 1993, Jabalpur Mw 5.8 in 1997 and Bhuj Mw 7.7 in 2001. Seismogenesis of these events is still not understood well. Detailed study of nine such earthquake localities (as appended in Table 1), indicates quite high P- and S- velocities (6.2–6.7 km/s and 3.65–3.90 km/s respectively) at a shallow depth of almost surface to six kilometers. These seismogenic regions appear to be in a state of continuous uplift and erosion since geological times, which brought mafic (granulitic/amphibolitic) crust to significantly shallow levels in which stresses are accummulated due to ongoing local uplift and a high input of heatflow from the mantle. These stresses act over and above to the regional compressive stresses generated by India-Eurasia collision. As against common belief, the role played by fluids in nucleation of such earthquakes, in the relatively denser and high velocity Indian crust (compared to the other global stable continental regions), appears limited.     

Lithium isotopic evidence for subduction of the Indian lower crust beneath southern Tibet

Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ⁷Li values: PVRs vary from −4.9‰ to +3.2‰, UPVs from −3.9‰ to +1.7‰, and MPRs from −1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ⁷Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (−4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau.     

Continental crust in Gorny Altai: nature and composition of protoliths

The history of the Vendian–Early Paleozoic formation of protoliths of continental crust in the Gorny Altai segment of the Central Asian fold belt is considered, and their composition, isotopic characteristics, and formation mechanisms are estimated. We have established two stages of crust-forming processes in Gorny Altai: Early and Late Caledonian, with the different structures of formed geoblocks and nature and compositions of crustal protoliths. At the Early Caledonian stage, fragments of oceanic lithosphere of basic composition (MORB, OIT, OIB) (T_Nd(DM-2st) = 0.65–1.1 Ga) formed, as well as island arcs with andesite-basaltic and andesitic protoliths with low contents of incompatible elements (T_Nd(DM-2st) = 0.7–0.9 Ga). At the Late Caledonian stage, the redistribution of the substance of these blocks and the external supply of material led to the formation of heterogeneous crust of turbidite basins with an oceanic basement and andesite-dacitic upper-crustal protoliths (T_Nd(DM-2st) varies from 0.8–0.9 Ga in the framing of the volcanic arc of Altaids to 1.4–1.6 Ga at the boundary of the Altai–Mongolian microcontinent).     

The nature and geological history of the deep crust under the Eifel, Germany

The crust ≈ 10–20 km under the Eifel is composed of amphibolite-facies metasediments and meta-igneous rocks of tonalitic to granodioritic composition; mafic granulites occupy the base of the crust down to a Moho depth between about 29 and 34 km. The meta-granodiorites and meta-tonalites have I-type chemical characteristics and appear to have formed approximately 400 Myr ago by partial melting of a lower crustal source. Amphibolite-facies metamorphism probably followed within the same orogeny. During the Quaternary, many amphibolite-facies rocks were subjected to contact heating in crustal magma chambers and/or during transport to the earth's surface. Contact heating is also recorded in radiogenic isotope compositions of minerals from one xenolith. A genetic link between meta-igneous amphibolites and the deeper crustal mafic granulites can neither be proven nor discounted by the isotope data. If there is a genetic relationship, it requires fractionation of a mafic magma in the lower crust and assimilation of metasediments and separation of a highly evolved melt.     

Chronology of late quaternary glaciations in Indian Himalaya: A critical review

The timing of glaciation is an important parameter that helps in the understanding of past climate change and provides valuable information for developing the predictive futuristic models. There are evidences to suggest that during the late Quaternary, Himalayan glaciers fluctuated considerably thus implying their sensitivity to changes in past climatic conditions. Although the Himalayan region is fed by two major weather systems viz. the southwest summer monsoon and the mid-latitude westerlies, however, the existing chronology (mostly exposure ages) indicates that irrespective of the geographical position, glaciers seem to grow during increased insolation and enhanced southwest summer monsoon including the mid-latitude westerly dominated north-western Himalayan glaciers (Ladakh and Karakoram). Considering the limited geographical coverage and the dating uncertainty, the above inferences should be treated as tentative.     

Elemental variations in glacier cryoconites of Indian Himalaya and Spitsbergen,Arctic

Cryoconite samples were collected from two different climatic domains i.e., the Sutri Dhaka glacier, western Himalaya India and Svalbard glaciers, the Spitsbergen, Arctic, to understand the elemental source and elemental deposition patterns. The data of geochemical analysis suggest that the Himalayan cryoconite samples accumulate higher concentrations as compared to the cryoconite samples of the Arctic glaciers. The concentration of lithophile elements (Cs, Li, Rb and U) was recorded higher in the cryoconite holes of the Himalayas, especially, in the lower to the higher parts of the glacier, whereas, lower concentrations were recorded in the Arctic samples. Chalcophile elements in the Himalayan cryoconites are enriched in As and Bi while the Arctic cryoconite samples show a higher concentration of Bi, Pb and As. The higher concentrations are responsible for influencing the ecosystem and in human health related issues. Siderophile elements (Co, Fe, Mn and Ni) show high concentrations in the Himalayan samples, whereas, the Arctic samples show minor variations and low elemental concentration in these elements, respectively. In addition, a few elements, such as Ag, Mg, and Ca show higher concentration in the Himalayan glacier samples. Ca also occurs in high concentrations in Arctic glacier samples. R-mode factor analysis of the Himalayas (Arctic) samples indicate that the elements are distributed in four (three) factors, explaining 89% (90%) of the variance in their elemental distribution. The Factor 1 suggests statistically significant positive loadings for most of the lithophile, chalcophile and siderophile elements of the “Himalayan” and the Arctic cryoconite samples. The sample-wise factor score distribution shows a considerable variation in the sampling locations along the glaciers of both the regions. Factors 2 and 3, demonstrate insignificant loading for most of the elements, except statistically significant positive loading in some of the elements of the both, Himalayan and Arctic “cryoconites”. The higher elemental concentration in the cryoconites of the Himalayan region may be an indicator of the natural processes and/or attributed to the rapid industrialization in the Asian countries.     

Mantle-derived mafic-ultramafic xenoliths and the nature of Indian sub-continental lithosphere

Mantle derived xenoliths in India are known to occur in the Proterozoic ultrapotassic rocks like kimberlites from Dharwar and Bastar craton and Mesozoic alkali igneous rocks like lamrophyres, nephelinites and basanites. The xenoliths in kimberlites are represented by garnet harzburgites, lherzolites, wehrlite, olivine clinopyroxenites and kyaniteeclogite varieties. The PT conditions estimated for xenoliths from the Dharwar craton suggest that the lithosphere was at least 185 km thick during the Mid-Proterozoic period. The ultrabasic and eclogite xenoliths have been derived from depths of 100–180 km and 75–150 km respectively. The Kalyandurg and Brahmanpalle clusters have sampled the typical Archaean subcontinental lithospheric mantle (SCLM) with a low geotherm (35 mW/m²) and harzburgitic to lherzolitic rocks with median X_mg ^olivine > 0.93. The base of the depleted lithosphere at 185–195 km depth is marked by a 10–15 km layer of strongly metasomatised peridotites (X_mg ^olivine > ∼0.88). The Anampalle and Wajrakarur clusters 60 km to the NW show a distinctly different SCLM; it has a higher geotherm (37.5 to 40 mW/m²) and contains few subcalcic harzburgites, and has a median X_mg ^olivine = 0.925. In contrast, the kimberlites of the Uravakonda and WK-7 clusters sampled quite fertile (median X_mg ^olivine ∼0.915) SCLM with an elevated geotherm (> 40 mW/m²). The lamrophyres, basanites and melanephelinites associated with the Deccan Volcanic Province entrain both ultramafic and mafic xenoliths. The ultramafic group is represented by (i) spinel lherzolites, harzburgites, and (ii) pyroxenites. Single pyroxene granulite and two pyroxene granulites constitutes the mafic group. Temperature estimates for the West Coast xenoliths indicate equilibration temperatures of 500–900°C while the pressure estimates vary between 6–11 kbar corresponding to depths of 20–35 km. This elevated geotherm implies that the region is characterized by abnormally high heat flow, which is also supported by the presence of linear array of hot springs along the West Coast. Spinel peridotite xenoliths entrained in the basanites and melanephelinites from the Kutch show low equilibrium temperatures (884–972°C). The estimated pressures obtained on the basis of the absence of both plagioclase and garnet in the xenoliths and by referring the temperatures to the West Coast geotherm is ∼ 15 kbar (40–45 km depth). The minimum heat flow of 60 to 70 mW/m² has been computed for the Kutch xenolith (Bhujia hill), which is closely comparable to the oceanic geotherm. Xenolith studies from the West Coast and Kutch indicate that the SCLM beneath is strongly metasomatised although the style of metasomatism is different from that below the Dharwar Craton.     

Gabbroic xenoliths in tuff-breccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy

Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.     

印度与欧亚板块碰撞以来东喜马拉雅构造结的演化

在野外填图,构造观察及前人研究的基础上,本文识别并描述了东喜马拉雅构造结中的推覆断裂、正断裂及走滑断裂、背斜(形)和向斜(形)等构造类型,讨论了这些构造位置及与印度板块挤入,印支地块旋转的关系,还探讨了东喜马拉雅构造结对印度板块持续向北推挤下的特殊应变调节方式。在印度大陆部分,东喜马拉雅构造结由3个向外逐渐变新的构造结组成,即北东向的南迦巴瓦峰复式背斜、北西向的桑复式向斜及北东向的阿萨母复式向斜。上述3个构造结是协调印度板块的挤入、喜马拉雅弧的扩展及印支地块的旋转的构造。在欧亚大陆内部的冈底斯岛弧,在派区及阿尼桥走滑断裂协调下,高喜马拉雅结晶岩的基底挤入冈底斯岛弧内部,在大拐弯顶端形成向上的挤出构造。在南迦巴瓦峰构造结的北西侧,由于掀斜式抬升及重力滑动,使得冈底斯盖层与结晶基底脱耦,上盘盖层沿东久向北西方向滑动。在南迦巴瓦峰构造结北东侧,由于印支地块的挤出和旋转,形成一系列的北西向走滑断裂,如实皆断裂、嘉黎—高黎贡断裂、澜沧江断裂及红河断裂等。