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Gupta H.,National Disaster Management Authority | Gupta H.,CSIR - Central Electrochemical Research Institute | Gahalaut V.K.,CSIR - Central Electrochemical Research Institute
Gondwana Research | Year: 2014

Bounded by the western and eastern syntaxes, the Himalayan region has experienced at least five M. ~. 8 earthquakes during a seismically very active phase from 1897 through 1952. However, there has been a paucity of M. ~. 8 earthquakes since 1952. Examining of various catalogues and seismograms from the Gottingen Observatory, it is established that this quiescence of M. ~. 8 earthquakes is real. While it has not been possible to forecast earthquakes, there has been a success in making a medium term forecast of an M. 7.3 earthquake in the adjoining Indo-Burmese arc. Similarly we find that in the central Himalayan region, earthquakes of M. >. 6.5 have been preceded by seismic swarms and quiescences. In the recent past, based on GPS data, estimates have been made of the accumulated strains and it is postulated that a number of M. ~. 8 earthquakes are imminent in the Himalayan region. We examine these estimates and find that while earthquakes of M. ~. 8 may occur in the region, however, the available GPS data and their interpretation do not necessarily suggest their size and time of occurrence and whether an earthquake in a particular segment will occur sooner in comparison to that in the neighboring segment. We also comment on the inference of occurrence of M. ~. 8 earthquakes based on M8 algorithm for the region. We conclude that while an M. ~. 8 earthquake could occur any time anywhere in the Himalayan region, there is no indication as of now as to where and when it would occur. We impress on the need for preparedness to mitigate the pending earthquake disaster in the region. © 2012 International Association for Gondwana Research. Source

Rao C.V.D.,National Disaster Management Authority | Chmielowski R.M.,University of Milan
Geological Journal | Year: 2011

The sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>900°C). Diagnostic UHT assemblages in these rocks include sapphirine+quartz, spinel+quartz and high alumina orthopyroxene+sillimanite+quartz. The stability of spinel+quartz, sapphirine+quartz and orthopyroxene+sillimanite+quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet-orthopyroxene is indicative of a subsequent high P-UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10wt% Al2O3) coexisting with garnet. Peak P-T conditions of ∼970°C and 9.5kbars are calculated from conventional garnet-orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet-orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P-T estimates indicate a three-stage evolution of the sapphirine granulites and associated garnet-orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low-pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet-orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene-cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet-orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo-barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P-T evolution of this sector of the EGB. © 2010 John Wiley & Sons, Ltd.. Source

The magmatic suite at Kondapalle represents a Mesoproterozoic (~ca. 1634Ma) magmatic arc emplaced in the southern sector of the Eastern Ghats Belt (EGB). Here we present new geological, mineralogical and geochemical data on the various lithological units in this complex including anorthosites, gabbronorites and pyroxenites. The major mineral constituents in these rocks are plagioclase (An98-57), amphibole (XMg 0.93-0.52), orthopyroxene (XMg 0.94-0.51), clinopyroxene (XMg 0.93-0.63) and chromite (XMg 0.20-50). The near-absence of plagioclase in the orthopyroxenites, early and abundant crystallization of orthopyroxene, and formation of gabbronorites rather than gabbro or olivine gabbro in the Kondapalle suite are correlated with the features of arc cumulates. The chemistry of chromian spinel and clinopyroxene also displays the trend for arc cumulates. The variations in the anorthite content of plagioclase vs. the Mg# of olivine attest to an arc-related magma source. The rocks display low abundance of incompatible trace elements (Ba, Rb, K and Zr) comparable to the values typically observed in subduction-related magmatic arcs. In trace element N-MORB normalized diagrams, all the rock units show Nb-Ta-Ti-Zr troughs reflecting the features characteristic of arc magmas. We interpret the Kondapalle rocks to represent the root zone of a deeply eroded magmatic arc built during the Mesoproterozoic associated with the subduction of an oceanic lithosphere in a long-lived convergent margin. We identify that the Mesoproterozoic subduction along the eastern margin of the Indian plate generated a wide arc-accretionary complex with an extruded high P-T metamorphic orogen during the final stage of collision. The subduction-accretion process is also supported by recent findings of Mesoproterozoic ophiolite mélanges from this zone, marking the history from the break-up of the Paleoproterozoic Columbia supercontinent to the assembly of the Neoproterozoic Rodinia supercontinent. © 2011 Elsevier B.V. Source

Dharma Rao C.V.,National Disaster Management Authority | Windley B.F.,University of Leicester | Choudhary A.K.,Indian Institute of Technology Roorkee
Journal of Asian Earth Sciences | Year: 2011

New major and trace element data on the Proterozoic Chimalpahad layered anorthositic Complex and associated basaltic amphibolites of the Nellore Schist Belt of South India provide new constraints on their petrogenesis and geodynamic setting. The Complex consists of layered anorthosites, leucogabbros, gabbros, ultramafic rocks and is spatially associated with basaltic amphibolites. Despite deformation and metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the Complex. Whereas the amphibolites display diverse REE systematics, the Chimalpahad anorthositic-gabbroic rocks are characterized by moderately depleted to strongly enriched LREE patterns and by flat to depleted HREE patterns. The field relations, major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic-gabbroic rocks by fractional crystallization. The anorthositic rocks and the basaltic amphibolites share the depletion of Nb relative to Th and La on primitive mantle-normalized diagrams. They exhibit signatures of arc magmatic rocks, such as high LILE and LREE relative to the HFSE and HREE, as well as high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios that mimic chondrite-normalized REE and primitive mantle-normalized trace element patterns of arc magmas. Similarly, on log-transformed tectonic discrimination diagrams, the Chimalpahad rocks plot within the field of Phanerozoic magmatic arcs, consistent with a subduction zone origin. On the basis of field relations and geochemical characteristics, the Chimalpahad Complex is interpreted as a fragment of a magma chamber of an island arc, which is tectonically juxtaposed against its original volcanic cover. A new preliminary Sm-Nd date of anorthosite from the Chimalpahad Complex indicates a model age of 1170. Ma. © 2010 Elsevier Ltd. Source

Dharma Rao C.V.,National Disaster Management Authority | Santosh M.,Kochi University | Sajeev K.,Indian Institute of Science | Windley B.F.,University of Leicester
Precambrian Research | Year: 2013

The Neoarchean layered anorthositic complex at Sittampundi in southern India is known for its chromitite layers that are mostly associated with anorthosite (An90-100). The chromitites contain FeAl-rich chromites concentrated in layers between amphibole-rich layers with a dominant mineralogy of amphibole-spinel-plagiocase±sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X-ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2O3 and Al2O3 contents varying between 34-40wt.% and 23-28wt.%. The chromite compositions are noticeably different from those in layered igneous intrusions of the Bushveld-Stillwater type. The existence of original highly calcic plagioclase, FeAl-rich chromite, and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra-subduction zone arc setting. In terms of mineralogy and field relations, the Sittampundi chromitites are remarkably similar to anorthosite-hosted chromitites in the Neoarchean Fiskenæsset anorthositic complex, Greenland. We propose that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone. This melting process produced hydrous, aluminous basalt, which fractionated at depth to give rise to a variety of high-alumina basalt compositions from which the anorthositic complex with its cumulate chromite-rich and amphibole-rich layers formed within the magma chamber of a supra-subduction zone arc. © 2011 Elsevier B.V. Source

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