Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P-T record. A high-T decompression from above 10 kbar to 8 kbar around 1100" C has been determined from Mg-rich metapelites (XMg > 0.60) with quartz-cordieriteorthopyroxene-sillimanite and cordierite-orthopyroxene-sapphirine-spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670" C at 8 kbar is evident. These changes are registered by the rim Compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene-quartz-garnet-plagioclase-cordierite assemblage, and are further supported by the garnet + quartz .$ orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650" C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P-T values are obtained from metapelites with a quartz-plagioclase-garnet-sillimanite-cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700" C and the rims 5 kbar, 650" C, suggesting a third decompression.The earliest deformation (Fl) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P-T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P-T path replicate parts of the P-T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats. Recent work on P-T-t paths indicates cooling from moderately high temperatures and decompression at ROCK TYPES A N D F I E L D RELATIONS medium temperatures in several sectors of the Eastern There are five major rock types in the Chilka area: Ghats. The paucity of evidence for the prograde path does metapelites (with intercalated quartzites and rarely 287 288 s. K . S E N Er A L '. ' : I Bf'3d 290 9 290
Dark patches of charnockitic rocks characterized by orthopyroxene occur within garnetiferous granite gneisses (leptynites) in a granulite-migmatite suite around the Chilka Lake, Orissa, within the Eastern Ghats belt in the Indian Precambrian. Analysis of structures of different scales observed in this terrain establishes the presence of three phases of deformation. S, is pervasive in the metapelitic granulites (mainly khondalite), while in the migmatite complex composed of leptynites, charnockites and quartzofeldspathic veins, Sj is present exclusively within the charnockite lenses and bands, and shows different stages of obliteration in the associated leptynites. Thus, the charnockite patches must be earlier than the surrounding migmatitic rocks. The charnockite patches and the surrounding leptynitic gneisses are chemically quite different and the two rock types are not related by any prograde or retrograde transformation. The shapes and disposition of charnockite patches in the mixed exposures are found to be largely controlled by the third phase of folding and locally associated shearing. The kinematics of this late deformation are not favourable for fluid ingress from deeper levels.
The Himalayan arc has an arcuate E-W trending geometry with reversal of trend at the terminal ends -Nanga-Parbat (western) syntaxis and Namcha-Barwa (eastern) syntaxis. Both ends are characterized by an actively deformed uplifted dome with its flanks bounded by active shear zones/faults that cause the majority of the seismicity. Compiled map data and seismo-geological depth sections around these two syntaxial zones have brought out active crustal structure and seismotectonic setup. The Nanga-Parbat syntaxis exhibits upward bending and subsequent thickening of the Indian plate with the cluster of seismicity along the NNE-SSW trending Raikhot fault/Diamer shear in its western margin and a comparatively less active Rupal-Chichi shear zone of N-S trend with diffused seismicity towards the east. The 2005 Kashmir earthquake is spawned due to interaction of the Main Boundary thrust and the Muzaffarabad fault. The Namcha-Barwa syntaxis displays a fault-bounded upliftment and thickening of the Indian plate where Canyon thrust marks the boundary between the Indian and Eurasian plates. The occurrence of the 1950 Assam earthquake in the vicinity of the eastern syntaxis is attributed to a regional right lateral strike-slip motion on the causative fault plane. The seismicity in the syntaxes is primarily controlled by strike-slip faults/shear zones along the flanks of popup antiforms.
An extraordinarily strong and persistent earthquake swarm (Andaman swarm 2005) originated in the Andaman back-arc following the aftershock sequences of the 26 December 2004 Sumatra earthquake. The swarm (n = 651, mb max = 5.9) came mainly in two phases: January 26-31 and Feb. -Aug. 2005, in an area of size 90 x 40 km 2 , at the centre of which lies a broad bathymetric depression and high gravity zone. The swarm demonstrates a complex faulting series, initially the strike-slip motion followed by normal faulting in repetitive sequences, whose representative fault planes orient at high angle to the regional faults. The swarm character as well as the distribution of stress-axes and their correlation to tectonic features lends speculation for formation of a nascent rift segment in NW-SE direction at the doorstep of the Sewell Seamount. The swarm has given rise to 21 episodes of rifting activities of variable time extent within 26 -31 January 2005. The r-t plots corresponding to the swarm data, modelled with variable hydraulic diffusivity (D) values 4, 6, 8 and 10 m 2 /s, suggest for excess pressure front from ascending magmatic fluid. This eventually heralded the rifting; causing pore pressure perturbations that propagated in accordance with known diffusion parabolic equations.
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