Articles written in Journal of Earth System Science

    • Internal geometry of a thrust sheet, eastern Proterozoic belt, Godavari Valley, South India

      Dilip Saha

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      A small thrust sheet, named Pedda Gutta thrust sheet, consisting of calcareous to cherty argillites and cherts, and juxtaposed against tidal-intertidal cross-bedded quartzites and stromatolitic and sileceous limestone in the eastern Proterozoic belt, Godavari Valley, exhibits structures comparable in style to those of the external zone of a fold-thrust mountain belt. A wide spectrum of periodic and aperiodic mesoscopic folds varying from upright ones with rounded hinges and attenuated limbs, through noncylindrical kinks to whalebacks and sheath-like forms have developed within the small volume of the thrust sheet, the preserved thickness of which is of the order of 50 metres (comparable in scale to cleavage duplexes). Cleavage development is also heterogeneous across the width of the sheet. Displacement transfer from faults to folds and vice-versa is a common feature.

      On the basis of the distribution of the mesoscopic structures of varying style within the sheet and localization of fault rocks, three slices (wedges) have been recognized, each bounded on the east by a thrust which is steep at the current erosion level but interpreted to be of listric form making the thrust network comparable in architecture, though not in scale, to a hinterland (west) dipping imbricate fan.

    • Some observations on brittle-ductile toggle

      Dilip Saha

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      In low temperature deformation of polymineralic rocks the constituent minerals often show contrasting deformation mechanisms. In naturally deformed arkoses, feldspathic quartzites and grits under greenschist to almandine-amphibolite fades condition, feldspar deforms by microboudinage (rigid-brittle behaviour), while quartz flows by a combination of dislocation creep, pressure solution and solution transfer. Boudin segments develop and separate in a phased sequential manner while quartz matrix flows in a ductile manner, indicating a brittle-ductile toggle during progressive deformation.

      Both the pressure solution and dislocation creep flows are volume-conservative. Therefore, a net volume increase during the above deformations is a necessity, unless compensated by a solution-transfer process. Hydrofracturing probably played a role in microboudinage formation as the ambient level of differential stress is estimated to be low around 45–75 MPa.

      To develop a synthetic flow law for the above type of deformation in arkoses, one needs to consider the significance of different rate-controlling mechanisms. As feldspar pull-aparts are syntectonically filled with quartz or metamorphic minerals crystallizing during progressive deformation, successive microboudin segmentation will depend on how fast/slow the matrix quartz moves to the open crack or the sealing takes place by transfer of appropriate solute components by pressure solution or solution transfer, the real rate-controlling process.

    • Flow laws in polymineralic aggregates deformed by a combination of diffusion creep and dislocation creep

      Dilip Saha

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      Microstructures in naturally deformed rocks in the upper crust demonstrate that creep strain in nature may be accommodated by a combination of dislocation creep, diffusion/dissolution processes and microcracking. A theoretical approach towards deriving an aggregate flow law is presented, where the strain in the constituent phases is assumed to occur by simultaneous operation of diffusive mass transfer and crystal plastic mechanisms (dislocation creep). Both uniform stress and uniform strain rate situations are considered.

    • Kinematics of large scale asymmetric folds and associated smaller scale brittle — Ductile structures in the proterozoic somnur formation, pranhita — Godavari Valley, South India

      Gautam Ghosh Dilip Saha

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      The development of structural elements and finite strain data are analysed to constrain kinematics of folds and faults at various scales within a Proterozoic fold-and-thrust belt in Pranhita-Godavari basin, south India. The first order structures in this belt are interpreted as large scale buckle folds above a subsurface decollement emphasizing the importance of detachment folding in thin skinned deformation of a sedimentary prism lying above a gneissic basement. That the folds have developed through fixed-hinge buckling is constrained by the nature of variation of mesoscopic fabric over large folds and finite strain data. Relatively low, irrotational flattening strain (X:Z-3.1-4.8, k<1) are associated with zones of near upright early mesoscopic folds and cleavage, whereas large flattening strain (X:Z-3.9-7.3, k<1) involving noncoaxiality are linked to domains of asymmetric, later inclined folds, faults and intense cleavage on the hanging wall of thrusts on the flanks of large folds. In the latter case, the bulk strain can be factorized to components of pure shear and simple shear with a maximum shearing strain of 3. The present work reiterates the importance of analysis of minor structures in conjunction with strain data to unravel the kinematic history of fold-and-thrust belts developed at shallow crustal level.

    • Advective heat transfer and fabric development in a shallow crustal intrusive granite – the case of Proterozoic Vellaturu granite, south India

      Dilip Saha Sukanya Chakraborti

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      Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric,although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt,south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton.The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c axis fabric in completely recrystallized quartz ribbons,dynamic recrystallization of perthites,and myrmekite indicative of fabric development under intermediate temperature (∼500-400° C). The weakly-deformed interior shows myrmekite,feldspar microfracturing and limited bulging recrystallization of quartz.The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature(∼300° C).The relative rates of cooling in fluenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development.The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss.Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamalai rocks.

    • Universal stage measurements in petrofabric analysis revisited


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      Once an important accessory for petrographic microscope and a valuable tool in mineralogical and petrofabric work, the Universal Stage (U-stage) has been dubbed obsolete with the advent of electron microprobe and XRD, and subsequent phasing out of the production of the optical equipment. Modern-day fabric analysis also relies more on far more expensive texture goniometers, neutron activation analysis and electron back-scattered diffraction analysis (EBSD) equipment. However, routine measurements of crystallographic fabric elements of uniaxial mineral aggregates (quartz- and calcite c-axis), principal axes of important biaxial minerals like olivine, strength of mica (001) fabric, paleostress estimate from mechanical twins (e-twin) in calcite, and evaluation of meteorite impact texture in quartz as well as impact history of asteroids, can still be done by U-stage knowledgeably and in a cost effective manner, subject to adequate training by efficient instructors. In the cases of (e-twin in individual calcite grains and planar deformation features in natural shocked quartz, U-stage measurement cannot still be replaced by EBSD. An abridged review of the science behind U-stage procedure with examples of actual application has been presented to rekindle the interest.

    • Overlapping A-type and S-type characters in late- to post-tectonic granites – petro-tectonic evolution of late Mesoproterozoic Andhra Konda granite, Nellore Schist Belt, southern India


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      As the debate continues on whether all A-type granites are anorogenic, we present here new structural and geochemical data and interpretation on a late Mesoproterozoic granitic pluton from the Nellore Schist Belt (NSB), southern India. The Andhra Konda granite, occurring in the central part of the NSB is intrusive into peripheral part of deformed volcano-sedimentary succession of the Vinjamuru Group, near its contact with the Kanigiri Ophiolite. The overall elongation of the granitic stock parallel to the regional strike in the NSB, parallelism of magmatic and mylonitic fabrics and continuum of hypersolidus to subsolidus crystal plastic deformation fabrics are comparable to features of late- to post-tectonic granites. The Andhra Konda granite is peraluminous, alkali-calcic to calc-alkalic and ferroan in composition, and enriched in light rare earth elements (LREE), Cs, Rb and depleted in Eu, Ba, Sr and P. Overlapping geochemical signatures suggest that the granitic magma could have evolved from either ocean island basalt magma or island arc basalt magma, or alternatively originated from partial melting of metasedimentary rocks, with involvement of continental crust in a subduction-related convergent setting. Based on mineral chemistry and geochemical signatures, and relatively higher crystallization temperature, Andhra Konda granite is proposed to be A-type granite, though some overlap in character with S-type exists. Geochemical discrimination also favours late- to post-collisional tectonic setting for the Andhra Konda granite. Andhra Konda granite and geochemically similar 1284 Ma old granitic stocks of Kanigiri in the central NSB possibly represent a phase of granitic magmatism distinct from supposedly rift-related alkaline magmatism in this region, and augment the notion that A-type granites may form and occur in diverse tectonic settings.


      $\bullet$ New geochemical data and tectonic interpretation from a late-Mesoproterozoic ferroan granite

      $\bullet$ Microstructures show a continuum of hypersolidus to solid-state deformation fabrics as in late-tectonic granites

      $\bullet$ Major and trace element ratios linked to subduction-related partial melting and crustal input

      $\bullet$ Post-tectonic Andhra Konda granite adds to the diversity of A-type granite character

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