Articles written in Journal of Earth System Science

    • Development of micro-scale joints in volcanic rocks under thermal stress

      Susanta Kumar Samanta

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      Petrographic studies of samples of the Rajmahal basalt reveal a variety of microscopic joints within phenocrysts, which seem to have developed under the influence of thermal stresses during cooling. The theoretical analysis shows that:

      •thermal stresses develop in a system only under unsteady state of cooling,

      •the stresses responsible for the development of joints are tensile in nature, and

      •the magnitude of stresses increases with increasing rates of cooling.

      In order to understand the effects of cooling rate on the mode of jointing, analogue model experiments were performed by quenching melt films of organic, crystalline materials under varying cooling conditions. In experiments, non-branching linear joints developed at relatively lower rates of cooling by a process of sub-critical crack propagation. On the other hand, at higher rates of cooling the mechanism of crack propagation was essentially supercritical leading to the development of branching joints.

    • Variation of strain pattern and its influence on the geometry of the uranium mineralized body in Bangurdih area, western part of the Singhbhum shear zone, eastern India


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      The mineralized (U bearing) rocks, near Bangurdih area, in the western part of the Singhbhum shear zone (SSZ) are strongly deformed and metamorphosed. The host and country rocks are mylonitic in nature with strong linear and planar fabrics. Surface studies indicate that the mineralized bodies are discontinuous in nature and occur sporadically. The thickness of the individual mineralized bodies is also not uniform. Present work is aimed at studying meso- and micro-scale deformational structures, their bearing on regional scale structure and their relation with the occurrences of U-mineralization in and around Bangurdih area. The area comprises predominantly of quartz–chlorite schist/quartz–chlorite–sericite schist with intercalated quartzite, meta-conglomerate and feldspathic schist of varying thickness. Apatite–tourmaline–magnetite-rich quartzite, chlorite schist and conglomerate are commonly radioactive. The uranium mineralized body at Bangurdih shows pinch-and-swell structure. Stretched thin quartz veins, observed within the sheared quartzite layers, have undergone layer parallel flow resulting in bidirectional pinch-and-swell structure lying on YZ and XZ sections of strain ellipsoid with their axes parallel and perpendicular to the shear lineation, respectively. Oriented samples were collected from quartz veins and the country rocks to prepare thin sections parallel to XY, YZ and XZ plane of strain ellipsoid from each sample for microstructural study. Aspect ratios of strain ellipses of XY, YZ and XZ sections are measured using Fry analysis. Flinn diagram shows that the rocks of the Bangurdih were subjected to bi-directional flattening (average k = 0.77), which indicates that in addition to simple shear deformation there is a shortening component perpendicular to the shear plane in this area. To correlate the strain pattern of Bangurdih with nearby areas, deformed pebbles from conglomeratic horizons and oriented samples of other rocks from Bhurkuli–Bundu and Rangamatia–Simulbera area were studied. Flinn diagram, prepared using all data, reveals that while the strain type in Bangurdih area shows intense flattening type of deformation, the Rangamatia–Simulbera area shows constrictional type of deformation (k values range from 0.57 to 0.88 and 1.02 to 2, respectively). Other locations (Bhurkuli–Bundu) in between Bangurdih and Rangamatia–Simulbera show both constriction and flattening type of strain (k values vary from 0.67 to 1.35). Therefore, it is concluded that the intense flattening type of simple shear deformation in Bangurdih area leads the ore rich layers to be stretched and detached in two directions (X and Y), which in turn, causes the mineralized body to occur intermittently with systematic trend on the surface and possibly in subsurface conditions too.

    • Low-effective fault strength of a blind detachment beneath the Indo-Burmese Arc (NE-India) induced by frictional–viscous flow


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      We present evidence for frictional–viscous flow (or steady creep) behaviour of the blind detachment below the Indo-Burmese wedge (IBW), as characterised by the fault gouge exposed along the Churachandpur– Mao Fault, which is one of the splay faults to the blind detachment. The petrography of the exhumed phyllonitic gouge material (considered the wedge material from the detachment and representative of the detachment) indicates evidence of pressure solution and viscous flow. We use a microphysical model involving frictional–viscous flow, and together with microstructural observations from the exhumed gouge we interpret low shear strength (${\le}$20 MPa) within the frictional–viscous transition zone. The results of the model also suggest that the geodetically estimated strain rate in this region lies in the domain of steady creep or velocity strengthening frictional behaviour. We suggest that the presence of phyllosilicate-rich rocks may significantly weaken mature fault cores along the detachment beneath the IBW and facilitate frictional–viscous flow at shallow depth. This new evidence suggests a significantly reduced seismic hazard across the megathrust.


      $\bullet$ Frictional–viscous flow along the blind detachment beneath the Indo-Burmese Arc.

      $\bullet$ Exhumed fault gouge indicates evidence of pressure solution and viscous flow.

      $\bullet$ Phyllosilicate rich rocks significantly weaken mature fault cores along this detachment.

      $\bullet$ Microphysical model imply low shear strength of the blind detachment.

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