Articles written in Journal of Earth System Science

    • Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

      Bhuban Mohan Behera V Thirukumaran Aishwaraya Soni Prasanta Kumar Mishra Tapas Kumar Biswal

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      Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 μm2) deviating from linearity. The shape of the clasts shows a high degree of roundness (<0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000∘C. Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

    • Palaeostress and magma pressure measurement of granite veins in the Neoproterozoic Ambaji granulite, South Delhi terrane, Aravalli–Delhi mobile belt, NW India: Implication towards the extension-driven exhumation of the middle–lower crustal rocks

      Sudheer Kumar Tiwari Tapas Kumar Biswal

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      Neoproterozoic Ambaji granulite, in the South Delhi terrane (SDT) of the Aravalli–Delhi mobile belt (ADMB), is multiply deformed at different stages of its exhumation from the middle–lower crust. It was intruded by four phases of granites, G$_{0-3}$, of which the G$_{3}$ was syntectonic with brittle shearing. The orientation of the G$_{3}$ granite veins was used, in this paper, to measure the palaeostress and magma pressure during its intrusion. The G$_{3}$ granite has a porphyritic texture with quartz, microcline and biotite minerals and contains magmatic foliation; biotites are aligned oblique to such foliation, suggesting the syntectonic intrusion of the granite with normal faulting. Stereoplot of the poles of the granite veins shows concentration in the NW and SW quadrants and lack data over an elliptical area at the centre, indicating a girdle pattern. This implies that the magma pressure $P_{\rm{m}}$ > $\sigma _{2}$, compression $\sigma _{1}$ was vertical and extension $\sigma _{3}$ was NW–SE horizontal. Values of $\theta_{2}$ = 16$^{\circ}$ and $\theta_{3}$ = 40$^{\circ}$ were obtained from the stereoplot, used to calculate stress ratio $\Phi$ = 0.81, driving pressure $R^{\prime}$ = 0.92 and the construction of the 3D Mohr plot for $P_{\rm{m}}$ and principal stresses. Result suggests that the $G_{3}$ granite intruded under extensional tectonics that probably contributed to the exhumation of the Ambaji granulite.

    • Dynamic recrystallization mechanisms and vorticity estimation of the Terrane Boundary Shear Zone (Lakhna shear zone): Implications on dynamics of juxtaposition of the Eastern Ghats Mobile Belt with the Bastar Craton, NW Odisha


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      The Terrane Boundary Shear Zones provide important information regarding tectonics of juxtaposition between different terranes. In this paper, we have studied the Lakhna shear zone, between the Eastern Ghats Mobile Belt and the Bastar Craton. Detailed structural study, sampling and strain analysis through measurement of size and orientation of dynamically recrystallized quartz grains indicate: (i) the Lakhna shear zone is developed on granitic protolith of the Bastar Craton, (ii) mylonites are quartzofeldspathic in composition, marked by SE dipping mylonitic foliation and down dip stretching lineation defined by biotite and quartz grains, (iii) S-C fabric and rotated porphyroclasts indicate a NW vergence thrust kinematics, (iv) recrystallization of quartz grains occurred by climb-assisted dislocation creep through BLG near craton boundary, SGR in the central part of the shear zone, GBM towards the Eastern Ghats margin, (v) temperature of deformation increases towards mobile belt ($380–560^{\circ}\rm{C}$) that suggests thrusting of granulitic hot slab over cold craton and (vi) $W_{m}$ estimates of 0.9 suggest simple shear deformation. The juxtaposition between the EGMB and the Bastar Craton occurred by simple shear thrusting when the granulitic slab was hot enough to create a difference in temperature and mechanism of dynamic recrystallization across the shear zone.

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