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Abstract <p>A brittle-ductile shear zone associated with the Barail Group of sedimentary rocks in the Pfutsero area, Nagaland (Indo-Myanmar Ranges), has a unique structure characterized by an array of sigmoidal tension gashes (STG). This structure provides valuable insights into the deformation mechanism of the shear zone, shear strain distribution, and their relationship with the regional tectonic setting. Ramsay's equation and graphical methods were employed to quantify shear strain (γ), yielding values of 0.76 and 0.243, respectively; graphical methods yield a volume change of 31%. The tips of the STGs align parallel to the Minimum Instantaneous Stretching Axis (ISA<sub>3</sub>), and the degree of co-axiality was measured. It reveals a kinematic vorticity number (W<sub>k</sub>) of 0.98, indicating simple shear dominated deformation within a wrenching regime.</p> Research highlights <p><OrderedList> <ListItem> <ItemNumber>1.</ItemNumber> <ItemContent> <p>Indo-Myanmar Ranges exhibit a complex deformation setting and preserved well developed meso-scale deformation structures that record multiple stages of tectonic activity.</p> </ItemContent> </ListItem> <ListItem> <ItemNumber>2.</ItemNumber> <ItemContent> <p>The brittle-ductile shear zone contains an array of sigmoidal tension gashes, which are used to quantify the magnitude of shear strain and the kinematic vorticity number of the shear zone.</p> </ItemContent> </ListItem> <ListItem> <ItemNumber>3.</ItemNumber> <ItemContent> <p>Shear strain estimates yield γ= 0.76 for the Ramsay model, whereas the folded bridge model indicates γ=0.243, with the volume increase of +31%.</p> </ItemContent> </ListItem> <ListItem> <ItemNumber>4.</ItemNumber> <ItemContent> <p>The calculated kinematic vorticity number (W<sub>k</sub> =0.93-0.99) suggests that deformation within the shear zone was strongly dominated by simple shear.</p> </ItemContent> </ListItem> <ListItem> <ItemNumber>5.</ItemNumber> <ItemContent> <p>The observed shear orientation and kinematic sense align with the regional Riedel shear fracture of the IMR, suggesting that the shear zone is formed in response to the Riedel fracture of the regional tectonics.</p> </ItemContent> </ListItem> </OrderedList></p>

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Shear strain and vorticity analysis of a brittle–ductile shear zone in the Pfutsero Area, Nagaland: implications for strain state and deformation mechanisms in the Indo-Myanmar Ranges

  • 期刊论文

  • 发表日期：

    2026年7月1日

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  • Thokchom Nilamani Singh,
  • Soibam Ibotombi,
  • Khundrakpam Kumarjit Singh

摘要

Abstract

A brittle-ductile shear zone associated with the Barail Group of sedimentary rocks in the Pfutsero area, Nagaland (Indo-Myanmar Ranges), has a unique structure characterized by an array of sigmoidal tension gashes (STG). This structure provides valuable insights into the deformation mechanism of the shear zone, shear strain distribution, and their relationship with the regional tectonic setting. Ramsay's equation and graphical methods were employed to quantify shear strain (γ), yielding values of 0.76 and 0.243, respectively; graphical methods yield a volume change of 31%. The tips of the STGs align parallel to the Minimum Instantaneous Stretching Axis (ISA3), and the degree of co-axiality was measured. It reveals a kinematic vorticity number (Wk) of 0.98, indicating simple shear dominated deformation within a wrenching regime.

Research highlights

1.

Indo-Myanmar Ranges exhibit a complex deformation setting and preserved well developed meso-scale deformation structures that record multiple stages of tectonic activity.

2.

The brittle-ductile shear zone contains an array of sigmoidal tension gashes, which are used to quantify the magnitude of shear strain and the kinematic vorticity number of the shear zone.

 3.

Shear strain estimates yield γ= 0.76 for the Ramsay model, whereas the folded bridge model indicates γ=0.243, with the volume increase of +31%.

 4.

The calculated kinematic vorticity number (Wk =0.93-0.99) suggests that deformation within the shear zone was strongly dominated by simple shear.

 5.

The observed shear orientation and kinematic sense align with the regional Riedel shear fracture of the IMR, suggesting that the shear zone is formed in response to the Riedel fracture of the regional tectonics.