Presentation Information
[T4-P-8]Structural Characteristics and Tectonic Setting of a Crustal Scale Shear Zone: The Gadag–Mandya Shear Zone, Dharwar Craton, India
*Manato Urakawa1, Lakshmanan Sreehari1, Yoshihiro Nakamura2 (1. Shimane Univ., 2. AIST)
Keywords:
Ductile deformation,Shear zone,Microstructure,Mylonite,EBSD
The Dharwar Craton, a Meso- to Neoarchean terrane (ca. 3500-2500 Ma) in southern India, comprises the Western (WDC), Central (CDC), and Eastern (EDC) Dharwar Cratons. The Gadag-Mandya Shear Zone (GMSZ), a >300 km long N-S to NW-SE trending structure, marks the boundary between the WDC and CDC. Transecting greenschist facies rocks in the north and granulite facies in the south, the GMSZ offers a rare opportunity to examine ductile deformation across crustal levels from the upper to lower crust. Despite its tectonic significance, detailed investigations of the internal architecture and deformation conditions of the GMSZ remain limited. This study integrates field observations, petrography, EBSD analysis, and fractal dimension measurements to characterize its deformation history and tectonic setting.
Microstructural and analytical data along the GMSZ reveal a systematic southward increase in deformation temperature and crustal depth. In the northern zone, quartz shows ribbon structures, chessboard extinction, myrmekite, and grain boundary migration (GBM) recrystallization, consistent with high-temperature deformation. EBSD c-axis pole figures exhibit type-II crossed girdle patterns, indicating dominant prism-a slip system at temperatures of ~600-650 °C. Fractal dimension (D) values are relatively low (1.12-1.14), supporting high-T deformation, likely synmagmatic.
The central zone is characterized by quartz-rich mylonites with well-developed S-C and C' fabrics, dominated by bulging (BLG) and subgrain rotation (SGR) recrystallization, indicating deformation at ~300-500 °C (Stipp et al., 2002). EBSD data show type-I crossed girdle patterns and continued dominance of prism-a slip system. D-values range from 1.12 to 1.19, suggesting temperatures up to ~650 °C. The presence of muscovite along foliation and minor biotite suggests syn-deformational hydration.
In the southern zone, high-grade shear zones host aligned amphibole and occasional orthopyroxene, with cm-scale shear zones and melt pockets observed in granulites. Ultramylonites occur along the margins of km-scale granitic plutons. Quartz microstructures display GBM and SGR features, with recrystallized feldspar and myrmekite suggesting deformation at ~700 °C. Low D-values (1.05-1.07) support high-T conditions; however, EBSD data indicate basal-a slip system, possibly reflecting overprinting by a later, lower-T deformation. Overprinting of amphibole by muscovite and biotite suggests hydration during this late-stage event.
These results highlight the progressive transition of the GMSZ from upper- to lower-crustal conditions. The shear zone width increases from a few centimeters in the north to several kilometers in the south, reflecting crustal-scale strain localization. Shear zones are commonly localized along granitic pluton margins, and in the southern granulite facies zone, melt infiltration along shear planes suggests a key role in magma ascent and granulitization, potentially contributing to cratonization of the Dharwar Craton.
Microstructural and analytical data along the GMSZ reveal a systematic southward increase in deformation temperature and crustal depth. In the northern zone, quartz shows ribbon structures, chessboard extinction, myrmekite, and grain boundary migration (GBM) recrystallization, consistent with high-temperature deformation. EBSD c-axis pole figures exhibit type-II crossed girdle patterns, indicating dominant prism-a slip system at temperatures of ~600-650 °C. Fractal dimension (D) values are relatively low (1.12-1.14), supporting high-T deformation, likely synmagmatic.
The central zone is characterized by quartz-rich mylonites with well-developed S-C and C' fabrics, dominated by bulging (BLG) and subgrain rotation (SGR) recrystallization, indicating deformation at ~300-500 °C (Stipp et al., 2002). EBSD data show type-I crossed girdle patterns and continued dominance of prism-a slip system. D-values range from 1.12 to 1.19, suggesting temperatures up to ~650 °C. The presence of muscovite along foliation and minor biotite suggests syn-deformational hydration.
In the southern zone, high-grade shear zones host aligned amphibole and occasional orthopyroxene, with cm-scale shear zones and melt pockets observed in granulites. Ultramylonites occur along the margins of km-scale granitic plutons. Quartz microstructures display GBM and SGR features, with recrystallized feldspar and myrmekite suggesting deformation at ~700 °C. Low D-values (1.05-1.07) support high-T conditions; however, EBSD data indicate basal-a slip system, possibly reflecting overprinting by a later, lower-T deformation. Overprinting of amphibole by muscovite and biotite suggests hydration during this late-stage event.
These results highlight the progressive transition of the GMSZ from upper- to lower-crustal conditions. The shear zone width increases from a few centimeters in the north to several kilometers in the south, reflecting crustal-scale strain localization. Shear zones are commonly localized along granitic pluton margins, and in the southern granulite facies zone, melt infiltration along shear planes suggests a key role in magma ascent and granulitization, potentially contributing to cratonization of the Dharwar Craton.