Controls on Deformation Style in a Neoarchean Crustal-Scale Shear Zone: Evidence from the Dharwar Craton

Variations in rock deformation style across crustal levels are a fundamental topic in structural geology, yet the factors controlling strain localization in long-lived shear zones remain debated. Here we present a detailed field-scale and microstructural study of the >200 km-long Neoarchean Gadag–Mandya shear zone in the Dharwar Craton, southern India, based on a continuous along-transect observations.

The significance of this shear zone lies in its pronounced metamorphic gradient, from dominantly greenschist-facies assemblages in the northern Western Dharwar Craton to amphibolite- and granulite-facies assemblages in the south. This framework allows us to investigate variations in deformation mechanisms and the factors governing deformation style at different crustal levels, dominantly within granitic basement rocks. The shear zone has also been interpreted as a major tectonic boundary related to Neoarchean subduction, making its internal architecture critical for understanding the tectonic evolution of the Dharwar Craton.

Our results show that shear-zone width varies markedly along the transect, from centimetre- to metre-scale zones in the greenschist-facies domain to kilometre-scale zones near the amphibolite–granulite transition. The wider domains are characterized by (i) strong strain localization within granitic intrusions, (ii) the presence of pseudotachylytes, ultramylonites, and hydrous mineral assemblages, and (iii) pervasive overprinting relationships. EBSD data and quartz microstructural analyses indicate overprinting of earlier high-temperature deformation by lower-temperature deformation, particularly in the southern sector, where amphibolite-facies assemblages are locally retrogressed to chlorite–muscovite-bearing fabrics.

Beyond the amphibolite–granulite transition, marked by the appearance of clinopyroxene within the foliation, the main shear zone becomes difficult to trace as a single continuous structure. Instead, multiple metre-scale shear zones with variable orientations are observed, commonly spatially associated with melt-rich domains. These observations highlight the critical role of rheological heterogeneity, melt and fluid infiltration, and inherited thermal structure in controlling shear-zone width, strain localization, and deformation style in Neoarchean crustal-scale shear zones. Rather than recording a simple depth-controlled transition, the Gadag–Mandya shear zone preserves a composite record of spatially and temporally variable deformation processes, in which localized seismic slip, viscous flow, and melt-assisted deformation coexist and overprint each other. This integrated field–microstructural dataset provides new constraints on the mechanical behavior of long-lived lithospheric shear zones in Archean continental crust.