KINEMATICS OF A TERRANE BOUNDARY STRIKE-SLIP SYSTEM FROM FIELD AND MICROSTRUCTURAL STUDIES: THE BARAKOT SHEAR ZONE, EASTERN INDIA

Ductile shear zones are often simply regarded as products of simple shearing, with the associated stretching lineation on the mylonitic shear foliation being considered to be the transport direction. However, observed stretching and transport directions may differ in natural shear zones, complicating kinematic interpretations. The Barakot Shear Zone (BSZ), the contact between the Mesoarchean Singhbhum Craton and the Neoarchean Rengali Province in the eastern Indian shield, has been variably regarded either as a thrust and or a strike-slip contact. This work aims to resolve the kinematics of this major terrane boundary shear zone from detailed field and microstructural studies using petrographic and electron backscatter diffraction (EBSD) analyses.

The mylonitic foliation in quartzites and mafic schists within the BSZ strikes ESE-WNW with a steep southerly dip. The associated lineation plunges down-dip, while strike-parallel sections of the outcrops display dextral shear-sense markers in quartzites and mafic schists, consistent with petrographic studies. Foliation normal, lineation parallel sections show stretched quartz and aligned actinolite needles defining the stretching lineation in quartzite and mafic schist, respectively. Shear sense indicators in these sections exhibit asymmetry consistent with an extensional component. Crystallographic preferred orientation plots in foliation normal, strike parallel sections show dextral asymmetry of axes girdles, while foliation normal, lineation parallel sections display extensional sense consistent with rhomb <a> activation in quartz and alignment of c-axes of actinolite, respectively. Crystallographic vorticity analyses conducted on EBSD data indicate a sub-vertical vorticity axis, suggesting strike-parallel transport during shearing. Hence, the plane of maximum asymmetry is sub-parallel to the strike-parallel section, consistent with field observations. These studies suggest that the BSZ is a strike-slip shear zone associated with an extensional sense. This confirms the earlier interpretation of some workers that the Rengali Province is a dilational step-over zone associated with strike-slip movement along the terrane boundary, which facilitated fluid influx that influenced the regional strain accommodation pattern and deformation mechanisms within the shear zone.