Magnetic fabric as a vorticity gauge in syntectonically deformed granitic rocks

• AMS data are used to quantify vorticity in syntectonic granite.
• Magnetic foliation traces fabric developed during final stage of ductile deformation.
• Shear zone adjacent to granite traces the extensional flow apophysis.
• Angle between magnetic foliation and shear zone gives kinematic vorticity number.

A concept is presented to quantify vorticity using magnetic fabric data determined from anisotropy of magnetic susceptibility (AMS) analysis in syntectonic granites, whose emplacement is synchronous with tectonics of adjacent shear zones. The latter is considered to define the direction of extensional flow apophysis (Ae). It is suggested that the magnetic foliation traces the direction of maximum instantaneous stretching axis during the final stage of ductile deformation. Hence, the angle between mean orientation of magnetic foliation and Ae gives the kinematic vorticity number (Wn). This concept is tested in two granites from India — Godhra Granite (western India) and Chakradharpur Granitoid (eastern India). The analysis explains the kinematics of fabric development within the granites and also the evolution of structural elements in the surrounding rocks. It is also suggested that in cases where granite margins get mylonitized synchronously with tectonic activity along adjacent shear zones, the angle between mean magnetic foliation of the granite margin samples and the shear zone can help calculate Wn. The example of Malanjkhand Granite (central India) is discussed to highlight this. Using magnetic fabric, a value of Wn = 0.98 is recorded for granite margin samples implying their deformation by dominantly simple shear. Oblique quartz foliation recorded on the microscale in the granite margin samples yields Wn = 0.94, which is similar to the value obtained using AMS. It is thus concluded that magnetic fabric provides a possibility to quantify vorticity in syntectonic granites in 2-dimension.