Fluid assisted rejuvenation of precursor brittle fractures as the habitats of ductile shear zones: An example from the ~2.6 Ga Bundelkhand Granitoid of north-central India. (December 2020)

• Record Type: Journal Article
• Authors: Sarkar, Goutam; Banerjee, Sayandeep; Maity, Sayan; Srivastava, Hari B.
• Is Part Of: Journal of structural geology. Volume 141(2020)
• Publication Date: 2020-12
• Subjects: Bundelkhand Craton -- Ductile shear zones -- Brittle fractures -- Microstructure -- Mylonite -- Fluid phase -- Fluid-rock interaction; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2020.104198
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: Ductile shear zones frequently nucleate on precursor fractures, and usually imitate their initial geometries. The fractures may either predate the ductile shearing episode or form at an early stage of the ductile shearing. In either case, fluids play a pivotal role in the transformation from brittle to ductile mechanisms. However, the exact mechanism(s) triggering the transition from brittle fracturing to ductile shearing remains elusive. The granitoid rocks of the Bundelkhand Craton preserve evidences of at least five growth stages of ductile shear zone, nucleated on precursor fractures. Field and microstructural investigations suggest that these ductile shear zones are always associated with epidote (~70–80% epidote + ~30-20% quartz ± ~5% chlorite) veins. Here, we argue that a fluid, now represented by epidote vein, originated from the breakdown of mafic minerals in the host granitoid, essentially percolated in the fractures that promoted cataclasis at first. The finer grain size resulted from cataclasis coupled with the fluid had weakened the host rock along fractures which eventually nucleated the ductile shear zone. Protracted supply of the fluid and syn-kinematic redistribution by the fluid phase widened the shear zone. Graphical abstract: Image 1 Highlights: First comprehensive report on the nucleation and growth of ductile shear zones in the Bundelkhand Craton. Transformation of precursor brittle fractures to mature ductile shear zones due to continued flux of the epidote vein. Syn-kinematic redistribution by a fluid phase is effective in the process of dissolution and precipitation of epidote in the mylonitized zone. Reduced grain size coupled with fluids of the epidote vein transform the rheology from brittle to ductile. High-strain domains and augmented porosity zones characterize the mylonitized zone of ductile shear zones.