Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: examples from the Gavilgarh–Tan Shear Zone, central India

• Published in: Geological magazine Vol. 145; no. 6; pp. 766 - 777
• Main Authors: CHATTOPADHYAY, A., KHASDEO, L., HOLDSWORTH, R. E., SMITH, S. A. F.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01.11.2008
• Subjects: Asia; brittle deformation; brittle-plastic transition; central India; crust; Deformation; fault reactivation; faults; Gavilgarh-Tan shear zone; Geology; India; Indian Peninsula; Indian Shield; Low temperature; Madhya Pradesh India; metamorphic rocks; microstructure; mylonite; mylonites; plastic deformation; Plastics; Plate tectonics; Precambrian; Proterozoic; pseudotachylite; reactivation; Rocks; Seismic activity; seismicity; Seismology; shear zones; Structural geology; Sulfides; upper crust; upper Precambrian; central India; brittle-plastic transition; pseudotachylite; mylonite; fault reactivation
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756808005074

In the sheared and foliated granitoids of the Proterozoic Gavilgarh–Tan Shear Zone (GTSZ) in central India, two types of pseudotachylite (Pt-M and Pt-C) are recognized. Pt-M layers are interbanded with mylonite and ultramylonite, show strong internal plastic deformation and buckle folding concurrent with the host rocks, and appear to have formed within the greenschist facies (300–400 °C) in the brittle–plastic transitional (semi-brittle) regime. Pt-C layers show sharp contacts with the host rock, exhibit abundant coeval cataclasis, preserve no evidence of subsequent plastic deformation, and formed at shallower depths, at temperature < 300 °C. Sulphide droplets and embayment of quartz grain margins in the pseudotachylite (Pt-C) matrix indicates a melt origin. Ductile shear sense criteria in the host mylonites are consistently sinistral, while those associated with the deformed pseudotachylite (Pt-M) layers are dextral. It appears therefore that the host mylonite/ultramylonite foliation experienced reactivated slip movement in the ‘semi-brittle’ zone when pseudotachylite was generated and subsequently ductilely deformed. The brittle pseudotachylite (Pt-C) layers were generated later at a shallower level, and at a lower temperature. They are spatially associated with a set of foliation-parallel brittle shears with sinistral-sense displacements. The multiple episodes of frictional melt generation within the Gavilgarh–Tan Shear Zone illustrate that it has a complex history of multiple reactivations. It therefore represents an important new area for the study of seismic behaviour of the upper crust along pre-existing structures and may facilitate a better geological understanding of the present seismic activity in the central Indian Shield.