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Geological Magazine; July 2006; v. 143; no. 4; p. 531-543; DOI: 10.1017/S0016756805001676
© 2006 Cambridge University Press (CUP)
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Grain-scale deformation in the Palaeoproterozoic Dongargarh Supergroup, central India: implications for shallow crustal deformation mechanisms from microstructural analysis

GAUTAM GHOSH*,{dagger}, SUKANYA CHAKRABORTY{ddagger}, JOYDIP MUKHOPADHYAY* and ARIJIT RAY*

Department of Geology, Presidency College, 86/1 College Street, Kolkata 700073, India
Geological Studies Unit, Indian Statistical Unit, 203 B.T. Road, Kolkata 700035, India

{dagger} Author for correspondence: gautam_0262{at}rediffmail.com

Analysis of the grain-scale deformation mechanisms in folded rocks of the Dongargarh Supergroup, central India, reveals that deformation was accomplished by a combination of pressure solution, microfracturing and dislocation creep processes. The finite strain was assessed using the Rf/{phi} method (X/Z {approx} 2). Partitioning of strain into various deformation mechanisms revealed that dislocation creep and pressure solution were the major contributors to the finite strain, followed by microfracturing. Analyses of microstructures suggest a sequence of dislocation creep followed by pressure solution and microfracturing, that ultimately gave way to microfracturing and limited crystallization or recrystallization. Overall constancy in volume during deformation is suggested from the balance between fracture-related grain-scale dilatancy and solution-related volume loss. Observations on cleavage spacing within various lithologies in a specific structural setting suggest that lithology played a vital role in cleavage development. Cleavage development in sandstones of the Dongargarh Supergroup required thin shale interbeds (for competency contrast) and grain size < 0.036 mm (4.75 {phi}).

Key Words: cleavage • grain scale deformation • Palaeoproterozoic • shallow crustal processes






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