Brittle tectonics and pore-fluid conditions in the evolution of the Great Boundary Fault around Chittaurgarh, Northwestern India

Abstract

This paper aims at reconstructing paleostress history and deciphering the pore-fluid pressure conditions during the reactivation of a regional-scale fault. Paleostress analyses of the mesoscopic structures suggest that three successive events of reactivation on the Great Boundary Fault occurred in thrust-type, strike-slip type and thrust-type tectonic-settings, respectively. Whereas the pore-fluid pressure was supralithostatic during the first and third events of reactivation, it was sublithostatic during the second event. Each event of reactivation induced a fluid pressure gradient, which resulted in the focused and enhanced flow of syntectonic fluids within the high permeability locales, primarily fractures and faults. Fluid inclusion study on strike-slip veins reveals that the syntectonic fluids were highly dense, Na-Ca-Cl brines of formational water origin. Stratigraphic evidence in favour of a 2-km-thick column of overburden above Kaimur sandstone beds implies that the strike-slip faulting occurred at 160-202°C temperature and 53 MPa pressure. Variation in homogenization temperature reflects fluctuation in pore-fluid pressure during entrapment of syntectonic fluids and points to seismic pumping as a possible mechanism of fluid flow during faulting. High paleogeothermal gradient, obtained by fluid inclusion data, is ascribed to the high heat flow due to crustal stretching during the Proterozoic rifting, the basal and intermittent volcanism in the basin, and occurrence of Berach granite as the basement.