A kinematic model of emplacement of quartz reefs and subsequent deformation patterns in the central Indian Bundelkhand batholith

Abstract

This paper attempts to show that the quartz reefs forming the principal fabric of a large number of granitoid diapirs in the central Indian Bundelkhand batholith are dominated by numerous sigmoidal tensile fissures generated as a result of an EW sinistral brittle-ductile inhomogeneous simple shear but the secretion of quartz veins within the reefs occurred under an approximately EW subhorizontal extension. The paper also discusses the disruption along the reefs and of the reefs themselves by later faulting under a rotational subhorizontal or gently plunging maximum principal compressive stress that seems to be intimately related to the diapinc rise of more and more acidic magmas at relatively deeper levels within the crust, the relationship with the deformation within the supracrustals and suggests that the overall prolate strain within the diapirs was built up gradually, with initial NE-SW shortening, followed by meridional shortening and finally culminating into the NW-SE shortening that opened a large number of tensile cracks occupied by late dyke swarms. The slight reverse component of strike-slip faults, of both dextral and sinistral, corroborates the overall constriction at the centres of individual diapirs at depth. The palaeostress analysis using slickensided striae corroborates the general conclusions presented. Indeed, the paper tries to demonstrate that the prolate strain in the central part of a diapir is not something that occurs simultaneously from all sides radially inwards but is a phased one. It depends upon the size and shape of the initial diapir, and the part of the supracrustals in which the deformation begins first and is controlled by pre-existing planes of weakness. The paper tries to demonstrate how the prolate strain at the centres of granitoid diapirs might express itself at higher crustal levels under a relatively less ductile or brittle-ductile and even brittle regime.