Rift and grain in two granites of the White Mountain magma series

Abstract

New England granites commonly have two subperpendicular weakness directions, called rift and grain, which are exploited during quarrying. Field and thin section study of granites from the White Mountain Batholith (Jurassic) and the Mount Ascutney Intrusive (Cretaceous) show that the rift and grain directions are consistently developed parallel to Mesozoic joints and suggests that the rift is the result of a preferentially oriented microcrack fabric and topographically controlled near‐surface in situ stresses. The microcrack fabric is made up of fluid inclusion planes and open microfractures. Fluid inclusions developed during postemplacement cooling and represent healed microfractures. Open microcracks developed during uplift by recracking fluid inclusion planes, so there is a strong similarity of orientation between elements in the microfracture fabric. The influence of gravity sets up downslope acting near‐surface stresses which favor the development of open microfractures striking perpendicular to the downslope stresses. Joints and microfractures are best developed perpendicular to the local slope directions. Replotted and field checked data from Dale (1923) demonstrate that in 53 turn‐of‐the‐century New England quarries, rift and grain are parallel and perpendicular, respectively, to the direction of topographic slope. Thus rift and grain in granites of the White Mountain Batholith (Jurassic) and the Mount Ascutney Intrusive (Cretaceous) result from a combination of a properly oriented microcrack fabric and topographically controlled near‐surface in situ stresses.