Discussion on the three-dimensional morphology of arrays of echelon and sigmoidal, mineral-filled fractures from north Cornwall

Abstract

N. S. Meadows writes: In a recent paper, Nicholson & Ejiofor (1987) discuss the morphology of sigmoidal quartz-filled veins forming en echelon arrays within lithologically homogeneous cobbles of sandstone derived from the Crackington Formation of north Cornwall. The authors cast some doubt on the previously accepted hypothesis that such arrays are the result of simple shear in the plane of the array ( Ramsay 1967 , pp. 88–91; Beach 1975 , 1977 ). They ascribe the formation of these particular veins to dilatation resulting from the effects of intruding hypothermal fluids. Work on apparently similar arrays of sigmoidal veins in the Lochalsh district of northwest Scotland has also shown that a ductile simple shear model alone is unlikely to account for both the formation and the propagation of such veins. In this case, however, simple shear does appear to be a major contributor in the development of en echelon vein arrays. It is unfortunate that the host rocks for the veins studied by Nicholson and Ejiofor do not possess any significant pre-vein fabric so that there is no marker by which any displacement across the vein arrays can be measured. The en echelon vein arrays, that are abundant on the Lochalsh peninsula, occur within Torridonian sandstones of the Diabaig Group. These rocks exhibit a regional penetrative grain fabric related to large-scale ductile shearing which pre-dated the initiation of the vein arrays. The deflection of this grain fabric can be seen as it passes through individual vein arrays and, in favourable outcrops, the amount