Geometries and mechanics of veins and dykes

Abstract

The mechanics of vein and dyke formation is investigated from field observations of natural fracture systems and by using techniques from rock fracture mechanics. Veins from the Culm Basin near Millook Haven, south-west England, are used to develop and test hypotheses that natural crack geometry can be used to infer paleostress conditions and propagation histories from the time of their initiation. It is suggested that veins initially propagate as essentially mode I (extension) fractures. Local crack- induced stresses generated through mechanical interaction, or by subsequent loading effects on pre- existing cracks may involve mixed-mode deformations, which can produce minor cracks and structures that accommodate shear displacements. Sigmoidal vein profiles and vein fillings are modelled as outgrowths of mechanical crack interaction, under fixed boundary conditions, and in the absence of high finite shear strains. IVIaps of overlapping veins with a wide range of geometries, sizes and spacing values are explained in terms of selective mechanical interaction and the stress shielding effect of longer fractures upon shorter fractures. Straight, strongly overlapping veins imply the controlling influence of a relatively high remote crack-parallel compressive stress. En echelon vein arrays are re-examined in terms of the kinematics of transtension-transpression. Determination of displacements across vein arrays and from fibres and wall-rock markers, indicate that there is a spectrum of vein geometries which can be directly related to wall-rock displacements. The role of tectonic stress and fluid pressure in the generation of an effective tensile stress for initial vein growth is investigated using examples of en echelon vein arrays. The results indicate that vein initiation and growth may be tectonically driven (deformation induced) and is not entirely dependent on high internal fluid pressures. The frequency and spatial distribution of vein thickness and spacing in sedimentary rocks is analysed and the mechanical role of layer thickness on fracture development examined. Negative exponential and power-law (fractal) distributions are the most appropriate models to explain the range of vein thickness and spacing values. Departures from the power-law model are not simply sampling artifacts, but instead represent real differences in maximum vein thickness and spacing values, which can be attributed to the effects of fracture propagation through layered rocks. Observations of Tertiary mafic dykes from around Easdale, north-west Scotland, indicate that many variably oriented dykes share common sub-horizontal opening directions. The stress control of dyke intrusion is examined, with analysis of the stress ratio for individual dykes indicating local transtensional loading conditions. Examination of the structure of the chilled margins, flow lineations and the textural zonation of the dykes, suggest that magma solidified on the dyke walls as it continued to flow in the dyke interior. Measurements of dyke thickness variation and estimates of Reynolds numbers yield values well within the regime for laminar flow.

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