Kattenhorn, S.A. (1994)

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Mechanisms of sill and dyke intrusion.


MSc thesis, University of Natal, Durban, KwaZulu-Natal, South Africa

Mechanisms of sill and dyke intrusion require an understanding of fracture growth, stress distributions and intensities, dilation, intrusion rates, hydraulic pressure, host-rock effects, en echelon fracture arrays, and flow direction. The methods of previous studies have been applied to natural sill and dyke examples of the Karoo Igneous Province in northern Natal.

An en echelon array of Jurassic dolerite sills occurs within Permian Ecca sediments along the Mhlatuze River, west of Empangeni. Dolerite emplacement occurred as two intrusive phases. The first phase resulted in thick, coarse-grained dolerite sills. The second phase produced relatively thinner, fine-grained sills. The intrusion of fine-grained dolerite into older sills is demonstrated by abrupt variations in the whole-rock and mineral geochemistry profiles across the sills. Syn-crystallisation effects such as crystal settling and fractionation, and post-crystallisation hydrothermal activity is also manifested in the mineralogical and geochemical changes across the sills. The fine-grained dolerite is associated with xenolithic dolerite which represents a contaminated magma propagation front of the fine-grained dolerite. The higher viscosity of the xenolithic dolerite hindered propagation, and was thus overtaken and engulfed by the main magma pulse.

Consistent sinistral offsetting of sill segments is interpreted to be the result of a fingered sill periphery intruding an en echelon fracture array. Dilation of individual segments, or fingers, occurred simultaneously. Subsequent interaction of near-tip stresses induced inwardly propagating curvature of adjacent segments in the array. Resultant linkage has produced a stepped-sill geometry; sill propagation and flow directions were orthogonal to the plane of linkage.

The flow direction is confirmed by shape preferred-orientations of acicular mineral grains within the chilled margins of the sills, indicating the direction of flow to be perpendicular to the plane of the en echelon array, and parallel to strike directions of offset surfaces that link adjacent sill segments.

Multiple dyke intrusion is examined at an outcrop of the Rooi Rand Dyke Swarm, along the Pongola River. Individual intrusive episodes are identifiable on the basis of chill-zone relationships. The pattern along the Pongola River suggests that younger intrusive episodes frequently intrude through the centres of older dykes. A three-dimensional analysis of en echelon dykelet segments allows a re-construction of the dilation history, and provides an explanation for the development of blunt-ended intrusion segments. Mineral geochemistry anomalies around dyke tips suggests possible facilitation of incipient fracture via decreases in mineral strengths manifested by geochemical changes. A statistical digital analysis of micro-phenocryst orientations within chilled dyke margins is shown to provide a viable method to ascertain magma flow directions within dykes, and may thus be a useful tool for future investigations.




Comments


This abstract describes the research undertaken for an M.Sc. degree at the University of Natal (Durban), South Africa. The work follows on from an Honours thesis produced in 1991, and seven months of researching from January to July, 1992. The thesis was submitted after an additional month of work in June 1994, and the M.Sc. degree was awarded in November 1994.



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Citations:


This thesis has been cited in the following works:


Watkeys, M.K., Sokoutis, D., 1998. Transtension in southeastern Africa associated with Gondwana breakup. In: Holdsworth, R.E., Strachan, R.A., Dewey, J.F., eds., Continental Transpressional and Transtensional Tectonics, GEOLOGICAL SOCIETY OF LONDON, SPECIAL PUBLICATIONS 135: 203-214.


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