The Bushveld Igneous Complex Of The Central Transvaal

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Download or read book Interference-folding of Bushveld-Igneous-Complex Age in the Transvaal System North of Marble Hall, Transvaal written by and published by . This book was released on 1969 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A geological investigation of the northern half of the Marble Hall Fragment revealed that the major geological feature is a block of folded metasediments of the Transvaal System, enclosed in the Bushveld granite. Irregular bodies of diorite and albite diorite, classified with the upper portion of the Main Plutonic Phase of the Bushveld Igneous Complex, and a sheet of pyroxene granulite of the Maruleng type (?) intruded in the Transvaal System. The Karroo System is represented by remnants of horizontal beds of the Ecca Series. A few dolerite dykes encountered in the mapped area are also grouped with the Karroo System. The interference-folding is the result of two successive stages of deformation. The first phase of deformation is believed to have taken place either just before or in the initial stages of the emplacement of the Main Plutonic Phase. It probably marked the first stages of instability of the floor of the Transvaal System before emplacement of the Complex. The deformation took place by a mechanism of shear-folding, and a series of north-south-trending folds ensued. Of these, the most notable was the Swartkop-Marble Hall Anticline that was overfolded to the west. The intensity of all these folds apparently decreased to the south and the west. The Swartkop- Marble Hall Anticline is considered to be the continuation of the Dennilton Plunging Anticline east of Dennilton. The folding took place at deep levels, presumably under high load pressures, so that the formation of b-lineations ensued. The second phase of deformation took place either during the period of emplacement of the Main Plutonic Phase or, less probably, during the emplacement of the Bushveld Granite. The mechanism was most probably that of oblique flexure-slip, and the geometrical arrangement that of a conical fold. This pattern of folding led to tension-faults in the outer limb of the Swartkop-Marble Hall Anticline and to thrust-type faulting in the core areas. The diorite and related rocks intruded after, or towards the end of, the second phase of deformation and caused widespread sodium metasomatism. Simultaneously, a medium grade of thermal metamorphism affected the country-rock. Finally, the Bushveld Granite enveloped the complex of folds and intrusions to give the present structure of the Marble Hall Fragment. At first, high-grade thermal metamorphism was superimposed on the earlier metamorphic phenomena, but later, regional retrogressive metamorphism followed.