The geology of the country round the south-eastern end of Lake Malawi with particular reference to the evolution of the Malawi Rift

Abstract

The thesis presents results of a study of over 900 square miles of country round the southern end of Lake Malawi, the geology of which was previously unknown. The complex structure of the Basement series of crystalline rocks which are exposed on either flank of the Rift results from two episodes of deformation producing superimposed sets of folds with NNW-SSE and NE-SW trending axial planes.These rocks, which are probably largely of sedimentary origin, show a common imprint of granulite facies metamorphism, but the widespread occurrence of disequilibrium textures and biotite in them shows them to have suffered retrogressive changes.The retrograde changes are associated with a growth of microperthitic felspar, extreme development of which gives rise to palingenetic perthositic syenites and quartz-syenites. These syenites are broadly syntectonic with the second deformation; an alkali metasomatism leading to syenitisation, reactivation and partial mobilisation of a pre-existing crystalline Basement is thus indicated. The repeatedly folded polymetamorphic Basement rocks have been intruded, apparently under epeirogenic conditions, by later series of microcline-granites and alkali-syenites. Recent work on radiometric dating has shown that episodes of intrusion of microcline-granites occurred in the region of the Malawi Rift in the Lower Silurian and the Middle Permian, and of alkali-syenites in the Lower Cretaceous, and correlation of the Fort Johnston intrusions with these events is suggested. An account of the minor intrusions of the area is also included.The marked structural asymmetry of this part of the Rift is demonstrated, and the Rift faulting and its relation with the pre-existing structures is examined and discussed.A description of the physiography of the area is given which includes the south-eastern extremity of the Lake, its basin, its shorelines, and its only outlet, which lies within the area mapped.