The photogeological recognition of ancient orogenic belts in Africa

Abstract

Photogeological interpretation, assisted by some ground control, has been used to delineate areas of the crystalline basement in which structures formed during a particular tectonism are predominant. Such areas are referred to as 'tectonic domains'. They are recognized principally by distinctive tectonic style, by characteristic trend and axial direction, by metamorphic grade and by the sequence ot imposition of tectonisms. These criteria may be supported by geophysical data. It is unusual for all the criteria to be satisfied photogeologically, but with some ground information it is possible to draw maps which show the distribution of tectonic domains. The tectonic domain map of Karamoja, north-east Uganda, shows four domains: an early granulite facies (charnockitic) east-striking domain ( A ); a second, north-striking, more widely developed amphibolite facies domain ( B ) in which A structures are reworked and partly obliterated; a C domain which cuts across the A and B domains in a series of straight, penetrative and partially cataclastic, north-west-trending structural belts which are parallel to the Aswa Zone of central Uganda; and the Karasuk domain of recumbent flaggy folds named after the Karasuk Group. Each domain is laterally bounded, replaced or 'overprinted' by another. These relationships are illuminated by those established elsewhere in northern Uganda, and the tectonic domain map is thought to depict the interplay in north-east Uganda of the Watian, Aruan and Ghuan tectonisms which may be orogenies, and to include the Mozambiquian front impinging upon older structures. The value of the method of mapping tectonic domains lies in presenting, with a minimum of field-work, something approaching the normal tectonic map and so depicting the outlines of the ancient orogenic belts. Data such as age-determinations can be attached to the tectonic domain maps.