A critique of "orogenic trends" in archaean correlation in India

Abstract

The currently accepted correlation of the Archaean formations of India is dependent on the validity of two concepts: 1. (1) The pegmatite hosts of the analyzed radioactive minerals were emplaced in the dying phase of orogenies which were attendant with metamorphism and migmatization; and 2. (2) the suggestion by Ramsay and Sederholm that each Archaean province has a distinctive pattern of "strike", with the pattern in one province intersecting that in another. The second concept must be broadened to take into account the characteristic orogenic patterns like syntaxis-virgation, deflection and linkage. Further, the "strike" or "orogenic trend" may be defined either as the trend of the parent geosyncline, or as the tectonic trend marked by the folds. Only the latter can be determined by field studies, palinspastic reconstruction being practically impossible in India at this stage. As used in the Indian Archaeans, however, "orogenic trend" is, in the majority of cases, either the physiographic trend or the generalized strike of foliation. Except in special cases of non-plunging and/or upright folds, the strike of neither the bedding plane foliation nor the axial plane foliation would indicate the trend of folds. Examples of determinable fold trends in particular terrains oblique to the accepted orogenic trends there are presented.