On the issue of establishing intrusions in folded areas that preceded folding

Abstract

The problem of dividing intrusions in folded areas according to their relation to folding is one of the most difficult in geology. It is usually difficult to answer the question of the time of intrusion emplacement - before, during or after folding. Particularly difficult is the identification of intrusions in folded areas that were intruded into the host rocks before folding and, as a result of the latter superposition, even if their relatively older age is established in comparison with the following intrusions, can be taken for earlier manifestations directly related to the same folding. An exceptional feature of intrusions, the formation of which occurs before folding, is that, being located in quietly lying layered deposits, they form characteristic forms of concordant intrusive deposits, sills, which practically appear in the layered structure as intrusive layers, with all the ensuing consequences. It is precisely this circumstance that gives us a real opportunity to recognize such intrusions in the folded structure. The factual material of this article gives grounds to conclude that special attention must be paid to basic and ultrabasic intrusions from the point of view of the time of their appearance in relation to folding.