Vertical movement in orogenic belts and the timing of metamorphism relative to deformation

Abstract

Summary Any phase of deformation in an orogenic belt that causes relative vertical movements of bodies of rock by a few kilometres or more should have a recognizable imprint in the metamorphic history. The effect seen in a P-T-t path will depend on whether the rock body examined moved relatively up or down in the orogenic pile. An upthrown block is predicted to cool soon after the onset of movement, but the resulting P-T path will be insignificantly different to that resulting from a cycle of thickening and erosion. A downthrown block will undergo renewed heating after movement, and evidence of the earlier metamorphic history will probably be overprinted. It is suggested that the recognition of phases of relative vertical movement in an orogenic belt is best made by combined studies of deformation-metamorphism textural relationships, and P-T-t paths. The Lukmanier Pass area of the Alps illustrates the textural patterns predicted: here, a phase of deformation immediately post-dates the metamorphic peak, suggesting uplift of the area relative to the Helvetic nappes to the north.