Multiple successions of foliation development in shallow versus deep orogenic regimes and their relationship to regional folding

Abstract

Fold axial plane traces and schistosity in the high temperature-low pressure regime PreCambrian rocks in Arkansas River region preserve the effects of a succession of 5 FIA trends (foliation intersection or inflection axes in porphyroblasts). Similar fold axial plane trace measurements in high temperature-medium pressure regime Palaeozoic rocks in Vermont preserve the effects of only the youngest two NNW-SSE and NNE-SSW trending FIAs of the 5 sets that developed in this region. The other three have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks tends to flatten folds preserving only those most recently developed or the folds.