Gravitational collapse and folding during orogenesis: A comparative study of FIA trends and fold axial plane traces

Abstract: The FIA trends (foliation intersection or inflection axes in porphyroblasts) measured from Siluro-Devonian garnet sillimanite gneiss in Rangeley, Appalachians, when plotted on a rose diagram, reflect the trends of axial planes of all map scale folds. Similar FIA trends data measured in high temperature-low pressure regime Precambrian rocks in the Texas Creek, Cordillera, are reflected by the fold axial plane traces and schistosity data in this region. But the fold axial plane trace data, which were measured in high temperature to medium pressure regime Palaeozoic rocks in Chester Dome, 200 km southwest of Rangeley, only preserve the effects of the youngest FIA of the all 5 FIA sets that were obtained in this region. This difference of the orogenic regimes from the shallow part to the depth reflects decreasing competency contrasts under greater pressures with collapse and unfolding of earlier formed folds. This investigation shows that the greater overlying loads of rocks flatten folds more obviously under higher pressures.