Frontal vs. basal accretion and contrasting particle paths in metamorphic thrust belts

Fold-and-thrust belts at the Appalachian and Grenvillian orogenic fronts developed in similar continental-margin tectonic settings but show marked contrasts in the relative timing of thrust stacking and metamorphism. In the Corner Brook I-ake thrust belt of the Appalachian orogen in Newfoundland, thrust stacking took place at shallow-crustal levels and was followed by the development of a foliation under increasing metamorphic conditions. The peak of metamorphism post-dated thrusting and overprinted the foliation. In the Grenvillian Gagnon thrust belt in western Labrador, development of a foliation and the stacking of the thrust sheets occurred at near-peak metamorphic conditions at upper- to middle-crustal levels. These contrasting deformational and metamorphic histories reflect different modes of accretion in metamorphic thrust wedges. In the Corner Brook Lake thrust belt, rocks were accreted at the toe of the wedge (frontal accretion) and subsequently were buried by deformation of the wedge. In the Gagnon belt, thrust sheets were accreted at the base of the wedge (basal accretion), after burial beneath the advancing wedge. The two examples demonstrate how basic field observations and petrographic tools can be used to determine the mode of accretion within a metamorphic thrust wedge.