Fold-and-thrust belts at the Appalachian and Grenvillian orogenic fron
ts developed in similar continental-margin tectonic settings but show
marked contrasts in the relative timing of thrust stacking and metamor
phism. 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 m
etamorphic conditions. The peak of metamorphism post-dated thrusting a
nd 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- t
o middle-crustal levels. These contrasting deformational and metamorph
ic histories reflect different modes of accretion in metamorphic thrus
t 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 a
ccreted at the base of the wedge (basal accretion), after burial benea
th the advancing wedge. The two examples demonstrate how basic field o
bservations and petrographic tools can be used to determine the mode o
f accretion within a metamorphic thrust wedge.