Abstract
Archean gneisses in part of West Greenland mainly formed from sheets of tonalite and granodiorite emplaced into more mafic volcanic and plutonic rocks. Sheet emplacement was syntectonic and associated with dilation and thrusting, and it led to considerable and irreversible thickening of the continental crust. Large recumbent nappelike folds of these rocks were compressed and refolded on steep axes while still ductile. They were then deformed by steep shear zones as they dried out, stiffened, and became part of a stable craton about 2,850 m.y. ago. Although similar in composition to many Phanerozoic intrusions, these Archean granitoid sheets appear to be unusually extensive. An analog for their generation is the present tectonic setting of the Himalayas and Tibetan plateau.
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