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New insight into the nature of the Acadian orogeny in eastern Maine has been gained by combining detailed field studies in six lithotectonic belts with geochemical data from the igneous rocks of the region. Revised stratigraphies and deformation histories of the tracts reveal their sedimentological and structural evolution from Ordovician through Early Devonian times, and variations in the isotope geochemistry of the igneous rocks permit delineation of the basement blocks beneath the supracrustal belts. Combined, these results yield a model for plate interactions that followed Taconian deformation and culminated in the Acadian orogeny.Large basins (e.g., Aroostook-Matapedia, Central Maine) formed immediately after the Taconic orogeny on the recently accreted eastern margin of ancestral North America. These filled with thick clastic sequences derived from post-Taconian highlands during Late Ordovician through at least Middle Silurian times and characteristically preserve complex facies patterns at their margins. At the same time, sedimentation continued in the Fredericton Trough, inferred to be the only remaining oceanic crust in the region. This ocean basin separated the composite North American terrane from an equally complex Avalonian continent. Closing of this basin resulted in the Acadian orogeny.The onset of the Acadian suturing of Avalon to North America is indicated by a change from local basin filling to a more homogeneous blanket of sandstones whose deposition appears to have begun in the east (Flume Ridge Formation) and migrated westward. Collision of basement blocks led first to westward thrusting of parts of the Avalonian continent over the Fredericton belt. Later Acadian thrusting caused by final collision between Avalon and ancestral North America transported supracrustal Miramichi belt strata eastward over the Fredericton belt and parts of the Fredericton belt eastward over the western edge of the Avalonian allochthon. Acadian thrusting has displaced the original boundaries between supracrustal belts in southeastern Maine so that they no longer coincide with boundaries between the basement blocks that originally lay beneath them.

From studies of structure, metamorphism, and geochronology we have evidence that the Norumbega Fault Zone represents a transition from mid-to shallow crustal levels in a dextral, transcurrent shear zone within the northern Appalachian Orogen. The Norumbega Fault Zone strikes parallel to the orogen (northeast-southwest), is ~5 to 30 km wide, and is characterized by distributed ductile dextral shear fabrics in the southwestern section with a transition to brittle fabrics toward the northeast. Within the zone of distributed shear, deformation is partitioned into local zones of very high strain. Upright, isoclinal folds are common in areas of high and low strain. Metamorphic grade decreases from amphibolite facies in the southwest to sub-greenschist facies in the northeast. ^®Ar/^Ar mineral ages from recrystallized minerals in a high strain zone, regional cooling ages in areas of lower strain, and metamorphic textures are consistent with a polyphase history of deformation. We inter pret a younging trend in ^A r /^A r cooling ages toward the northeast, together with the deformational fabrics and metamorphic features, to represent exhumation of the southwestern section of the Norumbega Fault Zone from mid-crustal levels during the polyphase history of this transcurrent zone. The Norumbega Fault Zone may there fore serve as a model for deformational processes at mid-to shallow crustal levels in active strike-slip systems.Des etudes de la structure, du metamorphisme et de la geochronologie nous fournissent la preuve que la zone faillee de Norumbega represente une transition des niveaux crustaux moyens a des niveaux peu profonds dans une zone de cisaillement transversal dextrale a 1'interieur de la partie septentrionale de l'orogene appalachien. La zone faillee de Norumbega suit une direction parallele a l'orogene (nord-est-sud-ouest); elle a cinq a 30 km de largeur; et elle est caracterisee par des structures cisaillees dextrales deformables se ramifiant dans la section sud-ouest avec une transition a des structures cassantes vers le nord-est. A 1'interieur de la zone de cisaillement ramifie, la deformation est divisee en zones locales de tres forte contrainte. Les plis droits isoclinaux sont courants dans les secteurs de forte et faible contrainte. L'intensite du metamorphisme decrott d'un facies a amphibolite dans le sud-ouest a un facies secondaire de schistes verts dans le nord-est. Les periodes min^rales ^A r /^A r j es mineraux recristallises dans une zone de forte contrainte, les periodes de refroidissement regional dans les secteurs de faible contrainte et les textures metamorphiques correspondent a une orogenese polyphasee. Nous interpretons la tendance de rajeunissement dans les periodes de refroidissement ^®Ar/^Ar vers le nord-est, de meme que les textures de deformation et les caracteristiques metamorphiques, comme une exhumation de la section sud-ouest de la zone faillee de Norumbega depuis les niveaux crustaux moyens pendant l'histoire polyphasee de cette zone transversale. La zone faillee de Norumbega pou...