Nature of the Acadian Orogeny in eastern Maine

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.