Overview

Maine was assembled through collisions of island arc systems during Middle Ordovician time (Taconic) and Middle to Late Devonian time (Acadian). Many of the sediments that were metamorphosed such as limestone and other marine sediments are believed to have formed during Late Proterozoic accretion of the Pannotian supercontinent. At the start of the Paleozoic Cambrian Period, sedimentation and limited volcanic activity occurred in the Iaepetus Ocean basin and during Early Ordovician time. By Late Ordovician time, subsidence and re-initiation of sedimentation was followed by deformation, metamorphism, plutonism, and uplift resulting from the Taconic collisions. Limited rifting along the Laurentian margin in the Silurian continued deposition, explosive volcanism, and intrusion. Acadian uplift in Middle and Late Devonian time resulted in major deformation, metamorphism, and plutonism occurring over multiple stages with impacts to the previous Taconic uplift. Carboniferous impacts included the assembly of Gondwana to Laurentia forming Pangea with additional effects on previous metamorphism and deformation occurring. By Triassic time, Pangea rifted apart and became a region of volcanic eruptions which allowed stretching of the crust through expansion and migration of molten rock (gabbro and diabase) to reach the surface. Northern and Central Maine were dominated by northeast to southwest linear belts of Cambrian to Ordovician volcanics and related sedimentary rocks erupted on the oceanic floor. Marine sediments were accreted to Maine's continental margin and metamorphosed and deformed into gneiss, schist, and marble during Taconic collisions. Ordovician to Silurian marine sandstone and slate grades into gneiss and schist representing inter-collision sedimentation occurring between Taconic and Acadian collisions. Devonian marine sandstone and slate appear in northern and central Maine with gneiss and schist in southwestern Maine. These rocks were accreted, metamorphosed, and intruded into during the Acadian uplift. Most plutonic intrusive impacts occurred during Acadian collisions at a time when the Acadian subduction zone was segmented by transform faults during Silurian time. Horizontal dispslacement along these faults provided the driving mechanism which pushed the Acadian islands into Maine's continental margin. By Devonian time, the suduction zone was segmented which resulted in variable direction of Acadian subduction zone directions and ultimately Acadian island collisions with southeastern Maine. Mount Desert Island is an example which provides evidence for plutonic intrusion. During the Carboniferous Period, collision of Gondwana with Laurentia assembled Pangea superimposed additional metamorphic, deformational, and plutonic intrusives into the prior Taconic and Acadian mountainous terranes. By Triassic time, rifting of Pangea provided crustal expansion for volcanic eruptions of basaltic lava and intrusion of diabase dikes into granitic plutons. Maine was almost a flat plain during this time.

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