page_title: Acadian orogeny
section_title: Collision
hierarchical_section_title: Acadian orogeny / Overview of orogenic events / Collision
context_page_description: The Acadian orogeny is a long-lasting mountain building event which began in the Middle Devonian, reaching a climax in the early Late Devonian. It was active for approximately 50 million years, beginning roughly around 375 million years ago, with deformational, plutonic, and metamorphic events extending into the Early Mississippian. The Acadian orogeny is the third of the four orogenies that created the Appalachian orogen and subsequent basin. The preceding orogenies consisted of the Potomac and Taconic orogeny, which followed a rift/drift stage in the Late Neoproterozoic. The Acadian orogeny involved the collision of a series of Avalonian continental fragments with the Laurasian continent. Geographically, the Acadian orogeny extended from the Canadian Maritime provinces migrating in a southwesterly direction toward Alabama. However, the Northern Appalachian region, from New England northeastward into Gaspé region of Canada, was the most greatly affected region by the collision.
  It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of the Great Basin.
context_section_description: As mentioned previously, the Acadian Orogeny formed from the oblique convergence or major transcurrent movement along a large strike-slip fault moving in a southeasterly direction.  Avalonian terranes accreted to the eastern margin of Laurentia during the Late to Middle Devonian.  This was a continent-continent collision with Baltica/Avalonia obliquely subducting below the eastern Laurentia margin, which closed the southern Iapetus ocean and created a high mountain belt.
  The collision initiated a sequence of events where the older rocks on Laurentia and Avalonia were subjected deformation, plutonism, metamorphism, and uplift that occurred over a large area of eastern Laurentia (Naylor).  During the course of the orogeny, new faults formed, while older faults were reactivated.  Acadian deformation and metamorphism were asymmetric across the strike of the orogen.  The Acadian plutons intruded every belt, unlike the deformation/metamorphism, of Avalonia which did not undergo much of the alteration shown in other localities (Bradley).  During the Middle Devonian, centers for volcanoes and uplift formed in the New England region and shed fine-grained clastic material into an inland seaway that covered a large part of southern and central Appalachia.
  Today, portions of the ancient Avalonia landmass occur in scattered outcrop belts along the eastern margin of North America. One belt occurs in Newfoundland, another forms the bedrock of much of the coastal region of New England from eastern Connecticut to northern Maine, where it is known as the Coastal Lithotectonic Block.
  The collision between Laurentia and Avalonian terranes is actually more complex than described above.  The collision is broken into three, or possibly four tectophases, which represent a successive collision of the Avalonian terranes with eastern Laurentia.
caption: The Appalachian Orogeny, a result of three separate continental collisions. USGS
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