Acadian orogeny
During the time of the Acadian orogeny, Middle Devonian (385 Ma), the paleolatitude of the Laurentia was in the southern hemisphere near the equator, between 0° to 30°S latitude.
[7] However, more recent research, from Scotese & McKerrow, suggests that in Late Devonian, the South Pole was in north-central Argentina rather than northern Africa, which was supported with paleoclimatic evidence.
[7] The collision initiating the Acadian orogeny resulted in the closing of the southern Iapetus Ocean and the formation of a high mountain belt.
[7] Major defining tectonic events include the Neoproterozoic rift sequence from the breakup of Grenville basement rocks, thermal subsidence related to the Early Cambrian to Middle Ordovician drift sequence during the opening of the Iapetus Ocean, the Appalachian accretionary events to the eastern continental margin, and the resulting foreland-basin and clastic wedges.
[6] Avalonia rifted from Gondwana during the onset of igneous activity in the Ardennes, Wales, and southeast Ireland that consumed the Tornquist Sea oceanic crust.
In the central to southern Appalachians, evidence for the Acadian orogeny is poor and is found primarily in the plutonism of the Blue Ridge and metamorphism of the Cat Square terrane.
[13] 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.
The southward migration of deformation reflected the third tectophase, which marks the collision of Avalon terranes with the Virginia promontory in the Middle to Late Devonian time.
[9] As the migration of deformation continued southward along the fault zone, during the Early Mississippian time, the final collision occurred with the Alabama promontory .
[14] Subsidence follows bulge movement and uplift and is produced on the cratonic side of the orogen due to regional isostatic adjustment to the load by the lithosphere.
[4] Once thrust propagation declines, substantial relief and drainage nets have had time to develop, and the as a resulting coarser clastic sediment is eroded and transported to the foreland basin.
These deposits extend from central New York and Pennsylvania westward to Ohio, and south along the Appalachian Mountains through Virginia and Tennessee to Alabama.
The Catskill paleogeography appears to consist of many small streams, which deposited their sedimentary load along a coastal alluvial plain that was hundreds of miles long.
As collision intensified, subsidence in the foreland basin declined, and sedimentation was replaced by an influx of calcareous silty shales and carbonates.
