Figure 1 Global paleogeographic reconstruction of the Earth in the middle Ordovician period 470 million years ago. Dr. Ron Blakey, http://jan.ucc.nau.edu/~rcb7/
The Iapetus Ocean was a Southern Hemisphere Ocean that existed between the continents of Laurentia, Baltica, and Avalonia.
Because the Ocean existed between the continents that would later become the Atlantic, the Ocean was named Iapetus, the
father of Atlas in Greek mythology.
Around 800 million years ago the supercontinent Rodinia contained the cratons that would later form the continents that would bound the Iapetus. Although the exact reason is unknown, intracratonic basins began forming during the Cryogenian due to lithospheric extension. Around this same time, 670-650 mya basaltic dykes were forming in Scandanavia. This is thought to be the signaling of the beginning of rifting in Rodinia and the birth of the Iapetus Ocean.
The ocean was bounded by Laurentia to the west, Baltica to the east, and Gondwana to the south from which a parade of microcontinents drifted out. In the southwest of the ocean, a subduction zone formed between two oceanic crustal plates. A volcanic arc developed above the subduction zone and was pulled northward across the Iapetus Oceanic Basin. This would become the Taconic volcanic arc and was heading to Laurentia.
The Taconic Orogeny occurred when the Taconic island arc in the Iapetus Ocean collided with Laurentia (Proto-North America). The Taconic lands were pulled up onto Larentia as the continent was subducted below the arc. The lands of the arc rode up onto the contenent, deformed, and would become Western New England, the Canadian Maritimes, as well as portions of the Eastern seaboard southward to North Carolina.
Figure 1 Taconic land rides up on Laurentia while the crust of Laurentia is partially subducted below the island arc.
The Taconic Orogeny occurred in multiple pulses. This occurred because Laurentia was not a smooth coast but had several promontories, those promontories being hit hardest by the oncoming landmass. The Taconic Arc also came in from the southeast hitting lands that would become Virginia millions of years before Pennsylvania. This resulted in inconsistent mountain building throughout the Eastern Seaboard. While Southern Virginia and Pennsylvania both experienced major deformation and thrust mountain building, areas in between were sheltered by being removed from the front and experienced gentle folding.
As Virginia experienced the first pulse of the Taconic a thrust belt was pushed up over the subducting Laurentide plate. Additional thrust faults were activated such as the Haysville-Fries-Rockfish Valley fault system which originated during the Grenville Orogeny. A foreland basin was created inland from the up thrust in Virginia as was another when the Taconic reached Pennsylvanian lands; each pulse of the Taconic, as it hit the Laurentide landmass initiated another thrust and foreland basin.
Figure 2 The foreland basin system, reversed from East Coast orogonies.
At the end of the Taconic, the mountains eroded immature sediment into the foreland basins. The orogeny had ended as the immature sediment, easily weathered particles and feldspars, decreased in availability. No new upthrusting was creating additional erodible material. Beyond this point we see mature, highly erosion resistant rock particles such as quartz layered above the immature layers of the orogeny. This is the signal of the end of the orogeny as the mountains have been eroded into stability and the foreland basins have filled, allowing uniformity in the layering of the mature layers.
A case study of the Taconic in North America is covered by fellow Emporia State student R. W. Shepherd in his examination of Betts Cove Ophiolite.
The Avalonian micro-continent docked with Baltica around 440 mya. As Avalonia continued its northwestern movement and Baltica progressed westward, the Iapetus crust subducted under Laurentia bringing both landmasses not only together but into Laurentia. Baltica was the first to hit Laurentia initiating the Caledonia Orogeny.
The Caledonian Orogeny consisted of three European pulses, the Grampian phase ran from approximately 480-465 mya, the intermediate followed around 465-435 mya, and finally the Scandian phase, 432-390 mya.
Literature on the Caledonian Orogeny can be problematic in that there seems to be little agreement on what exactly is the Caledonian Orogeny. McKerrow et al have included all orogonies involved in the closure of the Iapetus as a part of the Caledonian event, while others have assigned this to only the orogenic pulses that created the Northern British highlands, specifically the closure of the Northern Iapetus from the collision of Baltica with Laurentia. The divide appears to be based in part on which side of the Atlantic the writer is looking from, with British geologist being inclusive and American authors being exclusive. The collision forming the Grampian Highlands in Scotland was followed by the subsequent Acadian Orogeny which constituted the southern microcontinent of Avalonia's collision with Laurentia. While the Acadian and Caledonian were linked in movement and cause, the effects were not exactly coincedent nor were they acting on the same cratonic structures therefore it seems fair to catagorize the orogenies as seperate events.
An closer examination of the Caledonian Orogeny is discussed in a presentation by fellow Emporia State student Scott Jones in The Highland Boundary Fault of Scotland.
The lands of the Avalonian micro-continent became the eastern portions of Maine, Massachusetts, Connecticut, and all of Rhode Island as well as parts of South Carolina, the Maritimes, parts of Western Europe, and Morocco.
Figure 3 Avalon advances on Laurentia (notice the partial subduction of Laurentia under Taconian land to the west).
The Avalonian micro-continent collided into Laurentia obliquely moving along the eastward dipping subduction zone creating the most folding and thrusting in the Catskill region as well as Newfoundland and the Maritimes. This was the end of the Iapetus Ocean as water behind Avalon (or Armorica) was known as the Rheic Ocean. Later when the proto-Atlantic would rift out of this region, the Mid-Atlantic east of Washington DC to the continental shelf remained as Avalonian terrane sutured to Laurentia while the Western portions became the southern British Isles and portions of Europe. Again foreland basins were created and sedimentary fill from mountain erosion leaves the history of these orogenies.
The closure of the Iapetus was an important event in the geologic history of North America and Europe. While the mountains of the era are no longer prominent spires over the eastern coast of America, the legacy of this phase of mountain building continues to be seen. New England, the Mid-Atlantic and the Maritimes are exotic terranes gifted to Laurentia. The carbonate and silicate sedimentary layers of the foreland basins now sit on mountaintops after the folding of the Appalachian orogeny that came after these orogenic events.
The Atlantic would open through rifting in the area near the former location of the Iapetus in relation to the bounding cratons. This rifting leads into the current expansion of the Atlantic Basin. Thus we have a brief glimpse into the most recent history of the Laurentian craton boundary in respect to the continuing Wilson Cycle.
The Geological Evolution of the Virginia and the Mid-Atlantic Region, website, accessed 4/03/09. http://csmres.jmu.edu/geollab/vageol/vahist/
Geological History of Jamestown, RI, website, accessed 4/03/09. http://www.jamestown-ri.info/geological_history.htm
Mountain Building in Scotland, Open University text online, accessed 4/03/09. http://openlearn.open.ac.uk/course/view.php?id=3480
USGS NYC Regional Geology; The Highlands Province, website, access 4/03/09. http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html
Figure 1 - Originally Dr. Ron Blakey, http://jan.ucc.nau.edu/~rcb7/,This file was retrieved via wikimedia commons and is licensed under the Creative Commons Attribution ShareAlike 3.0, modified by Michael Lewis.
Figure 2 - This file was retrieved via wikimedia commons and is released into the public domain without restriction.
Figure 3 - Copyrighted, Lynn S. Fitcher & Steve J. Baedke © 1999, Dept. of Geology and Environmental Studies, James MAdison University. May be used by permission of authors for personal or educational purposes with acknoledgement.