Iapetus Ocean

The Iapetus Ocean was an ocean that existed in the Southern Hemisphere between Laurentia (Scotland and North America) and Baltica (Scandinavia) between 400 and 600 million years ago. As a sort of precursor of the Atlantic Ocean, it was named for the Titan Iapetus, who in Greek mythology was the father of Atlas.

The Iapetus Ocean and the geology of North America

The Taconic orogeny

The Taconic orogeny was a great mountain building period that perhaps had the greatest overall effect on the geologic structure of basement rocks within the New York Bight region. The effects of this orogeny are most apparent throughout New England, but the sediments derived from mountainous areas formed in the northeast can be traced throughout the Appalachian and Midcontinent regions of North America. The following discussion provides a summary of events leading to the culmination of this orogeny.

Beginning in Cambrian times (about 550 million years ago) the Iapetus Ocean began to grow progressively narrower. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to gradually fold downward. In this manner, shallow carbonate deposition that had persisted on the shelf margin through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. Sometime during this period a convergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along a subduction zone with an eastward-dipping-orientation. Partial melting of the down-going plate produced magma that returned to the surface to form the Taconic island arc offshore from the continent. By the Late Ordovician, this island arc had collided with the North American continent. The sedimentary and igneous rock between the land masses were intensely folded and faulted, and were subjected to varying degrees of intense metamorphism. This was the final episode of the long-lasting mountain-building period referred to as the Taconic Orogeny.

When the Taconic Orogeny subsided in the New York Bight region during Late Ordovician time (about 440 million years ago), subduction ended, culminating in the accretion of the Iapetus Terrane onto the eastern margin of the continent. This resulted in the formation of a great mountain range throughout New England and eastern Canada, and perhaps to a lesser degree, southward along the region that is now the Piedmont of eastern North America. The newly expanded continental margin gradually stabilized. Erosion continued to strip away sediments from upland areas. Inland seas covering the Midcontinent gradually expanded eastward into the New York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically-quiet period persisted until the Late Devonian time (about 360 million years ago) when the next period of mountain-building began, the Acadian Orogeny.

The Acadian orogeny

The Acadian orogeny is the name of a long-lasting mountain building disturbance that most greatly affected the Northern Appalachian region (New England northeastward into the Gaspé region of Canada). The "climax" of this orogeny is dated as early in the Late Devonian, but deformation, plutonism, and metamorphism related to this orogeny continued well into the Mississippian Period. The cause of this great period of deformation is a result of the plate-docking of a small continental landmass called Avalonia (named after the Avalon Peninsula of Newfoundland). The docking of Avalonia onto the margin of Laurentia resulted in the closing of a portion of the Iapetus Ocean. The Acadian Orogeny spanned a period of about 50 million years (beginning roughly 375 million years ago). During the course of the orogeny, older rocks were deformed and metamorphosed, and new faults formed and older faults were reactivated.

Avalonia was gradually torn apart as plate tectonic forces accreted the landmass onto the edge of the larger North American continent. 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 eastern Massachusetts.

The Iapetus Ocean and the geology of the British Isles

Formation

Iapetus Ocean, along with its sister ocean, Khanty Ocean, formed when the supercontinent of Proto-Laurasia rifted into three separate continents - Baltica, Laurentia, and Siberia. The ocean was situated between Laurentia and Baltica. The Iapetus Ocean began to close in the Ordovician Period and finally closed marking the end of the Silurian. Rocks of this age in present day GB include the Wenlock Limestones. The closing of the Iapetus Ocean resulted in the mountain building events of the Caledonian Orogeny. For more extensive geologic information see Ordovician paleogeography and the evolution of the Iapetus ocean. [http://www.earth.ox.ac.uk/~conallm/Iapetus.pdf]

ee also

*Avalonia
*Baltica
*London-Brabant Island
*Plate tectonics
*Geologic timescale
*Southern uplands of Scotland
*Khanty Ocean

External links

* [http://www.bbc.co.uk/education/rocks/programmes/prog1.shtml BBC Guide to Rocks: Slices of Scotland]

"Note: some material in this article has been adapted from public domain USGS material at http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm and http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html"