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Taconic orogeny
The Taconic orogeny was a mountain building period that ended 440 million years ago and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont off the East coast of the United States. As the mountain chain eroded in the Silurian and Devonian periods, sediments from the mountain chain spread throughout the present-day Appalachians and midcontinental North America.
==Development==
Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to close. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to fold gradually downward.〔 In this manner, shallow-water carbonate deposition that had persisted on the continental shelf margin through Late Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. In 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.〔 Dewatering of the down-going plate led to hydration of the peridotites in the overlying mantle wedge, lowering their melting point. This led to partial melting of the peridotites within the mantle wedge producing magma that returned to the surface to form the offshore Taconic (or Bronson Hill) island arc.
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 Taconic Orogeny.〔 Cameron's Line is the suture zone that is modern-day evidence of the collision of the island arc and the continent. Cameron's Line winds southward out of New England into Western Connecticut and passes through southern New York across the Bronx, following the general trend of the East River. It extends beneath sedimentary cover on Staten Island and southward beneath the Coastal Plain of New Jersey. In general, basement rocks to the west of Cameron's Line are regarded as autochthonous, meaning that they have not been significantly displaced by tectonic processes. The rocks to the west of Cameron's Line include metamorphosed sedimentary material originally comprising ancient continental slope, rise, and shelf deposits. The rocks to the east of Cameron's Line are allochthonous, which means they have been shoved westward over autochthonous basement rocks on the order of many tens or even hundreds of kilometers. These rocks were originally deposited as sediments in a deep water basin. Cameron's Line represents the trace of a subduction zone that ceased when the Taconic island arc collided with, and became accreted onto, the eastern margin of North America. Many of the rocks east of Cameron's Line were once part of the floor of the Iapetus Ocean.〔
When the Taconic Orogeny subsided 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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