Taconic orogeny
The Taconic orogeny was a mountain building period that ended 440 million years ago (Ma) and affected most of modern-day New England. A great mountain chain formed from eastern Canada down through what is now the Piedmont of the east coast of the United States. As the mountain chain eroded in the Silurian and Devonian periods, sediment spread throughout the present-day Appalachians and midcontinental North America.[1]
New England and Canada
Beginning in
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 metamorphism. This was the final episode of the Taconic orogeny.[1] Cameron's Line is the suture zone that is modern-day evidence of the collision of the island arc and the continent.[3] 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.[2]
When the Taconic orogeny subsided during the late Ordovician (about 440 Ma), 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 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 (about 360 Ma) when the next period of mountain-building began, the Acadian orogeny.[1]
Southern Appalachians
In the southern Appalachians of Alabama, Georgia, and North Carolina, the Taconic orogeny was not associated with collision of an island arc with ancient North America (Laurentia). Geologists working in these areas have long puzzled over the "missing" arc terrane typical of Taconic-aged rocks in New England and Canada.[4] Instead, the Iapetus margin of this part of Laurentia appears to have faced a back-arc basin during the Ordovician, suggesting that Iapetus oceanic crust was subducted beneath Laurentia—unlike the New England and Canadian segments of the margin, where Laurentia was on the subducting plate.[5][6]
In contrast to the Ordovician geologic history of New England, rocks in Alabama, Georgia, Tennessee, and North Carolina—including those of the Dahlonega gold belt[7] (Georgia and North Carolina), Talladega belt[8][9] (Alabama and Georgia), and eastern Blue Ridge (Georgia, Tennessee, and North Carolina)—are not typical of a volcanic arc in its strictest sense.[10] Instead, these rocks have geochemical and other characteristics typical of back-arc basins, which form behind the volcanic arc on the overriding plate.[5][6] The presence of these early-middle Ordovician (480 - 460 million year old) back-arc basin rocks in direct or faulted contact with rocks of the Laurentian shelf and slope-rise in the southern Appalachians suggests they were built on the margin of Laurentia, beyond the edge of the continental shelf-slope break.[9]
In the southern Appalachians, the Ordovician Laurentian margin probably resembled that seen in the modern Sea of Japan, with the continental mainland separated from a volcanic arc by a narrow, "marginal" seaway. Other lines of evidence supporting a back-arc, Sea of Japan-style tectonic model for the Taconic orogeny in the southern Appalachians include mixing of Ordovician and Grenville (ca. 1 billion year old) detrital zircons in metamorphosed sedimentary sequences, and interlayering of metamorphosed Ordovician volcanic rocks with sedimentary rocks derived from the Laurentian margin.[5][6]
Relation with the Famatinian orogeny
It has been suggested that the coeval
Aftermath
As the Taconic orogeny subsided in
Through Silurian time, the deposition of coarse
See also
- Ebenezer Emmons, the geologist who first described the Taconic system
- Queenston Delta, a clastic wedge of sediment deposited west of the Taconics at that time
Notes
- ^ In other words what is at present the southern end of the Taconic orogeny would have been connected with what is currently the northern end of the Famatinian orogeny.
References
- ^ a b c
This article incorporates public domain material from Valley and Ridge Province. United States Geological Survey.
- ^ a b c d e This article incorporates public domain material from The Highlands Province. United States Geological Survey.
- ^ Schneider, Daniel B. (August 22, 1999). "F.Y.I." New York Times. Retrieved 1 May 2010.
- ISSN 0072-1069.
- ^ ISSN 0016-7606.
- ^ ISSN 2333-0937.
- ISSN 0361-0128. Archived from the originalon 2019-09-13. Retrieved 2016-08-21.
- S2CID 130204532. Archived from the original(PDF) on 2020-02-20.
- ^ ISSN 2333-0937.
- ISSN 0016-7606.
- S2CID 128856492.
- hdl:11336/93714.
