Taconic Orogeny

The Taconic allochthons are a group of Cambrian through Middle ordovician slates resting alloch-thonously on the Cambro-ordovician carbonate platform. These allochthons are very different from the underlying rocks, implying that there have been substantial displacements on the thrust faults beneath the allochthons, probably on the order of 100 miles (160 km). The allochthons structurally overlie wild flysch breccias that are basically submarine slide breccias and mudflows derived from the allochthons.

Eastern sections of the Taconic-aged rocks in the Appalachians are more strongly deformed than those in the west. East of the Taconic foreland fold-thrust belts, a chain of uplifted basement with Grenville ages (about one billion years) extends discontinu-ously from Newfoundland to the Blue Ridge Mountains, and includes the Green Mountains of Vermont. These rocks generally mark the edge of the hinterland of the orogen and the transition into greenschist and higher metamorphic facies. some of these uplifted basement gneisses are very strongly deformed and metamorphosed, and they contain domal structures known as gneiss domes, with gneisses at the core and strongly deformed and metamorphosed Cambro-Ordovician marbles around their rims. These rocks were deformed at great depths.

Also close to the western edge of the orogen is a discontinuous belt of mafic and ultramafic rocks comprising an ophiolite suite, interpreted to be remnants of the ocean floor of the Iapetus Ocean that closed during the Taconic orogeny. spectacular examples of these ophiolites occur in Newfoundland, including the Bay of Islands ophiolite complex along Newfoundland's western shores.

Taconic Orogeny
Tectonic zone map of northern Appalachians showing the Early Paleozoic tectonic terranes

Further east in the Taconic orogen are rocks of the Bronson Hill anticlinorium or terrane that are strongly deformed and metamorphosed and have been affected by both the Taconic and Acadian orogenies. These rocks have proven very difficult to map and have been of controversial significance for more than a century. Perhaps the best interpretation is that they represent rocks of the Taconic island arc that collided with North America to produce the Taconic orogeny.

The Piscataquis volcanic arc is a belt of Devonian volcanic rocks that extends from central Massachusetts to the Gaspe Peninsula. These rocks are roughly coextensive with the Ordovician arc of the Bronson Hill anticlinorium and include basalts, andesites, dacites, and rhyolites. Both subaerial volcanics and subaquatic pillow lavas are found in the belt. The Greenville plutonic belt of Maine (including Mount Kathadin) is included in the Pis-cataquis arc and is interpreted by some workers to be post-Acadian, but is more typical of syntectonic arc plutons. The Acadian orogeny also deformed the eastern part of the Taconic orogenic belt, which contains some younger rocks deposited on top of the eroded Taconic island arc.

The Taconic allochthons turn out to be continental rise sediments that were scraped off the North American continental margin and transported on thrusts for 60-120 miles (100-200 km) during the

Taconic arc continent collision. A clastic wedge (Austin Glen and Normanskill formations) was deposited during emplacement of the allochthons by their erosion and spread out laterally in the foreland. As Taconic deformation proceeded, the clastic wedge, underlying carbonates, and the Grenville basement became involved in the deformation, rotating them, forming the Taconic angular unconformity.

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