Acadian Orogeny

The Acadian orogeny has historically been one of the most poorly understood aspects of the regional geology of the Appalachians. Some of the major problems in interpreting the Acadian orogeny include understanding the nature of pre-Acadian, post-Taconic basins such as the Kearsarge-Central Maine basin, Aroostook-Matapedia trough, and the Connecticut Valley-Gaspe trough. The existence and vergence of Acadian subduction zones is debated, and the relative amount of post-Acadian strike-slip movements is not well constrained.

Examining the regional geology of the northern Appalachians using only the rocks that are younger than the post-Taconic unconformity yields a picture of several distinctive tectonic belts including different rock types and structures. The North American craton includes Grenville gneisses and Paleozoic carbonates. The foreland basin includes a thick wedge of Devonian synorogenic clastic rocks, such as the

Catskill Mountains, that thicken toward the mountain belt. The Green Mountain anticlinorium is a basement thrust slice, and the Connecticut Valley-Gaspe trough is a post-Taconic basin with rapid Silurian subsidence and deposition. The Bronson Hill-Boundary Mountain anticlinorium (Piscataquis volcanic arc) is a Silurian-mid-Devonian volcanic belt formed along the North American continental margin. The Aroostook-Matapedia trough is a Silurian extensional basin, and the Miramichi massif represents remnants of a high-standing Ordovician (Taconic) arc. The Kearsarge-Central Maine basin (Merrimack trough) preserves Silurian deep-water sedimentary rocks, preserved in accretionary prisms, and is the most likely site where the Acadian ocean closed. The Fredericton trough is a continuation of the Merrimack trough, and the Avalon Composite terrane (coastal volcanic arc) contains Silurian-Early Devonian shallow marine volcanics built upon Pre-cambrian basement of Avalonia.

Synthesizing the geology of these complex belts, the tectonics of the Acadian orogeny in the Appalachian Mountains can be summarized as follows. The Grenville gneisses and some of the accreted Taconic orogen were overlain by a Paleozoic platform sequence, and by mid-Devonian times the region was buried beneath thick clastics of the Acadian foreland basin, best preserved in the Catskill Mountains. Nearly two miles (3 km) of fluvial sediments were deposited in 20 million years, derived from mountains to the east. Molasse and red beds of the Catskills once covered the Adirondack Mountains, as evidenced by pieces preserved in a diatreme in Montreal. These features are exposed along strike as the Old Red Sandstone in Scotland and on Spitzbergen Island.

The Connecticut Valley-Gaspe trough is a complex basin developed over the Taconic suture that was active from Silurian through Early Devonian. It is an extensional basin containing shallow marine sedimentary rocks and may have formed from oblique strike-slip after the Taconic collision, with subsidence in pull-apart basins. The Aroostook-Matapedia trough is an Ordovician-Silurian turbidite belt, probably a post-Taconic extensional basin, and perhaps a narrow oceanic basin.

The Miramichi massif contains Ordovician arc rocks intruded by Acadian plutons, and is part of the Taconic arc that persisted as a high area through Silurian times and became part of the Piscataquis volcanic arc in Silurian-Devonian times. The coastal volcanic arc (Avalon) is exposed in eastern Massachusetts though southern New Brunswick and includes about 5 miles (8 km) of basalt, andesites, rhyolite, and deep and shallow marine sediments. It is a volcanic arc that was built on Precambrian base ment that originated in the Avalonian or Gondwana side of the Iapetus Ocean.

The Kearsarge-Central Maine basin (Fredericton trough) is the location of a major post-Taconic, pre-Acadian ocean that closed to produce the Acadian orogeny. It contains polydeformed deepwater turbi-dites and black shales, mostly Silurian. The regional structural plunge results in low grades of metamor-phism in Maine and high grades in New Hampshire, Massachusetts, and Connecticut. There are a few dismembered ophiolites present in the belt, structurally incorporated in about 3 miles (5 km) of turbidites.

Volcanic belts on either side of the Merrimack trough are interpreted to be arcs built over contemporaneous subduction zones. In the Late Silurian, the Acadian Ocean basin was subducting on both sides, forming accretionary wedges of opposite vergence, and forming the Coastal and Piscataquis volcanic arcs. The Connecticut Valley-Gaspe trough is a zone of active strike-slip faulting and pull-apart basin formation behind the Piscataquis arc. In the Devonian, the accretionary prism complexes collide, and west-directed overthrusting produces a migrating flexural basin of turbidite deposition, including the widespread Seboomook and Littleton formations. The collision continued until the Late Devonian, then more plutons intruded, and dextral strike-slip faulting continued.

Acadian plutons intrude all over the different tectonic zones and are poorly understood. Some are related to arc magmatism, some to crustal thickening during collision. Late transpression in the Carboniferous includes abundant dextral strike-slip faults, disrupted zones, and formed pull-apart basins with local accumulations of several miles of sediments. About 200 miles (300 km) of dextral strike-slip offsets are estimated to have occurred across the orogen.

The Late Paleozoic Alleghenian orogeny in the Carboniferous and Permian included strong folding and thrusting in the southern Appalachians, and formed a fold/thrust belt with a ramp/flat geometry. In the southern Appalachians the foreland was shortened by about 50 percent during this event, with an estimated 120 miles (200 km) of shortening. The rocks highest in the thrust belt have been transported the farthest and are the most allochthonous. At the same time, motions in the northern Appalachians were dominantly dextral strike-slip in nature.

In the Late Triassic-Jurassic, rifting and normal faulting were associated with the formation of many small basins and the intrusion of mafic dike swarms related to the opening of the present day Atlantic Ocean.

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