DEFINITION OF APPALACHIANS

Definition of Appalachians A mountain belt that extends for 1,600

miles (1,000 km) along the east coast of North America,

stretching from the St. Lawrence Valley in Quebec, Canada,

to Alabama. Many classifications consider the Appalachians

to continue through Newfoundland in maritime Canada, and

before the Atlantic Ocean opened, the Appalachians were

continuous with the Caledonides of Europe. The Appalachians

are one of the best-studied mountain ranges in the world,

and understanding of their evolution was one of the factors

that led to the development and refinement of the paradigm

of plate tectonics in the early 1970s.

Rocks that form the Appalachians include those that

were deposited on or adjacent to North America and thrust

upon the continent during several orogenic events. For the

length of the Appalachians, the older continental crust consists

of Grenville Province gneisses, deformed and metamorphosed

about 1.0 billion years ago during the Grenville

orogeny. The Appalachians grew in several stages. After Late

Precambrian rifting, the Iapetus Ocean evolved and hosted

island arc growth, while a passive margin sequence was

deposited on the North American rifted margin in Cambrian-

Ordovician times. In the Middle Ordovician, the collision of

an island arc terrane with North America marks the Taconic

orogeny, followed by the Mid-Devonian Acadian orogeny,

which probably represents the collision of North America

with Avalonia, off the coast of Gondwana. This orogeny

formed huge molassic fan delta complexes of the Catskill

Mountains and was followed by strike-slip faulting. The Late

Paleozoic Alleghenian orogeny formed striking folds and

faults in the southern Appalachians but was dominated by

strike-slip faulting in the northern Appalachians. This event

appears to be related to the rotation of Africa to close the

remaining part of the open ocean in the southern Appalachians.

Late Triassic-Jurassic rifting reopened the Appalachians,

forming the present Atlantic Ocean.

The history of the Appalachians begins with rifting of

the one-billion-year-old Grenville gneisses and the formation

of an ocean basin known as Iapetus approximately 800–570

million years ago. Rifting was accompanied by the formation

of normal-fault systems and grabens and by the intrusion of

swarms of mafic dikes exposed in places in the Appalachians

such as in the Long Range dike swarm on Newfoundland’s

Long Range Peninsula. Rifting was also accompanied by the

deposition of sediments, first in rift basins, and then as a

Cambrian transgressive sequence that prograded onto the

North American craton. This unit is generally known as the

Potsdam Sandstone and is well-exposed around the Adirondack

dome in northern New York State. Basal parts of the

Potsdam sandstone typically consist of a quartz pebble conglomerate

and a clean quartzite.

Overlying the basal Cambrian transgressive sandstone is

a Cambrian-Ordovician sequence of carbonate rocks deposited

on a stable carbonate platform or passive margin, known

in the northern Appalachians as the Beekmantown Group.

Deposition on the passive margin was abruptly terminated in

the Middle Ordovician when the carbonate platform was

progressively uplifted above sea level from the east, then

migrated to the west, and then suddenly dropped down to

water depths too great to continue production of carbonates.

In this period, black shales of the Trenton and Black River

Groups were deposited, first in the east and then in the west.

During this time, a system of normal faults also migrated

across the continental margin, active first in the east and then

in the west. The next event in the history of the continental

margin is deposition of coarser-grained clastic rocks of the

Austin Glen and correlative formations, as a migrating clastic

wedge, with older rocks in the east and younger ones in the

west. Together, these diachronous events represent the first

stages of the Taconic orogeny, and they represent a response

to the emplacement of the Taconic allochthons on the North

American continental margin during Middle Ordovician arccontinent

collision.

The Taconic allochthons are a group of Cambrian

through Middle Ordovician slates resting allochthonously

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 discontinuously 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.

Further east in the Taconic orogen are rocks of the Bronson

Hill anticlinorium or terrane, which 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 Piscataquis arc, and interpreted

by some workers to be post-Acadian, but is more typical of

syn-tectonic arc plutons. The eastern part of the Taconic orogenic

belt was also deformed by the Acadian orogeny and

contains some younger rocks deposited on top of the eroded

Taconic island arc, then deformed in the Acadian orogeny.

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 and underlying carbonates

and Grenville basement became involved in the deformation,

rotating them, forming the Taconic angular unconformity.

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 Kearsage–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 importance

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 highstanding

Ordovician (Taconic) arc. The Kearsarge–Central

Maine basin (Merrimack trough) preserves Silurian deepwater

sedimentary rocks, preserved in accretionary prisms, and

is the most likely site where the Acadian Ocean closed. The

Fredrickton 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 Precambrian 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 and pieces are preserved in a diatreme in Montreal,

and they 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 and 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 basement that originated in

the Avalonian or Gondwana side of the Iapetus Ocean.

The Kearsage–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 turbidites and black shales, mostly Silurian.

The regional structural plunge results in low grades of

metamorphism in Maine, 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 sub-

ducting 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

collided, and west-directed overthrusting produced 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.

See also CALEDONIDES; PENOBSCOTTIAN OROGENY; PLATE

TECTONICS.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar