Acadian orogeny - AbsoluteAstronomy.com

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 of the East coast of the United States...

, Acadian and Alleghenian

Alleghenian orogeny

The Alleghenian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 1957....

orogenies are the three tectonic phases responsible for the formation of the present Appalachian Mountains. The Acadian orogeny is a middle Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

mountain building episode dating back 325-400 million years which should not be regarded as a single event but rather as a chain of mountain building events. The Acadian orogeny is mostly responsible for the deformation in the northern Appalachians between New York and Newfoundland. The tectonic stresses brought on by the collision of the Avalonian tectonic plate with the convergent boundary of northeastern Laurentia

Laurentia

Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...

began the formation of the early Appalachian orogeny. Its climax occurred in the Late Devonian with deformational, plutonic and metamorphic events extending into Early Mississippian time.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny

Variscan orogeny

The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...

of Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, with metamorphic events in southwestern Texas

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

and northern Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

, and with the Antler orogeny

Antler orogeny

The Antler orogeny is a mountain-building episode that is named for Antler Peak, at Battle Mountain, Nevada. The orogeny extensively deformed Paleozoic rocks of the Great Basin in Nevada and western Utah during Late Devonian and Early Mississippian time...

of the Great Basin

Great Basin

The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...

Geodynamics

The Acadian orogeny was caused by the convergence of a small continental landmass called the Avalonia island arc at an active margin on the northeastern portion of the Laurentia plate. The convergence of the Avalonia island arc created and reactivated numerous faults in the region and caused older rocks to be deformed and metamorphosed.
Avalonia islands gradually accreted onto the eastern margin of the Laurentia plate and created magmatic activity which can be found in scattered outcrop belts along parts of Newfoundland and much of the coastal region of New England from Connecticut to northern Maine. Convergence of the Taconic and Avalonia tectonic plates were early stages of the formation of a supercontinent called Pangea. The final process, termed the Alleghanian orogeny, occurred when the African and North American plates collided producing the final tectonic stresses attributed to the formation of the Appalachians.

When the Avalonia tectonic plate was accreted onto the Laurentian plate during the Acadian orogen, the land mass was pushed on top of the Laurentian plate creating a depressed area called a foreland basin or foredeep. The enormous stresses brought on by the collision of these two plates caused the foredeep to migrate westward as the pressures continued from the eastern direction.
Deposition of sediments in the migrating foredeep due to the erosion of the Avalonia terrane and the prevailing Acadian Mountains created an interesting combination of sediments called flysch and molasse. Flysch

Flysch

Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...

is deep water sediment indicating that the foredeep was completely inundated forming depositional environments of black shale later recognized as the Needmore and Millboro formations. Deposition of shallow water sediments known as molasse

Molasse

The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...

become more prevalent as the foredeep filled with sediment and became progressively shallower. The red rocks from the meandering river channels known as the Hampshire Formation carried and deposited large volumes of sediment directly into the basin as underwater avalanches which are preserved in the Brallier Formation. The marine life preserved in the Foreknobs formation was deposited on the continental shelf of the ancient inland sea.
The Avalonia land mass originally included parts of Great Britain and southern Europe but was rifted away from North America during the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

opening of the Atlantic Ocean. Avalon added the land lying under the coastal plain province to the edge of the continental shelf in the Mid-Atlantic region of North America. Large volumes of volcanic rocks and younger Iapetus Ocean sediments were accreted onto the North American margin as the land masses converged forming a band of younger terrane between the older North American and Avalonian terranes. The majority of this metamorphosed material can be found in the states of Maine, New Hampshire, Vermont and major portions of Massachusetts and Connecticut. These Avalonian rocks typically consists of schist

Schist

The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...

, phyllite

Phyllite

Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...

, gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

and granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

which can also be found in parts of South Carolina, Newfoundland, Nova Scotia as well as Morocco, Portugal, Spain, Ireland, Wales, southern England and the northern coasts of France, Belgium, Holland and Germany.

Avalonian tectonic plate converged on Laurentia obliquely from the northeast. The Laurentian coast had irregularities that the Avalonia island arc hit the hardest which resulted in a ragged type of mountain building that began in the north and migrated south through time as the Iapetus Ocean closed from northeast to southwest in direction. The northern collision in Canada is recognized by the severely deformed igneous and metamorphic rocks of Nova Scotia with sediments spreading southward.

The second impact, known as the Catskill, happened in the regions of New Jersey, southeastern New York and eastern Pennsylvania producing a large, deep Catskill foreland basin. The foreland basin deepens toward the north and is influenced by two processes, initial rapid basin subsidence and sediment filling the basin from bottom to top, incorporating distal sources in the mountains to the east and proximal meandering river systems of the Hampshire Formation transporting sediment toward the Kaskaskia Epicontinental Sea to the west. The mountains in the southeast are the major source of sediment, therefore the Catskill clastic wedge prograded toward the northwest into the foreland basin. As the foreland basin filled with sediment, the shelf environment widened progressively. Actually, as time passed, each environmental belt became wider and more distal environments usually were buried under the depositional environment prograding behind it. The Catskill clastic wedge in Central Pennsylvania is several thousands of feet thick and thins out steadily southwestward through Virginia and West Virginia indicating the diminishing effects of the collision. .

The third and final phase, known as the Pocono or Price impact, was smaller and took place in southern Virginia. The collision created a small foreland basin with a moderate volume of sediment spreading north and west from the collision. The sediments of the Price/Pocono clastic wedge are not very widespread or thick indicating the collision was small compared to the Catskill impact.

The cause of this great period of deformation is a result of the plate-docking of a small continental landmass called Avalonia

Avalonia

Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...

(named after the Avalon Peninsula

Avalon Peninsula

The Avalon Peninsula is a large peninsula that makes up the southeast portion of the island of Newfoundland.The peninsula is home to 257,223 people, which is approximately 51% of Newfoundland's population in 2009, and is the location of the provincial capital, St. John's. It is connected to the...

of Newfoundland). The docking of Avalonia onto the composite margin of Ganderia and Laurentia resulted in the closing of a portion of the Rheic Ocean

Rheic Ocean

The Rheic Ocean was a Paleozoic ocean between the large continent Gondwana to the south and the microcontinents Avalonia and others to the north...

.

Avalonia was gradually torn apart as plate tectonic forces accreted the landmass onto the edge of the larger North American continent. Today, portions of the ancient Avalonia landmass occur in scattered outcrop belts along the eastern margin of North America. One belt occurs in Newfoundland, another forms the bedrock of much of the coastal region of New England from eastern Connecticut to northern Maine.

A period of lithospheric

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

thinning that followed the Acadian orogeny created volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es, such as the large Mount Pleasant Caldera

Mount Pleasant Caldera

The Mount Pleasant Caldera is a large eroded Late Devonian volcanic caldera complex, located in the northern Appalachian Mountains of southwestern New Brunswick, Canada. It is one of few noticeable pre-Cenozoic calderas, and its formation is associated to a period of crustal thinning that followed...

in southwestern New Brunswick

New Brunswick

New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

Geodynamics

See also