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Orogenic Case Study: The Appalachians
The geology of the Appalachians dates back to the Ordovician, more than 480 million years ago. A look
at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust
faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor - strong evidence
that these rocks were deformed during plate collision. The birth of
the Appalachian ranges marks the first of several mountain building
plate collisions that culminated in the construction of the
supercontinent Pangaea with the Appalachians and
neighboring Anti-Atlas (now in Morocco) near the center. These
mountain ranges were once higher than today's Himalaya
mountain range, which was also formed by continental collision.
During the earliest Paleozoic Era, the continent that would later
become North America straddled the equator. The Appalachian
region was a passive plate margin, not unlike today's Atlantic
Coastal Plain Province. During this interval, the region was
periodically submerged beneath shallow seas. Thick layers of
sediment and carbonate rock were deposited on the shallow sea
bottom when the region was submerged. When seas receded,
terrestrial sedimentary deposits and erosion dominated.
During the middle Ordovician Period (about 480-440 million
years ago), a change in plate motions set the stage for the first
Paleozoic mountain building event (Taconic orogeny) in North
America.
Taconic Orogeny
The Taconic orogeny was a great mountain building period that perhaps had the greatest overall effect
on the geologic structure of basement rocks within the New York Bight region. The effects of this
orogeny are most apparent throughout New England, but the sediments derived from mountainous
areas formed in the northeast can be traced throughout the Appalachians and midcontinental North
America.
Beginning in Cambrian time, about 550 million years ago, the Iapetus Ocean began to grow progressively
narrower. The weight of accumulating sediments, in addition to compressional forces in the crust,
forced the eastern edge of the North American continent to fold gradually downward. In this manner,
shallow-water carbonate deposition that had persisted on the continental shelf margin through Late
Cambrian into Early Ordovician time, gave way to fine-grained clastic deposition and deeper water
conditions during the Middle Ordovician. Sometime during this period a convergent plate boundary
developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean
sank into the mantle along a subduction zone with an eastward-dipping orientation. Partial melting of
the down-going plate produced magma that returned to the surface to form the offshore Taconic island
arc. 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 intense metamorphism. This was the final episode of the long-lasting
mountain-building period referred to as the Taconic Orogeny.
When the Taconic Orogeny subsided in the New York Bight region during Late Ordovician time (about
440 million years ago), subduction ended, culminating in the accretion of the Iapetus Terraneonto 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 newly 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
time (about 360 million years ago) when the next period of mountain-building began, the Acadian
orogeny.
Acadian Orogeny
The Acadian orogeny is a middle Paleozoic mountain building event (orogeny), especially in the
northern Appalachians, between New York and Newfoundland. The Acadian orogeny most greatly
affected the Northern Appalachian region (New England northeastward into the Gaspé region of
Canada). The Acadian orogeny should not be regarded as a single tectonic event, but rather as an
orogenic era. It spanned a period of about 50 million years, from 375 to 325 million years ago.
In Gaspéand adjacent areas, its climax is dated as early in the Late Devonian, but
deformational, plutonic, and metamorphic events extended into Early Mississippian time. During the
course of the orogeny, older rocks were deformed and metamorphosed, and new faults formed and
older faults were reactivated.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with
metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of
theGreat Basin.
The cause of this great period of deformation is a result of the plate-docking of a small continental
landmass called Avalonia (named after the Avalon Peninsula of Newfoundland). The docking of Avalonia
onto the composite margin of Ganderia and Laurentia resulted in the closing of a portion of theRheic
Ocean.
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 thinning that followed the Acadian orogeny created volcanoes, such as the
large Mount Pleasant Caldera in southwestern New Brunswick, Canada.
Alleghenian (Appalachian) Orogeny
The Alleghenian orogeny or Appalachian
orogeny is one of the geological mountainforming events (orogeny) that formed the
Appalachian Mountains and Allegheny
Mountains. The term and spelling "Alleghany
Orogeny" (sic) originally proposed by H.P.
Woodward (1957, 1958) is preferred usage.
Approximately 350 million to 300 million
years ago, in the Carboniferous period, when
Gondwana (specifically what became Africa)
and what became North America collided,
forming Pangaea. This collision exerted
massive stress on what is today theEastern
Seaboard of North America, resulting in a
large-scale uplift of the entire region. Closer
to the boundary between the colliding plates,
tectonic stresses contributed to the
metamorphosizing of the rock (i.e. the
transformation of igneous and sedimentary
rock into metamorphic rock). These stresses
concurrently caused faults (mostly thrust
faults and some strike-slip faults) as well as
folding. The immense region involved in the
continental collision, the vast temporal length
of the orogeny and the thickness of the pile of
sediments and igneous rocks known to have
been involved are evidence that at the peak of the mountain-building process, the Appalachians could
have risen as high or perhaps even higher than the present-day Himalaya.
The Appalachian Orogeny is responsible for the creation of the mountains themselves and is not
responsible for the topography that now typifies the Piedmont and coastal plain regions east of the
mountain chain. The heavily-eroded hills of Piedmont are remnants of the sizeable mountain chain,
while the coastal plain is made up of the material that was washed away in that process. Thus, the
coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years
ago to the present.
Evidence for the Appalachian orogeny stretches for many hundreds of miles on the surface from
Alabama to New Jersey and can be traced further subsurface to the southwest. In the north it enters a
region of confused topography associated with earlier orogenies, but clearly the Applachian
deformation extends northeast to Newfoundland.
The mountains were once rugged and high, but in our time are now eroded into only a small
remnant. Sediments that were carried eastward form part of the continental shelf. Sediments that were
carried westward form the Allegheny and Cumberland Plateau, which in some areas are popularly called
mountains, but are actually simply uplifted and eroded plateaus. Carbonates and fine sediments from
these mountains were carried farther to form limey rocks in a shallow sea that was later uplifted and
forms the bulk of Tennessee, Kentucky, Ohio, and Indiana.
A portion of the Alleghenian mountain system departed with Africa when Pangaea broke up and the
Atlantic Ocean began to form. Today, this forms the Anti-Atlas mountains of Morocco. The Anti-Atlas
have been geologically uplifted in relatively recent times, and are today much more rugged than their
Alleghenian relatives.
Erosion
Pangea began to break up about 220 million years ago, in the Early Mesozoic Era (Late Triassic
Period). As Pangea rifted apart a new passive tectonic margin was born and the forces that created the
Appalachian, Ouachita, and Marathon Mountains were stilled. Weathering and erosion prevailed, and
the mountains began to wear away.
By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain.
It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the
present formed. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the
ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many
millions of years earlier. Other streams downcut so rapidly that they cut right across the resistant folded
rocks of the mountain core, carving canyons across rock layers and geologic structures.