Download The Appalachian Mountains: Deposition-Subduction

The Appalachian Mountains: Deposition-Subduction-Subduction-Rifting-Uplift (and Erosion is in
there too!)
The Appalachian region started out as a passive plate margin located near the equator; at this time thick
layers of sediment and carbonate rock were deposited. During the Taconic orogeny (445-435Ma) the
Appalachian region became an active plate boundary. Collision with and subduction of the Iapetus plate
(Europe) during the Taconic led to volcanism, and the formation of Laurentia. The Taconic was the
beginning of the growth of Pangea. Following the Taconic event about 300 Ma was another major
orogeny, the Alleghenian. The collision of the African (Gondwana) and North American plates raised the
Appalachians to Himalayan-scale ranges. Around 220Ma Pangea began rifting apart. Crustal stretching
produced fault block basins, which filled with red sediments, basaltic lava flows and shallow diabase
intrusions. At this time mountain building ceased and erosion ruled. Bye- bye Mountains. This would be
the end of the story if it hadn’t been for the Cenozoic uplift. Streams were rejuvenated and cut into the
bedrock. Northeast trending structures were superposed by east flowing streams and differential erosion
cut the extensive valley system between the sandstone ridges. The shale and limestone were easily eroded
because they are less resistant and that resulted in the development of trellis drainage pattern.
The ridge and valley province is a classic example of landscape formed on folded and thrust-faulted strata.
The extent of the Appalachian is 200km (it runs from central Alabama to Pennsylvania. Similar mountain
belts include the Zargas Mountains in Iran, the Urals in Russia, part of the Andes and the foothills of the
Himalayas.
Sources:
Van Diver, Bradford B. 1990. Roadside Geology of
Pennsylvania. Mountain Press Publishing Company.
352pp.
http://wrgis.wr.usgs.gov/docs/parks/province/appalach.html
Figure 1: Modified from Skinner and Porter, The Blue
Planet, 1995, p.186.
A. Six hundred million years ago, a small fragment of
continental crust (an island) lay offshore of North America.
Beyond the island was a subduction zone and an arc of
volcanoes.
B. A new subduction zone starts beneath the island about
500 million years ago.
C. Between 400 and 500 million years ago, the island
collides with North America (these rocks can be seen today
in North Carolina and Virginia), and the Appalachians start
to form.
D. Between 350 and 400 million years ago, the arc of
volcanoes collides with North America. The Iapetus Ocean
slowly closes as North Africa approaches and eventually
collides.
E. When Pangaea broke apart about 200 million years ago,
a fragment of Africa remained attached to North America.
The passive continental margin so formed (today's margin)
bounds the eastern edge of North America.
http://www-users.cs.umn.edu/~checheln/geo/exams/final1.html