Computer exercises in tectonics
or
Things that go bump in the night
Have the continents always been as they are today?
No. Data suggest that the continents have moved over the face of the earth.
But, more than that, continents may have undergone a process of formation,
growth, and destruction over time. Material has been added to continents
during collisions between continents and between continents and island arcs.
Mountain ranges and volcanoes may form during such collisions. Continents
have also been split apart during rifting, much as eastern Africa and Africa and
Saudi Arabia are being split apart today. Thus, even the form of continents has
altered through time, and pieces that are attached to one continent today may
have been part of another continent, or not attached to any continent, in the past
(these displaced pieces are called allochthons or exotic terranes).
Archean
Later Precambrian
Paleozoic
(
Mesozoic
Tertiary
Figure 1. North America shows a roughly concentric structure, with old
Precambrian shield at the center and progressively younger rocks outward. This
supports the hypothesis that North America has grown with time.
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Continental margin
Continental
slope
A
continent
Passive Margin
volcano
Folded orogenic
mountain belt
Active Margin
B
Microcontinent
or island arc
C
Continental margin
D
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Figure 2. Sequence of events in passive and convergent continental margins. (A) shows a
passive margin, much like the Atlantic margin today. (B, C, and D) show various phases of
active, convergent plate margins. (B) shows a simple convergent margin. Continental margin
sediments are folded into an orogenic zone, a linear belt of mountains, roughly parallel with
the plate margin. (C) shows the early stage of the collision between a continent and a microcontinent or island arc. (D) shows the micro-continent being accreted (welded on) to the
continental mass. The continental margin has migrated to a new position.
Figure 1 shows that the ages of rocks in North America and Greenland form
several, more-or-less concentric rings. Centered in Canada and Greenland is
the ancient Archean (early Precambrian) craton. The craton is an old, currently
stable, tectonically quiet, mass of rock, some of which has existed for more
than 2 billion years. Surrounding the craton are generally progressively
younger belts. The first is comprised of younger Precambrian rocks (about 1
billion years old). Generally surrounding that are Paleozoic and younger rocks.
One interpretation of this pattern is that North America started with a small
nucleus during the early Precambrian and has grown progressively by adding
new material during various tectonic episodes. Thus, some geologists believe
that the North American continent has grown with time.
Episodes of mountain building (tectonism or orogeny) and volcanism are often
related to the collision or break-up of continents. One part of this process is
shown in Figure 2. Figure 2A shows a cross-section of a passive continental
margin, similar to the Atlantic continental margin today. Sediments accumulate
on the continental shelf and slope of passive continental margins. After 200 to
250 million years the oceanic crust and lithosphere uncouple from the
continental lithosphere and begin to subduct. This converts the passive margin
into an active, convergent margin. Figure 2B shows oceanic lithosphere being
subducted under continental lithosphere. During this period, the sediments
accumulated on the passive margin are folded and faulted to form an orogenic
mountain belt, a line of mountains along the continental margin. Melting of the
subducting lithosphere may also form a line of volcanoes or igneous intrusives.
Figure 2C shows the beginning of a collision between this actively subducting
continental margin and another continent, island arc, or micro-continent.
Because the low-density continental material cannot be easily subducted the
collision causes the colliding fragment to be accreted or "welded" onto the
continent as an "exotic" or allochthonous (moved) terrane (Figure 2D). Thus
layers or rings of material may be added to continental masses as suggested by
Figure 1.
Tectonism in eastern North America, eastern Pennsylvania, and New
Jersey
Four great periods of tectonism have been part of the geologic history of
eastern North America and, more specifically, the eastern Pennsylvania and
New Jersey areas (Figure 3). The oldest of these (called the "Grenville")
occurred during the late Precambrian. During this period the Baltimore Gneiss
and other ancient rocks of the Philadelphia area were formed.
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Grabens - volcanoes
Alleghenian Orogeny
Acadian Orogeny
Taconic Orogeny
Grenville Orogeny
Figure 3. Major geological events in eastern North America and the eastern
Pennsylvania - New Jersey area. (Time scale graphic from Dr. Bob's Geologic Time
web page <http://oldsci.eiu.edu/geology/jorstad/geoltime.html>).
Three orogenic periods formed the Appalachian mountains: the Taconic (late
Ordovician to Silurian), Acadian (Devonian to early Carboniferous), and
Alleghenian (Permian) Orogenies. Very strong folding and faulting, including
formation of thrust faults (low angle faults), and metamorphism marked these
orogenies. Such compressive folding may be associated with collision of
continents or collision of a continent with an island arc or micro-continent. The
final tectonic episode occurred during the Triassic and Jurassic, and was
marked by formation of grabens and volcanism. Graben (rift valley) formation,
which is occuring today in western North America and eastern Africa, occurs
when areas are under tension, which may occur when continents are breaking
apart.
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This laboratory will explore how movement of continents and fragments of
continents explains major aspects of geologic history. This laboratory will
focus on several case studies:
 Eastern North America, the Philadelphia area, and the formation of the
Appalachians.
 India and the formation of the Himalayas
You will use a computer program, TimeTrek, which simulates the movements
of more than 100 plates over the last 700 million years. The simulations of
plate movements are based on paleomagnetic data and correlative rock units.
You should realize that, although this simulation is consistent with the data,
some ambiguities do exist, and this is not the only possible interpretation. Plate
movements have also been simplified somewhat for ease of presentation. For a
different interpretation use the web browser and go to
<http://www.scotese.com/earth.htm>. The maps on this site also show changes
in coastlines for different geologic periods.
Set the following selections from the tool bar:
 (Age Maximum) maximum age 700 my
 (Age Age Range) step of 5 my
 (File New Layer) Make sure that the 200 meter bathymetric (below sea
level) contour is turned on. The edge of the continental block is usually close to
this contour.
The following options allow you to track paleomagnetic signatures.
 Data  Add  Anomaly
 Options  unclick data labels (remove location names)
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1. Locate the east coast of North America (Click on the globe edge to rotate the
view). Follow the movements of the east coast of North America (including
Florida) as portrayed in TimeTrek over the last 700 million years. First run
the simulation from the present back into the past; you may need to stop the
simulation periodically and recentre the global view. Try to associate the four
Phanerozoic tectonic episodes with periods of continental collision or rifting
(Triassic).
a. Has the East Coast of North America, including the Philadelphia area,
always been attached to North America? If not, with what other continent or
continents was it formerly associated? When did the Philadelphia region first
become attached in its present position on North America?
b. Can you associate plate interactions with the four major orogenic events?
What continents or plate fragments are involved? What type of interactions
(convergent, divergent, transverse) are involved? When do the interactions
occur?
 Graben formation
 Alleghenian
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 Acadian
 Taconic
c. Has the East Coast fragment ever been near the south pole. If so, when?
2. Follow the movements of the Indian subcontinent over the last 300 million
years.
 Has India always been attached where it is now? What continent or
continents was it attached to in the past?
 The Himalayas are a result of a collision between India and the Eurasian
continent. How old are the Himalayas (When did the orogeny forming these
mountains occur)?
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 Permian and Triassic glacial deposits have been found in India. Is this
consistent with the movement of India? Which pole was India near at that
time? On what other continents, or in what other areas might you also find
glacial deposits of a similar age?
3. Chose an area of the world which is of interest to you (perhaps one which
you would like to visit someday, or one from which your ancestors came).
Follow movements of appropriate plates over the last 700 million years (and
possibly into the future as well). Go to the library or Internet (such as
http://www.scotese.com/earth.htm) and try to correlate any plate collisions or
break-ups in the simulation with the geologic history or the physiographic
nature (for example presence and orientation of mountain belts) of the area.
(Note: not all areas have geologic histories that are obviously interesting at
this scale or level of detail. If your first choice isn't interesting, try another
region).
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