Download 11.14 Where Will Mountains and Basins Form in This Region?

I N V E S T I G AT I O N
Where Will Mountains and Basins
Form in This Region?
11.14
The figure below shows part of a continent and adjacent ocean. There are no plate boundaries now, but a
subduction zone will form along the western coast of the continent, and the eastern part of the continent will
be rifted away. You will use the typical patterns that form along such boundaries to predict where mountains
and basins will form once the new plate boundaries are fully developed.
Goals of This Exercise:
• Observe the continent and ocean below, and read the descriptions of the types of features that will form in the future.
• Use your understanding of plate boundaries and the settings in which mountains and basins develop to predict where mountains
and basins will form. Sketch your predictions on a diagram of the region.
• Predict what the regional topography will be like in different parts of the region, identifying whether an area will rise or subside,
and what changes on the surface, within the crust, or in the mantle would cause this change in elevation.
1. This view shows a continent and ocean at some time, which we will call
Time 1. The western part of the region is a typical ocean basin and has no
trenches, mid-ocean ridges, or hot-spot islands.
11.14.a1
2. A small piece of continent lies offshore in the
middle of the ocean. When
the oceanic plate begins to
move, this piece of continent
will be carried toward and
will collide with the main
continent.
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3. The ocean-continent
edge is currently a passive margin, not a plate
boundary. It will become
an ocean-continent
convergent boundary, and
the oceanic material will be
subducted eastward below
the continent.
4. Once plate convergence
begins, a magmatic belt will
form inland from the coast,
near the position of the yellow
triangles. Farther inland a thrust
belt will form as shown by the
blue dashed line with teeth. In
the thrust belt, the western part
of the continent will be thrust
eastward over the central part
of the continent.
5. A continental rift has formed, with three
arms radiating out from a high central region,
which is a hot spot marked by voluminous
volcanism. This rift will split the continent into
two pieces. At some later time, the piece of
continent to the right will break away completely, and seafloor spreading will form a
new ocean basin. Even later, at a time we will
call Time 2, the edge of the continent will
have evolved into a passive margin (not a
plate boundary), and the spreading center
will be out of the region.
Mountains, Basins, and Continents
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Procedures
Use the data to complete the following steps, entering your answers in the worksheet or online.
1. Observe the regional features shown on the figure on the left page, which represents the situation at Time 1. Read the descriptions
associated with that figure and decide what each statement implies about the future topography (elevations) of the area.
2. For each feature (subduction zone, thrust belt, etc.) that will form by Time 2, think about how that feature is typically expressed in
the topography. Does it form a mountain range, a basin, or a mountain with a nearby basin?
3. On the worksheet, sketch your predictions about the area’s topography for Time 2 on the simplified figure below, which shows
the same area as the figure on the previous page. The figure shows the overall shape of the continent but not the topography.
Use the following letters: O for an oceanic trench, A for an accretionary prism, M for mountains, V for volcanoes in the continental
magmatic belt, B for a basin, and P for a passive margin. Feel free to sketch some simple lines to portray the locations of the
features. Your instructor may have you predict other features that might develop, such as a tectonic terrane (T) or features related
to a collision (C).
4. On the map below are letters A – D. A is along the coast, B is at the future position of the magmatic belt, C is within the future
fold and thrust belt, and D is along the coast from which the other piece of the continent was rifted. In the worksheet, predict
what will happen to the crustal thickness in each of the four locations, and identify the processes that could cause thickening or
thinning of the crust or the mantle part of the lithosphere beneath each site.
Perspective of the Region in the Future (Time 2)
This boundary is now marked by a subduction zone, where the oceanic plate is being
subducted beneath the continent.
By this time, subduction will result in a
magmatic belt inland from the coast and
a thrust belt farther into the continent.
The hot spot is no longer active, but
its former position is recorded by an
indentation in the continent.
D
A
B
C
11.14.a2
Rifting has produced a new coast where the
other piece of the continent was rifted away.
By Time 2, the edge of the continent is a passive margin, not a plate boundary.
11.14
The small piece of continent that was in the
ocean is not shown. You should predict where
it might have ended up and how it would have
changed the coastline (not shown).
The eastern part of the original continent
has been rifted away, leaving an ocean basin.
By Time 2, the mid-ocean ridge that formed
between the two continental pieces has
migrated out of the region as it added new
oceanic plate to the edge of the continent.