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Chapter 3 Geologic Structures and Processes141
GEOLOGIC STRUCTURES
An anticline rock face, part of a series of syncline and anticline folded carbonate rocks on Beavertail Mountain, NWT.
R
ock features that form as a result of deformation
are called geologic structures. Such structures
help geologists tell the story of a rock body. They
help scientists understand the type of stress and the
environmental conditions that rocks have experienced.
Geologic structures can be recognized at a variety
of scales. Some are seen only through a microscope,
some can be identified in a rock sample that you hold
in your hand, and others stretch over many hundreds
of kilometres.
The structures formed by plastic and brittle deformation are quite different. Plastic deformation usually
results in layers of rocks that are folded in some way,
while brittle deformation breaks the rock layers along
fault lines.
Another important type of structure is called an uncomformity. It is a type of contact that shows a gap in
the geologic record.
such as uplifting and erosion, push the strata back to
the surface.
Most folds have two sides, called limbs, and a crest
or trough. At the centre of the fold, between the two
limbs, runs an imaginary plane called the axial plane. It
intersects with the crest or trough at an imaginary line
called the axis of fold or the hinge line (see Figure 3.8).
Ax
is o
ld
Plunge
Structures Produced by Plastic
Deformation
When rocks are subject to plastic deformation, they are
folded upward or downward. Folds occur as a result of
compressive stresses.
Folds are usually found in sedimentary rocks, which
originally formed in horizontal strata. Metamorphic
rocks can also have folds, which may not be as distinctive. Buried sedimentary layers are bent by pressure
from movement in the crust. Further geologic action,
f fo
Axial plane
Figure 3.8 A block diagram showing the location of an axial
plane and its relationships to the axis of fold and plunge.
SCIENTIFIC TERMS
axial plane: in folded rocks, an imaginary plane that passes
through all points of maximum curvature of beds in an
anticline or syncline.
crest: highest part of a fold or wave.
trough: lowest part of a fold or wave.
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Crest
Anticline
Syncline
Anticline
Anticline Syncline
Limb
Open folding
Overturned folds
Axial Planes
Anticline
Syncline
(Above): Figure 3.9 A block diagram illustrating anticline and
syncline formations. (Below): Figure 3.10 Block diagrams
illustrating the cross-sections and map views of various
anticline and syncline situations.
Hinge line
Syncline 3-D
Limb 2
Syncline cross-section
Eroded syncline 3-D
e
urfac
line s
ync
ded s
Ero
view
Bisector
Limb 2
Anticline 3-D
Limb 1
Bisector
Limb 1
Anticline cross-section
Tight folding
Recumbent folding
Figure 3.11 The major types of folding and the relationship of
the axial planes to the horizontal.
An upfold—the upward bend in a fold—is called an
anticline, and a downfold is a syncline (see Figure 3.9).
Together, they make what looks like a wave frozen in
rock, as the crests and troughs alternate through the
strata. One limb of an anticline becomes a limb of the
next syncline.
Folds can take a variety of forms, as you can see in
Figure 3.10. They are classified according to the angle of
their axial planes.
If the forces on both sides are equal, the axial plane
along which a fold occurs is vertical. The resulting fold
is a symmetrical fold. If the axial plane is tilted, meaning that the force causing the bend was stronger on one
side, the fold is asymmetrical. Sometimes the force on
one side of the fold is so strong that the fold is pushed
over on its side and folds over itself. This is called recumbent folding. Measuring the strike and dip of the axial
surface and the trend and plunge of the fold axis can help
one interpret the forces and geometry in the structure.
Figure 3.11 illustrates, in cross-section, the major
fold types representative of anticline and syncline features. Knowing the orientation of the strata on the surface helps to explain the history, forces, and structures
below the surface.
SCIENTIFIC TERMS
Eroded anticline 3-D
icline
ant
oded
Er
view
ce
surfa
anticline: a fold, generally convex upward, whose core
contains the older rocks.
recumbent folding: the axis of the fold overturns, resulting in
a low-angle horizontal fold.
syncline: a fold, generally concave upward, whose core
contains the younger rocks.
Chapter 3 Geologic Structures and Processes143
Reading anticlines and synclines
An anticline that has not been modified by erosion has
a crest or arch at the top. The rock layers dip away from
the centre of the fold. (Think of the letter A to help you
remember the shape.) The maximum curve is located at
the hinge line. Geologists know that when an anticline
is eroded, the oldest exposed rocks are always at the
centre. Study the illustration in Figure 3.10 to determine why.
Synclines are downward-arching rock beds. The
rocks dip towards the centre of the fold. The rocks at
the centre of a syncline are the youngest exposed rocks.
The plan or map views of the anticlines and synclines in Figure 3.10 show parallel bands. Erosion removes parts of the top layer or layers of rock, leaving
parallel bands, as you will demonstrate in activity 8.
Both a plunging anticline and a plunging syncline
make a V-shaped outcrop pattern. With the anticline,
however, the V points towards the plunge, whereas in
the syncline the V points away from the plunge.
Mapping anticlines and synclines
Look at Figure 3.12 to see how anticlines and synclines
are represented on geologic maps. In this example,
strike and dip symbols show the attitude, in degrees, of
the limbs of the synclines.
The hinge line of the fold is shown as a line running
down the centre of a formation (indicated by the coloured regions).
If the bands were not labelled “anticline” and “syncline” at the top of the map, how would you be able to
tell the difference? Note the arrows beside the hinge
lines. Remember that on an anticline, the limbs dip
away from the hinge line. To indicate this on a map, the
arrows point away from the hinge. Similarly, on a syncline the layers dip towards the hinge line, so on a map
the arrows point towards each other.
Web Link
To find animations showing how anticlines and
synclines are formed, use the search terms “animation
anticline” and “animation syncline.”
North
syncline
anticline
anticline
40
38
55
54
0
10 km
5 km
Figure 3.12 A sample map of anticlines and synclines
indicating directions and magnitude of strikes and dips.
Domes and basins
Sometimes layers of rock are not folded in two directions but in all directions. If the rock is pushed upwards
as in an anticline, but all sides are lifted, it forms a
dome. If the folding pushes downward like a syncline,
but including all sides, it forms a depression called a
basin.
Structural domes and basins generally appear circular or elliptical in shape when seen from above or
drawn on a map. Sometimes these structures are found
as small features just a few metres across. Usually, however, they are large regional structures that may have
a diameter of many kilometres. These structures are
usually not visible in the field but must be identified by
mapping.
When domes and basins become eroded, the rock
exposure exhibits a pattern similar to anticlines and
synclines. An eroded dome’s oldest rocks are at the
centre, as the younger rock in the crest has been worn
away. A basin has younger rock in the centre, as the
outer edges are typically eroded first. Geologists use
these patterns to help them decide whether a circular
outcrop is a dome or a basin.
SCIENTIFIC TERMS
basin: a low area in the Earth’s crust, of tectonic origin, in
which sediment has accumulated.
dome: an uplift of anticlinal structure, circular or elliptical in
shape, in which the rocks dip gently away in all directions.
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Experiential Science 30—Freshwater Systems
3-D view of dome
Aerial view of dome
3-D view of basin
Aerial view of basin
Symbol of dome
Symbol of basin
Figure 3.13 Block diagrams, aerial views, and map symbols of
domes and basins.
On maps, domes and basins are recognizable by
their circular shape with a series of concentric layers.
The dip direction is indicated with arrows, as you can
see in the right-hand column in Figure 3.13.
An example of a dome in the Northwest Territories
is the Aylmer dome (Figure 3.14), northeast of Great
Slave Lake near Walmsley Lake. It has a core of older
granitoid rocks, surrounded by more recent volcanic
rocks. If you were to walk across the area, you would
never know that a dome structure lay beneath your feet.
It is 130 km north of the treeline, in a typical section of
flat barren lands. There is very little outcrop for geologists to study. However, they are able to use the magnetic properties of some of the rock to determine the
contacts between layers.
When kimberlite was discovered in the nearby Lac
de Gras area in the 1990s, the region of the Aylmer
dome was staked and explored by a diamond mining
company. Their exploration provided more detail on
the subsurface geology, which helped to identify it as
a dome.
Figure 3.14 A simplified geologic map of the Aylmer dome,
NWT. Research on the subsurface geology helped indentify it
as a dome, even though there are few rocky outcrops because
of glacial erosion.
Chapter 3 Geologic Structures and Processes145
Activity 8
field activity
4 lab activity
library activity
classroom activity
chapter project
4 research team activity
Modelling Fold Structures
Purpose
To demonstrate understanding of anticlines and synclines by
constructing models.
Materials and Equipment
• modelling clay or playdough (2 or 3 different colours)
• butter knife or string
• transparent drinking straws
Procedure
1. Work in your research team. Make three layers of clay
using alternating or different colours. These represent
three horizontal rock beds. Form a sandwich of layers
about 5 cm × 15 cm × 0.5 cm.
2. Use compression to push the ends together to form an
anticline. It should look like an arch. Draw a vertical crosssectional view of what the model looks like now.
3. Make another sandwich, this time making a syncline
(arching downwards).
4. Use the straws to take three core samples of the models.
You may need to work with a partner so that one person
holds the model while the other inserts the straws.
• Make sure you keep the straws vertical as you push
them into the model.
• When you take them out, make sure to keep them
aligned the same way as when they went in.
• Take one sample from the left side, one from the
middle, and one from the right side.
• Draw the core samples, showing the relative size,
shape, and colour of the layers.
5. Using a butter knife or string, cut off the tops of the models
horizontally to simulate erosion. Sketch the surface map
view of your model. This shows how it would look on a
geologic map.
6. Cut another layer off the top, but this time angle the plane
of your cut to represent a plunging fold. Sketch the map
view.
Reflections and Conclusions
1. What part of this activity demonstrates the principle of
original horizontality?
2. If these were real rock layers, which would be the
youngest rock and which would be the oldest? Which
geologic principle does this illustrate?
3. Does the order of the relative ages of the rock layers
change when they form a syncline or anticline? Explain
your answer.
4. Compare the core samples from each structure. How would
you be able to tell whether a sample was taken from an
anticline or from a syncline? Would you be able to tell from
just one sample?