CHAPTER 11 - GEOLOGIC STRUCTURES
Overview
Structural geology is concerned with shapes, arrangement and
interrelationships of rock units and the forces that cause them. Tectonic forces
cause stress in an object that is either compressional, tensional, or shear
(Figures 11.2-11.3 utilize a ball of silly putty and a deck of cards to illustrate
stress and strain). Stress produces strain that is either elastic, plastic, or
brittle. Present-day rock movements in California, such as those along a fault
running through Hollister, are very rapid (1 cm per year) compared to other
geologic processes (Fig. 11.5).
Structures formed in the geologic past have been exposed by erosion at the
earth's surface. They can be depicted and interpreted using geologic maps that
present the distribution and nature of rock units, structural features, ore
deposits and other geological features by symbols and patterns. Strike and
angle and direction of dip can be recorded using symbols that portray the
geometry of bed tilting in the area mapped (Figures 11.7, 11.9). Geologic
cross sections provide views of rocks below the surface and help to visualize
three-dimensional views of the earth's interior.
Folded rocks reflect plastic strain. The orientation of anticlines and synclines is
described by reference to hinge lines and axial planes (Figure 11.11). Plunging
folds have dipping hinge lines and V-shaped outcrop patterns (Figure 11.13).
Structural domes (doubly plunging anticlines) and structural basins (doubly
plunging synclines) are large features with beds dipping toward or away from
their centers respectively (Figure 11.15). As folding forces increase, folds
change from open to isoclinal, to overturned, to recumbent.
Brittle failure results in either joints with no displacement, or faults with
displacement. Faults are described by movements of the hanging wall along
the fault surface. Dip-slip faults are normal, reverse, or thrust. Strike-slip have
either left-lateral or right-lateral horizontal movement. These various faults
are illustrated diagrammatically in Figures 11.20-11.28.
Learning Objectives
1. Tectonic forces move and deform parts of the earth's crust. Stress is force
applied to an object, while strain is a change in size and shape or both, while
an object is undergoing stress. Compressional stress produces shortening
strain, tensional stress produces stretching or extensional strain, and shear
stress produces shear strain: parallel movement in opposite directions.
2. A body responding with elastic strain recovers its original shape. A body
responding with plastic strain does not return to its original shape, while brittle
strain produces fractures. Rocks at the earth's surface are typically brittle.
3. A geologic map depicts rock types and structures, and a geologic cross
section is a vertical representation of a portion of the earth. Strike is the
compass direction of the line formed by the intersection of an inclined bedding
plane with a horizontal plane. Dip is the angle formed by the bed and a
horizontal plane, and it is always measured perpendicular to strike. Strike and
dip are measured in the field with a Brunton pocket transit and are marked on
a map with symbols. Horizontal and vertical beds have special symbols.
4. Folds are bends in layered rock produced by plastic strain. The axial plane
(visualized as connecting the hinge lines formed by the bending of each bed in
the fold) divides the fold into limbs. An anticline is an arch in which the beds
dip away from the hinge line, while a syncline is a trough in which the beds dip
toward the hinge line. Erosion of an anticline will allow the oldest beds to be
exposed along the hinge line, while erosion of a syncline will expose the
youngest beds along the hinge line.
5. Plunging folds have hinge lines that dip and produce V-shaped patterns of
exposed strata.
6. Structural domes (doubly plunging anticlines) have beds that dip away from
a central point, while structural basins (doubly plunging synclines) have beds
that dip toward a central point.
7. Open folds have limbs with gentle dips; isoclinal folds have limbs parallel to
one another. Overturned folds have limbs that dip in the same direction.
Recumbent folds have limbs that are essentially horizontal.
8. Brittle strain produces fractures in rocks called joints, if no displacement
occurs. Columnar jointing and sheet jointing were mentioned in earlier
chapters. Compression can produce multiple joint sets.
9. Faults are fractures along which displacement occurs. The fault surface or
fault plane separates the two sides of the fault into a hanging wall (above the
fault plane) and a footwall (below the fault plane).
10. Dip-slip faults exhibit movement parallel to the dip of the fault plane.
Normal faults have a hanging wall that moved down in response to tensional
stress. Blocks bounded by normal faults produce grabens and rifts, if dropped
down, or horsts, if raised up. Reverse faults are dip-slip faults that have a
hanging wall that moved up in response to compressional stress. A thrust fault
is a reverse fault with a low angle fault plane. Strike-slip faults are associated
with shearing and have no vertical displacement. Left-lateral and right-lateral
movement is determined by looking at displacement across the fault plane.
Boxes
11.1 - IN GREATER DEPTH - STRUCTURES ASSOCIATED WITH SALT DOMES
Vertical compressive stress is associated with salt diapirs, bodies of rock salt, a
kilometer or more wide, that rise several kilometers through sedimentary rock
layers to form surface domes or anticlines. The beds on the sides of the salt
domes are upturned and pierced, forming suitable traps for petroleum, and the
rising salt diapir may form fault traps in the rock layers above it that can also
trap petroleum. The salt may actually penetrate the ground surface where it
will continue to flow plastically.
11.2 - IN GREATER DEPTH - IS THERE OIL BENEATH MY PROPERTY- FIRST CHECK THE
GEOLOGICAL STRUCTURE
Accumulations of crude oil and gas require a source rock, which is an organicrich sedimentary rock that produces petroleum as the organisms decompose
following burial. Petroleum migrates from the source rock until it encounters
impermeable rock or the earth's surface, where it is lost in seeps. Porous and
permeable reservoir rocks allow petroleum to accumulate below impermeable
layers. Geologic structures provide traps to further concentrate the
accumulating petroleum. Anticlines are the best traps and the first recognized
by geologists searching for oil. These structures can form "oil pools" with
stacked levels of water (lowest), oil and natural gas (highest) against the
impermeable rock above the reservoir. Other types of traps have been
recognized including faults, unconformities, and facies changes. Modern
petroleum exploration is sophisticated, but there is no way short of drilling to
determine the presence of oil. "Wildcat" wells only have a 1 in 10 chance of
discovery, which explains why future oil supplies will be more difficult and
costly to find.
11.3 - IN GREATER DEPTH - CALIFORNIA'S GREATEST FAULT - THE SAN ANDREAS
The San Andreas Fault is the longest of a series of right-lateral faults
comprising a system approximately 100 km wide and 1,300 km long. The
average rate of movement is approximately 2 cm per year and earthquakes
are common along the system. For most of its length, the fault is easily
identified by straight stream valleys and elongate lakes (sag ponds) eroded
into the weaker, broken rock of the fault zone. In southwestern San Francisco,
the fault zone has become hidden by recent building that may spell future
disaster. Two questions puzzle geologists: what is the total displacement along
the fault and how long has the San Andreas been active? Volcanic sequences
that are 23.5 million years old have been displaced 315 kilometers by the
fault. Older rocks appear to have been offset at least 560 kilometers. Evidence
of recent movements, particularly displaced stream channels, is abundant, and
displacement of the Sierra Nevada granitic batholiths suggest that the fault is
no older than 80 million years. The San Andreas is a transform boundary
separating the North American and Pacific plates, and it may have formed 30
million years ago when Baja California split from mainland Mexico.
Short Discussion/Essay
1. Explain how strike is determined in an outcrop of sedimentary rocks.
2. Categorize the strain responses to compressional and tensional stress.
3. Why do horizontal beds lack strike and dip?
4. Differentiate plunging anticlines and synclines by the dip of their limbs, age
relationships of their beds, and direction of their V-shaped outcrop patterns.
5. Explain how to determine which block of a fault forms the hanging wall.
Draw diagrams to illustrate the relative hanging wall movements for normal,
reverse, thrust, and oblique-slip faults.
Longer Discussion/Essay
1. Explain why strike-slip faults may actually lack a hanging wall and a
footwall.
2. How much displacement is required to constitute a fault, and why has this
become an issue in evaluating landfill and other waste disposal sites?
3. Explain how the width of a geological unit shown on a geologic map is
related to angle of dip.
4. Explain why most of the great mountain chains of the world result from
compressional stress.
5. Why are anticlines such good traps for petroleum?
Selected Readings
Structural geology is a basic subdiscipline of geology taught in virtually all
undergraduate curricula. There are numerous textbooks treating this subject.
The following are selected examples:
Davis, G.H. and Reynolds, S.J. 1996. Structural Geology of Rocks and Regions.
2nd Edition. New York: John Wiley and Sons.
Hatcher, R.D., Jr. 1995. Structural Geology. 2nd Edition. Englewood Cliffs, NJ:
Prentice-Hall.
Twiss, R.J. and Moores, E.M. 1992. Structural Geology. W.H. Freeman and
Company.
Some articles of general interest involving structural geology:
Bjornerud, M.G. 1991. "Conveying Principles of Finite Strain with Standard
Graphics Software," Journal of Geological Education 39(1):23-27.
Kenah, C. 1994. "Squashed coins illustrate the power of structural geology,"
Journal of Geological Education 42:118-124
Stern, R.S. and Yeats, R.S. 1989. "Hidden earthquakes," Scientific American
261: 48-57.
Wallace, R.E. ed. 1990. The San Andreas fault system. U.S. Geological Survey
Professional Paper 1515.