JOINTS AND FAULTS
Brittle strain in a rock can take two forms.
In one case the fracture separates two pieces of the crust, but there is no indication of
movement of one side relative to each other. Such fractures are called joints. While
interesting and somewhat informative, they are not sufficiently interesting and are too
difficult to interpret for the level of this course.
When there is some indication of relative movement of one or both sides the fracture is a
fault. These are much more interesting and informative than joints, and we will learn to
recognize three types.
Incidentally, relative movement means that the two sides appear to have slipped in
different directions, offsetting any features that were originally continuous across where
the fault now lies. This will become obvious from the diagrams that follow.
joint picture?
Faults
CROSS-SECTION (FACING NORTH, WHICH IS TRADITIONAL)
W
E
The fracture has separated
the rocks into two
discontinuous pieces called
blocks.
The fracture may not actually
be perfectly planar, but we
usually represent it on a
schematic diagram as if it is.
That is, we draw it as a
straight line down the side of
the cross section.
This is the west block of this
fault,
and this is the east
block of the fault.
CROSS-SECTION
Imagine separating the
two blocks from each
other. The fault surface
on each one would
define the edge of that
block. That edge is
referred to a wall of the
fault.
West
Wall
East
Wall
CROSS-SECTION
Recall that faults are
treated as essentially
planar features and that we
can describe planar features
by their strike and dip.
The movement on some
faults is essentially entirely
horizontal. In other words
we can measure offset on
the upper surface (map
views), but none on the
sides (cross-sections).
These are strike-slip faults.
The opposite, vertical offset
visible in cross-section, are
dip-slip faults.
Most of the faults we will be
interested in will be dip-slip.
These generally dip at
angles other than 90°, like
this one. However, we will
initially think about strike-slip
faults that are typically
approximately vertical – their
walls have dip angles of 90°.
CROSS-SECTION
On the surface of the Earth, the plane of the fault intersects the erosional surface and defines a
line. This line is called the fault trace. In the idealized faults we’ll see this line will be straight, but if
the fault is not perfectly planar or the ground not perfectly flat, the trace will not be perfectly straight.
The dip direction of the fault is indicated on the map by some symbol. In this class we will use an
arrow. On real geologic maps the symbol generally tells what type of fault it is.
Dip direction
Strike-slip faults
This photograph was taken not long after
the 1906 “great fire” of San Francisco.
The fire was actually started by an
earthquake with estimated Richter
magnitude of 8.3, but none of the
newspapers gave that fact more than
passing image. The famous San Andreas
Fault was the culprit.
The fence is in Marin Co, CA, north of San
Francisco. There is about 2.5m (8.5ft) of
offset on the original fence, repaired by
lashing or nailing poles across the gap.
Notice there is no vertical offset – only
horizontal. All the movement was along
the strike of the fault plane and none
along the dip.
This is characteristic of strike-slip faults.
The photographer was standing on one
side of the fault facing directly across.
Notice that the fence on the far side was
moved to the viewer’s right. If the viewer
were on the far side looking this way,
which way would the offset (on this side)
look?
Public use image courtesy of the United States Geological Survey.
If you were in car “w”,
you would see car “x”
out your left window.
w
If you were in car “y”,
you would see car “z”
out your right window.
y
x
If you were in car “x”,
you would see car “w”
out your left window.
z
If you were in car “z”,
you would see car “y”
out your right window.
A Left-lateral strike-slip fault
B Right-lateral strike-slip fault
Is this a right- or a
Left-lateral fault?
Public use image courtesy of the United States Geological Survey.
The ocean ridges are frequently offset by large-scale strike-slip faults like the ones shown here in the equatorial Atlantic.
These very large forms are called transform faults. The Romance Fracture Zone is one such fault. Is it right- or leftlateral? Can you spot one with the other sense of motion?
Map copyright belongs to The National Geographic Society © 1975.
Dip-slip faults
CROSS-SECTION
West Block
UNDER the fault at any point
East Block
OVER the fault
Faults generally make highly permeable pathways for mineral-rich fluids moving through or away from
metamorphic centers or igneous bodies. Consequently, mines are often located along a fault plane. Any
miner working in a mine here would have one block under his foot, and the other hanging over his head.
The former is called the footwall block and the latter the hanging wall block. The footwall block is the
one “under” the fault at any point, and the hanging wall block is above it.
WEST
FOOTWALL BLOCK
EAST
HANGING WALL BLOCK
NORMAL FAULTS
A normal fault is one in which the
hanging wall block is downthrown.
CROSS-SECTION
Fault scarp at surface makes sense of
motion obvious initially.
Footwall block seems to
have moved upward. It is
said to be “upthrown”.
Hanging wall block seems to
have moved downward. It is
said to be “downthrown”.
Arrows show the
“sense of motion”
Actually, the hanging wall
block is the one that
probably does most
or all of the moving
in most dip-slip
faults.
CROSS-SECTION
After erosion levels the fault scarp, offset is no longer obvious at the surface
However, originally continuous
features (like sedimentary
beds, intrusions, or older faults)
that are older than the fault will
show the offset in crosssection.
BEFORE FAULTING
7 units
13 units
Incipient fault
20 units
AFTER NORMAL FAULTING
7 units
13 units
2 units
22 units
AFTER NORMAL FAULTING
22 units
Normal faults result from tensional stress.
Both the sides of the active rifts in the axes of the ridges and the parallel “grooves” evident for
some distance away on either side are fault scarps of normal faults.
Map copyright belongs to The National Geographic Society © 1975.
Normal faults are also common in
continental rift zones, such as the
East African Rift Valley.
These faults are still active. If
they continue to be active, the rift
zone will eventually widen
enough to be an incipient ocean,
as the Red Sea and Gulf of Aden
have already done!
Map copyright belongs to The National Geographic Society © 1967.
REVERSE FAULTS
A reverse faults is one in which the hanging wall
block is upthrown.
(NOTE: there is nothing abnormal about a reverse fault. They are every bit
as common as normal faults.)
CROSS-SECTION
Overhanging fault scarp at surface
makes sense of motion obvious initially.
CROSS-SECTION
After erosion levels the fault scarp, offset is no longer obvious at the surface,
But it is easy to determine
in cross-section
BEFORE FAULTING
7 units
13 units
Incipient fault
20 units
AFTER REVERSE FAULTING
13 units
2 units
7 units
18 units
AFTER REVERSE FAULTING
7 units
18 units
Reverse faults result from compressional stress.
In most reverse faults the angle of dip is very low, often less than 25°.
Such faults are called thrust faults. The lateral offset (and therefore the
crustal shortening) can be many kilometers.
Reverse faults at Pinto, MD.
After the rocks had folded as
tightly as they could,
continued stress finally
fractured them.
View is about 1m across.
Reverse faults, actually mostly
thrusts, are typical of continental
mountain chains.
The red arrows indicate the typical
dip directions for thrusts in the
Appalachians and the Andes.
Because the compressional
stresses that create thrusts push
the hanging wall block up the fault
plane (as if it were a ramp), the dip
direction indicates the direction
toward the stress that created the
fault.
Map copyright belongs to The National Geographic Society © 1975.
Distinguishing dip-slip faults
on maps.
Recall the definitions of the dip-slip
fault types:
• A normal fault is one in which the hanging wall
block is downthrown.
• A reverse fault is one in which the hanging wall
block is upthrown.
• Note that both refer to the hanging wall block.
• We need to know two things to determine fault
type:
– which side is the hanging wall, and
– which side is upthrown and which downthrown.
Remember that there is a special map symbol that indicates the dip direction of the fault.
Remember too that the dip is defined as the direction and angle of tilt downward from the
horizontal, so it will always be an acute angle. (Unless the dip is vertical.)
Therefore it follows that the dip indicator will always point toward the hanging wall block.
Determining this part is easy.
Dip direction
FW
HW
N
Which side of this fault (in map view) is the hanging wall block?
Upthrown/downthrown is also pretty easy to determine once you know
what to look for.
Imagine that this stack of rocks is about to fault along the dotted line.
Cenozoic (CZ)
Incipient fault
Mesozoic (MZ)
Paleozoic (PZ)
Precambrian (PC)
If erosion cut down to level “A”, which side (west or east) would have older
rocks? WHY? What about level “B”? “C”? “D”?
D
C
B
A
DOWNTHROWN
UPTHROWN
Even at level “D” the same age relationship holds if we understand the
ages in enough detail. THE OLDER ROCK WILL ALWAYS BE ON THE
UPTHROWN BLOCK!
Oligocene
Eocene
Oligocene
Paleocene
Eocene
Paleocene
DOWNTHROWN
UPTHROWN
A
C
PZ (younger) PC (older)
(upthrown)
CZ (younger) MZ (older)
(upthrown)
Ol
CZ Eo
Pc
MZ
MZ (younger) PZ (older)
(upthrown)
Ol (younger) Pc (older)
(upthrown)
PZ
PC
B
D
If the fault were not vertical, but dipped eastward instead, what type of faults are these?
A
C
CZ (younger) MZ (older)
(upthrown)
PZ (younger) PC (older)
(upthrown)
Ol
CZ Eo
Pc
MZ
Ol (younger) Pc (older)
(upthrown)
MZ (younger) PZ (older)
(upthrown)
PZ
PC
B
D
In every case the east block is both the hanging wall block and upthrown.
A
C
PZ (younger) PC (older)
(upthrown)
CZ (younger) MZ (older)
(upthrown)
Ol
HW
HW
CZ Eo
Pc
MZ
MZ (younger) PZ (older)
(upthrown)
Ol (younger) Pc (older)
(upthrown)
HW
B
HW
D
PZ
PC