joints and faults - georgia southwestern state...

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)

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.

W E

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 East Wall 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.

A B

w

x

y

z

If you were in car “z”, you would see car “y” out your right window.

If you were in car “y”, you would see car “z” out your right window.

If you were in car “w”, you would see car “x” out your left window.

If you were in car “x”, you would see car “w” out your left window.

Left-lateral strike-slip fault 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 left-

lateral? 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 East Block UNDER the fault at any point OVER the fault

WEST EAST

FOOTWALL BLOCK HANGING WALL BLOCK

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.

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.

Arrows show the “sense of motion”

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”. 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 cross-section.

AFTER NORMAL FAULTING

22 units

2 units

7 units

13 units

BEFORE FAULTING

20 units

7 units 13 units

Incipient fault

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.

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!


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

20 units

7 units 13 units

Incipient fault

18 units

AFTER REVERSE FAULTING

7 units

13 units

2 units

18 units

AFTER REVERSE FAULTING

7 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.

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.

Dip direction

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.

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) Mesozoic (MZ) Paleozoic (PZ) Precambrian (PC)

Incipient fault

DOWNTHROWN UPTHROWN

D C B A

If erosion cut down to level “A”, which side (west or east) would have older rocks? WHY? What about level “B”? “C”? “D”?

DOWNTHROWN UPTHROWN

Oligocene Eocene Paleocene

Oligocene Eocene Paleocene

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!

A C B D

MZ (younger) PZ (older) (upthrown)

PZ (younger) PC (older) (upthrown)

CZ (younger) MZ (older) (upthrown)

Ol (younger) Pc (older) (upthrown)

Ol CZ Eo Pc MZ PZ PC

A C B D






If the fault were not vertical, but dipped eastward instead, what type of faults are these?

A C B D






HW HW

HW HW

In every case the east block is both the hanging wall block and upthrown.

joints and faults - georgia southwestern state...

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