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Rock Deformation and Geologic Structures
Rock Deformation and Geologic Structures
• How do we measure rock deformation?
• What causes rocks to deform?
• How do geologic structures relate to stress,
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strain, and rock strength?
How does rock strength vary in the lithosphere?
What is the connection between earthquakes
and rock deformation?
How do we detect and measure earthquakes?
How are earthquakes destructive?
Why study rock deformation?
• Earthquakes are a powerful and violent force
and cause great destruction.
• Geologic structures can act as traps for
economically important materials, e.g. crude oil.
• Magmas often rise to the surface along
structures such as faults.
• Geologic structures are a record of rock
deformation – which is caused by plate tectonic
forces.
How do we measure rock deformation?
• Describing the spatial orientation of rocks with
tilted or dipping beds:
• Strike: The compass direction of a line formed by
the intersection of a horizontal plane (at the
surface) with the dipping (inclined) bed.
• Dip: The angle between the surface horizontal
plane and the inclined rock layer.
How do we measure rock deformation?
Example of determination of strike and dip.
Strike and dip symbols are used on geologic maps to
indicate orientation of rocks.
How do we measure rock deformation?
• Describing the spatial orientation of rocks that are
folded:
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Limbs: The two dipping sides of a fold.
Axial plane: Plane separating the two dipping sides.
Hinge line: Line where the dipping limbs join.
Plunging fold: When the hinge line plunges downward into
the ground (is not horizontal).
• Anticline: A folded structure that arches upward in the center
(limbs dip away from axial plane).
• Syncline: A folded structure that arches down in the center
(limbs dip in towards axial plane).
How do we measure rock deformation?
Folding may occur over very different scales: folds can
be very large or extremely small.
How do we measure rock deformation?
How do we measure rock deformation?
Anticlines and synclines can be either horizontal or plunging.
At the center of a fold is the Hinge Line and the Axial Plane.
The sides of a fold are called Limbs.
Hinge line
How do we measure rock deformation?
How do we measure rock deformation?
How do we measure rock deformation?
Anticlines: Layered rocks that have been folded into arches.
Thus, after erosion the oldest rocks are at the center.
Synclines: Layered rocks that have been folded into troughs.
Thus, after erosion the youngest rocks are at the center.
They often occur next to each other and repeat over distances.
Strike and Dip Measurements Indicate Structure At Depth
How do we measure rock deformation?
A plunging anticline closes up in the direction of plunge.
A plunging syncline opens out in the direction of plunge.
How do we measure rock deformation?
Anticlines and synclines which are plunging to the left (northwest in photo).
How do we measure rock deformation?
Structural domes and basins may also form.
In a dome the strikes form a circular pattern and dips are away
from the center. The oldest rocks are at the center.
In a basin the strikes form a circular pattern and dips are
towards the center. The youngest rocks are at the center.
Aerial photo of a structural dome. Rocks dip away from the center.
The Michigan Basin - Youngest Rocks At Center
How do we measure rock deformation?
• Faults – Planar breaks through rocks along
which movement occurs.
• Footwall: The rock below the fault plane.
• Hanging wall: The rock above the fault plane.
• Dip-slip: Movement on a fault that is primarily
vertical displacement along the dip surface.
These can be either normal or reverse.
• Strike-slip: motion along a fault that results in
horizontal rock displacement.
• Oblique-slip fault: A combination of dip- and strikeslip displacements.
How do we measure rock deformation?
Footwall and hanging
wall terminology comes
from mining.
It refers to the rock
under a miners feet,
and the wall above the
miner (where you might
hang your lamp).
How do we measure rock deformation?
A normal fault (dip-slip) - the hanging wall has moved
down relative to the footwall. Produced by tensional
tectonic forces.
How do we measure rock deformation?
A reverse fault (dip-slip) - the hanging wall has moved
up relative to the foot wall. Produced by compressional
tectonic forces.
How do we measure rock deformation?
A thrust fault is a low-angle (<45o) reverse fault.
How do we measure rock deformation?
Strike-slip faults
and
Oblique-slip faults
What causes rocks to deform?
• Faulting and folding
occur at plate
boundaries.
• Compressive force:
reverse faults and
folding.
(convergent boundaries)
• Extensional force:
normal faults.
(divergent boundaries)
• Strike-slip force (shear
stress): strike-slip faults.
(transform boundaries)
What causes rocks to deform?
• Joints: Fractures created by tension or
changes in pressure in brittle rocks.
• No movement has occurred.
• Can form in two ways 1) Form by decompression as rock is
brought to the surface, the rock expands.
2) Form by cooling, e.g. a lava flow, the
rock contracts.
Columnar Joints in Basalt
Contraction during cooling.
Joints Developed in Granite
Expansion during decompression.
Exfoliation Dome:
A type of jointing
developed in
granites due to uplift
and decompression
of the rock.
Geologic Structures And Economic Deposits
Oil and gas traps: Oil, gas, and groundwater all move through
pores and all have different densities that cause them to stratify. If
an impermeable layer of rock stops the upward migration a crude
oil and gas deposit may form.
Geologic Structures And Economic Deposits
The crests of anticlines or domes may accumulate crude oil and
gas deposits in this way. Faults may also act as traps.
Geologic Structures And Economic Deposits
Hot fluids migrate along faults
and deposit ore minerals.
Miners often follow fault zones
to recover these ores.
What causes rocks to deform?
• Rocks strain when stress exceeds the yield strength.
• Stress - the force applied to rocks
• Strain - the deformation resulting from stress
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(a change in size and shape)
• Three types of stress and strain.
• Compressive stress – results in shortening strain
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- convergent plate boundaries • Tensional stress – results in elongation strain
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- divergent plate boundaries • Shear stress – results in shear strain
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- transform plate boundaries -
What causes rocks to deform?
What causes rocks to deform?
Faulting, folding, and plastic flow are all types of strain.
What controls whether a rock is brittle or ductile?
• Confining Pressure
• Near the surface pressure is low, rocks are more brittle.
• Deeper in Earth pressure is higher, rocks are more ductile.
• Temperature
• At cooler temperatures, rocks are more brittle.
• At higher temperatures, rocks are more ductile.
• Type of rock
• Some rocks are naturally more brittle or ductile than others.
• High strength = brittle deformation, low strength = ductile deformation.
• e.g., Shale and halite are generally ductile, granite and quartzite are
generally brittle in their response to stress.
• Rate of deformation (i.e. applied stress)
• Very slow rates of deformation (e.g. plate tectonic motions) produce
ductile behavior.
• Fast rates of deformation (e.g. during an earthquake) produce brittle
behavior.
What controls whether a rock is
brittle or ductile?
Ductile and Brittle Strain Diagram
As stress is applied 3 different responses:
Elastic strain –
reversible, rock can
return to original
shape.
Plastic strain – rock
flows, strain rate
increases faster as
stress goes up,
irreversible.
Brittle strain – rock
breaks suddenly as
stress increases,
irreversible.
Simple Illustration of Elastic and Brittle Strain
What controls whether a rock is
brittle or ductile?
Experiments show that confining pressure affects whether
brittle or ductile deformation will occur.
Summary of “Rock Mechanics”
• Some rocks are strong and others are not.
• Thus, some are prone to brittle fracture while
others deform plastically – at the same stress level.
• With increasing depth, temperature and confining
pressure brittle fracture decreases and plastic flow
increases for all rock types.
• So, faults are expected at shallow depths, whereas
plastic flow should occur at depth.
What controls whether a rock is
brittle or ductile?
Horst – uplifted block between two normal faults.
Graben – downdropped block between two normal faults.
What controls whether a rock is
brittle or ductile?