Geology 1100
Lecture 19 Wed. Mar. 08, 2017
• GeoNews
• Today and Friday:
– Chapter 11 Deformation of rocks
• Reading: Read Ch. 12 for Monday/Wednesday after break
1
Deformation Overview
• Patterns in deformation tell story of past forces acting on rocks
• Certain types of deformation trap gas, oil, or give clues to where economic minerals found
• Some types of deformation related to hazards, like earthquakes
• Two broad types of deformation:
– Folds (rocks layers bent, but not broken)
– Faults (rocks broken, often sliding past each other along a single "fault plane")
– and Intermediate and Special Cases of above
• Geologists have developed lots of specific names for different types of deformation,
and lots of terms for describing the deformation quantitatively
– Book starts by introducing terms for describing "deformation" (from horizontal): Dip, Strike
– I will instead jump ahead to show simple types of deformation,
then come back to terms for describing it
2
Simple 1-Dimensional Folds
• If rocks layers are compressed horizontally, they can bend up and
down in accordion like folds:
– Axis (of fold), and Limbs to either side of axis
• We usually can only see the surface, not the side cross-section shown
above, so we need to infer subsurface details from patterns in rocks are
exposed at the surface. In following, assume erosion slides off all
rocks above a certain horizontal level.
• Syncline: Downward bend.
Youngest rocks exposed at center, getting
older as you move away from center
• Anticline: Upward bend:
Oldest rocks exposed at center, getting
younger as you move away from center
• Geologists map surface rocks, and measuring dip and strike of rock
layers, and infer of folds.
– "T" like symbols show which way rock layers dip.
– Lines with arrows show axes, and whether it is syncline or anticline
3
Side view of anticlines and synclines
• In a few places river canyons,
road cuts, etc., do give you a
cross-section (side)
view of the rock layers
• Geologist often draw and
attach attach inferred
cross-sections to their
geological maps, to help
readers visualize the rock
structure.
4
Dip and Strike
• DIP: Tail of T points in direction of steepest dip (direction ball would roll on bedding plane).
– Angle of dip (called "The Dip") measures how steeply the bedding plane is tilted (0 0 = horizontal, 900 = vertical).
• Strike: Top of T is perpendicular to dip. On map is equivalent to line representing intersection of bedding plane and horizontal plane.
– Direction of that line (called "The Strike") is measured in degrees east of north (North = 0, East = 90, S = 180, W = 270).
IF the actual land surface was horizontal, that is the "compass heading" you would need to follow to stay on the Contact (the boundary)
between the two rock layers.
• Dip and Strike refer to the way the rock layers are tilted, not the way the land surface is tilted
– But we often measure that by finding place where the land surface IS (locally) the boundary between two rock layers.
– That happens when lower rock layer resists erosion and upper one does not (i.e. weak shale over hard sandstone)
5
Plunging Folds
• Take previous example of folds with horizontal axes -(so level of rocks was same at front of diagram as at the back)
and tilt the rocks to that the front of the diagram goes down
and the back goes up.
• Next, erode all the rocks to a level horizontal surface
• The axis of the fold "Plunges" as you move to the front of the
diagram:
– Indicate that by an arrow on the axis line of the map.
• The dip of the rocks will no longer be just side-to-side but will
be partly towards the front of the diagram (so the tails of the
T's will point partly that way as well)
• Because the rocks dip down toward the front -- after erosion,
older (originally lower) rocks will be more exposed towards
the back, and younger rocks will be exposed more towards the
front
6
Non-Plunging vs. Plunging Folds
• Above diagram is before surface is planed flat by erosion
• After erosion you get pattern as on left:
• Sheep Mountain (near Greybull),
a classic plunging anticline:
– older rocks near center
– younger as you move away from axes, or in the direction of
the plunge
7
Anticline dome: Hinge line plunges to both sides of a center point
From Schott,
Bakersfield
College
• Big
Medicine
Bow
Anticline
8
Joints: Cracks without displacement
9
Joints: Cracks without displacement
• Need to distinguish between:
• joints
• (cracks formed after the rock was "lithified")
•
• versus
• bedding planes and crossbeds, created as the
original sediment was deposited
10
Faults: Cracks where
rocks on opposite sides do move
• Dip-slip faults, one side moves up, the other down
(in dip direction)
– Dip of fault (like dip of rock layer) is angle from the horizontal:
00 = horizontal, 900 = vertical
– Footwall: Side on bottom of vertical line through fault
– Hanging wall: Side on top of vertical line through fault
• 3 names for dip-slip faults:
– Normal fault: Hanging wall goes down
• Indicates tension -- outer sides of box are moving apart
– Reverse and Thrust faults: Hanging wall goes up
• Indicates compression -- outer sides of box are moving together
• Thrust (mostly horizontal motion) Dip Angle > 450
• Reverse (mostly vertical motion) Dip Angle < 450
11
Faults: Cracks where
rocks on opposite sides do move
• Strike-slip faults:
opposite sides move
horizontally
(in "strike" direction)
– Left-lateral: Side opposite
you moves to your left
– Right-lateral: Side opposite
you moves to your right
• Oblique-slip faults:
Combination of horizontal
and vertical motion
12
Oil, Gas, Mineral Locations
• Gas and Oil migrate upwards through permeable (porous)
layers like sandstone, as they float on denser water
• They escape to the surface (and are destroyed) unless
they are stopped by impermeable layers (such as shale)
• Anticlines and certain fault arrangements can trap gas and
oil because all ways "up" are blocked.
• While the above fault has a special configuration which
creates a "cap", most faults and joints allow flow of fluids
along the fault.
– Flowing fluids can over time deposit minerals
13
Stress vs. Strain
• Stress: The FORCE which is applied across
a surface
– Compression (= shortening)
– Tension (= elongation = extension)
– Shear (sideways relative to surface)
• Strain: The DEFORMATION (motion) of
the rock in response to stress
– Also can be compression, tension, or shear
• Rock Strength: The rock's ability to
withstand stress without the rock
breaking or flowing
14
Folding vs. Faulting
• Elastic Deformation: Once the stress is
removed, the rock simple "bounces" back -like a spring.
When subject to stress beyond a given
Yield Strength
the rocks permanently deform.
(Varies with rock type, other facts)
• Faulting: The rock breaks and that on one
side of the fault(s)
moves relative to that on the other
• Flow: (Rock deforms smoothly -- also
called ductile or plastic deformation)
15
Strength of Rocks: Experiments
• At stress higher than Yield Strength rock will wither fracture or flow
• With Low Confining Pressure rock fractures
(breaks into pieces along fracture planes, and pieces move relative to each other)
• At High Confining Pressure rock flows
(individual mineral grains and atoms within minerals move relative to each other)
16
Experiments on given rock
at various confining pressures
• At surface (very low confining pressure):
strain (shortening) proportional to stress till, at yield strength, rock fractures.
• At 1km depth (moderately low confining pressure):
same as above, except
– confining pressure strengthens the rock, so you get slightly less strain for a given stress,
and the yield strength at which it finally breaks is higher.
• At 6 km depth (moderately high confining pressure):
same as above except, at yield strength, rock begins to flow instead of
fracture,
• At 12 km depth (high confining pressure):
same as above except
– confining pressure strengthens the rock, and once it does start to flow,
amount of flow is less than in above case
• If, before reaching yield strength, you remove stress,
then strain returns to zero. Above yield strength, strain is permanent
• Once rock begins to flow, amount of flow is related not just to stress,
but to how long you apply it
17
Different rocks, different depths
• Different rocks have different yield strengths (strong vs. weak)
• Different rocks have different preference for fracture vs. flow (brittle,
ductile)
• Different rocks flow by different amounts once they fail
• Effects of pressure, temperature
– Higher confining pressure strengthens rocks
– Higher temperature weakens rocks
• Because of above, for given rock:
– Given rock at surface tends to fracture
– Given rock at depth tends to flow
• Exception: Layers of rocks act differently than single rocks
(next slide)
18
Folding of layers of rock
• Previous experiments were on whole (solid) rocks
• Layers of different rocks can behave differently than a single, solid rock
– Single thick beam of wood behaves like single rock: deforms elastically till at yield strength it fractures
– Collection of individual layers of wood (if not glued together) or layers of paper can bend because
individual layers can slide relative to each other
• Sedimentary rocks folds often drape over an underling fault in crystalline basement rock
19
Patterns in Extension (tension)
• Plastic flow of rock at depth
• Reverse faults in upper layers
– Graben: Down-dropped valleys
– Horst: High blocks between valleys
• Occasional strike-slip faults offset horst or graben
• Basin and Range province in Western US are
whole series of horst and graben:
– basin = graben
– range = horst
20
Patterns in Compression
• Plastic flow of rock at depth
• Fold, thrust faults, reverse faults, in upper layers
• Occasional strike-slip faults offset
valleys and ridges
• Appalachian Mountains in Eastern US
21
Patterns in Shear
• Plastic flow of rock at depth
• Strike-slip faults in upper layers
– If strike-slip fault bends,
then at the bends you get extension and compression
(LA Basin, San Gabriel Mountains)
• San Andreas Fault, CA
22
Earthquakes
• Rocks along fault can creep in very small steps, without
storing up energy for earthquakes
• Elastic Rebound Theory
– If fault is strong, rocks cannot creep past each other
– Rocks deform, string up elastic energy, until at some place,
stress exceeds yield strength
– Rocks there fracture, transferring stress to other locations
which also then fracture
– Energy suddenly released along a long portion of the fault
23