CRUSTAL DEFORMATION

1
CRUSTAL DEFORMATION

• FOLDS FAULTS

2
Structural Geology study of the
architecture of the crust
.by examination of deformed
rocks DEFORMATION changes in location,
orientation, shape, volume results from
stresses that exceed rock strength
ductile (bends)
brittle (breaks)
same material (fence) deforms in different ways
from Davis and Reynolds, 1996
3
TERMS

• FOLD a form of ductile strain in which rock
  layers bend, usually as a result of compression
• FAULT a break in a body of rock along which one
  block slides relative to another
• Strain Any change in the shape or volume of a
  rock that results from stress

4
rocks, like the fence, will deform in different
ways, depending upon the properties of the rock
and the nature of the applied stress
folds (ductile)
faults (brittle)
5
macroscopic (map scale)
Keystone thrust (dark over light)
Waterpocket fold
left from Robert Varga, http//www.wooster.edu/g
eology/GEO313/Images/
right from http//dax.geo.arizona.edu/earthscope/
frontpage/images/ut18_lg.jpg
6
mesoscopic (outcrop)
all from J. Waldron, http//www.stmarys.ca/academ
ic/science/geology/structural/
7
TYPES OF FOLDS

• ANTICLINE a fold where the oldest rock is in the
  center and is generally in an arch shape
• SYNCLINE a fold where the youngest layer is in
  the center and is generally in the shape of a
  bowl (U)
• MONOCLINE a fold where both limbs are horizontal

8
Folds

• fold bends or wave-like features in layered rock
• anticline an upward arching fold
• hinge line the axis of the fold
• syncline a downward-arching counterpart of an
  anticline
• axial plane a plane containing all of the hinge
  lines of a fold

9
Plunging Folds

• plunging folds folds in which the hinge lines
  are not horizontal

Plunging folds anticline on left and right,
syncline in center. The hinge lines are at an
angle to the block diagram, penetrating
the surface and emerging from the front cross
section
10
Convergent Plate Boundaries and Folding
Continent-Continent collision forms Folded
Mountain BeltAlps, Himalayans, Appalachians
Ocean-Ocean collision forms Island Arc
Japan, Aleutians, Cent. Am.
11
Folded Rock Before Erosion
12
Folded Rock After Erosion
Eroded Anticline, older rocks in center. Syncline
is opposite.
13
Structural Domes and Structural Basins

• structural dome a structure in which the beds
  dip away from a central point
• structural basin a structure in which the beds
  dip toward a central point

Structural basin
Structural dome
14
the two limbs of this fold have the same strike
but different dips
cross-section
15
Folded Rocks (Dorset, England)Center has
overturned area
Older
Overturned Area
Younger
Lucky we have ways of recognizing right side
up What are they?
Source Tom Bean
Older
Younger
16
Anticlines and Oil
Early USA petroleum exploration, e.g.
Pennsylvania anticlines
17
TYPES OF FAULTS

• NORMAL a fault where the hanging wall moves
  downward relative to the foot wall pulled apart
  by tension at divergent boundaries
• REVERSE where compression causes the hanging
  wall to move upward with respect to the foot wall
  at convergent boundaries
• STRIKE-SLIP the rock on either side of the fault
  slides horizontally in response to shear stress
  at transform fault boundaries

18
Faults

• dip-slip fault movement is parallel to the dip
  of the fault surface
• strike-slip fault horizontal motion parallel to
  the strike of the fault surface
• oblique-slip fault both strike-slip and dip-slip
  components

19
Dip-Slip Faults

• footwall the underlying surface of an inclined
  fault plane
• hanging wall the overlying surface of an
  inclined fault plane

20
Dip-Slip Faults (cont.)

• normal fault a fault where the hanging-wall
  block has moved downward relative to the footwall
  block
• graben when a block bounded by normal faults
  drops down
• horst when a block bounded by normal faults is
  uplifted

21
Dip-Slip Faults (cont.)

• reverse fault when the hanging-wall block has
  moved upward relative to the footwall block
• thrust fault a reverse fault in which the dip of
  the fault plane is at a low angle to horizontal

A reverse fault. The fault is unaffected by
erosion. Arrows indicate compressive stress.
Diagram shows area after erosion dashed lines
indicate portion eroded away
Thrust fault due to horizontal compression.
22
Strike-slip Faults

• strike-slip fault a fault where the movement is
  predominantly horizontal and parallel to the
  strike of the fault
• right-lateral fault a strike-slip fault in which
  the block seen across the fault appears displaced
  to the right
• left-lateral fault a strike-slip fault in which
  the block seen across the fault appears displaced
  to the left

23
Dip-Slip Faults
Demo Cardboard Models
24
Reverse Fault (called Thrust Fault if shallow
angle)
Typical of convergent margins
(Hanging wall Up)
Structural Geology is taught by Dr. Krall
This guy is rich
Younger
What phase of magma fractionation would result in
the placement of this ore body? Which formed
first, the ore body or the fault? What common
mineral is mostly likely in the ore body?
Miners pay geologists to find their lost
orebody One friend earned enough to buy a house
This poor guy is out of luck
25
Normal Fault Hanging Wall Down
Foot wall under the fault plane
Hanging wall overhangs the fault plane
Key Bed
Source John S. Shelton
Especially common in divergent margins
26
Faults and Oil
27
TERMS

• STRESS The amount of force exerted on a unit of
  rock
• ISOTASY a condition of gravitational and buoyant
  equilibrium between the earths Lithosphere and
  the Athenosphere
• Deformation the bending, tilting and breaking of
  the Earths crust

28
importance of stress and strain
stress is the force applied and strain is the
deformation that results
stress is instantaneous and only exists when
applied strain is permanent change in a rock
29
TYPES OF STRESS

• Compression the type of stress that squeezes and
  shortens, commonly reduces the volume of the rock
• Tension stress that stretches and pulls the rock
  layer apart
• Shear stress that distorts a body by pushing
  parts of the body in opposite directions

30
Stress-Strain
31
Compression, Tension, and Shearing Stress
Convergent Divergent Transform
32
Shear stress and strain