Geology Lecture Outline –
Crustal Deformation –
(Ch 10)
I. Lecture Content
Introduction - Makings of High Places
Deformation - Stress and Strain
Structural Geology - Folding, Faulting, and Shearing
II. Introduction
A. Deformation and
1. Spectacular crustal
upheavals are the results of:
· · Same forces that power plate
tectonics
·
·
Interplay
between crustal rocks and the global
and
regional-scale deviatoric stresses created
by inter-plate
motions
2.
Deformation events are typically part of much larger
regional mountain building events called orogenies.
III. Deformation
A. Stress and Strain - Defined
1. STRESS Defined:
ü
ü
Force
applied to a given area of rock
ü
ü
Stress
= Force/unit area
·
·
Stress
causes strain (deformation) in rocks
·
·
Stress
applied to a rock can be exerted:
ü
ü
Equally
in all directions = Isostatic stress
ü
ü
Unequally
from 2 directions = Deviatoric stress
·
·
Deviatoric Stress can be exerted in several
ways
ü
ü Compressional
ü
ü Tensional
ü ü Shearing
2. STRAIN
·
·
Defined:
ü
ü
Changes
in the shape and/or volume of a rock
·
·
Strain means the same thing as "deformation"
·
·
Strain
is the result of stress
ü ü Stress is the cause
ü ü Strain is the effect
·
·
Three
types of strain are recognized -
ü
ü
Elastic
ü
ü
Plastic
ü
ü
Fracture
·
·
The
type of strain is dependent upon the response of
rocks to a
particular stress
·
·
Factors that control the type of strain in a rock
ü ü Kind of applied stress
ü ü Amount of pressure (force)
ü ü Temperature
ü ü Rock type
ü ü Length of time under stress
3.
Stress and Strain Illustrated
·
·
See
Figures
B. Three Types
of Deviatoric Stress
1. Compressional Stress
·
·
Defined:
ü
ü
External
forces applied toward one another
·
·
Rock under compressional stress results in:
ü
ü
Shortening
in the direction of stress
ü
ü
Thickening
perpendicular to the stress
ü
ü
Deformation
in the form of folding and faulting
2. Tensional Stress
·
·
Defined:
ü ü External forces acting in
opposite direction along
the same line
·
·
Rock under tensional stress results
in:
ü
ü
Lengthening/extension
in the direction of stress
ü
ü
Thinning
perpendicular to the stress
ü
ü
Deformation
in the form of faulting
3. Shear Stress
·
·
Defined:
ü
ü
External
forces acting parallel to one another
but in opposite
directions
·
·
Rock under shearing stress results
in:
ü
ü
Displacement
of adjacent layers along closely
spaced planes in
the direction of stress
ü
ü
Deformation
in the form of folding, faulting, and
shear zones
C. Three Types of Strain
1. Elastic Strain
·
·
Rocks
behave in an elastic fashion
ü
ü
Stressed
rock returns to its original shape and
volume when the
stress field is removed
·
·
Elastic behavior in rock common in the upper crust
2. Plastic Strain
·
·
Rocks
behave in a plastic fashion
ü ü Stressed rock remains deformed
after the stress
field is removed
·
·
Amount
of plastic deformation in rock depends on its
degree of ductility
ü
ü
Rocks
that are said to be "ductile" are able
undergo
large amounts
of plastic strain
ü
ü
Rocks
that are said to be "brittle" are not able to
undergo much plastic
strain
·
·
Plastic behavior common in the lower crust
3.
Fracture Strain
·
·
Rocks
behave in a brittle fashion
ü ü Stressed rock breaks into two or
more pieces
before the stress
field is removed
ü ü Rock remains deformed after
stress is removed
·
·
Brittle-Fracture behavior common in the upper crust.
IV. Structural Geology
A. Geologic Structures
1. Bodies of
rock that are distinguished by their geometric
arrangement of
both rock types and structural elements
such as size, layering, shape, and orientation.
2. Many
types of geologic structures are the result of
deformation
events such as uplift, folding and faulting
B. Orientation of Structural
Layering
1. Strike - Definition
2. Dip - Definition
3. Determining Strike and Dip
C. Graphic
Illustrations of Geologic Structures
1.
Geology Maps
2.
Block Diagrams
D. Fold
Structures
1. Fold defined:
·
·
Originally
planar feature that is now bent
ü ü Planer features like bedding
·
·
Bending process is plastic deformation
2. Anatomy of
a Fold
·
·
Fold limb = "cline"
·
·
Fold axis = Line separating limbs
·
·
Axial plane = Plane equally separating limbs
3.Three Basic Types
of Fold Structures
·
· Monoclines
ü ü Simple bend with only one bent
limb
ü ü Illustrated in Figure
·
· Anticlines
ü ü Convex-upward fold
ü ü Limbs dip away from each other
ü ü Oldest layers exposed in the core
part of fold
ü ü Illustrated in Figure
·
· Synclines
ü ü Concave-upward fold
ü ü Limbs dip towards each other
ü ü Youngest layers exposed in the
core part of fold
ü ü Illustrated in Figure
4. Fold structures can be
oriented in several different ways
·
·
Upright
fold
ü ü Fold axis is horizontal
ü ü Axial plane is vertical
ü ü Illustrated in Figure
·
·
Inclined
fold
ü ü Fold axis is horizontal
ü ü Axial plane is inclined
ü ü Fold limbs dip at different
angles
ü ü Illustrated in Figure
·
·
Overturned
fold
ü ü Fold axis is horizontal
ü ü Axial plane is inclined
ü ü Both fold limbs dip in the same
direction
ü ü Illustrated in Figure 13.13(b)
·
·
Recumbent
fold
ü ü Fold axis is horizontal
ü ü Axial plane is inclined to near
horizontal
ü ü Illustrated in Figure
·
·
Plunging fold
ü ü Fold axis is inclined (plunging)
ü ü Axial plane is vertical
ü ü Illustrated in Figure
5. Two special types of fold
structures
·
· Domes
ü
ü
Circular
to oval-shaped anticline
ü
ü
All
rock layers dip away from center of dome
ü
ü
Oldest
rocks exposed in center of structure
·
· Basins
ü
ü
Circular
to oval-shaped syncline
ü
ü
All
rock layers dip towards the center of basin
ü
ü
Youngest
rocks exposed in center of structure
6. Folds
are typically the result of compressional
stresses
acting on plastic-behaving layered rock.
E. Fault
Structures
1. Fault defined:
· · A fracture along
which opposing crustal blocks move
parallel to the fracture surface (plane of the
fault)
2. Anatomy of
a Fault - (See Figure )
·
· Hanging wall block
·
· Footwall block
·
·
Fault plane (dip and strike)
·
·
Fault scarp
3. Three Basic
Types of Fault Movement
·
·
Pure
Dip-slip
ü
ü
Relative
fault motion parallel to dip direction
ü
ü
Relative
fault motion can be either hanging wall
up or hanging
wall down
·
·
Pure
Strike-slip
ü
ü
Relative
fault motion parallel to strike direction
ü
ü
Relative
fault motion can be either right lateral or
left lateral
· ·
Oblique-slip
ü
ü
Relative
fault motion has both dip and strike
directional
components
ü
ü
Relative
fault motion can be any which way
4. Three Basic Types of Faults
·
·
ü
ü
Hanging
wall moves down relative to footwall
ü
ü
Caused
by tensional stress
ü
ü
Associated
with divergent tectonics
·
· Reverse faults
ü
ü
Hanging
wall moves down relative to footwall
ü
ü
Called
a thrust fault if fault plane angle < 45°
ü
ü
Caused
by tensional stress
ü
ü
Associated
with convergent tectonics
·
· Strike-slip faults
ü
ü
Opposing
blocks move laterally to each other -
either right-laterally or left-laterally
ü
ü
Caused
by shearing stress
ü
ü
Associated
with transform tectonics
5. Faults
are the result of deviatoric stresses acting on
brittle-behaving rock
F. Joint
Structures
1. Joint defined:
· · A fracture along
which no movement has occurred, or
movement
occurred perpendicular to joint surface
·
·
Lack
of movement along fracture plane is difference
between joints
and faults
2. Commonest strain-related structures in
rocks
3. Formation of joint fractures
·
·
Caused
by either compression, tension, or shear stress
·
·
Typically
caused by a release of pressure
·
·
Also
caused by the rapid cooling of magma or rock
·
·
Joint-forming
process a shallow crustal phenomenon
·
·
Typically
form either columnar or sheet-like patterns
V. Vocabulary – Crustal Deformation and Structural Geology - CH10
Anticline
Basin
Compression
Continental
accretion
Craton
Deformation
Dip
Dip-slip fault
Dome
Elastic strain
Fault
Fault plane
Footwall block
Fracture strain
Hanging wall
block
Joint
Monocline
Normal fault
Oblique-slip
fault
Orogeny
Plastic strain
Plunging fold
Reverse fault
Shear stress
Shield
Strain
Stress
Strike
Strike-slip
fault
Suture zone
Syncline
Tension
Terrane
Thrust fault