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Geologic Structures
• Changes in the shape and/or orientation of rocks in
response to applied stress
Figure 15.19
Can be as big as a ‘breadbox’
Or much bigger than a ‘breadbox’
Three basic types
Fractures >>> The rocks break but don’t move
Faults
>>> The rocks and move
Folds
>>> Rocks don’t break, but deform
ductiley
What forces are
involved?
Stress and Strain
• Stress is force per unit area
– The three basic types of stress are
compressive, tensional and shear
• Strain is a change in size or
shape in response to stress
– Geologic structures are indicative
of the type of stress and its rate of
application, as well the physical
properties of the rocks or sediments
Rock Deformation
• Stress is the pressure or force applied to rocks
that cause deformation to occur
• Uniform (confining) stress is equal in all
directions (hydrostatic)
– Rocks are confined by the rock around them
• Differential stress is not equal in all directions
(directional)
– This is what deforms rocks
Rock Deformation
• Three types of differential stress
– Tensional - pulling apart
– Compressional - squeezing together
– Shear - slipping, twisting, or wrenching
• Strain is the result of applying a stress to
a rock
– The change in size and/or shape of a solid
Tension and compression
Shear stress
Rock Deformation
• Strain produces a spectrum of
deformation
– Elastic deformation
• Rocks return to original shape
– Ductile deformation
• Irreversible change in size and/or shape
• Volume and density may change
– Brittle deformation - Fracture
• Stress exceeds the ductile limit
• Irreversible break
How Rocks Respond to Stress
• Rocks behave as elastic, ductile or brittle
materials depending on:
– amount and rate of stress application
– type of rock
– temperature and pressure
• If deformed materials return to original shape
after stress removal, they are behaving elastically
• However, once the stress exceeds the elastic limit
of a rock, it deforms permanently
– ductile deformation involves bending plastically
– brittle deformation involves fracturing
Orientation of Geologic Structures
• Geologic structures are most obvious
in deformed sedimentary rocks
• Tilted beds, joints, and faults are planar
features whose orientation is described
by their strike and dip
– Strike is the compass direction of a line
formed by the intersection of an inclined
plane with a horizontal plane
– Dip is the direction and angle from
horizontal in which a plane is oriented
How do we
describe rock
relationships in
nature?
Geometry of Rock Structures
• Structures may be defined by the
orientation of planes
– Dip – the angle of inclination downward
from a horizontal plane
– Strike – the compass bearing of a
horizontal line where the inclined plane
intersects an imaginary horizontal plane
Figure 15.7
Fig. 7.5.
Strike & Dip
Structures and Geologic Maps
• Rock structures are determined
on the ground by geologists
observing rock outcrops
– Outcrops are places where bedrock
is exposed at the surface
• Geologic maps use standardized
symbols and patterns to represent
rock types and geologic structures,
such as tilted beds, joints, faults
and folds
Figure 15.8
Figure 15.9
Geologic Structures:
Fractures and
Faults
Joints
• Fractures created by tension in brittle
rocks
– No shear or displacement has occurred
– Form as overburden is removed, confining
stress reduced
– Form by cooling of igneous rocks
– Often occur in sets
Joint systems
Faults
• Fractures that have been displaced
– Most faults are inclined at some angle
measured from horizontal
• The dip angle of the fault
• Two blocks are defined, one on either side of
the fault
Faults
• Fault geometry
– Imagine a horizontal tunnel cutting
through a fault in cross-section
Horizontal
Surface
Dip angle
Foot Wall
Hanging Wall
Fault plane
Fault Types
• Faults may be divided into three categories
– Normal faults
• Hanging wall moves down relative to foot wall
• Block slides down the dip angle
– Reverse faults
• Hanging wall moves up relative to foot wall
• Block moves in the reverse direction to what
seems normal
Fault Types
– Strike slip faults
• Displacement to sideways in a horizontal
direction
• Movement is parallel to the strike of the fault
plane
• Strike is the direction of the line formed by the
intersection of the fault plane with the Earth’s
surface
Major types of faults
Normal Faults
• Normal faults are created by tensional
forces, i.e. pulling apart
– Rifts are created by parallel normal faults
dipping toward each other
• The block in the center which drops down is
a graben
• The Rio Grande valley in New Mexico is a rift
graben
A normal fault?
A special type of normal fault
– Fault blocks, bounded by normal
faults, that drop down or are
uplifted are known as grabens and
horsts, respectively
• Grabens associated with divergent
plate boundaries are called rifts
Normal faults produce grabens & horsts
Extensional (Normal) Faults
reviewed
Reverse Faults
• Compressional stress usually causes
reverse faults to form
– Reverse faults are common at convergent
plate boundaries
– Reverse faults cause a thickening of the
crust as rocks are piled up
– Older rocks may be found above younger
rocks
Reverse Faults
• Thrust faults are a special kind of
reverse fault
– Shallow dip angle, > 45o
– Common in large mountain ranges
– Horizontal displacement may be many
tens of kilometers
– Evidence of thrust faults in sedimentary
rocks is seen when a sequence of the
same rocks are repeated
Strike-Slip Faults
• Strike-Slip faults
– Principle movement is horizontal
• Left or Right Lateral
• Little or no vertical movement
– Caused by shear stress
– Indicated by abrupt changes in drainage
patterns
Strike-slip faults offset drainage
Types of Faults
• Strike-slip faults have movement that is
predominantly horizontal and parallel to
the strike of the fault plane
– A viewer looking across to the other side of a
right-lateral strike-slip fault would observe it
to be offset to their right
– A viewer looking across to the other side of a
left-lateral strike-slip fault would observe it
to be offset to their left
• Oblique-slip faults have movement with
both vertical and horizontal components
Right-lateral San Andreas Fault
F aulitng.exe
Movement Along Faults
• Rarely exceeds a few meters in a single
event
• Small movements, cm scale, may occur
on a regular basis
– Tectonic creep
• Total displacement may be km, but does
not occur in a single event
Geologic Structures:
Folds
Folds
• Folds are wavelike bends in layered rock
– Represent rock strained in a ductile manner,
usually under compression
• The axial plane divides a fold into its
two limbs
– The surface trace of an axial plane is called
the hinge line (or axis) of the fold
• Anticlines are upward-arching folds, and
synclines are downward-arching folds
Folds
• Warps in rock strata due to ductile
deformation
– 3-D structures of wide ranging scale
– Generally indicate horizontal compression
– Multiple generations of folding may exist
Folds
• Three simple fold forms exist
– Synclines warp downward
– Anticlines warp upward
– Monoclines dip in one direction
Folds
• Folds are described by:
– The strike of their hinge line
• The hinge line is the intersection of the hinge
plane with the folded layer
• Hinge lines may be inclined in a plunging fold
– The angle of dip of their limbs
Fold geometry
Types of folds
Anticlines & Synclines
• The sequence of ages of strata indicate the
geologic structure in folds
– Anticlines have the oldest layers exposed at
the center of the fold along the axial plane
– Synclines have the youngest strata exposed at
the center along the axial plane
A series of anticlines & synclines
Fold Belts
• Orogenic belts consist of long linear
series of folds
– Fold geometry is not overly complex
– Pattern of outcrops may appear complex
– Complex folds may develop as folds are:
• Re-folded
• Cut by thrust faults
Orogenic belt with complex folding
Complex Folds
• Folds may be very complex
– Application of shear stress
– Multiple folding events
– Complex forms are created
Complex Folds
• Plunging folds occur when the folds axis is
dipping or plunging
• Limbs of Asymmetrical folds are not the same,
one dips more steeply than the other
• Overturned and Recumbent folds occur when
folding is so extreme that beds are turned
upside-down
A plunging anticline
Types of Folds
• Plunging folds are folds in which
the hinge line is not horizontal
– Where surfaces have been leveled by erosion,
plunging folds form V- or horseshoe-shaped
patterns of exposed rock layers (beds)
• Open folds have limbs that dip gently, whereas isoclinal folds
have parallel limbs
• Overturned folds have limbs that dip in the same directions, and
recumbent folds are overturned to the point of being horizontal
F olds.exe
Structural Domes and Basins
• Domes are structures in
which the beds dip away
from a central point
– Sometimes called doubly
plunging anticlines
• Basins are structures in
which the beds dip toward
a central point
– Sometimes called doubly
plunging synclines
Complex Folds
• Domes & Basins
– Generally occur in continental interiors
– Broadly warped regions
– Roughly circular pattern of outcrops
A small dome
Box 15.1 Figure 1a
Complex Folds
• Diapirs
– Less dense salt layers may rise up
– Some overlying strata may be pierced
– Salt diapir has an inverted teardrop shape
– Strata above diapir are domed upward
Mountain Belts and Earth’s Systems
• Mountain belts are chains of mountain
ranges that are 1000s of km long
– Commonly located at or near the edges of
continental landmasses
• Mountain belts are part of the geosphere
– Form and grow by tectonic and volcanic
processes over tens of millions of years
• As mountains grow higher and steeper,
erosion rates (particularly from running
water and ice - hydrosphere) increase
• Air (atmosphere) rising over mountain
ranges results in precipitation and erosion
Characteristics of Mountain Belts
• Mountain belts are very long compared
to their width
– The North American Cordillera runs from
southwestern Alaska down to Panama
• Older mountain ranges (Appalachians)
tend to be lower than younger ones
(Himalayas) due to erosion
– Young mountain belts are tens of millions of
years old, whereas older ones may be
hundreds of millions of years old
• Ancient mountain belts (billions of years
old) have eroded nearly flat to form the
stable cores (cratons) of the continents
– Shields - areas of cratons laid bare by erosion
Insert revised Fig. 20.4 h
Rock Patterns in Mountain Belts
• Mountain belts typically contain thick
sequences of folded and faulted
sedimentary rocks, often of marine origin
– May also contain great thicknesses of
volcanic rock
• Fold and thrust belts (composed of many
folds and reverse faults) indicate crustal
shortening (and thickening) produced by
compression
– Common at convergent boundaries
– Typically contain large amounts of
metamorphic rock
Rock Patterns in Mountain Belts
• Erosion-resistant batholiths may be left behind as
mountain ranges after long periods of erosion
• Localized tension in uplifting mountain belts can
result in normal faulting
– Horsts and grabens can produce mountains and valleys
• Earthquakes common along faults in mountain ranges
Evolution of Mountain Belts
• Rocks (sedimentary and volcanic) that
will later be uplifted into mountains are
deposited during accumulation stage
– Typically occurs in marine environment, at
opening ocean basin or convergent plate
boundary
• Mountains are uplifted at convergent
boundaries during the orogenic stage
– Result of ocean-continent, arc-continent, or
continent-continent convergence
– Subsequent gravitational collapse and spreading
may bring deep-seated rocks to the surface
Evolution of Mountain Belts
• After convergence stops, a long
period of erosion, uplift and
block-faulting occurs
– As erosion removes overlying rock,
the crustal root of a mountain range
rises by isostatic adjustment
– Tension in uplifting and spreading
crust results in normal faulting and
fault-block mountain ranges
Evolution of Mountain Belts
• Basin-and-Range province of
western North America may
be the result of delamination
– Overthickened mantle
lithosphere beneath old
mountain belt may detach and
sink into asthenosphere
– Resulting inflow of hot
asthenosphere can stretch and
thin overlying crust, producing
normal faults
Growth of Continents
• Continents grow larger as mountain
belts evolve along their margins
– Accumulation and igneous activity add
new continental crust
• New accreted terranes can be added
with each episode of convergence
– Western North America (especially
Alaska) contains many such terranes
– Numerous terranes, of gradually
decreasing age, surround older cratons
that form the cores of the continents