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EAS 2200
The Earth System
Spring 2012
Lecture 11
Faulting, Folding, and Mountain Building
Why it matters
Can you imagine living on a planet where there was no deformation?
A flat featureless plain, climate varying smoothly from pole to equator - how boring!
What kind of biological diversity would this produce?
Very little, assuming there were organisms to diversify!
No uplift -> no erosion -> no nutrients -> no life.
Structural Geology
What we will cover today is part of structural geology.
Structural geology focuses on deformation of the Earth’s surface.
The goal of this field, like that of other fields in geology, is to understand how the Earth
works.
Because of the vast scales of space and time on which it operates, understanding the Earth
is a non-trivial problem.
Mostly, we must resort to observing and trying to understand what has happened in the
past.
The procedure in structural geology is as follows:
First, describe the geometry
Second, kinematics - reconstruct the movements and distortions
Third, deduce the dynamics - the causes of motion.
Convergent Plate Boundaries (again)
When an oceanic plate collides with another oceanic plate or a continental plate, the denser
oceanic plate subducts back into the mantle.
What happens when two continental plates collide and neither wants to go down?
Himalayas provide an example.
India-Asia collision
Began about 45 Ma ago after the Tethyian Ocean is consumed.
India continued to plow into
Asia, pushing up the Himalayas
and the Tibetan Plateau
Isostacy
Let’s begin by asking why
continents rise above the
ocean, and why in particular,
mountains are high.
It is the same reason corks
float or icebergs rise above the
sea - they are less dense that
the fluid in which they are
immersed.
This concept is known as
isostacy.
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EAS 2200
The Earth System
Spring 2012
Lecture 11
Isostatic Equilibrium
When isostatic equilibrium is achieved, the total mass above the compensation depth must
be the same for both continents and oceans.
Consequently, mountains stick up a little bit and down a lot.
Once isostatic equilibrium is achieved, the height of a mountain belt will depend on the
density and total thickness of continental crust beneath it.
Bottom Line: Mountains stick up because the crust beneath them is thick.
How does the crust thicken?
Types of Deformation
Rigid-body Deformation
Translation
Rotation
Non-rigid Deformation (Strain)
Distortion
Dilation (compaction)
Classical vs. Plate Tectonic Views
Classical geological thinking before 1970 emphasized vertical displacements over
horizontal ones.
There could be no denying that marine sediments at the summit of Everest testify to >9 km
vertical displacement.
Plate Tectonics turned this thinking on its head.
In the plate tectonic view, most vertical
displacements occur as a result of horizontal
motions.
The Himalayas are the result of the collision of
the Indian and Asian Plates and the resulting
crustal thickening and shortening.
The 9 km vertical displacement is small potatoes
compares with the ~6000 km horizontal
displacement of India during the preceding 60
Ma.
The rigidly displaced Everest sediments sit atop a
very large pile of non-rigidly deformed rocks
beneath them.
Stress
Stress refers to the internal distribution of
pressure, or force per unit area, within a body.
(Stress has the same units as pressure, i.e.,
force per unit area).
In 3 dimensions, we can identify two kinds of
stress operating on a cube:
Normal stresses, σ, operating perpendicular to a
body.
Shear stress, τ, directed parallel to the face of
the cube.
There are thus 9 potential stresses that comprise
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Three Measures of Strain
EAS 2200
The Earth System
Spring 2012
Lecture 11
the stress tensor.
Strain
Strain is the deformation of a body that results from stress.
Like stress, strain is a tensor and has normal
and shear components.
We can measure strain in 3 ways:
Change in length (normal)
Change in volume (normal)
Change in angle (shear)
Deformation: Elastic Strain
Now let’s consider the relationship between
stress and strain.
There are 3 possibilities
Elastic strain
Brittle failure
Plastic (or ductile) deformation
Ideal elastic strain:
Deformation is proportional to stress
Deformation is instantaneous
Deformation is reversible
Elastic strain occurs when seismic waves pass
through rock.
Elastic strain
Brittle Failure
Many materials are elastic only up to a point.
The maximum stress that can be withstood is
called the yield strength.
Beyond that, the material undergoes
irreversible brittle failure.
Dutile Deformation
Brittle failure
Some materials deform plastically above a
certain yield strength.
In this situation, very little additional stress
results in a large, irreversible deformation.
Rocks typically deform brittlely at low
temperature and low confining pressure and
deform plastically at high confining pressure and
elevated temperature.
Three types of Deformation
Folding
Homogeneous Distortion
Faulting
Plastic deformation
Folding
Folding is a typical result of ductile deformation in inhomogeneous (e.g., layered) material.
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EAS 2200
The Earth System
Spring 2012
Lecture 11
Describing Folds
Anticline or Antiform: upward closing fold
Syncline or Synform: downward closing fold
Hinge Line: joins points of maximum curvature along the same layer
Axial Surface: contains the hinge lines of different layers
Plunging Folds
If the hinge line is not horizontal, the fold is said to plunge
Strike: azimuthal direction (i.e., compass bearing) of feature
(e.g., hinge line of fold)
value between 0 and 359˚
Dip: inclination (with respect to horizon) of feature
(e.g., hinge line of fold)
value between o and 90˚
Terms strike and dip can be used in describing just about any geologic feature (e.g., faults,
dikes, contacts, etc.)
Depending on attitude of axial surface of fold, it may be
Upright: vertical axial surface
Recumbent: horizontal axial surface
Inclined: axial surface between 0 and 90˚
Folds may be symmetric or asymmetric
Making Folds
Buckle Folds
Buckle folds occur in a strong layer embedded in a weaker layer
Shear Folds
Layer experiences shear stress but does not fail (later would produce fault)
Faults
Dip-slip faults
Normal faults: hanging wall moves down relative to footwall
Thrust or reverse faults: hanging wall moves up relative to footwall
Strike slip faults: Dominantly horizontal motion; left and right lateral
Hanging wall is rock above fault surface; footwall is rock below fault surface
Birth of a Mountain Range Case History: The Appalachian
Orogeny
The northeastern US was assembled through a series of collisions and breakups over
hundreds of millions of years.
Taconic Orogeny 450 Ma (Ordovician)
An island arc accretes to eastern North America to form the Taconic mountains
(easternmost NY)
Acadian Orogeny ~400 Ma (Silurian)
The Avalon Terrane is accreted to the eastern margin of North America to become New
England
Ithaca formation deposited in shallow sea west of mountains 370-350 Ma (Devonian
Allegheny Orogeny 340-270Ma
Gondwana collides with Laurentia to form Pangea
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EAS 2200
The Earth System
Spring 2012
Lecture 11
Mountains in Extensional Regimes: The Basin and Range
While most mountain belts form in regions of crustal shortening and thickening (e.g., the
Appalachians and the Himalayas), mountains can sometimes form over extensional
regimes.
The Basin and Range Province of the Western US provides an excellent example.
Basin & Range bounded on one side by the Wasatch Range of Utah and the Sierra Nevada
of California on the other.
Average elevation in the Basin & Range is 1.5 km, but crust is only 25 km thick. We can’t
explain the elevation with crustal isostacy.
Uplift and extension is due to upwelling buoyant asthenospheric mantle beneath the thin
lithosphere.
Faulting Patterns in the Basin and Range
Upthrown blocks called Horsts alternate with downthrown blocks called Grabens
This is a typical faulting pattern of extensional regimes.
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