Geologic structures

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Geologic structures
Deformation of rocks produces a range of geologic
structures - folds and faults. Mountains such as
the Himalayas are the result of deformation.
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Compressional tensional stress
Compression leads to shortening Tension leads to
stretching Style of deformation reflects applied
stress
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Shear stress
Shear stress - type of non-uniform stress. Stress
tangental to surface of body. Leads to shear
stain. Change in volume and shape, and involves
relative motion (movement) within body.
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Cleavage
Slaty cleavage in a slate
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Deformation textures
Foliation in this conglomerate involves the
flattening of clasts perpendicular to the maximum
applied stress.
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Brittle versus ductile behavior
Title
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Brittle versus ductile behavior
Title
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Brittle versus ductile behavior
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Material behavior
The way material behaves can change depending on
the rate of strain.
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Material behavior
Different material can behave differently
producing spectacular outcrop patterns.
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Folding
Deformation of rocks produces geologic structures
such as folds or faults
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Symmetric and assymmetric folds
Axial plane
Compressive stress
Symmetric fold
Axial plane
Asymmetric fold
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Asymmetric folds
Asymmetric anticline and syncline
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Plunging folds
Fold tilted along axis - produces distinctive
outcrop patterns
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Plunging folds
Plunging folds are common in Pennsylvania,
Virginia and West Virginia, and form the Valley
and Ridge Province of the Appalachians. The name
reflects the long ridges separated by valleys
formed by erosion of the folds. Fold axes strike
N to NNE, so that folding reflects E-W to NNW-SSE
compression
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Domes and Basins
Dome near Casper, Wyoming
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Fold complexity
Folds show varying degrees of complexity, from
open upright folds and isoclinal upright folds,
to overturned and recumbent folds. The complexity
correlates with the intensity of deformation.
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Open folds
Limbs dip away from fold axis
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Tight folds
Limbs dip away from fold axis
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Isoclinal folds
Limbs parallel to fold axis
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Overturned folds
Fold axis not vertical
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Recumbent folds
Fold axis horizontal. Beds overturned on one fold
limb
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Fault motion
Vertical motion - Dip-slip fault Horizontal
motion - strike-slip fault Vertical and
horizontal motion - oblique-slip fault Motion
along most faults is oblique-slip
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Fault
Normal faults
Block above fault plane has moved down relative
to block below fault plane
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Reverse fault
Block above fault plane has moved up relative to
block below fault plane
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Faults and Stress
Type of fault (normal or reversed) reflects type
of stress.
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Fault scarps
Erosion of faults commonly produces steep-sloped
scarps.
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Strike-Slip faults
Horizontal motion. Left lateral motion - block on
other side of fault moves to left. Right lateral
motion - block on other side of fault moves to
Right. (Note offset stream in diagram - left
lateral motion) Also known as transcurrent
faults. Motion reflects shear stress. Common
where plates slide past each other (transform
margins).
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San Andreas Fault-system
The San Andreas is an active system of
strike-slip faults in California.
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San Andreas Fault-system
The San Andreas fault system near San Francisco.
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San Andreas
This major fault system actually represent a
(transform) plate margin. It is responsible for
many of the major earthquakes that occur in
California (including the 1906 San Francisco
earthquake, the 1989 Loma Prieta earthquake, and
the 1994 Northridge earthquake).
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San Andreas - Motion
The offset streams in this photograph indicate
that motion along the San Andreas is
right-lateral.
Fault
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San Andreas - Motion
The offset of major geologic features such as the
Sierra Nevada granite batholith also indicates
right-lateral motion. Movement began about 30
million years ago, and the average movement is
about 1-2 cm per year, and the total movement of
the Pacific block relative to N. America is 315
km.
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San Andreas - Motion
N. American
Pacific
The San Andreas runs right through San Francisco.
The Pacific plate is on the left, the North
American plate is on the right. The USGS facility
at Menlo Park straddles the fault.