Chapter 15 Geologic Structures I

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Chapter 15Geologic Structures - I
Photo credit G. Mattioli
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Tectonic Forces at Work

• Stress and Strain in the Earths Crust
• Behavior of Rocks to Stress and Strain
• Present Deformation of the Crust

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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
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rocks, like the fence, will deform in different
ways, depending upon the properties of the rock
and the nature of the applied stress
folds
faults
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three categories of fundamental structures 1)
contacts--separate rock bodies normal
depositional contacts and unconformities
intrusive contacts fault contacts 2) primary
structures--develop during formation of rock
body in sediment before it becomes
sedimentary rock cross beds, ripple marks,
etc. in lava or magma before they become
igneous rock ropy texture in basalt,
vesicles, etc. 3) secondary structures--develop
after formation of rock joints, shear
fractures, and veins faults folds
foliations and lineations shear zones
secondary are theme of this class
sometimes distinguishing primary and secondary is
difficult
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Earths Plates
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CA Tectonic Elements
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Offset Ditch, Hollister, CA - 1975
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Offset Ditch, Hollister, CA - 1992
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Palmdale, CA Road Cut
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kinematic analysis 4 possible types of movements
time 1
time 2
translation (rigid body)
rotation (rigid body)
distortion strain (shape changes)
volume change (dilation, contraction)
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looking at it another way
dilation
translation
distortion (strain)
rotation
from Davis and Reynolds, 1996
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distinction between stress and strain stress
is instantaneous exists only when applied
strain is the deformation in the rock that
results (this is basis for detailed structural
analysis) an applied stress yields strain each
has its own terms associated with it
stress compression and tension strain
shortening (contraction) and lengthening
(extension)
What stress is responsible for formation of
geologic structures?
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Stress-Strain
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Shear stress and strain
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Stress-Strain Curves
Elastic Limit
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Structures as a Record of the Geologic Past

• Geologic Maps and Field Methods
• Strike and Dip
• Measuring Folds
• Other Strain Markers

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Steeply Dipping Beds,Pt. Arena, Ca
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Strikes and Dips
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Field Measurements
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Maps and X-sections
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Folds

• Geometry of Folds
• Fold Axis
• Axial Plane
• Limbs and Hinges
• Fold Types
• Upright
• Open
• Isoclinal
• Overturned
• Recumbant
• Interpreting Folds

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Fold Elements
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Fold Animation