Structural Geology
Introduction
Lecture 1 – Spring 2016
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What is Structural Geology?
• Structural Geology is the branch of geology
that deals with the form, arrangement, and
internal structure of the rocks, and
especially with the description,
representation, and analysis of structures,
chiefly on a moderate to small scale
• Similar to tectonics, but the latter term is
used for broader regional or historical
structures
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Geologic Structure
• A geometric feature in rock whose shape,
form, and distribution can be described
 There are several ways in which structures can
classified
 Different classification methods are useful in
specific circumstances so several will be
discussed
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Geometrical Classification
• A. Planar to subplanar
• B. Curviplanar
• C. Linear
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Geological Significance
Classification
• A. Primary - Formed by the formation of the rock itself
• B. Local gravity-driven - formed due to slip down an
inclined surface - Includes: Slumping, at any scale, driven
by gravitational potential which exceeds friction
• C. Local density-inversion driven - formed because of local
lateral rock-density variations, causing local buoyancy
forces
• D. Fluid-driven pressure: Injection of unconsolidated
material due to sudden release of pressure
• E. Tectonic - Formed due to interactions between
lithospheric plates, or regional interactions between the
asthenosphere and the lithosphere. The forces are
gravitational, trying to achieve isostatic equilibrium, at
crustal to lithospheric scales
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Debris Flow
• Flowing mixture is dominantly sediment
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Time of Formation Classification
• A. Syn-formational - Structure forms at the same
time as the material that forms the rock
• B. Penecontemporaneous - Formed after
deposition, but before full lithification
• C. Post-formational - Structure forms after the
rock has fully formed, as a consequence of forces
not related to the formation of the rock
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Formation Process Classification
• A. Fracturing - related to development or
coalescence of fractures in rock
• B. Frictional sliding - The slippage of one rock
past another, or of grains past each other, resisted
by frictional forces
• C. Plasticity - Deformation by the internal flow of
crystals w/o loss of cohesion, or non-frictional
sliding of crystals past each other
• D. Diffusion - material transport either in the
solid-state or by dissolution in a fluid
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Mesoscopic Cohesion Classification
• A. Brittle - loss of cohesion across a
mesoscopically discrete surface
• B. Ductile - w/o loss of cohesion across a
mesoscopically discrete surface
• C. Brittle/ductile - some combination of the
two behaviors
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Brittle vs. Ductile Response
• Marble squeezed in a compression test will either deform by cracking
(fracturing; brittle behavior on b) or flowing (ductile behavior on c)
depending on pressure and temperature conditions
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Scale of Observation
• The scale of observation is often divided into
three categories:
 Micro – visible optically in thin section, or at
even smaller dimensions seen in an electron
microscope
 Meso – visible in a single outcrop, but not
necessarily traceable from outcrop to outcrop
 Macro – Traceable from outcrop to outcrop, and
sometimes over whole mountain ranges
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“Significance of Strain” Classification
• A. Contractional - Shortening of a region
• B. Extensional - Lengthening (extension) of
a region
• C. Strike-slip - movement w/o dimensional
change
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Distribution of Deformation
Classification
• A. Continuous - deformation throughout the rock
body, on all scales
• B. Penetrative - At the scale of observation, occurs
throughout the rock body - at finer scales, there
may be gaps between structures
• C. Localized - Continuous or penetrative
throughout a definable sub region of the overall
structure
• D. Discrete - a structure which occurs as an
isolated feature
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Brittle/Ductile Distinction
• These types of deformation may be on the basis of
mesoscopic cohesiveness
 Brittle – Joints, veins some faults are primarily brittle
 Ductile – Cleavage, foliation, and folding are usually
ductile
 Strain rate is very important
 An increase in strain rate can transform ductile to brittle
deformation
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Plate Tectonic Features
• Figure 1.4 in text
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