Structural Geology 3443 Ch. 1 Overview

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Structural Geology (3443)Ch. 1 - Overview
Department of Geology University of Texas at
Arlington
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Structural Geology Ch. 1 - Overview

• Some Definitions
• Geologic Structures are features and shapes in
  and on the Earth that can be described
  geometrically (shape, size, position).
• Deformation is the response of any material
  (solid, liquid or gas) to forces that change its
  shape, size or position.

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Structural Geology (3443)

• Classification of structures
• Geometric elements that make up a structure
• Surfaces (planes or curved)
• Lines
• Volumes
• Cause of the structure (give examples)
• Primary formed by the same process that
  produced the rock.
• Tectonic formed as a result of plate tectonic
  processes
• Generated by gravity or fluid pressure

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Structural Geology (3443)

• Classification of structures
• Timing of structure
• Syn-formational (-depositional)
• Penecontemporaneous
• Post-formational (-depositional)
• Mechanisms forming the structure
• Fracturing frictional sliding
• Plasticity diffusion Crystal or grain
  deformation.
• Cohesiveness or penetration of deformation
• Brittle Isolated, discrete zones of deformation
• Ductile Continuous (distributed) deformation

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Structural Geology

• Classification of structures
• Type of Strain
• Contractional rock shortened horizontally,
  lengthened vertically
• Extensional rock elongated horizontally,
  shortened vertically
• Strike-slip rock both elongated and shortened
  horizontally along perpendicular directions
• Distribution of deformation in a volume
• Continuous, Penetrative deformation throughout
  the volume
• Localized Penetrative within a small domain
• Discrete An isolated structure

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Examples of structures (shapes, Patterns)

• Surfaces/Boundaries/ contacts between different
  rock types
• Sedimentary layering (below)
• Intrusive boundaries (left)

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Types of structures (shapes, Patterns)

• Surfaces contacts between different rock types
• Volcanic (Rhyolite) layering (left)
• Gneiss layering (below)

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Types of structures (shapes, Patterns)

• Primary structures formed about the same time
  the rock did

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Types of structures (shapes, Patterns)

• Primary structures formed about the same time
  the rock did.

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Types of structures (shapes, Patterns)

• Secondary deformational structures generated
  after the rock formed
• Folds, fractures

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Types of structures (shapes, Patterns)

• Secondary structures generated after the rock
  formed
• Cleavage (foliation), lineation

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Types of structures (shapes, Patterns)

• Secondary structures generated after the rock
  formed
• Dikes (right with thermal alteration), sills
  (left)

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Types of structures (shapes, Patterns)

• Secondary structures generated after the rock
  formed
• craters, unconformities, etc.

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Origin of Structures Plate Tectonics

• Convection Types of Plate Boundaries
• http//pubs.usgs.gov/publications/text/understandi
  ng.htmlanchor15039288

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Origin of Structures

• Forces in the Earth Are ultimately produced by
  the transfer of energy heat energy and
  gravitational energy (buoyancy)
• However, the explanation of deformational
  structures relies on force intensity (stress),
  not force alone.
• Stress (force intensity) is force divided by the
  area it acts over
• In a tiny volume of a material, how many areas
  are there?

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Origin of Structures

• The stress (force intensity) produces strain,
  which can be defined as
• Like stress, strain is defined in an
  infinitesimally small volume. How many line
  lengths are there in that small volume?

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Origin of Structures

• Because stress and strain are defined at an
  infinitesimal volume of a material, they can
  change value from one point to another.
• If the value of stress and strain are the same
  from point to point, they are called homogeneous.
  Otherwise they are heterogeneous

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Origin of Structures

• A reference frame is critical because the
  results can vary from one frame to another.
• For example the Coriolis force is necessary if
  the reference frame is fixed on the Earths
  surface. However, it disappears if the reference
  frame is fixed at the Earths center rotates
  with the Earth.
• Likewise stress and strain values change
  depending on whether the reference frame refers
  to the deformed state (Lagrangian) or the
  undeformed one (Eulerian).

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Structural Analysis

• Types of structural Analysis
• Geometric (Descriptive - below left)
• Kinematic (Motion - top right)
• Dynamic (Motion Forces - bottom right)

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Structural Analysis

• Structural Analysis
• Scale of observation and resolution Complete
  analysis needs to make observations on a variety
  of scales.

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Structural Analysis

• Descriptive analysis includes the following
• Type of rock or material
• Geometry of feature (linear Surface planar,
  curved volume)
• Position in space usually a map, cross section,
  block diagram (requires coordinates, orientation)
  
• Shape. Size
• Spatial Relationship to other features (distance,
  relative orientation)
• Time Relationship to other features
  (Stratigraphy, cross-cutting relationships,
  radiometric dating).
• Mathematically, the feature f(x,y,z.t)

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Structural Analysis

• Types of description
• Description of a Physical Object or feature
• Description of a geometric idealization of the
  physical object (e.g. bedding plane)
• Statistical descriptions of a group of similar
  physical objects in terms of averages and
  variation from the average (sets of structures)

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Structural Analysis

• Kinematic and strain analysis is measurement of
  displacements and includes the following
• Rigid Body Displacement (no change in shape or
  size)
• Translation (plate motion or slip of one side of
  a fault relative to the other
• Rotation (Tilting of beds)
• Change of shape and/or size (strain)
• Dilation (volume expansion or contraction)
• Strain (shape shifting)
• Timing
• How long did it take a particular displacement or
  deformation to occur?
• Sequencing what was the sequence of the various
  structural features.

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Structural Analysis

• Types of structural Analysis
• Kinematic analysis depends on scale
• At the atomic/molecular scale, everything is
  translation and rotation (discontinuous
  deformation) and is called mechanism analysis
  in the book
• At larger scales, molecular translation and
  rotation cannot be seen and it looks like
  penetrative (continuous) deformation (strain)
  so description methods change.
• On a global scale, plates look rigid and
  displace and rotate relative to each other. This
  is called tectonic analysis in the book.
  However, on a local scale, there are slipping
  faults (displacements) and distortions (strain)
  that cant be seen from a Satellite.

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Structural Analysis

• Types of structural Analysis
• Dynamic Analysis is an explanation of the
  structural features described geometrically and
  kinematically in terms of the forces acting on
  the rock materials.
• These forces cannot be measured directly because
  the event happened long ago. Therefore,
  structural geologists must rely on dynamic models
  to try to duplicate the geometric and kinematic
  observations.

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Structural Analysis

• Dynamic Analysis
• Two types of dynamic models
• Physical Scale Models Some material (often sand
  or clay) is used to represent the behavior of
  rock. Forces are applied and structures observed
  and compared to natural features. (Below)

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Structural Analysis
Dynamic Analysis Two types of dynamic models 2)
Mathematical/numerical Models A set of
mathematical equations are developed that predict
deformation. They are solved analytically or
numerically to generate structures and compare
with the natural features. (Below)
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Structural Analysis

• Structural History timing of deformational
  events/processes (Geochronology)
• Principles of Relative Timing
• Superposition
• Cross cutting relationships
• Original horizontality of sedimentary layers
• Faunal Succession
• Absolute timing Radioactive Decay

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Structural Analysis
What is the sequence of formation of the various
structures in the block diagram? Which principles
are used?
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Absolute Geologic Time
Prior to 1900, Geologists did not know how old
rocks were in an absolute sense. Around 1900,
Madam Curie discovered radioactivity Some
isotopes are unstable and spontaneously decay
(change) into other elements by expelling
particles and energy out of the nucleus (see
below)
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Absolute Geologic Time
Rate of decay is constant for a particular
isotope and is expressed as the half life the
time it takes 1/2 the atoms of an unstable
isotope (parent) to change into another isotope
(child).
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Absolute Geologic Time
Starting with 1000 atoms, how many parents
children are there after a) One ½ life? b) Two
½ Lives? c) Three ½ lives?
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Absolute Geologic Time
Half Lives for some isotopes found in
minerals Parent Child 1/2
Life U238 Pb206 4.5 billion K40 Ar40 1.3
billion Ru87 Sr87 47 billion C14 N14 5,
730 years
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Absolute Geologic Time

• Assumptions/requirements
• Rate of decay is constant and not affected by
  time, temperature or pressure.
• No gain or loss of parent or child isotopes from
  material being dated.
• Initial amounts of child isotope in material must
  be measured.