Fractures

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Fractures
cracks along which cohesion of the material has
been lost - planes of discontinuitymost common
type of geologic structure
Two types of fractures (1) Faults surface
across which the rock has been displaced in a
direction that is generally parallel to the
fracture
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(2) Joints a fracture with NO displacement
parallel to the fracture planedisplacement is
perpendicular to the fracturewill look at in
more detail in subsequent lecture Very commonly
filled with mineralization Fissure
special class of joints that have dilation gt20 cm
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Why study fractures?

• Engineering
• Fluid flow
• Earthquake hazards
• Stress-strain significance

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1. Engineering
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2. Fractures can control subsurface fluid flow
paths
Courtesy of Zion National Park
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3. Siesmic hazards
Freeway Damage from 1994 Northridge Earthquake
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4. Stress-strain signficance

• Fractures record tectonism
• Joints due to
• Faulting
• Mechanical Stratigraphy
• Fold Shape

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Describing Faults
Faults a fracture surface across which the rock
has been displaced in a direction that is
generally parallel to the surface
itself. Classification of faults - each type
of fault forms under different geologic
conditions - so, if we can characterize the
nature of the fault, we can conclude something
about the geologic history...
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Fault Classification
Fault classifications are based on two basic
observations (1) orientation of the fault
surface (2) nature of the displacement or
offset across the fault Also to a lesser
extent (3) Character of the fault surface (This
comes into recognizing the faultswell look at
this later)
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Orientation of fault surfaces
Strike Dip


Note for mechanical reasons fault planes are
seldom planar over long distancesthus, must take
MANY strike and dip along a fault
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Nature of slip along Faults
Basic Fault Terminology (more to come
later) Footwall block sitting below the fault
surface Hanging wall above the
fault surface
Thrust/Reverse Fault
Strike-Slip Fault
Normal Fault
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Nature of Fault Surfaces

• Discrete fracture or fault zone

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Nature of Fault Surfaces

• Slickensides
• Striations
• Grooves

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Nature of Fault Surfaces

• Joints and Microfaults
• Fault breccia
• Vein Filling
• Fault Gouge
• Pseudotachylite

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Nature of Fault Surfaces

• Deformation Bands
• Cataclastic rocks and mylonite
• Ductile shear zones

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Nature of Fault Shapes

• Faults are not infinite

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Terminology of Slip

• Slip actual relative displacement on a fault.
• Orientation of the slip line
• Magnitude of displacement
• Relative displacement

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Drag folds as slip indicators

• Distortion of layers consistent with sense of
  shear
• Reverse drag

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Recognition of faults

• 1. Truncation and offset of rock units
• 2. Inconsistent stratigraphic relationships

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Recognition of faults

• 3. Fault Scarps
• 4. Fault Line Scarps

Photos by M. Miller
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Recognition of faults

• 5. Triangular facets
• 6. Erosional traces

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Recognition of faults

• 7. Topographic irregularities
• Tectonic geomorphology

http//www.intermargins.org/tsunami1.htmla2
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Recognition of faults

• 8. Geophysics

From the University of Leeds, UK