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![Page 1: Fractures “cracks along which cohesion of the material has been lost” - planes of discontinuity…most common type of geologic structure Two types of fractures:](https://reader035.fdocuments.in/reader035/viewer/2022062803/56649cca5503460f949937eb/html5/thumbnails/1.jpg)
Fractures
“cracks along which cohesion of the material has been lost”- planes of discontinuity…most 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 plane…displacement is perpendicular to the fracture…will look at in more detail in subsequent lecture
Very commonly filled with mineralization…
Fissure: special class of joints that have dilation >20 cm
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Why study fractures?
EngineeringFluid flowEarthquake hazardsStress-strain significance
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1. Engineering
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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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 1994Northridge Earthquake
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4. Stress-strain signficance
Fractures record tectonism
Joints due to:1. Faulting2. Mechanical
Stratigraphy3. 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 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 faults…we’ll look at this later)
Fault Classification
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Orientation of fault surfaces:
Strike
Dip
strik
e
dip
Note: for mechanical reasons fault planes are seldom planar over long distances…thus, must take MANY strike and dip along a fault
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Basic Fault Terminology (more to come later):Footwall – block sitting below the fault surface
Hanging wall – “ “ “ “ above the fault surface
NormalFault
Foot
wal
l
Han
ging
wal
l
Footwall
Hangin
g wal
l
Thrust/ReverseFault
Strike-SlipFault
Nature of slip along Faults
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Nature of Fault Surfaces
Discrete fracture or fault zone
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Nature of Fault Surfaces
SlickensidesStriationsGrooves
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Reidel shears (photo by M.Miller)
Nature of Fault Surfaces
Joints and MicrofaultsFault brecciaVein FillingFault GougePseudotachylite
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Nature of Fault Surfaces
Deformation BandsCataclastic rocks
and myloniteDuctile shear zones
Adirondacks Jay, NY
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Nature of Fault Shapes
Faults are not infinite
Wytch Farm Oil Field, southern England (taken from Kttenhorn & Pollard, AAPG Bull, 2001)
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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 Scarps4. Fault Line Scarps
Photos by M. Miller
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Recognition of faults
5. Triangular facets6. Erosional traces
Death Valley (photo by M. Miller)
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Recognition of faults
7. Topographic irregularities Tectonic
geomorphology
http://www.intermargins.org/tsunami1.html#a2
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Recognition of faults
8. Geophysics
From the University of Leeds, UK
From the University of Leeds, UK