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Fractures

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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Fractures

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  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: (1) Faults: surface across which the rock has been displaced in a direction that is generally parallel to the fracture

  2. (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

  3. Why study fractures? • Engineering • Fluid flow • Earthquake hazards • Stress-strain significance

  4. 1. Engineering

  5. 2. Fractures can control subsurface fluid flow paths Courtesy of Zion National Park

  6. 3. Siesmic hazards Freeway Damage from 1994 Northridge Earthquake

  7. 4. Stress-strain signficance Fractures record tectonism Joints due to: • Faulting • Mechanical Stratigraphy • Fold Shape

  8. 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...

  9. 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 faults…we’ll look at this later)

  10. strike dip Orientation of fault surfaces: Strike Dip Note: for mechanical reasons fault planes are seldom planar over long distances…thus, must take MANY strike and dip along a fault

  11. Footwall Hanging wall Hanging wall Footwall 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

  12. Nature of Fault Surfaces • Discrete fracture or fault zone

  13. Nature of Fault Surfaces • Slickensides • Striations • Grooves

  14. Reidel shears (photo by M.Miller) Nature of Fault Surfaces • Joints and Microfaults • Fault breccia • Vein Filling • Fault Gouge • Pseudotachylite

  15. Adirondacks Jay, NY Nature of Fault Surfaces • Deformation Bands • Cataclastic rocks and mylonite • Ductile shear zones

  16. Wytch Farm Oil Field, southern England (taken from Kttenhorn & Pollard, AAPG Bull, 2001) Nature of Fault Shapes • Faults are not infinite

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

  18. Drag folds as slip indicators • Distortion of layers consistent with sense of shear • Reverse drag

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

  20. Recognition of faults • 3. Fault Scarps • 4. Fault Line Scarps Photos by M. Miller

  21. Death Valley (photo by M. Miller) Recognition of faults • 5. Triangular facets • 6. Erosional traces

  22. Recognition of faults • 7. Topographic irregularities • Tectonic geomorphology http://www.intermargins.org/tsunami1.html#a2

  23. From the University of Leeds, UK Recognition of faults • 8. Geophysics From the University of Leeds, UK

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