Faults: Introduction

Why do we need to know about faults? Hazards, Economics, Tectonics & Earth History

What Do We Need to Know about Faults:

  • Location
  • Displacement (direction, amount)
  • Timing
  • Hydrologic Characteristics
  • Understanding of Related Structures

What is a Fault

  • Surface across which measurable displacement has occurred. Brittle structure, a fracture on which slip has occurred.
  • Scales: Microfault, Mesoscopic Fault or just fault (handspecimen to outcrop), Macro (map scale or just fault)

Terminology Related to Type and Geometry:

Fault, Fault Zone, Ductile Shear Zone

Fault Scales: Top to Bottom


Mesoscopic (handspecimen to outcrop)

Map Scale

  • Hanging Wall (Block)
  • Foot Wall (Block)
  • Fault Types--List and sketch them

Displacement or Net Slip, Apparent Slip or Separation

  • Fault Slip (net slip in your book; also called displacement): True displacement between two formerly continuous points (piercing points) on either side of fault.

  • Piercing Point-linear feature providing tie across fault
  • Fault Separation: Distance between displaced parts of a marker horizon as measured along a specified line (sometimes called apparent slip). Can have many forms (dip, strike). Image below from Davis and Reynolds, Structural Geology, 1996
  • Throw and Heave: Throw up and heave out. Respectively vertical and horizontal components of dip separation. Terms valuable because at times it is convenient to measure horizontal or vertical separation, for example from maps or boreholes.

Principles of Stratigraphic Interpretation, Surface Expression, Terminations, Length-Displacement Relationships

  • Interpreting faults from boreholes (normally vertical)--thrust faults repeat section normal faults have section missing section and using Stratigraphy in map interpretation of faults. (figure below from Twiss and Moores, Structural Geology (1992)
  • Surface Expression: Scarp, Fault-line Scarp.(from Twiss and Moores (1992) Structural Geology
  • Terminations: surface, at tip lines where displacement dies out, blind--tip lines can occur at depth, beneath growing fold, for example.

    Displacement Map on fault shows value of 3D seismic in understanding structure.

    Figure from Davis and Reynolds (1996) Structural Geology

  • Terminations:

    Fault displacement increases with length.

    Fault width also increases with displacement.

    Can you think of an exception to the displacement-length rule at the plate tectonic scale?

Fault Surface Ornamentation: Fault displacement scratches and otherwise lineates fault surface and tells us commonly a linear direction of movement and sometimes and absolute direction of movement. Many indicators, some more subtle than others.  Some features below, others will be discussed as we cover other fault types and get a better understanding of mechanics:

  • Slickenlines or Slip Lineations (lineations on surface) that comprise Slickensides (striated fault surface).

  • Fiber growth in direction of fault displacement provide clear indications of relative offset

Drag Folds: Asymmetry of folds near of fault can indicate direction of transport.

Folded Gouge, San Gregorio Fault Zone, Moss Beach CA. Soft smectite-rich clays form a gouge zone 10-40? m wide that has enjoyed tens of km to displacement. Huge strains result in banding and drag folding seen here. (Photo by Meredith Lohr).

Basic Fault Rock Types: A variety of classifications in various books, some general characteristics described by your book. The grouping below diverges from your book. Always remember, when dealing with classification, if you use a word (i.e., classification) that is not widely known, define it.

Non-cohesive Fault Rocks, "fragmental" composed of broken materials, disaggregate easily, unless cemented or compacted

  • Gouge: fine grained, <1.0 mm, unconsolidated
  • .
  • Breccia: > 1.0 mm to several meters.
    Breccia on normal fault. Quaternary Klamath Lake Ore. Note colluvial deposit against fault scarp. Smoother areas of fault show finer gouge that has not been weathered out from between breccia-size fragments.

Cohesive Fault Rocks: Fragmental rocks, but dense and cohesive.

  • Indurated Gouge: compacted < 1.0 mm grain size.
  • Breccia: Fragments > 1mm to several meter in size
  • Vein-filled cemented Breccia: cemented fault rock with fragments > 1 mm. Note that gouges could be cemented but tend not to be because they are much less permeable than breccias
  • Cataclastite: A fragmental but cohesive (hard, dense) fault rock, usually < 1 cm particle size. Doesn't disintegrate when hit with a hammer.
    Cataclastic shear zones in sandstone-siltstone, Kodiak Islands, Alaska. In this setting subduction complex setting called "web structure", similar features refered to as shear bands in sandstones of Colorado Plateau
    Broken Grains in Sandstone, above. Cathodo-luminescence (visible light produced by bombarding minerals with electrons--like a TV tube. Good at distinguishing minerals and their growth phases.)
  • Scaly Mudstone or Argille Scagliose: Fine-grained clay-rich rock that is cut by wavy or "anastomosing" shear planes. Breaks into scales or platy flakes, common in subduction zones. Core from subduction zone off SW Japan.

Fault rocks with Metamorphic or Igneous Affinities

  • Mylonite: Cohesive fault rock formed dominantly by crystal-plastic processes, strong planar orientation of minerals and layering common (called foliation)

    Mylonite, eastern Australia: Note resemblance to metamorphic rock, with well developed planar layering, especially along planar surface to right of coin. (Photo courtesy of O. Tobisch)

  • Microscopic view of Mylonite. View about 2 mm. Dark areas are plastically deformed quartz. Ligher areas are pther minerals being broken, rotated and stretched by shear. Photograph after from Mike Higgins (1971) US Geological Survey
  • Pseudotachylyte: A glassy or microcrystalline material in fault zone, sometimes as injected dikes. Formed during frictional heating during fault slip. Paleoearthquake indicator.

Zonation of Fault Rocks in the Earth: Gouge, Breccia and Scaly Mudstone to Cataclastites and Pseudotachylites to Mylonites.

Mapping Faults: Why? document strain and permeability character of rocks, entent of fault, timing.

Character of Fault Zone

  • Core Zone, commonly gouge or cataclastite, very concentrated or localized deformation.
  • Breccia Zone: Adjacent to Core, larger-scale fragmentation
  • Damage Zone: Deformed and fractured at a greater frequency than country rocks

Extent of Fault:

  • Hazards, Earthquake Potential

Timing of Deformation:

  • Offset strata. Fault moved between time of deposition and now.
  • Offset igneous rocks.
  • Fault moved between age of rocks and now or later unconformity or crosscutting igneous rocks
  • Fault displacement formed metamorphic minerals that can be dated.


A Simple Theory (Anderson's) of Fault Mechanics:

  • From study of brittle failure in the lab we know that shear planes develop at about +30 degrees to Sigma 1. These are called conjugate shear fractures, are separated by about 60 degrees from each other and provide an immediate clue to stress orientations.
  • The earths surface is a free surface ( a contact between the ground and air or water) and cannot be subject to a shear stress. Therefore one of the principal stresses must be perpendicular to the free surface. Only planes of stress ellipse (or state of stress) without shear stress are principal planes, that is planes perpendicular to one of the maximum principal stresses.
  • When Sigma 1 is vertical normal faulting occurs with the intersection of the conjugate faults along Sigma 2.
  • When Sigma 1 is horizontal and Sigma 3 is vertical thrust faulting occurs.
  • When Sigma 2 is vertical (and Sigma 1 and Sigma 3 are horizontal) strike slip faulting occurs.


    See Fault Mechanics Images for various field examples, mostly of normal faulting (Sigma 1 vertical)

Anderson's theory of faulting explains a lot of what we see, but certainly not all faulting. For example, faults may preferentially develop on pre-existing fractures.