Why do we need to know about faults? Hazards, Economics, Tectonics & Earth History
What Do We Need to Know about Faults:
Terminology Related to Type and Geometry:
|Fault, Fault Zone, Ductile Shear Zone|
Fault Scales: Top to Bottom
Mesoscopic (handspecimen to outcrop)
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.|
Principles of Stratigraphic Interpretation, Surface Expression, Terminations, Length-Displacement Relationships
|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
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
|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.
|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.)|
Fault rocks with Metamorphic or Igneous Affinities
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
Extent of Fault:
Timing of Deformation:
A Simple Theory (Anderson's) of Fault Mechanics:
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.