Fault scaling relationships, and fluid flow through rocks.

Lecture index: Fault scaling relationships between variables. / Fractal distributions of fault variables. / Fluid flow through faults.

Readings: Note that these readings are from the primary literature. There may be aspects you don't understand yet. Read carefully and get what you can out of these. Focus on the graphs and diagrams. Ask questions.

Fault scaling relationships between variables.

While this topic is typically not treated at much length in undergraduate structural geology textbooks, much has been learned in the last few decades, and it is useful knowledge in the practice of geology. For these reasons, and just because it is interesting, we will take a look at them.

Possible aspects of faults that scale against each other:

This is explored some in your readings. Usually there is linear relationship in log-log space.

This photograph of a normal fault zone truncating a horizontal sandstone layer in the hanging wall is from the Arikaree Group in NW NE. Note the well developed fault breccia, along with thin white calcite slickensides at the base of the fault zone. Offset could not be measured for this outcrop since matching strata on both sides could not be identified. Could one estimate the amount of offset on this fault on the basis of the thickness of the fault zone? Scaling relationships could provide at least some basis for doing so.

 

Interestingly. most of the examples in the literature focus on normal faults. Arrays of strike-slip faults may show scaling relationships. Thrust faults often do not seem to follow these scaling relationships. For example, some thrust fault zones with abundant movement on them are very thin and so the fault zone thickness seems to bear little relationship to the amount of slip.

Map from USGS showing array of faults (red lines) in the Arbuckles of Oklahoma. What scaling relationships might exist here? What do you notice about the pattern of faulting. Source: http://ok.water.usgs.gov/arbsimp/

Fractal distributions of fault variables

Fractal perspective: Scaling relationships can also be found in the distribution of a single fault descriptor, and these can exhibit a fractal character. The reason for this is discussed in Turcotte's book - Fractals and chaos in geology and geophysics (Cambridge University Press). The best documented such relationship is for breccias.

Why are scaling relationships useful?

Fluid flow through faults

Evidence faults effect fluid flow:

Quartz veins concentrated in fault hanging wall. They could be considered as part of a damage zone. Source: http://geology.wr.usgs.gov/parks/olym/olym7.html

This is a USGS image where the colors represent subsidence due to "fault-controlled deformation from the dissipation of residual ground-water pore-fluid pressure changes in response to past underground nuclear weapons testing." in the Yucca Flat area. Note how the white lines, representing faults compartmentalize the pattern of subsidence. The faults are acting as barriers to fluid flow in this case. Source: http://pubs.usgs.gov/fs/fs06903/

Processes that effect the geohydrology of a fault:

Water, the seismic cycle, and seismic pumping:

Oxidizing vs. reducing waters - possible indicator of direction of fluid flow. Structural topography and direction of flow (normal faults vs. thrust faults).

Sealing faults vs. porous faults, and the significance in petroleum exploration.

The importance of damage zones.

Implications for seismic hazard assessment, given importance of pore pressure.

Additional references for reading for the interested:

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