Environmental Engineering Reference
In-Depth Information
FIGURE 6.55
Slickensides in dike of decomposed basalt (Highway BR 277, Curitiba, Parana, Brazil).
TABLE 6.9
Internal Evidence of Faulting
Feature
Characteristics
Figure
Stratum
Abrupt change in strata: discontinuous, omitted, or repeated
6.59
discontinuity
Polished and striated surfaces resulting from shearing forces;
6.54
Slickensides
characteristic of weaker rocks
6.55
Breccia
Angular to subangular fragments in a finely crushed matrix in the
6.56a
fault zone in strong rocks
Gouge
Pulverized material along the fault zone; typically clayey:
6.56b, 6.56c,
characteristic of stronger rocks
6.56d
Mineral
Groundwater deposits minerals in the pervious fault zone, often
6.57
alteration
substantially different from the local rock
Circulating waters can also remove materials
Radiometric dating of the altered minerals aids in dating the fault movement
Groundwater
Clayey gouge causes a groundwater barrier and results in a water table of
6.51
levels
varying depths on each side of the fault. The difference in water levels can
result in a marked difference in vegetation on either side of the fault, especially
in an arid climate. Tree lines in arid climates often follow faults
Foliation shear
Short faults caused by folding result in foliation shear in weaker layers in
metamorphic rocks (typically mica, chlorite, talc, or graphite schist in a
sequence of harder massive rocks) (Deere, 1974)
Shear zone thickness typically a few centimeters including the gouge and
crushed rock. Adjacent rock is often heavily jointed, altered, and slightly
sheared for a meter or so on each side. The zones can be continuous for several
hundred meters and can be spaced through the rock mass beds of
Shale mylonite
A bedding shear zone caused by differential movement between sedimentary
seam
rock during folding or during relief of lateral stress by valley cutting. Concen-
trated in the weaker beds such as shale, or along a thin seam of montmoril-
lonite or lignite, and bounded by stronger beds such as sandstone or limestone.
Sheared and crushed shale gouge is usually only a few centimeters thick
but it can be continuous for many tens of meters (Deere, 1974). Both foliation
shear and mylonite, when present in slopes, represent potential failure surfaces
