Jointed rocks in the Appalachian Plateau
So-called "minor" structures can have a major impact on
geoenvironmental problems. On the scales of the outcrop and
jobsite, fracture sets (joints) represent defects that reduce
rock mass strength to a value far below the strength of the intact
rock material. Open fractures also strongly influence the flow of
groundwater, petroleum, methane, and pollutants. Because the
fractures are preferentially orientated (not random), the permeability
and strength are also orientated -- thus rock masses acquire
anisotropic properties.
The photograph above shows edge fringe cracks propagating downward
into a thick shale from a parent joint in a siltstone bed. The
parent joint, striking 342 (azimuth), is typical of early dip joints
(that is, joint strike is approximately parallel to dip of the
mildly-folded beds), that propagated during layer-parallel shortening
of the Appalachian Plateau detachment sheet. The fringe cracks,
striking 351, suggest a clockwise stress field rotation accompanying
the Alleghanian Orogeny in both the Appalachian Plateau, and Valley
and Ridge provinces.
This example of joint development, photographed and analyzed by
PSU Geosciences Prof Terry Engelder, has appeared in several
publications including
Engelder (1985) Journal of Structural Geology
Pollard and Aydin (1988) Geological Society of America Bulletin
Helgeson and Aydin (1991) Journal of Structural Geology
Engelder (1993) Stress Regimes in the Lithosphere [book]
Davis and Reynolds (1996) Structural Geology [bbok]
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