Jointed rocks in the Appalachian Plateau


 Penn State


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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