Geology Reference
In-Depth Information
Fig. 1.27.
Strain and cleavage patterns in transverse contraction folds produced by differential vertical
displacement.
a
Strain distribution above a model salt dome (after Dixon 1975).
b
Cleavage or stylo-
lites parallel to bedding.
Arrows
show directions of the boundary displacements
Fig. 1.28.
Veins due to outer-arc bend-
ing stresses
blocks or in the steep limb of the drape fold over the fault zone. A soft forcing member
(i.e., salt) will distribute the curvature and strain widely over the uplifted region
(Fig. 1.27a). Because cleavage and stylolites form perpendicular to the shortening di-
rection, in folds produced by displacements at a high angle to bedding, the expected
cleavage and stylolite direction is parallel to bedding (Fig. 1.27b). In highly deformed
rocks, cleavage parallel to bedding might be the result of deformation caused by a large
amount of layer-parallel slip or by isoclinal refolding of an earlier axial-plane cleavage.
Extension fractures and veins may form due to the bending stresses in the outer arc
of a fold (Fig. 1.28). Such features should become narrower and die out toward the
neutral surface. The fracture plane is expected to be approximately parallel to the axis
of the fold and the fracture-bedding line of intersection should be parallel to the fold
axis. Bending fractures might occur in any type of fold.
1.6
Faults
A fault (Fig. 1.29) is a surface or narrow zone across which there has been relative dis-
placement of the two sides parallel to the zone (after Bates and Jackson 1987). The
term displacement is the general term for the relative movement of the two sides of the
fault, measured in any chosen direction. A shear zone is a general term for a relatively
narrow zone with subparallel boundaries in which shear strain is concentrated (Mitra
and Marshak 1988). As the terms are usually applied, a bed, foliation trend, or other
marker horizon maintains continuity across a shear zone but is broken and displaced
