Perhaps, one of the simplest deformational structures is that of monoclines. Surprisingly, there is not a lot of work done in this type of structures as far as fracturing in the monoclines is concerned. Reches (1976) presented one of the most comprehensive study of fractures in two monoclines in Israel. Later, Cooke (1996), Mollema and Aydin (1997), and Cooke et al. (2000) characterized and mapped joint clusters and bedding slip-based faults within the Mesozoic sandstone formations within the East Kaibab Monocline, Utah.

Figure 1 is a photograph of a highly tilted pavement showing bed-perpendicular and strike-prallel and dip-parallel joint sets, north of Hwy 89. Figure 2 shows a cross-section views with excellent examples of bed-parallel shearing and related splay fractures oblique to bedding.

Figure 1. View due north along the East Kaibab Monocline showing joint traces on a tilted pavement, north of Hwy 89. Joint sets parallel and perpendicular to the monocline axis are dominant.

Figure 2. Outcrop photos of bedding slip-based faults in Navajo Sandstone in East Kaibab Monocline with bedding plane fault and the splay joints pointed out. (b) shows the details of lower central part of (a). The bedding planes are dune boundaries. The fine-grained interdune material diminishes to the left has eroded to the right making the boundary recognizable from a distance. The oblique splay joints indicate top to the right or right-lateral slip sense along the dune boundary. Some of these splays are as much as 40 meters long. From Cooke (1996).

Fold-axis parallel and bedding oblique joints, shown in Figure 2, are inferred to be the splays of dune boundary slip, or sheared bedding-based faults. The regions prone to bedding-prallel slip along the bed interfaces occurs along the steeply dipping limb of the fold and in the middle of the layer (Figure 3).

Figure 3. Schematic illustration of fracture types associated with folding at East Kaibab Monocline, UT-AZ. Bedding plane-based fault as well as splay joints occurs in the inclined region. From Cooke (1996).

The bedding perpendicular and fold-axis parallel joints are inferred to have formed by bending-related stresses along the outer arc of the fold. The growth potential of those joints can be assessed by the maximum principle stress induced by the folding, which increases with fold amplitude. Figure 4 shows a plot of the calculated maximum principle stresses along a bended layer. Along the top surface of the fold, the highest stresses occur at the crest of the anticline; while along the fold's bottom surface, at the trough of the syncline.

Figure 4. The maximum principle stresses along the top and bottom of the layer flexed from 12 to 22 meter fold amplitude. The stresses are greater at the crest of the anticline than the trough of the syncline due to increased lithostatic compression. For rock tensile strengths of 5 to 25 MPa, curvature-related joints are expected to form perpendicular to bedding at 14 to 20 meter of fold amplitude. From Cooke (1996).

Also shown in Figure 5, while the fold shape is symmetric, the potential for joint growth is not. The large compressive lithostatic loads along the bottom surface of the fold decreases the maximum principal stress in the trough of the syncline relative to that in the crest of the anticline, so the maximum principal stress is greater in the anticlinal hinge than in the synclinal hinge.

Figure 5. Plotted field measurements of the slip distribution along non-uniformly distributed frictional interfaces of a 30 meter fold amplitude. The interface at 1725 m depth has the longest slip patch while the interface at 1765 m depth has the greatest offset, about 15 cm. The center of the slip patch lies within the region of steepest dip along the monocline. From Cooke (1996).

The case illustrates the importance of bedding plane slip during the flexture of multi-layers. Slip along interlayers effectively decouples layers allowing greater folding amplitude and may produce clusters of joints. Knowledge of the variation in layer thickness in a sequence is thus crucial and can be used to infer the sequence and pattern of slip along layer interfaces. Knowing the mechanical stratigraphy of rock strata, the contribution of both fold curvature and bedding plane slip to joint occurrence and localization can be evaluated. It is also important to consider the basement faults responsible for the monoclinal folding of the sedimentary cover. To this end, please see Fischer and Christensen (2004). These models provide the framework for predicting the location, orientation, and localization of sub-surface fracture distributions in monoclinal structures.

In summary, two types of joint clusters were documented as idealized in an 2D section in Figure 3: those perpendicular to bedding and parallel to the fold axis, and those parallel or perpendicular to the fold axis and oblique to bedding associated with bedding plane slip. The former occurs predominantly within the synclinal hinge of the monocline, while the latter occurs along the steeply dipping limb of the fold and within the middle of the formation.