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Geological Maps 2: Folded Strata
The same factors responsible for metamorphism (chiefly pressure and temperature) are also responsible for rock deformation; however, the actual processes of deformation are complex which necessitates additional discussion here. If you take a volume of strata and place it deep inside the Earth, those rocks will experience pressure related to the mass of the overlying rocks. Geologists refer to this as the confining pressure. Confining pressure is generally isostatic (i.e., it is more or less uniform in all directions) and in general, the deeper you go, the higher it becomes. Since the strata are equally squeezed from all sides, the result is a net decrease in volume, but there is no change in shape of the rock volume (Figure 1).
Not all pressure in the interior of the Earth is isostatic. Along a convergent plate boundary, for example, the pressure is directed between the two colliding plates. Pressure that is non-isostatic or directed is regarded as stress. There are three main types of stress that can affect rocks. Compressive stress (or compression ) occurs when rocks are squeezed together such as along convergent plate boundaries and subduction zones (Figure 1b). Tensional shear (or tension ) occurs when rocks are pulled apart (Figure 1c). This is the major force operating along divergent plate boundaries such as
Figure 1: Effects of pressure on the volume and shape of rock strata.
the Mid-Atlantic Ridge. The third type of stress, shear , occurs when rocks slide past one another (Figure 1d). This occurs along transform fault boundaries such as the San Andreas Fault in California or the Alpine Fault in New Zealand. It is important to note that while compression, tension and shear do operate along plate boundaries, they are by no means restricted to these features. Stress can occur anywhere, provided that circumstances permit it. The earthquake that occurred north of Mobile in 1997 resulted from stress that built up wholly within a continent not at a plate boundary. Like stress, earthquakes can also occur anywhere on the Earth.
Figure 2: Schematic diagrams of ideal fold structures. (A) symmetrical anticline; (B) symmetrical syncline; (C) monocline.
Figure 3: Schematic diagrams of symmetrical, asymmetrical, overturned and recumbent anticlines and synclines.
orientated. This class of folds are said to be plunging and they are among the most difficult of the geological structures to visualize and to interpret (Figure 4).
Two other fold structures that give students the heebee jeebees are domes and basins (Figure 5). These geological features are perhaps best described as doubly folded folds. Domes consist of strata that have been folded upwards where as basins consist of strata that is inclined downwards (down-warped). Domes can be very large, in fact whole mountain ranges may consist of a single dome structure (e.g., the Ozark Mountains). Basins are formed through a much different mechanism more related to sedimentation than to simple rock deformation. Down-warping is produced by subsidence which may or may not be tectonic in origin. The main point is that down-warping produces a depression which gradually becomes filled in with sediment. The Law of Superposition applies here. The oldest strata occurs at the bottom of the basin (e.g., Figure 5).
Folds are really not all that difficult to interpret, but some of the terminology is trying. The next section summarizes everything that you will need to know to identify and label vital components of folds on block diagrams. Section 5 which follows it summarizes
Figure 4: Schematic diagrams illustrating horizontal and plunging anticlines.
Figure 6: Oblique schematic of a horizontal symmetrical anticline with all important parts and features labeled. Refer to Table 1 and to your lecture notes for discussion of each of these features. The inset summarizes a plane-section (map view) of the anticline. Note the orientation of the strike and dip symbols for each fold limb relative to the fold axis.
orientated in a vertical fashion. An asymmetrical or overturned fold may have an inclined axial plane (Figure 7) and axial planes may even be horizontal if the fold is recumbent.
Axial planes are vital for describing the orientation of folds, but they are hard to visualize as a surface expression on plane sections (e.g., geological maps). Fortunately (or unfortunately depending upon your opinion of geological maps), axial planes do leave their mark. Remember anything from high school geometry? When any two planes intersect, they do so in a line. The axial plane usually intersects the surface of
Figure 7: Schematic block diagrams and plane sections of non-symmetrical folds. (A) asymmetrical anticline; (B) overturned syncline. Compare the orientations of the strike and dip symbols for each fold with those in Figure
the Earth (another plane) and they do so in a line called the fold axis (Figure 6). There are two symbols used to identify fold axes depending upon whether the structure is an anticline or a syncline (Table 5.3). The symbols are modified for instances when the folds plunge.
Plunging folds, as stated previously, are inclined relative to a horizontal plane such as the surface of the Earth (Figure 4). This wasn't explained all that well earlier because it was necessary to first explain the concept of axial planes and fold axes. The previous example (Figure 6), was a horizontal anticline. Note that the fold axis is also horizontal.
Figure 8: Oblique schematic of a plunging symmetrical anticline with all important parts and features labeled. The inset summarizes a plane-section (map view) of the anticline. Note the variation in orientation of the strike and dip symbols for each fold limb relative to the fold axis and the plunge of the fold axis.
has demonstrated one major thing about real folds; they seldom occur as isolated structures. More often than not, folds occur in complex sequences of anticlines and synclines. Interpreting these complexes is where the real fun begins in structural geology.
Figure 9: Strike and dip trends of strata associated with a (A) plunging anticline and (B) a plunging syncline.
Figure 10: Strike and dip trends of strata associated with a (A) dome and (B) a basin.
do is to identify the main structures from the information available on the surface of the block diagram. The age data tells you which parts of the strata are the oldest and which are youngest. Careful examination of the example will reveal that there are two patterns of repetition. One repeats in the following fashion: Su, Do, Ma, Do, Su. The other repeats as follows: Su, Osp, C 1 , Osp, Su. The first repeats around Ma which is the youngest layer in the sequence. This defines a syncline. The second repeats around the oldest bed (C 1 ) indicating an anticline. Once you identify all of the structures, add the relevant fold axis to the block diagram (Figure 13a). There really isn't much more information that you can use here to better refine your interpretation, so the best that you can do is to sketch in approximate orientations of bedding in the vertical faces of the block diagram and add strike and dip symbols on the upper (planar) surface (Figure 13b). Sometimes you will get much more information than this. There may be strike and dip symbols already on the block diagram and/or there may be a river crossing geological contacts (don't forget the Law of the "V"s from Chapter 5!). You will be given several other examples in class by which to refine your interpretive geological mapping skills. There are also several do-it-yourself cutout block diagrams in the optional exercises section of this chapter. They may help you if you are having problems visualizing some of the structures.
Figure 12: Schematic block diagram of a syncline-anticline pair.
Figure 13: The same schematic block diagram of a syncline-anticline pair as shown in Figure 12 with added fold axes (A) and interpreted cross-section (B).
B) Basin
C) Complex horizontally-orientated folds
