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 0 Geological Maps 2 Folds 1 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 Geological Maps 2 Folds 2 1 1 Types of Rock Deformation When rocks experience stress compression tension or shear they respond to it by deforming The rocks tend to give a little bit especially at the beginning of the deformation If the pressure is eased during this period of elasticity the rocks return to their original volume and shape If however the stress builds up past the elastic limit of the strata deformation becomes permanent Rocks will permanently deform in one of two ways depending upon their properties and the confining pressure they experience Strata will break if the rocks behave in a brittle fashion and or the confining pressure is low Brittle deformation is particularly common in rocks that contain hard and non giving minerals like quartz e g quartz arenite sandstone or calcite e g limestone that are shallowly buried low confining pressure It results in fractures joints and faults features that you will have the pleasure Ha of dealing with when we get to faults If strata are ductile like modeling clay and or the confining pressure is high the deformation is more likely to result in bending or folding Rocks containing pliable minerals like gypsum evaporites or clays shales or rocks that are deeply buried 10 km depth become folded when subjected to stress There are numerous types and shapes of folds but most can be classified into three basic types 1 2 Types of Fold Structures The three broad classes of folds are 1 anticlines 2 synclines and 3 monoclines The term anticline is used for any fold structure consisting of two limbs spread apart in a downward fashion concave downward Figure 2a Synclines are bi limbed folds where the limbs open upward concave upward Figure 2b Monoclines as the name implies have only one limb Figure 2c The symmetry and orientation of these fold structures can be highly variable They can be symmetrical Figure 2a b asymmetrical or overturned Figure 3 In some instances anticlines and synclines may lie on their sides These folds are said to be recumbent Figure 3c They may also be inclined rather than horizontally Geological Maps 2 Folds 3 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 Geological Maps 2 Folds 4 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 Geological Maps 2 Folds 5 Figure 5 Schematic diagrams illustrating a dome and a basin vital information by which to recognize more complex fold structures on geological maps Refer to Table 5 3 for additional fold symbols 1 3 Folds on Block Diagrams Figure 6 shows a schematic diagram of an anticline with key components labeled First observe that the anticline is symmetrical both limbs dip the same amount but in opposite directions In a way you can consider fold limbs to be two sets of inclined strata which dip in different Each limb can be described using a single strike and dip symbol Figure 6 Note since the fold is symmetrical the dips are equal Had the fold been asymmetrical the dips would have been different Figure 7 Overturned folds would have