{"id":531,"date":"2019-06-11T14:51:38","date_gmt":"2019-06-11T14:51:38","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-2-folding\/"},"modified":"2021-12-08T18:08:54","modified_gmt":"2021-12-08T18:08:54","slug":"12-2-folding","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-2-folding\/","title":{"raw":"12.2 Folding","rendered":"12.2 Folding"},"content":{"raw":"When a body of rock, especially sedimentary rock, is squeezed from the sides by tectonic forces, it is likely to fracture and\/or become faulted if it is cold and brittle, or become folded if it is warm enough to behave in a plastic manner.\r\n\r\nThe nomenclature and geometry of folds are summarized on Figure 12.2.1.\u00a0 An upward fold is called an [pb_glossary id=\"1694\"]anticline[\/pb_glossary]<\/strong>\u00a0(or, more accurately, an antiform if we don't know if the beds have been overturned or not), while a downward fold is called a [pb_glossary id=\"1695\"]syncline[\/pb_glossary]<\/strong>, (or a synform if we don't if the beds have been overturned).\u00a0 In many areas it\u2019s common to find a series of antiforms and synforms (as in Figure 12.5), although some sequences of rocks are folded into a single antiform or synform.\u00a0A plane drawn through the crest of a fold in a series of beds is called the [pb_glossary id=\"1696\"]axial plane[\/pb_glossary]<\/strong> of the fold.\u00a0The sloping beds on either side of an axial plane are [pb_glossary id=\"1698\"]limbs[\/pb_glossary]<\/strong>.\u00a0An antiform or synform is described as [pb_glossary id=\"1697\"]symmetrical[\/pb_glossary]<\/strong> if the angles between each of limb and the axial plane are generally similar, and [pb_glossary id=\"1699\"]asymmetrical[\/pb_glossary]<\/strong> if they are not.\u00a0If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an [pb_glossary id=\"1700\"]overturned[\/pb_glossary]<\/strong> antiform or synform.\r\n\r\n[caption id=\"attachment_526\" align=\"aligncenter\" width=\"900\"]\"\"<\/a> Figure 12.2.1 Examples of different types of folds and fold nomenclature.\u00a0Axial planes are only shown for the antiforms, but synforms also have axial planes.[\/caption]\r\n\r\n[caption id=\"attachment_527\" align=\"alignright\" width=\"400\"]\"""\"<\/a> Figure 12.2.2 An isoclinal recumbent fold.[\/caption]\r\n\r\nA very tight fold, in which the limbs are parallel or nearly parallel to one another is called an [pb_glossary id=\"1701\"]isoclinal fold[\/pb_glossary]<\/strong> (Figure 12.2.2).\u00a0Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called [pb_glossary id=\"1702\"]recumbent folds[\/pb_glossary]<\/strong>.\r\n\r\nFolds can be of any size, and it\u2019s very common to have smaller folds within larger folds (Figure 12.2.3).\u00a0 Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope.\r\n\r\n[caption id=\"attachment_528\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 12.2.3 Folded limestone (grey) and chert (rust-coloured) in Triassic Quatsino Formation rocks on Quadra Island, B.C.\u00a0 The image is about 1 metre across.[\/caption]\r\n\r\nAntiforms are not necessarily, or even typically, expressed as ridges in the terrain, nor synforms as valleys.\u00a0Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion (Figure 12.2.4).\r\n\r\n[caption id=\"attachment_529\" align=\"aligncenter\" width=\"900\"]\"\"<\/a> Figure 12.2.4 Example of the topography in an area of folded rocks that has been eroded.\u00a0In this case the blue and green rocks are most resistant to erosion, and are represented by hills.[\/caption]\r\n\r\n
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Exercise 12.1 Folding style<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nFigure 12.2.5 shows folding in the same area of the Rocky Mountains as Figure 12.0.1.\u00a0 Describe the types of folds using the appropriate terms from above (symmetrical, asymmetrical, isoclinal, overturned, recumbent etc.).\u00a0 You might find it useful to first sketch in the axial planes.\r\n\r\n[caption id=\"attachment_530\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 12.2.5[\/caption]\r\n\r\nSee Appendix 3 for Exercise 12.1 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n

Media Attributions<\/h3>\r\n