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# ECU GEOL 1500 - Structural Geology - Part II - Folds - Fall 2014

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FOLDSANTICLINESSYNCLINESFOLDSMost folds are produced by compressional stress during mountain building at active Convergent Boundaries. Folding usually occurs at depth during plastic deformation. Note themain parts of a fold, limbs, axial plane, and fold axis in the diagram below. The axial plane isa plane that divides a fold into 2 equal and symmetrical halves. The fold axis, which can be horizontal or plunging, is the line of intersection of maximum curvature of any bed with theaxial plane. Note that the fold axis (a line) must be a line that lies within the axial plane (a plane) of any given fold. Remember that all sedimentary rocks are oriented horizontally before they are folded with the oldest unit on the bottom and the youngest unit on the top! There are twomain types of folds, Anticlines and Synclines.Geometry of Folds ANTICLINESAnticlines are upfolds in which the beds are dipping down and away from the axial plane (from a top to bottom perspective) and the oldest bed is in the center of the fold on theEarth’s surface after erosion (map view) [See diagram below]. A Dome is a special category of an anticline in which all beds are dipping down and away from a central point rather thanan axial plane. Thus, in the map view a Dome will have a circular outcrop pattern with the oldest bed in the center of the fold (See diagram below).AnticlineBlock Diagram and Geologic Map of Non-Plunging Anticline Block Diagrams of a Dome (top) and Basin (bottom)SYNCLINESSynclines are downfolds in which the beds are dipping down and toward the axial plane (from a top to bottom perspective) and the youngest bed is in the center of the fold on the Earth’s surface after erosion (map view) [See diagram below]. A Basin is a special category of a syncline in which all beds are dipping down and toward a central point rather than an axial plane. Thus, in the map view a Basin will have a circular outcrop pattern withthe youngest bed in the center of the fold (See diagram above).SynclineNote that anticlines and synclines are plunging when the fold axis is tilted from the horizontal, resulting in a U- or V-shaped outcrop pattern in map view. For anticlines this U-or V-shaped outcrop pattern points in the direction of plunge whereas the opposite is true for a syncline (See diagrams below). Plunging FoldsComplete the top and side cross sectional view of the Plunging Anticline and Syncline below Also, from a cross-sectional perspective note the orientation of folds: anticlines and synclines can be Upright (axial plane is vertical), overturned (axial plane is tilted from the vertical), or can be recumbent (completely rolled over with horizontal to sub-horizontal axial plane). See diagram belowTwo Anticlines that are InclinedRecumbent FoldsLastly, note that anticlines/structural domes do not necessarily make ridges or hills nor do synclines/structural basins form valleys or topographic basins. Thus, topography is more a function of differential weathering and erosion than geologic structure.GEOLOGIC MAPS, BLOCK DIAGRAMS,AND CROSS SECTIONSA geologic map is a representation of the distribution of rock units and geologic structures, such as faults and folds, based on field observations and reconnaissance. Geologic maps can be plotted either on topographic base maps or on aerial photographs such that one can ascertain the relationship of rock units and geologic structures to both natural physiographic features, such as mountain ranges, and to man-made features, such as cities. Geologic Map and Cross Section of a Small AreaThus, the most important skill you must learn in structural geology is to interpret the geology below the Earth’s surface based on what is observed on its surface. To perfect this skill will require visualizing in 3-dimensions, a little imagination, and patience! So, when frustration sets in, try again rather than give up!To show what happens to rock units and geologic structures below the Earth’s surface, we must use Geologic Cross Sections (see illustration above) and block diagrams. Geologic Cross sections and block diagrams are interpretations of what occurs below the Earth’s surface based on what we can observe on its surface and a little imagination and a geologist’s artistic

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