GY 111 Lecture Notes D Haywick 2008 097 1 GY 111 Lecture Notes Folds Lecture Goals A Types of folds B Anatomy of a fold terminology C Geological maps 2 folds on maps Reference Press et al 2004 Chapter 11 Grotzinger et al 2007 Chapter 7 p 158 160 GY 111 Lab manual Chapter 6 A Types of folds As we discussed in class last time permanent ductile deformation results in folds There are three basic types of folds 1 anticlines 2 synclines and 3 monoclines The adjacent diagram should quickly demonstrate how the basic folds differ from one another but should you need additional memory stimulation consider this anticlines close up think vnticline and synclines open up think swncline and monoclines just have one limb In GY 111 we more or less ignore monoclines so the rest of this lecture and all of the Chapter 6 exercises will be restricted to anticlines and synclines Once you understand the basic difference between anticlines and synclines the rest of fold morphology is fairly consistent Folds can be symmetrical or asymmetrical The former is when the fold limbs have an equal but opposite angle of dip Asymmetrical folds are those where one limb dips at a different amount than the other Many folds are overturned both limbs dip in the same direction Lastly intensely folded rocks might even be tilted right back to horizontal These recumbent folds are frequently difficult to recognize in outcrop because the bedding appears horizontal Close examination will however reveal that half of the rocks are upside down remember the Principle of Superposition and that the sedimentary sequence is repeated see cartoon below GY 111 Lecture Notes D Haywick 2008 097 2 So far we have restricted ourselves to folds in 2 dimensions but as I explained in an earlier lecture to properly understand structural geology you need to envision these features in 3 dimensions A anticline B syncline C monocline All of the previous diagrams were drawn with a horizontal orientation It is important to note however that folds need not always be horizontal The might plunge Plunging folds are simply folds that have been tilted in one direction The concept is relatively simple to GY 111 Lecture Notes D Haywick 2008 097 3 understand but the real complication comes from the rock pattern that develops at the Earth s surface when folds plunge Interpretation of maps containing plunging folds is one of the happy tasks that you will get to do in the lab We will touch on it in this lecture shortly as well However before we do that I need to give you some additional basic info about folds Folds are seldom isolated Rather they occur in multiples Limbs are shared between successive folds In the cartoon below three anticlines and three synclines are shown Note that each anticline shares its limbs with adjacent synclines and each synclines shares its limbs with adjacent anticlines Multiple folds like these are important components of mountain belts throughout the worlds They are called big surprise here fold mountains GY 111 Lecture Notes D Haywick 2008 097 4 Since we are discussing mountains we should probably reintroduce another important concept at this point erosion If you recall our lecture on weathering I mentioned that mountains or areas of high topography erode more rapidly than valleys low topography Following this reasoning anticlines would be expected to preferentially erode in fold mountains There is of course more to the story than just this Rocks that are hard and composed of weathering resistant minerals like quartz e g a well cemented quartz arenite layer Pg in the adjacent cartoon will hang around a long time even if they are atop of a mountain Weaker rocks like shale Layer Trx will erode more rapidly even if they crop out in a valley It is this differing rock competence that leads to the varying topography that typifies most mountain belts fold mountains included Two scales of real fold structures in the rock record The top image is outcrop scale it s a road cut and the bottom image is a mere 10 cm across Both images are from http www globalchange umich edu GY 111 Lecture Notes D Haywick 2008 097 5 B Anatomy of a fold terminology The next diagram is one of those nasty 3dimensional cartoons that makes most GY 111 students cringe It is also a favorite of mine and frequently appears in the fill inthe blanks component of the final lecture exam It portrays a horizontal symmetrical anticline You will notice that the fold is intersected by a horizontal plane If you recall the lecture on strike and dip of inclined planes you will be able to identify the strike and dip components of each of the fold s limbs Perhaps now you can appreciate the description of a symmetrical fold Notice that both limbs have the same degree of tilt the same dip but that the dip direction for each limb is in opposite directions The strike of each fold limb is exactly the same So a symmetrical fold can be simply be envisioned as two inclined beds that dip the same amount in opposite directions There is however a major addition that necessary in order to fully describe folds A second plane is added to the diagram that bisects the fold limbs For a symmetrical fold this axial plane is perpendicular to the horizontal plane that contains the strike lines of the fold For asymmetrical and overturned folds the axial plane is itself inclined The axial plane is horizontal in recumbent folds The fold axis is the line that forms where the axial plane intersects the fold For a horizontal fold the fold axis is horizontal As you will soon see fold axes of plunging folds dip in the direction of plunge When structural geologists define the fold axis or the axial plane they use different and some would say more confusing definitions The fold axis is defined as the line that connects the points of maximum curvature of a fold at the surface of the Earth The axial plane is defined as the plane that results when all of the points of maximum curvature in all of the beds comprising a fold are joined Maximum curvature is the point on a fold where the dip direction changes Think of a cross section of a roof cartoon below The crown of the roof represents the point of maximum curvature The fold axis is simply a series of points located along the crown For a roof the fold axis is a straight line but for a real fold on the surface of the Earth the fold axis may curve GY 111 Lecture Notes Plunging folds add an additional element of complexity that is best just shown in cartoon fashion like