111-36 - GY 111 Lecture Notes D. Haywick (2008-097) 1 GY...

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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 v nticline) and synclines open up (think s w ncline) 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).
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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
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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
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111-36 - GY 111 Lecture Notes D. Haywick (2008-097) 1 GY...

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