7.4: Folds
- Page ID
- 36918
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Fold Creation
Compressional stress is the dominant force in mountain building. As plates collide, compressional stress causes the crust to buckle to produce folds. A compressional fold belt can be found in all mountain ranges around the world which have been caused by tectonic plate collision. View the Google Earth image below of the Ridge and Valley region in the Appalachian Mountains of Pennsylvania. The rock in this region includes mostly ductile sedimentary rock layers which were folded during the collision of Africa with North America as the Pangean supercontinent was being pieced together. We’re viewing this section of the Appalachians from an altitude of about 200 km. The folding pattern displayed is quite a spectacular feature seen from space.
Fold Classification
Folds are classified based on their geometry and fall into three general categories: monoclines, anticlines and synclines.
Anticlines and Synclines
Anticlines (upfolds) and synclines (downfolds) are very common geologic structures that form in pairs in response to compression. Anticlines and synclines will share a limb of the fold. Click on the starred placemarks in the gigapixel image from eastern West Virginia, below, to learn more about specific characteristics of anticlines and synclines.
Monoclines
Monoclines are not as common and usually form as an adjustment in the surface layers to some type of tectonic activity happening deep within the Earth’s surface. An example of this would be an uplift of a block of deeply buried “basement” rock of the continental crust.
Overturned and Recumbent Folds
The stress of tectonic compression is commonly applied more forcefully in one direction as one tectonic plate collides with another. The directionally applied stress may result in folding that appears to be “pushed” from one direction. When this happens, folds may begin to turn over on themselves and be called overturned. A fold will become increasingly asymmetric as the axial plane tips toward 90\(^{\circ}\) from vertical at which time it will be classified as recumbent.
Overturned Fold
Recumbent Fold
- anticline – a fold in rock layers that is arch-shaped, with the oldest rocks located at the core of the fold and the limbs dipping away from the center
- axial plane – an imaginary surface that divides a fold into two roughly symmetrical halves, passing through the center of each layer
- limb – the side or arm of a fold, consisting of rock layers that dip away from (or toward) the center of the fold
- monocline – a simple fold in which otherwise horizontal rock layers bend in one direction, creating a single step-like feature
- overturned fold – a fold in which one or both limbs are tilted beyond vertical, so that the rock layers on one limb are upside down.
- recumbent fold – a fold that has been rotated so much that its axial plane is nearly horizontal, causing the limbs to be almost parallel
- syncline – a fold in rock layers that is trough-shaped, with the youngest rocks located at the core of the fold and the limbs dipping inward toward the hinge