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Earth and Space Sciences · Glossary

What is Fold; anticline; syncline; fold axis?

Also known as: fold · fold axis · anticline · syncline

Definition 20.4 High School Earth and Space Sciences · Chapter 20 — Building Mountains

A fold is a bend in layers of rock. Its sloping sides are the limbs; they meet at the hinge, where the layers are most curved, and the line along the hinge is the fold axis. An anticline is a fold whose layers bend upward, its limbs dipping away from the axis; a syncline is a fold whose layers bend downward, its limbs dipping toward the axis.

An anticline and a syncline, cut by a flat eroded surface (block diagram, not to scale). The fold axes run along the hinges; the dashed lines are the traces of the axial planes. On the top surface the eroded anticline shows older layers in its core, the syncline younger ones.
An anticline and a syncline, cut by a flat eroded surface (block diagram, not to scale). The fold axes run along the hinges; the dashed lines are the traces of the axial planes. On the top surface the eroded anticline shows older layers in its core, the syncline younger ones.
Folded layers of rock in a cliff of the Himalaya, about 3\, km north-east of Jomsom in the Kali Gandaki gorge, Nepal. Photograph Jean-Marie Hullot (CC BY 2.0).
Folded layers of rock in a cliff of the Himalaya, about 3 km3\,\mathrm{km} north-east of Jomsom in the Kali Gandaki gorge, Nepal. Photograph Jean-Marie Hullot (CC BY 2.0).

Examples

Example 20.10 (The Varden folds)

The limbs of the Varden folds dip 12∘12{}^{\circ}, so the cover was shortened by e=1−cos⁡12∘=1−0.978=0.022e = 1 - \cos12{}^{\circ} = 1 - 0.978 = 0.022, about 2.2 %2.2\,\%: a stretch of the region 10 km10\,\mathrm{km} wide today was 10/cos⁡12∘≈10.22 km10/\cos12{}^{\circ} \approx 10.22\,\mathrm{km} wide before folding. Measured on section A–B as drawn, the limbs would look about 28∘28{}^{\circ} steep, because the vertical scale is stretched 2.5 times: tan⁡θ\tan\theta is multiplied by 2.5, and arctan⁡(2.5tan⁡12∘)≈28∘\arctan(2.5 \tan12{}^{\circ}) \approx 28{}^{\circ}. Read at face value, that would give a shortening of 12 %12\,\%, more than five times too much.

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