A CROSS-SECTION the internal shape of folds. This is what you would see in a roadcut or a canyon wall. It is a view from the side.

FOLDS

Basic Fold Geometry

A CROSS-SECTION the internal shape of folds. This is what you would see in a roadcut or a canyon wall. It is a view from the side. Closer to Earth s surface Deeper in Earth

HINGE HINGE

HINGE LINE HINGE PLANE UP DOWN

Continuous in both directions for considerable distance!

LIMB SHARED LIMB LIMB TRACES OF HINGE PLANES ON FACE

ANTICLINE Limbs dip away from hinge SYNCLINE Limbs dip toward hinge

Both Pinto, MD Anticline Minnehaha Springs, WV Syncline Sideling Hill, MD

Monocline (only 1 limb) near Mexican Hat, UT. (The only time you ll see one this term.)

What if we can only see the top??

MAP VIEW a birds-eye view. These are much more commonly encountered than cross sections. Which way do the beds tilt?

We have symbols for this. Once someone has measured the strikes and dips of each bed the information is recorded on the map like this: N West 30 30 30 30 30 30 30 East 30

30 30 30 30 30 30 30 30 What does it look like here?

30 30 30 30 30 30 30 30 30

30 30 30 30 30 30 30 30

VARIATIONS ON BASIC GEOMETRY 1. Plunge

HINGE 4 rings 9 9 4

NOSE

or Like an anticline Like a syncline

Cut on top part of cylinder, not parallel to its axis. (Like an anticline)

Cut on bottom part of cylinder, not parallel to its axis. (Like a syncline)

The plane of the cut was not parallel to the axis of the tree. What is the evidence for this assertion?

~FLAT EROSIONAL SURFACE CUTTING AN ANTICLINE THAT PLUNGES TO THE EAST NOSE NOSE NOSE NOSE NOSE NOSE NOSES POINT TOWARD THE DIRECTION OF PLUNGE

NOSES Point in the direction of plunge NOSE NOSE NOSE NOSE NOSE NOSE

~FLAT EROSIONAL SURFACE CUTTING A SYNCLINE THAT PLUNGES TO THE EAST NOSE NOSE NOSE NOSES POINT OPPOSITE THE PLUNGE DIRECTION

Fold Symmetry and Asymmetry

These two folds are symmetrical. Hinge planes are vertical ~20 ~20 ~20 ~20 Dip angle degrees are similar on both limbs. (Though in opposite directions, of course.)

The Wills Mountain Anticline at Germany Valley is asymmetrical. Seneca Rocks Hinge plane is not vertical and limb dip angles differ substantially. North Fork Mountain ~90 ~20 (We could determine the strike and dip of the hinge plane!)

NOSE Notice how much wider the ridge is on the gently dipping limb View in previous slide ~2 miles

Here s why. Erosion of softer rocks to this level

W E These folds are beyond asymmetric. The hinge planes are not vertical and the limbs all dip eastward in the same direction! Notice that the two unshared limbs are overturned. The red arrows point up. The shared limb is upright, but dips very steeply. These are overturned folds.

In some places folds have bent the rocks to the point that the limbs are ~parallel to each other and the rocks are all almost flat. These are recumbent folds.

It is tempting to suppose that folds form from bending stresses because the rocks involved are obviously bent. However, there is a simpler explanation that can bend things without applying stress in multiple directions. Folds result from compressional stress. The original segment length of the bedding plane traced by the curve between the two lines was a bit over 10% longer than the present distance. We say that the crust has been shortened by ~10%. To make something shorter, you squeeze it.

Though we can t be certain of the accuracy of our guesstimate of where the rocks originally were, my best estimate of crustal shortening in Germany Valley is about 30% -- nearly three times that in the previous picture.

Shortening between the two indicated points is about 130%!

Crustal shortening here is at least 160%. Notice that as the shortening increases the folds go from symmetric to asymmetric to overturned to recumbent.

In any fold belt the direction in which folds are asymmetric and overturned is very consistent. In the Appalachian Valley and Ridge, for example, the folds generally lean or are overturned to the west. The four types of folds we ve seen simply record different degrees to which a compressional stress from one direction has distorted them. A little stress from the east gives 30% shortening and a symmetric fold. More stress from the east gives more shortening & asymmetry (to west). Even more stress from the east gives more shortening & overturns (to west). and so on.

Ways to tell folds apart

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NOSE NOSE INDICATE PLUNGE DIRECTION