Geologic Structures Diagrams 

This graph shows the response to increasing stress as applied to two different rock types: BRITTLE vs. DUCTILE/PLASTIC

Definitions:

• STRESS: The force applied to a plane divided by the area of the plane.

• COMPRESSIVE STRESS: The stress generated by forces directed toward one another on opposite sides of a real or imaginary plane.

• TENSILE STRESS: The stress generated by forces directed away from one another on opposite sides of a real or imaginary plane.

• SHEAR STRESS: Stress (force per unit area) that acts parallel to a (fault) plane and tends to cause the rocks on either side of the plane to slide by one another.

• STRAIN: The result of stress applied to a body, causing the deformation of its shape and/or a change of volume.

• ELASTIC RESPONSE: The deformation of a body in proportion to the applied stress and its recovery once the stress is removed.

• ELASTIC LIMIT: The maximum amount of stress a material can withstand before it deforms permanently. 

• DUCTILE RESPONSE: The permanent deformation, without fracture in the shape of a solid.

• BRITTLE RESPONSE: The fracturing of a rock in response to stress with little or no permanent deformation prior to its rupture.

• FOLD: Permanent wavelike deformation in layered rock or sediment.

• FAULT: A fracture in bedrock along which rocks on one side have moved relative to the other side.

• JOINT: A fracture on a rock without noticeable movement.

This Diagram depicts the types of stresses available.

Modified from: Page 374

George H. Davis:  Structural Geology of Rocks and Regions     Copyright  C 1984, by John Wiley & Sons, Inc.

This diagram depicts some common fold types.

Acetate 54 (Figure 14-13)

Syncline and Anticline

C 1992 West Publishing Company

This diagram depicts an adjacent ANTICLINE and SYNCLINE with their representative FOLD AXIS and AXIAL PLANES.

Figure 1.4

Press and Siever:  Understanding Earth           Copyright C 1994 W. H. Freeman and Company

This Diagram depicts some of the differences between Asymmetrical, Symmetrical, and OVERTURNED folds.

More Fold types

Figures 11.17 and 11.18 from

George H. Davis:  Structural Geology of Rocks and Regions     Copyright  C 1984, by John Wiley & Sons, Inc.

These diagrams show the affect of plunge on the fold axis.

More Fold Types

59 Strike and Dip Diagram

Plummer, Charles C., and David McGeary, Physical Geology, 6/e.  Copyright C 1993 Wm. C Brown Publishers, Dubuque, Iowa.  All Rights Reserved.

STRIKE: The direction of the line formed by the intersection of a horizontal plane with a bedding or fault plane.  The trend of the rock/fault outcrop.

DIP: The angle formed by the intersection of a bedding or fault plane and the horizontal plane; measured in a vertical plane perpendicular to the strike.

This diagram uses Strike and Dip of repeating rock units to produce a geologic map and to infer the underlying fold.

Notice the differential weathering of different rock layers, especially on the right side of the image.  This differential weathering allows the tilted/dipping limbs to be more noticeable.  Notice the V-shape of the outcrop pattern.  This ANTICLINE is plunging toward the top of the picture, therefore, it is a PLUNGING ANTICLINE!

Folding produced by fault action typically produces angular and/or box-folds

Notice the affect of differential weathering on the joints in the background on the left side of the image.

Fault Nomenclature

Fault Types

A and B are REVERSE faults and C is a low-angle reverse fault, typically called a THRUST fault.

A, B, and C depict Normal Faulting.  In D, normal faulting has produced HORSTS and GRABENS.  Horsts are the up-thrown blocks and the Grabens are the down-thrown blocks.  In other words, the Horsts are the ridges and the Grabens are the valleys.

Strike-Slip Faults typically have near-vertical fault surfaces.  They also come in two varieties: Left-Lateral Strike-Slip and Right-Lateral Strike-Slip Faults.  Notice the pond in C.  This is a SAG POND, which is usually due to INTERSEISMIC SUBSIDENCE.  Following the release in stress as the result of an Earthquake, the rocks/ground relaxes and subsides, thus forming sag ponds in some situations.