Structural Geology Lab

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Last Modified: 08/20/09

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Structural Geology is perhaps one of the hardest subjects for beginning geology students to learn. It deals with geologic structures (faults and folds) that form when stresses (tension, compression, shear) act upon a body of rock. The difficulty is that it requires that the student think in 3-D -- which is a difficult skill to learn. However, there is hope! Knowing a few simple rules (and a lot of terminology) can allow even the beginning geologist to handle some rather complex structural problems. Let's get to it!


Click on the topic below to jump to that section:

Terms to Know  
Broken Rocks  
Folded Rocks  
Map Symbols  
Important Principles to Know  
Block Diagrams  

Terms to know:

Unconformity

Stress and Strain

Compression

Tension

Shear

Strike

Dip

Joint

Fault

Slickensides

Normal Fault

Reverse Fault

Thrust Fault

Axial Plane

Fold Axis

Limb

Fault Plane

Horst/Graben

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BROKEN ROCKS

Joint: A break in a rock (crack) in which there is no relative movement of either side across the break.

Fault: A break in the Earth in which the rocks on either side of the break have been displaced (vertically and/or horizontally) relative to each other (see pictures below).

Strike-Slip fault: Movement of the Foot Wall and the Hanging Wall blocks is parallel to the strike of the fault plane. These faults form from shear stresses.

[Left and Right-Lateral Strike-Slip faults]

How do you name the strike-slip fault?

Just imagine you are standing on one side of the fault. How did the other side move relative to the side you're standing on? If it appeared to move left, it is a left-lateral strike-slip fault. If it appeared to move right, it is a right-lateral strike-slip fault.

Dip-Slip fault: Movement of the Footwall and the Hanging Wall blocks is parallel to the dip direction of the fault plane. These faults form from tensional stresses or compressional stresses.

[(Reverse) Fault Anatomy]

The above picture displays the "anatomy" of a Dip-Slip fault. Let's go over it in more detail...

Notice the terms 'Footwall Block' and 'Hanging wall Block'

The Footwall and Hanging Wall Blocks are separated by the Fault Plane

There are 2 kinds of dip-slip faults: Normal and Reverse Dip-Slip Faults...

In Normal Faults, the Footwall Block moves UP relative to the Hanging Wall Block
In Reverse Faults, the Footwall Block moves DOWN relative to the Hanging Wall Block

Another important observation regarding Dip-Slip faults is that they move older rocks next to younger rocks.

* Up to Structural Geology Topic List *


FOLDED ROCKS

Hard, brittle rock may fold and not break if the stress is applied slowly and continuously over a very long period of time. Folds form purely from compressional stresses. Let's look at the anatomy of a fold...

[Anatomy of a fold]

Fold Types:

Syncline: Limbs dip towards the axial plane (shaped like a Smile - side view)
Anticline: Limbs dip away from the axial plane (shaped like an 'A' - side view)

[Syncline] [Anticline]

Anticlines and Synclines may be:

[Asymmetrical Anticline]

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Map Symbols:

Dip and Strike: [Dip and strike symbol]

Horizontal Beds: [Horizontal Bed symbol]

Vertical beds: [Vertical Bed Symbol]

Normal Fault: (Note that the arrow points in the direction the fault plane is dipping; it also (therefore) always points to the hanging wall block!)

[Normal Fault symbol]

Reverse Fault (Thrust): (Note that the 'teeth' point in the direction the fault plane is dipping; it also (therefore) points to the hanging wall block!)

[Symbol for Thrust Fault]

Synclines: [Syncline symbol](Dark line marks fold axis; arrows denote direction limbs are dipping)

Anticlines: [Anticline symbol](Dark line marks fold axis; arrows denote direction limbs are dipping)

* Up to Structural Geology Topic List *


A Guide to Block Diagrams

[Guide to Block Diagrams]

Information Given by the Block Diagram:

1) The top of the block is a HORIZONTAL PLANE

- A dipping bed that intersects it forms a STRIKE LINE

2) The sides of the block are VERTICAL walls

- Will show the DIP DIRECTION of the beds

- Will show the DIP angle of the beds

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Basic (Important) Principles to Know:

Older beds dip toward younger beds (unless overturned).

The lower the number the older the bed (1=Oldest)

How to draw block diagrams for synclines, dipping beds, etc.

How to recognize sequences (ex. 1234321 - a syncline)

How to determine the type of fault you're looking at (i.e., a Reverse Fault, Normal Fault, Right Lateral Strike Slip, etc.)

* Up to Structural Geology Topic List *


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