Geological Faults

Objective: By the end of this session you should be able to use

diagrams and examples to describe and explain the different

types of geological faults

Types of Geological Faults

Vertical Movement (Dip-Slip)• Normal (East African Rift Valley)• Reverse (Salt Lake City)Horizontal Movement (Strike-Slip)• Tear Faults (The Great Glen Scotland)• Transform Faults (San Andreas Fault)Thrust Faults• Overfold or Nappe (cliffs Broadhaven

Dyfed)

You have one minute to draw and label a normal fault

Normal FaultsTENSION / Pulling motion = LENGTHENING

Throw = Vertical DisplacementHeave = Horizontal Displacement

H

T

Fault Plane

Fault Scarp

Upthrow and Downthrow

A UPTHROW

B DOWNTHROW

1. A moved up while B moved down2.3.4.5.

Throw is therefore a RELATIVE position

Give five ways that the blocks could have moved?

Reverse Faults

Fault Plane

T

H

COMPRESSION / Pushing motion = SHORTENING

Rift Valleys

Graben or Rift Valleys are the result of subsidence between Normal Faults (i.e. TENSION/pulling) Horsts are left upstanding after subsidence on either side.

Fault Scarp

Name a famous rift valley:________________________________________

Strike-slip faults (horizontal faults)

• Tear Faults - the result of shearing forces applied to rocks this can be famously seen at the Great Glen Fault where there is SINISTRAL movement (to the left) of a Granite Intrusion split in two. The opposite of sinistral movement is DEXTRAL movement (to the r_____).

Strike-slip faults (horizontal faults)

• Transform Faults - associated with plate margins. Famously seen at the S___ A____ Fault. These faults are also common at right angles to a mid-ocean ridge. Plate motion can also be seen at transform faults.

Thrust Faults• Intense c__________•Beds so contorted movement can only occur through fracture•Overfold or Nappe moves up and over lower part•A type of reverse fault with an almost horizontal fault plane•Crustal s_________ takes place•Associated with mountain building•Older rocks can be pushed over y_____ rocks•At the Moine Thrust Fault in NW Scotland Pre-Cambrian rocks are found above Cambrian rocks

Nappe

Other Fault Features• Fault Breccia: crushed and broken fragments along

fault planes• Slickenslides: deep grooves along fault planes showing

direction of movement• Gaps along fault planes: often filled with fluids such as

magma giving rise to dykes or hydrothermal solutions carrying minerals (such as veins of galena and fluorite found in the Pennines)

• Erosion: valleys can form along weak fault lines; upthrow can be eroded to the same level as downthrow which can leave misleading geological patterns

1. Draw and label a Dip-Slip Fault2. Name a specific example of a Normal

Fault3. Is dextral movement to the left or right?4. What types of faults involve Nappes?

What have you remembered?(Blank paper, two minutes!!!)

What have you remembered?

Exam Question from Summer 2007

Mark Scheme for Exam Question

• Specific type of fold/fault e.g. "reverse fault", "anticline" (1)

• Quality of diagram/description (2)• Reference to appropriate scale (1)• Specific location of feature (1)

• Correct name of stress involved (1) R• e.g. Extension/Tension for normal fault, • Compression for reverse/thrust fault or fold• Shear for tear fault