EPS 101 Spring 2003 – Adrian Brearley – Lecture 22,23

Folds and Faults.

Folded metamorphic gneisses from Northwest Scotland

Key concepts

Know that:

1. Rocks on the Earth are subjected to a variety of deforming forces – compressional, tensional and shear.
2. These forces produce faults and joints in rocks near the surface of the Earth and folds at depth in the Earth.
3. There are different kinds of major fold types and fault types – know what they are.
4. Be able to define the terms strike and dip.

Folding – the bending of rocks in the Earth as a result of compressional tectonic forces such as continental collisions. Occurs at depth in the Earth.

Faulting – fracturing of rocks in the Earth causing breakage and slip parallel to the fracture. Caused by compressional, extensional and shear forces. Occurs close to surface of the Earth.

Geological measurements in the field

Strike – direction of intersection of a rock layer with a horizontal surface.

Dip – measured at right angles to the strike is amount of tilting. i.e. angle that the bed inclines from the horizontal.

Forces that cause deformation

Compressional – forces that squeeze rocks together and cause shortening.

Tensional – forces that stretch and tend to pull a body apart, extensional.

Shearing – forces that push two sides of an object in opposite directions.

Brittle and ductile behavior

Brittle behavior – Materials that are brittle suddenly fracture when forces (stresses) acting on them become high enough, i.e. glass at room temperature. Rocks close to surface of the Earth.

Ductile behavior – Materials that are ductile can accommodate stresses by flowing or deforming. Glass at high temperatures will flow. Molten chocolate. Rocks at depth in the Earth.

Types of folds

Anticline– upfolds or arches of folded, layered rocks.

Syncline – downfolds or troughs of folded, layered rocks.

Limbs – The two sides of a fold.

Axial plane – imaginary surface that divides a fold as symmetrically as possible, with one limb on either side of the plane.

Fold axis – line made by the lengthwise interaction of the axial plane with the beds of the fold.

Plunging fold – a fold whose fold axis is not horizontal, but dips at some angle.

Asymmetrical fold – fold that has one limb dipping more steeply than the other. Caused by increased horizontal forces.

Overturned fold – fold that has been subjected to intense deformation and has one limb that has been tilted beyond the vertical.

Faults

Fault plane – the surface along which a rock formation fractures and slips.

Dip-slip fault – relative movement up or down the fault plane.

Normal fault – dip slip fault where the rocks below the fault plane move upwards – caused by extensional forces.

Reverse fault – dip slip fault whare the rocks below the fault plane move downwards – caused by compressional forces.

Strike-slip fault – fault whose movement is horizontal, parallel to the strike of the fault plane – caused by shearing. Right lateral fault – strike slip with movement to the right, Left lateral fault, strike slip fault with movement to the left.

Oblique-slip fault – fault where move is simultaneously along strike and either up or down – movement at an angle along the fault plane.

Thrust fault – very low angle reverse fault where one formation is thrust over another – process called overthrusting.

Rift valley – depression in the Earth caused by extensional forces causing normal faults that allow a large block of the Earth to drop. Creates a long narrow trough in the Earth bounded by fault planes – East African rift valley, Rio Grande Rift.