Forces that cause deformation Ductile Brittle

Behaviour of Rocks to Stress & Strain Stress - a force per unit area at a particular point. Strain - the change in size (volume) or shape, or both, while an object is undergoing stress. Behaviour of rocks with increasing stress and strain. Elastic behaviour occurs along the straight line portions (blue) At stresses greater than the elastic limit (red points) the rock will either deform as a ductile material or break, as shown in the deformed rock cylinders.

Brittle: Faults

Ductile: Folds

Components of a fold Axis/Nose: most sharply curved part • Limb: sides of a fold

Kinds of fold sets

ANOTHER TYPE OF FOLDS

Folded Rock Before Erosion

Folded Rock After Erosion Eroded Anticline, older rocks in center. Syncline is opposite.

3-D: Dome and Basin

UNCONFORMITY A surface (or contact) represents a gap in the geologic record, with the rock unit immediately above the contact being considerablely younger than the rock beneath Most unconformities are burried erosion surfaces. 3 types

Disconformity An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition

Angular unconformity An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity

Angular unconformity Angular unconformity

Nonconformity A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock

Nonconformity

Measuring Attitude of Rocks Dip - Angle of bed with the horizontal Strike - Bearing (compass direction) of line of intersection between horizontal plane and the inclined bed. Dip Direction – is the compass direction in which the angle of dip is measured. Attitude of planar structures (bedding, faults, joints, foliations, etc.) is often depicted by the reading of strike and dip, or dip direction and dip.