Rock Deformation
Stress and Strain
Stress = force per unit area. Higher stress for same force if applied over a smaller area.
Compressive, tensional, shear
Strain = change in shape, size, or orientation
Strain (deformation) can be brittle (breaking), ductile (bending and flowing), or elastic
(recoverable, like a rubber band)
Type of deformation depends on: confining pressure (higher = ductile), temperature
(higher = ductile), rock type (stronger = brittle), and strain rate (faster = brittle).
Depicting the orientation of layers using STRIKE AND DIP. Strike is the direction of a
horizontal line that is drawn on the layer, and dip is the maximum slope on the bed. Useful way
to convey the information of rock deformation and determine the types of folds and the direction
of stresses that deform the rocks.
Know what a strike and dip symbol represents, and how to draw a simple one.
FOLDS-typically formed by compressional stress.
Hinge is the break in slope of the fold
Fold axis (also sometimes called the hinge line) runs along the crest of the fold
Axial plane is the plane that bisects the fold
Fold types
Anticlines: beds dip away from the center of the fold, oldest beds are exposed in the center of
the fold.
Synclines: beds dip toward the center of the fold, youngest beds are exposed in the center of the
fold.
Monoclines: step-like folds with two hinges
Plunging folds have a fold axis that is not horizontal
Overturned folds have one limb that is upside down or overturned
DOMES and BASINS-similar to anticlines and synclines to determine the type.
FAULTS-brittle deformation
Strike-slip faults: movement along the strike direction (horizontal). They are either RIGHT- or
LEFT-LATERAL (San Andreas Fault system is an example)
Dip Slip Faults (Vertical movments “along the dip”)
NORMAL faults-extensional faults, hangingwall moves down relative to the footwall
REVERSE faults-compressional faults, hangingwall moves up relative to the footwall
THRUST faults are low-angle reverse faults, so also compressional.
Be able to distinguish these faults and identify the hanging wall and footwall of a fault.