GSC 1530 Chapter 10

Crustal Deformation

Crustal Deformation

• Crustal deformation processes have produced some of the world’s most beautiful scenery (e.g., the Himalaya Mountains) but also some of its most deadly events (e.g., the earthquake which induced the massive 2004 Indian Ocean tsunami)

Terminology

• Stress – amount of force applied to a given area of a body

• Three main types of non-uniform (differential) stress: compressive ( ), tensional ( ) and shear ( )

• Strain – change in shape of a body resulting from the applied stress

• Three main types: brittle, elastic and plastic (ductile); see classroom examples and slides

Joints and Faults

Joints and faults result from brittle rock behavior

Joints – rock fractures along which no appreciable displacement has occurred

Earthquake Origin?

•Fault – a crack in rock along which rock shifting (displacement) has occurred•Earthquakes result from brittle rock behavior which produces faults•Elastic Rebound Theory: earthquakes result from the rapid release of elastic strain energy that produces rock rupture and displacement (shifting)•See classroom examples, including slide illustrating the elastic rebound theory

Elastic ReboundPhenomenonand EarthquakeProduction

Elastic Rebound

Earthquakes and Faults

• An initial earthquake produces the fault and subsequent movement can occur along the fault if sufficient strain energy accumulates to cause brittle behavior

More Earthquake Terminology

• Focus – subsurface origin point of earthquake; earthquake (seismic) energy radiates outward in all directions from the focus

• Epicenter – point on the Earth’s surface directly above the focus; seismic surface waves radiate outward from this point

• Fault scarp – relatively steep landform produced from fault motion with a substantial vertical component

• Seismic waves – the vibrational energy produced during an earthquake

Fault Types

• Faults can be broadly classified as dip-slip (rock displacement is primarily vertical) and strike-slip (rock displacement is primarily horizontal)

• Dip-slip faults can be further subclassified as reverse or normal faults

• To correctly subclassify a dip-slip fault, you need to know the relative motion of the hanging wall and footwall blocks (see slide)

Reverse fault – hanging wall block moves upwardrelative to the footwallblock

Normal fault – hangingwall block movesdownward relativeto the footwallblock

Hanging Wall

Footwall

Hanging WallFootwall

Type of dip-slip fault?

Type of dip-slip fault?

Horst – uplifted block of crust bounded by normal faults that divergewith depth

Graben – a basin (valley) formed by a subsided block of crust bounded by normal faults that converge with depth

Major landforms produced by normal faulting

Strike-slip fault

Lateral displacement (~8.5 feet) of fence during the 1906 San Francisco Earthquake

Major Fold Types

• Rocks, under certain conditions, may fold (ductile deformation) instead of faulting

• What are the four major factors that determine whether a rock will fold or fault?

• Rock temperature• Amount of stress exerted upon rock• Rate stress is exerted upon the rock• Rock composition

Major Fold Types

• There are three major folds you should be able to describe: anticlines, synclines and monoclines

• Anticline – an arch-like package of folded rocks

• Syncline – a trough-like package of folded rock

Major Fold Types

• Monoclines – strata exhibiting a single inclination direction (see slide)

Monocline

Related Fold Types

• Domes - large-scale upwarping of sedimentary rocks

• Basins - large-scale downwarping of sedimentary rocks (see slides)

Michigan’sLowerPeninsula isoften describedby geologistsas the MichiganBasin

Folds and Faults

• Folded and faulted rock often coexists; the rocks fold until their brittle strain threshold is exceeded and then fault (see slide)

Folded and FaultedSedimentary Rock