Ch10 structural geology_fall2007

CRUSTAL DEFORMATION and Geologic Structures

Deformation

• Deformation involves:– Stress – the amount of force applied to

a given area.

– Types of Stress:– Confining Stress – stress applied

equally in all directions.– Differential Stress – stress applied

unequally in different directions.

Deformational Stress• Types of Differential Stress:

(1) Compressional Stress – shortens and thickens a rock body (associated with convergent plate boundaries).

(2) Tensional Stress – tends to elongate and thin or pull apart a rock unit (associated with divergent plate boundaries).

(3) Shear Stress – produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom (associated with transform plate boundaries).

Deformation of the Earth’s Crust Caused by Tectonic Forces and Associated Differential

Stresses

Deformation

• Differential stress applied to rocks during tectonic activity causes rocks to respond via deformation.

• Strain – changes in the shape or size of a rock body caused by stress.

• Strained rock bodies do not retain their original configuration during deformation.

How Do Rocks Deform?• Rocks subjected to stresses greater than

their own strength begin to deform usually by folding, flowing, or fracturing.

– General Characteristics of Rock Deformation:

• Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed.

• Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

• Factors that influence the strength of a rock and how it will deform:

• Depth• Temperature• Confining Pressure• Rock Type• Availability of Fluids• Time

How Do Rocks Deform?

• Rocks near the surface, where confining pressures and temperatures are low, will behave as a brittle solid and fracture once their strength is exceeded.

• Rocks at depth, where confining pressures and temperatures are high, will exhibit ductile behavior or solid-state flow, in which changes occur without fracturing.

How Do Rocks Deform?

Crustal Structures• Folds – During crustal deformation rocks

are often bent into a series of wave-like undulations. – Anticlines and Synclines– Domes and Basins– Monoclines

• Characteristics of Folds:• Most folds result from compressional

stresses which shorten and thicken the crust.• Most of them occur in a series.

Anatomy of a Fold• Limbs – Refers to the two sides of a fold.

• Axis (or Hinge) – A line drawn down the points of maximum curvature of each layer.

• Axial Plane – An imaginary surface that divides a fold symmetrically.

• Plunge – In complex folding, the axis is often inclined at an angle called plunge.

(A) Horizontal Anticline

and(B) Plunging

Anticline

Common Types of Folds

• Anticline – upfolded or arched rock layers.

• Syncline – downfolds or troughs of rock layers.

Photo courtesy of J. T. Daniels http://disc.gsfc.nasa.gov/geomorphology/GEO_2/GEO_PLATE_T-42.shtml

Photo courtesy of Brennan T. Jordan, Department of Earth Sciences, University of South Dakota http://www.usd.edu/~Brennan.Jordan/

Common Types of Folds• Depending on their orientation, anticlines and

synclines can be described as…• Symmetrical, asymmetrical, overturned, recumbent

(a type of overturned fold – “lying on its side”), or plunging.

Formationof Folds

Insert Animation #30:

Folds

Name the Folds Below

Plunging Anticlines

and Synclines

(Note: the outcrop pattern of an

anticline points in the direction it is

plunging, whereas the opposite is

true for a syncline)

Sheep Mountain, A Plunging Anticline

Formationof Folds

Insert Animation #30:

Plunging Folds

Other Types of Folds• Monoclines

• Large, step-like folds in otherwise horizontal sedimentary strata. • Closely associated with faulting.

Other Types of Folds• Dome

• Upwarped displacement of rocks.

• Circular or slightly elongated structure.

• Oldest rocks in center, younger rocks on the flanks.

Other Types of Folds• Basin

• Circular or slightly elongated structure.

• Downwarped displacement of rocks.

• Youngest rocks are found near the center, oldest rocks on the flanks.

Crustal Structures• Faults – Fractures in rocks along which appreciable

displacement has taken place.• Fault Zone – Displacements along multiple interconnected

faults.• Sudden movements along faults are the cause of most

earthquakes.

Types of Faults• Classified by their relative movement

which can be Horizontal, Vertical, or Oblique.

Summary of Fault Types

• Dip-Slip Faults:• Normal (gravity) – associated with divergent plate

boundaries.

• Reverse and Thrust – associated with convergent plate boundaries.

• Strike-Slip Faults:• Lateral (right and left) – associated with transform

plate boundaries.

Dip-Slip Faults

• Movement is mainly parallel to the dip of the fault surface.

• Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault).

Dip-Slip Faults• Normal Fault (gravity)

Dip-Slip Faults– Hanging wall block moves

down relative to the footwall block.

– Tensional stress– Accommodate lengthening or

extension and thinning of the crust.

– Associated with divergent plate boundaries.

– Most are small with displacements of a meter or so.

– Larger scale normal faults are associated with structures called fault-block mountains (Teton Range in Wyoming, Basin and Range Province in Nevada).

Formationof Normal

Faults

Insert Animation #29:

Faults – Normal

Normal Faulting – Fault Block Mountains• Fault-Block Mountains – Basin and Range Province

in Nevada – topography generated by a system of roughly north to south trending normal faults.

• Movements along these faults have produced alternating uplifted blocks called horsts (form elevated ranges) and down-dropped blocks called grabens (form basins).

• Half-Grabens – a tilted fault block in which the higher side forms mountainous topography and the lower side forms a basin that fills with sediment.

• Detachment Fault – nearly horizontal fault extending up to hundreds of kilometers into the subsurface. Smaller faults are connected to this larger fault. Boundary between ductile and brittle deformation.

Dip-Slip Faults• Reverse and Thrust Dip-Slip Faults

– Hanging wall block moves up relative to the footwall block.

– Reverse faults have dips greater than 45o

– Thrust faults have dips less than 45o.

• Strong compressional stress.

• Accommodate shortening and thickening of the crust.

• Associated with convergent plate boundaries.

Formationof Reverse

Faults

Insert Animation #29:

Faults – Reverse

Idealized Development of Lewis Overthrust Fault near Glacier National Park

Strike-Slip Faults

• Dominant displacement is horizontal and parallel to the strike of the fault.

• May produce broad zones of roughly parallel fractures up too several kilometers in width.

• Shear stress.

• Associated with transform plate boundaries.

Formationof Strike-Slip

Faults

Insert Animation #29:

Faults – Strike-Slip

Types of Strike-Slip Faults• Right-Lateral – as you face the fault, the

opposite side of the fault moves to the right.• Left-Lateral – as you face the fault, the

opposite side of the fault moves to the left.

http://www.pbs.org/wnet/savageearth/animations/

Animations:

Right-Lateral Strike-Slip Fault

Types of Strike-Slip Faults

• Transform Fault– Large strike-slip

fault that cuts through accommodates motion between two large crustal plates.

– Example: San Andreas Fault System

Name the Type of Fault Below

Name the Type of Fault Below

Name the Type of Fault Below

Insert Animation #28: Exposing Metamorphic Rock

Name the Type of Fault Below

Name the Type of Fault Below

Name the Structure

Name the Structure

Mapping Geologic Structures

• Geologists measure the orientation or attitude of a rock layers or fault/fracture surfaces in order to describe and map geologic structures that result from deformation.

Mapping Geologic Structures– Strike (Trend)

• The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane.

• Generally expressed an an angle relative to north.

• Example: N10ºE

Mapping Geologic Structures– Dip (Inclination)

• The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane.

• Includes both an inclination and a direction toward which the rock is inclined.

• Example: 30ºSE

A Geologic Map Showing Strike and Dip of Structures

By knowing the strike and dip, geologists can predict the nature of rock structures hidden beneath the surface.

Geologist Measuring the Dip of Strata in a Roadcut