CRUSTAL DEFORMATION d G l i St tand Geologic Structures

Chapter 10

DeformationDeformation

D f ti i l• Deformation involves:– Stress – the amount of force applied to

a given area.– Types of Stress:yp

– Confining Stress – stress applied equally in all directions.q y

– Differential Stress – stress applied unequally in different directions.q y

Deformational Stress• Types of Differential Stress:

(1)C i l St h t d(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).with divergent plate boundaries).

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

Deformation of the Earth’sof the Earth s Crust Caused b T t iby Tectonic Forces and Associated DifferentialDifferential

Stresses

D f tiDeformation

• Differential stress applied to rocks during tectonic activity causes rocks to respond viad f tideformation.

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

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

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

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

– General Characteristics of Rock Deformation:• Elastic deformation – the rock returns to

nearly its original size and shape when the stress is removed.stress is removed.

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

How Do Rocks Deform?

F t th t i fl th t th

How Do Rocks Deform?

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

• Depth• Temperature• Confining Pressure• Rock Typeyp• Availability of Fluids• TimeTime

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 exceededfracture 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 withoutsolid-state flow, in which changes occur without fracturing.

Mapping Geologic Structurespp g g• When conducting a study of a region, a

geologist identifies and describes the dominantgeologist identifies and describes the dominant rock structures.

• Describing and mapping the orientation orDescribing and mapping the orientation or attitude of a rock layer or fault/ fracture surface involves determining the features.– Strike (Trend)

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

• Generally expressed an an angle relative to north (for example N10ºE)(for example, N10ºE).

Mapping Geologic StructuresMapping Geologic Structures– Dip (Inclination)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 di ti t d hi h th k idirection toward which the rock is inclined (for example, 30ºSE).

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

Strike and Dip of a Rock LayerStrike and Dip of a Rock Layer

A Geologic Map Showing Strike and Dip of Structures

G l iGeologist Measuring gthe Dip of Strata in aStrata in a Roadcut

Crustal StructuresCrustal Structures• Folds – During crustal deformation rocks

ft b t i t i f likare often bent into a series of wave-like undulations.

A ti li d S li– Anticlines and Synclines– Domes and Basins

Monoclines– Monoclines• Characteristics of Folds:

M t f ld lt f i l• Most folds result from compressional stresses which shorten and thicken the crust.

• Most of them occur in a series.Most of them occur in a series.

Anatomy of a Foldy• Limbs – Refers to the two sides of a

f ldfold.• 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.that divides a fold symmetrically.

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

Horizontal(A) and(A) and

Plunging (B) Anticlines

Common Types of FoldsCommon Types of Folds• Anticline – upfolded or arched rock t c e up o ded o a c ed oc

layers.• Syncline – downfolds or troughs of• Syncline – downfolds or troughs of

rock layers.Depending on their orientation anticlines– Depending on their orientation, anticlines and synclines can be described as• Symmetrical asymmetrical overturned• Symmetrical, asymmetrical, overturned,

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

Animations

A ti li d S liAnticlines and Synclines

F tiFormationof Folds

Insert Animation #30: Folds of FoldsFolds

Syncline (left) and A ti li ( i ht)Anticline (right)

Plunging Anticlines

andand Synclines

(Note: the outcrop pattern of an

anticline points inanticline points in the direction it is

plunging, whereasplunging, whereas the opposite is

true for a syncline)

Sheep Mountain, A Plunging Anticline

F tiFormationof Folds

Insert Animation #30: Plunging Folds of FoldsPlunging Folds

Other Types of Folds• Monoclines

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

Other Types of Foldsyp• Dome

• Upwarped displacement of rocks• Upwarped displacement of rocks.• Circular or slightly elongated structure.• Oldest rocks in center, younger rocks on the flanks., y g

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 flanksrocks on the flanks.

Crustal StructuresCrustal Structures• Faults – Fractures in rocks along which g

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

interconnected faults.• Sudden movements along faults are the• Sudden movements along faults are the

cause of most earthquakes.Cl ifi d b th i l ti t hi h• Classified by their relative movement which can be

• Horizontal, Vertical, or Oblique

Types of Faults

Summary of Fault Typesy yp

• Dip-Slip Faults: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

l t b d iplate boundaries.

Dip-Slip Faults• Movement is

mainly parallel tomainly parallel to the dip of the fault surface.

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

Dip-Slip Faults• Normal Fault (gravity) Dip-Slip Faults

• Hanging wall block moves down relative to theHanging 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• 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 ProvinceRange in Wyoming, Basin and Range Province in Nevada).

NormalFaulting

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1103/es1103page01.cfm?chapter_no=visualizationhttp://www.iris.edu/gifs/animations/faults.htmhttp://www.pbs.org/wnet/savageearth/animations/

Animations:

Formationof Normal

F lInsert Animation #29: Faults – Normal FaultsFaults – Normal

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

in Nevada – topography generated by a system of hl th t th t di l f ltroughly north to south trending normal faults.

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

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

• Detachment Fault – nearly horizontal fault extending up to hundreds of kilometers into the subsurface. S ll f lt t d t thiSmaller faults are connected to this larger fault. Boundary between ductile and brittle deformation.

Dip-Slip FaultsDip Slip Faults• Reverse and Thrust Dip-Slip Faultsp p

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

• Reverse faults have dips greater than 45o

and thrust faults have dips less than 45o. Strong compressional stress• Strong compressional stress.

• Accommodate shortening and thickening of the crust.thickening of the crust.

• Associated with convergent plate boundaries.

Reverse Fault

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

Formationof Reverse

F lInsert Animation #29: Faults – Reverse FaultsFaults – Reverse

Idealized Development of Lewis Overthrust Fault near Glacier National ParkFault near Glacier National Park

Fault-Associated Folding

Strike-Slip FaultsStrike-Slip Faults

• Dominant displacement is horizontal• Dominant displacement is horizontal and parallel to the strike of the fault.M d b d f hl• May produce broad zones of roughly parallel fractures up too several kil t i idthkilometers in width.

• Shear stress.• Associated with transform plate

boundaries.

Formationof Strike-Slip

F lInsert Animation #29: Faults – Strike-Slip FaultsFaults – Strike-Slip

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

h i id f h f lthe 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.

• Transform Fault – Large strike-slip g pfault that cuts through accommodates motion between two large crustal plates g p(San Andreas Fault).

A Right-Lateral Strike-Slip FaultA Right-Lateral Strike-Slip Fault

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

The SanAndreas

FaultFault System

Name the Type of Fault Belowyp

Name the Type of Fault BelowName the Type of Fault Below

Name the Type of Fault Below

Insert Animation #28: Exposing Metamorphic RockExposing Metamorphic Rock

Name the Type of Fault Below

Name the Type fof Fault Below

Name the St tStructure

Name the St tStructure