op ch11 lecture_earth3, orogeny

Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Chapter 11Crags, Cracks, and Crumples:

Crustal Deformation and Mountain Building

Crags, Cracks, and Crumples: Crustal Deformation and Mountain BuildingCrags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana

Prepared by

Ronald Parker Earlham College Department of Geosciences

Richmond, Indiana


MountainsMountains Attractive landscape features for humans.Attractive landscape features for humans.

Provide beautiful scenery.Provide beautiful scenery. Refuge from the mundane.Refuge from the mundane. Inspire poetry and art.Inspire poetry and art.

Mountains provide vivid evidence of tectonic activity.Mountains provide vivid evidence of tectonic activity. They embody… They embody…

Uplift. Uplift. Deformation.Deformation. Metamorphism.Metamorphism.


Mountain BeltsMountain Belts Mountains frequently occur in elongate, linear belts.Mountains frequently occur in elongate, linear belts. Mountains are constructed by tectonic plate interactions Mountains are constructed by tectonic plate interactions

in a process called orogenesis.in a process called orogenesis.


MountainsMountains Mountain building involves…Mountain building involves…

Structural deformation.Structural deformation. Jointing.Jointing. Faulting.Faulting. Folding.Folding. Partial melting.Partial melting. Foliation.Foliation. Metamorphism.Metamorphism. Glaciation. Glaciation. Erosion.Erosion. Sedimentation.Sedimentation.

Constructive processes build mountains up; destructive Constructive processes build mountains up; destructive processes tear them back down again.processes tear them back down again.


Orogenic BeltsOrogenic Belts Mountains are born and have a finite lifespan.Mountains are born and have a finite lifespan.

Young mountains are high, steep, and growing upward.Young mountains are high, steep, and growing upward. Middle-aged mountains are dissected by erosion.Middle-aged mountains are dissected by erosion. Old mountains are deeply eroded and often buried.Old mountains are deeply eroded and often buried.

Ancient orogenic belts are found in continental interiors. Ancient orogenic belts are found in continental interiors. Orogenic continental crust is too buoyant to subduct. Orogenic continental crust is too buoyant to subduct. Hence, if it escapes erosion it is usually preserved. Hence, if it escapes erosion it is usually preserved.


Crustal DeformationCrustal Deformation Orogenesis causes deformation, consisting of…Orogenesis causes deformation, consisting of…

Bending.Bending. Breaking.Breaking. Tilting.Tilting. Squashing.Squashing. Stretching.Stretching. Shearing.Shearing.

Deformation is a force applied to rock. Deformation is a force applied to rock. Changes in shape via deformation are called strain.Changes in shape via deformation are called strain. The study of deformation is called structural geology.The study of deformation is called structural geology.


DeformationDeformation Deformation strain creates geologic structures.Deformation strain creates geologic structures.

Joints – Fractures that have no offset.Joints – Fractures that have no offset. Folds – Layers that are bent by slow plastic flow.Folds – Layers that are bent by slow plastic flow. Faults – Fractures that are offset.Faults – Fractures that are offset. Foliation – Planar metamorphic fabric.Foliation – Planar metamorphic fabric.


DeformationDeformation Deformed terrane passes into undeformed terrane.Deformed terrane passes into undeformed terrane.

Undeformed (unstrained).Undeformed (unstrained).Horizontal beds.Horizontal beds.Spherical sand grains.Spherical sand grains.Flat-lying clays.Flat-lying clays.No folds or faults.No folds or faults.


DeformationDeformation Deformed terrane passes into undeformed terrane.Deformed terrane passes into undeformed terrane.

Deformed (strained).Deformed (strained).Tilted beds.Tilted beds.Metamorphic alteration.Metamorphic alteration.

Sand = Quartzite.Sand = Quartzite. Clay = Slate, phyllite, schist, or gneiss.Clay = Slate, phyllite, schist, or gneiss.

Folding and faulting.Folding and faulting.


DeformationDeformation Deformation results in one or all of the following...Deformation results in one or all of the following...

Translation – Change in location.Translation – Change in location. Rotation – Change in orientation.Rotation – Change in orientation. Distortion – Change in shape.Distortion – Change in shape.

Deformation is often easy to see.Deformation is often easy to see.


StrainStrain Changes in shape caused by deformation.Changes in shape caused by deformation.

Stretching – Pulling apart.Stretching – Pulling apart. Shortening – Compressing, squeezing.Shortening – Compressing, squeezing. Shear – Sliding past.Shear – Sliding past.

Elastic strain – Reversible change in shape.Elastic strain – Reversible change in shape. Permanent strain – Irreversible change in shape.Permanent strain – Irreversible change in shape. Two types of permanent strain: brittle and ductile.Two types of permanent strain: brittle and ductile.


Deformation TypesDeformation Types Two major types: brittle and ductile.Two major types: brittle and ductile.

Brittle deformation – Rocks break by fracturing. Brittle deformation – Rocks break by fracturing. Brittle deformation occurs in the shallow crust.Brittle deformation occurs in the shallow crust.

A transition between the two occurs at 10-15 km depth. A transition between the two occurs at 10-15 km depth.


Deformation TypesDeformation Types Two major types: brittle and ductile.Two major types: brittle and ductile.

Ductile deformation – Rocks deform by flow and folding. Ductile deformation – Rocks deform by flow and folding. Ductile deformation occurs in the deeper crust.Ductile deformation occurs in the deeper crust.

A transition between the two occurs at 10-15 km depth. A transition between the two occurs at 10-15 km depth.


Causes of Deformation Causes of Deformation Strain is the result of deformation, but what causes it?Strain is the result of deformation, but what causes it?

Caused by force acting on rock, a phenomenon called stress.Caused by force acting on rock, a phenomenon called stress. Stress is the force applied across an area.Stress is the force applied across an area.

A large force per area results in much deformation.A large force per area results in much deformation. A small force per area results in little deformation.A small force per area results in little deformation.


Causes of DeformationCauses of Deformation Types of stress:Types of stress:

Compressional – Squeezing.Compressional – Squeezing. Tensional – Pulling apart.Tensional – Pulling apart. Shear – Sliding past.Shear – Sliding past.

Tectonic collision produces horizontal compression.Tectonic collision produces horizontal compression. Large scale.Large scale. Most common type of deformation.Most common type of deformation.


StressStress Pressure – Object feels the same stress on all sides. Pressure – Object feels the same stress on all sides.


Compression – Squeezing (greater stress in 1 direction).Compression – Squeezing (greater stress in 1 direction). Tends to thicken material.Tends to thicken material.

StressStress


Extension – Pull apart (greater stress in 1 direction).Extension – Pull apart (greater stress in 1 direction). Tends to thin material. Tends to thin material.

StressStress


Shear – Blocks of rock sliding past one another.Shear – Blocks of rock sliding past one another. Crust is neither thickened or thinned.Crust is neither thickened or thinned.

StressStress


Geologic StructuresGeologic Structures Geometric features created by deformation.Geometric features created by deformation.

Folds, faults, joints, etc.Folds, faults, joints, etc. Often preserve information about stress fields. Often preserve information about stress fields.

3-D structural orientation is described by strike and dip.3-D structural orientation is described by strike and dip. Strike – Horizontal intersection with a tilted surface.Strike – Horizontal intersection with a tilted surface. Dip – Angle of surface down from the horizontal.Dip – Angle of surface down from the horizontal.


Measuring StructuresMeasuring Structures Dip is always…Dip is always…

Perpendicular to strike.Perpendicular to strike. Measured downslope.Measured downslope.

Linear structures measure similar properties.Linear structures measure similar properties. Bearing (compass direction).Bearing (compass direction). Plunge – Angle from the horizontal. Plunge – Angle from the horizontal.

Strike and dip measurements are common.Strike and dip measurements are common.


JointsJoints Planar rock fractures without offset. Planar rock fractures without offset. Result from tensional tectonic stresses.Result from tensional tectonic stresses. Systematic joints occur in parallel sets.Systematic joints occur in parallel sets. Minerals can fill joints to form veins. Minerals can fill joints to form veins. Joints control weathering of rock.Joints control weathering of rock.


FaultsFaults Planar fractures offset by movement across the break. Planar fractures offset by movement across the break. Faults are abundant and occur at a variety of scales.Faults are abundant and occur at a variety of scales. Faults may be active or inactive. Faults may be active or inactive. Sudden movements along faults cause earthquakes.Sudden movements along faults cause earthquakes. Faults vary by type of stress and crustal level. Faults vary by type of stress and crustal level.

Compression.Compression. Tension.Tension. Shear.Shear. Brittle (shallow). Brittle (shallow). Ductile (deep).Ductile (deep).


FaultsFaults Faults may offset large blocks of Earth.Faults may offset large blocks of Earth. The amount of offset is a measure called displacement. The amount of offset is a measure called displacement. The San Andreas (below) - Displacement of 100s of kms.The San Andreas (below) - Displacement of 100s of kms.

The recently developed stream is offset ~ 100m. The recently developed stream is offset ~ 100m.


Fault MovementFault Movement The direction of relative block motion…The direction of relative block motion…

Reflects the dominant type of crustal stress.Reflects the dominant type of crustal stress. Defines the type of fault.Defines the type of fault.

All motion is relative.All motion is relative. To help visualize fault motion…To help visualize fault motion…

Imagine that one block is stationary (fixed in place).Imagine that one block is stationary (fixed in place).Then, imagine that faulting moves the other block. Then, imagine that faulting moves the other block.


Fault ClassificationFault Classification Fault plane orientation.Fault plane orientation.

Vertical.Vertical. Horizontal.Horizontal. Dipping.Dipping.

Relative motion of the offset blocks. Relative motion of the offset blocks. Dip slip – Blocks move parallel to fault plane dip. Dip slip – Blocks move parallel to fault plane dip. Strike slip – Blocks move parallel to fault plane strike. Strike slip – Blocks move parallel to fault plane strike. Oblique slip – Combination of dip-slip and strike-slip. Oblique slip – Combination of dip-slip and strike-slip.


Fault OrientationFault Orientation On a dipping fault, the blocks are classified as the…On a dipping fault, the blocks are classified as the…

Hanging wall block (above the fault), and the...Hanging wall block (above the fault), and the... Footwall block (below the fault).Footwall block (below the fault).

Standing in a tunnel excavated along the fault…Standing in a tunnel excavated along the fault… Your head is near the hanging wall block.Your head is near the hanging wall block. You are standing on the foot wall block. You are standing on the foot wall block.


Recognizing FaultsRecognizing Faults Continuous layers in rock are displaced across a fault. Continuous layers in rock are displaced across a fault. Faults may juxtapose different kinds of rock. Faults may juxtapose different kinds of rock. Scarps may form where faults intersect the surface. Scarps may form where faults intersect the surface. Fault friction motion may bend rocks into drag folds.Fault friction motion may bend rocks into drag folds. Fault-broken rocks may be more easily eroded.Fault-broken rocks may be more easily eroded. Minerals may grow on fault surfaces due to fluid flow. Minerals may grow on fault surfaces due to fluid flow.


Brittle faults can be distinguished from ductile faults. Brittle faults can be distinguished from ductile faults. Brittle fault motion results in shattered and crushed rock. Brittle fault motion results in shattered and crushed rock.

Fault breccia – Fault zone preserving broken fragments of rock.Fault breccia – Fault zone preserving broken fragments of rock. Fault gouge – Fault zone preserving pulverized, powdered rock.Fault gouge – Fault zone preserving pulverized, powdered rock. Slickensides – Surface polished by fault motion.Slickensides – Surface polished by fault motion. Slip lineations – Linear grooves on a fault preserving direction. Slip lineations – Linear grooves on a fault preserving direction.

Recognizing FaultsRecognizing Faults


Ductile faults can be distinguished from brittle faults. Ductile faults can be distinguished from brittle faults. Ductile fault motion results in plastically deformed rocks.Ductile fault motion results in plastically deformed rocks.

Rocks do not break; instead, they are intensely sheared. Rocks do not break; instead, they are intensely sheared. Rocks from ductile shear zones are called mylonites. Rocks from ductile shear zones are called mylonites. Mylonites typify detachment faults in collisional orogens.Mylonites typify detachment faults in collisional orogens.

Recognizing FaultsRecognizing Faults


Dip-Slip FaultsDip-Slip Faults Sliding is parallel to fault plane dip. Sliding is parallel to fault plane dip. Thus, blocks move up or down the slope of the fault.Thus, blocks move up or down the slope of the fault. Two kinds of dip-slip fault depend on relative motion.Two kinds of dip-slip fault depend on relative motion.

Reverse fault – Hanging wall moves up.Reverse fault – Hanging wall moves up.Thrust fault (a special type of reverse fault). Thrust fault (a special type of reverse fault).

Normal fault – Hanging wall moves down.Normal fault – Hanging wall moves down.


Normal FaultNormal Fault Hanging wall moves down relative to the footwall.Hanging wall moves down relative to the footwall. Accommodate crustal extension (pulling apart).Accommodate crustal extension (pulling apart). The fault below shows displacement and drag folding.The fault below shows displacement and drag folding.

Normal FaultsNormal Faults


Reverse and Thrust FaultsReverse and Thrust Faults Hanging wall moves up the footwall. Hanging wall moves up the footwall. Reverse faults – Fault dip is steeper than 35Reverse faults – Fault dip is steeper than 35oo. . Thrust faults – Fault dip is less than 35Thrust faults – Fault dip is less than 35oo. . Accommodate crustal shortening (compression).Accommodate crustal shortening (compression).


Thrust FaultsThrust Faults Bring old rocks up and over younger rocks.Bring old rocks up and over younger rocks. Common at the leading edge of orogenic deformation.Common at the leading edge of orogenic deformation. Can transport thrust sheets 100s of kilometers.Can transport thrust sheets 100s of kilometers. Act to shorten and thicken mountain belts.Act to shorten and thicken mountain belts.


Strike-Slip Faults Strike-Slip Faults Fault motion is parallel to the strike of the fault. Fault motion is parallel to the strike of the fault. Classified by the relative sense of motion. To find this…Classified by the relative sense of motion. To find this…

Imagine standing on one block looking across the fault.Imagine standing on one block looking across the fault. Which way does the opposite block move? Which way does the opposite block move?

Right lateral – Opposite block moves to observer’s right.Right lateral – Opposite block moves to observer’s right. Left lateral – Opposite block moves to observer’s left.Left lateral – Opposite block moves to observer’s left. Large strike-slip faults may slice the entire lithosphere.Large strike-slip faults may slice the entire lithosphere.


Fault SystemsFault Systems Faults commonly occur in groups called fault systems.Faults commonly occur in groups called fault systems.

Due to regional stresses that create many similar faults. Due to regional stresses that create many similar faults. May diverge from a common horizontal detachment fault.May diverge from a common horizontal detachment fault.

Thrust fault systems.Thrust fault systems. Stack fault blocks on top of one another.Stack fault blocks on top of one another. Act to shorten and thicken the crust.Act to shorten and thicken the crust. Result from horizontal compression.Result from horizontal compression.


Fault SystemsFault Systems Normal fault systems.Normal fault systems.

Fault blocks slide away from one another. Fault blocks slide away from one another. Fault dips often decrease with depth, joining a detachment.Fault dips often decrease with depth, joining a detachment. Blocks rotate on faults and create half-graben basins. Blocks rotate on faults and create half-graben basins. Act to stretch and thin the crust.Act to stretch and thin the crust. Result from horizontal extension (pull-apart) stress. Result from horizontal extension (pull-apart) stress.


FoldsFolds Layered rocks may be deformed into curves called folds.Layered rocks may be deformed into curves called folds. Folds occur in a variety of shapes, sizes, and geometries.Folds occur in a variety of shapes, sizes, and geometries. A special terminology is used to describe folds. A special terminology is used to describe folds.

Hinge – Portion of maximum curvature on a fold.Hinge – Portion of maximum curvature on a fold. Limb – Less-curved “sides” of a foldLimb – Less-curved “sides” of a fold Axial plane – Imaginary surface defined by connecting Axial plane – Imaginary surface defined by connecting

hinges of successively nested folds. hinges of successively nested folds.


FoldsFolds Folds often occur in a series. Folds often occur in a series. Folding may result in extremely complex geometries. Folding may result in extremely complex geometries. Orogenic settings produce large volumes of folded rock. Orogenic settings produce large volumes of folded rock. Deformed rock often experiences multiple events. Deformed rock often experiences multiple events.


Fold IdentificationFold Identification Anticline – Arch-like fold; limbs dip away from the hinge.Anticline – Arch-like fold; limbs dip away from the hinge. Syncline – Trough-like fold; limbs dip toward the hinge.Syncline – Trough-like fold; limbs dip toward the hinge. Anticlines and synclines frequently alternate in series.Anticlines and synclines frequently alternate in series.

Anticline


Fold IdentificationFold Identification Monocline – A fold like a carpet draped over a stairstep. Monocline – A fold like a carpet draped over a stairstep.

Generated by blind faults in the basement rock. Generated by blind faults in the basement rock. These faults do not cut through to the surface.These faults do not cut through to the surface. Instead, displacement folds overlying sedimentary cover. Instead, displacement folds overlying sedimentary cover.


Fold IdentificationFold Identification Folds are described by the severity of folding. Folds are described by the severity of folding.

Open fold – Has a large angle between limbs.Open fold – Has a large angle between limbs. Tight fold – Has a small angle between limbs. Tight fold – Has a small angle between limbs.


Fold IdentificationFold Identification Folds are described by hinge geometry. Folds are described by hinge geometry.

Plunging fold – Has a hinge that is tilted. Plunging fold – Has a hinge that is tilted. Non-plunging fold – Has a horizontal hinge. Non-plunging fold – Has a horizontal hinge.


Fold IdentificationFold Identification Erosion of plunging folds can create zig-zag outcrops. Erosion of plunging folds can create zig-zag outcrops.


Fold IdentificationFold Identification Folds are described by their 3-dimensional shape. Folds are described by their 3-dimensional shape.

Dome – Fold with appearance of an overturned bowl.Dome – Fold with appearance of an overturned bowl.Erode to expose old rocks in center; younger rocks outside.Erode to expose old rocks in center; younger rocks outside.Result from crustal upwarping.Result from crustal upwarping.

Basin – Fold shaped like a bowl. Basin – Fold shaped like a bowl. Erode to expose young rocks in center; older outside.Erode to expose young rocks in center; older outside.Result from crustal subsidence.Result from crustal subsidence.

Domes and basins result from vertical crustal motions.Domes and basins result from vertical crustal motions.


Forming FoldsForming Folds Folds develop in two ways. Folds develop in two ways.

Flexural folds – Layers slip as stratified rocks are bent. Flexural folds – Layers slip as stratified rocks are bent. Analogous to shear as a deck of cards is bent.Analogous to shear as a deck of cards is bent.


Forming FoldsForming Folds Folds develop in two ways. Folds develop in two ways.

Flow folds – Form by ductile flow of hot, soft rock.Flow folds – Form by ductile flow of hot, soft rock.


Forming FoldsForming Folds

Horizontal compression causes rocks to buckle. Horizontal compression causes rocks to buckle. Shear causes rocks to smear out.Shear causes rocks to smear out.


Tectonic FoliationTectonic Foliation Foliation develops via compressional deformation.Foliation develops via compressional deformation. Flattening – Develops perpendicular to shortening strain.Flattening – Develops perpendicular to shortening strain.

Sand grains flatten and elongate; clays reorient.Sand grains flatten and elongate; clays reorient. Foliation parallels fold axial planes. Foliation parallels fold axial planes.


Tectonic FoliationTectonic Foliation Foliation can develop as the result of shearing.Foliation can develop as the result of shearing.

Foliation created as ductile rock is smeared. Foliation created as ductile rock is smeared. Shear foliation is not perpendicular to compression.Shear foliation is not perpendicular to compression. Rocks that are sheared have a distinctive appearance.Rocks that are sheared have a distinctive appearance.


Orogenesis and Rock GenesisOrogenesis and Rock Genesis Orogenic events create many kinds of rocks. Orogenic events create many kinds of rocks.

Igneous rocks – Intrusive and extrusive.Igneous rocks – Intrusive and extrusive.Subduction-related volcanic arc.Subduction-related volcanic arc.Rift-related decompressional melting.Rift-related decompressional melting.

Metamorphic rocks – Regional and contact.Metamorphic rocks – Regional and contact.Igneous intrusion.Igneous intrusion.Deep burial. Deep burial. Horizontal compression.Horizontal compression.


Orogenesis and Rock GenesisOrogenesis and Rock Genesis Orogenic events create many kinds of rocks. Orogenic events create many kinds of rocks.

Sedimentary rocks – Weathering and erosion.Sedimentary rocks – Weathering and erosion.Erosional debris is shed to adjacent regions. Erosional debris is shed to adjacent regions. Sediments accumulate in basins created by crustal flexure.Sediments accumulate in basins created by crustal flexure.Sediments can preserve evidence of mountains eroded away.Sediments can preserve evidence of mountains eroded away.


UpliftUplift Construction of mountains requires substantial uplift.Construction of mountains requires substantial uplift.

Mt. Everest (8.85 km above sea level).Mt. Everest (8.85 km above sea level). Comprised of marine sediments (formed below sea level).Comprised of marine sediments (formed below sea level).

Lofty mountains are supported by a thickened crust. Lofty mountains are supported by a thickened crust.


Crustal RootsCrustal Roots High mountains are supported by thickened lithosphere. High mountains are supported by thickened lithosphere. Thickening is caused by collisional orogenesis.Thickening is caused by collisional orogenesis.

Average continental crust – 35 to 40 km thick.Average continental crust – 35 to 40 km thick. Beneath orogenic belts – 50 to 70 km thick.Beneath orogenic belts – 50 to 70 km thick.

This thickened crust helps buoy the mountains upward.This thickened crust helps buoy the mountains upward.


IsostacyIsostacy Surface elevation represents a balance between forces.Surface elevation represents a balance between forces.

Gravitational attraction – Pushes plate into the mantle. Gravitational attraction – Pushes plate into the mantle. Buoyancy – Causes plate to float higher on the mantle.Buoyancy – Causes plate to float higher on the mantle.

The term The term isostatic equilibriumisostatic equilibrium describes this balance. describes this balance. Isostacy is compensated after a disturbance. Isostacy is compensated after a disturbance.

Adding weight pushes the lithosphere down.Adding weight pushes the lithosphere down. Removing weight causes isostatic rebound.Removing weight causes isostatic rebound.

Compensation is slow, requiring asthenospheric flow.Compensation is slow, requiring asthenospheric flow.


©2008 W. W. Norton & Company, Inc.

Portrait of a PlanetThird Edition

earthearth

LECTURE OUTLINE

Chapter 11Crags, Cracks, and Crumples:

Crustal Deformation and Mountain Building

This concludes the

op ch11 lecture_earth3, orogeny

Education

deformation deformation

deformation types

structural deformation

study of deformation

result of deformation

crustal deformation

brittle deformation

ductile deformation