Op ch11 lecture_earth3, Orogeny

Chapter 11
Crags, Cracks, and Crumples:
Crustal Deformation and Mountain Building

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 2nd edition, by Stephen Marshak

Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Prepared by

Ronald Parker
Earlham College Department of Geosciences
Richmond, Indiana

Mountains
 Attractive landscape features for humans.
 Provide beautiful scenery.
 Refuge from the mundane.
 Inspire poetry and art.

 Mountains provide vivid evidence of tectonic activity.
 They embody…
 Uplift.
 Deformation.
 Metamorphism.



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

in a process called orogenesis.



Mountains
 Mountain building involves…
 Structural deformation.
 Jointing.
 Faulting.
 Folding.
 Partial melting.
 Foliation.
 Metamorphism.
 Glaciation.
 Erosion.
 Sedimentation.

 Constructive processes build mountains up; destructive

processes tear them back down again.


Orogenic Belts
 Mountains are born and have a finite lifespan.
 Young mountains are high, steep, and growing upward.
 Middle-aged mountains are dissected by erosion.
 Old mountains are deeply eroded and often buried.

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



Crustal Deformation
 Orogenesis causes deformation, consisting of…
 Bending.
 Breaking.
 Tilting.
 Squashing.
 Stretching.
 Shearing.

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


Deformation
 Deformation strain creates geologic structures.
 Joints – Fractures that have no offset.
 Folds – Layers that are bent by slow plastic flow.
 Faults – Fractures that are offset.
 Foliation – Planar metamorphic fabric.



Deformation
 Deformed terrane passes into undeformed terrane.
 Undeformed (unstrained).

Horizontal beds.
Spherical sand grains.
Flat-lying clays.
No folds or faults.



Deformation
 Deformed terrane passes into undeformed terrane.
 Deformed (strained).

Tilted beds.
Metamorphic alteration.
Sand = Quartzite.
Clay = Slate, phyllite, schist, or gneiss.

Folding and faulting.



Deformation
 Deformation results in one or all of the following...
 Translation – Change in location.
 Rotation – Change in orientation.
 Distortion – Change in shape.

 Deformation is often easy to see.



Strain
 Changes in shape caused by deformation.
 Stretching – Pulling apart.
 Shortening – Compressing, squeezing.
 Shear – Sliding past.

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


Deformation Types
 Two major types: brittle and ductile.
 Brittle deformation – Rocks break by fracturing.

Brittle deformation occurs in the shallow crust.
 A transition between the two occurs at 10-15 km depth.



Deformation Types
 Two major types: brittle and ductile.
 Ductile deformation – Rocks deform by flow and folding.

Ductile deformation occurs in the deeper crust.
 A transition between the two occurs at 10-15 km depth.



Causes of Deformation
 Strain is the result of deformation, but what causes it?
 Caused by force acting on rock, a phenomenon called stress.

 Stress is the force applied across an area.
 A large force per area results in much deformation.
 A small force per area results in little deformation.



Causes of Deformation
 Types of stress:
 Compressional – Squeezing.
 Tensional – Pulling apart.
 Shear – Sliding past.

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



Stress
 Pressure – Object feels the same stress on all sides.



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



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



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



Geologic Structures
 Geometric features created by deformation.
 Folds, faults, joints, etc.
 Often preserve information about stress fields.

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



Measuring Structures
 Dip is always…
 Perpendicular to strike.
 Measured downslope.

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

 Strike and dip measurements are common.



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



Faults
 Planar fractures offset by movement across the break.
 Faults are abundant and occur at a variety of scales.
 Faults may be active or inactive.
 Sudden movements along faults cause earthquakes.
 Faults vary by type of stress and crustal level.
 Compression.
 Tension.
 Shear.
 Brittle (shallow).
 Ductile (deep).



Faults
 Faults may offset large blocks of Earth.
 The amount of offset is a measure called displacement.
 The San Andreas (below) - Displacement of 100s of kms.
 The recently developed stream is offset ~ 100m.



Fault Movement
 The direction of relative block motion…
 Reflects the dominant type of crustal stress.
 Defines the type of fault.

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

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



Fault Classification
 Fault plane orientation.
 Vertical.
 Horizontal.
 Dipping.

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



Fault Orientation
 On a dipping fault, the blocks are classified as the…
 Hanging wall block (above the fault), and the...
 Footwall block (below the fault).

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



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



Recognizing Faults
 Brittle faults can be distinguished from ductile faults.
 Brittle fault motion results in shattered and crushed rock.
Fault breccia – Fault zone preserving broken fragments of rock.
Fault gouge – Fault zone preserving pulverized, powdered rock.
Slickensides – Surface polished by fault motion.
Slip lineations – Linear grooves on a fault preserving direction.



Recognizing Faults
 Ductile faults can be distinguished from brittle faults.
 Ductile fault motion results in plastically deformed rocks.

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



Dip-Slip Faults
 Sliding is parallel to fault plane dip.
 Thus, blocks move up or down the slope of the fault.
 Two kinds of dip-slip fault depend on relative motion.
 Reverse fault – Hanging wall moves up.

Thrust fault (a special type of reverse fault).
 Normal fault – Hanging wall moves down.



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



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



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



Strike-Slip Faults
 Fault motion is parallel to the strike of the fault.
 Classified by the relative sense of motion. To find this…
 Imagine standing on one block looking across the fault.
 Which way does the opposite block move?
 Right lateral – Opposite block moves to observer’s right.
 Left lateral – Opposite block moves to observer’s left.
 Large strike-slip faults may slice the entire lithosphere.



Fault Systems
 Faults commonly occur in groups called fault systems.
 Due to regional stresses that create many similar faults.
 May diverge from a common horizontal detachment fault.

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



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



Folds
 Layered rocks may be deformed into curves called folds.
 Folds occur in a variety of shapes, sizes, and geometries.
 A special terminology is used to describe folds.
 Hinge – Portion of maximum curvature on a fold.
 Limb – Less-curved “sides” of a fold
 Axial plane – Imaginary surface defined by connecting

hinges of successively nested folds.



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



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

Anticline



Fold Identification
 Monocline – A fold like a carpet draped over a stairstep.
 Generated by blind faults in the basement rock.
 These faults do not cut through to the surface.
 Instead, displacement folds overlying sedimentary cover.



Fold Identification
 Folds are described by the severity of folding.
 Open fold – Has a large angle between limbs.
 Tight fold – Has a small angle between limbs.



Fold Identification
 Folds are described by hinge geometry.
 Plunging fold – Has a hinge that is tilted.
 Non-plunging fold – Has a horizontal hinge.



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



Fold Identification
 Folds are described by their 3-dimensional shape.
 Dome – Fold with appearance of an overturned bowl.

Erode to expose old rocks in center; younger rocks outside.
Result from crustal upwarping.
 Basin – Fold shaped like a bowl.

Erode to expose young rocks in center; older outside.
Result from crustal subsidence.
 Domes and basins result from vertical crustal motions.



Forming Folds
 Folds develop in two ways.
 Flexural folds – Layers slip as stratified rocks are bent.
 Analogous to shear as a deck of cards is bent.



Forming Folds
 Folds develop in two ways.
 Flow folds – Form by ductile flow of hot, soft rock.



Forming Folds
 Horizontal compression causes rocks to buckle.
 Shear causes rocks to smear out.



Tectonic Foliation
 Foliation develops via compressional deformation.
 Flattening – Develops perpendicular to shortening strain.
 Sand grains flatten and elongate; clays reorient.
 Foliation parallels fold axial planes.



Tectonic Foliation
 Foliation can develop as the result of shearing.
 Foliation created as ductile rock is smeared.
 Shear foliation is not perpendicular to compression.
 Rocks that are sheared have a distinctive appearance.



Orogenesis and Rock Genesis
 Orogenic events create many kinds of rocks.
 Igneous rocks – Intrusive and extrusive.

Subduction-related volcanic arc.
Rift-related decompressional melting.
 Metamorphic rocks – Regional and contact.

Igneous intrusion.
Deep burial.
Horizontal compression.



Orogenesis and Rock Genesis
 Orogenic events create many kinds of rocks.
 Sedimentary rocks – Weathering and erosion.

Erosional debris is shed to adjacent regions.
Sediments accumulate in basins created by crustal flexure.
Sediments can preserve evidence of mountains eroded
away.



Uplift
 Construction of mountains requires substantial uplift.
 Mt. Everest (8.85 km above sea level).
 Comprised of marine sediments (formed below sea level).

 Lofty mountains are supported by a thickened crust.



Crustal Roots
 High mountains are supported by thickened lithosphere.
 Thickening is caused by collisional orogenesis.
 Average continental crust – 35 to 40 km thick.
 Beneath orogenic belts – 50 to 70 km thick.

 This thickened crust helps buoy the mountains upward.



Isostacy
 Surface elevation represents a balance between forces.
 Gravitational attraction – Pushes plate into the mantle.
 Buoyancy – Causes plate to float higher on the mantle.

 The term isostatic equilibrium describes this balance.
 Isostacy is compensated after a disturbance.
 Adding weight pushes the lithosphere down.
 Removing weight causes isostatic rebound.

 Compensation is slow, requiring asthenospheric flow.



This concludes the
Chapter 11

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.


Op ch11 lecture_earth3, Orogeny

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