Crustal Deformation and Mountain Building

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Transcript Crustal Deformation and Mountain Building

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

LECTURE OUTLINE

earth

Portrait of a Planet Third Edition

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

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.

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Refuge from the mundane.

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Inspire poetry and art.



Mountains provide vivid evidence of tectonic activity.



They embody…



Uplift.



Deformation.



Metamorphism.

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

Mountain Belts

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Mountains frequently occur in elongate, linear belts.

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Mountains are constructed by tectonic plate interactions in a process called orogenesis.

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

Mountains

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Mountain building involves…

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Structural deformation.

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Jointing.

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Faulting.

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Folding.

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Partial melting.

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Foliation.

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Metamorphism.

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Glaciation.

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Erosion.

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Sedimentation.

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

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

Orogenic Belts

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Mountains are born and have a finite lifespan.

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Young mountains are high, steep, and growing upward.

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Middle-aged mountains are dissected by erosion.

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Old mountains are deeply eroded and often buried.

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Ancient orogenic belts are found in continental interiors.

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Orogenic continental crust is too buoyant to subduct.

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Hence, if it escapes erosion it is usually preserved. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Crustal Deformation

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Orogenesis causes deformation, consisting of…

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Bending.

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Breaking.

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Tilting.

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Squashing.

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Stretching.

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Shearing.

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Deformation is a force applied to rock.

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Changes in shape via deformation are called strain.

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The study of deformation is called structural geology.

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

Deformation

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Deformation strain creates geologic structures.

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Joints – Fractures that have no offset.

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Folds – Layers that are bent by slow plastic flow.

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Faults – Fractures that are offset.

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Foliation – Planar metamorphic fabric.

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

Deformation

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Deformed terrane passes into undeformed terrane.

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Undeformed (unstrained).

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Horizontal beds.

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Spherical sand grains.

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Flat-lying clays.

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No folds or faults.

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

Deformation

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Deformed terrane passes into undeformed terrane.

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Deformed (strained).

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Tilted beds.

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Metamorphic alteration.

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Sand = Quartzite.



Clay = Slate, phyllite, schist, or gneiss.

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Folding and faulting.

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

Deformation

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

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Translation – Change in location.

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Rotation – Change in orientation.

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Distortion – Change in shape.

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Deformation is often easy to see.

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

Strain

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Changes in shape caused by deformation.

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Stretching – Pulling apart.

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Shortening – Compressing, squeezing.

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Shear – Sliding past.

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Elastic strain – Reversible change in shape.

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Permanent strain – Irreversible change in shape.

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Two types of permanent strain: brittle and ductile.

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

Deformation Types

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Two major types: brittle and ductile.

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Brittle deformation – Rocks break by fracturing.

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Brittle deformation occurs in the shallow crust.

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A transition between the two occurs at 10-15 km depth. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Deformation Types

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Two major types: brittle and ductile.

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Ductile deformation – Rocks deform by flow and folding.

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Ductile deformation occurs in the deeper crust.

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A transition between the two occurs at 10-15 km depth. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Deformation

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

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Caused by force acting on rock, a phenomenon called stress.

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Stress is the force applied across an area.

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A large force per area results in much deformation.

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A small force per area results in little deformation.

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

Causes of Deformation

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Types of stress:

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Compressional – Squeezing.

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Tensional – Pulling apart.

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Shear – Sliding past.

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Tectonic collision produces horizontal compression.

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Large scale.

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Most common type of deformation.

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

Stress

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Pressure – Object feels the same stress on all sides. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Stress

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Compression – Squeezing (greater stress in 1 direction).

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Tends to thicken material.

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

Stress

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Extension – Pull apart (greater stress in 1 direction).

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

Stress

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Shear – Blocks of rock sliding past one another.

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Crust is neither thickened or thinned.

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

Geologic Structures

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Geometric features created by deformation.

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Folds, faults, joints, etc.

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Often preserve information about stress fields.

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3-D structural orientation is described by strike and dip.

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Strike – Horizontal intersection with a tilted surface.

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Dip – Angle of surface down from the horizontal.

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

Measuring Structures

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Dip is always…

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Perpendicular to strike.

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Measured downslope.

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Linear structures measure similar properties.

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Bearing (compass direction).

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Plunge – Angle from the horizontal.

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Strike and dip measurements are common.

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

Joints

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Planar rock fractures without offset.

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Result from tensional tectonic stresses.

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Systematic joints occur in parallel sets.

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Minerals can fill joints to form veins.

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Joints control weathering of rock.

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

Faults

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Planar fractures offset by movement across the break.

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Faults are abundant and occur at a variety of scales.

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Faults may be active or inactive.

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Sudden movements along faults cause earthquakes.

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Faults vary by type of stress and crustal level.

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Compression.

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Tension.

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Shear.

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Brittle (shallow).

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Ductile (deep).

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

Faults

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Faults may offset large blocks of Earth.

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The amount of offset is a measure called displacement.

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The San Andreas (below) - Displacement of 100s of kms.

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The recently developed stream is offset ~ 100m. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Movement

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The direction of relative block motion…

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Reflects the dominant type of crustal stress.

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Defines the type of fault.

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All motion is relative.

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To help visualize fault motion…

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Imagine that one block is stationary (fixed in place).

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Then, imagine that faulting moves the other block. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Classification

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Fault plane orientation.

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Vertical.

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Horizontal.

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Dipping.

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Relative motion of the offset blocks.

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Dip slip – Blocks move parallel to fault plane dip.

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Strike slip – Blocks move parallel to fault plane strike.

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Oblique slip – Combination of dip-slip and strike-slip. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fault Orientation

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On a dipping fault, the blocks are classified as the…

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

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Footwall block (below the fault).

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Standing in a tunnel excavated along the fault…

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Your head is near the hanging wall block.

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You are standing on the foot wall block. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults

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Continuous layers in rock are displaced across a fault.

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Faults may juxtapose different kinds of rock.

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Scarps may form where faults intersect the surface.

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Fault friction motion may bend rocks into drag folds.

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Fault-broken rocks may be more easily eroded.

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Minerals may grow on fault surfaces due to fluid flow. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults

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Brittle faults can be distinguished from ductile faults.

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Brittle fault motion results in shattered and crushed rock.

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Fault breccia – Fault zone preserving broken fragments of rock.

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Fault gouge – Fault zone preserving pulverized, powdered rock.

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Slickensides – Surface polished by fault motion.

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Slip lineations – Linear grooves on a fault preserving direction. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Recognizing Faults

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Ductile faults can be distinguished from brittle faults.

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Ductile fault motion results in plastically deformed rocks.

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Rocks do not break; instead, they are intensely sheared.

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Rocks from ductile shear zones are called mylonites.

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Mylonites typify detachment faults in collisional orogens.

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

Dip-Slip Faults

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Sliding is parallel to fault plane dip.

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Thus, blocks move up or down the slope of the fault.

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Two kinds of dip-slip fault depend on relative motion.

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Reverse fault – Hanging wall moves up.

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Thrust fault (a special type of reverse fault).

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Normal fault – Hanging wall moves down.

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

Normal Fault

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Hanging wall moves down relative to the footwall.

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Accommodate crustal extension (pulling apart).

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The fault below shows displacement and drag folding.

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

Normal Faults

Reverse and Thrust Faults

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Hanging wall moves up the footwall.

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Reverse faults – Fault dip is steeper than 35 o .

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Thrust faults – Fault dip is less than 35 o .

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Accommodate crustal shortening (compression).

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

Thrust Faults

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Bring old rocks up and over younger rocks.

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Common at the leading edge of orogenic deformation.

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Can transport thrust sheets 100s of kilometers.

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Act to shorten and thicken mountain belts.

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

Strike-Slip Faults

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Fault motion is parallel to the strike of the fault.

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Classified by the relative sense of motion. To find this…

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Imagine standing on one block looking across the fault.

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Which way does the opposite block move?

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Right lateral – Opposite block moves to observer’s right.

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Left lateral – Opposite block moves to observer’s left.

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Large strike-slip faults may slice the entire lithosphere.

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

Fault Systems

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Faults commonly occur in groups called fault systems.

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Due to regional stresses that create many similar faults.

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May diverge from a common horizontal detachment fault.

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Thrust fault systems.

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Stack fault blocks on top of one another.

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Act to shorten and thicken the crust.

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Result from horizontal compression.

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

Fault Systems

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Normal fault systems.

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Fault blocks slide away from one another.

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Fault dips often decrease with depth, joining a detachment.

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Blocks rotate on faults and create half-graben basins.

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Act to stretch and thin the crust.

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Result from horizontal extension (pull-apart) stress. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Folds

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Layered rocks may be deformed into curves called folds.

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Folds occur in a variety of shapes, sizes, and geometries.

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A special terminology is used to describe folds.

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Hinge – Portion of maximum curvature on a fold.

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Limb – Less-curved “sides” of a fold

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Axial plane – Imaginary surface defined by connecting hinges of successively nested folds. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Folds

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Folds often occur in a series.

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Folding may result in extremely complex geometries.

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Orogenic settings produce large volumes of folded rock.

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Deformed rock often experiences multiple events. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification

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Anticline – Arch-like fold; limbs dip away from the hinge.

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Syncline – Trough-like fold; limbs dip toward the hinge.

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Anticlines and synclines frequently alternate in series.

Anticline

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

Fold Identification

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Monocline – A fold like a carpet draped over a stairstep.

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Generated by blind faults in the basement rock.

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These faults do not cut through to the surface.

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Instead, displacement folds overlying sedimentary cover. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification

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Folds are described by the severity of folding.

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Open fold – Has a large angle between limbs.

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Tight fold – Has a small angle between limbs. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification

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Folds are described by hinge geometry.

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Plunging fold – Has a hinge that is tilted.

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Non-plunging fold – Has a horizontal hinge. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification

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Erosion of plunging folds can create zig-zag outcrops. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Fold Identification

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Folds are described by their 3-dimensional shape.

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Dome – Fold with appearance of an overturned bowl.

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Erode to expose old rocks in center; younger rocks outside.

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Result from crustal upwarping.

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Basin – Fold shaped like a bowl.

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Erode to expose young rocks in center; older outside.

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Result from crustal subsidence.



Domes and basins result from vertical crustal motions.

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

Forming Folds

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Folds develop in two ways.

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Flexural folds – Layers slip as stratified rocks are bent.

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Analogous to shear as a deck of cards is bent.

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

Forming Folds

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Folds develop in two ways.

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

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

Forming Folds

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Horizontal compression causes rocks to buckle.

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Shear causes rocks to smear out.

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

Tectonic Foliation

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Foliation develops via compressional deformation.

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Flattening – Develops perpendicular to shortening strain.

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Sand grains flatten and elongate; clays reorient.

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Foliation parallels fold axial planes. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Tectonic Foliation

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Foliation can develop as the result of shearing.

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Foliation created as ductile rock is smeared.

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Shear foliation is not perpendicular to compression.

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Rocks that are sheared have a distinctive appearance.

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

Orogenesis and Rock Genesis

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Orogenic events create many kinds of rocks.

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Igneous rocks – Intrusive and extrusive.

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Subduction-related volcanic arc.

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Rift-related decompressional melting.

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Metamorphic rocks – Regional and contact.

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Igneous intrusion.

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Deep burial.



Horizontal compression.

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

Orogenesis and Rock Genesis

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Orogenic events create many kinds of rocks.

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Sedimentary rocks – Weathering and erosion.

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Erosional debris is shed to adjacent regions.



Sediments accumulate in basins created by crustal flexure.

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Sediments can preserve evidence of mountains eroded away.

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

Uplift

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Construction of mountains requires substantial uplift.

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Mt. Everest (8.85 km above sea level).

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Comprised of marine sediments (formed below sea level).

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Lofty mountains are supported by a thickened crust. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Crustal Roots

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High mountains are supported by thickened lithosphere.

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Thickening is caused by collisional orogenesis.

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Average continental crust – 35 to 40 km thick.



Beneath orogenic belts – 50 to 70 km thick.

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This thickened crust helps buoy the mountains upward.

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

Isostacy



Surface elevation represents a balance between forces.

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Gravitational attraction – Pushes plate into the mantle.

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Buoyancy – Causes plate to float higher on the mantle.

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The term isostatic equilibrium describes this balance.

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Isostacy is compensated after a disturbance.

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Adding weight pushes the lithosphere down.

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Removing weight causes isostatic rebound.



Compensation is slow, requiring asthenospheric flow.

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

Gravity Anomalies



Gravimeters determine departure from predicted gravity.



Geophysicists can model crustal density changes.

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

Erosional Sculpting



Mountains are steep and jagged from erosion.



Mountains reflect a balance between uplift and erosion.

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Rock structures can affect erosion.



Resistant layers form cliffs.



Easily eroded rocks form slopes.

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

Orogenic Collapse

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The Himalayas are the maximum height possible. Why?

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There is an upper limit to mountain heights.

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Erosion accelerates with height.

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Weight of high mountains overwhelms rock strength.

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Deep, hot rocks eventually flow out from beneath mountains.

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The mountains then collapse downward like soft cheese.

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Uplift, erosion, and collapse exhume deep crustal rocks. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis

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Mountain building is driven by plate tectonics.

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Convergent plate boundaries.

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Continental collisions.

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Rifting.

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Orogenic phases may last several hundred Ma.



Ancient mountains are deeply dissected by erosion.

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

Causes of Orogenesis

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Convergent tectonic boundaries create mountains.

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Subduction-related volcanic arcs grow on overriding plate.

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Accretionary prisms (off-scraped sediment) grow upward.



Compression stacks thrust faults on the far side of mountain belt.

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

Causes of Orogenesis

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Island fragments of continental lithosphere won’t subduct.

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Added to the overriding plate.

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Exotic terranes – Geologic history differs from surroundings. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Causes of Orogenesis

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Continental collisions.

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Oceanic lithosphere can completely subduct.

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This closes the pre-existing ocean basin.

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Brings two blocks of continental crust together.



Buoyant continental crust will not subduct.



Instead, subduction is extinguished.

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

Causes of Orogenesis



Continent-on continent collision…

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Creates a broad welt of crustal thickening.



Thickening due to thrust faulting and flow folding.

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Center of belt consists of high-grade metamorphic rocks.



Fold and thrust belts extend outward on either side.



The resulting high mountains may eventually collapse. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Sand Model of Orogenesis

Causes of Orogenesis



Continental rifting.



Continental crust is uplifted in rift settings.



Thinned crust is less heavy; mantle responds isostatically.



Decompressional melting adds asthenospheric magma.



Increased heat flow from magma expands and uplifts rocks.



Rifting creates linear fault block mountains and linear basins. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Cratons

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A craton is crust that hasn’t been deformed in 1 Ga.



Low-geothermal gradient; cool, strong, and stable crust.



Two cratonic provinces.



Shields – Outcropping Precambrian igneous and metamorphic rocks.



Platforms – Shields covered by layers of Phanerozoic strata. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Cratonic Platforms



Sedimentary rocks covering Pre-Cambrian basement.



Exhibit domes and basins.



From vertical crustal adjustment.



Created by stresses transmitted to from active margin to interior.

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

Case Study - Appalachians



A complex orogenic belt formed by 3 orogenic pulses.



Preserves a complete Wilson cycle.



The Appalachians today are eroded remnants.

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

Appalachians



A giant orogenic belt existed before the Appalachians.



The Grenville orogeny (1.1 Ga) formed a supercontinent.



By 600 Ma, much of this orogenic belt had eroded away.

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

Appalachians



The Grenville orogenic belt rifted apart 600 Ma.



This formed a new ocean (the proto-Atlantic).



Eastern North America developed as a passive margin.



A thick pile of sediments accumulated along this margin.



An east-dipping subduction zone built up an island arc. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Appalachians



Subduction carried the margin into the island arc.



The collision resulted in the Taconic orogeny 420 Ma.



A doubly dipping subduction zone developed.



Exotic blocks of continental crust were carried in.



These blocks were added to the margin during the Acadian orogeny 370 Ma.

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

Appalachians



East-dipping subduction continued to close the ocean.



Alleghenian orogeny (270 Ma): Africa collided with N. Am.



Created a huge fold and thrust belt and mountain range.



Assembled the supercontinent of Pangaea.

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

Appalachians



Pangaea began to rift apart 180 Ma.



Faulting and stretching thinned the lithosphere.



Rifting led to development of a divergent margin.



Sea-floor spreading created the Atlantic Ocean. Earth: Portrait of a Planet, 2 nd edition, by Stephen Marshak Chapter 11: Crags, Cracks, and Crumples: Crustal Deformation and Mountain Building

Modern Orogenesis



Modern instrumentation can measure mountain growth.



Global positioning systems (GPS) measure rates of…



Horizontal compression.



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

This concludes the

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

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