SIO 226: SIO 226: Introduction to Introduction to

Marine GeophysicsMarine GeophysicsPlate TectonicsPlate Tectonics

John HildebrandJohn HildebrandScripps Institution of Scripps Institution of

OceanographyOceanographyMay, 2004May, 2004

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Plate Tectonics, Sea-Floor Plate Tectonics, Sea-Floor Spreading and Continental Spreading and Continental

DriftDrift

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Main ParodyMain Parody

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Plate GeographyPlate Geography

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Lineated Magnetic Anomalies in the Lineated Magnetic Anomalies in the Oceanic CrustOceanic Crust

From:R.G. Mason andA.D. Raff

“Magnetic AnomaliesOff the West CoastOf North America”

GSA Bulletin 72, 12591961.

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Three Interpretations of Magnetic Three Interpretations of Magnetic ProfilesProfiles

Volcanic IntrusionsVolcanic Intrusions Basement FaultingBasement Faulting Variations in Variations in

MagnetizationMagnetization

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Horizontal Displacements in the Floor of the Horizontal Displacements in the Floor of the Pacific OceanPacific Ocean

V. D. Vacquier, “Magnetic Evidence for Horizontal Displacements in The Floor of the Pacific Ocean” GSA Bulletin 72, 1250, 1961.

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Magnetic Anomalies Offset by Deep-Sea Magnetic Anomalies Offset by Deep-Sea EscarpmentsEscarpments

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Magnetic Anomalies Over Oceanic Magnetic Anomalies Over Oceanic RidgesRidgesF. J. Vine and D. H. Matthews -- Nature 199, 947, 1963F. J. Vine and D. H. Matthews -- Nature 199, 947, 1963

1.1. Magnetic Field ReversalsMagnetic Field Reversals2.2. Ocean Crust Records Field DirectionOcean Crust Records Field Direction3.3. Sea Floor Spreading at RidgesSea Floor Spreading at Ridges

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Magnetic Reversal Time Magnetic Reversal Time ScaleScale

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Plate Tectonics: How it Works –Cox and HartPlate Tectonics: How it Works –Cox and HartDefinitions and SymbolsDefinitions and Symbols

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Plate Tectonics – How it Plate Tectonics – How it WorksWorks

Motion of B is oblique to allboundaries

Motion of B is parallel to oneboundary – a transform.

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Plate Tectonics – How it Plate Tectonics – How it WorksWorks

Linear velocity of point b

The velocity fields of (b) and (c) are both consistent with the assumed relative motionof plates A and B about Euler pole E.

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Plate Tectonics – How it Plate Tectonics – How it WorksWorks

The Velocity Field of a Geologically Realistic Plate

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Plate Tectonics – How it Plate Tectonics – How it WorksWorks

Two sets of plates with the same spreading rates, but different relative velocities

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Plate Tectonics – How it Plate Tectonics – How it WorksWorks

Find a velocity field consistent with all boundaries …Which are impossible?

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Juan de Fuca Plate Juan de Fuca Plate MotionsMotions

The Juan de Fuca Ridge is The Juan de Fuca Ridge is adjacent to a series of adjacent to a series of transforms that are not all transforms that are not all parallel. parallel.

See (1), (2), (3), (4) and See (1), (2), (3), (4) and (5). (5).

Is the JdF Ridge at the Is the JdF Ridge at the boundary between the boundary between the

PacificPacificPlate and the North Plate and the North

AmericaAmericaPlate? Plate?

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Juan de Fuca Plate Juan de Fuca Plate MotionsMotions

Introducing the JDF Introducing the JDF PlatePlateeliminates the eliminates the inconsistency in the inconsistency in the directions of the directions of the transforms.transforms.

Taking the NA Plate as Taking the NA Plate as fixed, the velocity of the fixed, the velocity of the JDF Plate relative to NA JDF Plate relative to NA is toward the northeast, is toward the northeast, a convergent margina convergent margin

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Mercator ProjectionMercator Projection•Projection of a globe upon a cylinder•Progressive distortion at high latitudes•Great circle paths are generally NOT straight lines

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Oblique Mercator Oblique Mercator ProjectionProjection

•Defines a new “pole” for projection and “equator” of new pole is a straight line on the resulting map

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Mercator Projection of Fracture Mercator Projection of Fracture ZonesZones

After LePichon et al.

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Oblique Mercator Oblique Mercator Projection as a tool for Projection as a tool for finding poles relative finding poles relative

motionmotion

Pole of projection for South Pacific 69-N 157-W

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Absolute Motion: Hot Spot Absolute Motion: Hot Spot TracksTracks

Model of formation of Hawaiian Island chain by Hot Spot Volcanism

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Absolute Motion: Hot Spot Absolute Motion: Hot Spot TracksTracks

Age in Ma for volcanoes in the Hawaiian-Emperor Chain

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Comparison of Hawaiian and Comparison of Hawaiian and Louisville Louisville

Hot Spot ChainsHot Spot Chains Matching of Matching of

hotspot chains in hotspot chains in the North and the North and South Pacific South Pacific about inferred about inferred poles of Pacific poles of Pacific plate motionplate motion

After: Lonsdale JGRAfter: Lonsdale JGR

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Comparison of Age Comparison of Age ProgressionProgression

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Plate Driving Forces: Thermal Plate Driving Forces: Thermal ConvectionConvection

Do Ridges and Trenches delimit the limbs of a convection cell?

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Driving Forces Acting on Plates: Driving Forces Acting on Plates: After After Forsyth and UyedaForsyth and Uyeda

Mantle and Continental Drag Forces – proportional to the Mantle and Continental Drag Forces – proportional to the area and velocity of platearea and velocity of plate

Ridge Push – gravitational potential energyRidge Push – gravitational potential energy Slab Pull – subducting slab densitySlab Pull – subducting slab density Transform Fault Resistance – faulting occurs at critical yield Transform Fault Resistance – faulting occurs at critical yield

so force is velocity independentso force is velocity independent Colliding Resistance – faulting occurs, velocity independentColliding Resistance – faulting occurs, velocity independent Trench Suction – flow of astenosphere related to subductionTrench Suction – flow of astenosphere related to subduction

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Review of Plate Tectonic Review of Plate Tectonic ModelModel

Oceanic crust is created at ridge crestsOceanic crust is created at ridge crests Ridge crest is segmented by transform faultsRidge crest is segmented by transform faults Oceanic crust is re-cycled at subduction zonesOceanic crust is re-cycled at subduction zones