Marine Geology and Geophysics
From sea floor spreading to plate tectonics
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Sea floor morphology and bathymetry
Seismic structure of the oceanic crust
Gravity anomalies
Oceanic heat flow
Marine magnetic anomalies
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Magnetic field
Magnetic properties of rocks
Inversion of the earth magnetic field
Reversal chronometry
Sea floor spreading. Vine Matthews Morley hypothesis
Transform faults
Trenches and subduction zones
Velocity structure from seismic refraction
VP (km s-1)
Thickness (km)
water
1.5
4.5
Layer 1
1.6-2.5
0.4
Layer 2
4.0-6.0
1.5
Layer 3
6.4-7.0
5.0
Mantle
7.4-8.6
Gravity => Isostatic compensation
Mid-oceanic ridge system
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Submarine mountain chain
35,000km long
Volcanic rocks
Central rift valley
Isostatically compensated
Trenches
-Trenches
10km deep
-Concavity
-gravity
doublet
Heat flow
Roger Revelle et Edward Bullard ->
Harry Hess’s
sea floor spreading hypothesis
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Guyots are sea mounts with flat top on flanks
of ridges
Erosion at sea level?
Too deep for sea level changes
Harry Hess (1962) hypothesis of sea floor
spreading
Sea floor moves away from ridge and sinks
slowly with seamounts
Evidence: Marine magnetic anomalies
Magnetic field is dipolar
Magnetic field is a vector (3
components)
• Vector is defined by 3
components in Cartesian
coordinate system
• Or by magnitude and
direction (2 angles,
inclination, declination)
• Magnetic anomaly usually
refers to total field (i.e.
magnitude of the field).
• Magnetic anomaly
symmetric only at the
pole.
Reference magnetic field
Mechanism of magnetization at molecular level
(alignment of dipolar moment of atoms)
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2 ways to remove
magnetization:
- Magnetic field in
opposite direction
- Heating
Effect of heating (Thermo-remanence)
Magnetisation remanente
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Temperature de Curie: toutes les proprietes magnetiques
disparaissent si la temperature depasse la temperature de Curie
Sous la temperature de Curie, magnetisation induite par champ
ambiant.
Sous la temperature de blocage, la magnetisation ne change
plus.
Magnetisation est acquise entre temperatures de Curie et de
blocage.
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Temperature de Curie ~ 450-650 C (depend du mineral)
Temperature de blocage ~100 degres plus basse que Curie
Inversions of the Earth magnetic field
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Brunhes (1904) found rocks in Auvergne with inverse
magnetization
Was magnetic field inverted in the past?
Or did magnetization of rocks invert?
How to demonstrate?
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No physical mechanism possible
Find field evidence that is impossible
Show synchroneity of magnetic polarity world-wide?
Requires absolute dating!!!
Evidence from study of lavas on land!
Pioneering work by Matuyama (1929) who showed
that all rocks with normal polarity were Pleistocene.
Reversals chronometry
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Established by dating lava flows on land.
Possible because of K-Ar dating method was
developed in late 50s. (need to date accurately
volcanic rocks less than 1,000,000yrs).
1959-1962
First studies of magnetic
reversals in lavas from Italy
(Mount Etna).
Systematic studies of polarity
world wide.
Jaramillo!
Identification of Jaramillo event
in lava from New Mexico
moved the beginning of the
normal Brunhes epoch at
700ka.
Allan
Cox (Stanford
a
University) major
contributor to the reversal
chronometry
1962-1966
Magnetic stratigraphy
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Epoch (~1My)
Events (<50ky)
Excursions(<5ky)
Sea floor spreading
Vine-Matthews-Morley (1963)
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Plancher oceanique se forme
par magmatisme le long des
dorsales.
En s’ecartant de la dorsale,
le plancher oceanique se
refroidit.
Acquiert magnetisation
remanente et enregistre la
polarite du champ
magnetique terrestre.
Hypothese implicite:
PLAQUES
Interpretation des anomalies
Vitesse d’expansion
Pour determiner la vitesse d’expansion, identifier
isochrone de part et d’autre d’une dorsale
active, mesurer la distance, diviser par l’age.
Si la dorsale est inactive, mesurer distance entre
deux isochrones, et diviser par difference
d’age. On a ainsi une demi vitesse.
En principe, l’anomalie depend de l’inclinaison du champ magnetique sauf au pole
ou le champ est vertical.
Each magnetic anomaly defines an isochron
Structure de la croute oceanique
Verification: Ophiolites- section of oceanic crust and mantle
“obducted” on land (Oman, Appalachian, Chypre, etc…)
Test
Transform faults (1)
Transform faults (2)
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J Tuzo Wilson, Prof of
(geo)physics, U. of
Toronto
Problems with the
transcurrent fault model
Where does it end? You
should see deformation
at the ends of the faults
Transform faults (3)
Different types of transform faults
Transform faults (4)
The geodynamo
Inversion mechanism
Supplementary materials
Magnetic anomaly => Sea floor age
Regarder la polarite, et non l’anomalie.
(Forme de l’anomalie varie suivant geometrie)
1. Dorsale a km 250, Jaramillo km 200 et 300,
Terminaison = 5 Ma.
Taux d’expansion = 100km/Ma