Reconstruction of former plate motions'

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Reconstruction of former plate motions. Based on
magnetic anomaly-patterns in the oceans it is
possible to reconstruct the face of the earth
back in time for the period we have oceanic
lithospere preserved
We can see that the oldest ocean floor is ca
180Ma, how Can we reconstruct plate-motions
before mid-Jurassic time?
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• We see that if magnetic anomalies in the oceans
  were the only basis for
• plate-reconstructions we would have no
  quantitative way to reconstruct
• older plate-configurations.
• Paleomagnetism is the most quanititative
  important method to produce
• older reconstructions.
• Other methods include faunal provinciality,
  geological correlation,
• jig-saw type coast-fitting
• Preconditions for palmag
• The earths magnetic poles coinside with the
  geographical poles over time.
• The magnetic field is vertical at the poles and
  horizontal at equator, and the
• inclination (I) changes progressively with
  magnetic latitude (?) (tan I 2 tan ??.
• The earths magnetic field may be frozen in as
  remanent magnetizim in a rock, and
• this remanence may be preserved during
  geological time.
• A database of well-defined and well-dated
  magnetic remanences from various time-
• periods for one definite area may be used to
  determine the same areas position with
• respect to the magnetic poles (or vice versa)
  for those periods for which we have
• well-defined remanences.
• Paleomagnetic remanence shows latitude and
  direction (azimut) to a magnetic pol.

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The magnetic field varies in orietation and
intencity, but over geological time it coinsides
with the geographical poles (rotation poles)
Mag. pole positions thorugh the past 2000 yr And
with the average shown.
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Magnetic inclination map
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The present-day total magnetic field (H) at a
site is decomposed as shown in a vertical and
horizontal component. These components give the
Inclination and Declination of the total field.
Normalized over time the declination will
show The direction to the geograpical poles, and
the inclination will be a function of latidtude.
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A magnetic pole determined by paleomagnetism is
defined by the average of poles from one area and
from rocks with well-known ages. The magnetic
pole is positioned on a great circle defined by
the declination.
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The magnetic pole will sit on a great circle
definde by the declination.
Uncertainty elipse
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N-American poles fra mezo- og cenozoikum. Ages
for poles and their 95 confidence is shown.
Notice the dotted curve constructed through the
uncertainty ellipses.
4 Mid-Cretaceous poles from 4 sites in
N-America. If there is no qualitiative
difference in the poles, the average of
these poles will define the Mid- Cretaceous pole
of N-America.
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SUCH A BEST FIT CURVE IS KNOWN AS APPARENT-POLAR
-WANDER PATH or APWP The APWP describes the
relative motion of a continent to a pole or vice
verca.
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Principle for paleogeographical reconstruction by
a APWP
APWP for the plate M in the period 0 to 80 Ma is
shown as a red curve
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• APWPs for N-America og Eurasia from the
  Ordovician to the Jurassic.
• N-America og Eurasia rotated back to the position
  before the
• N-Atlantic opened (Bullard fit)
• NOTICE THE COINCIDENCE OF THE APWPs FOR THE 2
  CONTINENTS!

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• Paleomagnetism, methods and data quality
• keywords
• Curie temperature, Magnetite 5800C, Hematite
  6800C
• Thermal Remanent Magnetism (TRM),
• Sediment Remanent Magnetism (SRM)
• Thermo-chemical Remanent Magnetism (CRM),
• Field tests
• Fold test
• Conglomerate test
• Contact test
• Data quality
• Sampling and sample-handling
• Thermal demagnetisation (thermal cleaning)
• Alternating field demagnetisation
• Instrument quality

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Fold test
Fold test
Magnetization is older than the folding (positive
fold test)
Magnetization is younger than the
folding (negativ folde test)
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Conglomerate test
Magnetization is close to arbitary in the pebbles
and probably primary i.e. than the conglomerate
(positive conglomerate test)
Magnetization is parallel in the pebbles and
probably secondary (negative conglomerate test)
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Contact test
The dyke has its own remanence, and remagnetizes
the wall-rocks near the contact. A positive
contact test indicating that magnetization is
primary.
The remanence in the dyke cannot be
distinguished from the magnetization of the
wall-rocks. A negative contact test.
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Sample collection and handling Samples are
drilled out in cores with non-magnetic equipment
Samples are oriented accurately with compass and
if possible sun-compass
a
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Handling, demag. in a room without a magnetic
field
Spinner- or supercondutive magnetometer Magnetic
suseptibility Thermal demagnetization Alternatio
ng field demag.
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NMR WITH SEVERAL COMPONENTS
b) Vertical plane
a) Horizontal plane
c) Horizontale og vertical projection into one
vektor component figure.
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NMR WITH TWO COMPONENTS, here shown in RED
og BLUE
The poles of both components can be determined,
and if they are distinctly and
statistically different we can assume that the
magnetizations were formed at two
different geographical locations! If the APWP is
well determinded will the poles location on the
APWP give information about the age of the
magnetizations.
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500 Ma
Caledonian orogenic cycle in brief
BALTICA a separate continent 550-425Ma
Notice that traditional Wilson-cycle
tectonics does not work to explain formation of
the Caledonides
http//www.geodynamics.no/platemotions/500-400/
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OTHER APPLICATIONS OF PALMAG
DATE EVENTS STUDY PALEOCLIMATE BELTS (SNOWBALL-EAR
TH HYPOTHESIS) ETC. HERE IS AN EXAMPLE
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CASE STUDY LÆRDAL-GJENDE FAULT
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CASE STUDY LÆRDAL-GJENDE FAULT
Breccias along a fault cutting all major
tectonic units in south Norway, can we find the
age of brecciation and therefore movement?
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• The results
• Example of 2-component magnetization
• The components plotted in stereograms

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LGF-case study The poles for the two
magnetizations are plotted on the APWP for
EURASIA. Vi can see that the comparison gives
a Permian and late-Jurassic to Cretaceous age
for the two components respectively. We
interpret these to represent two stages of
movement on the LGF