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
180
155 130
We can see that the oldest ocean floor is ca ≈180Ma, how
Can we reconstruct plate-motions before mid-Jurassic time?
We see that if magnetic anomalies in the oceans were the only basis for
plate-reconstructions we would have no quantitative way of reconstructing
older plate-configurations.
Paleomagnetism is the only quantitative important method to produce
older reconstructions (before present day ocean-floor mag anomalies)
Preconditions for using paleomag:
1) The earth’s magnetic poles coinside with the geographical poles over time.
2) The magnetic field is vertical at the poles and horizontal at the equator, and the
inclination (I) changes progressively with magnetic latitude () according to
tan I = 2 tan .
3) The earth’s magnetic field may be frozen-in as remanent magnetism in a rock,
and the remanence may be preserved through geological time.
4) A database of well-defined and well-dated magnetic remanences from various
time periods for one defined area may be used to determine the same area’s
position with respect toe the magnetic poles (or vice versa) for the periods for
which we have well-defined remanences.
5) Paleomagnetic remanence shows latitude and direction (azimuth) to a magnetic
pole. Paleomagnetic data thus only give information on latitude and plate-velocity
across latitude. Assumptions on paleo-longitude must be based on other
criteria, such as a fixed point (hot-spot, fixed continent etc) for the time-period
in question.
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.
Magnetic inclination map
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.
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.
The magnetic pole will sit on a great circle definde by the declination.
Uncertainty
elipse
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 MidCretaceous pole of N-America.
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.
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
a) APWPs for N-America og Eurasia from the Ordovician to the Jurassic.
b) 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!
•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
Fold test
Magnetization is older than the folding
(positive fold test)
Magnetization is younger than the folding
(negativ folde test)
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)
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.
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
Handling, demag. in a room without a magnetic field
Spinner- or supercondutive
magnetometer
Magnetic suseptibility
Thermal demagnetization
Alternationg field demag.
NMR WITH SEVERAL COMPONENTS
a) Horizontal plane
c) Horizontale og
vertical projection
into one vektor
component figure.
b) Vertical plane
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 pole´s
location on the APWP give
information about the age of
the magnetizations.
500 Ma
460 Ma
BALTICA a separate
continent ≈ 550-425Ma
Caledonian
orogenic cycle
in brief
440 Ma
400 Ma
http://www.geodynamics.no/platemotions/500-400/
420 Ma
Notice that traditional
Wilson-cycle tectonics
does not work to explain
formation of the
Caledonides
OTHER APPLICATIONS
OF PALMAG
DATE EVENTS
STUDY PALEOCLIMATE BELTS
(SNOWBALL-EARTH HYPOTHESIS)
ETC.
HERE IS AN EXAMPLE:
CASE STUDY:
LÆRDAL-GJENDE
FAULT
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?
The results:
A) Example of 2-component magnetization
B) The components plotted in stereograms
horizontal
vertical
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
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
180
155 130
We can see that the oldest ocean floor is ca ≈180Ma, how
Can we reconstruct plate-motions before mid-Jurassic time?