Reconstruction of former plate motions .
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Transcript of Reconstruction of former plate motions .
Reconstruction of former plate motionsReconstruction of former plate motions..Based on magnetic anomaly-patterns in the oceans it is possible to reconstruct theBased on magnetic anomaly-patterns in the oceans it is possible to reconstruct theface of the earth back in time for the period we have oceanic lithospere preservedface 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, howCan 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 to reconstruct older plate-configurations.
Paleomagnetism is the most quanititative important method to produceolder reconstructions. Other methods include faunal provinciality, geological correlation, jig-saw type coast-fitting
Preconditions for palmag: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 equator, and the
inclination (I) changes progressively with magnetic latitude () (tanI = 2 tan.3) The earth´s magnetic field may be ”frozen in” as remanent magnetizim in a rock, and
this remanence may be preserved during geological time. 4) 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 area´s position withrespect 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. Paleomagnetic data thus only give information on latidude and plate-velocities acrosslattidude! Assumtions on paleo-longitude must be based on other criteria such as assuming a fixed point (hot-spot, fixed continent etc) for the timeperiod in question.
The magnetic field varies in orietation and intencity, but over geological The magnetic field varies in orietation and intencity, but over geological time it coinsides with the geographical poles (rotation poles)time it coinsides with the geographical poles (rotation poles)
Mag. pole positions thorugh the past 2000 yrAnd with the average shown.
Magnetic inclination mapMagnetic inclination map
The present-day total magnetic field (H) at a site isThe present-day total magnetic field (H) at a site isdecomposed as shown in a vertical and horizontaldecomposed as shown in a vertical and horizontalcomponent. These components give the component. These components give the InclinationInclinationand and DeclinationDeclination of the total field. of the total field. Normalized over time the declination will showNormalized over time the declination will showThe direction to the geograpical poles, and the The direction to the geograpical poles, and the inclination will be a function of latidtude.inclination will be a function of latidtude.
A magnetic pole determined by paleomagnetismA magnetic pole determined by paleomagnetismis defined by the average of poles from one areais defined by the average of poles from one areaand from rocks with well-known ages. and from rocks with well-known ages. The magnetic pole is positioned on a great circleThe magnetic pole is positioned on a great circledefined by the declination.defined by the declination.
The magnetic pole will sit on a great circle definde by the declination.
Uncertaintyelipse
4 Mid-Cretaceous poles from 4 sites in N-America. If there is no qualitiative difference in thepoles, the average of thesepoles will define the Mid-Cretaceous pole of N-America.
N-American poles fra mezo- ogcenozoikum. Ages for poles and their95% confidence is shown. Notice the dotted curve constructed throughthe uncertainty ellipses.
SUCH A BEST FIT CURVEIS KNOWN AS:”APPARENT-POLAR-WANDERPATH” or APWPThe APWP describes the relativemotion of a continent to a pole orvice verca.
APWP for the plate M in the period 0 to 80 Mais shown as a red curve
Principle for paleogeographicalreconstruction by a APWP
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 testFold test
Magnetization is older than the folding(positive fold test)
Magnetization is younger than the folding(negativ folde test)
Conglomerate testConglomerate test
Magnetization is close to arbitary in the pebbles and probably primaryi.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 indicatingthat magnetization is primary.
The remanence in the dykecannot be distinguishedfrom the magnetization ofthe wall-rocks. A negativecontact test.
Sample collection and handlingSamples are drilled out in cores with non-magnetic equipment
Samples are oriented accurately with compass and if possible sun-compass
Handling, demag. in a room without a magnetic field
Spinner- or supercondutivemagnetometer
Magnetic suseptibility
Thermal demagnetization
Alternationg field demag.
b) Vertical plane
a) Horizontal plane
c) Horizontale ogvertical projectioninto one vektorcomponent figure.
NMR WITH SEVERAL COMPONENTS
NMR WITH TWO COMPONENTS, here shown inRED og BLUE
The poles of both components can be determined, and if they are distinctly and statisticallydifferent we can assume that the magnetizations wereformed at two differentgeographical locations! If the APWP is well determinded will the pole´slocation on the APWP giveinformation about the age ofthe magnetizations.
http://www.geodynamics.no/platemotions/500-400/
BALTICA a separatecontinent ≈ 550-425Ma
500 Ma460 Ma
440 Ma
420 Ma400 Ma
Caledonian orogenic cyclein brief
Notice that traditionalWilson-cycle tectonicsdoes not work to explainformation of the Caledonides
OTHER APPLICATIONS OF PALMAG
DATE EVENTSSTUDY PALEOCLIMATE BELTS
(SNOWBALL-EARTH HYPOTHESIS)ETC.
HERE IS AN EXAMPLE:
CASE STUDY: LÆRDAL-GJENDE
FAULT
CASE STUDY:LÆRDAL-GJENDEFAULT
Breccias along a faultcutting all major tectonic units in southNorway, can we findthe age of brecciationand therefore movement?
The results: A) Example of 2-component magnetizationB) The components plotted in stereograms
LGF-case study: The poles for the two magnetizations areplotted on theAPWP for EURASIA.Vi can see that the comparison gives a Permian and late-Jurassicto Cretaceous age for the twocomponentsrespectively.We interpret theseto represent two stages of movement on the LGF