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 of reconstructingolder plate-configurations.

Paleomagnetism is the only quantitative important method to produceolder 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 totan I = 2 tan .

• The earth’s magnetic field may be frozen-in as remanent magnetism in a rock,and the remanence may be preserved through geological time.

• A database of well-defined and well-dated magnetic remanences from varioustime periods for one defined area may be used to determine the same area’sposition with respect toe the magnetic poles (or vice versa) for the periods forwhich we have well-defined remanences.

• Paleomagnetic remanence shows latitude and direction (azimuth) to a magneticpole. Paleomagnetic data thus only give information on latitude and plate-velocityacross latitude. Assumptions on paleo-longitude must be based on other criteria, such as a fixed point (hot-spot, fixed continent etc) for the time-periodin 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

horizontal

vertical

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

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?