Rock magnetizations that are most useful for paleomagnetism

Detrital Remanent Magnetization (DRM)• formed during or soon after deposition of sediments• locked in by compaction and lithification to sedimentary rock• relatively weak

Thermo-remanent Magnetization (TRM)• formed in basic igneous rocks (e.g., basalt) upon cooling through Curie temperature• locked in upon further cooling• very strong

Ternary diagram of the iron-titanium oxide solid solution magnetic minerals

“ferromagnetic” minerals:

Paleomagnetic measurements

Collect many suitable samples (e.g. basalt) with in situ orientation determined

Paleomagnetic measurements

For each sample measure stable component of magnetization intensity and direction (declination, D, and inclination, I) in laboratory (“cleaning” samples by removing unstable superimposed magnetizations.

alternating current (AC) demagnetization thermal demagnetization

Paleomagnetic measurements

Determine VGP locations, average VGP for specified time in past, and error

Fisher statistics for VGP’s

•Consider each VGP as the intersection of a unit vector at the center of the earth with the earth’s surface. The latitude and longitude of the VGP can be transformed into a cartesian coordinate system at the earth’s center, with the x axis pointing to 0N, 0W, the y axis pointing to 0N, 90E, and the z axis pointing to the North Pole.

• For a set of N VGP’s., one can add up the unit vectors and find the resultant direction. The intersection of this resultant vector with the earth’s surface is the best estimate of the average VGP for the N data.

• The magnitude of the resultant vector, say, is L. If all the data pointed in the same direction (same VGP location), then L = N. Otherwise, L is less than N, and is the key measure of the clustering of the data about the average.

• Precision parameter K = (N-1)/(N-L)

• angular standard deviation S = 81°/sqrt(K)

• 95% cone of confidence about the mean vector is a small circle centered on the mean vector with angle (measured from the mean vector) given approximately by

95 = 140°/sqrt(KN)

Paleomagnetic measurements

Plot time sequence of VGP locations on equal area projection of earth

Apparent polar wander paths for North America and Europe

N

S

W E

This is a projection of the earth’s surface, looking from the outside downwards, with the axis of rotation intersecting the surface at

the pole of rotation shown by the green circle. Again, the great and small circles are NOT geographic coordinates, but are arrayed relative to the pole of rotation.

Rotation on a sphere

N

S

W E

Examples of two abitrary great circles emanating from the pole of rotation. These two great circles represent a rotation of 20 degrees about the pole of rotation. For positive rotation, the left hand great circle 1 rotates to the right hand great circle 2.

1

2

N

S

W E

Examples of two abitrary small circles (perpendicular to the great circles) are shown in blue. For a rotation about the pole, all small circes concentric to the pole give possible paths for rotation about the pole.

N

S

W E

Point, P1, moves to position shown by P2 as a result of the 20 degree rotation about the pole of rotation.

P1P2

N

S

W E

Now we show the situation in a geographic framework, with the pole of rotation located somewhere at mid-latitudes in the northern hemisphere. Again, we are looking down on the earth, with the grid now representing latitudes and longitudes.

P1

P2

Pole of rotation

T0 to T

4

Re-assembling continental fragments

T0 to T

4

Re-assembling continental fragments

T0 to T

4T4 to T8

Re-assembling continental fragments

A

T0 to T

4T4 to T8

Note that the VGP’s are rigidly connected to the continental fragments, and are rotated along with the fragments

Re-assembling continental fragments

A

T0 to T

4T4 to T8

Re-assembling continental fragments

Note that the VGP’s are rigidly connected to the continental fragments, and are rotated along with the fragments

Europe rotated by 38 degrees about rotation pole at 88.5N, 27.7E

Closing the North AtlanticOrdovician to Jurassic (500-200 Ma) “Apparent polar wander” (APW) paths for North America and Europe

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

4600 My 600 My 65 My

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

600 My

Europe rotated by 38 degrees about rotation pole at 88.5N, 27.7E

Closing the North AtlanticOrdovician to Jurassic (500-200 Ma) “Apparent polar wander” (APW) paths for North America and Europe

Errors (95 cones of confidence)

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

4600 My 600 My 65 My

seafl

oor

tap

e r

ecord

er

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

4600 My 600 My 65 My

Paleozoic Continental Drift

• Paleomagnetic VGP’s and APW’s

• Geological structures

• Paleo-environments and paleo climatic zones

See continental reconstructions from PLATES project of the University of Texas