## Info

AFrom McFadden and McElhinny (1995). m, number of paleomagnetic poles N, total number of sites A95, radius of circle of 95 confidence about the mean pole position. The mean age in each case is given to the nearest 5 Myr. aFrom McFadden and McElhinny (1995). m, number of paleomagnetic poles N, total number of sites A95, radius of circle of 95 confidence about the mean pole position. The mean age in each case is given to the nearest 5 Myr. Fig. 6.12. Phanerozoic APWP for Laurentia using the mean...

## Xha

A consistent estimate is obtained if k is chosen such that This can be achieved by numerical iteration using some starting value of k until (6.4.4) holds. Having determined k (equivalent to determining R), the circle of 95 confidence may then be determined from (3.2.13). In the following sections of this chapter the above method is used to calculate mean paleomagnetic poles to enable APWPs to be determined for each of the major blocks for any given time interval. Other methods have been tried...

## GaA Gacga L

Similarly, if the angular velocity Ao> B of another plate B relative to the plate A is known then the only measurable component cilB of the angular velocity of B relative to the geographic pole is given by and it also passes through the equator. Consequently, when reconstructing past positions using paleomagnetic information, rotation about a construction pole on the equator is most commonly used. Three examples of the application of (7.1.2) are illustrated in Fig. 7.2. Continent a is taken...

## Paleomagnetism and Rodinia

The key to the reconstruction of Rodinia is the fit between the eastern margin of Australia and the western margin of Laurentia as they were constituted at about 1100 Ma. At that time Rodinia included East Gondwana (Australia, Antarctica, and India) as a unit (Fig. 7.20). Therefore, a viable paleomagnetic test of the Australia-Laurentia connection can be made by comparing poles from East Gondwana with Laurentia. In a seminal paper, Powell et al. (1993) carried out the first such test using...

## M

Difference vector method of analysis for three components A, B, C with varying overlap in their coercivity or blocking temperature spectra (left) and corresponding stereoplots of the difference vector paths (right). After Hoffman and Day (1978), with permission from Elsevier Science. demagnetization because of almost complete overlap of its coercivity spectrum with that of A. However, the blocking temperature spectrum of B hardly overlaps with that of A and component B is therefore...

## Sampling and Measurement

3.1.1 Sample Collection in the Field The typical sampling scheme for a land-based paleomagnetic study is hierarchical, consisting of specimens cut from independently oriented samples collected at different sites distributed throughout a rock unit. There are no hard and fast rules the details of any specific scheme are dictated by the goals of the study, the rock type, accessibility of exposure, and the statistical properties of the magnetization (see 3.2). The rock unit would be a set of...

## Evolution of Oceanic Plates

Most of the basaltic volcanism on the Earth's surface occurs at plate boundaries, either as mid-ocean ridge basalts or as volcanic arc basalts at subduction zones. Fig. 5.24. Apparent polar wander path for the Indian plate for the time interval 70-20 Ma derived from DSDP cores as listed in Table 5.4. Fig. 5.24. Apparent polar wander path for the Indian plate for the time interval 70-20 Ma derived from DSDP cores as listed in Table 5.4. Basaltic volcanism also occurs within plates on a much...

## Apparent Polar Wander

6.4.1 The Concept of Apparent Polar Wander From a tectonic perspective one of the purposes of paleomagnetic investigations is to provide information relating to the way in which parts of the Earth's crust have moved apart or come together during Earth history. In order to achieve this, it is necessary to have paleomagnetic data from localities well distributed both in space and in time and to have a method by which these data can be presented and analyzed. There are two principal ways of...

## Marine Magnetic Anomalies

5.1.1 Sea-Floor Spreading and Plate Tectonics The sea-floor spreading hypothesis was first formulated by Hess (1960, 1962). The mid-ocean ridges, which are characterized by unusually high heat flow along their crests, are the largest topographic features on the surface of the Earth. On Hess's model the mid-ocean ridges are interpreted as representing the rising limbs of mantle convection where hot magma comes right through to the surface and new oceanic crust is formed as the magma cools. It...

## Precambrian Cratons

In the 1960s and 1970s there was considerable interest in Archean and Proterozoic paleomagnetism, and studies were made in all the major cratons of the present continents. The general method of presentation of these studies was to allow the paleomagnetic poles to fall within an APWP swathe of -20 width. These swathes were then drawn so as to incorporate all the pole positions by making suitable bends and curves in the swathe where necessary. As a result of this approach Piper et al. (1973)...

## Table

High-Temperature (Deuteric) Oxidation Classification Scheme from Wilson and Watkins (1967) I Homogeneous (single-phase) titanomagnetites II Titanomagnetites contain a few exsolved ilmenite lamellae in 111 planes III Abundant ilmenite lamellae with equilibrium two-phase intergrowths IV Ilmenite lamellae oxidized to rutile + hematite V Residual titanomagnetite and ilmenite oxidized to rutile + hematite VI Total oxidation to pseudobrookite and hematite and or rutile A useful empirical...

## The Geomagnetic Polarity Time Scale

4.2.1 Polarity Dating of Lava Flows 0-6 Ma Mercanton (1926) first realized that if rocks containing reverse magnetizations were due to reversals of the Earth's magnetic field, then this should be registered in rocks worldwide and so he obtained samples from Spitsbergen, Greenland, Iceland, the Faroe Islands, Mull, Jan Mayen Land and Australia as a test. He found that some were magnetized in the same sense as the present Earth's field and others were roughly reversed from it. Matuyama (1929)...

## Non Plate Tectonic Hypotheses

In paleomagnetism it is important to distinguish between the concepts of polar wandering and continental drift. All measurements in paleomagnetism are made with respect to the geographic pole as the frame of reference. Therefore, in plate tectonics the concept of polar wander may appear to be meaningless because all one is saying is that the reference frame itself is in motion. True polar wander, TPW, on the other hand, has been defined as the displacement of the entire Earth with respect to...

## Field Tests for Stability

3.3.1 Constraining the Age of Magnetization In favorable circumstances it is possible to arrange paleomagnetic sampling so that field tests can be performed to constrain the age of magnetization. Graham 1949 proposed two such tests referred to as the fold test and the conglomerate test. Later, Everitt and Clegg 1962 introduced the baked contact test. The circumstances for each of these tests are illustrated in Fig. 3.4. More recently Kirschvink 1978 proposed the unconformity test, which is only...

## S

Methods for determining the Earth's paleoradius. a The paleomeridian method of Egyed 1960 . Cross-section through the Earth with two sampling sites S, and S2 lying on a paleomeridian separated by distance d. If their paleolatitudes are gt ., and .2, the ancient radius a is determined from 7.1.13 . b The minimum dispersion method of Ward 1963 . The positions of rock units X , lt j gt are transformed to a new co-ordinate system using the center of the continent as the pole center of...