An apparent polar-wander path for a continent is a record of the continent’s changing orientation relative to the Earth’s north and south magnetic poles over a period of time. It is typically represented as a curve on a map that shows the continent’s longitude and latitude relative to the poles. The curve reflects changes in the continent’s position over time due to continental drift or other tectonic processes.

Apparent polar wander

apparent polar deviation (APW) is the perceived motion of the Earthin paleomagnetic poles in relation to a continent while considering the studied continent as fixed in position. It is often displayed on the current latitude-longitude map as a path connecting the locations of geomagnetic polesinferred at different times using paleomagnetic techniques.

In reality, relative polar motion can be real polar wander or continental drift (or a combination of both). Data from around the world are needed to isolate or distinguish between the two. However, the magnetic poles rarely deviate from the planet’s geographic poles; instead they tend to follow real polar walk. Therefore, the concept of apparent polar shift is very useful in plate tectonics, as it can retrace the relative motion of continents, as well as the formation and separation of supercontinents.

History

It has long been known that the geomagnetic field varies over time, and records of its direction and magnitude have been kept in different locations since the 19th century. The technique of plotting the apparent polar shift was first developed by Creer et al. (1954), and was a major step towards acceptance of plate tectonics theory. Since then, many discoveries have been made in this field, and the apparent polar shift has become better understood with the evolution of the geocentric axial dipole (GAD) theory and model. From 2010[update]there were more than 10,000 paleomagnetic poles registered in the database.

paleomagnetic poles

Lots of research on paleomagnetism aims to find paleomagnetic poles for different continents and at different times in order to assemble them into apparent polar path tracks (APWP). The paleomagnetic poles have the advantage of having the same value at each observation site based on the geocentric axial dipole model. Thus, they can be used to compare paleomagnetic results from widely separated localities.

rock magnetism

Fossil magnetization in rocks is the key to locating a paleomagnetic pole. At the time of formation, some rocks retain the direction of the magnetic field. The inclination (Im) and declination (Dm) vectors are preserved and therefore the paleolatitude (λp) and paleolongitude (φp) of the pole can be found.

lock temperature

The reason why field characteristics are conserved comes from the concept of blocking temperature (also known as closing temperature in geochronology). This temperature is where the system is locked against thermal agitation at lower temperatures. Therefore, some minerals exhibit remanent magnetization. A problem that arises in determining remnant (or fossil) magnetization is that if the temperature rises above this point, the magnetic history is destroyed. However, in theory, it should be possible to relate the magnetic blocking temperature to the isotope closing temperature, so that it can be checked whether a sample can be used or not.

Tracks

APWP trails often represent the movement of a plate relative to a fixed point (paleomagnetic pole). The usual pattern observed consists of long, smoothly curved segments connected by short, strongly curved segments. These correspond, respectively, to time intervals of constant plate movement versus variable plate movement.

These segments are described by rotation about a pivot point, which is called the paleomagnetic Euler pole (see Euler Rotation Theorem). Relative motion between two plates is also described by rotation around an Euler pole. In recent times, it is easier to determine finite rotations, as transforms and crests are respectively perpendicular and parallel to the direction of a finite rotation pole. In this way, reconstructions of the last 200 million years (Ma) depend mainly on marine geophysical data. For earlier dates, other forms must be used, such as paleomagnetic poles and adjustment of geological observations.

Determining paleomagnetic poles is a complicated process, as with increasing time more uncertainties come into play. The reliability of the poles has been the subject of debate for many years. The paleomagnetic poles are usually a group average determined from different samples in order to average the secular variation over time to respect the GAD hypothesis. Data processing is a big step and involves many statistical calculations to obtain a valid paleomagnetic pole.

When applied to continents, it is possible to define finite rotation with paleomagnetic poles; that is, to describe the certain movement of a continent based on the records of its paleomagnetic poles. However, there are two main problems with constraining finite rotation:

  • Because of random magnetic reversals, the north magnetic pole at any given time can be either in the northern hemisphere or the southern hemisphere. Without context, it is impossible to know what the north direction of the magnetic vectors is. Again, in recent times there is often better context, but after 300 Ma it becomes increasingly difficult.
  • Paleolongitude cannot be restricted from the pole only. Therefore, data from different locations are required, as it reduces the degrees of freedom. With the apparent high-fidelity polar deviation path, however, the paleolongitude can be constrained by the paleomagnetic Euler rotations (poles and angles of rotation) estimated from the circle modeling for the APWP tracks.

The aim of many paleomagnetic surveys is to mount poles on APWPs for the different continental fragments, which is the first step in reconstructing the paleogeography. The two main issues in this construction are the selection of reliable poles (criteria V90, BC02) and curve fitting. The first question was approached with general selection criteria. The common ones were described by Van der Voo (1990; V90). These include uncertainty about ages, number of samples, positive field tests to constrain the age of the magnetization relative to the age of the rock (eg bending test) and pole positions. Besse and Courtillot (2002; BC02) brought some modifications to these criteria for specific applications.

Once the poles are selected and assigned a certain degree of confidence, the curve-fitting task remains in order to define the apparent polar deviation paths. Different approaches have been used for this process: discrete windows, key poles, moving windows, splines, Euler pole paleomagnetic analysis (PEP), master path and slope-only data. These differ in the way the poles are separated, in the relative importance given to some poles and in the general shape of the resulting curves.

References


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