True Polar Wander

In Section 3.6 it was demonstrated that paleomagnetic techniques can be used to construct apparent polar wandering paths which track the motions of plates with respect to the magnetic north pole and hence, using an axial geocentric dipole model, the spin axis of the Earth. In Section 5.5 it was suggested that hotspots are nearly

Figure 5.11 Predicted hotspot track assuming that the Iceland hotspot is fixed relative to the other Indo-Atlantic hotspots of Fig. 5.9. Position of hotspot at 10 Ma intervals is indicated by solid dots. AHI, Axel Heiberg Island; EI, Ellesmere Island; MR, Mendeleyev Ridge. Dashed line, continent-ocean boundary based on bathymetry. Gap between 70 Ma positions results from sea floor created after the passage of the Labrador Sea Ridge over the hotspot at 70 Ma (modified and redrawn from Lawver & Müller, 1994, courtesy of the Geological Society of America).

Figure 5.11 Predicted hotspot track assuming that the Iceland hotspot is fixed relative to the other Indo-Atlantic hotspots of Fig. 5.9. Position of hotspot at 10 Ma intervals is indicated by solid dots. AHI, Axel Heiberg Island; EI, Ellesmere Island; MR, Mendeleyev Ridge. Dashed line, continent-ocean boundary based on bathymetry. Gap between 70 Ma positions results from sea floor created after the passage of the Labrador Sea Ridge over the hotspot at 70 Ma (modified and redrawn from Lawver & Müller, 1994, courtesy of the Geological Society of America).

stationary in the mantle, and so their trajectories provide a record of the motions of plates with respect to the mantle. A combination of these two methods can be used to test if there has been any relative movement between the mantle and the Earth's spin axis. This phenomenon is known as true polar wander (TPW).

The method employed to investigate TPW is as follows. Paleomagnetic pole positions for the past 200 Ma are compiled for a number of continents that are separated by spreading oceans so that their relative motions can be reconstructed from magnetic lineation data (Section 4.1.7). The pole positions are then corrected for the rotations relative to a single continent (usually Africa) experienced as a result of sea floor spreading since the time for which they apply. In this way a composite or global apparent polar wander path is obtained. This is then compared with the track of the axis of the hotspot reference frame as viewed from the fixed continent. The TPW path is then determined by calculating the angular rotation that shifts the global mean paleomagnetic pole of a certain age to the north pole, and then applying the same rotation to the hotspot pole of the same age (Courtillot & Besse, 1987).

The TPW path for the past 200 Ma, obtained by Besse & Courtillot (2002), is shown in Fig. 5.12. Their analysis utilizes paleomagnetic data from six continents, sea floor spreading data from the Atlantic and Indian Oceans, and the Indo-Atlantic hotspot reference frame of Müller et al. (1993) for the past 130 Ma, and of Morgan (1983) for the period from 130 to 200 Ma. They conclude that as much as 30° of true polar wander has occurred in the past 200 Ma, and that the movement of the pole

Polar Wandering

Figure 5.12 True Polar Wander (TPW) path for the past 200 Ma. TPW is defined as the movement of the "geographic" pole of the Indo-Atlantic hotspot reference frame with respect to the magnetic pole defined by paleomagnetic data, the latter being equated to the Earth's rotational axis (redrawn from Besse & Courtillot, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

Figure 5.12 True Polar Wander (TPW) path for the past 200 Ma. TPW is defined as the movement of the "geographic" pole of the Indo-Atlantic hotspot reference frame with respect to the magnetic pole defined by paleomagnetic data, the latter being equated to the Earth's rotational axis (redrawn from Besse & Courtillot, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

has been episodic. A period of relatively fast TPW, averaging 30 mm a-1, separates periods of quasi-standstill between 10 and 50 Ma, and 130 and 160 Ma. During the past 5-10 Ma the rate has been high, of the order of 100 mm a-1. This analysis does not include the oceanic plates of the Pacific hemisphere. This is because there are problems with the quality and quantity of data from the Pacific, and doubts about the fixity of the Pacific hotspots relative to the Indo-Atlantic hotspots (Section 5.5). Notwithstanding these problems, Besse & Courtillot (2002) carried out an analysis for the Pacific plate using nine paleomagnetic poles, between 26 and 126 Ma, derived from analyses of the pattern of the linear magnetic anomalies and the magnetic anomalies developed over seamounts (Petronotis & Gordon, 1999). They assumed the hotspot kinematic model for the Pacific plate of Engebretson et al. (1985), and derived a TPW path for this period of time that is remarkably similar in length and direction to that of the path shown in Fig. 5.12, but offset from it in a way that is compatible with the southward motion of the Hawaiian hotspot discussed in Section 5.5. This, taken together with the similarities between the path shown in Fig. 5.12 and those derived in earlier analyses, based on smaller data sets (e.g. Livermore et al., 1984; Besse & Courtillot, 1991; Prevot et al., 2000), suggests a robust result. One must bear in mind however that these conclusions are only as good as the underlying assumptions: the axial dipole nature of the Earth's magnetic field, and hotspot tracks as indicators of the motion of plates with respect to the Earth's deep interior throughout the past 200 Ma.

The relative motion between the mantle and the rotation axis, as illustrated by the TPW path, may be interpreted as a shifting of the whole or part of the Earth in response to some form of internal mass redistribution that causes a change in the direction about which the moment of inertia of the mantle is a maximum (Andrews, 1985). For example, Anderson (1982) relates TPW to the development of elevations of the Earth's surface resulting from the insulating effect of supercontinents that prevents heat loss from the underlying mantle. It is possible that only the lithosphere or the mantle or both lithosphere and mantle together shift during polar wander. It is highly unlikely that the lithosphere and mantle are sufficiently decoupled to move independently, and so it appears probable that shifting of lithosphere and mantle as a single unit takes place during TPW. Indeed, if there is coupling between core and mantle, the whole Earth may be affected. Andrews's interpretation of TPW is supported by astronomical data which shows that during the 20th century the location of the Earth's rotational axis has moved at a rate similar to that computed from paleo-magnetic and hotspot data, namely about 1° Ma-1. This suggests that at least part of the mass redistribution takes place in the mantle, as the continents do not move this rapidly. Sabadini & Yuen (1989) have shown that both viscosity and chemical stratification in the mantle are important in determining the rate of polar wander. Another mechanism proposed for driving TPW is the surface mass redistribution arising from major glaciations and deglaciations (Sabadini et al., 1982). However, mantle flow is required to explain TPW during periods with no evidence of significant continental glaciation, and, indeed, may be responsible for the majority of TPW. It has also been suggested that TPW is excited by the mass redistributions associated with subduction zones (Section 12.9) (Spada et al., 1992), mountain building, and erosion (Vermeersen & Vlaar, 1993).

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

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