Page 118 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 105

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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, aver- plate of Engebretson et al. (1985), and derived a TPW
−1
aging 30 mm a , separates periods of quasi-standstill path for this period of time that is remarkably similar
between 10 and 50 Ma, and 130 and 160 Ma. During the in length and direction to that of the path shown in Fig.
past 5–10 Ma the rate has been high, of the order of 5.12, but offset from it in a way that is compatible with
−1
100 mm a . This analysis does not include the oceanic the southward motion of the Hawaiian hotspot dis-
plates of the Pacific hemisphere. This is because there cussed in Section 5.5. This, taken together with the

are problems with the quality and quantity of data from similarities between the path shown in Fig. 5.12 and
the Pacific, and doubts about the fixity of the Pacifi c those derived in earlier analyses, based on smaller data

hotspots relative to the Indo-Atlantic hotspots (Section sets (e.g. Livermore et al., 1984; Besse & Courtillot,
5.5). Notwithstanding these problems, Besse & Cour- 1991; Prevot et al., 2000), suggests a robust result. One
tillot (2002) carried out an analysis for the Pacifi c plate must bear in mind however that these conclusions are
using nine paleomagnetic poles, between 26 and 126 Ma, only as good as the underlying assumptions: the axial

derived from analyses of the pattern of the linear mag- dipole nature of the Earth’s magnetic field, and hotspot
netic anomalies and the magnetic anomalies developed tracks as indicators of the motion of plates with respect
over seamounts (Petronotis & Gordon, 1999). They to the Earth’s deep interior throughout the past
assumed the hotspot kinematic model for the Pacifi c 200 Ma.