palaeomagnetism and continental drift

palaeomagnetism and continental drift The development of a very sensitive rock magnetometer in the mid-1940s paved the way for much palaeomagnetic research, including measurements on weakly magnetized sedimentary rocks, some as old as Precambrian: more than about 545 million years (Ma) old. In the 1950s, workers from England, including Keith Runcorn, then at Cambridge University, began to acquire palaeomagnetic data from the Phanerozoic rocks of western Europe and North America to test hypotheses of continental drift and true magnetic polar wandering. By 1956, data sets showed systematic discrepancies of about 25° between palaeomagnetic poles inferred for Europe and North America. Thus, the palaeomagnetic database supported the idea of continental drift well before an actual mechanism for drift had been proposed and accepted, a not surprising situation.

The Vine–Mathews, Morley–Larochelle hypothesis of the early 1960s explained marine magnetic anomaly patterns by proposing that oceanic crust was created at mid-ocean ridges. The ocean crust, to a first approximation, faithfully records the direction and polarity of the geomagnetic field. By the late 1960s, a sufficiently detailed geomagnetic polarity timescale for the past few million years of its Earth's history had been compiled and was used to verify that marine magnetic anomaly records described accurately the polarity history of the planet. The paradigm of plate tectonics was finally accepted by the majority of the geoscience community and the use of palaeomagnetic data to interpret continental drift began in earnest.

Palaeomagnetic poles became available for all the continents, and their temporal distribution on the globe defined paths of apparent polar wander (APW). Each continent exhibited an APW path of different geometry and the conclusion was reached that, to a first approximation, the paths recorded latitudinal shifts and rigid-body vertical-axis rotation of the continents, rather than true polar wander. Increasingly refined comparisons of APW paths from different continents clearly indicated that they recorded the motion in the geological past, often of large magnitude, of what had by then become known as lithospheric plates. Symmetry arguments show that the position of a palaeomagnetic pole relative to a sampled continent does not constrain the palaeolongitude of the continent. However, and very importantly, the distance from a palaeomagnetic pole and the sampled area of the continent, as reflected by palaeomagnetic inclination, provides a direct estimate of the palaeolatitude of the continent.

The APW path for North America in the Phanerozoic (Fig. 1) records, to a first approximation, several global tectonic events. Palaeomagnetic poles for much of the Palaeozoic and early Mesozoic are about 90° from the interior of North America, and are consistent with a near-equatorial position for the continent, as the stratigraphical record indicates, for this period of time. The early Palaeozoic part of the APW path suggests a significant counterclockwise rotation. Poles for the late Palaeozoic and earliest Triassic are nearly identical, implying relatively quiescent motion of the continent; they probably reflect the final closure of Iaepetus and the stabilization of the supercontinent Pangaea. From Middle Triassic to earliest Jurassic times, North America again experienced counterclockwise rotation. At the beginning of the Jurassic, however, a major change in plate motion occurred, because the continents bordering the central Atlantic began to separate. Although there is controversy about the details of the APW path and the ages assigned to particular palaeomagnetic poles, clockwise rotation, ultimately combined with northward translation, became the dominant motion. North America was closest to the pole in the mid-Cretaceous, between about 110 and 80 Ma ago. Since the latest Cretaceous, a slight counterclockwise rotation and southward translation has occurred.



Phanerozoic APW paths have been established for most of the continents, but unfortunately the database for them is not as comprehensive as that for North America. Nonetheless, sufficient palaeomagnetic data exist to reveal many features of the past tectonic history of the continents. For example, the database for India documents its large-magnitude northward drift since the break-up of Gondwanaland, beginning in Early Jurassic time. In contrast to North America, the Mesozoic and Cenozoic parts of the APW path for South America are of exceptionally low amplitude. The limited Palaeozoic data from South America, however, indicate considerable motion.

A fundamental consequence of plate tectonic movements is that continents not only move but also grow with time, by the accretion of fragments of previous continents, oceanic islands, or parts of ocean floors being subducted. While many North American palaeomagnetists were devoted to establishing an APW path by studying rocks in the interior of the continent, the western margin of North America also became the focus of numerous palaeomagnetic studies. Most palaeomagnetic poles from these rocks did not agree with the reference poles of appropriate age from the APW path. Typically, they lay to the right of or to the far side of the APW path. These discrepancies were interpreted by Myrl Beck of Western Washington University and others to indicate, in an absolute sense with respect to North America, clockwise rotation (eastward declinations) and northward translation (gentler inclinations) of much, if not most, of the crust comprising the western margin of the continent. Rocks from Baja California to south and central Alaska yielded data interpreted to suggest the impressive rates at which fragments of crust can be transported before and during final accretion with a larger continent. Just as with the interpretation of APW paths, much debate still centres on the interpretation of the palaeomagnetic data; for example, the data for intrusive igneous rocks could be explained by local tilting, rather than latitudinal transport and rotation.

John W M Geissman

Bibliography

Butler, R. F. (1992) Palaeomagnetism: magnetic domains to geologic terranes. Blackwell Scientific Publications, Oxford.
Van der Voo, R. (1993) Palaeomagnetism of the Atlantic, Tethys, and Iapetus Oceans. Cambridge University Press.