MICROPLATES AND EXOTIC, DISPLACED TERRANES: CONTRIBUTIONS FROM PALEOMAGNETISM

Reliable paleomagnetic results can yield precise determinations of declinations and inclinations that are useful for terrane analysis. I will discuss three accretionary domains, one in the Mediterranean, one in the Pangea core of the northern Appalachian range, and one on the western margin of Laurentia. The peculiarities of each will be briefly summarized.

My research in paleomagnetism got started with Permian and younger sample collections from Spain, Portugal, Lebanon and Turkey. Other PhD students at the University of Utrecht had collected in Sardinia and the Southern Alps in Italy, with the goal of contributing a datapoint as a chapter of their thesis. The collective results formed a coherent pattern of counterclockwise deviating declinations, logically interpreted as caused by block-rotations. Following an idea of paleomagnetist Myrl Beck, this pattern was attributed to a “ball-bearing” movement in a geodynamic sense caused by the sinistral motion between Africa and Stable Europe as the Atlantic opened. Later research showed that the rotations were real and definitely sinistral, but not simultaneous. The idea of a ball-bearing was clearly too naïve.

Collisions resulting in closure of the early Paleozoic Iapetus Ocean involved a substantial continental ribbon continent (Avalonia), which joined Laurentia and Baltica in Silurian time, having swept up several exotic terranes now within the Dunnage and Gander belts. The entire process had a strong south to north component, which was ideal for paleomagnetism, with its unique (albeit non-linear) correspondence between inclinations and paleolatitudes. Paleomagnetic data could yield an intra-Iapetan distribution of terranes, owing to the availability of stratified, low-grade, igneous rocks.

A similar south to north displacement pattern was revealed for Laurentia’s west margin, with abundant evidence for accretion of terranes, although uncertainties in the paleohorizontal of some of the rocks rendered several inclinations less reliable. The inevitable additional uncertainty about west to east movements added an additional degree of freedom in the pre-accretion history of the terranes.