REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 384-394, APRIL 1991 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1987-1990 Paleomagnetic Reference Poles, Apparent Polar Wander Paths, Paleomagnetic Euler Pole Analysis, and True Polar Wander JOHN WM. GEISSMAN Department o/Geology, University 0/ New Mexico, Albuquerque, New Mexico RICHARD G. GORDON Department o/Geological Sciences, Northwestern University, Evanston, Illinois INTRODUCTION We review progress over the past four years in assess- ing Phanerozoic and Precambrian apparent polar wander (APW). The prior quadrennial report concluded that cra- tonic reference poles and APW paths are less accurate than was assumed in the late 1970's and early 1980's [Kent and May, 1987]. Although this statement remains true, important contributions to APW paths were made during both the 1983-1986 and the 1987-1990 quadren- nia. Paleomagnetic investigations of orogenic belts, which were reviewed by Beck [1989], continue to spur research on cratonic reference poles and APW paths. Irv- ing [1988] has recently reviewed paleomagnetic research and APW path construction from 1948 to 1960, while summarizing the role of paleomagnetic investigations in the confirmation of continental drift. Herein we review new paleomagnetic reference poles, re-interpretations of older data, new approaches to the analysis of APW paths, and true polar wander. Before summarizing new data, we highlight important issues considered over the past four years. REVISED APW PATHS, REINTERPRETATIONS, AND !MPUCATIONS Pre-Carboniferous APW and Paleogeography 0/ the Atlantic Bordering Continents In the early and mid-1980's, revisions to Paleozoic paleomagnetic reference poles for North America required revisions to reconstructions of Atlantic-bordering continents and terranes. Although these reconstructions depend on Gondwana's poorly constrained, early to mid- Paleozoic APW path, a consensus on some issues has emerged: In early to mid-Paleozoic time, North America was in an equatorial to moderately southern latitude while rotating counterclockwise. By Late Devonian time, a wide ocean at mid-southerly latitudes separated North America and western Europe from Africa and the other Gondwana continents [e.g., Van der Voo, 1988; Kent and Van der Voo, 1990]. Other aspects of Paleozoic circum-Atlantic reconstruc- tions are disputed, and in several cases tentative Copyright 1991 by the American Geophysical Union. Paper number 91RG00632 8755-1209/91/9lRG00632 $15.00 conclusions are rapidly superseded by later data and con- clusions. For example, Briden et al. [1988] evaluate Late Proterozoic and pre-Permian, Paleozoic paleomagnetic data and the implied constraints on the evolution of the Caledonian-Appalachian orogen. In Late Proterozoic time, the Laurentian and Baltic shields were close together, but drifted apart by mid-Ordovician time. Paleolatitude differences between the North American and British/Scandinavian margins of the Caledonides appear to be small; therefore Silurian convergence must have been mainly east-west. However, recent results from Britain [e.g., McCabe and Channell, 1990] chal- lenge this interpretation. For mid-Paleozoic time, Briden et al. [1988] noted that the available paleomagnetic data could not be used to resolve the final collision of Gondwana with the Old Red Continent, which consisted of Euramerica and Armorica. Kent and Keppie [1988] proposed reconstructions for Late Silurian, Early Carboniferous, and Late Carboniferous-Early Permian time. They interpreted oro- geny during Late Silurian time as the east-west closure of the Iapetus Ocean. They also suggest that mid-Devonian sinistral shearing along the Caledonian orogen placed Africa against eastern North America and the Old Red Continent by early Carboniferous time. Van der Voo [1988] proposed reconstructions for Ordovician through Late Devonian time. His Late Devonian reconstruction places the Gondwana continents at high southerly lati- tudes. Many intervening displaced terranes (e.g., Armor- ica and Avalonian basement rocks) were located adjacent to the northern margin of Gondwana in Late Ordovician time. Late Silurian to Early Devonian collision affixed several terranes, as well as smaller fragments of Iapetus [Van der Pluijm et al., 1990], next to North America. Van der Voo [1988] discussed the general agreement among paleomagnetic, paleoclimatologic, and biogeo- graphic data in these reconstructions. Kent and Van der Voo [1990] offer reconstructions for Siluro-Devonian, Early Devonian, and Late Devonian time. The transpressive collision of the northwest margin of South America against the eastern margin of Laurentia in the Siluro-Devonian is an important difference between their model and prior models [e.g., Kent and Keppie, 1988]. Following this collision, Gondwana retreated in the Early Devonian, opening a wide ocean by the Late Devonian. Later, by late Early Carboniferous (end of Visean), Gondwana moves into a Pangea configuration. 384