A Discussion on global tectonics in Proterozoic times - Proterozoic magnetostratigraphy and the tectonic evolution of Laurentia*

Abstract There have been two major orogenic cycles in the Proterozoic of Laurentia which culminated at about -1850 (Hudsonian) and -1000 Ma (Grenvillian). A third event, the so-called Elsonian ‘Orogeny’ ( —1400Ma) was dominantly a phase of igneous intrusion. The palaeomagnetic poles from Laurentia are reviewed, and an attempt is made to order them into a path of apparent polar wander (a.p.w.). We have constructed this path so as to minimize its length without violating the palaeomagnetic or geological observations. This a.p.w. path defines a magnetic stratigraphy for the Proterozoic of Laurentia which bears on the nature of the above orogenic events and the tectonic evolution that they may signify. If the results from the Grenville Structural Province are excluded, a single polar path for the interval —2200 to -1300 Ma can be constructed, indicating that Laurentia has not been dismembered and that the Hudsonian Orogeny occurred by internal deformation. The geological evidence is consistent with this view, and also indicates that deformation occurred marginally in the Coronation Geosyncline at about 1800 Ma. Poles from the Grenville Province available at present are displaced from the poles from the rest of Laurentia, and may indicate that the southern part of the Grenville Province was displaced 5000 km at about -1150 Ma. The geological evidence is insufficient to determine whether or not such a reconstruction is correct, but it is notable that the Grenville orogenic cycle is preceded by, and is in part contemporaneous with, extensive rift systems, which developed following the Elsonian ‘Orogeny’. The Grenville Province may then be a product of marginal tectonics and the first instance of the opening and closing of an ocean basin whose descendant is the present day North Atlantic. It is concluded that during the interval —2200 to —1400 Ma both marginal and internal tectonics occurred in Laurentia, whereas in the later Proterozoic marginal tectonics dominated. The Elsonian ‘Orogeny’ was apparently the time of changeover from one regime to the other. Nothing can yet be concluded from the palaeomagnetic evidence about the nature of orogenesis in the earliest Proterozoic ( -2600 to -2200 Ma). It must be emphasized that these conclusions flow from the basic assumption (that of minimizing polar path length) used in constructing our polar path. The data are also compatible with other more complex reconstructions, requiring greater polar path length, and which do not require this tectonic evolution.