Proterozoic Geology: Selected Papers from an International Proterozoic Symposium
J.D.A. Piper, 1983. "Dynamics of the continental crust in Proterozoic times", Proterozoic Geology: Selected Papers from an International Proterozoic Symposium, L. G. Medaris, Jr., C. W. Byers, D. M. Mickelson, W. C. Shanks
Download citation file:
Studies of recent years, notably from metamorphic terrains, have resolved the magnitude and direction of apparent polar wandering (APW) motions over much of Proterozoic times. This paper re-examines the case for a Proterozoic supercontinent, and shows that both the polarities and the positions of the Proterozoic paleopoles from the major shields conform to a single APW path using a unique reconstruction. These data imply that the continental crust was amassed together as a single lens-shaped body as heat loss by extensive small-scale mantle convection correlating with low rates of APW was replaced by a large-scale mantle convection system responsible for the Proterozoic mobile belt regime and correlating with high rates of APW. The later greenstone belts (ca. 2900-2200 Ma old) formed in permobile environments and early Proterozoic straight belts (> 2200 Ma old) formed between larger stabilized divisions of continental crust are oriented parallel to the long axis of the continent. This alignment is preserved by younger mobile belts and most tectonic trends. There was a progressive contraction of rapakivi-massive anorthosite magmatism towards one margin of the supercontinent as temperature gradients declined; most features linked to ocean lithosphere subduction are associated with this margin. Large-scale fracturing with the formation of aulacogens took place along the opposite colder margin. Peripheral parts of the supercontinent broke up apparently without major separation 1100 Ma ago, and the central parts broke up at the beginning of Cambrian times with the formation of a large ocean basin between the Gondwanaland and the Laurentian, Fennoscandian, and Siberian Shields; this event marking the end of Proterozoic times is defined by a number of chemical changes linked to the faunal diversification. Both of these episodes are defined by widespread alkaline magmatism and rifting, and the latter is linked to subdivision of a simple roll mantle convection system which appears to have pertained in some form during most of Proterozoic times.
The characteristic signature of the Proterozoic APW path is a closed loop with a hairpin near the apex. These features can no longer be correlated with tectonic/magmatic episodes and appear to reflect overturn in the driving mantle system. The ensialic mobile episodes follow periods when APW movements were relatively small and may be linked to a thermal blanketing effect of the continental crust. Since 2700-to 2200-Ma-old paleopoles from Africa, Laurentia, Australia, and India are not significantly different from one another on the supercontinent reconstruction, the continental crust has evidently been a highly coherent unit since late Archean times. Movements across later mobile belts do not appear to have been on a scale large enough to be detected by paleomagnetism.