Pre-Permian Global Tectonics: A Tectonic Test

Post-Permian sea-floor spreading coupled with fragmentation and widespread drift (> 1,000 km) of large segments (>107 km2) of continental crust appear established. However, studies of the nature and patterns of orogenic belts formed from the early Archean to the Recent suggest the post-Permian, large-scale dispersive movements of major continental fragments are uncommon, possibly unique in the geological record.

Widespread drift and agglomeration of arcs, inter-arc basins, seas, and protocontinental segments also are indicated in conjunction with numerous subparallel spreading centers and subduction zones in the relict Archean rock complexes formed between 2.5 and 3.5 b.y. (b.y. = 109 yrs). But the Archean-type global tectonics were characterized by both spreading and accretive movements of evolving arcs, intervening basins, and protocontinental segments. Accretive motions dominated throughout the final 500 m.y. of Archean time, culminating in perhaps one, or, at most, three major protocontinents at about 2.5 aeons ago.

The ensialic nature of most Proterozoic orogenic belts and their lithic components, as well as their interrelations to Archean terranes, indicate that they evolved in large part on and between the closely spaced Archean protocontinental clusters. These underwent deformation, refractionation, and thickening as the Proterozoic orogens evolved, but without the major fragmentation, widespread dispersion, and recollision of continents typical of the post-Permian.

Late Proterozoic and early Paleozoic orogenic patterns and features, including ophi-olitic series within some orogens, indicate rifting, rotation, subordinate drift, and collision of large fragments of the evolving megacontinental masses. But the dominating continental component in associated orogenic belts indicates the scale of drift was limited to “second-order” movements commonly involving much less than 1,000-km drift, and no more than 20° rotation of these fragments. The rifting and collisions indicated in the late Proterozoic and early Paleozoic appear to be the incipient phase of the global dispersal of major continents in the post-Permian.

Structural evidence for these conclusions appears in the accordant tectonic patterns that result when the Americas, Africa, India, and Australia are reclustered into the pre-Triassic Gondwana and western Laurasia defined by many independent lines of evidence. In this megacontinent, especially in Gondwana, fold belts of Paleozoic, Proterozoic, and Archean ages are juxtaposed into subparallel, coherent entities. In general the fold belts decrease in age from north to south. These first-order features seem to require a consistent interrelation and orientation of southern Gondwana with remarkably uniform regional force fields in the mantle, under, and seaward beyond Gondwana throughout at least 3.5 b.y. of geologic time.

The further accordance, along great circles or parallels, of the Archean fold belts of Permian North America with those in Permian Gondwana suggests either a single mega-Archean orogenic system, or two complementary subparallel systems which have undergone little differential reorientation throughout the entire Proterozoic and Paleozoic.

Because the motions necessary to generate the subparallel mega-Archean fold systems of the world did not deviate widely from the contemporary meridians of latitude, they seem largely of internal (radiogenic?) origin or only indirectly related to obvious extraterrestrial forces, such as changes in rates of rotation of the earth.

Field of study: Geology