A COMPARISON OF THE TECTONIC EVOLUTION OF THE EASTERN MARGIN OF NORTH AMERICA AND EASTERN SOUTH CHINA

Supercontinents form through the amalgamation of continental blocks along suture zones or orogenic belts. These regions often serve as zones of weakness, where continental breakup is most likely to occur. Similarly, within individual orogenic systems, smaller blocks or terranes can undergo cycles of assembly and dispersal, also exploiting the structural weaknesses of suture zones. As a result of this cyclical process, an ancient orogen preserved between two blocks (or along a continental margin) may represent only a portion of an initial orogen. It may be a result of the collision between the two bounding blocks, but it can also result from an earlier collision cycle unrelated to one of the blocks.

This phenomenon is well illustrated by the tectonic evolution of eastern North America. Here, (1) Amazonia collided with, and later rifted away from, Laurentia during the assembly and breakup of Rodinia, and (2) Africa collided with, and subsequently rifted away from, composite Laurentia during the assembly and breakup of Pangea. The resulting orogens—the ca. 1.0 Ga Grenville orogen and the Paleozoic Appalachians—are only partially preserved in North America.

The Tonian Jiangnan orogen in South China, located between the Yangtze and Cathaysia blocks, is generally thought to have formed through the collision of these two blocks. However, the contrasting geological histories and paleogeographic positions of the two blocks suggest that they did not amalgamate until the Ordovician. We propose that the Jiangnan orogen resulted from a Tonian collision and the closure of the Paleo-Huanan Ocean between the Yangtze block and a third continental block, the Greater Huaiyu Terrane (GHT), which was part of Greater India at the time. Subsequent rifting along the orogen led to the opening of the Huanan Ocean and the separation of Yangtze from GHT and Greater India. The Huanan Ocean was later closed during the Cathaysia-Yangtze amalgamation.

The tectonic evolution of South China is characterized by cycles of collision and ocean closure, followed by rifting and ocean opening, similar to the history of the eastern margin of North America. The Jiangnan orogen is only partially preserved in South China, and its formation was unrelated to the Cathaysia block.