PALEOMAGNETIC CONSTRAINTS ON THE CRUSTAL EVOLUTION OF THE YANGTZE BLOCK, SOUTHEASTERN CHINA

Paleomagnetic poles for the Yangtze block, ranging in age from Early Cambrian to Quaternary, are compiled up to the end of 1989. The data are classified into A-, B-, C- or D-quality categories by applying a specific reliability grading scale. A filtering technique is then used to separate out the less reliable from the more reliable poles. Unit weight is finally assigned to the remaining results to establish an APW path for this block. The new APW path reveals two sharp hairpins in the Permian-Jurassic APW segment, which were not observed in previous APW paths. The first hairpin occurs around the Permian-Triassic boundary which may be related to the rapid subduction of the Qinling Ocean beneath the Sino-Korean block. The second one occurs around the Triassic-Jurassic boundary, corresponding to the late Indosinian orogeny. In addition, the Early Paleozoic path roughly describes an anticlockwise, self-closing loop which is opposite in sense to that found in the APW path for Gondwana Land, and implies that the Yangtze block had been separated from Gondwana Land since that time. The crustal evolution for the Yangtze block is presented in terms of paleolatitude and paleorotation. During the Paleozoic and the Early Mesozoic, the block was located near the equator. After the Late Permian, the block underwent considerable poleward motion until it approached its present, relative latitudinal position and orientation in the Cretaceous. Relative motions of the Yangtze block with respect to Eurasia are discussed by comparing their APW paths, showing that the Yangtze block was not part of the Eurasian continent until the Cretaceous and that since then no substantial motions occurred between them.