A new Mesozoic apparent polar wander loop for South China: paleomagnetism of Middle Triassic rocks from Guizhou Province

Abstract

Middle Triassic marine platform carbonates and sandstones in southeastern Guizhou province (South China Block) were sampled for a paleomagnetic study. A total of 91 samples from 14 sites were collected, including sandstone samples from two limbs of a fold. Progressive thermal demagnetization isolated single characteristic component magnetizations in the limestone unit and two components in the sandstones; all components show mixed polarities. The mean direction in the limestone unit is 352/ - 34 (in-situ) and 1/ - 4 (tilt-corrected) ([alpha]95 = 12[deg]). Component 1 in the sandstones isolated within higher temperature ranges exhibits consistent directions in five sites out of six, yielding an overall site mean direction of 36/75, k = 10, [alpha]95 = 45 (in-situ) and 355/13, k = 178, [alpha]95 = 5.2 (tilt-corrected) and passes a fold test at the 99% confidence level. The directions of component 2 with lower unblocking temperatures in the sandstones are best grouped at 10% unfolding (k = 278, [alpha]95 = 6), giving a mean direction of 351/18 that conforms to the mean directions of the characteristic component in the limestone unit as well as component 1 in the first five sandstone sites. Our interpretation, therefore, is that all the magnetizations in the both limestones and the sandstones were acquired at the time of folding. Our paleomagnetic directions from Guizhou are similar to those of Paleozoic rocks, from which folding associated remagnetizations were also observed, in the vicinity of Nanjing in the northeastern part of the Yangtze Block. The folding associated remagnetizations in both areas thus most likely have the same age, i.e., Late Triassic/Jurassic. We further infer that the southern Guizhou province has not been rotated since Late Triassic/Jurassic times by considering the similarity of the pole positions obtained from remagnetizations in Yunnan, Guizhou and the Nanjing area. Moreover, we suggest a new early Mesozoic apparent polar wander loop for the South China Block, going through the poles of the Triassic/Jurassic remagnetizations. This loop points to rotations of the South China Block as a whole during the late Triassic-Jurassic period.