A new Mesozoic apparent polar wander loop for South China: paleomagnetism of Middle Triassic rocks from Guizhou Province
A new Mesozoic apparent polar wander loop for South China: paleomagnetism of Middle Triassic rocks from Guizhou Province
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华南地区新的中生代极地漂移环：贵州中三叠世岩石的古地磁
DOI：
10.1016/0012-821x(93)90208-q
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发表时间：
1993
影响因子：
5.3
通讯作者：
Yanggeng Wang
中科院分区：
文献类型：
--
作者：
Zhongmin Wang;R. Voo;Yanggeng Wang
对贵州东南部(中国地块南部)中三叠世海相台地碳酸盐岩和砂岩进行了古地磁研究。共采集了14个地点的91个样品，其中包括一个褶皱的两个肢体的砂岩样品。逐级热退磁在石灰岩单元中孤立的单一特征成分磁化作用和在砂岩中的两种成分磁化作用；所有成分都表现出混合的极性。石灰岩单元的平均方向为352/−34(原地)和1/−4(倾斜校正)(α95=12°)。在较高温度范围内分离的砂岩中的组分1在6个地点中有5个显示出一致的方向，总体地点平均方向为36/75，k=10，α95=45(原地)和355/13，k=178，α95=5.2(倾斜校正)，并在99%的置信度水平下通过褶皱测试。砂岩中解堵温度较低的组分2的方向最好集中在10%展开时(k=2 78，α95=6)，其平均方向为35 1/18，与灰岩单元中特征组分的平均方向以及前5个砂岩部位的组分1的平均方向一致。因此，我们的解释是，石灰岩和砂岩中的所有磁化作用都是在褶皱时获得的。我们来自贵州的古地磁方向与扬子地块东北部南京附近的古生代岩石相似，从中也观察到了褶皱伴生的再磁化作用。因此，这两个地区的褶皱伴生再磁化作用很可能具有相同的年龄，即晚三叠世/侏罗纪。考虑到云南、贵州和南京地区磁化作用获得的磁极位置的相似性，我们进一步推断贵州南部地区自晚三叠世/侏罗纪以来就没有发生过旋转。此外，我们还提出了一个新的早中生代视极游走环，它穿过三叠纪/侏罗纪重磁化的两极，形成于中国地块南部。该旋回反映了中国地块在晚三叠世-侏罗纪时期的整体旋转。
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) (α95= 12°). 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,α95= 45(in-situ) and 355/13,k = 178,α95= 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,α95= 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.