Previous study has showed that Late Triassic and Early Jurassic marine sediments exist in the northern margin of the South China Sea (SCS), the eastern part of Nansha Block and the western part of the Kalimantan Islands. Similarities of lithofacies and fossil types between these areas indicate that they probably belonged to a unified basin in the Late Triassic and Early Jurassic. To test this hypothesis, paleomagnetic data are required from these three crustal blocks, and here we report Early Jurassic paleomagnetic data from the northern margin of the SCS. Seven sites (98 samples) of volcaniclastic sediments and basalts were collected from Meixian; two sites (48 samples) of siltstones were collected from Huizhou and Dongguan. Single, recent overprint component was found in the volcaniclastic sediments from Meixian. Low temperature component (LTC) and high temperature component (HTC) can be isolated from basalts in Meixian. LTC, intermediate temperature component (ITC) and HTC can be isolated from siltstones in Dongguan. The LTC directions from Meixian and Dongguan are distributed near the present geomagnetic field, indicating that they are recent viscous remanence. HTCs can be isolated from these two sites. However, it is hard to know whether the HTCs are primary components because fold tests are not available. If reference poles of Early Jurassic, Mid-Late Jurassic and Cretaceous from the South China Block (SCB) were used to confine the magnetic inclination of Meixian and Dongguan, we could estimate which phase the components were formed in during 0?100% unfolding of the strata by comparing the confined inclination to the magnetic inclination during unfolding. With this method, we conclude that the ITC of Dongguan was formed when the strata was unfold by 70%?80% and HTC of Dongguan was probably formed before the folding, but we cannot determine when the HTC of Meixian was formed because the range of magnetic inclination during unfolding is wide. Results of anisotropy of magnetic susceptibility (AMS) are consistent with lava flow fabrics, sediment deposition and compaction fabrics. Curves of isothemal remanent magnetism (IRM), thermal demagnetization and alternative field demagnetization indicate that the main magnetic mineral of red beds from Dongguan is hematite and that the main magnetic mineral of some specimen from Meixian is magnetite while the main magnetic mineral of other specimen from Meixian is hematite. Though our result can’t answer the question that whether the northern margin of SCS, the eastern part of Nansha Block and the western part of the Kalimantan Islands belonged to the same basin in the Late Triassic and Early Jurassic due to the limited paleomagnetic data, it is still important to report the result to provide information for further paleomagnetic research of those areas.
ZHANG Huo-Dai
,
TAN Xiao-Dong
,
ZHOU Di
,
LI Ming-Han
,
CHEN Han-Zong
,
SHANG Xian-Zan
. Paleomagnetic results and magnetic characteristics of rocks from Lower Jurassic rocks in the northern margin of the South China Sea[J]. Journal of Tropical Oceanography, 2012
, 31(3)
: 103
-112
.
DOI: 10.11978/j.issn.1009-5470.2012.03.014
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