The Remagnetization of Marine Carbonate Rocks of the Late Ordovician in Pingliang Section,Southwest Ordos(China)

The natural remnant magnetization(NRM)of carbonate rocks in particular must be understood for the construction of a meaningful Paleozoic geomagnetic polarity time scale.This would enable evaluating geomagnetic features(e.g.reversal frequency)and would provide a paleomagnetic dating tool(Voo,1990).

On the Reliability (Remagnetization?) of Paleomagnetic Poles Obtained from Permo-Silurian Rocks from Oaxaca Mexico, Belize and Guatemala: Insights from Rock Magnetic Studies

In the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples is required particularly for Curie point, for grain-size analyses in addition to thermal and alternating field (a.f.) demagnetization experiments. We present the comparative results of 20 sites drilled at 3 different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico;the Late Silurian (~418 Ma) Mountain Pine Ridge Granite, the Hummingbird Granite in Belize, and the Early Leonardian Chochal Limestone in Guatemala. The samples of all 20 sites were subjected to a.f. and thermal demagnetization in 16 steps from NRM to 100 mT, the thermally demagnetized samples were cleaned in 15 increasing temperature steps from NRM up to 675℃. Principal component analysis was applied to the samples in order to obtain their respective mean directions. Saturation Isothermal Remanent Magnetization (SIRM), hysteresis loops, and coercivity experiments performed indicate that about 90% of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were in the Pseudo-Single Domain (PSD) range of the Day diagram. Curie point determinations results ranged from 190℃ to 660℃, indicating the presence of titanomagnetites as well as hematite. In the Juchatengo area reliable poles were obtained from 3 sites, in Belize 3 sites and only 2 sites of the Permian Chochal Formation, Guatemala yielded coherent and useful results. All the “reliable” paleopoles obtained do not agree with the APWP of North America.

Remagnetization of the Early Paleozoic rocks from southern part of the Huabei basin(Ⅱ)——Rock magnetic results

The opinions vary on the secondary remagnetization mechanisms of carbonate rocks. The prevalent interpretations of magnetic remanences are either chemical in origin that invoked the large scale migration of orogenic fluids, or thermoviscous magnetization involved burial diagenetic processes. The paleomagnetic study carried out in northwestern part of Henan Province (north of the Qinling orogenic belt) reveals that 90% of carbonate rocks have suffered from the secondary remagnetization. On the basis of rock magnetic experiments, the assemblage of the magnetic minerals, remanent carriers and its formation process, and the possible mechanisms of remagnetization in these carbonate rocks are discussed.

Remagnetization of the Early Paleozoic rocks from southern part of the Huabei basin (Ⅰ)——Paleomagnetic result and its implications

Although paleomagnetic study of the Early Paleozoic for the North China Block (NCB) has witnessed rapid progress since the 1980s, significant difference in the results can be found from the widespread areas in North China. Besides the paleomagnetic techniques used in the laboratories, the difference of these Paleozoic poles could also be due to the early and late Mesozoic remagnetization in the eastern part of China. It is therefore necessary to carry out systematic paleomagnetic and rock magnetic studies for the Early Paleozoic rocks in the NCB. The remagnetization results from the northwestern part of Henan Province are reported, and related geological implications are discussed.

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province, China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103.17°E). The results indicate the magnetic minerals are dominant by multi domain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.

Evidence for Himalayan remagnetization in Tarim Basin

Himalayan remagnetization in the Tarim Basin was found to be widespread in Paleozoic, Mesozoic and Ce-nozoic rocks. Rock magnetism was performed to study the magnetic carriers. The authors believe that tectonic fluid in the Himalayan stage caused the rock remagnetization. The framboidal pyrites in bitumen and hydrocarbon-rich rocks may transform to framboidal magnetite in the later alkali environment, which leads to remagnetization.

Inclination Deepening of Paleogene Red Beds in the Qiangtang Terrane,the Hinterland of the Qinghai-Tibet Plateau

In this research,one hundred and ten samples（ten sampling localities）were drilled by a portable petrol-driven drill in the Tongtianhe area along the highway where developes Yaxicuo Group,Oligocene. Complete demagnetization were finished in University of California,Santa Cruz（UCSC）.These remanence data were collected:Dg=334.2,Ig=52.3,α95=7.1,k=7.3（before structural correction）;Ds=27.2, Is=66.6,α95=7.3,k=7.0（after structural correction）.