New Middle Cambrian and Middle Ordovician palaeomagnetic data from Siberia: Llandelian magnetostratigraphy and relative rotation between the Aldan and Anabar-Angara blocks

摘要

Whether Siberia was a coherent tectonic unit throughout the Phanerozoic or whether significant rotations occurred in early Palaeozoic times between the Anabar-Angara and Aldan blocks is still a matter of debate. Addressing this problem a detailed palaeomagnetic study of sections of mid Cambrian and Llandeillian age has been carried out along the river Lena, river Maya, river Kulumbe, and river Stolbovaya, with a total of 403 collected hand-samples. A section of Llandeilian age was also investigated on the Russian Platform, near St. Petersburg. The studied rocks are carbonates or fine grained terrigenous sediments of reddish and greyish colors. Stepwise thermal demagnetization experiments reveal a rather simple directional behavior. In most samples, after removal of a secondary magnetizations at low to intermediate temperatures, a high temperature component is isolated, which is carried by magnetite and/or hematite. Positive reversal tests and fold tests clearly demonstrate the primary character of the latter component. While only reversed polarity directions are obtained in the lower two thirds of the Llandeilo, six magnetozones of normal polarity are found in the Upper Llandeilo, which mark the end of the Moyero reverse superchron. Together with previously published paleomagnetic data our new results show rather systematic difference in declinations of -20° between the Aldan and Anabar-Angara blocks. This argues in favor of a relative rotation between these two blocks, with an Euler pole of rotation located at latitude -62°N and Longitude -117°. Our interpretation is further supported by existing results from deep sounding seismic experiments, yielding a very similar pole of rotation.