A paleomagnetic study of Tertiary formations from the Kyrgyz Tien‐Shan and its tectonic implications

As part of a general investigation of Cenozoic deformation in Central Asia, we studied the paleomagnetism of Tertiary red beds and lava flows from intermontane basins in the Tien-Shan region of Kyrgyzstan. We collected 532 cores and hand samples from 78 sites at 12 localities and progressively demagnetized them, thermally or with alternating fields. For most sites, there are magnetic components with unblocking temperatures higher than 600°C. We infer that the magnetic carriers are mainly hematite and magnetite. For most localities, the high-temperature component appears to predate tectonic folding. For all localities, mean inclinations are shallower than expected from apparent polar wander paths. Inclination anomalies range from 16°±5° for the Issyk-Kul basin, to 26°±7° for the Fergana basin. If due to changes in latitude, these anomalies imply at least 2000 km of northward displacement of the Tien-Shan during the Tertiary, for which there is no tectonic evidence. We thus consider that the paleomagnetic reference directions cannot be directly compared with our Tertiary data. We explored other possible reasons for this anomaly, which has also been reported from other parts of the Alpine belt, but we could not find a satisfactory explanation. Absolute rotations cannot be accurately determined, because of problems with the reference direction. Nevertheless, the mean declination for the Fergana basin lies counterclockwise by 20°±11° from the mean declination of the Issyk-Kul basin. This result is consistent with the counterclockwise rotation inferred for the Fergana basin from the pattern of Cenozoic faults and folds. It suggests a Cenozoic right-lateral displacement of 110±60 km on the Talas-Fergana fault.

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