CRETACEOUS PALEOMAGNETISM OF THE FERGANA BASIN AND ADJACENT RANGES, CENTRAL-ASIA - TECTONIC IMPLICATIONS

Paleomagnetic results have been obtained from Cretaceous red beds sampled at fifteen localities in the Fergana basin and adjacent ranges of the Tien Shan (Central Asia). The natural remanent magnetization of these rocks was accounted for by an Late Cretaceous overprint of normal polarity at fourteen localities and dual polarity, presumably primary component, at seven localities. No definite correlation was found between the declinations of the two components and structural trends; thus implying no oroclinal bending of the arcuate structures in the area studied. Inclination data are difficult to interpret due to probable tectonic movements, inclination errors of various types, and ambiguities in the Cretaceous reference data. All locality mean declinations were found to be deflected westward with respect to the Eurasian paleomeridian. The proposed counter-clockwise rotation of the Fergana block during the late Cenozoic is shown to account for the main features of the Alpine tectonics. In particular, the 200 km offset along the Talas-Fergana fault is connected with rotation, which contradicts the common idea that this displacement is of Late Paleozoic age. The horizontal movements are most probably not older than 10 Ma.