PALEOMAGNETIC EVIDENCE FOR CENOZOIC BLOCK ROTATIONS IN THE TAJIK DEPRESSION (CENTRAL-ASIA)

This paper presents results of a paleomagnetic study of Oligo-Miocene red beds of the Tadjik depression in Central Asia. We sampled about 53 0 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high-temperature components appear to predate folding of upper te rtiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the we stern margin of the depression. However, they are shallower by about 3 0-degrees than the inclination predicted from the reference apparent p olar wander path. This appears to indicate a 23-degrees difference in latitude, which is incompatible with paleogeographic reconstructions f or the Tertiary. A sound interpretation of this anomaly would require a better-constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significant ly rotated, counterclockwise with respect to those measured on the Tur an platform. The eastern part of the depression is a domain of large r otation (52-degrees +/- 13-degrees to 46-degrees +/- 15-degrees), wher eas smaller amounts of rotation have occurred in the western part (27- degrees +/- 14-degrees to 14-degrees +/- 15-degrees). The similarity b etween Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and wester n parts of the depression. Paleomagnetic and structural data suggest t hat block rotations in the Tadjik depression are associated with inden tation of the Pamirs into stable Asia. At a larger scale, observed rot ations are compatible with a model of regional sinistral wrenching, al ong a strip running from the Gulf of Oman to Lake Baikal.