ANOMALOUS LOW PALEOMAGNETIC INCLINATIONS FROM OLIGOCENE LOWER MIOCENERED BEDS OF THE SOUTH-WEST TIEN-SHAN, CENTRAL-ASIA

We studied Oligocene to Lower Miocene red beds from the western margin s of the South-West Ghissar Range (the westernmost part of the Tien Sh an fold belt, Central Asia), A total of 19 sites were sampled on both limbs of a gentle syncline. After removal of an unstable component at 200 degrees to 300 degrees C, the natural remanent magnetization in th ese red beds from 18 sites is accounted for by one characteristic comp onent, ChRM, which persists up to 680 degrees C, Intermediate directio ns of the geomagnetic field were recorded at five sites; after tilt co rrection, the overall mean direction for the remaining 13 sites is D=3 degrees, I=30 degrees, k=19, alpha(95)=8.9 degrees. Both the reversal and the fold tests are positive, Haematite is found to be the main re manence carrier in these red beds, The calculated mean declination fit s well with the reference data (Besse & Courtillot 1991), and no rotat ion with respect to Eurasia is detected, in accord with geological dat a. In contrast, the mean inclination is about 30 degrees shallower tha n expected. We argue that this result is not due to large-scale northw ard transport of the area, Mean inclinations showing a similar amount of shallowing were also reported from the Tadzhik and Fergana basins a nd Kirghiz Tien Shan from various rock types (Thomas et al. 1993, 1994 ), and we are of the opinion that neither inclination error nor compac tion could have led to such a consistent pattern, The analysis of ail reliable Early Tertiary palaeomagnetic results extracted from the Glob al Paleo Magnetic data base for the northern part of the Eurasian plat e reveals that inclination anomalies are very common in this data set, whatever the rock type. The general tendency is a progressive increas e in the anomaly amplitude from the Atlantic Ocean to Central Asia. Th e size and localization of this Tertiary field anomaly is comparable w ith the present-day non-dipole field anomaly. This suggests that there may have been a long-term standing non-dipole anomaly in the Tertiary Earth magnetic field, perhaps correlated to major features of the con temporaneous plate tectonics, for example the India-Asia collision.