Anomalous low palaeomagnetic inclinations from Oligocene-Lower Miocene red beds of the south-west Tien Shan, Central Asia

Abstract

SUMMARY We studied Oligocene to Lower Miocene red beds from the western margins of the South-West Ghissar Range (the westernmost part of the Tien Shan 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° to 300°C., the natural remanent magnetization in these red beds from 18 sites is accounted for by one characteristic component, ChRM, which persists up to 680°C. Intermediate directions of the geomagnetic field were recorded at five sites; after tilt correction, the overall mean direction for the remaining 13 sites is D= 3°, I= 30°, k= 19, α95= 8.9°. Both the reversal and the fold tests are positive. Haematite is found to be the main remanence carrier in these red beds. The calculated mean declination fits well with the reference data (Besse & Courtillot 1991), and no rotation with respect to Eurasia is detected, in accord with geological data. In contrast, the mean inclination is about 30° shallower than expected. We argue that this result is not due to large-scale northward transport of the area. Mean inclinations showing a similar amount of shallowing were also reported from the Tadzhik and Fergana basins and Kirghiz Tien Shan from various rock types (Thomas et al. 1993, 1994), and we are of the opinion that neither inclination error nor compaction could have led to such a consistent pattern. The analysis of all reliable Early Tertiary palaeomagnetic results extracted from the Global PaleoMagnetic data base for the northern part of the Eurasian plate reveals that inclination anomalies are very common in this data set, whatever the rock type. The general tendency is a progressive increase in the anomaly amplitude from the Atlantic Ocean to Central Asia. The size and localization of this Tertiary field anomaly is comparable with 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 contemporaneous plate tectonics, for example the India-Asia collision.