Too-low Magnetic Inclinations in Central Asia: An Indication of a Long-term Tertiary Non-dipole Field?

Observed Tertiary paleolatitudes in central Asia are more southerly (by about 1600 km on average) than those predicted from the Eurasian reference paleopoles. Subsequent northward displacements of the central Asian terranes are unlikely to have been this large. In this study we analyze to what extent non-dipole fields, especially octupole fields, can explain this phenomenon. A global (zonal) octupole field manifests itself in two ways. (1) Because the reference APWP is based mostly on results from the U.K. and North America, its paleopoles will be far-sided as seen from the North Atlantic, but near-sided as seen from eastern Asia, giving predicted paleolatitudes that are too high. (2) An octupole field contribution produces observed paleolatitudes, as calculated with the dipole formula for central Asia, that are too low. Both effects, therefore, increase the paleolatitude anomalies in Asia. We find that an octupole/dipole field ratio (G3) of 0.06 or greater will reduce the paleolatitude discrepancies significantly and is of the same magnitude as the G3 estimate of recent analyses of Early Tertiary European and North American paleomagnetic and hot spot data.