The Matuyama-Jaramillo R-N Transition Recorded in a Loess Section near Lanzhou, P. R. China.

Abstract

A detailed record of the Matuyama-Jaramillo (reversed-normal) transition has been obtained from a loess section near the city of Lanzhou, in the Gansu province of China. At the position in the section where the reversal occurs, the accumulation rate has been estimated at 28 cm per thousand years. A continuous section of 6 m was collected for investigation using both thermal and alternating field demagnetisation. Results indicate that the reversal in direction had a duration of approximately 5000 years while the field strength was much reduced for a period of at least 9000 years. Secular variation in both direction and strength of the field is important throughout the sequence, being particularly strong while the field strength was low. As a consequence, the stability of the field leading up to the reversal in direction is poor, with a number of apparent short duration “reversals” due to a relative enhancement of the non-dipole field. Fourier analysis of the record shows an important periodicity close to 750 years in the field strength and periodicities in the horizontal and vertical field components which can be correlated with more recent secular variation. During the reversal, the VGP path is restricted in longitude, initially following the coast of Africa until it reaches the equator. It then swings west to the Caribbean before moving up through North America to complete the reversal. The second part of this path is within one of the preferred longitude bands recently highlighted by LAJ et al. (1991).