Earth and Planetary Science Letters, 87 (1988) 463-472 463 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [41 The Matuyama-Brunhes transition recorded from Lake Tecopa sediments (California) Jean-Pierre Valet *, Lisa Tauxe and David R. Clark Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.) Received September 1, 1987; revised version accepted December 17, 1987 The paleomagnetic record of the Matuyama-Brunhes reversal in the sediment of ancient lake Tecopa provided the first evidence that the geomagnetic field during transition is not dipolar. We resampled the section to increase the time resolution of the record. Our new data show that the characteristic direction could not be completely isolated by AF demagnetization. The transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non-dipolar remains. The VGP paths lying close to the site longitude is consistent with an axisymmetric transition field. 1. Introduction The work of Hillhouse and Cox [1] on the paleomagnetic record of the Matuyama-Brunhes reversal in sediments from the ancient lake Tecopa constituted a major step in our knowledge of the behaviour of the geomagnetic field during a rever- sal. These authors showed that the trajectories of virtual geomagnetic poles (VGPs) obtained for this transition in California and Japan [2] traced completely different paths, indicating that the transitional field is not dipolar. Hillhouse and Cox also observed that the dominant direction of the non-dipolar transitional component was parallel to the present non-dipolar field. Furthermore, it had the same direction during an excursion which occurred prior to the reversal. They interpreted these results in terms of the standing field model which invokes the predominance of a standing portion of the non-dipole field during the main part of the transition while the axial dipole field decays to zero and then grows back in the oppo- site sense. * Permanent address: Centre des Faibles Radioactivitrs, Laboratoire mixte CNRS-CEA, Avenue de la Terrasse, Parc du CNRS, 91190 Gif-sur-Yvette, France. 0012-821X/88/$03.50 © 1988 Elsevier Science Publishers B.V. Since this classic study, a large number of records of the Matuyama-Brunhes transition have been obtained from sediments cored from the deep sea or sampled on land [3-5]. Many of these are poorly defined by very few intermediate direc- tions. In fact Tecopa may still be regarded as one of the most detailed studies of the last reversal. However, improved instrumentation and advances in techniques have made possible a considerable increase in the time resolution of the record. It was with this in mind that we resampled the Tecopa section. In addition, we hoped to obtain a detailed record of field variation before the transi- tion in order to assess the relationship of the transitional field to secular variation. 2. Geologic setting and sampling Lake Tecopa is located just to the east of Death Valley about 100 km west of Las Vegas. The geologic setting of the lake deposits has been extensively described by Sheppard and Gude [6] and also summarized by HiUhouse and Cox [1]. Lake Tecopa existed in the Middle Pleistocene during which time mudstone and sandstone inter- bedded with tuff layers accumulated until 500,000 years ago. Three diagenetic facies controlled by