Evidence for weak geomagnetic field intensity prior to the Cretaceous normal superchron
Introduction
Although palaeointensity data are critical for understanding the workings of the earth’s dynamo (e.g. Prévot et al., 1990), studies of recorded palaeomagnetic variations are predominantly concerned with descriptions of directional behaviour. The full palaeomagnetic vector is far less often addressed, a fact that is easily understood when one considers the labour intensiveness necessary for the determination of absolute palaeointensity in lavas (e.g. Carlut and Quidelleur, 2000). By their very nature, polarity superchrons have been the source of considerable interest, especially with regard to the relationship between reversal rate and palaeointensity (e.g. Glatzmaier et al., 1999, Tarduno et al., 2001). Some authors have proposed the presence of strong fields during superchrons (e.g. Larson and Olson, 1991, Tarduno et al., 2001), while others have favoured a weak dipole moment (e.g. Loper and McCartney, 1986, Pick and Tauxe, 1993). The picture is further complicated by the contention that the dipole strength during the Mesozoic was only one-third that of the Cenozoic value (Prévot et al., 1990). As a consequence, Prévot et al. (1990) suggest further that the terrestrial dynamo may exhibit different long-term stable states dependent upon the thermal structure in the lowermost mantle. Clearly, more palaeointensity determinations are needed for pre-Cenozoic times to understand the causal mechanisms that may be involved with field reversal initiation. In this paper we report new absolute palaeointensity estimates for a basalt lava sequence erupted just before the onset of the pre-Cretaceous normal superchron (CNS). Following this we offer a preliminary analysis relating long-term variations in palaeointensity with reversal frequency.
Section snippets
Sampling
The Beipiao basin in the West Liaoning province is situated in the western area of the Tanlu fault zone, a major structural belt in the eastern China (Fig. 1). Volcanism in this basin occurred from the Early Jurassic to Early Cretaceous times (Chen et al., 1997). The basin has attracted the attention of geologists and palaeontologists because of its well-developed Late Mesozoic strata rich in fossils and volcanic activity (Pan et al., 2001, Xu et al., 2001, Ji et al., 2001).
The Sihetun section
Rock magnetic investigation
Magnetic properties of the lava samples were analyzed in order to identify the magnetic carriers of the NRM and determine suitability for palaeointensity experiments. Thermomagnetic curves of small amounts of powdered samples from each flow were measured in a steady field of 800 mT on a magnetic-measurement variable field translation balance (MMVFTB). Based on the results, three groups of samples are identified (Fig. 2): first, 10 of the 19 lava flows (Nos. 3, 4, 5, 6, 8, 10, 13, 14, 18, 19) are
Directions of remanent magnetization
Remanent measurements were made on a 2G cryogenic magnetometer situated in a field free space (<300 nT) of the palaeomagnetic laboratory at the Institute of Geology and Geophysics (IGG) in Beijing. A total of 153 samples (one from each core) were stepwise thermal demagnetized at 14 steps from room temperature to 585 °C. At each step the magnetic susceptibility was measured using a Bartington susceptibility meter to check for magnetic changes. Most of the studied samples showed at most
Sample selection
Pre-selection of samples suitable for palaeointensity determination was primarily based on the behavioural characteristics observed during both demagnetization and high-field thermomagnetic runs. Samples were selected when they indicated (1) an easily removable secondary overprint; (2) a ChRM direction in close proximity to the mean direction associated with all measured samples from that particular lava flow; and (3) thermomagnetic behaviour characterized by a single high Curie temperature and
Palaeodirections and tectonics
The present study provides high quality full-vector palaeomagnetic data from West Liaoning, China, spanning some 9 Myr of volcanic activity—between 133 and 124 Ma—just before the onset of the Cretaceous normal superchron. However, before we more fully explore our findings, especially with regard to the variation in palaeointensity during this time, it is necessary to compare our palaeodirectional results with other published Early Cretaceous data from the region. Indeed, the basic
Acknowledgements
This work was supported by the National Science Foundation of China (49834001), the NKHRSF project of China (G1999075509) and CAS (KZCX1-07).
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