Records of geomagnetic reversals from volcanic islands of French Polynesia 2. Paleomagnetic study of a flow sequence (1.2-0.6 Ma) from the island of Tahiti and discussion of reversal models

copyrighted by American Geophysical UnionA volcanic sequence almost 700 meters thick has been sampled in the Punaruu valley on the island of\ud Tahiti, southern central Pacific Ocean. Detailed paleomagnetic results have been obtained from 123 sites. Three\ud reversals are recorded in this sequence. Age determinations (K-Ar) indicate that the youngest reversal corresponds\ud to the Matuyama-Brunhes transition while the two other transitions limit the Jaramillo normal polarity\ud subchron. An apparent R-T-R excursion has been identified lower in the volcanic sequence and K-Ar age\ud determinations around 1.1 Ma suggest that it corresponds to the Cobb Mountain subchron, but no normal\ud paleomagnetic directions were discovered. The Matuyama-Brunhes and the lower JaramiIlo transition are defined\ud by only a few intermediate directions while many intermediate directions are observed for the upper Jaramillo\ud transition and the Cobb Mountain excursion. We attribute these differences to variations in the rate of eruption\ud of the volcanic rocks. The lower Jaramillo record is characterized by a steepening of the inclination at the\ud beginning of the reversal suggesting a possible axisymmetric control of the field at this stage. However, the\ud transition path for the most detailed record (upper Jaramillo) is characterized by large loops; this prevents simple\ud modeling of the transition by low order zonal harmonics at all stages of a reversal. Paleointensity determinations\ud were attempted on 48 samples with reliable results obtained for 26 of them. Paleointensities for the transitional\ud field range from 3 to 8 μT. Such very low field strengths were first suggested by the low intensity of the natural\ud remanent magnetization associated with intermediate directions. An analysis of the variation of the intensity of\ud magnetization with the angular departure from the central axial field, including all other available data from\ud Polynesia, indicates that (1) a paleomagnetic direction should be considered as intermediate when it is more than\ud 30° from the expected axial dipole direction, and (2) the average transitional geomagnetic field intensity is about\ud 1/5 of its value during stable polarity states. Comparison of these results observed in Polynesia with previous\ud studies from Iceland suggests an increase of the average intensity of the intermediate field with latitude