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

A volcanic sequence almost 700 meters thick has been sampled in the Punaruu valley on the island of Tahiti, southern central Pacific Ocean. Detailed paleomagnetic results have been obtained from 123 sites. Three reversals are recorded in this sequence. Age determinations (K-Ar) indicate that the youngest reversal corresponds to the Matuyama-Brunhes transition while the two other transitions limit the Jaramillo normal polarity subchron. An apparent R-T-R excursion has been identified lower in the volcanic sequence and K-Ar age determinations around 1.1 Ma suggest that it corresponds to the Cobb Mountain subchron, but no normal paleomagnetic directions were discovered. An analysis of the variation of the intensity of magnetization with the angular departure from the central axial field, including all other available data from Polynesia, indicates that i) a paleomagnetic direction should be considered as intermediate when it is more than 30° from the expected axial dipole direction, and 2) the average transitional geomagnetic field intensity is about 1/5 of its value duing stable polarity states. -from Authors