Abstract:
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. The Matuyama-Brunhes and the lower JaramiIlo transition are defined
by only a few intermediate directions while many intermediate directions are observed for the upper Jaramillo
transition and the Cobb Mountain excursion. We attribute these differences to variations in the rate of eruption
of the volcanic rocks. The lower Jaramillo record is characterized by a steepening of the inclination at the
beginning of the reversal suggesting a possible axisymmetric control of the field at this stage. However, the
transition path for the most detailed record (upper Jaramillo) is characterized by large loops; this prevents simple
modeling of the transition by low order zonal harmonics at all stages of a reversal. Paleointensity determinations
were attempted on 48 samples with reliable results obtained for 26 of them. Paleointensities for the transitional
field range from 3 to 8 μT. Such very low field strengths were first suggested by the low intensity of the natural
remanent magnetization associated with intermediate directions. 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 (1) 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 during stable polarity states. Comparison of these results observed in Polynesia with previous
studies from Iceland suggests an increase of the average intensity of the intermediate field with latitude.
