A PALEOMAGNETIC STUDY OF THE POHUE BAY FLOW AND ITS ASSOCIATED COASTAL CONES, MAUNA-LOA VOLCANO, HAWAII - CONSTRAINTS ON THEIR ORIGIN AND TEMPORAL RELATIONSHIPS

Detailed paleomagnetic and rock-magnetic studies of the Pohue Bay flow and associated cones on its coastal flat were made to determine if th e origin of the cones was due to primary (volcanic) or secondary (litt oral) processes. We used paleomagnetism to determine the temporal rela tionships between the flow and cones. If the flows and cones are from the same eruption, the littoral origin of the cones is strongly favore d. A total of 530 specimens from 232 core samples were collected and s tudied from the Pohue Bay flow and from lava ponded within the cones. Remanent magnetizations are very stable to stepwise alternating field demagnetization and show small angular dispersion and well-defined cha racteristic magnetizations. Magnetic carriers correspond to members of the titanomagnetite series with single or pseudo-single domain states . The overall mean directions for the Pohue Bay flow (Dec = 10.8 degre es, Inc = 23.6 degrees, k = 287.4 and alpha(95) = 2.3 degrees) and the lavas ponded within the cones (Dec = 12.8 degrees, Inc = 25.2 degrees , k = 353 and alpha(95) = 4.9 degrees) are statistically indistinguish able and have been drawn from a common Fisherian distribution, support ing a close age relationship. Additionally, the angular dispersion for the combination of the main flow and lavas ponded in cones is small, with a paleosecular variation (PSV) estimate of S-f = 4.2 degrees. Thi s small PSV value supports the temporal association between the Pohue Bay flow and associated cones, They can thus be assigned to the same e ruption timeframe. Because the source vent of the Pohue Bay flow is fa r upslope, the cones must therefore be littoral in origin, formed when the Pohue Bay flow entered the ocean. From the secular variation curv e and comparison with age-dated flows with similar paleomagnetic direc tions, we estimate that the Pohue Bay flow was erupted approximately 1 300 years ago. We were also able to distinguish both a possibly younge r lava flow that later utilized the main tube of the Pohue Bay flow an d an earlier sub-set of cones that were possibly formed before the Poh ue Bay eruption.