LATE QUATERNARY GEOMAGNETIC SECULAR VARIATION FROM HISTORICAL AND C-14 DATED LAVA FLOWS ON HAWAII

A paleomagnetic record of geomagnetic paleosecular variation (PSV) is constructed for the last 4400 years based on 191 sites in historical a nd C-14-dated lava flows from Mauna Loa, Kilauea, and Hualalai Volcano es on the island of Hawaii. The features of this new record are simila r to those recorded by sediments from Lake Waiau near the summit of Ma una Kea Volcano, but overall mean inclinations for the lava flows (31 degrees to 33 degrees, depending on window size) are nearer the expect ed dipole-field value (35 degrees) than is that for the sediments (27 degrees). Divergence of the inclination records with increasing age su ggests that the Lake Waiau values at depths below 2 m have been affect ed by compaction-related inclination shallowing, although magnetic ter rain effects cannot be ruled out. The rate of PSV indicated by the rec ord presented here is highly variable (<0.5 degrees/century to >20 deg rees/century), and a pronounced shift in inclination from 25 degrees t o 40 degrees occurred between similar to 1030 and similar to 975 years B.P. Paleomagnetic directions from undated materials can be correlate d with our calibrated curve, but the resolution is largely dependent o n the PSV rate and data densities for both the reference and unknown d irections. The upper part of the Puna Basalt (18 lava flows), previous ly sampled for paleomagnetism along the northern wall of Kilauea's cal dera (Uwekahuna Bluff), was likely deposited sometime between 1030 and 750 years B.P., but the lowest two flows beneath the Uwekahuna Ash (s imilar to 2100 years B.P.) are correlated with an age of similar to 30 34 years B.P. Paleomagnetic data for 54 lava flows of the Ka'u Basalt, exposed in the northwest wall of Mauna Loa's summit caldera (Mokuaweo weo), indicate that they probably accumulated over a relatively short time interval (similar to 200+ years) and are assigned to a 1000 to 11 99 year B.P. time window. The mean of ages within this window is simil ar to 1030 years B.P., but mapping and other C-14 dates indicate that these summit overflows are probably closer to similar to 1200 years B. P. in age.