Transitional field behavior during the Gilbert-Gauss and Lower Mammoth reversals recorded in lavas from the Wai'anae volcano, O'ahu, Hawaii

Using a portable fluxgate magnetometer, we identified two polarity transiti ons, one a reversed to normed (R-N) in a 600-m section called Pu'u Heleakal a and second a normal to reversed (N-R) in a 300-m section known as Pu'u Ka mai'leunu of superposed basalt flows on the western flank of Wai'anae volca no, one of the two volcanoes that compose the Hawaiian island of O'ahu. Vol canic stratigraphic studies suggest that the R-N reversal corresponds to th e Gilbert-Gauss polarity boundary and the N-R reversal correlates with the Lower Mammoth polarity transition. We drilled an average of six to eight sa mples from each of the lava flows (36 flows from the Heleakala and 29 from the Kamai'leunu) spanning the two sections. Lightning-induced secondary mag netization is common in some flows, requiring detailed alternating field de magnetization to remove it and to isolate the primary directions. Thermal d emagnetization proved to be ineffective and, if employed, could lead to inc orrect identification of lightning-dominated remanence as primary. Our labo ratory experiments indicate that we have successfully identified one transi tion zone on each section, composed of at least seven to eight transitional lava flows. The transitional VGPs of the Gilbert-Gauss reversal are locate d in northwestern Africa, Sri Lanka, and Borneo, whereas for the Lower Mamm oth reversal the transitional VGPs are located in northern Africa and in th e southern Atlantic. These observations are not in agreement with the hypot hesized antipodal longitudinal bands over the Americas and the Western Paci fic Rim. The two transitions are characterized by periods of high negative inclinations corresponding to VGPs in Africa about 180 degrees away from th e site longitude. The obtained VGP paths could be explained by a transition al field affected by anomalous geomagnetic fields that exist beneath the Pa cific hemisphere. These recurrent nonaxisymmetric components in the transit ional fields suggest that lateral variations in lower mantle properties inf luence the geodynamo.