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 transitions, one a reversed to normal (R‐N) in a 600‐m section called Pu'u Heleakala and second a normal to reversed (N‐R) in a 300‐m section known as Pu'u Kamai'leunu of superposed basalt flows on the western flank of Wai'anae volcano, one of the two volcanoes that compose the Hawaiian island of O'ahu. Volcanic stratigraphic studies suggest that the R‐N reversal corresponds to the Gilbert‐Gauss polarity boundary and the N‐R reversal correlates with the Lower Mammoth polarity transition. We drilled an average of six to eight samples from each of the lava flows (36 flows from the Heleakala and 29 from the Kamai'leunu) spanning the two sections. Lightning‐induced secondary magnetization is common in some flows, requiring detailed alternating field demagnetization to remove it and to isolate the primary directions. Thermal demagnetization proved to be ineffective and, if employed, could lead to incorrect identification of lightning‐dominated remanence as primary. Our laboratory experiments indicate that we have successfully identified one transition zone on each section, composed of at least seven to eight transitional lava flows. The transitional VGPs of the Gilbert‐Gauss reversal are located in northwestern Africa, Sri Lanka, and Borneo, whereas for the Lower Mammoth reversal the transitional VGPs are located in northern Africa and in the southern Atlantic. These observations are not in agreement with the hypothesized antipodal longitudinal bands over the Americas and the Western Pacific Rim. The two transitions are characterized by periods of high negative inclinations corresponding to VGPs in Africa about 180° away from the site longitude. The obtained VGP paths could be explained by a transitional field affected by anomalous geomagnetic fields that exist beneath the Pacific hemisphere. These recurrent nonaxisymmetric components in the transitional fields suggest that lateral variations in lower mantle properties influence the geodynamo.