Paleomagnetic directions and K/Ar dating of 0 to 1 Ma lava flows from La Guadeloupe Island (French West Indias): Implications for time-averaged fieldmodels

Twenty-six lava flows spanning the last million years were sampled in La Gu adeloupe, French West Indies. Because of the lack of continuous volcano-str atigraphic sections in La Guadeloupe, dating is necessary in order to descr ibe the temporal evolution of the geomagnetic field in this time interval. New K/Ar ages ranging from 50 ka to 1 Ma have been obtained on andesites us ing the Cassignol-Gillot technique at the Universite Paris Sud Institut de Physique du Globe de Paris Orsay laboratory. Additional flows (with K/Ar ag es obtained using the same dating technique) were also sampled for paleomag netic investigations. More than 200 samples were analyzed using both altern ating field AF and thermal stepwise demagnetization techniques. Duplicate s amplings of two flows at three different sites demonstrate that within-flow dispersion is negligible for the andesitic lava flows sampled in this stud y. Direct comparison with an earlier paleomagnetic study performed on the i sland indicates that, for the three investigated flows, modern demagnetizat ion techniques yield much better defined paleomagnetic directions. The Matu yama-Brunhes transition was recorded in a three-flow section and is dated a t 781 +/- 18 ka, in good agreement with other recent radiometric age determ inations. The mean paleomagnetic pole calculated from the 23 normal polarit y flows is indistinguishable from geographic north, which implies that no s ignificant persistent axial quadrupole term can be identified at this site for the last million years. This result contradicts earlier results and has important implications for models of the time-averaged field (TAF). An Occ am algorithm was used to construct a TAF model from the global volcanic dat abase from the last 5 Ma. Substitution of the mean direction calculated fro m the earlier study for the Lesser Antilles by our new mean value reduces t he quadrupole term by more than 30%. This effect, which was produced by cha nging data from a single site, demonstrates that older paleomagnetic sites may need to be reinvestigated. Furthermore, it also highlights the limitati ons of TAF models that can be inferred From paleomagnetic databases in thei r present state.