HR: 1330h
AN: V32A-0992    [PDF]
TI: Two Forms of Hawaiian Volcanism Generated by Mantle Decompression Beneath Flexural Uplift of the Lithosphere
AU: * Bianco, T A
EM: rixctf@soest.hawaii.edu
AF: Department of Geology and Geophysics; SOEST; University of Hawaii, Manoa, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Ito, G
EM: gito@hawaii.edu
AF: Department of Geology and Geophysics; SOEST; University of Hawaii, Manoa, 1680 East-West Road, Honolulu, HI 96822 United States
AB: Two forms of Hawaiian volcanism are poorly understood: post-erosional eruptions and the Hawaiian Arch flows. Various hypotheses have been forwarded to explain these magmatic events; one such mechanism that is relatively undeveloped associates volcanism with the topographic uplift caused by lithospheric flexure. We propose a model in which melt is generated as a direct consequence of the flexural uplift, which surrounds new volcanic shields as they grow. This uplift causes upward flow and decompression of the underlying asthenosphere. We assume that a thick ($\sim$50 km) layer of asthenosphere is near its solidus, a condition resulting from mantle plume material that first melted partially beneath the shield, but has since flowed laterally away from the shield. One prediction of our model is similar geochemical characteristics between the two types of volcanism, and this is consistent with results of recent geochemical studies. Another prediction is volcanism occurs where the lithosphere is actively rising, during the loading of a volcanic shield. Using age estimates of the post-erosional and arch volcanics, we find that they occurred $\sim$200-400 km away from the volcanic shield(s) active at the same time. This range of distance is consistent with the zone of flexural uplift predicted by loading a lithosphere with an effective elastic plate thickness of 30 km. To evaluate the volume and composition of melts generated, we calculate the flow of asthenosphere driven by the rate of lithospheric flexure. We simplify volcano loading as a growing point load and calculate the axisymmetric flexure of the lithosphere, which is the surface boundary condition on an isoviscous (asthenospheric) half-space. Two analytical methods are used to compute asthenospheric flow. The first method uses Fourier transforms, and the second used Hankel transforms. The two methods show good agreement, and both predict uplift beneath the arch, which decays with depth, but is present throughout the zone of near-solidus asthenosphere. From these calculations we evaluate the melting conditions required to generate the observed eruption volumes and many key aspects of the magma geochemistry.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3210 Modeling
DE: 3299 General or miscellaneous
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting