Vertical Motions of the Hawaiian Islands during the last 400 ka and their Implications for Plate-Plume Interactions

Radiometric dating on drowned coral reefs and sub-aerial lavas including those from the Hilo scientific drilling hole [e.g., Lipman and Moore, 1996] indicates that Hawaii has subsided for ~1.2 km in the last 450 ka. Studies by Grigg and Jones [1997] of elevated coral deposits on Lanai and Molokai, which are located ~250 km from Hawaii, indicate up to ~60-80 meters uplift in the last 300 ka for these islands. Similar studies show that Oahu which is ~340 km from Hawaii has experienced smaller amount of uplift during approximately the same period. The observed patterns of vertical motions have been attributed to the flexural effects of loading of Hawaii on an elastic plate overlying an inviscid substratum. Such a model, however, is time-invariant and does not take into account any changes that may occur in the subsidence and uplift history during loading. We have therefore formulated a viscoelastic model to investigate the vertical motion induced by the loading of Hawaii. The viscosity structure is determined from thermal age and a Newtonian flow law with activation energy 120 KJ/mol and reference viscosity 10²⁰ Pa s [Watts and Zhong, 2000]. A feature of the model is that at the time-scales appropriate to loading at Hawaii a significant portion of the lithosphere with viscosity greater than 10²² Pa s may still support stresses, which results in a larger apparent elastic thickness than has been deduced from flexural loading studies. Our studies show that while a viscoelastic model with a variable loading history can explain the subsidence at Hawaii, it fails to account for the uplift that is observed at Lanai and Molokai. One possibility is that these islands have been influenced by dynamic uplift due to an ascending plume beneath Hawaii. Our 3-D convection calculations show that the 10 cm/year motion of the Pacific plate causes the topography induced by a plume to peak "downstream" of Hawaii in the region of Lanai and Molokai. The vertical motion of the Hawaiian islands is, therefore, a consequence of the interaction between load-induced stress relaxation in the lithosphere and dynamic effects in the underlying mantle. Future work should enable the vertical motion data to be used not only as constraints on models for rheology of the lithosphere but also on the nature of plume-plate interactions. Grigg and Jones, Marine Geol., 141, 11-25, 1997. Lipman and Moore, J. Geophys. Res., 101, 11631-11641, 1996. Watts and Zhong, Geophys. J. Int., 142, 855-875, 2000.