Variation in Elastic Thickness along the Emperor Seamount Chain

The Hawaii-Emperor seamount chain stretches westward from the “Big Island” of Hawaii for over 6000 km until the oldest part of the Emperor chain is subducted at the Kuril and Aleutian trenches. Still regarded as the iconic hotspot-generated seamount chain it has been sampled, mapped, and studied to give insights into numerous oceanic phenomena, such as seamount and volcano formation and associated intraplate magma budgets, the past absolute motions of the Pacific plate and the drift of the Hawaiian plume, and the thermal and mechanical properties of oceanic lithosphere. Much early work on determining the flexural rigidity and equivalent elastic plate thickness that supports the large volcano loads that comprise the chain was focussed on the Hawaiian Ridge, with a major multichannel seismic expedition to the Hawaiian Islands in 1982 providing clear and direct evidence of plate flexure, as well as the indirect effect this deformation has on Earth’s gravity field. Numerous studies have since followed. However, the older part of the chain, beyond the ~50 Ma “bend”, has been much less well studied due to its remoteness, but recent expeditions have provided new marine seismic data to allow an estimation of elastic thickness along the Emperor chain and how they compare to the information we have along the Hawaiian Ridge. Here, we present preliminary work on determining the elastic thickness beneath the Emperor Seamounts. Unlike the Hawaiian Ridge, where the age of the lithosphere at the time of loading (i.e., the difference in age between the underlying seafloor and the formation age of a seamount or oceanic island) is remarkably constant, along the Emperor chain there are major variations in the age of loading, compounded by higher uncertainty due to limited seamount age sampling and the chain’s location within the Cretaceous Quiet Zone. Thus, models with variable elastic thickness as a function of location along the Emperor chain are required. In this presentation, we discuss several models that seek to account for the new seismic imaging of the top and base of flexed oceanic crust (i.e. Moho) at Jimmu guyot while at the same time honouring the characteristic gravimetric signature of the Emperor seamount edifices and their flanking moats. The Optimal Regional Separation (ORS) method is used to isolate the flexural loads, while seismic tomography and different velocity/density relations are explored for assigning suitable load and infill densities that vary spatially, and we search for optimal density and elastic parameters which minimize the misfit to both the residual gravity as well as the seismically observed flexure in the vicinity of Jimmu guyot. The first-order result is a clear thinning of the elastic thickness as we move from south to north: the implications of which we examine here for the tectonic evolution of the northwest Pacific Ocean and the long-term (>10⁶ a) mechanical properties of oceanic lithosphere.