IN SITU RHEOLOGY OF THE OCEANIC LITHOSPHERE ALONG THE HAWAI‘IAN RIDGE

Abstract

The Hawaiian Ridge beyond the main islands is composed of volcanoes with a large range of sizes, all formed on lithosphere of about the same age. This provides an unusually favorable setting to use plate flexure observations to examine the rheology of the oceanic lithosphere under different load stresses and a nearly uniform age-dependent temperature structure. We carry out such a study using new and archived shipboard gravity and multibeam bathymetry, combined with satellite-derived gravity. Flexure is simulated using thin-plate theory with curvature-dependent flexural rigidity, the strength of which is limited by brittle failure in the shallow lithosphere and low-temperature plasticity in the lower lithosphere. Misfits between model and observed residual gravity anomaly are combined with prior information to obtain posterior probabilities for model parameters: the frictional coefficient for brittle failure μ_f, a pre-exponential factor F for modulating the strength of four flow laws for low-temperature plasticity, average topographic crustal density ρ_c, thermal age which controls the geotherm, and a regional horizontal compression perpendicular to the ridge axis. Results show that all of the low-temperature plasticity laws require substantial weakening (F= 102 – 108) when using a geotherm for the age of the surrounding seafloor (90 Myr). A relatively simple solution is that the Hawaiian hotspot has thermally weakened the lithosphere. Extreme thermal rejuvenation (40-50 Myr geotherm) is required for the dry low-temperature plasticity laws of Mei et al. [2010] and Boioli et al. [2015]. Moderate thermal rejuvenation (60-70 Myr geotherm) is required for Katayama & Karato’s [2008] wet rheology. The dry flow law of Kranjc et al. [2016] in conjunction with minimal thermal rejuvenation (70-80 Myr) has highest probability and is thus our favored model for the Hawaiian Ridge.