Long‐term growth and subsidence of Ascension Island: Constraints on the rheology of young oceanic lithosphere

Abstract

The dating of material from deep boreholes drilled in volcanic ocean islands allows constraints to be placed on their growth and long‐term subsidence rates. We dated lavas from a 3 km geothermal borehole at Ascension Island by the laser‐heating 40Ar/39Ar technique. The samples yield ages of up to 3.4 Ma and volcanic growth rates of ∼0.4 km/Myr. The transition from submarine to subaerial eruption occurs at ∼710 m below present sea level and 2.5 Ma. Since 2.5 Ma, there has been ∼430–500 m of subsidence over and above the expected ∼190–260 m due to lithospheric cooling. Plausible elastic thicknesses and growth histories would generate a maximum elastic subsidence since 2.5 Ma of ∼200 m. We infer that the subsidence includes a component of viscous relaxation resulting from rapid loading prior to 2.5 Ma, and place constraints on the timescale of this relaxation, and hence the viscosity of the underlying lithosphere.