Long-term growth and subsidence of Ascension Island: constraints on the rheology of young oceanic lithosphere

Minshull, T.A., Ishizuka, O. and Garcia-Castellanos, D. (2010) Long-term growth and subsidence of Ascension Island: constraints on the rheology of young oceanic lithosphere. Geophysical Research Letters, 37 (23), L23306-[5pp]. (doi:10.1029/2010GL045112).

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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.

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