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

By T.A. Minshull, O. Ishizuka and D. Garcia-Castellanos


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

Topics: GC
Year: 2010
OAI identifier:
Provided by: e-Prints Soton

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