HR: 09:20h
AN: V11B-06    [PDF]
TI: Vertical Motions and Lithosphere Rheology at Ascension Island
AU: * Minshull, T A
EM: tmin@soc.soton.ac.uk
AF: Southampton Oceanography Centre, SOES, European Way, Southampton, SO14 3ZH United Kingdom
AU: Ishizuka, O
EM: o-ishizuka@aist.go.jp
AF: Institute of Geoscience, Geological Survey of Japan/AIST Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: Mitchell, N C
EM: neil@ocean.cf.ac.uk
AF: School of Earth, Ocean and Planetary Sciences, Cardiff University, PO Box 914, Cardiff, CF10 3YE United Kingdom
AU: Evangelidis, C
EM: ce1@soc.soton.ac.uk
AF: Southampton Oceanography Centre, SOES, European Way, Southampton, SO14 3ZH United Kingdom
AB: Ascension Island is a 4 km volcano built on 7 Ma lithosphere 90 km west of the Mid-Atlantic Ridge. The island's western shelf includes terraces to 400 m depth, which are difficult to correlate unambiguously with glaciogenic sealevel fluctuations but nevertheless suggest significant subsidence over the past few hundred ka. A geothermal exploration borehole drilled in 1986 penetrated 3 km into the volcanic edifice, and cuttings where archived for each 3 m interval. Despite strong alteration down most of the hole, we found several samples from a range of depths that were reasonably fresh. Glass-free fresh groundmass of aphyric lavas were dated by the laser-heating 40Ar/39Ar dating technique. Stepwise-heating analysis on a several mg aliquot of each sample returned plateau ages with a 1$\sigma$ error of 3-4%. Although published ages of surface samples from the island are all 1.5 Ma or less, the borehole samples yield ages of up to 3.6 Ma. Volcanic growth rates in the hole are $\sim 0.4$ km/Myr since 2.5 Ma and $\sim 1.0$ km/Myr between 2.5 and 3.6 Ma. The transition from submarine to subaerial eruption occurs at $\sim 900$ m depth in the hole and $\sim 720$ m below present sea level, and at an age of 2.5 Ma. Hence since 2.5 Ma, there has been $\sim 530$ m of subsidence over and above the expected $\sim 190$ m due to lithospheric cooling. Although this anomalous subsidence is consistent with Airy isostasy, gravity and seismic tomographic data indicate that the lithosphere has had significant flexural strength during the growth of the island. Plausible elastic thicknesses and volcanic growth histories would generate a maximum elastic subsidence since 2.5 Ma of $\sim 200$ m. We infer that the subsidence since 2.5 Ma includes a component of viscoelastic relaxation resulting from the period of more rapid load emplacement prior to 2.5 Ma, and use our data to place constraints on the time constant of this relaxation.
DE: 3025 Marine seismics (0935)
DE: 7220 Oceanic crust
DE: 8159 Rheology--crust and lithosphere
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting