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Lithospheric flexure at the Hawaiian Islands and its implications for mantle rheology

Lithospheric flexure at the Hawaiian Islands and its implications for mantle rheology. Perspective view (to the NW) of the satellite-derived free-air gravity anomaly field along the Hawaiian Islands S. J. Zhong (University of Colorado, Boulder, USA) A. B. Watts (University of Oxford, UK).

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Lithospheric flexure at the Hawaiian Islands and its implications for mantle rheology

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  1. Lithospheric flexure at the Hawaiian Islands and its implications for mantle rheology Perspective view (to the NW) of the satellite-derived free-air gravity anomaly field along the Hawaiian Islands S. J. Zhong (University of Colorado, Boulder, USA) A. B. Watts (University of Oxford, UK) Kauai Oahu Molokai Hawaii

  2. The Hawaiian Islands Two-Ship Seismic Experiment Watts et al. (1985)

  3. Depth converted seismic reflection profile data and flexure along the Kauai/Molokai transects Watts & ten Brink (1989)

  4. Linear elasticity Inviscid substrate ρload = 2800 kg m-3 ρinfill = 2600 kg m-3 ρmantle = 3330 kg m-3 E = 1012 Pa ν = 0 .25 The elastic plate (flexure) model • Corrections applied • to reflectors: • Seafloor age • Swell height • FZ crustal structure Te = 28 km Hunter et al. (2014)

  5. The behavior of oceanic lithosphere on seismic to geologic time-scales Oahu Load age = 3-4 Ma Seafloor age = 80 Ma Seamounts and Oceanic Islands Deep-sea trench – Outer Rise (Corrected for yielding) There is a competition between thermal cooling which strengthens the lithosphere and a stress-induced relaxation which weakens the lithosphere Te~1/3 Tse

  6. Viscoelastic plate models and the time scales of isostatic adjustment n = 1 (i.e. diffusion creep) Karato & Wu (2003) Fast isostasy Slow isostasy Watts & Zhong (2001)

  7. Rheology of oceanic lithosphere: A laboratory perspective • Frictional sliding (shallow depths) or Byerlee’s law: • Semi-brittle (transitional regime, poorly understood) • Plastic flow: • a) Low-temperature plasticity (<~800 oC): • b) High-temperature creep (>~800 oC):

  8. Viscoelastic plate flexure and laboratory-based mantle rheology Case R1 Seafloor age = 80 Ma Multilayered viscosity Loading history Byerlee friction law μ = 0.7 Low temperature (<800 oC) plasticity (Mei et al 2010) n=3.5, Ep=320 KJ/mol High temperature (>800 oC) creep (Hirth & Kohlstedt 1996). Ec=360 KJ/mol Case R14 Seafloor age = 80 Ma Multilayered viscosity Loading history Byerlee friction law μ = 0.7 Low temperature (<800 oC) plasticity (Mei et al 2010) n=3.5, Ep=320 KJ/mol High temperature (>800 oC) creep (Hirth & Kohlstedt 1996). Ec=360 KJ/mol + 108 weakening Zhong & Watts (2013)

  9. Misfit between observed and calculated plate flexure Zhong & Watts (2013)

  10. Hawaii Stress, viscosity and strain rate Zhong & Watts (2013)

  11. Seismicity – PLUME + HVO (2005-2007) Oahu Molokai Hawaii Wolfe et al. (2004)

  12. Progressive flexural loading and the timescales of isostatic adjustment

  13. Conclusions • Seismic and earthquake data at the Hawaiian Islands have been used, together with viscoelastic plate modelling, to constrain laboratory-based rheological laws. • Flow laws such as Mei et al. that combine low and high temperature creep, fit the seismic data, but require significant weakening. • A weakened Mei et al. predicts a ~30 km thick double seismic zone and a maximum bending stress of ~100-200 MPa. • Earthquake data are most sensitive to frictional laws (e.g. Byerlee) and suggest coefficients in the range of 0.25-0.70. • Models that incorporate the best fit rheological laws suggest the subsidence induced by volcano loads is more than ~30% achieved ~350 ka after loading and is essentially complete by ~750 ka.

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