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 Hawaii Molokai Oahu

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. The elastic plate (flexure) model T e = 28 km Corrections applied to reflectors: Seafloor age Swell height FZ crustal structure Linear elasticity Inviscid substrate ρ load = 2800 kg m -3 ρ infill = 2600 kg m -3 ρ mantle = 3330 kg m -3 E = 10 12 Pa ν = 0.25 Hunter et al. (2014)

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

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 o C): b) High-temperature creep (>~800 o C):

8. Zhong & Watts (2013) 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 o C) plasticity (Mei et al 2010) n=3.5, E p =320 KJ/mol High temperature (>800 o C) creep (Hirth & Kohlstedt 1996). E c =360 KJ/mol Case R14 Seafloor age = 80 Ma Multilayered viscosity Loading history Byerlee friction law μ = 0.7 Low temperature (<800 o C) plasticity (Mei et al 2010) n=3.5, E p =320 KJ/mol High temperature (>800 o C) creep (Hirth & Kohlstedt 1996). E c =360 KJ/mol + 10 8 weakening

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

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

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

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.