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

MolokaiOahu

The Hawaiian Islands Two-Ship Seismic Experiment

Watts et al. (1985)

Depth converted seismic reflection profile data and flexure along the Kauai/Molokai transects

Watts & ten Brink (1989)

The elastic plate (flexure) model

Te = 28 km

Corrections applied to reflectors:

•Seafloor age•Swell height

•FZ crustal structure

Linear elasticityInviscid substrate

ρload = 2800 kg m-3

ρinfill = 2600 kg m-3 ρmantle = 3330 kg m-3

E = 1012 Paν = 0 .25

Hunter et al. (2014)

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

Te ~1/3 Tse

OahuLoad age = 3-4 Ma

Seafloor age = 80 Ma

Deep-sea trench – Outer Rise (Corrected for yielding)

Seamounts and OceanicIslands

Viscoelastic plate models and the time scales of isostatic adjustment

n = 1 (i.e. diffusion creep)

ε.

=τ s

μ

⎛⎝⎜

⎞⎠⎟

n

Aexp −Qp / RT( )

Karato & Wu (2003)

Fast isostasy

Slow isostasy

Watts & Zhong (2001)

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):

τ yield = c0 + μρgh

ε.

= Aσ n exp −Ep

RT1−

σ

σ p

⎛

⎝⎜

⎞

⎠⎟

p⎛

⎝⎜⎜

⎞

⎠⎟⎟

q⎧

⎨⎪

⎩⎪

⎫

⎬⎪

⎭⎪

ε.

= Aσ n exp −Ec

RT⎧⎨⎩

⎫⎬⎭

Zhong & Watts (2013)

Viscoelastic plate flexure and laboratory-based mantle rheology

Case R1Seafloor age = 80 MaMultilayered viscosity

Loading history

Byerlee friction lawμ = 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 R14Seafloor age = 80 MaMultilayered viscosity

Loading history

Byerlee friction lawμ = 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)

Misfit between observed and calculated plate flexure

Stress, viscosity and strain rate

Zhong & Watts (2013)

Hawaii

Seismicity – PLUME + HVO (2005-2007)

Wolfe et al. (2004)

Molokai

Oahu

Hawaii

Progressive flexural loading and the timescales of isostatic adjustment

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.