Hawaiian hot-spot swell structure from seafloor MT sounding

Project Investigator

Doctor Graham Heinson

Project Collaborator

Steven Constable (Scripps Institution of Oceanography, USA)

Project Details

Two prominent features mark the passage of oceanic lithosphere over a hot-spot. The first is the initiation of oceanic volcanism leading to a chain of islands or seamounts. The second is the generation of a 1 km high, 1000 km wide bathymetric swell around the volcanic island chain. The origin of hotspot swells is still largely unknown. At least three different mechanisms have been proposed for swell generation:

1. thermal reheating (rejuvenation) of the lithosphere within a 1000 km region centered on the hotspot (Crough, 1978);

2. compositional underplating of depleted mantle residue from hotspot melting (Robinson, 1988; Phipps Morgan et al., 1995) and

3. dragging of hot plume asthenosphere by the overriding lithosphere (Sleep, 1990).

The primary reason for the multiplicity of theoretical models is that there are few geophysical constraints on the structure of the lithosphere and sub-lithosphere beneath a swell (Sleep, 1990). In this study seafloor magnetotelluric (MT) data was collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain (Fig. 1).

The Hawaiian swell is an excellent place to study the interactions of a mantle plume with oceanic upper lithosphere; large volumes of melt are being produced and the area is geographically isolated from coastlines, mid-ocean ridges, and subduction zones.

Results

All data is consistent with an electrical strike direction of 3008, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, dplumeT connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Vm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.

A seismic low velocity region (LVR) observed by Laske et al. (1999) at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume.

Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 ohm-m, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of (10)2 ohm-m (1450–1500 'C) within the seismic LVR increasing to an off-swell resistivity of >(10)3 ohm-m (<1300 'C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR (Fig. 2). This suggests a 'hot, dry lithosphere' thermal model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry.

Publications

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Constable, S.C., Heinson, G.S. 2004. 'Hawaiian hot-spot swell structure from seafloor MT sounding'. Tectnophysics. vol 389. pp 111- 124.