Hawaiian Hot-spot Swell Structure from Seafloor MT Sounding

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the HawaiianEmperor Island chain. All data are consistent with an electrical strike direction of 300◦, 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, ‘plume’ 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 Ωm, 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 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 100 Ωm (1450–1500◦C) within the seismic LVR increasing to an off-swell resistivity of >1000 Ωm (<1300◦C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ 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 measurments is in quantitative agreement with the swell bathymetry. Introduction 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 hot-spot swells is still largely unknown. At least three different mechanisms have been proposed for swell generation; (i) thermal reheating (rejuvenation) of the lithosphere within a 1000 km region centered on the hotspot (Cough, 1979; Detrick and Crough, 1978); (ii) compositional underplating of depleted mantle residue from hotspot melting (Robinson, 1988; Phipps Morgan et al., 1995); and (iii) 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). Constraints from both global and regional seismic studies are poor, since most current global models cannot reliably resolve features of diameters less than 500 km. The Hawaiian swell is an excellent place to study the interactions of a mantle plume with oceanic upper lithosphere due to the very high volumes of melt produced and geographic isolation from coastlines, mid-ocean ridges and subduction zones. The Hawaiian islands are almost in the center of the Pacific plate and are surrounded by lithosphere of 90-110 Ma age moving at a velocity of 83 mm.yr−1 (Gordon and Jurdy, 1986). In 1997, the SWELL (Seismic Wave Exploration of the Lower Lithosphere beneath the Hawaiian Swell) experiment took place, a pilot study to deploy long period hydrophones across the Hawaiian swell (Laske et al., 1999). We were able to piggyback the cruise and collect data from seven marine magnetotelluric (MT) instruments deployed in coordination with the seismic hydrophones between April and December 1997 from the RV Moana Wave. Figure 1 shows the swell (bounded by the 5000 m depth contour) and locations of MT sites (listed in Table 1). Location of sites was primarily chosen for seismic surface-wave analyses, and hence a hexagonal array with a central instrument at the ODP Borehole 843B was used (Laske et al., 1999). However, MT sites were positioned so that four MT responses were obtained above the swell (<5000 m), and three in the deeper ocean to the south of the swell, with site spacing of the order of 250 km. All instruments were successfully recovered with data. Table 1: Instrument positions and depths. Instrument Latitude Longitude Depth Data Opus 19◦ 50.98 158◦ 05.43 4350 m E/B Noddy 19◦ 05.42 157◦ 21.53 4570 m E/B Lolita 17◦ 33.60 157◦ 15.60 4760 m B Kermit 19◦ 03.60 160◦ 26.71 4950 m E/B Rhonda 17◦ 26.98 159◦ 20.40 5290 m E/B Trevor 16◦ 33.63 159◦ 46.80 5640 m E/B Ulysses 15◦ 34.88 160◦ 57.05 5620 m E/B