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The Hawaiian SWELL Pilot Experiment—Evidence for Lithosphere Rejuvenation from Ocean Bottom Surface Wave Data

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The Hawaiian SWELL Pilot Experiment—Evidence for Lithosphere Rejuvenation from Ocean Bottom Surface Wave Data The Geological Society of America Special Paper 430 2007 The Hawaiian SWELL pilot experiment—Evidence for lithosphere rejuvenation from ocean bottom surface wave data Gabi Laske Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0225, USA Jason Phipps Morgan Department of Earth & Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York, 14853-1504 John A. Orcutt Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0225, USA ABSTRACT During the roughly year-long Seismic Wave Exploration in the Lower Lithosphere (SWELL) pilot experiment in 1997/1998, eight ocean bottom instruments deployed to the southwest of the Hawaiian Islands recorded teleseismic Rayleigh waves with peri- ods between 15 and 70 s. Such data are capable of resolving structural variations in the oceanic lithosphere and upper asthenosphere and therefore help understand the mechanism that supports the Hawaiian Swell relief. The pilot experiment was a tech- nical as well as a scientific feasibility study and consisted of a hexagonal array of Scripps Low-Cost Hardware for Earth Applications and Physical Oceanography (L-CHEAPO) instruments using differential pressure sensors. The analysis of eighty- four earthquakes provided numerous high-precision phase velocity curves over an un- precedentedly wide period range. We find a rather uniform (unaltered) lid at the top of the lithosphere that is underlain by a strongly heterogeneous lower lithosphere and upper asthenosphere. Strong slow anomalies appear within ~300 km of the island chain and indicate that the lithosphere has most likely been altered by the same process that causes the Hawaiian volcanism. The anomalies increase with depth and reach well into the asthenosphere, suggesting a sublithospheric dynamic source for the swell re- lief. The imaged velocity variations are consistent with thermal rejuvenation, but our array does not appear to have covered the melt-generating region of the Hawaiian hotspot. Keywords: lithosphere, surface waves, dispersion, seusmic tomography, Hawaii hotspot *E-mail: [email protected]. Laske, G., Phipps Morgan, J., and Orcutt, J.A., 2007, The Hawaiian SWELL pilot experiment—Evidence for lithosphere rejuvenation from ocean bottom surface wave data, in Foulger, G.R., and Jurdy, D.M., eds., Plates, plumes, and planetary processes: Geological Society of America Special Paper 430, p. 209–233, doi: 10.1130/2007.2430(11). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved. 209 210 Laske et al. INTRODUCTION Possible Causes for Swell Relief The Hawaiian hotspot and its island chain are thought to be In the thermal rejuvenation model, the lithosphere reheats the textbook example of a hotspot located over a deep-rooted and thins when a plate moves over a hotspot (Fig. 1A). It ex- mantle plume (Wilson, 1963; Morgan, 1971). Because plume plains the uplift of the seafloor and the age-dependent subsi- material is expected to ascend in a much more viscous sur- dence of seamounts along the Hawaiian Island chain (Crough, rounding mantle, it is expected to stagnate near the top and ex- 1978; Detrick and Crough, 1978). This model was reported to hibit a sizable plume head that eventually leads to the uplift of be consistent with gravity and geoid anomalies, and observa- the overlying seafloor (e.g., Olson, 1990). A hotspot on a sta- tions suggest a compensation depth of only 40–90 km (instead tionary plate may then develop a dome-shaped swell (e.g., Cape of the 120 km for 90-Ma-old lithosphere). Initially, rapid heat- Verde), whereas a plate moving above a plume would shear it ing of the lower lithosphere (40–50 km) within 5 m.y. and sub- and drag some of its material downstream, creating an elongated sequent cooling appeared broadly consistent with heatflow data swell (Olson, 1990; Sleep, 1990). Hawaii’s isolated location along the swell (von Herzen et al., 1982), though Detrick and within a plate, away from plate boundaries, should give scien- Crough (1978) had recognized that the reheating model does not tists the opportunity to test most basic hypotheses on plume- offer a mechanism for the rapid heating. The heatflow argument plate interaction and related volcanism. Yet the lack of many was later revised when no significant anomaly was found across crucial geophysical data has recently revived the discussions on the swell southeast of Midway (von Herzen et al., 1989), though whether even the Hawaiian hotspot volcanism is related to a the interpretation of those data is still subject of debate (M. deep-seated mantle plume or is rather an expression of propa- McNutt, personal commun., 2002). The thermal rejuvenation gating cracks in the lithosphere (Natland and Winterer, 2005). model has received extensive criticism from geodynamicists, as Similarly, the dominant cause of the Hawaiian Swell relief has it is unable to explain the rapid initial heat loss by conduction not yet been conclusively determined. At least four mechanisms alone, and modeling attempts fail to erode the lithosphere sig- have been proposed (see, e.g., Phipps Morgan et al., 1995; Fig. nificantly if heating were the only mechanism involved (e.g., 1)—thermal rejuvenation, dynamic support, compositional Ribe and Christensen, 1994; Moore et al., 1998). The dynamic buoyancy, and propagating crack—but none of them is univer- support model is a result of early efforts to reconcile gravity and sally accepted as the single dominant mechanism. All these bathymetry observations of the Hawaiian Swell (Watts, 1976). mechanisms create a buoyant lithosphere and so can explain the Ponding, or pancaking, of ascending hot asthenosphere causes bathymetric anomalies, but they have distinct geophysical re- an unaltered lithosphere to rise. A moving Pacific plate shears sponses, and each model currently appears to be inconsistent the ponding mantle material and drags it along the island chain, with at least one observable. thereby causing the elongated Hawaiian Swell (Olson, 1990; Mechanisms Causing Swell Relief A Thermal Rejuventation and Lithosphere Thinning C Compositional Buoyancy Plate Motion slow normal normal seismically slow Lithosphere and thinned lithosphere Lithosphere unaltered lithosphere faster slow asthenosphere Figure 1. Concept figure for four possi- slower slow asthenosphere Asthenosphere Asthenosphere ble mechanisms for hotspot swells. Ther- slow slower slow + faster "residue" mal rejuvenation and the propagating Plate Motion crack model predict a significant impact Heat Source on the lithosphere but the latter is asso- Heat Source ciated with normal asthenosphere. The B Dynamic Support D Propagating Crack dynamic support and the compositional buoyancy model have an unaltered litho- Plate Motion swell? sphere but an anomalous asthenosphere. Plate Motion normal slow seismically slow Lithosphere unaltered lithosphere Lithosphere normal and thinned lithosphere slower normal asthenosphere slow asthenosphere Asthenosphere Asthenosphere slow slower slow + slower "pond" Heat Source The Hawaiian SWELL pilot experiment 211 Sleep, 1990). The compensation depth for this model remains at 2002; Schutt and Humphreys, 2004), but similar studies at Hawaii 120 km depth. An unaltered lithosphere is, however, inconsis- are extremely limited because of the nearly linear alignment of tent with the heatflow data along the swell (von Herzen et al., the islands (e.g., Wolfe et al., 2002). Such studies usually also do 1989) and the geoid. A recent hybrid model—dynamic thinning not have the resolution within the lithosphere and shallow as- —in which secondary convection in the ponding asthenosphere thenosphere to distinguish between the four models proposed for erodes the lithosphere downstream (Ribe, 2004), appears to find the swell uplift, but surface waves studies do. The reheating and support in a recent seismic study (Li et al., 2004). The third the propagating crack models cause low seismic velocities in the model, compositional buoyancy, was suggested by Jordan (1979) lower lithosphere, whereas normal velocities would be found for and is based on the idea that the extraction of melt by basaltic the dynamical support model (Fig. 1). The compositional buoy- volcanism leaves behind a buoyant, low-density mantle residue ancy model predicts high velocities, which are claimed to have (see also Robinson, 1988). Of the models described here, this is been found by Katzman et al. (1998) near the end of a corridor the only one that predicts high seismic velocities in the litho- between Fiji/Tonga and Hawaii. We would be able to distinguish sphere. At this point, it is unclear whether a fourth model of between the reheating and the propagating crack models, as the propagating cracks in the lithosphere could produce enough latter leaves seismic velocities in the asthenosphere unchanged, buoyant material for a swell (J. Winterer, personal commun., whereas a plume would lower the velocities in the reheating 2007). A cracking lithosphere would most likely have the seis- model. Surface wave studies along the Hawaiian Islands have mic signature of some degree of rejuventation, but the as- found no evidence for lithospheric thinning (Woods et al., 1991; thenosphere below should be normal (Fig. 1D). Woods and Okal, 1996; Priestley
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