Crustal structure, flexure, and subsidence history of the Hawaiian Islands

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Abstract
Seismic reflection profile data have been used to estimate the long‐term (>106 years) mechanical properties of the oceanic lithosphere underlying the Hawaiian Islands. The data show prominent reflectors associated with the top of the oceanic crust (reflector 2), the M discontinuity, and the base of the crust (reflector 3). The two‐way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three‐dimensional elastic plate models. A good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere Te of 40 km. The main discrepancies with the refraction data occur beneath Oahu and Molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of Hawaii where it is too deep. We attribute the thickened crust beneath Oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of Hawaii to crustal melting beneath the youngest part of the chain. The effective Te of 40 km is higher than expected for 80 m.y. oceanic lithosphere, possibly due to the effects of subsurface loads that act at or near the base of the flexed crust. According to free‐air gravity anomaly and bathymetry data, Te does not change significantly along the island chain, retaining a similar value for Hawaii as it does for Oahu. We have used this result to estimate the pattern of subsidence and uplift that would be expected along the Hawaiian Islands because of flexural loading of Hawaii. The observed depth of the Haleakala wave‐cut terrace northeast of Maui can be generally explained by the flexure model. The main discrepancies between observed and the predicted depths occur northwest of Hawaii where they suggest a somewhat higher value of Te, possibly due to incomplete adjustment of the lithosphere. We suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle “hotspot” is accommodated by progressive flexing “downstream,” causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge. The progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the Hawaiian Islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat. A progressive flexure model may also apply to the case of a continental plate moving over a mantle “hotspot” and could explain the observations of (1) a “clinoform” pattern of stratigraphy in some flood basalt provinces and (2) a seaward dipping reflector sequence at some passive continental margins.