Spots yes, hot barely or not

1Seth Stein & 2Carol Stein

1Dept. of Geological Sciences, Northwestern University, Evanston, IL 60208,

2Dept. of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607-7059,

A change in ideas about the origin of midplate hotspot swells has been the recognition that the elevated topography does not reflect significant excess heating of the shallow portions of the lithosphere. Initial models of individual swells like that at Hawaii [Crough, 1978] and and broad South Pacific superswell [McNutt & Judge, 1990] implied that the lithosphere was thinned dramatically, such that temperatures within its upper parts were raised by hundreds of degrees. Although analyses of heat flow data were interpreted as showing the expected elevated heat flow, subsequent analysis showed that the assumed-high swell heat flow was at most only slightly higher than that of oceanic lithosphere of comparable age elsewhere [Stein & Stein, AGU Geophysical Monograph 77, 1993]. This analysis, which has been confirmed by subsequent studies, precludes the possibility that swell lithosphere at shallow depths is significantly hotter than elsewhere for comparable age, as assumed in the thinning and reheating models, in accord with the observation that neither the maximum depth of earthquakes at Hawaii [Wiens & Stein, 1983] or the velocity structure along the Hawaiian swell shown by surface wave dispersion [Woods & Okal, 1996] differ from those observed for lithosphere of comparable age elsewhere. Although the simplest interpretation of these data is that lithospheric temperatures at swells have not been elevated significantly, because these data types are sensitive to temperatures in the upper lithosphere, they are consistent with models which predict some heating of the lower lithosphere so long as it occurs at depths great enough that the additional heat has not had time to be conducted upward and raise temperatures in the shallow lithosphere.