Thermal Evolution of the Lithosphere

Seth Stein (Northwestern University) and I are analyzing the variations in seafloor depth and heat flow with age that provide the primary constraints on the thermal structure and evolution of the oceanic lithosphere. Moreover, regions of midplate volcanism and swells are identified by shallow seafloor depths. In turn, investigation of the processes giving rise to these regions rely on assessments of how the depths, heat flow, and flexural properties differ from those for unperturbed lithosphere. Such comparisons have been inhibited because reference thermal models assumed to describe unperturbed lithosphere predict deeper depths and lower heat flow than typically observed for lithosphere older than 70 Ma. As a result, depth and heat flow anomalies can be significantly overestimated. To address this difficulty, we derive a new model, GDH1, by joint fitting of heat flow and bathymetry, which implies that the lithosphere is hotter at depth and thinner than previously assumed. GDH1 fits the data, including that data from older (& >70 Ma) lithosphere previously treated as anomalous, significantly better than previous models. GDH1 thus facilitates analysis of processes including midplate volcanism and swells, differences in regional subsidence, and hydrothermal circulation near spreading centers.

The new model reduces inferred anomalies at swells like Hawaii, implying little or no reheating of the lithosphere. Hence it is important for the ongoing debate about whether hotspot swells are due to plumes rising from deep the the mantle, or are instead due to shallow process. This leads naturally to consideration of "Superswells", such as the Darwin Rise region shown, where multiple hotspot tracks may indicate that in the Cretaceous (pre-70 Myr) an unusual outpouring of mantle heat produced a broad upwelling. Depth anomalies relative to different reference models yield quite different maps, and hence tectonic inferences. The entire Rise is shallow relative to a halfspace. Relative to PSM, much of the area is also shallow, suggesting a remnant regional thermal signature of the volcanism that formed the swells. However, because almost all lithosphere of this age is shallower than these models predict, the anomalies need not indicate that the Rise presently differs from lithosphere of this age elsewhere. In contrast, relative to GDH1, swells associated with volcanic chains are shallow, whereas depths between them are within a standard deviation of that predicted. Because of the three models GDH1 best describes average old lithosphere, it indicates that much of the Darwin Rise is not significantly deeper than lithosphere of the same age elsewhere, implying that the region between the swells retains no significant large-scale thermal signature of the Cretaceous events.

These studies have been extended, in joint research with Phil Richardson (Richardson et al., 1995) and John Delaughter (DeLaughter et al., 1999; 2005), using the earth's geoid measured from satellite altimetry. The geoid is very valuable for these studies, because it gives a constraint on the geotherm complementary to those given by bathymetry and heat flow, and useful for resolution of deep thermal structure.

References

DeLaughter, J., S. Stein, and C. Stein, Extraction of the lithospheric aging signal from satellite geoid data, Earth. Planet Sci. Lett., 174, 173-181, 1999. For pdf click here

DeLaughter, J., C. A. Stein, and S. Stein, Hotspots: a view from the swells, in Foulger, G. R., J. Natland, D. C. Presnall, and D. L. Anderson (eds.), Plates, plumes, and paradigms,Geol. Soc. Am. Sp. Paper 388, 257-278, doi: 10.1130/2005.2388(16), 2005. For pdf click here

Richardson, P., S. Stein, C. Stein, and M. Zuber, Geoid data and the thermal structure of oceanic lithosphere, Geophys. Res. Lett., 22, 1913-1916, 1995. For pdf click here

Stein, C. and S. Stein, A model for the global variation in oceanic depth and heat flow with lithospheric age, Nature, 359, 123-128, 1992. For pdf click here

Stein, C. and S. Stein, Constraints on Pacific midplate swells from global depth-age and heat flow-age models, in Pringle, M., W. Sager, W. Sliter, and S. Stein (eds), The Mesozoic Pacific, Geophysical Monograph 76, 53-76, American Geophysical Union, 1993. For pdf click here

Stein, S., and C. Stein, Ocean depths and the Lake Wobegone Effect, Science, 275, 1613-1614, 1997. For html click here