Thermal structure of continental upper mantle inferred from S-wave velocity and surface heat flow

Results from seismic tomography provide information on the thermal structur e of the continental upper mantle. This is borne out by the good agreement between tectonic age, surface heat flow and a tomographic S-wave velocity m odel for depths less than 180 km. The velocity anomalies of tomographic lay ers deeper than 230 km have relatively small amplitudes and show little cor relation with surface heat flow or shallow velocities. We associate the dro p in correlation and amplitude of the velocity perturbations between 180 an d 230 km depth with the maximum thickness of the thermal boundary layer (TB L), in which larger variations in temperature and possibly composition than in the underlying convecting mantle can be sustained. Velocity profiles fo r different tectonic provinces are converted to temperature using mineralog ical data. Both anharmonic and anelastic effects on the wave speeds are tak en into account. The resulting geotherms differ most at depths of 60-120 km with variations of up to 900 degrees C. Below 230 km, differences do not e xceed 300 degrees C. These geotherms agree well with one-dimensional conduc tive geotherms for the observed range of continental heat flow values using the empirical relationship that 40% of the surface heat flux stems from up per crustal radiogenic heat production. The S-wave velocity in the continen tal upper mantle appears to be adequately explained (within the uncertainti es of the tomography and the conversion to temperature) by a thermal signat ure. A compositional component can, however, not be ruled out as it may hav e only a minor effect on the velocity and the heat flow. The surface heat f low is controlled by the shallow heat production and the thickness of the T BL. Seismology helps to determine the relative importance of the two factor s and our results confirm the similar importance of both factors. Variation s of TBL thickness could be controlled by compositional differences and/or by the effect of temperature on the rheology. (C) 2000 Elsevier Science B.V . All rights reserved.