Lateral variations in upper mantle thermal structure inferred from three‐dimensional seismic inversion models

Global shear wave velocity variations in the upper mantle (Woodhouse and Dziewonski, 1984) are employed as a basis for inferring lateral heterogeneities in temperature or composition at a depth of 150 km. Using experimentally determined elastic moduli for the olivine solid solution and Hashin‐Shtrikman bound theory, V_s and its partial derivatives with respect to temperature or composition (iron content) are calculated for the depth of interest. Based on these results, the observed seismic V_s variations are interpreted as temperature and compositional deviations relative to a spherically symmetric reference model. If it is assumed that the V_s variations primarily represent lateral temperature changes, then the present results can be used to construct a global surface heat flow map from the geotherm families of Pollack and Chapman (1977). Our inferred heat flow patterns, calculated independently of actual heat flow data, show clear agreement with the surface heat flow maps proposed by Chapman and Pollack (1975) and Chapman (1985). This result is consistent with and supports thermal structure as the primary governing factor in the generation of upper mantle lateral velocity variations. Compositional changes appear to correlate with temperature, but affect velocity structure to a lesser extent.