Extraction of a lithospheric cooling signal from oceanwide geoid data John DeLaughter a , Seth Stein b; *, Carol A. Stein c a Chevron Petroleum Technology Company, San Ramon, CA 94583, USA b Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA c Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607-7059, USA Received 4 January 1999; received in revised form 16 August 1999; accepted 8 October 1999 Abstract The geoid, an equipotential surface of Earth's gravity field, reflects the distribution of mass within the planet and hence a variety of geodynamic processes. Because these data are dominated by sublithospheric processes, notably mantle convection, they have not played a major role in the ongoing debate concerning models of the thermal evolution of oceanic lithosphere. Application of spatial filtering to the age derivative of the geoid, however, extracts an age- dependent signal which reflects lithospheric thermal evolution. The data are much better fit by a thin (about 100 km thick) thermal plate than by a cooling halfspace, and so provide a valuable constraint complementary to and consistent with the variations in oceanic depth and heat flow with age. ß 1999 Elsevier Science B.V. All rights reserved. Keywords: geoid; lithosphere; thermal history; plate tectonics 1. Introduction A remarkable feature of Earth's gravity ¢eld is that its major features bear little resemblance to the near-surface features of mantle convection ex- pressed at the surface by plate tectonics, the pri- mary mode of heat transfer from Earth's interior [1], which makes Earth's tectonics so di¡erent from our neighboring planets [2]. This di¤culty is shown by comparison of Fig. 1A, a global map of lithospheric ages [3], to Fig. 1B, showing geoid anomalies, the deviation of the sealevel equipotential surface of gravity from the ellipsoi- dal shape of a rotating Earth [4]. The geoid anomalies are dominated by long wavelength fea- tures [5] interpreted as largely re£ecting deep mantle convection [6,7]. Although some e¡ects of subducting slabs are visible, there is little cor- respondence between the distribution of litho- spheric age and the geoid. Hence oceanic spread- ing centers and the resulting cooling of oceanic lithosphere as it moves toward subduction zones are not obvious in the geoid, although this cool- ing is evident from the increase in ocean depths and decrease in sea£oor heat £ow with lithospher- ic age [8^11]. In theory, cooling of the lithosphere should cause the geoid to vary with age, as it re£ects the integral of the thermal density anomaly 0012-821X / 99 / $ ^ see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII:S0012-821X(99)00247-2 * Corresponding author. Tel.: +1-847-491-5265; Fax: +1-847-491-8060; E-mail: seth@earth.nwu.edu Earth and Planetary Science Letters 174 (1999) 173^181 www.elsevier.com/locate/epsl