Pacific Plate gravity lineaments: Diffuse extension or thermal contraction?

[ 1] We detect strain very close to zero on the Pacific Plate relative to the Nazca Plate for Anomalies C8-C13. Our observations are in the region of the satellite gravity lineaments, 10degrees-40degreesS in the central Pacific. Strain was measured by comparing fracture zone spacing on the Pacific and Nazca Plates in an approach similar to that of Goodwillie and Parsons [ 1992] but with an improved data set which allowed us to place tighter tolerances on estimated strain. We used a standard finite rotation technique that rotates points of a given isochron and corresponding active transform fault from the Pacific Plate to the Nazca Plate. Our strain measurements discount the diffuse extension hypothesis for gravity lineament formation which requires at least 10% strain [Sandwell et al., 1995]. The best known hypothesis that small-scale convection rolls in the upper mantle aligned in the direction of plate motion produce the lineaments was rejected because of observations of seamount chains in the gravity lows and observations of lineaments in seafloor topography younger than 2-3 Ma. We propose a third hypothesis that the gravity lineaments are signatures of the bending stresses resulting from vertical variations in cooling rate. We interpret the lineament lows as areas of brittle failure and the highs as convex upward flexure due to greater contraction of the base of the plate. Association of the lows with volcanoes could result either from both being caused by cracks or by the load of volcanoes causing brittle failure. We explored the feasibility of this idea using the equations for thermal bending moment and vertical lithosphere displacement as presented by Turcotte [ 1974] and the equation for thermal stress over elastic thickness presented by Parmentier and Haxby [ 1986]. Results of this simple model show vertical displacement or convex upward flexure ranging from 80 m to 140 m with associated gravity anomalies of 6-10 mGal. These amplitudes are within a factor of 2 of topographic and free-air gravity anomaly amplitudes observed in the region of the gravity lineaments.