Constraints on thermal and mechanical structure of the oceanic lithosphere at the Bermuda Rise from geoid height and depth anomalies

Abstract

The Bermuda Rise is a broad area of anomalously shallow seafloor presumed to be of thermal origin dating to the middle Eocene. This and other midplate swells have been modeled both by elevated temperature in a convecting layer at the base of the plate and as thermal expansion within the conducting portion of the lithosphere. Using recently developed techniques for the estimation of lithospheric flexure and temperature structure from geoid, bathymetry, and heat flow data, we place constraints on the depth of the thermal load and limit the possible thermal mechanisms causing this swell. Our data include depth anomalies calculated from DBDB5C bathymetry data, geoid anomalies using data from both Seasat and GEOS-3 altimeter missions, and published heat flow values. The first technique we employ to determine the thermo-mechanical properties of the swell is a forward filtering technique with which we determine flexural rigidity and compensation depth by successive fits to topography filtered to separate subsurface and surface loading. The second method is an admittance technique, with which we directly determine the geoid to topography ratios corresponding to the volcano and the swell. Both linear filtering and admittance approaches yield similar conclusions concerning the elastic plate thickness and compensation depth of the Bermuda Rise, indicating that the effective elastic thickness for the plate supporting the volcanoes is 30 ± 5 km and the compensation depth for the swell is 55 ± 10 km. Finally, we use the linear programming technique to obtain extremal bounds on temperature as a function of depth and time in the lithosphere. We find that the depth and geoid anomalies, heat flow, and elastic thickness observations are inconsistent with a mechanism for formation of the Bermuda Rise which consists solely of thermal anomalies confined to depths greater than 50 km at the initiation of hotspot activity. If we use thermal rejuvenation alone to model the Bermuda Rise, the observations require that it be accompanied by substantial reheating to shallow depth in the lithosphere. On the basis of the data presented in this paper we cannot distinguish between a reheating model and a convection model for the origin of the Bermuda Rise though based on our geothermal modeling we preclude convective support models that do not change temperatures within the thermal plate.