Dynamic Compensation in the Central Pacific Ocean 4 Juan

ABsTRAcr Thc intermediate-wavelength geoid (A #)oo km) and sea-fbor topography fields in the central Pacific Ocean have been studied in terms of static and dynamic compensation models. Topographic fcatures on the sea-floor with As lo00 km haw been found to be wmpcnsatcd both regionally, by thc elastic strength of the lithosphere, and locally, by displacing mantle material to reach isostatic adjustment. The larger-scale sea-floor topography and the corresponding gcoid anomalies with X uloo km cannot be explained by either local or regional .compensation. The topography and thc resulting geoid anomaly at this wavelength have becn modeled by considering the dynamic eflccts arising from Viscous stresses in a layer of fluid with a highly temperature-dependent viscosity for thc cases of i) surface cooling, and ii) basal heating In this model, the mechanical properties of the elastic part of the lithosphere have been taken into account by considering an activation energy of about 52fl kJ/mol in the Arrhenius law for the viscosity.~ume&al’predictions of the topography, total gcoid anomaly, and admittance have been obtained, and the results show that the thermal perturbation in thc layer, which accounts for the m a s dcficit, must bc located dose to the surface to compensate thc gravitational effea of the surface deformation. For the case of basal heat-ing, the temperature dependence of viscosity results in a separation of the upper, quasi-rigid lid from the lower mobile fluid, hen= inhibiting the development of a compensating thermal perturbation at shallow depths. The results dearly d e out smaII-scale, upper-mantle c o n d o n as the source of these anomalies. Instcad, the gcophysical obsenrables can be well expl&d by a shallow, transient thermal perturbation.