Apparent Discrepancy Between Observed and Calculated Mass Excess of the Subducted Slab Under the Central Andes

The Airy isostatic gravity anomaly for the Central Andes is of the order of 0 to 80 mgal. Seismic studies suggest that the Central Andes are in approximate Airy isostatic equilibrium and as a consequence the Airy isostatic gravity anomaly provides an estimate of the deep sub-crustal gravity anomaly derived from subduction and related process. A comparison of this observed mantle residual gravity anomaly with that predicted by simple thermal models of the subducted Nazca plate under the Central Andes shows that the modelled gravity anomaly from the subduction process appears to be substantially greater than that observed by approximately 125 mgal. Key uncertainties in the prediction of the gravity anomaly from a thermal model of the subducted slab arise from uncertainties in the thermal expansion coefficient, and subduction geometry and dip. Sensitivity tests to model parameters show that the discrepancy between observed and modelled gravity anomaly may be as great as 300 mgal or as little as 40 mgal. This discrepancy between observed and modelled mantle gravity anomaly beneath the Central Andes may be resolved by the inclusion of other subduction related mass anomalies, in addition to that of the cold descending Nazca plate, as suggested by geophysical observations and thermo-dynamic considerations. A density model additionally incorporating the mineral phase changes within the slab at 410- and 660-km, petrological transformations of the subducting oceanic crust, and a hot asthenospheric wedge beneath the Andean crust due to local upwelling of the lithosphere-asthenosphere boundary, is able to generate a gravity anomaly that fits the observed Airy isostatic residual gravity anomaly within the uncertainty range. Dynamic fluid flow modelling of the preferred mantle density model predicts a dynamic subsidence at the base of the Central Andean crust that deviates the crust from local isostasy by as much as 6-10 km and 4-6 km in the Eastern and Western Cordilleras respectively. The inclusion of this dynamic subsidence in the determination of the observed residual gravity anomaly greatly improves the fit between observed and predicted mantle residual gravity anomalies from the mantle over the Central Andes. This suggests that dynamic topography and the depression of the Moho beneath the Central Andes may be important in compensating part of the mass excess arising from the cold subducting slab. Apart from the slab itself, the most important body affecting the wavelength and amplitude of the dynamic topography is the asthenospheric wedge. Results from combined dynamic and gravity modelling suggest that gravity alone can not be used to distinguish whether the asthenospheric wedge is present. Geophysical studies of Moho topography beneath the Central Andes may help clarify the existence and dimensions of asthenospheric material in the mantle wedge.