A gravity chart to quickly evaluate the probable contribution of lithospheric heating to the isostatic balance in the central Andes

The existence of a strong lithospheric heating beneath the Central Andes has been recognized by many authors. Isacks proposed in 1988 a mechanism that combines crustal thickening and lithospheric heating to justify both the Andean build up and its isostatic behaviour. With the purpose of including gravity in such a model, we present here a chart that simplifies the calculation of isostatic corrections for the thermal hypothesis (Pratt system). This chart was constructed from the 3-D gravity effect on a spherical earth of a thermal lithospheric \"root\" 400 km wide, extending between nearly 70 and 140 km depth, and whose flanks conform to the Nazca Plate subduction angle. The density contrast was assumed to be +30 kg/m³ . The largest isostatic-thermal correction exceeds +59 mGal (about 1/7 of the maximum Bouguer anomaly in the area), and a corrected Bouguer anomaly can be obtained by adding chart corrections to the observed values. Thus, via gravity inversion of this anomaly, a better determination of the real crustal root and the isostatic condition at the bottom of the lithosphere is possible. The observed Bouguer anomaly reaches about -400 mGal in the Central Andes. Of this maximum value, -60 mGal may be justified by means of expansive thermal effects taking place in the lower half of the lithosphere. The remaining -340 mGal correspond to a crustal root of 25 km due to orogenic shortening. Thus, the two classical hypotheses (Airy in the crust and Pratt in the lithosphere) are combined to explain the isostatic balance in this Andean sector. This gravity model is consistent with the seismic crustal results in an Andean section at latitude 24.5° south.