The gravity field of the southern Central Andes and their eastern foreland between 20° and 30° S has been investigated with regard to the isostatic state, the crustal density structure of the orogeny and the rigidity of the lithosphere. The gravity database and constraining information for a 3D density model are derived from geophysical field data acquired in the Central Andes over many years. The data cover both the area of the northern steep subduction zone and the flat-slab area in the south. All gravity data are tied to the IGSN71 gravity datum and are terrain corrected. Topographic corrections and analysis of Andean topography is based on the 1 km × 1 km mean elevation data grid of the USGS (GTOPO30) and our own data. The gravity effect of the downgoing Nazca Plate was removed from both Bouguer and isostatic residual anomalies (Airy and Vening-Meinesz type) and then the anomalies were correlated with mean topographic heights to identify areas of disturbed isostatic equilibrium. Most of the morphological Andean units are close to isostatic equilibrium and, in particular, we find Airy-type equilibrium in the Main Cordillera. Departures from isostatic equilibirum are found in the Bolivian Altiplano/Argentina Puna, the SE foreland, in the Coastal Cordillera and along a large NW- to SE-striking zone crossing the volcanic arc. This structure can be linked to high density crustal remnants of Mesozoic rifting. Novel methods for 2D modelling of lithospheric rigidity which can account for surface and subsurface loads have been applied to calculate flexural rigidity. Rather low values (about 10 21 to 10 23 Nm, corresponding to 10–15 km effective elastic thickness) were obtained in the area of active Andean volcanoes. These low values reflect the elevated temperatures and crustal melting associated with the arc. Rigidities of 10 23 to 5×10 23 Nm (effective elastic thickness 35–45 km, and in some cases more than 50 km) are found for the backarc region. The Andean forearc region is characterized by a relatively high flexural rigidity of about 10 24 Nm, which is consistent with the existence of a cold and rigid lithospheric domain.
Isostasy and lithospheric rigidity of the Central Andes (20 deg. S to 30 deg. S)
BRAITENBERG, CARLA;
2003-01-01
Abstract
The gravity field of the southern Central Andes and their eastern foreland between 20° and 30° S has been investigated with regard to the isostatic state, the crustal density structure of the orogeny and the rigidity of the lithosphere. The gravity database and constraining information for a 3D density model are derived from geophysical field data acquired in the Central Andes over many years. The data cover both the area of the northern steep subduction zone and the flat-slab area in the south. All gravity data are tied to the IGSN71 gravity datum and are terrain corrected. Topographic corrections and analysis of Andean topography is based on the 1 km × 1 km mean elevation data grid of the USGS (GTOPO30) and our own data. The gravity effect of the downgoing Nazca Plate was removed from both Bouguer and isostatic residual anomalies (Airy and Vening-Meinesz type) and then the anomalies were correlated with mean topographic heights to identify areas of disturbed isostatic equilibrium. Most of the morphological Andean units are close to isostatic equilibrium and, in particular, we find Airy-type equilibrium in the Main Cordillera. Departures from isostatic equilibirum are found in the Bolivian Altiplano/Argentina Puna, the SE foreland, in the Coastal Cordillera and along a large NW- to SE-striking zone crossing the volcanic arc. This structure can be linked to high density crustal remnants of Mesozoic rifting. Novel methods for 2D modelling of lithospheric rigidity which can account for surface and subsurface loads have been applied to calculate flexural rigidity. Rather low values (about 10 21 to 10 23 Nm, corresponding to 10–15 km effective elastic thickness) were obtained in the area of active Andean volcanoes. These low values reflect the elevated temperatures and crustal melting associated with the arc. Rigidities of 10 23 to 5×10 23 Nm (effective elastic thickness 35–45 km, and in some cases more than 50 km) are found for the backarc region. The Andean forearc region is characterized by a relatively high flexural rigidity of about 10 24 Nm, which is consistent with the existence of a cold and rigid lithospheric domain.Pubblicazioni consigliate
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