Gravity and Thermal modeling of the North Patagonian Massif.

MARÍA LAURA GÓMEZ DACAL; JUDITH SIPPEL; MAGDALENA SCHECK-WENDEROTH; CLAUDIA TOCHO; EUGENIO ARAGÓN

San Juan

Congreso; XXVII Reunión científica de la Asociación Argentina de Geofísicos y Geodestas; 2014

Universidad Nacional de San Juan

The North Patagonian Massif (NPM), Patagonia, Argentina is a plateau, covering 100.000 km2 , that has an average elevation of 1200 meters above sea level and stands out 500 to 700 m higher in altitude than the surrounding areas (Figure 1). Its present-day elevation was caused by a sudden uplift without noticeable internal deformation during the Oligocene (Aragón et al., 2011). These interesting characteristics of the geodynamic evolution of the area can be better understood if we know more about its present-day configuration and compare its specific characteristics with adjacent areas. The NPM is mostly surrounded by basins (Figure 1), of which the best known and studied one is the Neuquen Basin. This basin is also one of the most productive basins (in terms of oil production) in Argentina. Therefore, many studies were carried out, though all of them focused on the sedimentary fill of the basin (i.e. shallow studies).With the aim to characterize the present-day structure of the NPM in terms of density and temperature, we compare its characteristics with those of the surroundings. Therefore, 3D gravity and thermal models have been developed for the NPM (Figure 2), including its neighboring Neuquen Basin.The 3D density model (Figure 2a) was derived using the software IGMAS+ (Interactive Geophysical Modelling ASsistant; Götze et al., 2010). Bouguer anomalies, extracted from EGM2008 (Pavlis et al., 2012), were used as the measured gravity field to which the field generated by the model was compared. The surfaces of the model were defined using a sediment thickness model (Heine, personal communication), an intracrustal discontinuity from CRUST 1.0 (Laske et al., 2013) and a calculated Moho, assuming Airy isostasy. The selection of densities assigned to the different units was supported by results from xenolithe studies.The 3D thermal modeling (Figure 2b) was carried out with the software GMS (Geological Modelling System), developed at the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences (Bayer et al., 1997; Scheck and Bayer, 1999), using a 3D finite-element method to solve the equation of steady state conductive heat transport. The resulting calculated 3D temperature distribution is sensitive to the values of the thermal properties and to the choice of boundary conditions that were assigned based on literature.Both models were validated with independent data: point constrains of Moho depth from seismic experiments (Figure 1, Assumpção et al., 2013) and bottom hole temperature data (Figure 1), provided by YPF S.A. The isostatic Moho fits very well the independent Moho data and the gravity observations. In the density model, a good fit between measured and calculated gravity anomalies can be noticed for the long wavelength-anomalies. The same is found for the thermal model, where there is a small difference between predicted and observed temperature. To fit the temperature data, two different boundary conditions for the lower model boundary had to be selected: for the Northern part, a heat flow in the Moho of 30 mW/m and for the Southern part 40 mW/m. It should however, be noted, that available temperature data are sparse and no temperature measurements were available inside the NPM (Figure 1). Hence no validation of the model was possible in the area.From the combination of both models, some preliminary conclusions can be reached: (i) The isostatic Moho correlates with the structures (The NPM, Neuquen Basin and the Andes); therefore, the crust beneath NPM is thicker than the one in the adjacent areas; (ii) a heterogeneous crust in isostatic equilibrium is modeled within the NPM and Neuquén Basin; (iii) in general, in the South of the study area a higher temperature isnoticed at the same depth. These results can be used as a starting point of the reconstruction of the origin of the plateau.