Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling
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ORIGINAL PAPER
Lithospheric density structure of the southern Central Andes constrained by 3D data‑integrative gravity modelling Constanza Rodriguez Piceda1,2 · Magdalena Scheck Wenderoth1,3 · Maria Laura Gomez Dacal1 · Judith Bott1 · Claudia Beatriz Prezzi4 · Manfred R. Strecker2 Received: 26 March 2020 / Accepted: 10 November 2020 © The Author(s) 2020
Abstract The southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from subhorizontal ( 5e−8 y−1) are shown. Blue lines indicate compression. Red lines indicate extension. Boundaries of the main morphotectonic provinces are also marked: for abbreviations, see Fig. 1c
( 30 mGal, indicating that the alternative models are not able to accurately reproduce the observed gravity in this part of the study area (Fig. 12c, d). Third, we investigated the effects of varying the density of the lower continental crystalline crust by ± 100 kg/m3. Reducing the density of this lower crystalline unit resulted in an increase in its thickness of up to 17 km in the northern part of the orogen (Fig. 11e). In contrast, increasing the density by the same amount resulted in a lower crust that was up to 12.5 km thinner (Fig. 11f). With regard to the gravity residuals of these last two configurations, the inversion yielded a less accurate match with the observed gravity than the final model and the other alternatives described above in this section (RMSE = 33 mGal for the
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International Journal of Earth Sciences
Fig. 12 Gravity residual (difference between observed and calculated gravity) from models with density variations in selected modelled units after inversion: a continental sediments = 2300 kg/m3; b continental sediments = 2600 kg/m3; c continental upper crystalline crust = 2750 kg/m3; d continental upper crystalline crust = 2850 kg/ m3; e continental lower crystalline crust = 3000 kg/m3; f conti-
nental lower crystalline crust = 3200 kg/m3; g homogeneous mantle = 3360 kg/m3; h density distribution within the mantle obtained through conversion of S-wave velocities from the SEMum2 tomographic model (French et al. 2013). RMSE (root-mean-square error) in mGal. Boundaries of the main morphotectonic provinces are also marked: for abbreviations, see Fig. 1c
less dense lower continental crystalline crust and 25 mGal for the denser lower unit; Fig. 11e, f). The final model (Sect. 4.3) was therefore preferred over these alternative configurations. The fourth step in the sensitivity analysis involved investigating the effect of varying the density distribution in the mantle. We first considered a constant mantle density of 3360 kg/m3. Following inversion, the lower crust was up to 3.5 km thicker along the western part of the study area than in the final model (Fig. 11g). T
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