Causes of spatially variable tectonic subsidence in the Miocene Bermejo Foreland Basin, Argentina

Tectonic subsidence in the 20-9 Ma Bermejo basin resulted from spatially variable crustal loading on a lithosphere of spatially variable strength (e.g. elastic thickness). Reconstruction of the crustal loads added between 20 and 9 Ma, and assessment of the effects of these loads on an elastic, isotropic lithosphere confirm this hypothesis. Elastic models effectively explain tectonic subsidence east of the Iglesia-Calingasta basin, but west of it crustal loads were locally compensated. Elastic models also prove that the 20-9 Ma Frontal Cordillera loading is of no importance in the mechanical system of the Bermejo basin. 2D and 3D elastic models of a uniformly strong lithosphere under 20-9 Ma crustal loads corrected for post-9 Ma erosion successfully replicate the 9 Ma Bermejo basin's proximal palaeotopography. However, they fail to replicate the 9 Ma basin's medial and distal palaeotopography. A 3D finite element model of a lithosphere with bimodal strength (weak below the Bermejo basin and west of the Precordillera, and strong below the Precordillera and east of the Valle Fertil lineament) successfully replicates the 9 Ma basin's palaeotopography. That variable strength model introduces a southward decrease in the wavelength of flexural deformation, which results in a basin that narrows southward, consistent with the 9 Ma Bermejo basin. The preferred 9 Ma lithospheric strength distribution is similar to the present lithospheric strength field derived from gravity data, suggesting that the bimodal strength signature was retained throughout the entire basin's evolution. Late Miocene flattening of the subducting slab, tectonic change to a broken foreland, or deposition of a thick (similar to8-10 km) sedimentary cover did not affect the strength of the lithosphere underlying the Bermejo basin. The long-term bimodal strength field does not correlate with the documented thickness of the seismogenic crust.