Structure of the Lomas de Olmedo sub-basin (Salta rift) and synorogenic deposits in the Central Andes based on gravity data

Abstract

Northwestern Argentina in the Andean retroarc zone hosts Paleozoic, Cretaceous-Paleogene rift, and Paleogene-Neogene foreland units, representing the main hydrocarbon potential of the Central Andes. This study focuses on the Lomas de Olmedo sub-basin, the eastern branch of the Cretaceous rift system, with structures oriented parallel to regional compressive stresses that acted during the Andean orogenic phase. The considerable thickness of the synorogenic sedimentary sequences in the depocenter obscures the underlying basement structure, the main focus of the present work. Gravity data were used to model the lithospheric structure of the sub-basin through two 2D density models constrained by seismic, density, and structural data. The cross-depocenter model reveals 31 km of crustal thinning, inherited from extensional periods, whereas seismological models show a more regional, off-axis thinning pattern that does not fit the gravimetric data. The model parallel to the depocenter axis characterizes synorogenic and pre-orogenic sequences, where three overlapping sedimentary cycles reach a total thickness of about 10 km. The sub-basin lies within a transitional zone between a broken foreland basin in the south and a typical flexural foreland basin in the north. The modeled synorogenic sequences display the typical morphology of a foreland basin. A 2D flexural model with moderate rigidity (Te: 5-10 km) successfully reproduces basin architecture. Low Te correlates with westward-retracted deep seismicity, consistent with interpreted crustal thinning and reduced deformation front migration. This suggests that inherited rift-related structures, oriented parallel to regional stresses, may have controlled the limited eastward advance of the deformation front. Post-rift foreland sequences exhibit doming in transverse depocenter seismic lines and gravity models, consistent with flexural model forebulge predictions, suggesting a contribution to Neogene doming. The foreland basin is better analyzed using 2D flexural models, since, depending on lithospheric rigidity, they may have influenced synorogenic foreland deposition, modifying the stress field.