WP(3): Basin Modeling

The Andes exhibit first-order, along-strike variations in morphotectonic provinces, width, and deformation style. Allmendinger and Gubbels (1996) have suggested that the shortening style of the whole system might be controlled by the north-to-south variations in the lithospheric mechanical properties, which are sensitive to temperature and lithology. This inference was subsequently confirmed by thermo-mechanical modeling¹, emphasizing how mechanical weakening and failure of the foreland sedimentary cover drastically affect the rate and style of tectonic deformation and explain extensive understhrusting of the foreland in the Subandes. It is not known if these processes are promoted by increased rainfall or sedimentation rates, but in this context, it has been suggested that hydrocarbon maturation may have influenced the onset of the thin-skinned tectonics in the Subandes².

Other decisive factors in the location and evolution of sedimentary basins in the Andean realm include co-varying changes in magmatism and the geometry of the subducting slab³. Variations in the latter lead to horizontal translation of the ~100-km-deep zone of slab  dehydration and mantle-wedge melting⁴ and/or frontal tectonic erosion, which can also account for migration of the arc towards the foreland⁵. Slab flattening has been attributed to changes in buoyancy due to subduction of oceanic crust thickened by aseismic ridges⁶. A diagnostic feature of this process might be the occurrence of a foreland-ward shift in magmatic activity culminating with a magmatic lull. Basement faulting in the foreland, far from the plate boundary, may also occur, although its diagnostic value is disputed. The oblique subduction of the Nazca plate and the Juán Fernandez Ridge, may thus have led to a spatiotemporal migration of deformation, uplift, and basin formation. However, this scenario has never been fully tested. Additional aspects, such as the thermo-mechanical evolution of the subduction zone and associated dehydration processes in different segments with variable subduction angles may fundamentally impact upper-plate deformation, magma formation and emplacement (with implications for metallogenesis, see WP4), variations in the temperature field of the foreland basin, and thus the generation of hydrocarbons. Collectively, all these issues are well represented by the different evolutionary stages of the hydrocarbon-bearing Chaco-Paraná basin in NW Argentina and SE Bolivia.

The Chaco-Paraná Basin has been developing since the Eo-Oligocene; it comprises the undeformed Chaco-Parana plain to the east, and the Subandes, Sierras Pampeanas, and the ranges of the Santa Barbara System to the west. Crustal thicknesses of 42 km and 32-35 km have been inferred for the eastern Paraná and Chaco basins, respectively. Because of a highvelocity upper-mantle lid and a lack of a resolvable low-velocity zone (at least down to 200 km), the Paraná Basin has been regarded as “cratonic” in character⁷. In the Chaco Basin, on the contrary, upper-mantle S-wave velocities are low, indicating an “asthenospheric” character. In line with this, S receiver function analysis suggests a shallowing of the lithosphere-asthenosphere boundary from ~120 km in the NE Chaco-Paraná Basin to ~80 km in the central Chaco-Paraná Basin⁸. A W-to-E transect across the foreland reveals four juxtaposed depozones that can be distinguished based on sediment-fill geometries, average elevation, and Bouguer gravity anomalies. These zones include (1) the 1- to 3-km thick Cenozoic deposits in the Subandes, (2) a 3- to 4-km thick foredeep prism that tapers toward the eastern Chaco Plain, (3) a thin veneer of Quaternary alluvium forming the forebulge, and (4) a thin (0.5 km) accumulation of sediment in the back-bulge zone.

The Chaco-Paraná basin comprises >10 km of sediments spanning the Silurian to the Present. It is asymmetric, with a total stratigraphic thickness of Cenozoic rocks >7.5 km deposited at the western margin⁹ that thin eastward. The Subandes of the Chaco-Paraná foreland basin are characterized by mainly in-sequence, thin-skinned thrusting that includes ramp anticlines and passive roof duplexes^10,11 separated by thrust faults and synclines, although Plio-Pleistocene out-of-sequence thrusting has also been reported¹². The main decollement dips ~3°W and involves Silurian shales¹³. Intermediate detachment levels in Devonian shales generate lift-off structures and decoupling of the lower and upper structural levels¹⁴. Estimated total shortening in the Subandes is between 67 and 100 km^15,16.

The Subandes are home to several major gas discoveries during the last 20 years. Basin-wide assessments of resource potential are based on a variety of techniques. Production/analogue based assessments¹⁷ use existing or analogue based production-rate curves to estimate ultimate recoverable resources. Volumetric or in-place methods¹⁸ use available data on source-rock quality, type, extent, and maturity to derive estimates of potential volumes of undiscovered petroleum resources. By combining 3D petroleum system modeling techniques with detailed source rock and petroleum-accumulation information, volumetric assessments of petroleum potential can be produced at different scales. Recent publications on basin-wide generated, migrated, accumulated, and lost petroleum masses show how dynamic basin evolution is linked to the present-day resource potential¹⁹, for both conventional and unconventional petroleum resources.