The modern Pampean Plain foreland basin system at 31 SL: Depozones controlled by crystalline basement thrusting?

DAVILA, F.M.

Jujuy

Congreso; Congreso Geológico Argentino; 2008

Between 31º-33º SL, along the modern flat slab segment of the south-Central Andes, the DeCelles and Giles (1996) foreland basin system model was never fully tested. In this region, most studies have focused only on the most proximal depocenters, close to the Precordillera fold and thrust belt. Evidently, the occurrence of double vergent, deep–angle, crystalline basement thrusts partitioning the foreland plains; complicates the simple DeCelles and Giles’ configuration. For this reason, this foreland has been described in the literature as “atypical” (Ford, 2004), and named “broken foreland”. Eastward from the easternmost basement range of the Sierras Pampeanas, a thick Cenozoic succession (locally >500 m) lays in the subsurface of the Pampean Plain, lapping onto Paleozoic and/or Mesozoic units. This basin system has been interpreted as “intracratonic” due to its position close to Proterozoic cratons. Nevertheless, the subsidence mechanisms are still not well understood. Recently, Dávila et al. (2005, 2007) proposed that non-isostatic loads, related to mantle dynamics (dynamic topography), played an important role in the configuration of pericratonic and cratonic foreland basins of Argentina. Although, a dynamic component is likely influencing on the longest wavelength subsidence (>500 km), it is crucial to understand whether supracrustal topographic loads influenced at shorter-wavelengths (300x300 km) mostly developed in the Santa Fe province. The distance between mountain system and the San Guillermo High is ~230 km. On the base of previous studies (García-Castellanos et al., 2002), for a lithospheric effective elastic thickness (Te) close to 40-50 km (value used in this work), the maximum length of loads affecting a basin is around 150-100 km (independent of the net load). Due to the distance from the Cordoba Pampean Plain to the western most Sierras de Cordoba range (Pocho range) is ~120 km, and the next basement range to the west is located to >200 km, this model only considers the Sierras de Cordoba as the main topographic load. Thus, the Andean Cordillera does not influence on the accommodation of the sedimentary systems of the Pampean Plain. Two experiments were tested using the 2-D Cardozo and Jordan (2001) approach: (1) assuming the modern topography as the total load and (2) estimating the Tertiary uplift in the Sierras de Cordoba, based on thermochronology. In the first case, the topographic profile from the westernmost to the easternmost part of the Sierras de Cordoba was re-drawn in discrete rectangles, which represents the tectonic + sedimentary loads. When basement rocks dominate density is 2700 kg/m-3, whereas in the foothill of the Sierras, where alluvial fans and Cenozoic rock dominate, density is 2600 kg/m-3. The model assumed flexural compensation to the mantle (∂=3300 kg/m-3). An effective elastic thickness of 50 km was used in the calculation, coherent with recent geophysical results (Tassara and Yañez, 2003). This experiments reproduced a foredeep of ~250 km, a maximum accommodation amplitude (between the topography and the bulge) of ~200 m and a peripheral bulge amplitude of ~25 m. These magnitudes match remarkably with the morphotectonic regions previously described, not only in wavelengths and amplitudes but also in the expected depositional systems (see DeCelles and Giles, 1996). Hence, in the DeCelles and Giles’ model, the Sierras de Córdoba would represent the wedge top (main load), the foredeep would correspond to the megafan-dominated bajada, the San Guillermo High constitutes the forebulge, and the backbulge by the wetland region. In this context, the Mar Chiquita lake depression represents a terminal alluvial belt defeated by a topographic closure: the San Guillermo forebulge upwarping. The previous experiment assumes that the modern topography of the Sierras de Cordoba is a consequence of Andean deformation. However, thermochronological studies suggest the topography of this region was a resultant of various orogenic events as old as Mesozoic or even Paleozoic. The youngest apatite fission track age in the Sierras de Cordoba is ca. 115-111 Ma (Early Cretaceous, Jordan et al., 1989; Costa et al., 2006). Assuming a constant exhumation with paleogradients ~17°C/km (Collo and Dávila, 2008), the total exhumation since the Cretaceous would have been of 5-7 km and the mean exhumation rate of 44 – 62 m/my. Because the middle Miocene marine Paraná Fm rests subhorizontally in subsurface, we can assume this deposition pre-dates the major deformation event along the Pampean region. If we assume the major uplifting started at ~7 Ma, coeval with Pocho volcanism within the foreland, the Late Miocene-Pliocene exhumation would range between 189 and 434 m. Experiment using load geometry of 100 km (length) x 434 m (height), and similar lithospheric parameters than the previous model, the maximum bending would be only of 0.16 m and the bulge amplitude almost flat. This flexural analysis, along 31° SL, suggests the Pampean Plain is a foreland basin partly controlled (see below) by the west-vergent crystalline basement thrusting, uplifting and loading of the Sierras de Cordoba. Note the Pampean Plain basin is located backward of the basement thrust sheets, in opposed position respect to DeCelles and Giles’ model. But the influence of the Sierras de Cordoba topography is not only evident in the flexural analysis. A depocenter plan-view configuration shows that the forebulge develops coincidently along strike with the occurrence of the Sierras de Cordoba morphostructure. Regarding to the sediment supply/accommodation space ratio, the presence of an exhumed forebulge (San Guillermo high) indicates the predicted foredeep, represented in nature by megafan deposits, is an outstanding natural case of underfilled basin. This underfilling was suggested as well by sediment supply calculations (Pelletier, 2007). It is important to notice that although the experiments reproduced the observed wavelengths, they underestimated the accommodation space magnitudes suggesting a deficit of loads. Additional loads would be needed to explain the upper Miocene to Modern sequences. We could alternately think that the sedimentary spaces, in part, were inherited from earlier subsidence stages (early Tertiary or Mesozoic). However, considering that the Miocene ingression layer was bended during the forebulge upwarping, I would prefer an interpretation where the loading and major subsidence episodes postdate the middle Miocene marine incursion. Other alternative is to suppose we underestimated the topographic load and an important volume of topography was unroofed and unconsidered in the calculations. This would be possible only under unrealistic lower paleogradients (