The nature of the North-South change of the magnitude of tectonic shortening in Central Andes at Altiplano-Puna latitudes: a thermomechanical modeling approach
The first order features of the Central Andes tectonics are change of tectonic styles and magnitude of tectonic shortening from Altiplano to Puna latitudes. While causes of the change of tectonic styles were successfully investigated by comprehensive thermomechanical modeling in the first phase of the [G 3.2] StRATEGy project, the cause of change of shortening magnitude remains poorly understood. Our working hypothesis is that difference in strength of the upper plate that caused difference in tectonic styles also resulted in different rates of trench roll-back. To test this hypothesis and investigate other possibilities, in the new phase of the project we intend to extend our current models to the west including subducting Nazca plate. This will require more than doubling of the modeling area. We will build our models using experience of the first project phase and as previously will first run high-resolution 2D models along the Altiplano and Puna cross-sections and finally full 3D model including the entire region. To do that we will incorporate the updated versions of 3D thermomechanical codes LAMEM and ASPECT and 3D model of the lithospheric structure in the Central Andes developed in the partner project [G 3.3] and its extension.
Controls of Cenozoic foreland-deformation patterns
The Andean foreland demonstrates a pronounced N-S oriented differentiation in the style and magnitude of deformation and the nature of sedimentary basins. East of the Altiplano-Puna plateau and the Eastern Cordillera, a wedge-shaped, ~250-km-wide thin-skinned fold-and-thrust belt defines the eastern border of the orogen, which then transitions into the contiguous Chaco-Paraná foredeep. The spatial extent of the fold-and-thrust belt in Bolivia correlates with thick Paleozoic units, in which a series of Silurian, Devonian, and Carboniferous detachments define the basal decollément of the orogenic wedge. However, south of approximately 24°S, these mechanically weak layers thin and eventually disappear, and the thin-skinned style of deformation is replaced by the thick-skinned deformation of Santa Barbara and Sierras Pampeanas broken forelands. The gradual increase of the wavelength of foreland structures from north to south is accompanied by a decrease in accumulated shortening, a decrease in the width of orogenic plateau, and increasing lithospheric temperature. North-south trends are also evident in other key aspects of the South Central Andes, such as the N-S directed passage of Juán Fernandez Ridge during last 26 Ma and the widening of the magmatic arc and the start of an ignimbrite flare-up in the Puna. These observations suggest intimate relationships between style and intensity of deformation involving both shallow and deep lithospheric structures and processes. Supported by the results of numerous field observations, large-scale numerical thermomechanical modeling provides a valuable tool for testing geological and geodynamical hypotheses that are built on the apparent correlations between the different phenomena observed in the Central Andes. Previous 2D-models of the Central Andes already attempted to quantify some of these correlations. Recent progress in numerical modeling techniques, however, allows for an extension of this research towards 2.5- and 3D-lithospheric models. We will study the Andes from ca. 18 to 35°S, integrating the characteristics of the Nazca plate and the Andean foreland. Our numerical modeling will incorporate spatially varying age and geometry of the Nazca plate and thickness of the overriding South American plate to better constrain interplate stress transfer. Both deep and shallow model geometries and structures will be constrained by new data gathered in the framework of this initiative. Our project is tightly coupled with two other modeling projects: [G 3.3] on the 3D structural and thermal reconstruction of the evolution of the Chaco-Paraná Basin. The two projects, which cover somewhat different scales and/or locations, will exchange their input data and results, thus providing valuable constraints for each individual effort. Other important regional constraints are expected from field-studies planned in project [G 3.4], aimed at revealing long-term controls of sedimentary basin architecture in the brokenforeland realm.