WP(2): Interplay between tectonics & inherited crustal inhomogeneities

The region east of the 6000 m high Puna plateau and the Eastern Cordillera of NW Argentina is characeterized by a wedge-shaped ~250-km-wide fold-and-thrust belt, which defines the eastern border of the orogen and transitions into the unrestricted Chaco-Paraná foredeep. The spatial extent of the fold-and-thrust belt correlates with thick Paleozoic units that provide the basal decollement of the wedge¹. South of 24°S, however, these mechanically weak layers thin out and disappear, and the thinskinned style of deformation terminates^2,3. Instead, normal faults and transfer structures of the Cretaceous Salta Rift or Paleozoic metamorphic fabrics have accommodated shortening during the Cenozoic^4-9. The reactivated, inherited anisotropies have produced discrete ranges that occur both along the eastern flank of the plateau and far to the east within the otherwise undeformed foreland¹⁰. Virtually all of the ranges in the so-called “broken foreland” of the Sierras Pampeanas and the Santa Barbara System are bounded by faults that have been active during the Cenozoic, but range uplifts have been highly disparate in time and space, and there is no clear deformation front as in the foreland fold-and-thrust belt to the north. The gradual increase in the wavelength of foreland structures from north to south is accompanied by decreasing amounts of shortening and plateau width, but increasing lithospheric temperatures, suggesting a close relationship between structural inheritance and the style and intensity of deformation, even at the level of deep lithospheric structures¹¹.

At the foundation of understanding the tectonic and sedimentary processes in foreland realms lie the crucial questions of which structures may rupture, what magnitude of shaking can occur, how seismicity and climate change might affect the surface-process regime in the near future, and what aspects of the long-term geomorphic and depositional characteristics influence resource generation. While we can place bounds on the answers to the seismicity related questions in plate-boundary settings, the full array of earthquake ruptures, long-term tectonic deformation, interaction between faults, and depositional systems that may be generated in broken foreland settings may mask the desired answers, largely because we do not understand the basic processes that govern them.

These complex aspects of broken foreland regions are very well expressed in the Andean foreland. Some ranges in the broken foreland constitute large, tectonically active fault blocks^12,13 or anticlines that have formed over blind thrusts, such as the growing anticlines west of the town of Salta that were associated with a M6.3 earthquake in February, 2010¹⁴. The diachronous nature of long-term range uplift and deformation of intermontane basins is mirrored by the seemingly random occurrence of earthquakes associated with basement faults¹⁵, the manifestations of paleo-earthquakes¹⁶, and on longer timescales, the evolution of regional unconformities and disconnected depocenters. A further complication includes the potential for changes in climatic boundary conditions to influence the surface process system, particularly as the growth of orographic barriers can change atmospheric circulation and rainfall patterns. In this context, there appears to be a relationship between sediment removal and ensuing changes in crustal stresses along the E flank of the Central Andes¹⁷. Potential fault reactivation resulting from associated stress changes within and along basin margins may follow the removal of the sedimentary load, further complicating the evolution of foreland deformation, subsidence patterns, sediment transport, and ultimately, the representation of orogenic processes in the foreland stratigraphic records. Such complex relationships have not been elucidated by coupling landscape evolution and large-scale thermo-mechanical models that would allow for a detailed 3D characterization of the present-day state of the lithosphere¹⁸.