"Building an age-elevation profile of the Eastern Cordillera of Argentina using U-Th/He dating"

Emilyn Gjertsen, Thesis 2016

Abstract: The Altiplano-Puna Plateau is an area of high topography and low relief stretching from Bolivia through Argentina. This prominent geologic feature of the central Andes is comparable to the Tibetan Plateau of Southeast Asia and represents an ideal natural laboratory to explore the development of orogenic plateaus such as these (Barnes and Ehlers, 2009, Rahl et al., unpublished). The study of orogenic plateaus has become of wide interest because of a growing recognition that orogenic plateaus, like the Mesozoic Nevadaplano, existed in the past (DeCelles, 2004).

In the field of tectonics, the origin and evolution of orogenic plateaus is largely unknown despite recent study. Although other alternatives exist, two models dominate the discussion about
the formation of the Altiplano-Puna and other orogenic plateaus. Plateau growth may be the outcome of deep-seated mantle processes. The removal of lithosphere on the underside of the
crust and the expansion of hot asthenosphere many kilometers beneath a continent would cause the overlying area to “pop up,” creating an area of high topography (Garzione et al., 2006, Kay
and Mahlburg-Kay, 1991). This situation has little answer for the problem of frontal range growth and expansion into a foreland basin, and is therefore regarded as an unlikely scenario in
the Altiplano-Puna and Andes.

Alternatively, uplift in an orogenic plateau may begin with lateral or wedge growth along an upper-crustal detachment and mountain chain topography formed through thrust faults of consistent vergence (Siks and Horton, 2011). The wedge continues to grow, moving the front margin of the plateau across the foreland basin in the form of a thin-skinned fold and thrust belt mountain range, the Eastern Cordillera. As its margin expands toward the craton interior, the plateau itself expands in area. There is a growing body of evidence that the progressive growth model is a more likely scenario for uplift in the Altiplano-Puna (Siks and Horton, 2011, Barnes and Ehlers, 2009, Reiners et al., 2015).

Eastward growth of the Altiplano-Puna implies a complicated relationship between uplift and erosion at the plateau margin. The interior of the plateau is dry due to a rain shadow effect resulting from high topography to the east and has little relief. Most of its extensive area is internally drained, leaving little opportunity for the shedding of sediment through erosion (Reiners et al., 2015). It is likely accurate to limit the effects of physical erosion in the Altiplano-Puna to within a few hundred kilometers of the high elevation and relief of the Eastern Cordillera in a study of uplift and erosion dynamics (Reiners et al., 2015). At the eastern margin itself, high precipitation and steep slopes drive high erosion rates. For example, in the Rio Iryua basin, recent climate change and human disturbance have led to a high denudation rate of 3-6 mm/a in the steep headwaters aided by landslide-triggered mudflows (Rahl et al., unpublished). Rates of erosion this high should drive the plateau margin westward and increase exhumation in the Eastern Cordillera.

The nature of these interactions determines the topography of the region and the eastward growth of the frontal range into the foreland basin. Quantifying erosion rates in the Eastern Cordillera will shed light on the question of plateau growth in the Altiplano-Puna, particularly given that exhumation is occurring almost entirely due to erosion (Rahl et al., unpublished). Continued plateau growth to the east despite high modern erosion rates suggests that the relationship between the two physical processes is complex. Orogenic plateau formation and mountain building in the Andes created high topography and relief in the region, leading to an environment conducive to physical erosion. Using zircon (U-Th)/He thermochronology to capture the thermal history of a vertical column of rock, my research seeks to relate the results to erosional history of the upper crust in the Eastern Cordillera, a link which reflects tectonic activity. From these results, we can gain insight on erosion rates that are roughly equivalent with exhumation rates. Quantifying this process will offer data on Altiplano-Puna that can add to the growing body of evidence used to model the most likely uplift scenarios in the history of this orogenic plateau.

Full thesis available. Contact the Geology Department at 540-458-8800.