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THE ANDES-NAZCA DYNAMIC TOPOGRAPHY EXPERIMENT

Final Report Summary - ANDYN (THE ANDES-NAZCA DYNAMIC TOPOGRAPHY EXPERIMENT)

Crustal tectonics and mantle flow are the main driving mechanisms of Earth's topography. While tectonics associates with isostatic topography, mantle convection with dynamically supported long wavelength deflections (or dynamic topography). South America is an ideal natural laboratory to analyse and quantify these two contrasting components. Along the western margin, the Andes belt is a still active orogenic system affected by complex geodynamic processes like the subduction of oceanic ridges with thicker than average oceanic crust (Nazca, Juan Fernandez and Chile, among others). These subducting features affect not only the convergence dynamics and stresses along the entire margin but also the distribution of mass anomalies in the mantle, which drive sublithospheric flow and generate dynamic topography.
During this Marie Curie IIF project the fellow demonstrated using numerical models (flow and flexural calculations), direct observation and proxies that most of the South American continent has been uncompensated since the beginning of the Cenozoic (when the Andes started to grow) and that additional forces, such as mantle downwellings and upwellings, would be required to account for the observed topography. These results were synthetized in five articles, two already published in 2013 (JSAES and Geology), one under review (EPSL) and two in the process of submission (Nature Geosciences and J. Geodynamics).
Although all earlier dynamic topography models predict large negative and subsiding areas along most of South America (~1000 km across, tens of kilometres along and thousands of meters in amplitude), associated with the subduction history of the extinct Farallon Plate, no such subsidence could be reproduced with the fellow model and/or observed in the geological and geophysical records. Previous studies did not account for the large along-strike variations of the slab dip angle in South America or the supracrustal loading effects on the total topography, especially close to the Cordillera. These large along-strike dip changes manifest themselves as two large flat segments, one in Perú (14˚ S) and one in Argentina (32 °S). Previous work on the effects of flat subduction on dynamic topography suggested that slab flattening induced greater subsidence than normal subduction. The results were used to suggest that slab flattening was one of the major driving mechanisms of long-wavelength subsidence and intracontinental marine flooding during the Cretaceous-Early Cenozoic of North America. The fellow studies, nevertheless, found the exact opposite, in good agreement with the geological record of South America.
During the Income Phase of the project in London, the fellow designed three numerical flow models. A present-day model (PDM), representative of the evolution from the Mio-Pliocene to the present-day, where the subduction angle varied along strike from flat (0˚) to normal (~30˚) and the density contrast (∆ρ) of the slab (with respect to the mantle) was controlled by the slab age, the inferred crustal thickness and the Benioff zone geometry. In order to explore time variation of dynamic topography, he also constructed two simplified models: a Cenozoic model (CM) in which the slab dips 15˚ (Southern Andes) to 30˚ (Central and Northern Andes) eastward homogeneously along strike and a pre-Cenozoic model (PCM), where the slab is subvertical along strike.
The main results demonstrated that the geometry and density distribution of the Nazca slab had a strong effect on the pattern and amplitude of dynamic topography. The temporal evolution of dynamic topography induced by progressive flattening of the slab by comparing the CM and PDM, and computing the change in dynamic topography after the break up of the Farallon Plate and the creation of Nazca (Neogene) produced positive signals, consistent with dynamic uplift, especially over subducting ridges. The dynamic topography evolution shows that the proximal forelands (foredeeps) of South America would have profoundly subsided as a consequence of a progressive slab shallowing (from PCM to CM) in its early stages and then uplifted and rotated clockwise (<0.5˚) during the Miocene to Present.
The predicted dynamic topographies were compared with geological and geophysical observations that are good proxies of the topographic evolution and, hence, of the ‘observed’ dynamic signals:
1. Non-isostatic residual topography calculations across a representative swath in the flat-slab segment of Argentina agree with the general trend of flow models, i.e. positive residual topography on the flat slabs and negative where slabs plunge with 30° dip.
2. Trench bathymetry (corrected topography to the top of the oceanic basement) and a total relief analysis in the stable foreland of Argentina (outside the influence of Andean tectonics) also support the main conclusions.
3. The morphology of the distal foreland helps in calibrating models and proposing an “observed dynamic topography” value of ≤300 m. This ‘observed’ value correlates remarkably well with the ‘predicted’ dynamic topography calculated across the South American plains.
4. Observations (geological and geophysical) across the still active flat-slab segments of South America agree with model predictions. Dynamic uplift is more consistent than subsidence across segments affected by ridge dynamics. In fact, in the southern part of the Peruvian foreland up to >700 km from the trench, an elevated and long-wavelength bulge: the Fitzcarrald Arch. Further south, above the Argentine-Chilean flat-slab segment, the distal foreland (>600 km eastward) also shows a high-elevation system: the Sierras Pampeanas broken foreland. A sensitive element within the Sierras Pampeanas is the uplift of the Paranaense Sea marker bed, today exposed at 1 km on sea level in regions poorly affected by tectonics. These feature date to the latest Miocene-Pliocene, coeval with the advent of flat subduction. The same occurs in Patagonia, where an emerging plateau has developed since the arrival of the Chile seismic ridge.
In the southernmost South America, the model however could not reproduce the very deep Argentine abyssal basin. Neither do models with accurate reconstructions of the slab morphology in the upper mantle. Models that use long-wavelength seismic tomography that produce subsidence over all of South America do show strong negative dynamic topography over the Argentine basin but without the right geometry and only as a result of very large regional subsidence that does not match the geological and geophysical data. The fellow current working hypothesis is that this area would be located on an anomalous lithosphere, likely over thickened continental lithosphere, instead of a thin oceanic lithosphere.
The fellow results have had broader implications for understanding the topography in foreland, platform and oceanic regions, no only for academic purposes but also for exploration strategies. The fellow work allowed a cooperation project with the National Petroleum Company of Argentina, YPF and new lines of research in the Centro de Ciencias de la Tierra, CONICET. He is developing a new research group on Geodynamics in Cordoba. His new ideas have also formed part of teaching and short courses since the beginning of the project. He started as well new projects (funded by SECyT, FONCyT and CONICET) transferring the new hypotheses on unexplored regions of northern South America and Patagonia, which will allow a more fluent link between industry and science. A postdoc student started to work in Patagonia to test the fellow results and a master thesis is being finished in Peru, based on these ideas.