@phdthesis{Meessen2019,
  author    = {Meeßen, Christian},
  title     = {The thermal and rheological state of the Northern Argentinian foreland basins},
  doi       = {10.25932/publishup-43994},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-439945},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xviii, 151},
  year      = {2019},
  abstract  = {The foreland of the Andes in South America is characterised by distinct along strike changes in surface deformational styles. These styles are classified into two end-members, the thin-skinned and the thick-skinned style. The superficial expression of thin-skinned deformation is a succession of narrowly spaced hills and valleys, that form laterally continuous ranges on the foreland facing side of the orogen. Each of the hills is defined by a reverse fault that roots in a basal d{\´e}collement surface within the sedimentary cover, and acted as thrusting ramp to stack the sedimentary pile. Thick-skinned deformation is morphologically characterised by spatially disparate, basement-cored mountain ranges. These mountain ranges are uplifted along reactivated high-angle crustal-scale discontinuities, such as suture zones between different tectonic terranes. Amongst proposed causes for the observed variation are variations in the dip angle of the Nazca plate, variation in sediment thickness, lithospheric thickening, volcanism or compositional differences. The proposed mechanisms are predominantly based on geological observations or numerical thermomechanical modelling, but there has been no attempt to understand the mechanisms from a point of data-integrative 3D modelling. The aim of this dissertation is therefore to understand how lithospheric structure controls the deformational behaviour. The integration of independent data into a consistent model of the lithosphere allows to obtain additional evidence that helps to understand the causes for the different deformational styles. Northern Argentina encompasses the transition from the thin-skinned fold-and-thrust belt in Bolivia, to the thick-skinned Sierras Pampeanas province, which makes this area a well suited location for such a study. The general workflow followed in this study first involves data-constrained structural- and density-modelling in order to obtain a model of the study area. This model was then used to predict the steady-state thermal field, which was then used to assess the present-day rheological state in northern Argentina. The structural configuration of the lithosphere in northern Argentina was determined by means of data-integrative, 3D density modelling verified by Bouguer gravity. The model delineates the first-order density contrasts in the lithosphere in the uppermost 200 km, and discriminates bodies for the sediments, the crystalline crust, the lithospheric mantle and the subducting Nazca plate. To obtain the intra-crustal density structure, an automated inversion approach was developed and applied to a starting structural model that assumed a homogeneously dense crust. The resulting final structural model indicates that the crustal structure can be represented by an upper crust with a density of 2800 kg/m³, and a lower crust of 3100 kg/m³. The Transbrazilian Lineament, which separates the Pampia terrane from the R{\´i}o de la Plata craton, is expressed as a zone of low average crustal densities. In an excursion, we demonstrate in another study, that the gravity inversion method developed to obtain intra-crustal density structures, is also applicable to obtain density variations in the uppermost lithospheric mantle. Densities in such sub-crustal depths are difficult to constrain from seismic tomographic models due to smearing of crustal velocities. With the application to the uppermost lithospheric mantle in the north Atlantic, we demonstrate in Tan et al. (2018) that lateral density trends of at least 125\,km width are robustly recovered by the inversion method, thereby providing an important tool for the delineation of subcrustal density trends. Due to the genetic link between subduction, orogenesis and retroarc foreland basins the question rises whether the steady-state assumption is valid in such a dynamic setting. To answer this question, I analysed (i) the impact of subduction on the conductive thermal field of the overlying continental plate, (ii) the differences between the transient and steady-state thermal fields of a geodynamic coupled model. Both studies indicate that the assumption of a thermal steady-state is applicable in most parts of the study area. Within the orogenic wedge, where the assumption cannot be applied, I estimated the transient thermal field based on the results of the conducted analyses. Accordingly, the structural model that had been obtained in the first step, could be used to obtain a 3D conductive steady-state thermal field. The rheological assessment based on this thermal field indicates that the lithosphere of the thin-skinned Subandean ranges is characterised by a relatively strong crust and a weak mantle. Contrarily, the adjacent foreland basin consists of a fully coupled, very strong lithosphere. Thus, shortening in northern Argentina can only be accommodated within the weak lithosphere of the orogen and the Subandean ranges. The analysis suggests that the d{\´e}collements of the fold-and-thrust belt are the shallow continuation of shear zones that reside in the ductile sections of the orogenic crust. Furthermore, the localisation of the faults that provide strain transfer between the deeper ductile crust and the shallower d{\´e}collement is strongly influenced by crustal weak zones such as foliation. In contrast to the northern foreland, the lithosphere of the thick-skinned Sierras Pampeanas is fully coupled and characterised by a strong crust and mantle. The high overall strength prevents the generation of crustal-scale faults by tectonic stresses. Even inherited crustal-scale discontinuities, such as sutures, cannot sufficiently reduce the strength of the lithosphere in order to be reactivated. Therefore, magmatism that had been identified to be a precursor of basement uplift in the Sierras Pampeanas, is the key factor that leads to the broken foreland of this province. Due to thermal weakening, and potentially lubrication of the inherited discontinuities, the lithosphere is locally weakened such that tectonic stresses can uplift the basement blocks. This hypothesis explains both the spatially disparate character of the broken foreland, as well as the observed temporal delay between volcanism and basement block uplift. This dissertation provides for the first time a data-driven 3D model that is consistent with geophysical data and geological observations, and that is able to causally link the thermo-rheological structure of the lithosphere to the observed variation of surface deformation styles in the retroarc foreland of northern Argentina.},
  language  = {en}
}
@phdthesis{Pons2023,
  author    = {Pons, Micha{\"e}l},
  title     = {The Nature of the tectonic shortening in Central Andes},
  doi       = {10.25932/publishup-60089},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-600892},
  school      = {Universit{\"a}t Potsdam},
  pages     = {160},
  year      = {2023},
  abstract  = {The Andean Cordillera is a mountain range located at the western South American margin and is part of the Eastern- Circum-Pacific orogenic Belt. The ~7000 km long mountain range is one of the longest on Earth and hosts the second largest orogenic plateau in the world, the Altiplano-Puna plateau. The Andes are known as a non-collisional subduction-type orogen which developed as a result of the interaction between the subducted oceanic Nazca plate and the South American continental plate. The different Andean segments exhibit along-strike variations of morphotectonic provinces characterized by different elevations, volcanic activity, deformation styles, crustal thickness, shortening magnitude and oceanic plate geometry. Most of the present-day elevation can be explained by crustal shortening in the last ~50 Ma, with the shortening magnitude decreasing from ~300 km in the central (15°S-30°S) segment to less than half that in the southern part (30°S-40°S). Several factors were proposed that might control the magnitude and acceleration of shortening of the Central Andes in the last 15 Ma. One important factor is likely the slab geometry. At 27-33°S, the slab dips horizontally at ~100 km depth due to the subduction of the buoyant Juan Fernandez Ridge, forming the Pampean flat-slab. This horizontal subduction is thought to influence the thermo-mechanical state of the Sierras Pampeanas foreland, for instance, by strengthening the lithosphere and promoting the thick-skinned propagation of deformation to the east, resulting in the uplift of the Sierras Pampeanas basement blocks. The flat-slab has migrated southwards from the Altiplano latitude at ~30 Ma to its present-day position and the processes and consequences associated to its passage on the contemporaneous acceleration of the shortening rate in Central Andes remain unclear. Although the passage of the flat-slab could offer an explanation to the acceleration of the shortening, the timing does not explain the two pulses of shortening at about 15 Ma and 4 Ma that are suggested from geological observations. I hypothesize that deformation in the Central Andes is controlled by a complex interaction between the subduction dynamics of the Nazca plate and the dynamic strengthening and weakening of the South American plate due to several upper plate processes. To test this hypothesis, a detailed investigation into the role of the flat-slab, the structural inheritance of the continental plate, and the subduction dynamics in the Andes is needed. Therefore, I have built two classes of numerical thermo-mechanical models: (i) The first class of models are a series of generic E-W-oriented high-resolution 2D subduction models thatinclude flat subduction in order to investigate the role of the subduction dynamics on the temporal variability of the shortening rate in the Central Andes at Altiplano latitudes (~21°S). The shortening rate from the models was then validated with the observed tectonic shortening rate in the Central Andes. (ii) The second class of models are a series of 3D data-driven models of the present-day Pampean flat-slab configuration and the Sierras Pampeanas (26-42°S). The models aim to investigate the relative contribution of the present-day flat subduction and inherited structures in the continental lithosphere on the strain localization. Both model classes were built using the advanced finite element geodynamic code ASPECT. The first main finding of this work is to suggest that the temporal variability of shortening in the Central Andes is primarily controlled by the subduction dynamics of the Nazca plate while it penetrates into the mantle transition zone. These dynamics depends on the westward velocity of the South American plate that provides the main crustal shortening force to the Andes and forces the trench to retreat. When the subducting plate reaches the lower mantle, it buckles on it-self until the forced trench retreat causes the slab to steepen in the upper mantle in contrast with the classical slab-anchoring model. The steepening of the slab hinders the trench causing it to resist the advancing South American plate, resulting in the pulsatile shortening. This buckling and steepening subduction regime could have been initiated because of the overall decrease in the westwards velocity of the South American plate. In addition, the passage of the flat-slab is required to promote the shortening of the continental plate because flat subduction scrapes the mantle lithosphere, thus weakening the continental plate. This process contributes to the efficient shortening when the trench is hindered, followed by mantle lithosphere delamination at ~20 Ma. Finally, the underthrusting of the Brazilian cratonic shield beneath the orogen occurs at ~11 Ma due to the mechanical weakening of the thick sediments covered the shield margin, and due to the decreasing resistance of the weakened lithosphere of the orogen. The second main finding of this work is to suggest that the cold flat-slab strengthens the overriding continental lithosphere and prevents strain localization. Therefore, the deformation is transmitted to the eastern front of the flat-slab segment by the shear stress operating at the subduction interface, thus the flat-slab acts like an indenter that "bulldozes" the mantle-keel of the continental lithosphere. The offset in the propagation of deformation to the east between the flat and steeper slab segments in the south causes the formation of a transpressive dextral shear zone. Here, inherited faults of past tectonic events are reactivated and further localize the deformation in an en-echelon strike-slip shear zone, through a mechanism that I refer to as "flat-slab conveyor". Specifically, the shallowing of the flat-slab causes the lateral deformation, which explains the timing of multiple geological events preceding the arrival of the flat-slab at 33°S. These include the onset of the compression and of the transition between thin to thick-skinned deformation styles resulting from the crustal contraction of the crust in the Sierras Pampeanas some 10 and 6 Myr before the Juan Fernandez Ridge collision at that latitude, respectively.},
  language  = {en}
}