We present the results of a numerical modeling study of the Pyrenees Mountains investigating the spatio-temporal variation in lithospheric flexure in response to the developing orogen, with the aim of setting constraints on paleotopography. We employ a finite-element method to model the 3D flexural deformation of the lithosphere beneath the Pyrenean orogen since the onset of convergence in the Late Cretaceous. Using subsurface and structural data, we describe the evolving geometry of both the northern Aquitaine and southern Ebro foreland basins at the Paleocene (early orogenic phase), the end-and mid-Eocene (peak orogenic phase), the Oligocene (late orogenic phase), and the present (postorogenic phase). The flexural modeling provides insight into how both the rigidity of the

The crustal structure of overriding plates in subduction settings around the world varies between a wide range of deformation styles, ranging from extensional structures and backarc opening as in the Tonga or Hellenic subduction zone to large, plateau-like orogens such as the central Andes. Both end-member types have been intensively studied over the last decades, and several hypotheses have been proposed to explain their characteristics. Here we model ocean-continent collision using high-resolution, upper mantle scale plane-strain thermo-mechanical models, accounting for phase changes of rocks that enter the eclogite stability field and the phase transition at the 660 km mantle discontinuity. We test model sensitivity to varying plate velocities and backarc lithospheric strength as the main variables affecting the strain regime of the overriding plate in subduction zones. With our small set of variables, we reproduce both overriding plate extension and shortening and provide insight into the dynamics behind those processes. We find that absolute plate velocities determine the possible strain regimes in the overriding plate, where overriding plate movement toward the trench inhibits backarc extension and promotes overriding plate shortening. Additionally, a weak and removed backarc lithospheric mantle is required for backarc extension and facilitates overriding plate shortening. Comparison of the models with natural subduction systems, specifically the Andes and Hellenic subduction zones, corroborates that lithospheric removal and absolute plate velocities guide overriding plate deformation. the combined convergence rates of the subducting plate (v oc ) and the overriding plate (v cont ), and deformation occurs mostly at the subduction interface and not inside the plates (Capitanio et al., 2010). However, tectonic plates are no isolated features, have lateral density and viscosity variations, possibly several subducting edges (e.g., Pacific plate), and are dragged or pushed by topographic differences (e.g., ridge push) or sublithospheric mantle flow. These factors modify plate velocities and generate tensional or compressional tectonic stresses which can induce extension or shortening in the overriding plate.

Continent-continent collision creates mountain belts that grow in size with increasing amounts of plate convergence. Orogenic temperature is directly related to the amount of crustal thickening by horizontal shortening, as heat producing elements are concentrated in upper and middle crustal rocks (e.g., Hacker et al., 2015). Crustal heating, in turn, has a weakening effect on crustal rheology and controls regional metamorphism; influencing mountain-belt structure and deformation. During orogenic growth, surface processes shape mountain-belt topography, fill the evolving foreland basins and exhume metamorphic rocks. In the end-member case, erosion may even fully balance orogenic growth, creating a flux steady state between tectonics and erosion (Stolar et al., 2007; Willett & Brandon, 2002). Beaumont et al. (2006) propose a temperature-magnitude relationship for orogenic growth from small and cold to large and hot, in analogy to the Hertzsprung-Russel diagram of stellar classification. The temperature-magnitude diagram, however, does not explain whether there is a characteristic distribution of shortening and structural style (e.g., thinand thickskinned deformation, thrusting style) as a function of orogen size, and its controlling factors. In this study, we use thermo-mechanical models that are fully coupled to a landscape-evolution model, to investigate the first-order factors controlling the distribution of shortening and the different structural styles related to orogenic growth from small and cold to large and hot during continent-continent collision. A typical example of a small and cold orogen is the Pyrenean mountain belt, which grew by inversion of a rift system with additional crustal shortening and plate convergence of at most 165 km (Beaumont et al., 2000; Muñoz, 1992). The Alps, an intermediate size orogen, have a more complex history, that includes

High-elevation, low-relief surfaces are widespread in many mountain belts. However, the origin of these surfaces has long been debated. In particular, the southeast Tibetan Plateau has extensive low-relief surfaces perched above deep valleys and in the headwaters of three of the world’s largest rivers (Salween, Mekong, and Yangtze Rivers). Various geologic data and geodynamic models show that many mountain belts grow first to a certain height and then laterally in an outward propagation sequence. By translating this information into a kinematic propagating uplift function in a landscape evolution model, we propose that the high-elevation, low-relief surfaces in the southeast Tibetan Plateau are simply a consequence of mountain growth and do not require a special process to form. The propagating uplift forms an elongated river network geometry with broad high-elevation, low-relief headwaters and interfluves that persist for tens of millions of years, consistent with the observed geochronology. We suggest that the low-relief interfluves can be long-lived because they lack the drainage networks necessary to keep pace with the rapid incision of the large main-stem rivers. The propagating uplift also produces spatial and temporal exhumation patterns and river profile morphologies that match observations. Our modeling therefore reconciles geomorphic observations with geodynamic models of uplift of the southeast Tibetan Plateau, and it provides a simple mechanism to explain the low-relief surfaces observed in several mountain belts on Earth.

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