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Numerical models offer powerful insights into tectonic processes, especially when their validity can be tested against geological and geophysical observations from natural orogenic belts. Here we explain some of the criteria for success in integrating orogenic models with data, using examples from the Grenville Orogen. Model designs must be simplified by comparison with nature to illuminate the first-order processes that control orogenic evolution, which limits the extent to which model results can reproduce geological observations. For the western Grenville Orogen, observed variations in geological properties are represented by lower crustal blocks with strength decreasing from the exterior to the interior of the model. GO-series models with this design reproduce the first-order crustal architecture of the Georgian Bay and Montreal--Val d'Or Lithoprobe transects. Both constant-convergence and stop-convergence models produce similar geometries, but only stop-convergence models produce normal-sense shear zones like those observed. EGO-series models, incorporating an initial weak zone bounded by stronger lower crustal blocks, predict exhumation of high-pressure rocks as observed in the eastern Grenville Orogen, although other aspects of these model results are not as successful. The single most important test of a geodynamic model is its ability to integrate diverse and independent observations in a self- consistent manner. Other criteria include consistency with crustal-scale geometry and structural and metamorphic histories. By these criteria, the present models account reasonably well for the syn- and post-convergent evolution of the western Grenville Orogen, but further work is required to produce a fully satisfactory model for the eastern end of the system. Plusieurs modeles numeeriques offrent des apercus reeveelateurs de processus tectoniques, surtout lorsqu'ils peuvent etre validees par rapport a des observations geologiques et geeophysiques de ceintures orogeniques naturelles. Dans cet article, nous expliquons quelques criteres pour reussir l'inteegration de modeles orogeniques avec les donneees, en utilisant des exemples de l'orogene de Grenville. Les conceptions des modeles doivent etre simplifieees par rapport a la nature afin de souligner les processus de premier ordre qui controlent revolution de l'orogene, ce qui limite l'etendue selon laquelle les resultats des modeles peuvent reproduire les observations geologiques. En ce qui concerne la partie ouest de l'orogene de Grenville, les variations observees dans les proprieeteesgeologiques sont representees par des blocs de la croute infeerieure dont la puissance decroit de l'exteerieur vers l'interieur du modele. Les modeles de series de l'orogene de Grenville conjus de cette maniere reproduisent l'architecture de premier ordre de la croute des transects Lithoprobe de la baie Georgienne et de Montreal--Val-d'Or. Les modeles de convergence constante et de convergence par a-coups produisent des geometries similaires, mais seuls les modeles par a-coups produisent des zones de cisaillement distensif comme celles observees. Les modeles des series de l'orogene de Grenville oriental, incorporant une zone initiale faible limitee par des blocs plus forts de la croute, exhument les roches de haute pression tel qu'observe dans l'orogene de Grenville oriental, bien que d'autres aspects des resultats de ces modeles ne soient pas aussi reussis. La verification la plus importante d'un modele geodynamique est sa capacite d'inteegrer des observations diverses et independantes d'une maniere autoconsistante. D'autres criteres comprennent la conformite avec la geeomeetrie a l'echelle de la croflte et les historiques de structure et de metamorphisme. Selon ces criteres, les modeles actuels tiennent raisonnablement compte de revolution syn- et post-convergente de l'orogene de Grenville occidental mais il faudrait plus de travaux pour etablir un modele pleinement satisfaisant pour l'extreemitee est du systeme. [Traduit par la Redaction], Introduction Numerical models offer powerful insights into tectonic processes by providing quantitative tests of concepts. Ideally, they are experiments designed to identify the first-order physical principles controlling orogenesis, rather than [...]

Geodynamic modeling demonstrates various modes of behaviour of the tectonically active continental mantle lithosphere. At continental collision, mantle lithosphere below thickening crust can be accommodated by mixed subduction-like consumption and viscous drip-like instability, depending on the material rheology, temperature, and convergence velocity. Late-stage slab steepening, dual-sided and ablative consumption, and breakoff can occur as the buoyant crust resists subduction. Removal of accreted crust by erosion can modify how even the deepest portions of the mantle lithosphere evolves during contraction. When gravitational forcing rather than plate shortening dominates, mantle lithosphere may be removed through viscous dripping-like instability or delamination. The removal induces crustal heating, modified topography, and deformation, but distinctive styles of these develop depending on whether mantle lithosphere delaminates or drips. With a modified density stratification postulated for the Archean, relatively buoyant mantle lithosphere may undergo an in-situ overturn when triggered by unstable dense eclogite and basal traction. This causes a pulse of rapid crustal heating as hot lowermost lithosphere is brought into contact with the base of the crust. As an interpretive tool, the geodynamic experiments illustrate some of the dynamically feasible modes of behaviour and controlling parameters for the continental mantle lithosphere in ancient to modern tectonic environments. La modelisation geodynamique offre divers modeles de comportement du manteau lithospherique continental tectoniquement actif. Lors de la collision continentale, le manteau lithospherique sous la croflte qui s'eepaissit peut s'adapter par une consommation combineee de type subduction et d'instabilite de type egouttement visqueux, selon la rheologie du materiel, la temperature et la vitesse de convergence. Une accentuation de phase tardive de la pente de la dalle, une consommation ablative--a deux cotes etledeecrochement de la dalle peuvent avoir lieu alors que la croflte flottante resiste a la subduction. Le retrait par erosion de la croflte accretee peut modifier meme la maniere dont les portions les plus profondes du manteau lithospherique eevoluent durant la contraction. Lorsque le fonjage gravitationnel domine plutot que le raccourcissement des plaques, il peut y avoir retrait du manteau lithospherique par une instabilite de type egouttement visqueux ou par delamination. Le retrait cause l'echauffement de la croflte, la modification de la topographie et de la deformation mais les styles distincts de ces effets dependent de la delamination ou de l'egouttement de la lithosphere. En postulant une stratification a densite modifiee pour l'Archeen, la lithosphere dumanteau, relativement legere et flottante, peut subir un renversement in situ s'il est declenche par une eclogite dense et instable et de la traction a la base. Cela cause un echauffement rapide de la croute alors que la chaude lithosphere infeerieure est amenee en contact avec la base de la croflte. Servant d'outils d'interpretation, les essais geeodynamiques illustrent quelques-uns des modes de comportement dynamiquement realisables et des parametres de controle de la lithosphere du manteau continental dans des environnements tectoniques anciens a modernes. [Traduit par la Redaction], Introduction The sub-crustal portion of the lithosphere, the mantle lithosphere, comprises the bulk of the continental lithosphere. Compared to the crust, the structure, composition, and dynamics of the continental mantle [...]

[1] We present results from numerical models for a convergent orogen with laterally variable lower crustal strength, representing a simplified orogenic system in which a strong craton, flanked by progressively weaker terranes, collides with another continent. With progressive convergence, crustal thickening, and thermal relaxation, lower crust becomes decoupled from upper and middle crust, forming a ductile orogenic infrastructure beneath a stronger superstructure. Collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core, creating allochthonous terranes overlying a lower crustal indentor. The extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate. The western Grenville orogen displays across-strike variations in age, tectonic history, and protolith association, suggesting a systematic variation in precollision crustal strength. The Laurentian craton, margin, and accreted terranes were variably reworked at synorogenic depths of 25-35 km during the Ottawan orogeny. Deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest. A comparison between numerical model results and crustal-scale cross sections from the Georgian Bay and Montreal-Val d'Or transects shows close correspondence between crustal structure and model geometry. This indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western Grenville orogen. Contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record. Citation: Jamieson, R. A., C. Beaumont, M. H. Nguyen, and N. G. Culshaw (2007), Synconvergent ductile flow in variable-strength continental crust: Numerical models with application to the western Grenville orogen, Tectonics, 26, TC5005, doi: 10.1029/2006TC002036.

The historical superstructure-infrastructure concept (S-I) expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels. Two-dimensional thermal-mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening (phase 1), thermal relaxation and rheologieal weakening (phase 2), and ductile flow at depth (phase 3). Results predict an upper-crustal superstructure, dominated by early steep structures, separated across a subhorizontal high-strain zone from a ductile infrastructure with late gently dipping structures; this is consistent with observations from the western Superior Province. These models can account for contrasts in structural style, metamorphic grade, seismic reflectivity, and age between upper- and lower-crustal levels. In contrast to conventional thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive convergence, thereby encouraging reassessment of standard seismic reflection interpretations. Keywords: tectonics, crustal structure, superstructure, infrastructure, seismic reflectivity, Superior Province.

Author(s): C. Beaumont (corresponding author) [1]; R. A. Jamieson [2]; M. H. Nguyen [1, 2]; B. Lee [1] Recent interpretations of Himalayan-Tibetan tectonics have proposed that channel flow in the [...]