Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two‐dimensional, visco‐elasto‐plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold‐and‐thrust belts dominated by out‐of‐sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold‐and‐thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach

Understanding the tectonic evolution and crustal-scale structure of fold-thrust belts is crucial for exploring geological resources and evaluating seismic hazards. We conducted a series of finite-difference two-dimensional thermo-mechanical numerical models with visco-elasto-plastic/brittle rheology to decipher how the interaction of inherited basement faults and salt décollement levels control the deformation process and structural style of the Fars Arc in the Zagros fold -thrust belt, during tectonic inversion. Results indicate that initial rifting is controlled by the geometry of inherited faults. During the convergence phase, fold-and-thrust belts display folding at two scales: large wavelength folds induced by basement deformation in the form of fault-propagation faults, and small wavelength folds and thrust systems emerge above the salt layer as detachment folds. Reactivated faults can serve as pathways for stress transfer, resulting in the emergence of new faults and thus seismic activity. The tectonic events in orogenic belts like the Zagros do not adhere to a fixed pattern; they are shaped by factors such as the properties of basement rocks and the orientation of faults. Shallow earthquakes predominantly occur along décollement anticlines, while deeper and larger ones are associated with basement faults. Additionally, we observe variations in resistance to deformation based on salt rheology and fault geometry, with listric faults minimizing resistance. The degree of basement involvement in deformation directly influences the model's resistance, with greater involvement facilitating easier deformation. Our results showing the temporal-spatial relationship between thin- and thick-skinned tectonics can work as an analogue for similar orogenic belts worldwide, such as Taiwan, the Pyrenees, the Alps, the Appalachians, and the Kopet Dagh

Stretching and contraction of extensional basins with pre-rift salt: a numerical modelling approach

We present a series of 2D thermo-mechanical numerical experiments of thick-skinned crustal extension including a pre-rift salt horizon and subsequent thin-, thick-skinned, or mixed styles of convergence accompanied by surface processes. Extension localization along steep basement faults produces half-graben structures and leads to variations in the original distribution of pre-rift salt. Thick-skinned extension rate and salt rheology control hanging wall accommodation space as well as the locus and timing of minibasin grounding. Upon shortening, extension-related basement steps hinder forward propagation of evolving shallow thrust systems; conversely, if full basin inversion takes place along every individual fault, the regional salt layer is placed back to its pre-extensional configuration, constituting a regionally continuous décollement. Continued shortening and basement involvement deform the shallow fold-thrust structures and locally breaches the shallow décollement. We aim at obtaining a series of structural, stratigraphic and kinematic templates of fold-and-thrust belts involving rift basins with an intervening pre-rift salt horizon. Numerical results are compared to natural cases of salt-related inversion tectonics to better understand their structural evolution

Mobility in a Globalised World 2015

The term mobility has different meanings in the following science disciplines. In economics, mobility is the ability of an individual or a group to improve their economic status in relation to income and wealth within their lifetime or between generations. In information systems and computer science, mobility is used for the concept of mobile computing, in which a computer is transported by a person during normal use. Logistics creates by the design of logistics networks the infrastructure for the mobility of people and goods. Electric mobility is one of today’s solutions from engineering perspective to reduce the need of energy resources and environmental impact. Moreover, for urban planning, mobility is the crunch question about how to optimise the different needs for mobility and how to link different transportation systems. In this publication we collected the ideas of practitioners, researchers, and government officials regarding the different modes of mobility in a globalised world, focusing on both domestic and international issues

Effect of fluid pressure distribution on the structural evolution of accretionary wedges

Numerical experiments on evolving accretionary wedges usually implement predefined weak basal décollements and constant strength parameters for overlying compressed sequences, although fluid pressure ratio, and therefore brittle strength, can vary strongly in sedimentary basins. A two-dimensional finite difference model with a visco-elasto-plastic rheology is used to investigate the influence of different simplified fluid pressure ratio distributions on the structural evolution of accretionary wedge systems. Results show that a linear increase in fluid pressure ratio towards the base leads to toeward-verging thrust sheets and underplating of strata, while simulations with a predefined décollement form conjugate shear zones supporting box-fold-type frontal accretion. Surface tapers are in agreement with the critical wedge theory, which here is modified for cases of varying fluid pressure ratio. Furthermore, the numerical results resemble findings from natural examples of accretionary wedges. © 2017 John Wiley & Sons Ltd.Financial support was provided by the Swiss National Science Foundation (grant 2-77297-15).Peer reviewe

Numerical modeling of tectonic underplating in accretionary wedge systems

Many fossil and active accretionary wedge systems show signs of tectonic underplating, which denotes accretion of underthrust material to the base of the wedge. Underplating is a viable process for thickening of the rear part of accretionary wedges, for example as a response to horizontal growth perpendicular to strike. Here, numerical experiments with a visco-elasto-plastic rheology are applied to test the importance of backstop geometry, flexural rigidity, décollement strength, and surface erosion on the structural evolution of accretionary wedges undergoing different modes of sediment accretion, where underplating is introduced by the implementation of two, a basal and an intermediate, décollement levels. Results demonstrate that intense erosion and a strong lower plate hamper thickening of a wedge at the rear, enhancing localized underplating, antiformal stacking, and subsequent exhumation to sustain its critical taper. Furthermore, large strength contrasts between basal and intermediate décollements have an important morphological impact on wedge growth due to different resulting critical taper angles. Presented numerical experiments are compared to natural examples of accretionary wedges and are able to recreate first-order structural observations related to underplating.ISSN:1553-040

Effects of fault-weakening processes on oblique intracontinental rifting and subsequent tectonic inversion

In many mountain belts, deformation concentrates along mechanically weak fault zones inherited from earlier tectonic events. This work investigates the effects of two modes of structural weakening on the orientation of rifting and later tectonic inversion with respect to the imposed divergence/convergence direction in a high-resolution 3D finite difference model with a viscous-frictional rheology. In the first set of experiments, weakening consists in a decrease in frictional strength with increasing shear strain. The generated normal faults strike orthogonal to the imposed divergence direction. These faults are reactivated during tectonic inversion and absorb 50 to 70 percent of accumulated strain. In the second set of experiments, frictional strength is a decreasing function of shear strain rate. The generated faults are oblique to the divergence direction, implying oblique fault slip. Fault reactivation depends on the obliquity of the inverted rift to convergence direction, where larger obliquity leads to more intense fault reactivation. These new numerical results are compared to previous analogue and numerical models on the one hand, and natural examples of intracontinental mountain ranges due to tectonic inversion on the other hand. These comparisons demonstrate that both modes of frictional weakening should be taken into account when seeking to understand large-scale rifting and inversion tectonics.Financial support was provided by the Swiss National Science Foundation (grant 2-77297-15).Peer reviewe

Numerical modelling of inversion tectonics in fold-and-thrust belts

This work presents numerical experiments of inversion of rift basins and consequent sub-thrust imbrication in tectonic wedges. Half-graben basins initially develop and then are covered with a post-rift sequence bearing a décollement-prone horizon (i.e., the upper décollement). A total of twelve models of tectonic inversion have been conducted varying (i)the strength of inherited extensional fault arrays and (ii)applying different fluid pressure ratios (i.e., strength)within syn-rift strata. Combinations of those were simulated using different internal angles of friction for the inherited faults, different strengths for the syn-rift infill and for the upper décollement. Results show that changes in relative strength between inherited faults, syn-rift deposits and the upper crustal décollement leads to important variations in structural styles. Weak faults systematically favour the compressional reactivation of inherited extensional faults. Weak syn-rift sediments favour hanging wall by-pass structures instead of fault reactivation and less internal deformation of the syn-rift deposits. Weak upper décollements supports the accretion of basement in a hinterland antiformal stack, decoupling of basement and cover, and forward tectonic transport of rift basins. Strong upper crustal décollements favours basement and cover coupling, can lead to fault reactivation in the absence of weak faults and syn-rift sediments, however combinations of weak faults and strong upper décollement shows fault reactivation, weak syn-rift sediments and strong upper décollement form hanging wall by-pass structures. Modelling results are compared to natural case studies. © 2019This is a contribution of the Institut de Recerca Geomodels and the Geodinàmica i Analisi de Conques research group (2014SGR467SGR). Jonas B. Ruh was supported by the Swiss National Science Foundation grant number 2-77297-15 .Peer reviewe