ALBERT

Description: The first-order characteristics of collisional mountain belts and the potential feedback with surface processes are predicted by critical taper theory. While the feedback between erosion and mountain-belt structure has been fairly extensively studied, less attention has been given to the potential role of synorogenic deposition. For thin-skinned fold-and-thrust belts, recent studies indicate a strong control of syntectonic deposition on structure, as sedimentation tends to stabilize the thin-skinned wedge. However, the factors controlling basement deformation below fold-and-thrust belts, as evident for example in the Zagros Mountains or in the Swiss Alps, remain largely unknown. Previous work has suggested that such variations in orogenic structure may be explained by the thermo-tectonic “age” of the deforming lithosphere and hence its rheology. Here we demonstrate that sediment loading of the foreland basin area provides an additional control and may explain the variable basement involvement in orogenic belts. When examining the role of sedimentation we identify two end-members: 1) sediment-starved orogenic systems with thick-skinned basement deformation in an axial orogenic core and thin-skinned deformation in the bordering forelands; and 2) sediment-loaded orogens with thick packages of synorogenic deposits, derived from the axial basement zone, deposited on the surrounding foreland fold-and-thrust belts and characterized by basement deformation below the foreland. Using high resolution thermo-mechanical models, we demonstrate a strong feedback between deposition and crustal scale thick-skinned deformation. Our results show that the loading effects of syntectonic sediments lead to long crustal-scale thrust sheets beneath the orogenic foreland and explain the contrasting characteristics of sediment-starved and sediment-loaded orogens, showing for the first time how both thin- and thick-skinned crustal deformation are linked to sediment deposition in these orogenic systems. We show that the observed model behavior is consistent with observations from a number of natural orogenic systems.

Description: [1]  Numerous studies in the Central Pyrenees have provided evidence for a rapid phase of exhumation of this mountain belt during the Late Eocene (37–30 Ma). Simultaneously, the closure of the Ebro foreland basin allowed the accumulation of sediments at the southern Piedmont, which partially covered the fold-and-thrust belt from Late Eocene ( e . g . when it was still actively deforming) to Miocene times. We aim here at understanding the consequences of such syn-tectonic sedimentation on the Southern Pyrenean fold-and-thrust belt by using a 2-D numerical model that reproduces the development of a thin-skinned wedge subject to different modes of sedimentation and erosion. The results show contrasting fold-and-thrust belt behavior when applying aggrading or prograding sedimentation, which we link to the critical state of the wedge. When the sediments are sourced from the hinterland (progradation), the thrusting propagates toward the foreland; whereas when the sediments aggrade from the basin, the thrusting sequence migrates backward. This latter mode shows patterns of deformation that compare favorably to the Pyrenean thrusting sequence observed during Eocene-Miocene times.

Description: Abstract Here we present high‐resolution 2‐D coupled tectonic‐surface processes modeling of extensional basin formation. We focus on understanding feedbacks between erosion and deposition and tectonics during rift and passive margin formation. We test the combined effects of crustal rheology and varying surface process efficiency on structural style of rift and passive margin formation. The forward models presented here allow to identify the following four feedback relations between surface processes and tectonic deformation during rifted margin formation. (1) Erosion and deposition promote strain localization and enhance large offset asymmetric normal fault growth. (2) Progressive infill from proximal to more distal half‐grabens promotes the formation of synthetic sets of basin ward dipping normal faults for intermediate crustal strength cases. (3) Sediment loading on top of undeformed crustal rafts in weak crust cases enhances mid and lower crustal flow resulting in sag basin subsidence. (4) Interaction of high sediment supply to the distal margin in very weak crust cases results in detachment based rollover sedimentary basins. Our models further show that erosion efficiency and drainage area provide a first order control on sediment supply during rifting where rift related topography is relatively quickly eroded. Long term sustained sediment supply to the rift basins requires elevated onshore drainage basins. We discuss similar variations in structural style observed in natural systems and compare them with the feedbacks identified here.

Description: The strength of the crust and the mantle lithosphere are strongly influenced by the temperature, the thickness and the composition of the crust and mantle lithosphere, and by inherited weaknesses. It consequently strongly depends on the geodynamic setting and is then expected to be different from one orogen to another, which might explain the diversity of deformation styles. Here we use 2D thermomechanical models at lithospheric scale to study the effect of the strength of the upper and middle crust on the geometry of contractional systems. The role of extensional inheritance is included by forward modeling the formation of rift basin or passive margin and using the resulting extensional structure as initial condition for lithospheric scale inversion. Our results demonstrate that: 1) crustal strength strongly controls the width and the elevation of orogenic wedge; 2) relative weak crust facilitates efficient propagating of thick-skinned crustal scale thrusts; 3) contraction of a strong crust leads to the formation of long thrust sheets and an anti-formal stack in the core of the orogen or the accretion of upper crust depending on the depth of the decoupling zone in the mid crust; 4) erosion processes favor the localization of shortening in a narrow crustal orogen; 5) rifting inheritance can explain the presence at shallower depth of lower crustal or mantle material previously upwelled and facilitates the propagation of the deformation in the external part of the chain. The range of predicted behaviors is compared to first order with the Zagros, the Alps and the Pyrenees, three natural examples for which the crustal structure is well constrained by geophysical data.

Description: Bimodal Plio–Quaternary glacial erosion of fjords and low-relief surfaces in Scandinavia Nature Geoscience 5, 635 (2012). doi:10.1038/ngeo1549 Authors: Philippe Steer, Ritske S. Huismans, Pierre G. Valla, Sébastien Gac & Frédéric Herman Glacial landscapes are characterized by dramatic local relief, but they also commonly exhibit high-elevation, low-relief surfaces. These surfaces have been attributed to glacial headward erosion in Alpine settings. However, the timing and processes responsible for their formation in northern high-latitude regions remain elusive. Here we estimate the rate of fjord erosion from geophysical relief and compare that with the erosion reflected by offshore sedimentation in western Scandinavia during the late Pliocene and Quaternary glaciations (0–2.8 million years ago). We find that the sediments generated by fjord erosion over the entire western Scandinavia accounts for only 35–55% of the total sediment volume deposited off the coast of Norway. This large mismatch implies that during this period, significant erosion must have also taken place away from the fjords at high elevation and indicates a bimodal distribution of glacial erosion. Furthermore, comparing the distribution of the high-elevation, low-relief surfaces with estimates of the long-term glacier equilibrium line altitude supports the idea that effective erosion in extensively glaciated areas limits topographic height, a process known as the glacial buzzsaw. We therefore conclude that glacial and periglacial processes have a substantial impact on the formation of low-relief surfaces observed in glaciated mountain belts and high-latitude continental margins.

Description: Very large subsidence, with up to 20 km thick sediment layers, is observed in the East Barents Sea basin. Subsidence started in early Paleozoic, accelerated in Permo-Triassic times, finished during the middle Cretaceous, and was followed by moderate uplift in Cenozoic times. The observed gravity signal suggests that the East Barents Sea is at present in isostatic balance and indicates that a mass excess is required in the lithosphere to produce the observed large subsidence. Several origins have been proposed for the mass excess. We use 1-D thermokinematic modeling and 2-D isostatic density models of continental lithosphere to evaluate these competing hypotheses. The crustal density in 2-D thermokinematic models resulting from pressure-, temperature-, and composition-dependent phase change models is computed along transects crossing the East Barents Sea. The results indicate the following. (1) Extension can only explain the observed subsidence provided that a 10 km thick serpentinized mantle lens beneath the basin center is present. We conclude that this is unlikely given that this highly serpentinized layer should be formed below a sedimentary basin with more than 10 km of sediments and crust at least 10 km thick. (2) Phase changes in a compositionally homogeneous crust do not provide enough mass excess to explain the present-day basin geometry. (3) Phase change induced densification of a preexisting lower crustal gabbroic body, interpreted as a mafic magmatic underplate, can explain the basin geometry and observed gravity anomalies. The following model is proposed for the formation of the East Barents Sea basin: (1) Devonian rifting and extension related magmatism resulted in moderate thinning of the crust and a mafic underplate below the central basin area explaining initial late Paleozoic subsidence. (2) East-west shortening during the Permian and Triassic resulted in densification of the previously emplaced mafic underplated body and enhanced subsidence dramatically, explaining the present-day deep basin geometry.

Description: The way individual faults and rift segments link up is a fundamental aspect of lithosphere extension and continental break-up. Little is known however about the factors that control the selection of the different modes of rift interaction observed in nature. Here we use state-of-the-art large deformation 3D numerical models to examine the controls on the style and geometry of rift linkage between rift segments during extension of crustal brittle-ductile coupled systems. We focus on the effect of viscosity of the lower layer, the offset between the rift basins and the amount of strain weakening on the efficiency of rift linkage and rift propagation and the style of extension. The models predict three main modes of rift interaction: 1) oblique to transform linking graben systems for small to moderate rift offset and low lower layer viscosity, 2) propagating but non linking and overlapping primary grabens for larger offset and intermediate lower layer viscosity, and 3) formation of multiple graben systems with inefficient rift propagation for high lower layer viscosity. The transition between the linking (Mode 1) and non-linking mode (Mode 2) is controlled by the trade-off between the rift offset, the strength of brittle-ductile coupling, and the amount of strain weakening. The mode transition from overlapping non-connecting rift segments (Mode 2) to distributed deformation (Mode 3) is mainly controlled by the viscosity of the lower layer and can be understood from minimum energy dissipation analysis arguments.

Description: We focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models. To date, few 3-D models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones. We use an arbitrary Lagrangian-Eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions. Plastic materials weaken with accumulating strain. To localize deformation, a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions. Controls on the geometry and spacing of three-dimensional plastic shear zones are investigated. The sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested. The model results indicate the primary controls of strain-dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction. The amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other, resulting in a trade-off effect between the amount of offset between the initial grabens and the strain weakening ratio. The three-dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions: (1) small-offset grabens with a single relay zone, (2) intermediate-offset grabens with one or more secondary step-over graben segments, and (3) large-offset grabens with limited to no significant segment interaction.

Description: Abstract Subduction zones are the main entry points of water into Earth's mantle and play an important role in the global water cycle. The progressive release of water by metamorphic dehydration induces important physical‐chemical processes, including subduction zone earthquakes. Yet, how water migrates in subduction zones is not well understood. We investigate this problem by explicitly modeling two‐phase flow processes, in which fluids migrate through a compacting and decompacting solid matrix. As our results show, water migration is strongly affected by subduction dynamics, which exhibits three characteristic stages in our models: (1) an early stage of subduction initiation; (2) an intermediate stage of gravity‐driven steepening of the slab; and (3) a late stage of quasi‐steady state subduction. Two main water pathways are found in the models: trench‐ward and arc‐ward. They form in the first two stages and become steady in the third stage. Depending on the depth of water release from the subducting slab, water migration focuses in different pathways: a shallow release depth (e.g. 40 km) leads the water mainly through the trench‐ward pathway; a deep release depth (e.g. 120 km) promotes an arc‐ward pathway and a long horizontal migration distance (~ 300 km) from the trench; an intermediate release depth (e.g. 80 km) leads water to both pathways. We compare our models with seismic studies from southeast Japan (Saita et al., 2015) and the west Hellenic subduction zone (Halpaap, et al 2018) and provide geodynamical explanations for these seismic observations in natural subduction environments.

Description: Abstract 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 back‐arc 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 towards 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.