Mountain Building or Backarc Extension in Ocean-Continent Subduction Systems: A Function of Backarc Lithospheric Strength and Absolute Plate Velocities

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.publishedVersio

Long‐Term Coupling and Feedback Between Tectonics and Surface Processes During Non‐Volcanic Rifted Margin Formation

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 basinward dipping normal faults for intermediate crustal strength cases. (3) Sediment loading on top of undeformed crustal rafts in weak crust cases enhances middle 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.publishedVersio

Magmatism at Passive Margins: Effects of Depth-Dependent Wide Rifting and Lithospheric Counterflow

Rifted passive margins exhibit a large variety in the timing, distribution, and amount of magmatism. The factors controlling magmatism during rifted margin formation, remain, however, incompletely understood, partly owing to the complex rifting styles. In this study, we use 2-Dimensional numerical models to investigate the effects of depth-dependent wide rifting and lithospheric counterflow on magmatism during rifted margin formation. Results show that a strong crust promotes narrow margins with a sharp transition to normal thickness oceanic crust whereas a weak crust promotes depth-dependent wide rifting, with preferential removal of mantle lithosphere, leading to formation of wide margins with over-thickened (>18 km) igneous crust in the distal margin. Counterflow of depleted lithospheric mantle, in contrast, may delay syn-rift magmatism, and result in exhumed a-magmatic continental mantle at narrow margins. The combination of wide rifting and lithospheric counterflow results in magma-poor wide margins, with in some cases a transition to excess magmatic activity at breakup. Our models provide an explanation for the contrasting magmatic productivity at narrow and wide rifted margins, such as observed in the Lofoten-Vesterålen, Newfoundland, Kwanza Basin, and Orange Basin margins.publishedVersio

Water Migration in the Subduction Mantle Wedge: A Two-Phase Flow Approach

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. Our results show that 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: trenchward and arcward. 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 trenchward pathway, a deep release depth (e.g., 120 km) promotes an arcward pathway and a long horizontal migration distance (~300 km) from the trench, and 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, https://doi.org/10.1002/2015GL063084) and the west Hellenic subduction zone (Halpaap et al., 2018, https://doi.org/10.1002/2017JB015154) and provide geodynamical explanations for these seismic observations in natural subduction environments.publishedVersio

Wide Versus Narrow Back-Arc Rifting: Control of Subduction Velocity and Convective Back-Arc Thinning

Back-arc basins such as the ones behind the island-arcs of the Western Pacific Ocean or the ones in the Mediterranean Sea are ubiquitous structures of the Earth. They are extensional basins forming in the overriding plate behind subduction zones and similarly to continental rifts, they can exhibit different structural styles from narrow, localized rifting to wide-rift extension. While these different structural styles have been long recognized, the factors controlling the style of extension in these basins have not been explored properly. We use thermo-mechanical models to investigate how the relative rates of progressive build-up of slab-pull force and of convective thinning and thermal weakening of the overriding plate control the style of back-arc rifting. Following subduction-initiation, a high subducting plate velocity results in rapid build-up of the slab-pull force. The relatively low rate of convectively thinning and associated moderate weakening of the overriding plate require slab-pull to build up to close to its maximum value to overcome the high back-arc integrated strength resulting in a narrow back-arc rift. In turn a low subducting plate velocity in comparison with the timescale of convective thinning of the overriding plate allows for significant back-arc weakening before the slab-pull force becomes large enough to drive back-arc extension. In this case, the back-arc exhibits a wide rifting style as extension occurs at significantly reduced overriding plate integrated strength. Our model results provide an explanation why some subduction zones exhibit wide, distributed extension in the overriding plate such as for instance observed in the Pannonian basin.publishedVersio

Mantle exhumation at magma-poor rifted margins controlled by frictional shear zones

The transition zone from continental crust to the mature mid-ocean ridge spreading center of the Iberia-Newfoundland magma-poor rifted margins is mostly composed of exhumed mantle characterized by highs and domes with varying elevation, spacing and shape. The mechanism controlling strain localization and fault migration explaining the geometry of these peridotite ridges is poorly understood. Here we show using forward geodynamic models that multiple out-of-sequence detachments with recurring dip reversal form during magma-poor rifting and mantle exhumation as a consequence of the strength competition between weak frictional-plastic shear zones and the thermally weakened necking domain beneath the exhuming footwall explaining geometry of these peridotite ridges. Model behaviour also shows that fault types and detachment styles vary with spreading rate and fault strength and confirm that these results can be compared to other magma poor passive margins such as along Antarctica-Australia and to ultra-slow mid-ocean spreading systems as the South-West Indian Ridge.publishedVersio

The Role of Subduction Interface and Upper Plate Strength on Back-Arc Extension: Application to Mediterranean Back-Arc Basins

While there has been a lot of work focusing on improving our understanding of divergent and convergent plate boundaries, the intricate nature of back-arc extension, where subduction and large-scale extension occur and interact in close, is yet to be explored properly. It has long been proposed that the strength of the subduction interface, which depends among others on the amount of subducted sediments, plays a pivotal role in subduction dynamics. Here, we investigate the role of back-arc rheology and subduction interface strength on the deformation style of the overriding plate. Using two-dimensional thermomechanical model experiments, we demonstrate, that the presence of a weak mantle–lithospheric domain in the overriding plate can result in back-arc breakup even during the subduction of narrow, land-locked oceanic basins such as those found in the Mediterranean region. The thinning of the back-arc mantle–lithosphere results in a weaker overriding plate, hence a lower slab-pull force is sufficient to initiate back-arc extension. Convective thinning at the subduction interface also reduces the length of the interface, reducing the portion of slab-pull lost as energy dissipation. A weak plate interface, can also reduce the energy dissipated along the subduction zone, leading to earlier extension. A detailed analysis of the forces shaping the overriding plate stress field shows that transmission of slab-pull force has a predominant role while viscous basal drag has a negligible effect in our experiments. Our results compare favorably with large-scale characteristics of land-locked Mediterranean back-arc basins such as the North Tyrrhenian basin and the Pannonian basin.publishedVersio

How post-salt sediment flux and progradation rate influence salt tectonics on rifted margins: Insights from geodynamic modelling

Continental rifted margins can be associated with widespread and thick salt deposits, which are often formed during the final stages of rifting, prior to breakup. These salt-bearing margins are typically characterized by pronounced post-rift salt tectonics with variable and complex structural styles and evolution. We use a lithosphere-scale geodynamic numerical model to investigate the role of varying post-rift sediment fluxes and progradation rates on rifted margin salt tectonics. We focus on a single, intermediate, rifted margin type and salt basin geometry to explore scenarios with different: (i) constant and (ii) time-varying post-salt sediment fluxes. We demonstrate that these promote significant contrasts in the style and magnitude of salt tectonics in the proximal, transitional and distal margin domains. The differences are primarily controlled by the relationship between the rates of sediment progradation (Vprog) and salt flow (Vs). When Vprog > Vs, the salt is rapidly buried and both vertical and lateral salt flow are suppressed across the entire margin. When Vprog < Vs, the salt flows vertically and seaward faster than sediments prograde producing major diapirism in the proximal domain and major distal nappe advance, but only moderate overburden extension and distal diapirism. When Vprog ~ Vs, there is moderate proximal diapirism and distal nappe advance, but major updip extension and downdip shortening, which produces major distal diapirism. Modelling results are comparable to various natural systems and help improve our understanding of the controls and dynamics of salt tectonics along salt-bearing rifted margins.publishedVersio

Links Between Faulting, Topography, and Sediment Production During Continental Rifting: Insights From Coupled Surface Process, Thermomechanical Modeling

Continental rifts form by extension, and their subsequent evolution depends on the tectonic and climatic boundary conditions. We investigate how faulting, topography, and the evolution of the sediment flux during rifting are affected by these boundary conditions, in particular whether it is possible to correlate tectonic activity, topography, and sediment flux on long timescales (40 Myr). We use a thermomechanical model coupled with a landscape evolution model and present a series of 14 models, testing the sensitivity of the models to crustal strength, extension rate, and fluvial erodibility. The degree of strain localization drives the structural evolution of the modeled rifts: slow extension, high crustal strength, and efficient surface processes promote a high degree of strain localization, resulting in fewer active faults with larger offset. Overall, the magnitude of sediment production correlates with the degree of strain localization. In case of unchanged erosional power and similar amount of extension, systems with slower extension produce more sediment owing to a stronger positive feedback between erosion and fault offset. We observe a characteristic sequence of events, reflecting the morpho-tectonic evolution of rifts: the highest rock uplift rates are observed before the maximum elevation, and the highest sediment flux postdates the peak in elevation. Our results indicate that for natural systems, the evolution of the sediment flux is a good proxy for the evolution of topography, and that a time lag of 2–5 Myr between the peaks in main tectonic activity and sediment flux can exist.publishedVersio

Relative continent - mid-ocean ridge elevation: a reference case for isostasy in geodynamics

The choice of crustal and mantle densities in numerical geodynamic models is usually based on convention. The isostatic component of the topography is not calibrated to fit observations resulting in not very well constrained elevations. The density distribution on Earth is not easy to constrain because it involves multiple variables (temperature, pressure, composition, and deformation). We aim in this study to provide a reference case for geodynamic modelling where crustal and mantle densities are calibrated to fit the relative continent/mid-ocean ridge elevation in agreement with observations. We first review observed Earth topography of stable continents and of active mid-ocean ridges and define the characteristic average elevation of these domains. We use self-consistent thermodynamic calculations of dry mantle rocks that include partial melting to calibrate densities of the continental lithospheric mantle and beneath the mid-ocean ridge. The thermodynamic solutions are coupled with a 2-D incompressible plane strain finite element method for viscous-plastic creeping flows to solve for the dynamic evolution during extension from continental rifting to mid-ocean spreading. The combined results from 2-D thermo-mechanical models and 1-D isostatic calculations show that the relative elevation difference between mid-ocean ridges and continents depends on crustal density, mantle composition, and the degree of depletion of the lithospheric mantle. Based on these results we calibrate the reference density that only depends on temperature, which can be used in classic thermo-mechanical models based on the Boussinesq approximation. Finally the model calibration provides a solution that fits (1) the elevation of active mid-ocean ridges far from hotspots (-2750 ± 250 m), (2) the elevation of stable continents far from hotspots (+400 ± 400 m), (3) the average depletion buoyancy of the continental lithospheric mantle (between -20 and -50 ± 15 kg/m3 depending on lithospheric thickness) and (4) the average continental crust density (2835 ± 35 kg/m3 for a 35 km thick crust).publishedVersio