Many rifted margins have formed in areas that have previously experienced subduction and orogenesis, completing the Wilson cycle of closing and opening oceans. Often the subduction phase is accompanied by the accretion of bathymetric highs, such as oceanic plateaus, continental fragments, seamounts and microcontinents. Such accretionary orogeneses result in a more complex structural, rheological and thermal inheritance than continent-continent collision without terranes. Here we use 2D thermo-mechanical numerical models to investigate how accretionary, rather than collisional orogens, affect a subsequent phase of continental rifting. Our models build an orogen through subduction, terrane accretion and collision before the onset of rifting. We examine the structure of the resulting rifted margins and the degree in which inherited compressional structures are utilized.

For rifting of collisional systems without terrane accretion, we find that there is a competition between structural and thermal inheritance that has a first order control on rifted margin architectures. For smaller, colder collisional systems, localized reactivation of the old subduction interface promotes the formation of narrow margins. Conversely, in larger, hotter collisional orogens, wide margins develop through distributed extension, initiating away from the inherited suture in the hot, weak regions of the pre-rift orogen. This dynamic persists even in the presence of accreted terranes, where the orogens preserve multiple suture-zones that dissect the lithosphere. In smaller orogens, the optimally oriented, steepest and as a result shortest, and hence weakest suture experiences the highest degree of inversion, localizing the rifting.. In larger, hotter accretionary orogens, deformation is not primarily focused on inherited shear zones but is instead concentrated in the thickest, hottest part of the orogen. We interpret this as thermal inheritance dominating over the influence of structural inheritance. Depending on the pre-rift lithosphere configuration, accreted terranes can be preserved in one or both rifted margins. Our results show that the size of the accretionary orogen prior to extension has the strongest influence on the style of the resulting rifted margins and that the presence of multiple sutures between the accreted terranes plays a smaller role in localizing extension.

Our experiments demonstrate that a wide range of features such as continental fragments, allochthons or hyper-extended segments that can form in the presence of inherited compressional structures and emphasize the importance of the deformation history in the evolution of continental rifting. These results can be further used to understand how various stages of the Wilson cycle affect each other. 

How to cite: Erdős, Z., Buiter, S., and Tetreault, J.: Rifting in the presence of accreted terranes – a numerical modelling study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1771, https://doi.org/10.5194/egusphere-egu24-1771, 2024.

The Wilson Cycle describes the repeated opening and closure of oceanic basins from continental rifting to continent-continent collision. The correlation between ancient orogenic belts and young rift systems highlights the significance of orogenic inheritance in shaping the complexities of rifted margins. Orogenic belts can be classified as either pure shear double-vergent or simple shear single-vergent orogens based on their rheological properties and lithospheric deformation mechanisms during lithospheric shortening. Therefore, their resulting pre-rift conditions differ significantly by providing varying inherited structure. The actual inversion from orogen to rift remains poorly understood. For instance, how does inheritance from orogenic processes affect the evolution and final architecture of rifted margins? 
To investigate this, a numerical forward model was applied that integrates geodynamic thermo-mechanical and landscape evolution software. The simulations include continental collision, post-orogenic collapse and continental rifting, and breakup, through velocity boundary conditions that vary from compression to extension over time. The two end-member orogens are generated by the adjustment of crustal rheology and erosion efficiency. For comparative analysis, we also simulate the extension of laterally homogeneous lithosphere without orogenic inheritance.
Results show that collision in cold and strong continental crust generally produces single-vergent orogens. The double-vergent orogen is formed in weak and hot continental crust with low erosional efficiency. However, a transition in the orogenic dynamics occurs under high erosional efficiency, leading to the development of single-vergent orogens for weak and hot crust. The double-vergent orogen features a wide zone of shortening (~350 km) with a large number of conjugate thrust faults. These faults all tend to reactivate as normal-faults during the subsequent phase of rifting and breakup generally occurs around an inherited, overthickened crustal root. These orogens produce largely symmetric rifts. In contrast, the single-vergent orogen is asymmetric with most shortening accommodated along one dominant interface during the orogenic stage. During rifting, this subduction interface is fully reactivated, accommodating most of the extension and determining the crustal breakup location. These orogens produce an asymmetric rift. In conclusion, orogenic inheritance controls the localization of deformation along pre-existing structural weaknesses and reactivation mechanisms, resulting in complex rifted margins.

How to cite: Li, K., Brune, S., Erdős, Z., and Glerum, A.: Numerical modelling of the Wilson Cycle: effects of orogenic inheritance on the formation of rifted continental margins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7743, https://doi.org/10.5194/egusphere-egu24-7743, 2024.

Continental rifting is a fundamental process of plate tectonics and the Wilson Cycle where weak zones within the continental lithosphere are exploited by both far-field and near-field forces to break-up the continental lithosphere (e.g Molnar et al., 2019). These pre-existing weak-zones are remnants of past tectonic deformation, delineated by shear zones, faults, and/or mobile belts. Reactivation of such inherited structures from previous tectonic phases has been attributed to several continental rift systems, for example, the Rhine graben, Rio Grande rift, Main Ethiopian Rift, Malawi Rift, and the Red Sea. In geodynamic modeling of continental rifts, these weak zones are often approximated by lithospheric thermal perturbation or a weak seed/fault to facilitate strain localization and initiate rifting in response to uniform stretching of the lithosphere. Here, we adopt a different approach building upon models by Salazar-Mora and Sacek (2022) and Peron-Pinvidic et al. (2022) to implement the inherited structures. We start with a geodynamic simulation of continental collision and orogenesis prior to extension but include the effect of temperature-dependent strain healing in the mantle (e.g. Fuchs and Becker, 2021) and time dependent plastic strain healing in the crust (e.g. Olive et al., 2016). We use a 2D geodynamic model ThermoMech (e.g. Xue et al., 2023) coupled to a landscape evolution model FastScape (Yuan et al., 2019), to explore the parameter space in an effort to understand the longevity of weak zones and their implications for rift initiation.

Fuchs, L. & Becker, T. W. (2021). Deformation Memory in the Lithosphere: A Comparison of Damage-dependent Weakening and Grain‐Size Sensitive Rheologies. J. Geophys. Res.: Solid Earth 126.

Molnar, N. E., Cruden, A. R., & Betts, P. G. (2019). Interactions between propagating rifts and linear weaknesses in the lower crust. Geosphere, 15(5), 1617–1640.

Olive, J.-A., Behn, M. D., Mittelstaedt, E., Ito, G. & Klein, B. Z. (2016). The role of elasticity in simulating long-term tectonic extension. Geophys. J. Int. 205, 728–743.

Peron-Pinvidic, G., Fourel, L. & Buiter, S. J. H.  (2022). The influence of orogenic collision inheritance on rifted margin architecture: Insights from comparing numerical experiments to the Mid-Norwegian margin. Tectonophysics 828, 229273.

Salazar-Mora, C. A. & Sacek, V. (2023). Effects of Tectonic Quiescence Between Orogeny and Rifting. Tectonics 42.

Xue, L., Muirhead, J. D., Moucha, R., Wright, L. J. M. & Scholz, C. A. (2023). The Impact of Climate-Driven Lake Level Changes on Mantle Melting in Continental Rifts. Geophys. Res. Lett. 50.

Yuan, X. P., Braun, J., Guerit, L., Rouby, D., & Cordonnier, G. (2019). A New Efficient Method to Solve the Stream Power Law Model Taking Into Account Sediment Deposition. J. Geophys. Res.: Earth Surface, 124(6), 1346–1365.

How to cite: Moucha, R. and Xue, L.: Modeling inherited structures and their effects on strain localization during continental rifting , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13268, https://doi.org/10.5194/egusphere-egu24-13268, 2024.

The development of asymmetric conjugate rifted margins has been explained by processes such as rift migration and sequential faulting (Brune et al., 2014; Ranero & Pérez-Gussinyé, 2010), and by the effects of lithospheric strength and strain-softening (Svartman Dias et al., 2015; Huismans & Beaumont, 2003) during rifting. Briefly, rift migration consists of sequential faulting of the upper crust that moves oceanward and is associated with lower crustal flow. Nonetheless, there are other thermal and dynamic parameters that might either facilitate or hinder the construction of an asymmetric margin, also depending on the coupling degree between the continental and mantle lithosphere. Since there are a considerable number of asymmetric margins around the world, mostly associated to petroleum fields, and more recently emerging as green hydrogen reservoirs, there is a need to understand which and how much the parameters influence the construction of asymmetric margins during the rifting phase. For that reason, this work aims to contribute to the understanding of this subject through thermo-mechanical numerical models. Velocity and thermal structure were the principal factors considered in the context of a decoupled lithosphere. Our models show that rift velocity is the principal parameter that controls width and margin asymmetry, being followed by thermal structure. High rift velocities (~5 cm/year) developed wide and asymmetric margins, while a thick upper crust is shown to be crucial to develop the distal domain in the late stages of rifting. When both parameters are combined, the generated margins can reach about 360 km long. In some scenarios, the margin width is up to 550 km, with a distal domain which exceeds 130 km long.

Funded by Petrobras Project 2022/00157-6.

Brune, S. et al. Rift migration explains continental margin asymmetry and crustal hyper-extension. Nature communications, v. 5, n. 1, p. 4014, 2014.

Huismans, R. S. & Beaumont, C. Symmetric and asymmetric lithospheric extension: Relative effects of frictional-plastic and viscous strain softening. Journal of Geophysical Research: Solid Earth, v. 108, n. B10, 2003.

Ranero, C. R. & Pérez-Gussinyé, M. Sequential faulting explains the asymmetry and extension discrepancy of conjugate margins. Nature, v. 468, n. 7321, p. 294-299, 2010.

Svartman Dias, A. E. et al. Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes. Journal of Geophysical Research: Solid Earth, v. 120, n. 12, p. 8672-8700, 2015.

How to cite: dos Santos Souza, S., Salazar-Mora, C. A., and Sacek, V.: The role of velocity and thermal structure in the construction of asymmetric rifted margins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2366, https://doi.org/10.5194/egusphere-egu24-2366, 2024.

During the breakup of the Pangea Supercontinent, rifting localized in portions of the continental lithosphere that comprised orogenic structural inheritances. This heterogeneous orogenic lithosphere is a result of mountain-building processes followed by post-orogenic tectonic quiescence. In the case of the Atlantic Ocean opening in its North, Central, and South segments, the time span between Gondwana-Pangea amalgamation and the onset of rifting is largely different, ranging from tens of Myrs to hundreds of Myrs. In this contribution, we discuss the effects of different tectonic quiescence periods of time on the pre-rift continental lithosphere and consequent variable conjugate rifted margin configurations. Here we present 2D thermo-mechanical numerical models that simulate a sequence of extension, contraction, quiescence, and final extension (i.e. accordion-like models). Through this process, our models self-consistently create the orogenic inheritance that undergoes quiescence and final rifting. We explored wide orogenic structures (i.e. without erosion) and narrow ones (i.e. with erosion). In the case of wide orogens, our models showed that tectonic quiescence periods between 30-60 Myrs developed symmetric conjugate rifted margins, where the lithospheric mantle broke up before the continental crust, which, in turn, hyperextended. Nearly 50% of the previously subducted continental crust remained in the fossil subduction zone after rifting. In the case of wide orogens with 100-300 Myrs of tectonic quiescence, the conjugate rifted margins are strongly asymmetric with one ultra-wide side. Nearly 80% of the previously subducted crust was educted during extension. Still in the wide orogens, but now with less than 30 Myrs of quiescence, the resulting rifted margins are asymmetric, not developing ultra-wide sides and having up to 90% of the previously subducted crust educted. Finally, the narrow orogens were not significantly influenced by tectonic quiescence periods in the construction of the final rifted margins, which resulted all asymmetric and rather narrow. In this case, the longer the quiescence, the more continental crust was preserved in the fossil subduction zone. These simulations show that the final rifted conjugates are strongly affected by an interplay between structural and thermal inheritances in the orogenic lithosphere. Wide orogens are hot due to high concentrations of heat-producing elements and grow laterally by orogenic spreading during longer periods of quiescence. Contrastingly, narrow orogens are cold and lack crustal material for wide rifted conjugates.

Funded by Petrobras Project 2022/00157-6.

How to cite: Salazar-Mora, C. A. and Sacek, V.: The role of pre-rift tectonic quiescence on rifted margins configuration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2390, https://doi.org/10.5194/egusphere-egu24-2390, 2024.

Transform faults most commonly exhibit offsets of 100 to 200 km, with a minority defined as mega-transforms with >200 km offsets. Consequently, these mega-transforms represent a relatively understudied feature of plate tectonics with our understanding of their formation and development currently incomplete. In this study, we use the numerical modelling software ASPECT (Advanced Solver for Problems in Earth's ConvecTion) to create high resolution 3D simulations of mega-transforms following oblique changes in plate motion. Specifically, we determine how inducing transpression and transtension across a mega-transform fault affects the development of new transforms and mid-ocean ridge segments. Our numerical models all implement an initial stage of orthogonal extension and continental break up along an offset rift, followed by a second stage of oblique extension across a wide range of extension azimuths (-75° to 75°). Here, we find that small transpressional changes in plate motion (-15°) lead to the development of a short 130 km long transform, whilst larger (-75°) changes in plate motion led to the development of a longer 300 km transform. Alternatively, increasingly oblique, transtensional deformation leads to increased rifting between the offset ridges with a >60° change in the extension orientation leading to continental rifting across the old transform margin. These results are analogous to real world examples such as the Davie (West Somali Basin) and Ungava Fault Zones (Davis Strait) where we also highlight the role of plate motion changes on continental cleaving. Additionally, the orientation of mid-ocean ridges and transforms in the Labrador Sea suggests a late phase of E-W extension prior to the cessation of spreading.

How to cite: Longley, L., Phethean, J., and Heron, P.: Deciphering the Role of Plate Motion Changes and Inherited Structures in Mega-Transform Fault Development Using Geodynamic Numerical Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4090, https://doi.org/10.5194/egusphere-egu24-4090, 2024.

During continental breakup, the width of a developing rift system is thought to be primarily controlled by crustal rheology, where weak and decoupled crust may develop into a wide (up to 300 km) rift system and strong crust can lead to localised thinning. Simultaneously, the development of magma-rich margins is increasingly being recognised to result from lithospheric mantle thinning prior to crustal thinning, allowing the development of both narrow and wide magma-rich continental margin systems. Seaward Dipping Reflectors (SDRs) and flat lying flows (FLFs) at magma-rich margins are generally considered to develop above rifting upper continental crust and flowing ductile lower continental crust, respectively, which in many instances contribute to isostatic buoyancy and therefore the subaerial eruption of lavas. Subsequent to continental breakup, therefore, ocean basin flooding readily occurs, leading to the production of classical oceanic crustal structure in a submerged basin (i.e. pillow basalts, sheeted dykes, and gabbro). What happens, however, if basin flooding is significantly delayed relative to breakup of the continental lithosphere? Here, we review evidence from the Mozambique Basin (and other magma-rich basins around the globe) to understand if basin flooding can postdate continental breakup and lead to the development of SDRs outboard of the continent ocean transition. In the Mozambique Basin, we find this unusual situation may have occurred locally despite the basin likely residing below sea level. This circumstance was facilitated by long-offset continent-continent transform faults isolating the basin within the continent interior during plate separation. Our findings have implications for the development of appropriate models of crustal structure at magma-rich continental margins and, therefore, our ability to appropriately interpret geophysical datasets, which often permit contrasting interpretations of crustal composition and distribution.

How to cite: Phethean, J. J. J. and Peace, A. L.: Dry ocean formation: Might some SDRs represent post-breakup non-classical oceanic crust?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20807, https://doi.org/10.5194/egusphere-egu24-20807, 2024.

The structure of the oceanic crust generated by the ultraslow-spreading mid-ocean Knipovich Ridge still remains relatively uninvestigated compared to the other North Atlantic spreading ridges further south. The complexity of the Knipovich Ridge, with its oblique ultraslow-spreading and segmentation, makes this end-member of Spreading Ridge Systems an important and challenging ridge to investigate. At spreading rates below 20 mm/y, ultraslow spreading ridges are characterized by a low melt supply. The Ocean Bottom Seismometer (OBS) data along a refraction/reflection profile (~280 km) crossing the Knipovich Ridge off the western Barents Sea was acquired by use of RV G.O. Sars on July 24 - August 6, 2019. The project partners are University of Bergen, Institute of Geophysics, Polish Academy of Sciences, and Hokkaido University. The seismic energy was emitted every 200 m by an array of air-guns with total volume of 80 l. To receive and record the seismic waves at the seafloor, ocean bottom seismometers were deployed at 12 positions with about 15-km spacing in 2 deployments. All the stations were recovered and correctly recorded data. Seismic energy from airgun shots were obtained up to 50 km from the OBSs. The profile provides information on the seismic crustal structure of the Knipovich Ridge and oceanic and continental crust in the transition zone. Seismic record sections were analyzed with 2D trial-and-error forward seismic modeling. This profile is a prolongation of the previously acquired profile AWI-20090200 (Hermann & Jokat 2013) and together will allow to interpret of ~535 km long transect crossing the Knipovich Ridge from the American to the European plate. This work is supported by the National Science Centre, Poland according to the agreement UMO-2017/25/B/ST10/00488. The cruise was funded by University of Bergen.

Hermann, T. and Jokat, W., 2013. Crustal structures of the Boreas Basin and the Knipovich Ridge, North Atlantic. Geophys. J. Int., 193, 1399–1414, doi: 10.1093/gji/ggt048

How to cite: Czuba, W., Mjelde, R., Murai, Y., and Janik, T.: Ocean Bottom Seismic Model in the Knipovich Ridge area, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3455, https://doi.org/10.5194/egusphere-egu24-3455, 2024.

In this field-based study, we investigate the Notre Dame Bay Magmatic Province (NDBMP) located in the Dunnage Zone, north-central Newfoundland, Canada. The NDBMP is a collection of rift-related intrusions dated at ca. 148 Ma (Late Jurassic, Tithonian), including the gabbroic Budgell Harbour Stock (BHS) and an associated lamprophyre dyke swarm. The host rock, composed of Ordovician-aged sedimentary and volcanic back-arc sequences, is metamorphosed to greenschist and locally amphibolite facies. The host rocks were deformed during the Ordovician-Silurian closure of the Iapetus Ocean. The primary Iapetus suture divides peri-Laurentian and peri-Gondwanan terranes in the Newfoundland Appalachians, and forms a Z-shaped flexure across the study area.

Our research focuses on three primary aspects: 1) investigating the relationship between pre-existing orogenic structures and rift-related magmatism, 2) assessing the impact of this magmatism on the host rock, and 3) analysing the post-intrusive deformation of lamprophyres. The dataset includes 178 structural measurements of lamprophyres, and host rock structures, petrographic analysis of thin sections of the BHS, lamprophyres, and host rocks, and 3D structural models created from drone-based photogrammetry for selected outcrops.

Our findings indicate that structures dating from the Ordovician to Silurian, associated with the Iapetus suture and Notre Dame Bay oroclinal flexure, significantly impacted the location and pathways of magmatism. This influence occurred at local scales, where dykes were deflected along bedding, foliation, and fold hinges, and on a larger scale along the Iapetus Suture. Additionally, multiple instances of magmatism affecting the host rock, including fracturing occurring subparallel to dykes, hydrothermal alteration, and brecciation were observed. Our investigation also identified three instances where dykes underwent brittle and ductile deformation due to the reactivation of pre-rift south-east dipping thrust faults with an oblique dextral motion towards the northeast. This movement is consistent with the direction of extensional forces the region experienced during Mesozoic rifting.

Preliminary findings suggest that the reactivation of these Ordovician-Silurian thrust faults reflect larger scale transtensional reactivation of the Iapetus suture zone during Mesozoic rifting and opening of the Atlantic Ocean. These results enhance our understanding of structural inheritance, which is essential for accurately modelling rifting processes and reconstructing the opening of the North Atlantic Ocean.

How to cite: Keefe, E., Peace, A., Guna, A. G., and McCausland, P.: Impact of pre-existing structures on the emplacement and post-intrusion deformation of the Late Jurassic rift-related Notre Dame Bay Magmatic Province, Newfoundland, Canada, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4212, https://doi.org/10.5194/egusphere-egu24-4212, 2024.

The Izu-Bonin-Mariana (IBM) subduction zone has one of the most significant advancing trenches on Earth, but the mechanism responsible for its trench advance remains in dispute. Slab pull from the Ryukyu subduction zone may have provided the main driver for this significant trench advance. However, it is unclear whether this slab-pull force can transmit through the weak zones, such as the young Shikoku and Parece Vela basins, the active Izu-Bonin rifts, and the continuous spreading Mariana Trough, and then act on the IBM trench. To figure out this issue, we conduct slab subduction numerical models to reproduce the spatio-temporal tectonic evolution of the Philippine Sea Plate. Model results show that the stretching rate of 2.5 cm/yr during rifting/spreading represents the critical threshold for the transmission of slab pull. Additionally, the lithospheric strengthening and weakening effects cancel out each other during the rift stage so that the slab pull from the Ryukyu Trench can transmit through the weak fossil spreading centers and intra-arc rifts and drive the Izu-Bonin Trench's advance. In contrast, lithospheric weakening overwhelms lithospheric strengthening and impedes stress transfer in the back-arc spreading stage, suggesting that the slab pull cannot directly pull the Mariana Trench to advance at present. We suggest that the Mariana Trench advance is driven by the continuous Izu-Bonin Trench advance from the north, which is supported by the fact that the Mariana Trench is further east than the Izu-Bonin Trench and that the IBM trench advance rate decreases southward.

How to cite: Jian, H. and Yang, T.: Slab pull drives IBM Trench advance despite the weakened Philippine Sea Plate, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4479, https://doi.org/10.5194/egusphere-egu24-4479, 2024.

Initiation of new plate boundary can be related to a spatio-temporal evolution of an intraplate vast diffuse deformation towards a narrow highly deforming boundary. It can also occur by reactivation of an inherited weak zone. In all cases, breaking a plate requires a weakening of the lithospheric cold mantle, whose rheological parameterisation often features a « yield strength » formulation that is not clearly related to actual deformation mechanisms. On the other hand, the bulk effective viscosity for mantle rocks, either cold lithospheric mantle or the hotter asthenosphere underneath, have multiple dependencies, that may co-evolve in geodynamic settings, e.g. temperature and strain rate increase during asthenosphere upwelling associated with plate extension.

In dynamic models, the (output) pattern of deformation localization cannot be directly predicted from the (input) flow laws governing material weakening, e.g. viscosity decrease when strain-rate or temperature increase. We currently lack diagnostics to quantify which rheological dependency weakens lithosphere through time.

Using finite-element Fluidity code, we designed 2-D upper-mantle thermomechanical models of plate extension. Simulations were run for various background strain rates (associated to various horizontal velocity profiles imposed along vertical sides) and for various rheological parameterizations featuring Newtonian diffusion creep, non-Newtonian low/high temperature dislocation creep, and/or yield stress. We propose diagnostics to quantify, through space and time, the weakening efficiency associated to thermomechanical parameters (here either strain-rate, or temperature) . The weakening efficiency is defined as the temporal variation of viscosity relative to only strain rate (or only temperature), normalized to the total viscosity variation. It is used to characterize the chronological sequence and feedbacks leading to deformation localization, and compare them for different rheological parameterizations. From these diagnostics, we discuss which deformation mechanisms are activated during plate extension and thinning, and the characteristic time-scale of successful or failed localization for various rheologies. We compare especially simulations featuring an ad hoc yield strength parameterization vs. low-temperature dislocation creep.

How to cite: Van Broeck, E., Thoraval, C., Garel, F., Arcay, D., and Davies, R.: From intraplate weakening to plate boundary: New diagnostics to quantify rheological controls on deformation localization in a simple extension set-up at lithospheric scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9359, https://doi.org/10.5194/egusphere-egu24-9359, 2024.

Three boreholes drilled during the International Ocean Discovery Program (IODP) Expedition 396 have yielded unexpected findings of altered granitic rocks covered by basalt flows, interbedded sediments, and glacial mud on the Kolga High situated near the continent-ocean transition on the mid-Norwegian margin.  To assess basin and basement structures near Kolga High in relation to the broader regional setting, a potential field forward modelling study was conducted. One specific goal was to evaluate the density distribution beneath the Kolga granite. The necessity of low-density crustal material beneath the Kolga High challenges the hypothesis of an old, thick, dense, and inherited basement high directly beneath the basalt, given the low gravity signal observed. In our potential field model, the rock density underneath the basalt remains relatively low (2.4 g.cm^-3 in average). Based on onshore measurements, Caledonian or Precambrian ‘fresh’ granitoids and other inherited basement rocks typically exhibit bulk densities usually exceeding 2.65-2.75 g.cm^-3. The gravity signal observed on Kolga High, along with the low-density necessary to fit it, suggests that the inherited basement should be situated at a considerably greater depth (~up to 10 km), which is approximately 5-7 km deeper than the drilled Kolga granite/basalt interface. To unravel the weathering chronology for this enigmatic granite, the K-Ar method was selected to date fine-grained clay minerals. X-ray diffraction was performed on different grain size fractions to identify both protolithic and authigenically formed K-bearing minerals derived from the IODP rock samples (Holes U1565A and U1566A). K-Ar geochronology was then performed on five grain size fractions (<0.1, 0.1-0.4, 0.4-2, 2–6, and 6–10 µm).  Finally, the crystallisation age of the granite was verified by conducting mineral analysis on 104 zircons using laser ablation inductively coupled with mass spectrometry (LA-ICP-MS). The K-Ar dating indicates that the alteration of the Kolga granite occurred between 54.7 ± 1 and 37.1 ± 1 Ma suggesting a long period of near surface exposure after the breakup. Based on U-Pb dating of zircon, the granite’s crystallization age is determined at 56.3 ± 0.2 Ma, which aligns with the Paleocene-Eocene Thermal Maximum (around 56 Ma). Collectively, insights from the gravity model and geochronology indicate that the Kolga granite is a Paleocene intrusion, likely emplaced under exceptionally shallow conditions, possibly preceding the breakup and opening of the Norwegian-Greenland Sea. The geochronological results indicate a remarkably short period of time between the granite emplacement, its near surface weathering, and the basaltic lava flows emplacement above the paleosurface. Incidentally, this intrusion also represents the most distal and youngest granite discovered in Norway. This study provide crucial paleogeographic constraints and helps to refine the mode of breakup of a nascent volcanic margin.

How to cite: Gernigon, L., Knies, J., Schönenberger, J., Piraquive, A., van der Lelij, R., Huyskens, M. H., Planke, S., Berndt, C., Jones, M., Millett, J. M., Mohn, G., and Alvarez Zarikian, C. A.: Understanding Volcanic Margin Evolution through the Lens of Norway's Youngest Granite discovered by IODP Expedition 396, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9699, https://doi.org/10.5194/egusphere-egu24-9699, 2024.

Former plate boundaries (sutures) are usually considered to be future locations for continental breakup, but this is not always the case. For example, continental rifting can generate a crustal fragment, where a sliver of a plate diverges from its component part and remains attached to another plate. Despite the prevalence of continental fragments and accreted terranes in the geological record, the underlying tectonic processes leading to their formation remain poorly understood. Previous geodynamic models have indicated structural and rheological heterogeneities inherited from past tectonic events as a key mechanism driving the initiation of continental breakup. Most of these studies have primarily focused on the styles of rifted margins, but limited attention is given to the mechanism of continental fragment formation.

In this study, we present a suite of over 100 different numerical models of inherited structures with the tectonic potential to generate a new continental fragment during continental extension. Our models show the first-order impact of structural inheritance on the evolution of rifting and continental fragmentation. Here, the size of the fragment is influenced by the extent and geometry of the inherited structures. By analyzing our models using novel data science techniques, we are able to quantify the impact of different initial conditions on generating a continental fragment. Our models provide a range of new physical constraints for the formation of continental fragments. Most importantly, they highlight the potential role of different forms of structural inheritance in controlling deformation within complex tectonic plate margins. Finally, we apply these findings to some real-world examples of continental fragments.

How to cite: Yu, A., Gün, E., McCaffrey, K., and Heron, P.: A recipe for continental fragment formation: big data analysis of rift models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12945, https://doi.org/10.5194/egusphere-egu24-12945, 2024.

The southern rifted margins of the South Atlantic are commonly regarded as some of the best examples of magma-rich margins with the Pelotas, Uruguay, Argentine and Namibia margins showing prominent Seaward Dipping Reflectors (SDRs). These volcanic SDRs are commonly interpreted as resulting from enhanced decompression melting during rifting and breakup from regionally elevated asthenosphere temperatures associated with the Parana-Etendeka mantle plume. We investigate the lateral variability of breakup volcanic addition along-strike of the Pelotas segment of the southern South Atlantic rifted margin offshore SE Brazil. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of approximately 300 km.

In the north of the Pelotas margin, where SDRs are thickest, the Torres High shows SDRs up to  20 km thickness. In contrast, in the south of the Pelotas margin, the magmatic addition is normal and SDRs are very thin or absent. Further south of the Pelotas margin, offshore Uruguay and northern Argentina, margins are again magma-rich with SDRs thickness reaching 10 km or more.The very thick SDRs of the northern Pelotas margin lay offshore of the thick Serra Geral volcanics of similar Cretaceaous age found onshore in the Santa Catalina, Parana, Sao Paulo and northern Rio Grande do Sul states of SE Brazil. Further south, Serra Geral volcanics are absent in the cratonic southern Rio Grande do Sul, which is onshore of the southern Pelotas margin with thin or absent SDRs and normal magmatic addition. The abrupt decrease in rift and breakup decompression melting from north to south along the Pelotas margin, and its increase to the south on the Uruguay and northern Argentina margins is inconsistent with the simple Parana-Etendeka mantle plume model. The correlation of magma-normal breakup in the southern Pelotas margin with cratonic geology onshore implies a significant contribution of lithosphere inheritance to decompression melting during rifting and breakup to form the southern South Atlantic margins.

A relationship is observed between the amount of volcanic material and the two way travel time (TWTT) of first proximal volcanics in seismic sections.  First volcanics are observed at 1.25s TWTT for the highly magmatic Torres High profile while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed at 4.2s TWTT or deeper. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions of a simple isostatic model of continental lithosphere thinning and decompression melting during breakup. This relationship between TWTT of first volcanics in seismic sections and the magnitude of magmatic addition may provide an effective means of mapping the distribution of breakup magmatic volume for the southern South Atlantic margins and its correlation with onshore geological inheritance.

How to cite: Colling Cassel, M., Kusznir, N., Manatschal, G., and Sauter, D.: Rapid Along-strike Variation of Breakup Volcanism on the Pelotas Margin, Offshore SE Brazil, South Atlantic and its Control by Lithosphere Inheritance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15801, https://doi.org/10.5194/egusphere-egu24-15801, 2024.

Recent 3D seismic reflection imaging has provided new insights into lithosphere extensional deformation processes in the hyper-extended domain of magma-poor rifted margins where extensional faults penetrate through the thinned continental crust into the topmost mantle. Seismic analysis shows that high-angle extensional faults sole out into a sub-horizontal reflector (the S-type reflector) in the top-most mantle. This reflector is interpreted as a horizontal detachment and has been shown to develop progressively oceanward with the in-sequence extensional faulting above.

We examine the evolution of fault geometries during extensional faulting in the hyper-extended domain. We show that the predictions of a recursive flexural rolling-hinge model  of planar faulting of thinned continental crust soling out into a horizontal detachment in the top-most mantle are consistent with the seismic interpretations. Our modelling shows that initially high-angle extensional faults are isostatically rotated to low-angle by oceanward in-sequence faulting and that their deeper segments form a continuous sub-horizontal structure in the top-most mantle corresponding to the S-type reflector imaged by seismic data.

Both 3D seismic interpretation and our modelling indicate that the sub-horizontal detachment imaged as the S-type reflector, and forming an apparent regional detachment, is not active simultaneously over its whole length in the dip-direction but that it developed oceanward incrementally together with the in-sequence high-angle extensional faulting above.

How to cite: Kusznir, N. and Gomez-Romeu, J.: A “Rolling Hinge” Model of the Incremental Oceanward Development of the S-type Reflector Horizontal Detachment at Magma-Poor Rifted Margins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15890, https://doi.org/10.5194/egusphere-egu24-15890, 2024.