How syn-rift sedimentation promotes the formation of hyper-extended margins

2020 ◽  
Author(s):  
Susanne Buiter
Keyword(s):  
Sedimentary Basins ◽  
Surface Processes ◽  
Tectonic Activity ◽  
Tectonic Deformation ◽  
Continental Margins ◽  
Rifted Margins ◽  
Continental Rifts ◽  
Break Up ◽  
Mountain Belts ◽  
Sediment Layers

<p>Seismic observations show that some rifted continental margins may have substantial amounts of offshore sediments. For example, sediment layers of several kilometres thick are found on the margins of Mid Norway, Namibia and Angola. Intriguingly, these margins are wide, being characterised by distances of several hundreds of kilometres from typical continental crustal thicknesses of 30-40 km to clearly identifiable oceanic crust. On the other hand, some margins that are sediment-starved, such as Goban Spur, Flemish Cap and Northern Norway, have short onshore-to-offshore transitions. Variations in the amount of sediments not only impact the development of offshore sedimentary basins, but the changes in mass balance by erosion and sedimentation can also interact with extensional tectonic processes. In convergent settings, such feedback relationships between erosion and tectonic deformation have long been highlighted: Erosion reduces the elevation and width of mountain belts and in turn tectonic activity and exhumation are focused at regions of enhanced erosion. But what is the role played by surface processes during formation of rifted continental margins?</p><p>I use geodynamic finite-element experiments to explore the response of continental rifts to erosion and sedimentation from initial rifting to continental break-up. The experiments predict that rifted margins with thick syn-rift sedimentary packages are more likely to form hyper-extended crust and require more stretching to achieve continental break-up than sediment-starved margins. These findings imply that surface processes can control the style of continental break-up and that the role of sedimentation in rifted margin evolution goes far beyond the simple exertion of a passive weight.</p>

A discussion on how, when and where surface processes interplay with extensional tectonic deformation

2021 ◽  
Author(s):  
Susanne Buiter
Keyword(s):  
Numerical Experiments ◽  
Large Scale ◽  
Source Region ◽  
Surface Processes ◽  
Tectonic Deformation ◽  
Main Process ◽  
Brittle Strength ◽  
Local Climate ◽  
Rifted Margins ◽  
Mountain Belts

<p>Fascinating feedback relationships between surface processes and tectonic deformation have long been highlighted for convergent settings. Mountains influence local climate, with precipitation increasing with mountain height and focusing at windward-facing slopes. The resulting erosion reduces the elevation and width of mountain belts, in turn leading to a focussing of tectonic deformation and exhumation at eroding regions. Thus, in convergent settings, erosion and tectonic deformation show positive feedback by enhancing each other. In comparison, the role of surface processes in extensional settings has received less attention, which does not mean that erosion or sedimentation might not equally affect tectonics deformation during extension. In this presentation, i will review theoretical expectations, discuss numerical experiments, and pose questions on how, when, and where surface processes interplay with tectonic deformation during extension.</p><p>How: The removal of material by erosion is expected to decrease vertical crustal stress and reduce brittle strength (which is the main process leading to focussing of deformation in shortening). Sedimentation conversely increases brittle strength. However, sediments of low thermal conductivity in extensional basins can trap heat, increasing crustal temperatures, and reducing viscous crustal strength. Will brittle strengthening or viscous weakening dominate during sedimentation? And during rifting, is erosion the controlling surface process, or sedimentation, or both?</p><p>When: Usually, subsidence needs to create accommodation space before sedimentation occurs and rocks should uplift before they can be eroded. This would imply that surface processes need time to start up and cannot play a decisive role in initial stages of deformation. This then begs the question: once an extensional system starts to deform in a certain style, can surface processes still change the style? For rift basins, we find from numerical experiments that sedimentation favours symmetric basins over asymmetric half-graben and single basins over distributed deformation. For rifted margins, i have found that sedimentation promotes hyperextension by forming wide areas of thinned continental crust, thus supressing early break-up. These experiments point out that surface processes seem to be able to exert a control on the style of rifting. But at which stage in rift evolution do surface processes start to play a role? And is there a crucial timing, after which erosion and sedimentation no longer influence the extensional style?</p><p>Where: Analogous to convergent tectonic settings, erosion of rift footwalls can enhance tectonic deformation and, on a large-scale, turn a ‘passive’ margin ‘active’ in a tectonic sense. Footwall uplift provides a sediment source region, linking erosion to offshore sedimentation. For rifted margins, where does deposition of sediments (whether they are brittle strengthening or viscous weakening) play the most influential role in the rifting process? Can strong near-footwall sedimentation suppress footwall uplift, thus providing a negative feedback in the system?</p>

scholarly journals Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins

2013 ◽  
Vol 30 ◽  
pp. 1-150 ◽  
Author(s):  
Paul F. Green ◽  
Karna Lidmar-Bergström ◽  
Peter Japsen ◽  
Johan M. Bonow ◽  
James A. Chalmers
Keyword(s):  
Sea Level ◽  
Sedimentary Basins ◽  
Landscape Analysis ◽  
Direct Result ◽  
Continental Margins ◽  
West Greenland ◽  
Base Level ◽  
Break Up ◽  
Geological Constraints ◽  
High Topography

The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs) around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene) from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view that peneplains grade towards base level, and that in the absence of other options (e.g. widespread resistant lithologies), the most likely base level is sea level. This is particularly so at continental margins due to their proximity to the adjacent ocean. Studies in which EPCMs are interpreted as related to rifting or break-up commonly favour histories involving continuous denudation of margins following rifting, and interpretation of thermochronology data in terms of monotonic cooling histories. However, in several regions, including southern Africa, south-east Australia and eastern Brazil, geological constraints demonstrate that such scenarios are inappropriate, and an episodic development involving post-breakup subsidence and burial followed later by uplift and denudation is more realistic. Such development is also indicated by the presence in sedimentary basins adjacent to many EPCMs of major erosional unconformities within the post-breakup sedimentary section which correlate with onshore denudation episodes. The nature of the processes responsible is not yet understood, but it seems likely that plate-scale forces are required in order to explain the regional extent of the effects involved. New geodynamic models are required to explain the episodic development of EPCMs, accommodating post-breakup subsidence and burial as well as subsequent uplift and denudation, long after break-up which created the characteristic, modern-day EPCM landscapes.

Kinematics and extent of the Liguro-Piemont Ocean

2020 ◽  
Author(s):  
Eline Le Breton ◽  
Sascha Brune ◽  
Kamil Ustaszewski ◽  
Sabin Zahirovic ◽  
Maria Seton ◽  
...  
Keyword(s):  
Kinematic Model ◽  
Proximal Part ◽  
Upper Jurassic ◽  
Western Mediterranean ◽  
Continental Margins ◽  
Geological Evidence ◽  
Adriatic Plate ◽  
Rifted Margins ◽  
Oceanic Spreading ◽  
Mountain Belts

<p>Assessing the extent of a former ocean, of which only remnants are found in mountain belts, is challenging but crucial to understand subduction and exhumation processes. Here we present new constraints on the opening and width of the Liguro-Piemont (LP) Ocean (or Alpine Tethys) in Mesozoic time using plate kinematic reconstructions of the Western Mediterranean-Alpine area.</p><p>Our kinematic model is based on a compilation of geological-geophysical data and published reconstructions of the opening of the Atlantic for the motion of Europe, Africa and Iberia, and of the Cenozoic deformation along fold-and-thrust belts (Alps, Apennines, Dinarides, Provence) and extensional basins (Liguro-Provencal Basin and Sicily Channel Rift Zone) for the motion of the Adriatic plate (Adria) and Sardinia-Corsica. For Jurassic and Cretaceous times, our main assumption is to avoid significant convergence or divergence between Adria and Africa and between Iberia and Sardinia-Corsica, as there is no geological evidence for such deformation. This implies in return strike-slip motion between southern France and Iberia-Sardinia-Corsica and within the Adriatic plate.</p><p>Our model shows that the LP basin opened in three phases: (1) first a slow extensional phase of c. 4 mm/yr (full rate) in Lower-Middle Jurassic between 200-165 Ma, followed by (2) a faster (up to 1.5 cm/yr) oblique extension in Middle-Upper Jurassic between 165-154 Ma, which coincides with emplacement ages of gabbros and pillow-lavas, and (3) a final main extensional phase in Upper Jurassic between 154 and 145 Ma, with rates up to 2.3 cm/yr. At 145 Ma, Iberia starts to move relative to Europe and thus extension in the LP domain decreases rapidly till it ceases completely at about 130 Ma. We interpret the first phase as rifting of the proximal part of the continental margins (200-165 Ma) followed by hyper-extension and formation of the ocean-continent transition zone (165-154 Ma), and break-up and ultra-slow oceanic spreading during the final third phase (mainly 154-145 Ma). Along a NW-SE transect between Corsica and northern Adria, we estimate the width of the LP Ocean to a maximum of ~ 240 km (oceanic domain) and the extent of the whole rifted margins to ~ 500 km, subdivided into ~380 km for the proximal and necking zones, and ~120 km for the hyper-extended and ocean-continent transition zones. Our results are supported by high-resolution thermo-mechanical modelling of the rifting phase that, using our kinematic constraints, reproduces very well the geometry of the Adriatic margin, as obtained by published geological reconstructions of the Southern Alps.</p><p>We test other kinematic scenarios for the motion of Sardinia-Corsica and for the opening of the Ionian Basin which would increase the obliquity of rifting and reduce even more the width of the extended domain. Therefore, our calculated extent of the LP Ocean constitutes a maximum estimate providing crucial constraints for geodynamic modelling and a better understanding of subduction processes during the Alpine Orogeny.<span> </span></p>

scholarly journals Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

2021 ◽  
Vol 9 ◽  
Author(s):  
Elmar Albers ◽  
Wolfgang Bach ◽  
Marta Pérez-Gussinyé ◽  
Catherine McCammon ◽  
Thomas Frederichs
Keyword(s):  
H2 Production ◽  
Continental Margins ◽  
The West ◽  
Microbial Life ◽  
Ocean Ridges ◽  
Rock Types ◽  
Rifted Margins ◽  
Slow Spreading ◽  
Break Up ◽  
Ocean Ridge

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) < 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to >300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at >200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.

scholarly journals Evolution of rift systems and their fault networks in response to surface processes

2021 ◽  
Author(s):  
Derek Neuharth ◽  
Sascha Brune ◽  
Thilo Wrona ◽  
Anne Glerum ◽  
Jean Braun ◽  
...  
Keyword(s):  
Sedimentary Basins ◽  
Field Studies ◽  
Fault Analysis ◽  
Growth Patterns ◽  
Surface Processes ◽  
Fault System ◽  
Normal Faults ◽  
Continental Margins ◽  
Geodynamic Models ◽  
Fault Growth

Continental rifting is responsible for the generation of major sedimentary basins, both during rift inception and during the formation of rifted continental margins. Geophysical and field studies revealed that rifts feature complex networks of normal faults but the factors controlling fault network properties and their evolution are still matter of debate. Here, we employ high-resolution 2D geodynamic models (ASPECT) including two-way coupling to a surface processes code (FastScape) to conduct 12 models of major rift types that are exposed to various degrees of erosion and sedimentation. We further present a novel quantitative fault analysis toolbox (Fatbox), which allows us to isolate fault growth patterns, the number of faults, and their length and displacement throughout rift history. Our analysis reveals that rift fault networks may evolve through five major phases: 1) distributed deformation and coalescence, 2) fault system growth, 3) fault system decline and basinward localization, 4) rift migration, and 5) breakup. These phases can be correlated to distinct rifted margin domains. Models of asymmetric rifting suggest rift migration is facilitated through both ductile and brittle deformation within a weak exhumation channel that rotates subhorizontally and remains active at low angles. In sedimentation-starved settings, this channel satisfies the conditions for serpentinization. We find that surface processes are not only able to enhance strain localization and to increase fault longevity but that they also reduce the total length of the fault system, prolong rift phases and delay continental breakup.

3D fault network development at the Galicia magma-poor margin, North-Atlantic

2020 ◽  
Author(s):  
Derren Cresswell ◽  
Gaël Lymer ◽  
Tim Reston
Keyword(s):  
Continental Crust ◽  
Active Faults ◽  
Detachment Fault ◽  
Continental Margins ◽  
Network Development ◽  
Stratigraphic Sequence ◽  
Rifted Margins ◽  
3D Data ◽  
Break Up ◽  
Fault Network

<p>We investigate the structures of hyper-extended continental crust and the 3D nature of the development of syn-rift fault networks at the Galicia margin, West of Spain, based on observations from a 3D multi-channel seismic reflection dataset acquired in 2013. This seismic volume provides, for the first time, 3D high-resolution imaging of a fault network geometry above a detachment fault (The “S reflector”) in the distal setting of a continental margin. The Galicia margin is sediment-starved, magma-poor and salt-free, thus providing optimal observations of the structures through seismic data.</p><p>We use the 3D data to observe the geometries of the faults, to analyse the fault heaves at different levels of the litho-stratigraphic sequence (i.e. at the top of the crystalline basement, at the top of the pre-rift/early syn-rift sediments and at the top of the syn-kinematic sediments), and to make a stratigraphic analysis to constrain the dynamics and the kinematics of fault activity within the successive half-grabens.</p><p>Our 3D interpretations demonstrate that the continental crust thins to zero during the rifting by the simultaneous development of initially individual fault planes, which progressively link with adjacent faults to form a network of active faults. The linked roots of the faults altogether form the surface of the S at depth, and allow the oceanward propagation of the detachment fault during the rifting. The faults throughout the network remained active and progressively rotated with further extension, until their deactivation when they acquired an angle of ~30°. Whereupon, a new network of active, initially isolated, faults developed and linked one step (~10 km) oceanward. The system repeats until the break-up of the continental crust, resulting in the progressive focussing of the locus of the extension toward the ocean, where the continental crust is the thinnest. </p><p>Given the similitude of the features observed at the Galicia margin with other magma poor continental margins, we expect that most margins worldwide might have formed following similar processes, thus representing a paradigm shift in the global understanding of late fault network development at rifted margins during continental break-up.</p>

scholarly journals Rheology and deep tectonics

1997 ◽  
Vol 40 (3) ◽  
Author(s):  
G. Ranalli
Keyword(s):  
Rheological Properties ◽  
Sedimentary Basins ◽  
Resolving Power ◽  
Tectonic Deformation ◽  
Continental Margins ◽  
First Order ◽  
Tectonic Processes ◽  
Tectonic Inversion

The distribution of the rheological properties of the lithosphere in space, and their variations in time, have a profound effect on the resulting tectonic deformation. A classical way of estimating these properties makes use of rheological profiles (strength envelopes). Although rheological profiles are based on assumptions and approximations which limit their resolving power, they are an efficient first-order tool for the study of lithosphere rheology, and their application clarifies the dynamics of tectonic processes. Two examples of the interaction of rheology and tectonics are discussed, namely, the post-orogenic relaxation of Moho topography (which is an additional factor to be considered in tectonic inversion), and the strength control on the level of necking in extension (which may lead to apparent local isostasy at passive continental margins and in sedimentary basins).

How submarine channels (re)shape continental margins

2020 ◽  
Vol 90 (11) ◽  
pp. 1581-1600
Author(s):  
Luke A. Pettinga ◽  
Zane R. Jobe
Keyword(s):  
Continental Margin ◽  
Evolution Model ◽  
Sediment Supply ◽  
Tectonic Deformation ◽  
Continental Margins ◽  
Sedimentary Processes ◽  
Submarine Channels ◽  
Active Tectonic ◽  
Tectonic Settings ◽  
Longitudinal Profiles

ABSTRACT Submarine landscapes, like their terrestrial counterparts, are sculpted by autogenic sedimentary processes toward morphologies at equilibrium with their allogenic controls. While submarine channels and nearby, inter-channel continental-margin areas share boundary conditions (e.g., terrestrial sediment supply, tectonic deformation), there are significant differences between the style, recurrence, and magnitude of their respective autogenic sedimentary processes. We predict that these process-based differences affect the rates of geomorphic change and equilibrium (i.e., graded) morphologies of submarine-channel and continental-margin longitudinal profiles. To gain insight into this proposed relationship, we document, classify (using machine learning), and analyze longitudinal profiles from 50 siliciclastic continental margins and associated submarine channels which represent a range of sediment-supply regimes and tectonic settings. These profiles tend to evolve toward smooth, lower-gradient longitudinal profiles, and we created a “smoothness” metric as a proxy for the relative maturity of these profiles toward the idealized equilibrium profile. Generally, higher smoothness values occur in systems with larger sediment supply, and the smoothness of channels typically exceeds that of the associated continental margin. We propose that the high rates of erosion, bypass, and deposition via sediment gravity flows act to smooth and mature channel profiles more rapidly than the surrounding continental margin, which is dominated by less-energetic diffusive sedimentary processes. Additionally, tectonic deformation will act to reduce the smoothness of these longitudinal profiles. Importantly, the relationship between total sediment supply and the difference between smoothness values of associated continental margins and submarine channels (the “smoothness Δ”) follows separate trends in passive and active tectonic settings, which we attribute to the variability in relative rates of smoothness development between channelized and inter-channel environments in the presence or absence of tectonic deformation. We propose two endmember pathways by which continental margins and submarine channels coevolve towards their respective equilibrium profiles with increased sediment supply: 1) Coupled Evolution Model (common in passive tectonic settings), in which the smoothness Δ increases only slightly before remaining static, and 2) Decoupled Evolution Model (common in active tectonic settings), in which the smoothness Δ increases more rapidly and to a greater final value. Our analysis indicates that the interaction of the allogenic factors of sediment supply and tectonic deformation with the autogenic sedimentary processes characteristic of channelized and inter-channel areas of the continental margin may account for much of the variability between coevolution pathways and depositional architectures.

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