Rifting, lithosphere breakup and volcanism: Comparison of magma-poor and volcanic rifted margins

Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.

The isostatic evolution and bathymetry of rifted margins depends on thinning of continental crust, the volume of magmatic additions, lithosphere thermal perturbation during rifting and its post-rift re-equilibration, and sediment loading. Additionally, at some margins, bathymetric evolution may also be affected by basin isolation, where eustatic variations are not controlled by global sea-level changes, and mantle plume dynamic uplift and its collapse. The relative influence of these contributors to rifted margin bathymetric evolution varies from example to example.Here we investigate the parameters controlling the palaeobathymetric evolution of the Nova Scotian rifted margin during the early stages of the opening of the Central Atlantic Ocean, following Triassic rifting, salt deposition and early Jurassic continental breakup. We use a 3D flexural backstripping technique which incorporates decompaction and post-breakup reverse thermal subsidence modelling to provide palaeobathymetric predictions through the Cretaceous down to the Late Triassic base salt.Quantitative analysis of seismic reflection and gravity anomaly data together with residual depth anomaly analyses have also been used to determine variations of crustal thickness and crustal type as well as volumes of magmatic addition emplaced during rifting and continental breakup. We show the magma-rich to magma-poor transition of the Nova Scotian margin, characterized by seaward dipping reflectors (SDRs) in the SW, while in the NE mantle is possibly exhumed.Comparison of our palaeobathymetric predictions with seismic observations and palaeoenvironments deduced from biostratigraphy of drill samples are in good agreement over the continental shelf. As expected, discrepancies exist more distally related to salt withdrawal and sediment gravity-driven sliding. Palaeobathymetries predicted seaward, on the first oceanic crust, range from 2 to 2.5 km; values in the range of those observed at young oceanic ridges.The oceanic crust of the SW Nova Scotian margin shows well developed sequences of SDRs. Their morphology resembles that of inner SDRs of volcanic margins like the Norwegian and Greenland margins (North Atlantic), where drilling results indicate that they correspond to lava-flows emplaced near or above sea-level. Our predicted palaeobathymetry of top SDRs at breakup is nearly ~2km deeper than the expected near sea-level. This discrepancy suggests that the subsidence of this thick oceanic crust with SDRs requires an additional mechanism in addition to post-rift thermal subsidence.Mantle plume uplift and collapse likely occurs at volcanic margins and has a long wavelength of the order of 500 km or more. However, the subsidence discrepancy we observe has a shorter wavelength and seems focused along the nascent spreading axis. Thinning of the thick oceanic crust after SDR emplacement by oceanward lateral flow of molten and ductile lower crust is an alternative possibility and may be a common occurrence at volcanic rifted margins after continental breakup.

The binary magma-rich vs. magma-poor classification of rifted margins was introduced to distinguish between margins showing markedly different crustal architectures, in particular related to the occurrence of magmatic products: the “magma-poor” qualifier is attributed to margins that display a domain of exhumed mantle and whose crustal wedge is exclusively made of continental material, while margins whose continental crust is heavily intruded and overlain by extrusive magmatic flows (e.g., seaward dipping reflections (SDRs) in seismic sections) are regarded as “magma-rich”. Yet, distinguishing between inherited continental crust, newly created magmatic crust and serpentinized mantle in seismic data is challenging due to the comparable geophysical properties (density and seismic velocity). The only interfaces that can usually be identified with some confidence on seismic images are the top of the pre-rift basement and seismic Moho, which allow the determination of the first-order crustal shape of rifted margins. We investigate what the shape of rifted margins can tell us about the timing and volume of magma emplacement during rifting. We use a simple geometric/kinematic model to explore how the volume of magma and the timing of emplacement relative to crustal thinning impact the crustal shape and discuss how this approach may help us to better interpret and understand the tectono-magmatic processes at play during rifting.We show that crustal shape and inflection points at distal margins can be used to identify magma-poor rifted margins and the occurrence of exhumed mantle. Moreover, the crustal shape and inflection points of magma-poor rifted margins provide direct insights into the dominant processes controlling crustal thinning (e.g., pure-shear stretching, viscoplastic necking, and Coulomb controlled hyperextension) and also the delay of magma emplacement with respect to crustal thinning (e.g., inherited depleted subcontinental mantle, extension rate).In contrast, shapes of magma-rich margins are more challenging to interpret due to the difficulty to distinguish between continental and magmatic material. We show that different factors may impact the budget and/or timing of magma emplacement and control their distinctive shape, including: (1) the initial conditions from inheritance (e.g., mantle temperature, fertility, and water content); (2) the mode of lithosphere extension (e.g., pure shear vs. depth-dependent lithosphere thinning); and (3) external rift-independent factors (e.g., elevated temperature from mantle plumes).Crustal shapes allow us to define modes and conditions of crustal thinning at so-called magma-poor rifted margins. In contrast, to interpret crustal shapes of so-called magma-rich rifted margins and understand their tectono-magmatic evolution requires additional information such as timing and budget of magma-emplacement in the crustal wedge, paleo-bathymetry and subsidence history.

Volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift. This is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting. However, seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track. Here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and/or lithospheric stretching rates, variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift. Using three-dimensional numerical experiments, we find that South Atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the Tristan Plume that "drove" this volcanic rifted margin, and that the region where plume material drains into the rift should experience long-lived uplift during rifting-both as observed. We conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin, but lateral along-rift flow from this plume, not a broad starting plume head, is what controls when and where a volcanic rifted margin will form.

Cenozoic rifting in the South China Sea developed after a Mesozoic Andean-type orogeny (i.e., Yanshanian orogen) which led to structural, compositional, and thermal inheritance.These inherited lithospheric weaknesses can control the inception and evolution of rifting, as well as the final architecture of the rifted continental margin. In order to better understand these processes, recent studies have utilized seismic profiles, drill cores, and geochronological analysis to identify Mesozoic strata, magmatic rocks related to a former arc, and pre-Cenozoic fault systems in the region. These findings reveal that the pre-rift lithosphere was heterogeneous and that inherited structures affected the subsequent Cenozoic rift evolution.Here we use multi-stage models to investigate the impact of tectonic inheritance on the spatiotemporal evolution and final rift margin architecture in the South China Sea. We employ a numerical forward model that includes a two-way coupling strategy (Neuharth et al., 2022) linking the geodynamic code ASPECT and the landscape evolution model FastScape. We reproduce the first-order kinematic evolution of the South China Sea by imposing accordion type models of continental collision, followed by extension. We present a reference model that incorporates orogenic topography, thrust fault distribution, and the architecture of the rifted margin, while also accounting for realistic crustal thicknesses, heat flow, and lithosphere-asthenosphere boundary (LAB) properties. This model was derived by conducting a systematic evaluation of a suite of models that varied in terms of lithosphere rheology, convergence velocity, heat production, erosion rate, and random initial noise distribution.Our reference model reproduces a range of observations including continental collision, post-orogenic collapse, continental rifting and lithospheric breakup. During orogeny, the lithosphere undergoes thrust faulting, and crustal thickening, leading to the formation of inherited weakness in the crust. From orogenic collapse to continental rifting, pre-existing thrust faults serve as nucleation sites for normal faults, and their interaction with later rift-related normal faults can locally modify the regional stress field. During rifting, low-angle detachment faults which connect the reactivated thrust faults contribute to the overall deformation of the lithosphere. In this model, crustal thickening led to increasing temperature, which resulted in a more ductile lower crust with a rheological transition from brittle to ductile deformation. This thermal weakening of the lower crust allows for increased deformation and strain accommodation during lithospheric stretching. The presence of pre-existing thrust faults and a more ductile lower crust ultimately led to the formation of wide rifted margin of the South China Sea. We suggest that this finding is applicable to other post-orogenic, wide rifts worldwide, such as the Basin and Range Province, the Aegean Sea and the West Anatolian extensional system.[1] Neuharth, D., Brune, S., Wrona, T., Glerum, A., Braun, J., & Yuan, X. (2022). Evolution of rift systems and their fault networks in response to surface processes. Tectonics, 41(3), e2021TC007166.

Volcanic rifted margins evolve by a combination of extrusive flood volcanism, intrusive magmatism, extension, uplift, and erosion. The temporal and spatial relationships between these processes are influenced by the plate tectonic regime; the preexisting lithosphere (thickness, composition, geothermal gradient); the upper mantle (temperature and character); the magma production rate; and the prevailing climatic system. Of the Atlantic rifted margins, 75% are believed to be volcanic, the cumulative expression of thermotectonic processes over 200 m.y. Volcanic rifted margins also characterize Ethiopia-Yemen, India-Australia, and Africa-Madagascar. The transition from continental flood volcanism (or formation of a large igneous province) to ocean ridge processes (mid-ocean ridge basalt) is marked by a prerift to synrift transition with formation of a subaerial and/or submarine seaward-dipping reflector series and a significant thickness (to 15 km) of juvenile, high-velocity lower crust seaboard of the continental rifted margin. Herein we outline the similarities and differences between volcanic rifted margins worldwide and list some of their diagnostic features.

ABSTRACTMost works propose a genetic “wide‐rift” model for the northern South China Sea (SCS) rifted margin, where low‐angle detachment faults accommodate significant deformation during crustal extension. However, a new seismic grid along the northern SCS shows along‐strike changes in tectonics. At least two distinct tectonic domains, i.e., “wide‐rift” Eastern Domain and “narrow‐rift” Western Domain, have been revealed, which indicates that the current conceptual SCS rift models are likely too simplistic. Whereas, the Western Domain remains little explored because of the lack of available 3D seismic data and boreholes here. The 3D rift architecture in this tectonic domain is therefore inadequately constrained, which leads to insufficient knowledge of the syn‐tectonic rift evolution in this region. Based on an unpublished 2D deep‐penetration grid of seismic reflection sections, we have investigated the faulting style, sedimentary structure, and crustal architecture in the Western Domain of the mid‐northern SCS. Our data display that the Western Domain contains the Changchang and Heshan Segments separated by a transfer fault zone. The aborted Changchang Segment is characterised by landward‐dipping faults and younger T60 breakup unconformity. The neighbouring Heshan Segment, reaching the final continental breakup, is characterised by oceanward‐dipping faults and older T70 breakup unconformity. The observations imply abrupt along‐strike rifting changes not contemplated by current models, requiring unexplained crustal or mantle heterogeneity during extension.

Lower crustal bodies in the Møre volcanic rifted margin: Geophysical determination and geological implications

Rifted margins form by the thinning and stretching of continental lithosphere until it ruptures, forming new oceanic crust and lithosphere, which can be accompanied by decompression melting and the addition of magmatic material. Despite numerous studies on magma-rich margins, we still do not fully understand how and when magmatic features form at rifted margins. To address this I investigate the formation of various magmatic features on three rifted margins, the East Indian margin, the Pelotas margin and the Southeast (SE) Greenland margin, using a range of quantitative techniques. The East Indian margin is an often-cited example of a magma-poor margin; however, some interpretations suggest the transition from exhumed mantle to oceanic crust consists of 9 km thick magmatic crust. Gravity inversion, RDA analysis, subsidence analysis and joint inversion of seismic and gravity data alongside seismic observations, reveal the presence of magma-poor and magma-rich characteristics in the form of exhumed mantle and 9 km thick magmatic crust juxtaposed against each other, resulting from a two-stage breakup. Juxtaposition of end-member characteristics suggests that the use of end-member terminology based on volumes of magma alone is misleading. The Pelotas margin in the South Atlantic shows an extraordinarily thick sequence of seaward dipping reflectors (SDRs), of which the composition and formation is poorly understood. I investigate these SDRs using gravity inversion with a sensitivity to basalt/sediment composition, flexural backstripping and reverse thermal subsidence modelling, joint inversion of seismic and gravity data and seismic observations. I show there are two types of SDRs present on the Pelotas margin, an inner subaerial set of SDRs formed of basalt during pre-breakup intra-continental rifting and an outer set of SDRs formed of a mix of volcaniclastics and basalts during breakup in a subaqueous environment at an embryonic mid-ocean ridge. The SE Greenland margin in the North Atlantic has a broad region of ~15 km thick crust and exhibits strong crustal asymmetry with its conjugate Hatton Bank similar to magma-poor margins. I investigate whether the SE Greenland margin consists of magmatic crust or hyper-extended continental crust sandwiched by magmatic material. Gravity inversion, joint inversion of seismic and gravity data as well as seismic velocity analysis suggest the 15 km thick crust on the SE Greenland margin is magmatic rather than a sandwich of thinned continental crust and magmatic additions. This interpretation requires a sharp continent-ocean boundary, similar to Hatton Bank. Together, these case studies investigate the relationship between magmatism and breakup at rifted margins. The East Indian margin suggests that the use of end-member terminology in the classification of rifted margins is misleading when based only on the magmatic budget. The Pelotas margin shows how extrusive magmatism can record different stages in margin formation. Finally, the SE Greenland margin shows the importance of using quantitative techniques to interpret margin structure and subsequent formation processes.

Seaward‐dipping sequences (SDS) are typically observed in sediment‐driven progradation of slope deposits in post‐rift sequences, or along magma‐rich rifted margins, where they are primarily formed by subaerial lava flows and referred to as seaward dipping reflections (SDRs). However, new seismic reflection data from the sediment‐rich northwestern South China Sea (NW‐SCS) reveal the existence of SDS in sediment‐rich syn‐rift sequences, here referred to as syn‐rift sedimentary SDS. Based on a detailed tectono‐stratigraphic analysis, we describe how these syn‐rift sedimentary SDS differ from magmatic SDRs and propose a conceptual model for their formation. We show that the formation of these sequences is most likely controlled by a combination of high sediment supply rates, rapid subsidence and creation of horizontal accommodation space, first along large‐offset detachment faults and later, during breakup, by magmatic accretion. While inner magmatic SDRs typically form in subaerial conditions above continent‐dipping faults, we suggest that syn‐rift sedimentary SDS can derive from turbiditic flows in a relatively deep‐water environment above oceanward‐dipping detachment faults and/or interfinger with magmatic additions. Finally, we discuss the implications of syn‐rift sedimentary SDS for the interpretation of rifted margins in general.

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.

Break-up–related extrusive magmatism, imaged in reflection seismic data as seaward-dipping reflectors (SDRs), extends symmetrically along the volcanic margins of the Atlantic Ocean. Recent research found distinct along-margin variations in the distribution of SDRs, and abundance of volcanic material was found to be spatially linked to transfer fault systems. These segmented the propagating rift that later developed into the ocean, and are interpreted as rift propagation barriers. Based on these observations, we develop a numerical model, which shows that rift-parallel mantle flow and locally enhanced rates of volcanism are the result of delays in rift propagation and segmented opening. Our model suggests that segmentation is one of the major factors in the distribution and localization of riftrelated extrusive magmatism. We conclude that in addition to mantle temperature and inherited crustal structures (e.g., weaknesses from previous rift episodes), rift propagation delay plays an important role in the distribution of extrusive volcanism at volcanic passive margins by controlling the mantle flow beneath the rift axis.

The processes related to hyperextension, exhumed mantle domains, lithospheric breakup, and formation of first unequivocal oceanic crust at magma-poor rifted margins are yet poorly understood. In this paper, we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal Australian-Antarctic rifted margins. We propose a new interpretation, linking the sedimentary architectures to the nature and type of basement units, including hyperextended crust, exhumed mantle, embryonic, and steady state oceanic crusts. One major implication of our study is that terms like prerift, synrift, and postrift cannot be used in such polyphase settings, which also invalidates the concept of breakup unconformity. Integration and correlation of all available data, particular seismic and potential field data, allows us to propose a new model to explain the evolution of magma-poor distal rifted margins involving multiple and complex detachment systems. We propose that lithospheric breakup occurs after a phase of proto-oceanic crust formation, associated with a substantial magma supply. First steady state oceanic crust may therefore not have been emplaced before ~53.3 Ma corresponding to magnetic anomaly C24. Observations of magma amount and its distribution along the margins highlight a close magma-fault relationship during the development of these margins.

The continental margin of SW Africa is typical of a volcanic rifted margin associated with a hotspot trail characterized by a large volcanic ridge, the Walvis Ridge, defining the hotspot migration, and extensive extrusive volcanism that produced seaward-dipping reflectors (SDR). Previously unpublished seismic data show two significant anomalies of the SW African Margin when compared to other typical volcanic rifted margins: (1) Hyaloclastitic outer highs are rare, and (2) the SDR in the North dip towards the Walvis Ridge. We explain these anomalies by a major transform segment close to the centre of volcanism combined with pulsed volcanism. The Walvis Ridge represents an east-west striking extrusive centre which produced a SDR sequence. Following break-up the northern boundary of the Walvis Ridge became a left lateral transform fault. Our data support the idea that a transform fault system interacting with a ridge jump were responsible for the accretion of the Sao Paulo Plateau to the American plate.

<p>The transition between continental and oceanic lithosphere in rifted margins can display a wide range of characteristics, which primarily depend on the regional tectonic evolution. Rifted margins form when continents rift apart and are commonly characterized by a thinned transition zone between the continental crust and the oceanic crust. The velocity and duration of the rifting process influence the dimensions and geometry of the passive margin. Rifted (or passive) margins are often subdivided in a magma-rich type and a magma-poor type, where the magma-rich are characterized by large input of mafic melt, derived from the mantle, into the crust. Magma-poor rifted margins on the other hand are characterized by much less magma production during the rifting process. This causes high variability in the geometry and rheology of passive margins.</p><p>The aim of this work is to understand how different types of passive margins can influence the dynamics of continental collision. We modelled subduction using the finite element code Citcom and to describe the dynamics of continental collision we mainly focused on the time and position of the slab break-off after the collision and on the fate of the passive margin material.</p><p>We compared these models as a function of various parameters (e.g., margin length, density, and viscosity), in order to understand how the architecture of a passive margin affects the dynamics of continental collision. We find that passive margins have a noticeable impact on subduction, as we observe a large variability in slab break-off times (about 10–70 Myr after continental collision) and depth (about 200–450 km). Furthermore, the factor that shows the largest impact on subduction dynamics is the rheology of the passive margin. Our results show that for both magma-poor and magma-rich margins, part of the margin does not subduct but, instead, exhumes and accretes to the overriding plate. Importantly, the amount of accreted material to the overriding plate is much larger when the passive margin is magma-poor compared to the magma-rich case. This is consistent with geological observations that fossil magma-poor passive margins are preserved in many mountain ranges, such as the Alps and the Scandinavian Caledonides, whereas remnants of magma-rich rifted margins are scarce. Because, in our models, the slab break-off occurs inboard of the LCB, magma-rich rifted margin may only be preserved when the density of the LCB is similar to that of the rest of the continental plate. Therefore magma-rich rifted margins are prone to be subducted and recycled into the mantle. Importantly, our results show that rifted margin type controls the architecture of the subsequent collisional phase of the Wilson cycle.</p>