The tectono-stratigraphic and magmatic evolution of conjugate rifted margins

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Types of rifted margins

Drilling of the North Atlantic distal margins during the different ODP Legs in the late 1980ies and in particular the 1990ies enabled to establish two main observations: 1) the occurrence of wide zones of exhumed subcontinental mantle linked to large offset normal faults and creation of new real estate in the southern North Atlantic (Boillot et al., 1980), and 2) the drilling of SDRs confirming that they formed as subaerial basaltic flows in the northern North Atlantic. These observations set the stage for the subsequent subdivision of the margins into volcanic and non-volcanic (Boillot et al., 1987; Endholm et al., 1989), referred later as magma-poor or magma-rich (for reviews see Péron-Pinvidic et al., 2013; Franke et al., 2013). While the Iberia rifted margin became the archetype of a non-volcanic/magma-poor margin (Whitmarsh et al., 2001), the Norwegian-Greenland margins became the archetypes of volcanic/magma-rich margins (Eldholm et al., 1989). More recent drilling of the South China Sea (SCS) during IODP Legs 367/368, enabled to identify an intermediate type of rifted margin, which will be the focus of this study. Before concentrating on this new type and the SCS, I first provide an overview of the main three archetypes of rifted margins that are the magma-poor, magma-rich and transform margins.

Magma-poor margins

Magma-poor margins, as shown in Fig. I.4a, show broad zones of continental thinning, also referred to as crustal wedging, including faults and the formation of tilted blocks (Manatschal, 2004a, 2004b; Reston, 2007; Peron-Pinvindic et al., 2019). Hyperextended margins refer to margins where the continental crust is thinned to less than 10 km, the crust is embrittled and wedges out oceanward. Fluids hydrate the residual thin crust and underlying mantle, resulting in weak faults and a serpentinized uppermost mantle, which enable the development of long offset extensional detachment faults (Doré and Lundin, 2015; Tugend et al., 2015). Conjugate margins are often asymmetric: one margin is the footwall of the major detachment system, resulting in a significantly wider lower plate margin, and the other forms the hanging wall of the detachment system, resulting in a sharper upper plate margin (see Fig. I.3c; for original concept see Lister et al., 1991). At magma-poor rifted margins, crustal separation precedes magma-emplacement and formation of first mature oceanic crust, resulting in a wide zone of exhumed subcontinental mantle (Manatschal, 2004). Magma-poor rifted margins have been associated with slow extension (e.g., Bown and White, 1995; Perez-Gussinyé et al., 2006; Doré and Lundin, 2015), cold mantle (Boillot et al. 1987) or depleted mantle (Müntener and Manatschal, 2006; Chenin et al., 2018). Models for magma-poor margins include several stages comprising both pure-shear and simple-shear phases (Fig. I.3c). Based on geophysical and geological observations from the West Iberian margin and geological mapping in the Alps, Whitemarsh et al. (2001) presented a conceptual model of magma-poor rifted margins that includes a phase of lithospheric necking and diffuse faulting, followed by a later phase of localization of rifting and the onset of seafloor spreading. Later models tried to explain the apparent discrepancy between displacement along the faults and the amount of extension through exhumation of mid-crustal and mantle rocks (Lavier and Manatschal, 2006; Mohn et al., 2012) or through a later phase of deformation after embrittlement of the crust (Reston, 2007; Gussinyer-Perez and Raneiro, 2010). Péron-Pinvidic and Manatschal (2009) proposed a conceptual model to explain the evolution of the margins, integrating most observations and ideas derived from the study of the Newfoundland-Iberian and Alpine Tethys margins. This model, shown in Fig. I.3c; describes a succession of modes, referred to as stretching, necking and hyperextension. These modes refer to the amount and style of crustal/lithospheric thinning (Sutra et al., 2013), where stretching is characterized by low ß values of < 1,5, necking is characterized by a thinning of the crust to ≤ 10 km and hyperextension starts when the crust has been stretched to full embrittlement, usually at a crustal stretching factor of ca. 3–4 (Perez-Gussinyé and Reston, 2001). Hyperextension ends either when subcontinental mantle starts to be exhumed or when magma is emplaced and leads to lithospheric breakup and steady-state seafloor spreading, as shown in this study.

Magma-rich margins

At magma-rich margins (Fig. I.4b) onset of magmatism resulting in the formation of Seaward Dipping Reflections (SDR) can bypass all extensional stages and force the lithosphere to breakup contemporaneously with the separation of the continental crust. At these margins, onset of magma formation occurs prior to crustal separation. In such examples mantle rocks are not exhumed and the transition between the continent and the first oceanic crust is characterized by large amounts of extrusive lavas forming thick SDR sequences and intrusives flooring the lower crust, often referred to as high-velocity bodies (e. g., Hinz et al., 1993; Eldholm and Grue, 1994; Thybo et al., 2013;
Franke., 2013; Geoffroy et al., 2015). Most magma-rich margins show crustal thinning prior to onset of the first magma, however, it remains yet unclear if crustal thinning continues after onset of magma (Tomasi et al., 2021), or alternatively if the crust is literally separated by the arrival of first magma, as suggested by Buck (2007). While in the latter case the contact between continental and fully magmatic crust is sharp and breakup is localized and fast in the former case crustal thinning can occur simultaneous to magmatic accretion resulting in a diffuse breakup including hybrid crust as observed in this study. For a discussion about the magmatic budget during breakup see also Tugend et al., (2020). Another debated question is if serpentinized upper mantle can occur underneath the SDR sequences on volcanic margins and if magma-rich rifted margins can start to evolve from a typical magma-poor margin (Lundin et al., 2018; Ros et al., 2017). Zhang et al. (2020) also showed that even in the absence of SDRs, large amounts of magma can pound in sediments forming syn-rift laccolith. Another question is related to the link between rifting and plume or hotspots. If rifts are associated with plumes (e.g., Tomasi et al. 2021), the emplacement of magma can happen before or during early stages of rifting and can force the breakup. Franke et al. (2013) proposed a link between rifting and plumes. Buiter and Torsvik, (2014) suggested that rifting precedes plume emplacement indicating a tectonic control. Thus, at present the understanding of magmatic processes at rifted margins is mainly hampered by the lack of the understanding of how magmatic systems interact with extensional systems, a subject that I will address in this study.

Transform margins

A third group of continental margins, of lesser importance for this study, are the so-called transform or highly oblique margins (for a review see Mercier de Lépinay et al., 2016). Transform/highly oblique rifting might be one of the most efficient ways to break-up continents since lower forces are required to break obliquely through cratonic crust (Lundin and Doré, 2019; Brune et al., 2012). The kinematic evolution of transform margins is suggested to include three stages (Mascle and Blarez, 1987): 1) the intra-continental transform fault stage such as for example the San Andreas and Dead Sea faults resulting in pull apart basins; 2) the active transform margin stage, when continental lithosphere slides against oceanic lithosphere; and 3) the passive transform margin, after the spreading center has passed along the margin and the transform margin becomes inactive. The length of a transform margin depends on three parameters: the obliquity between the regional trend and the relative plate motion, the width of the continental rift and timing of the deformation partitioning (Basile, 2015). One characteristic feature of many transform margins is the existence of marginal ridges parallel to the transform margin (Basile et al., 1996) and transform marginal plateaus that correspond to bathymetric highs at continental margins, situated at the limit between oceanic basins of different ages (Loncke et al., 2020). Examples in the Atlantic Ocean are the Falkland, Demerara and Guinea plateaus (Schimschal and Jokat, 2018, 2019; Mercier de Lepinay, 2016; Parsiegla et al., 2008; Olyphant et al., 2017). Several hypotheses have tried to explain the existence of these marginal ridges such as: (1) crustal thickening from transpression in the intra-continental transform zone (e.g., Huguen et al., 2001); (2) thermal uplift due to heating of the continental lithosphere by the hotter oceanic lithosphere (e.g., Scrutton, 1979; Reid, 1989); and (3) flexural response of the lithosphere due to unloading by erosion (e.g., Basile and Allemand, 2002). Modern numerical studies hypothesize that most margin segments (more than 70%) do open in an oblique manner (exceed an obliquity of 20%) (e.g., Brune, 2014; Brune et al., 2014, 2018), an idea that can be tested in the propagator of the NW-SCS.

Distribution of types of margins

In the early 2000ies, with the access to more and higher quality reflection seismic data, mainly from ION, TGS and other geophysical companies, the world margins have been classified as either non-volcanic, volcanic or transform margins (Fig. I.5). This enabled to show that there is approximately as many non-volcanic rifted margins as volcanic margins. The mapping was mainly based on two criteria: 1) the existence of SDRs forming smooth top basement dipping down onto oceanic crust, and 2) the occurrence of a structured top basement stepping up unto oceanic crust. While the former is characteristic of volcanic margins, the later qualifies a margin as non-volcanic (Fig. I.4). The distribution of volcanic margins appears to be linked to large igneous provinces and elevated potential mantle temperatures (Smallwood and White, 2002). However, the thermal influence of a hotspot is not well determined (Austin et al., 1990) and the relation between volcanic margins and the occurrence of hotspots is not straightforward (e.g., Franke et al., 2013). It is frequently suggested that non-volcanic margins result from slow extension, so it could be assumed that volcanic margins are associated with rapid extension. However, the South and Central Atlantic, which are both characterized by volcanic and non-volcanic segments, have been opening at ultra-slow to slow rates. At the North Atlantic margin, opening occurred at a rate of ca.
25 mm/yr half spreading rate, but subsequently half-spreading rates rapidly decreased to about 10 mm/yr (Le Breton et al., 2012). Other explanations for volcanic margins, apart from the mantle temperature during extension are the inherited fertile nature of the mantle. The relative ratio between extruded vs intruded magmatic additions, another little understood parameter in magmatic systems, may be controlled by factors such as magma composition, tectono-magmatic interactions, crustal rheology and composition and presence of fluids (e.g., Koopmann et al., 2014a; Tugend et al., 2020; Gouiza and Paton, 2019; Zhang et al., 2020).
Data Center) showing the distribution of magma-poor, magma-rich and transform margins. Transform margins can also be classified as magma-poor or magma-rich, however, the present state of knowledge about these margins is meager and prevent such a classification at the moment. Modified after Haupert et al. (2016).
The present classification of magma-poor vs magma-rich rifted margins is mainly based on the contrasting magmatic budget identified at their distal settings. This binary view put the magmatic budget and the occurrence or absence of SDRs in the center of all studies, an evolution that continues until present. However, this way of looking rifted margins underscores the observation that all margins show magmatic additions as well as evidence for crustal thinning. Thus, the two-fold division may have been overly simplistic (Mutter, 1993). Müntener et al. (2010) demonstrated that at so-called magma-poor margins magma was produced during rifting but was not extracted, and therefore it is difficult to recognize. In contrast, the magmatic budget of volcanic margins may have been overestimated (van Wijk et al., 2004; Tugend et al., 2020). Thus, the use of the more flexible terms “magma-poor” and “magma-rich” is more appropriate than that of non-volcanic and volcanic margins and it is likely that the full spectrum of margins between these end members may exist (e.g., Franke, 2013). The results of my PhD show that neither SDRs nor exhumed mantle domains exist in the NW-SCS, and that rifts can fail even after breakup, pointing out that rifted margins are far more complex than the commonly used binary view described above. While the magma-rich vs magma-poor classification appears to work for many present-day Atlantic rifted margins, it does not for the SCS. Thus, one of the aims of this study is to make detailed observations on high-resolution seismic sections from the NW-SCS Ocean Continent Transition (OCT) with the aim to provide additional observations to better characterize the nature of its rifted margins.

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Rifted margins: ongoing research and open questions

The hydrocarbon industry and related service companies have undoubtedly been main drivers for the research at rifted margins. The improvement of reflection seismic imaging techniques and their global application enabled to better describe the architecture of rifts and rifted margins. The combination of new observations with the development of new modelling approaches and the study of margin remnants in collisional orogens enabled to develop a better understanding of the processes controlling the evolution of rift systems. The joint efforts between industry and academia made that at present we can explore and better understand the sedimentary archives of passive margins, which can help address some of the most pressing environmental issues presently faced by humanity. These include, for example, an understanding of rapid climate change and the link between slow and fast greenhouse gas cycles (methane and CO2). More recent studies showed that, magma production during rifting and breakup has also been related to massive CO2 production, which could explain natural excursions of methane and CO2 in Earth history and may in turn explain climate change, such as the Paleocene-Eocene Thermal Maximum (PETM) that occurred simultaneous to the formation of the conjugate North Atlantic Volcanic margins ca. 55 Ma (e.g., Svensen et al., 2004; Brune et al., 2017b). Pinto et al., (2017) and Abers et al. (2021) also suggested that mantle exhumation at magma-poor rifted margins and related serpentinization can account for substantial mass transfer of elements from the mantle into the hydrosphere, as well as the production of high volumes of methane and native hydrogen. Future research on rifted margins will be necessary to provide further details and confirm these results. However, a prerequisite for such studies is to understand how to interpret critical events such as magmatic breakup and/or serpentinization and to find the time equivalent deposits that may record these events.
Another challenge that the margin community is facing it the understanding of continental/lithospheric breakup. These studies have two main implications. A first one is linked to the opening of gateways that exerts a strong influence on the circulation of oceanic currents, and hence on the evolution and distribution of life (e.g., IODP Expedition 388). This evolution also controls the restriction of vast basins during rifting resulting in the formation of giant evaporite provinces favoring the preservation of organic material, as observed in many hyperextended rifted margins (e.g., South Atlantic, Gulf of Mexico, Central Atlantic) (Rowan, 2014). Thus, the formation of rifted margins, in particular during the stages preceding breakup, can have a fundamental impact on the paleo-climate, oceanography, and the redistribution of elements, gazes and source rocks in the Earth system. A second aspect of breakup relates to the tectono-magmatic processes that lead to plate separation and onset of seafloor spreading. While in the past the academic community was more focused on understanding the evolution of either rifts or mid ocean ridges, the breakup process has been less investigated. The fact that present-day rifted margins are not tectonically active makes it more difficult to image and understand the processes that are at the origin of crustal and lithospheric breakup. One of the aims of my PhD thesis will be to investigate the architecture of the OCT and the related processes controlling breakup.
A last, and more general theme of current research is the testing and generalization of existing rift models that have been mainly developed for magma-poor rifting in the North Atlantic to other rifted margins (Mjelde et al., 2002; Péron-Pinvindic et al., 2013; Tugend et al., 2018; Sapin et al. 2021). This leads to the fundamental question of what controls rifting. Existing observations suggest a complex link between magmatic/asthenospheric and structural/lithospheric processes that cannot be easily predicted. The along strike spatial and temporal changes observed in the evolution of rift systems reflect the interplay between their inheritance (innate/ »genetic code ») and the physical processes at play (acquired/external factors) nicely documented in the example of the North Atlantic but difficult to generalize and apply to other margins such as those in the SCS. Thus, the aim of this study is to use long offset, high-resolution reflection seismic data to investigate the margins in the NW SCS and to compare the results with present-day models. Of key importance are the observation of the structural and magmatic variability along rifted margins and to link them with the stratigraphic record. In order to do so, I use a methodological approach to describe rifting evolution that will be further described in in section 1.6.
The processes controlling the formation of deep-water sedimentary basins, lithospheric breakup and first oceanic accretion are among the most fundamental processes in Earth tectonics. A prerequisite to understand these processes is to first describe the nature and structure of the basement (crust and mantle) and overlying sediments. However, only few examples exist where scientific drilling penetrated these zones or high-quality seismic data imaged their crustal structure. Among these few places are the Iberia-Newfoundland and Greenland-Norwegian margins in the North Atlantic that became the type localities of magma-poor and magma-rich margins, respectively. Studies on fossil margins exposed in orogens have been developed in the Alps and Pyrenees and provided some additional observations. However, at present rift models are biased due to the lack of studied examples where interpretations are data-driven rather than model-driven. From this perspective, the South China Sea (SCS) offers a unique opportunity. The International Ocean Discovery Program (IODP) implemented three and a half drilling expeditions (IODP 349, 367, 368, 368X) over the past 5 years to explore the processes of crustal and lithospheric breakup. The acoustic basement was penetrated at 8 of the 12 drilled sites in early oceanic crust and the OCT, all in water depths exceeding 3700 m. This is the second largest drilling campaign at passive margins after the four Ocean Drilling Program (ODP) Legs (103, 149, 173 and 210) in the southern North Atlantic drilled in the 1990ies and early 2000th (Tucholke et al. 2007). The access to new drilling results and the excellent seismic data available from the SCS attracted the interest of the global margin community, which is also evidenced by the numerous new publications. Thus, the choice of working in the SCS is strategic and not only enables to use one of the best natural laboratories to study rifting and continental breakup, but also to reinforce the interactions of the Strasbourg group with the Chinese margin community that is at the origin of my PhD project.

Table of contents :

Chapter I: Introduction
1. Extensional systems in the Wilson cycle
2 Rifted margins
2.1 Concepts and models
2.2 Types of rifted margins
2.3 Distribution of types of margins
2.4 Rifted margins: ongoing research and open questions
3 The SCS: a natural laboratory to study rifting and breakup processes
3.1 Geographical setting
3.2. Geological setting and tectonic evolution
4. The study area
5. Open questions and aim of the thesis
5.1 The syn-rift mega-sequence and its link to the tectono-magmatic evolution in a polyphase rift system: how to define, describe and interpret?
5.2 From crustal separation to onset of seafloor spreading: how to define breakup?
5.3 Crustal breakup: how does it propagate?
5.4 The SCS: similarities and differences to Atlantic-type margins
6. Data, approaches and methods
6.1 Data set used in the PhD
6.2 Method and used approach
References
Chapter II: The tectono-stratigraphic and magmatic evolution of conjugate rifted margins: insights from the NW South China Sea
Abstract
1. Introduction
2. Geological setting
2.1. Geological evolution of the SCS
2.2. Structure of the NW-SCS
3. Data, methods and terminology
3.1. Data used and acquisition parameters
3.2. Method
3.3. Terminology
4. Seismic interpretation
4.1. Main interfaces
4.2. First order crustal structure and rift domains
4.3. Intra-sediment reflections and stratigraphic units
4.4. Intra-basement reflections and fault structures
4.5. Magmatic additions
5. Kinematic restoration of section CGN-1: methodological approach
6. Discussion
6.1. Rift architecture, H block and upper- vs. lower plate
6.2. Kinematic restoration of section CGN-1: tectono-structural evolution
6.3. Time frame for rifting and related syn-rift tracts and magmatic units
6.4. Strain rates
6.5. The stratigraphic tape recorder of rifting: the Wheeler approach
6.6. Strain localization during rifting and individualisation and dismembering of the H-block
7. Conclusion
Acknowledgments
References
Chapter III: The transition from continental to lithospheric breakup recorded in proto-oceanic crust: Insights from the NW South China Sea
Abstract
1. Introduction
2. Geological setting
3. Data, methods, and terminology
3.1 Data used and acquisition parameters
3.2 Methods
3.3 Terminology
4. Seismic interpretation of the OCT
4.1. Seismic observations and location of the OCT
4.2. Sediment architecture in the OCT
4.3. Nature of basement in the OCT
4.4. Faults in the OCT
5. Discussion
5.1. Major characteristics of OCT and definition of proto-oceanic crust
5.2. The syn-breakup sedimentary record
5.3. From continental to lithospheric breakup: the link between faults, sediments and magma
6. Conclusion
References
Chapter IV: A 3D snapshot of crustal breakup deduced from seismic analysis of the tip of the NW South China Sea
Abstract
1. Introduction
2. Geological setting
3. Data, methods and terminology
3.1 Data used and acquisition parameters
3.2 Methods
3.3 Terminology and definitions used in this study
4. Reflection seismic lines: from observations to seismic interpretations
4.1. First-order seismic interfaces and rift domain boundaries
4.2. Sedimentary sequences
5. Mapping rift domains at the tip of the NW SCS
5.1. Mapping rift domains based on reflection seismic data
5.2. Nature of crust at the tip of the propagator: constraints from refraction seismic data
5.3. Comparison with existing OCB maps and potential field data
5.4. Distribution of sedimentary sequences at the tip of the NW SCS
6. Discussion
6.1. 3D crustal architecture at the tip of the NW SCS
6.2. Tectono-magmatic evolution during breakup at the tip of the SW SCS
6.3. Propagating vs retrograding: a new kinematic model for the NW SCS
7. Conclusion
References
Chapter V: Discussion
1. The syn-rift mega-sequence and its link to the tectono-magmatic evolution in a polyphase rift system: how to define, describe and interpret?
2. From crustal separation to onset of seafloor spreading: how to define breakup?
3. Crustal breakup: how does it propagate?
References
Chapter VI: Conclusion
Chapter VII: Outlook
1. Time space correlations, unconformities and tectono-stratigraphic concepts
2. Evidence for a ridge jump in the NW SCS?
3. Sedimentation rates vs magmatic budget during rifting: towards a new classification
4. Lower crustal flow during crust necking
5. Extensional tectonic and crustal accretion at oceanic transform faults
6. From foreland basins to rifted margins: an example from the NE SCS
References

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