Rifting and continental break-up are major research topics within geosciences, and a thorough understanding of the processes involved as well as of the associated natural hazards and natural resources is of great importance to both science and society. As a result, a large body of knowledge is available in the literature, with most of this previous research being focused on tectonic and geodynamic processes and their links to the evolution of rift systems. We believe that the key task for researchers is to make our knowledge of rift systems available and applicable to face current and future societal challenges. In particular, we should embrace a system analysis approach and aim to apply our knowledge to better understand the links between rift processes, natural hazards, and the geo-resources that are of critical importance to realise the energy transition and a sustainable future.
The aim of this paper is therefore to provide a first-order framework for such an approach by providing an up-to-date summary of rifting processes, hazards, and geo-resources, followed by an assessment of future challenges and opportunities for research. We address the varied terminology used to characterise rifting in the scientific literature, followed by a description of rifting processes with a focus on the impact of (1) rheology and strain rates, (2) inheritance in three dimensions, (3) magmatism, and (4) surface processes. Subsequently, we describe the considerable natural hazards that occur in rift settings, which are linked to (1) seismicity, (2) magmatism, and (3) mass wasting, and provide some insights into how the impacts of these hazards can be mitigated. Moreover, we classify and describe the geo-resources occurring in rift environments as (1) non-energy resources, (2) geo-energy resources, (3) water and soils, and (4) opportunities for geological storage. Finally, we discuss the main challenges for the future linked to the aforementioned themes and identify numerous opportunities for follow-up research and knowledge application. In particular, we see great potential in systematic knowledge transfer and collaboration between researchers, industry partners, and government bodies, which may be the key to future successes and advancements.
Key processes controlling rift evolution
The study of rifting has a long history, reaching back to the late 1500s, when cartographers first noticed the apparent fit between the coastlines on both sides of the South Atlantic (Romm, 1994). Since then, our understanding has greatly advanced, and a broad variety of methods has been applied to study rifting over the past centuries. These methods include geological mapping and sampling, borehole logging, interpretation of 2D and 3D seismic and other regional geophysical datasets, and aerial and satellite observation, as well as analogue and numerical modelling of rifting processes.
As a consequence of using different analytical and exploratory methods in distinct areas of the world, researchers have historically developed a plethora of overlapping terminology to describe rifting processes and the different stages of rift evolution, which range from initial thinning of the lithosphere and the associated formation of rift basins to the eventual development of rifted margins flanking oceanic basins (e.g., Corti, 2012; Péron-Pinvidic and Manatschal, 2009) (Fig. 3). In this paper, the term ‘rifting’ refers to the development of an extensional tectonic setting due to divergent tectonic plate motion, be it in a continental or oceanic environment (i.e. before and after break-up of the continental lithosphere and the establishment of an oceanic lithosphere, respectively). Similarly, we use the term ‘rift system’, ‘rift environment’, or ‘rift settings’ for extensional tectonic systems in both continental and oceanic contexts. We also apply the broad term ‘rifted margin’, where synonyms are ‘passive margins’ (in contrast to active subduction margins, even though ‘passive’ rifted margins are often actively deforming), ‘extensional margins’, ‘divergent margins’, or ‘Atlantic margins’ (in contrast to the Pacific subduction margins).
Lithospheric rheology and strain rates
The type of lithospheric deformation occurring in a rift system will largely control its evolution. The general type of deformation is broadly dependent on the interplay between lithospheric rheology, timing (the specific rifting stage), and plate motion velocities. Firstly, the overall rheology of the lithosphere is strongly impacted by the presence of weak ductile layers in the lithosphere, most significant of which is the ductile lower crust (e.g., Brun, 1999; Burov and Watts, 2006; Fig. 4), though clay and evaporite layers can have a similar effect on a smaller scale. When present, as in standard continental lithosphere with some 40-km of crust on top of 100-km mantle lithosphere, such a layer can decouple brittle deformation in the mantle lithosphere, which represents the strongest (i.e. most competent) part of the lithosphere, from brittle deformation in the overlying, competent upper crust. Increased decoupling in a hotter lithosphere means that deformation is free to localise throughout the upper crust and parts of the upper lithospheric mantle, leading to distributed or wide rifting and even core complex development (e.g., in the Basin and Range Province in the USA or in the Aegean Sea; Kydonakis et al., 2015; Brun et al., 2018; Fig. 4).
Secondly, the progression of rifting is of importance as the rheology of the lithosphere changes over time. A very thick ductile lower crust that can lead to core complex formation is characteristic of systems with a thickened crust, for instance, after the development of a mountain range in which a surplus of radiogenic heating occurs prior to rifting, i.e. during the pre-rift stage (Figs. 3 and 4) (Buck, 1991; Brun et al., 2018). As rifting progresses, the lithospheric layers, including the lower crust, start to thin, so the decoupling effect decreases in importance and the mantle influence on upper crustal deformation starts to increase, leading to a strongly localised stretching regime, heralding the start of the second rifting stage (necking stage; Fig. 3).
The influence of strain rate is expressed in the altered behaviour of ductile layers in the lithosphere compared to its brittle parts above given that the brittle deformation of competent lithospheric layers is not dependent on the strain rate. When strain rates are low, ductile materials in the lithosphere generally weaken, whereas higher strain rates cause them to become more competent (e.g., Brun, 1999). This has a direct impact on the decoupling caused by a weak ductile lower crust during the stretching and necking stages of a rift system: slower plate motion tends to reduce coupling between brittle and ductile parts of the lithosphere, promoting wide rifting, whereas faster plate motion increases coupling between brittle and ductile lithospheric rock, promoting narrow rifting (Fig. 4).
Structural inheritance
The long and complex history of the Earth’s continental lithosphere leaves us with various types of inheritance that weaken the lithosphere and may affect strain localisation during rifting. Structural inheritance can come in the shape of pre-rift structures such as discrete faults or shear zones; pervasive fabrics in basement rocks; variations in lithospheric strength or layering between, for instance, a craton and adjacent terranes; compositional variations due to chemical alteration; or thermal variations due to mantle activity or previous lithospheric thinning (Schiffer et al., 2020; Glerum et al., 2020; Gouiza and Naliboff, 2021; Samsu et al., 2022).
The 3D orientation of the structural inheritance has a large impact; even a well-developed inherited shear zone may not reactivate when not oriented favourably for reactivation in terms of dip and strike with respect to the regional stress field (e.g., Maestrelli et al., 2020; Bonini et al., 2023). As a consequence, the structural inheritance the individual rift segments follow can result in different structural styles along each rift segment, ranging from orthogonal rifts to oblique and even transform systems (e.g., Corti et al., 2007; Agostini et al., 2011).
Magmatism
Multiple rift systems have experienced extensive magmatism at some point during their development, classifying them as magma-rich systems (in contrast to magma-poor systems; e.g., Franke, 2013; Tugend et al., 2015). Magmatism is often the result of mantle anomalies (Peace et al., 2020), and the timing of magmatism is of key importance to understand its impact on the evolution of rifting, continental break-up, and rifted margin architecture (e.g., Buiter and Torsvik, 2014; Sapin et al., 2021).
A key impact of such syn-rift magmatism is the significant reduction in lithospheric strength that can occur when intruded by magma (Buck, 2006) (Fig. 5). This strength reduction allows the efficient localisation of deformation along the rift axis, accompanied by only limited faulting, altogether referred to as ‘magma-accommodated rifting’, which allows rifting to remain rather symmetrical.
Surface processes
As rifts evolve, erosion and sedimentation (surface processes) actively shape their surface features. The associated transport and redistribution of lithospheric material can, in turn, influence large-scale tectonic processes by sedimentary loading in the basins and unloading of the eroding highs (Fig. 6; Burov and Cloetingh, 1997). Furthermore, the deposition of evaporites enables complex salt tectonic deformation (Fig. 7), and surface processes can even modify the thermal profile of the lithosphere.
The stage of rifting, location in the rift basin, and the provenance of the sediments largely determine the sedimentary infill of a given site, which nevertheless may record lower-order (minor) regressive episodes during periods of falling sea level or high relative sedimentation and subsidence rates (Martins-Neto and Catuneanu, 2010). Climate is known to influence rift systems by controlling the rates of denudation and erosion of evolving basins in time and space (Friedmann and Burbank, 1995; Leeder et al., 1998; Salgado et al., 2016; McNeill et al., 2019).
Natural hazards in rift settings
A number of natural hazards related to seismicity, volcanism, and mass-wasting processes can occur during the evolution of the various rifting stages. Wherever active tectonic deformation is occurring, earthquakes are likely to occur as well (Fig. 8). Although natural seismicity is an integral part of rifting and continental break-up processes, the magnitude of rift-related earthquakes remains often limited to ca. M_W 7 (e.g., Yang and Chen, 2010).
Significant continental rift-related seismicity is also found outside Africa. In western Europe, the year 1356 saw the total destruction of medieval Basel, near the southern tip of the Upper Rhine Graben (Meghraoui et al., 2001; Fig. 8c). Other examples of active continental rift systems posing hazards are the Gulf of Suez rift (Mohamed and Abd El-Aal, 2018) and the Trans-Mexican Volcanic Belt, the latter of which contains various major cities such as Mexico City (Maestrelli et al., 2020, and references therein).
In addition to the direct volcanism-related hazards such as explosive eruptions, magma-induced earthquakes, lava flows, pyroclastic flows, and lahars/mudflows that can lay waste to large swatches of land, magmatic activity in rift settings is accompanied by substantial hydrothermal circulation and degassing (e.g., Sawyer et al., 2008). These can lead to the development of mud volcanoes and geothermal fields that can pose serious hazards to local populations (e.g., Vereb et al., 2020).
Rifting causes the development of topography and thus of unstable slopes, either subareal or submarine, that can collapse in mass-wasting events. Particular areas at risk are the escarpments bordering the Ethiopian Highlands (Fubelli and Dramis, 2015; Martínek et al., 2021) and large parts of the western branch of the East African Rift (Stanley and Kirschbaum, 2017), for instance, along the shores of Lake Kivu (Jacobs et al., 2018; Depicker et al., 2021).
Geo-resources in rift environments
Rift systems contain a wealth of geo-resources that will be greatly needed for the energy transition and the establishment of a sustainable economy in the 21st century. These include (1) non-energy mineral resources, (2) geo-energy resources, (3) water and soils, and (4) opportunities for geological storage.
Non-energy mineral resources
The mineral deposits related to rift systems can be divided into various categories depending on when and how they formed (e.g., Groves and Bierlein, 2007; Zappettini et al., 2017; Fig. 11). These include pre-rift deposits, sedimentary mineral deposits, and sediment-hosted hydrothermal ore deposits. Prominent examples are diamond deposits offshore Namibia, sediment-hosted lead-zinc deposits, and stratiform copper deposits.
Geo-energy resources
Rift systems provide ideal environments for the development of conventional petroleum systems, with massive oil and natural gas deposits found in petroleum provinces such as those in the North Sea rift and UK-Norwegian margin and in the Gulf of Mexico (e.g., Levell and Bowman, 2018; Snedden and Galloway, 2019; Fig. 12). In addition, rifts can host unconventional petroleum systems, including shale oil/gas deposits and gas hydrates.
Another promising geo-energy source in rift settings is natural hydrogen gas (H2), which is generated through serpentinisation of (ultra)mafic rocks (e.g., Albers et al., 2021; Liu et al., 2023). Large amounts of natural H2 are released during the more advanced stages of rifting (i.e. break-up and drifting, when mantle material is being exhumed and serpentinised).
Geothermal energy production is a developing industry in rift basins around the world (Jolie et al., 2021). The thinning of the lithosphere and the rise of hot mantle material towards the surface create an elevated geothermal gradient, and the resulting higher heat flow can be exploited, especially in the East African Rift System and the Afar Triple Junction region.
Water and soils
Rift environments also provide geo-resources that are crucial to sustain life, such as fresh water and fertile soils. Meteoric water precipitates on rift shoulders or uplifted rifted margins and can accumulate in aquifers or lakes, while offshore freshened groundwater is found along rifted margins. The presence of rift-related volcanic rocks and soils also allows extensive agriculture in regions like Ethiopia.
Geological storage
Rift environments provide numerous opportunities for temporary and permanent geological storage, which is important for energy security and carbon sequestration. Depleted hydrocarbon fields, porous rock layers, and salt caverns in rift settings can be used to store natural gas, hydrogen, compressed air, and CO2.
Future challenges and opportunities
Rifting and continental break-up form a key research topic within geosciences. In this review, we provided an up-to-date summary of these processes, hazards, and resources. We discussed the key challenges for the future and identified opportunities for research and knowledge application, where especially knowledge transfer between science, industry, and government can help realise breakthroughs.
Some of the main future challenges include:
– Improving our understanding of lithospheric rheology, inheritance, and the role of the mantle in rifting processes
– Advancing data acquisition methods and increasing collaboration between academia, industry, and policymakers
– Enhancing risk assessment and mitigation for natural hazards like earthquakes, volcanism, and mass wasting
– Meeting the growing demand for geo-resources needed for the energy transition, including minerals, hydrocarbons, hydrogen, and geothermal energy
By embracing a system analysis approach and applying our knowledge of rift systems to these current and future societal challenges, we believe geoscientists can make invaluable contributions towards a sustainable future.