Continental Breakup Process Research Articles

Shallow Thermal Anomalies and Their Role in the Breakup Evolution Along the Conjugate Margins of the Fram Strait (Svalbard and Eastern North Greenland), Indicated by Low‐Temperature Thermochronology

AbstractWe investigated highly mature sedimentary rocks exposed along both sides of the Fram Strait in the northern North Atlantic using apatite fission track and (U‐Th)/He thermochronology to obtain information on the thermal imprint of rifting and continental breakup processes along a sheared margin. Our data showed that the conjugate margins experienced several heating episodes, which we explain as resulting from heat transfer along segments of the De Geer Fracture Zone, a large continental transform system which connected magmatic centers north and south of the Fram Strait. Heating occurred prior to and during the Eurekan intraplate orogeny, which occupied the position of the present‐day Fram Strait during the Eocene. Heat transfer may have caused or contributed to lithospheric weak zones, which focussed deformation during intraplate orogeny. Movements along the transform fault system continued during the Oligocene, after the end of the Eurekan Orogeny, causing further structural weakening of pre‐existing fault zones. These were exploited during the final continental breakup leading to the opening of the Fram Strait. No unambiguous thermal signature associated with this latest stage of breakup was detected. Our data underline recent studies on the importance of structural inheritance and continental transform faults for the prolonged and complex processes of continental rifting and breakup.

Exploring rift geodynamics in Ethiopia through olivine-spinel Al-exchange thermometry and rare-earth element distributions

Understanding the process of continental break-up requires knowledge of the geodynamics of mature rift systems close to the point of plate rupture. In Ethiopia, late-stage continental rifting occurs in a magma-rich setting where extensional processes correlate closely with magmatic and volcanic activity. Unravelling the role that mantle and lithospheric dynamics play in sustaining rifting in Ethiopia is key to improving models of late-stage continental rift evolution. In this study we provide petrological constraints on the physical characteristics of magma production using volcanic samples from the northern Main Ethiopian Rift (MER) and central Afar. Olivine crystallisation temperatures provide information about the thermal state of mantle-derived magmas generated by the mantle during upwelling and melting. These temperatures, determined by Al-exchange thermometry, are used in conjunction with other geochemical and geophysical constraints in inversion models of melting a multi-lithology mantle to find the best-fitting geodynamic parameters that can reproduce observed magma compositions and melt volumes. Our model results suggest that the potential temperature of the Ethiopian mantle is hotter than ambient mantle by ≥ 150∘C, and is elevated to a similar degree across the MER and Afar. We predict significant variations in lithospheric thickness between the MER (90 km) and Afar (50-70 km), with Afar also likely to have a higher portion of fusible mantle domains. This thinner lithosphere and/or more productive mantle is required to generate the larger volumes of magma inferred to have been intruded into the Afar crust. The geodynamic differences between these two settings can be attributed to the more-evolved state of the Afar rift and its proximity to the centre of the Afar plume.

A tectono-stratigraphic review of continental breakup on intraplate continental margins and its impact on resultant hydrocarbon systems

Intraplate Continental Margins comprise fully rifted areas of continents and cratons recording the tectonic events associated with plate breakup and subsequent continental drift. In contrast to extensional basins, this tectonism post-dates the initial stages of continental rifting of tectonic plates and is specifically associated with the breakup and ‘drifting’ of continents. Seismic and outcrop data from eight (8) Intraplate Continental Margins are reviewed in this work and tectonic-subsidence models are compiled to highlight their large-scale structural and depositional variability. Specific geological aspects that enhance the hydrocarbon potential of Intraplate Continental margins include: a) syn-rift strata that are strikingly thicker, and presumed older, than previously assumed on vintage seismic profiles, b) an important control of basement type and fabric on tectonic subsidence and subsequent reactivation after continental breakup is achieved, and c) widespread extensional collapse accompanying the continental breakup process to generate a phase of important sediment bypass into relatively deeper, distal depocentres. There is evidence on seismic data that the continental breakup process per se overprints older syn-rift structures to create a complex mosaic of uplifting and subsiding basins that may not coincide in location and extent with early syn-rift depocentres. This character has the potential of generating vast amounts of reservoir strata during tectonically active periods, at the same time enhancing the influx of organic matter to basins distant from point-sources of sediment during continental breakup. As a corollary, a classification of breakup sequences is presented in this work and a new case-study from South California is developed to acknowledge the varied hydrocarbon-generating settings of Intraplate Continental Margins when continental breakup is established.

Moho Structure of the Southwest Sub-Basin, South China Sea, from a Multichannel Seismic Reflection Profile NH973-1

Moho structure provides important clues for understanding crustal structure, isostatic state and magmatic flux from mantle to surface. Across-basin Moho structure of the South China Sea (SCS) is important for understanding crustal evolution mechanisms of both continental break-up and seafloor spreading processes. Southwest Sub-basin (SWSB) opened up the latest and has the closest continental margins, making it the best to study the across-basin structure. Multichannel seismic (MCS) reflection data of line NH973-1 that crosses SWSB in NW-SE direction were reprocessed in order to image Moho structure. In MCS data, Moho reflectors are observed in places, which were not revealed in prior researches. The Moho generally shows symmetric structure on both sides of the central rift valley (CRV) and with variations in crustal thickness. Around CRV, the Moho is 2 seconds depth in two-way travel time (TWTT) beneath the igneous basement, which corresponds to 7 km depth, indicating normal oceanic crustal accretion during the ending of seafloor spreading. Close to the continent-ocean boundary (COB), the Moho becomes shallow to 1 second depth in TWTT (3.5 km), implying strong crustal thinning towards the continent, probably because of poor magma supply at the beginning of seafloor spreading. At south COB, the Moho depth under the crust almost reaches zero, which could be explained as a result of exhumed mantle. In addition, two low-angle, deep-penetrating normal faults are observed at south COB. The faults cut across the Moho into the upper mantle, which may be attributed to lithospheric hyper-stretching at COB during continental break-up.

Crustal Architecture and Nature of Continental Breakup Along a Transform Margin: New Insights From Tanzania‐Mozambique Margin

AbstractThe Tanzania‐North Mozambique continental margin is a transform segment associated with Davie Fracture Zone (DFZ). The DFZ is described as an elongated linear oceanic fracture zone, commonly linked with the breakup between Eastern and Western Gondwana. We conducted a synthesized study using gravity, magnetic, and seismic data presenting the crustal architecture, geometry, and the kinematic nature of continental breakup along a transform margin. The crustal nature of DFZ, its role in forming kinematic linkage between two extensional margins during continental breakup processes, is focus of our study. The two extensional margins, Somalia‐Majunga and North Mozambique‐Antarctica, were linked via a 2,600‐km‐long dextral transform segment, partially overlapping with DFZ. Absence of classical rift indicators, weak signs of hyperextension, and abrupt ocean‐continent boundary suggest transform margin architecture. We redefined this feature as the Davie Transform System. The nature of deformation varies from transtensional pull‐apart in Tanzania to almost pure strike slip in North Mozambique. The southern transform segment exhibits abrupt change in ocean continent transition with a narrow zone of continental extension. This variation is recognized through the newly interpreted ocean‐continent boundary along this entire transform segment. Notably, within large pull‐apart systems in the north, the presence of fossilized incipient spreading center suggests that the extension had reached at quite advanced stages, characterized by significant thermal weakening as a consequence of strong magmatic activity. Through a series of reconstruction snapshots, we show the geodynamic evolution along the Tanzania‐North Mozambique margin explaining the role of Davie Transform System in the southward movement of Madagascar.

Basin modelling of the SW Barents Sea

Abstract The SW Barents Sea is an epicontinental platform consisting of N- to NNE-oriented basins separated by basement highs. The basins were formed during four distinct rift-phases in the Carboniferous, Late Permian, Late Jurassic-Early Cretaceous, and Paleocene-Eocene. Progressive rifting culminated in continental breakup and seafloor spreading along the North Atlantic axis approximately at 55 Ma. We present tectonic and thermal models of basin evolution along two seismic profiles crossing the SW Barents Sea. The thermal and isostatic history of basins is constrained through time-forward basin modelling based on an automated inverse basin reconstruction approach. We estimate the effects of continental breakup and near-margin processes (magmatic underplating and sill intrusions) on the thermal history and kerogen maturity below the Vestbakken Volcanic Province. Basin models are calibrated against well data. The results imply that both breakup and underplating alter the thermal and isostatic history of sediments along the margin. The hydrocarbon potential of source rocks modelled along the margin suggests breakup has a permanent thermal effect on the present-day sediments promoted by lateral heat flow, while heat conduction by underplating is more diffuse, inhibited to some degree by deeply-buried, low-conductivity shales and limestones.

Non-Wilsonian break-up predisposed by transforms: examples from the North Atlantic and Arctic

Abstract The Atlantic Ocean margins formed the basis for the seminal Wilson cycle concept, which suggests that oceans close, form fold belts, and later reopen in a concertina-like fashion. However, we observe that continental break-up of the North Atlantic–Arctic region only weakly reflects Wilson's concept. Rather than utilizing fold belts, transforms have been the dominant weaknesses that guided break-up, primarily because less force is required to break a plate via strike-slip related shearing than via rifting. Some transforms were inherited features, whereas others formed as part of the continental break-up process. Regardless of cause, once a transform has formed, the plate is broken and further rifting is not required before seafloor spreading can start. This is particularly well expressed in the NE Atlantic, where the line of Early Eocene break-up is very sharp, with minor or no preceding Paleocene rifting. Other examples include the De Geer, Ungava and Lomonosov transforms. We propose that the transform break-up mechanism is an important adjunct to the Wilson cycle theory and that it provides an explanation for ‘non-Wilson’ oceans, where old collision zones are not reactivated.

A phase of transient subsidence, sediment bypass and deposition of regressive–transgressive cycles during the breakup of Iberia and Newfoundland

Seismic, outcrop and well data from West Iberia and Newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence. In West Iberia, two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present-day continental shelf. This character is similar to Newfoundland, where correlative strata comprise Lower Cretaceous–Cenomanian coarse-grained siliciclastics accumulated around local sediment-source areas. The interpreted data reveal that the two breakup sequences: 1) materialise sediment bypass onto continental-slope depocentres that experienced important tectonic subsidence during continental breakup, but without showing typical syn-rift growth packages; 2) generate specific forced-regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin. Subsidence and sediment stacking patterns in both West Iberia and Newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere- to their mantle-breakup stages. On both margins, coarse-grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep-water basins. This work also confirms that in a setting dominated by a significant sediment influx, yet lacking the burial rates of continental slope basins in Newfoundland, West Iberia comprised accommodation-driven basins during continental breakup, not necessarily sediment starved. As a corollary of our analysis, we classify breakup sequences around the world based on the characteristic lithologies of their regressive–transgressive depositional cycles.

Slip history of the La Cruz fault: Development of a late Miocene transform in response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~19–17Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5±2km of dextral slip ~8–4Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6Ma onset of transform faulting and basin formation along >1000km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K–Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean–continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

Evidence of Gondwana early rifting process recorded by Resende-Ilha Grande Dike Swarm, southern Rio de Janeiro, Brazil

Continental flood basalts and dike swarm have been related to continental breakup process through geological time. The Resende – Ilha Grande Dike swarm (RIGDS) located in the southeast Brazil, is related the Gondwana breakup and composed of dikes/sills intruded in Precambrian gneiss. The dikes have three distinguish orientations: NNW more inland; NS-NNE in the central segment and NE orientation in the coast line, consistent with Precambrian structural lineaments. The swarm comprises high-TiO2 tholeiitic basalts divided into three suites based on REE and Sr and Nd isotope data. The Resende and Volta Redonda suites present higher initial 87Sr/86Sr ratios between 0.7077 and 0.7065, while Angra dos Reis suite presents values of 0.7066 to 0.7057. Geochemical and isotopic data support the sub-continental lithospheric mantle (SCLM) as the main source for the high-TiO2 basalts. The suites heterogeneities are explained by different compositions of SCLM in accreted Precambrian terranes and/or different degree of partial melting and fractional. 40Ar/39Ar data indicate age interval between ca. 156 to 144 Ma for the swarm, older than the average for Gondwana breakup (ca. 130-120 Ma). The age interval places the RIGDS between the Karoo magmatism (181-178 Ma) and the Paraná-Etendeka magmatism (133–134 Ma) and indicates that extensional process affected the supercontinent prior the break-up.

Use of a high-precision gravity survey to understand the formation of oceanic crust and the role of melt at the southern Red Sea rift in Afar, Ethiopia

Abstract The Red Sea arm of the triple junction in northeastern Ethiopia provides an opportunity to investigate rift-forming processes at divergent boundaries. In an attempt to study the subsurface, especially the distribution and role of melt in the rifting process, we carried out a high-precision gravity survey with a mean-square error of 0.011 mgal, assisted by differential global positioning system measurements. The profile is 162 km long and strikes ENE–WSW across the southern part of the Red Sea rift at a latitude of approximately 11.75° N. Modelling of the Bouguer anomaly, constrained by a priori information, showed detailed in-rift variations in the crustal structure and the distribution of melt beneath the rift axis. Our interpretation suggested that the process of continental break-up is governed by crustal stretching and rifting accompanied by the emplacement of melt into the lower crust above a lower density upper mantle. In addition, we interpreted the thickness of the crust beneath this part of the rift axis to be 25 km. The subsurface distribution of density beneath the profile shows that the south-central part of the Red Sea rift has modified thinned crust, intruded by high-density material, which resembles the crust formed during seafloor spreading.

Geophysical and geochemical constraints on Cretaceous-Cenozoic magmatism along the southern Australian margin

Increasing levels of exploration along rifted continental margins, such as the southern Australian margin, has led to growing recognition of the detrimental impacts of magmatic activity on hydrocarbon prospectivity. Key exploration risks include the impact of intrusions on seal integrity, reservoir quality, source rock maturation and migration pathways. However, the extent and distribution of volcanic rocks along continental margins, such as the Australian southern margin, and the processes by which magma is transported through sedimentary basins are still poorly understood despite the wealth of available seismic datasets. Although classified as a 'non-volcanic' rifted margin, our analysis shows that an extensive and largely undescribed record of Cretaceous-Cenozoic magmatic activity is preserved within the sedimentary successions of the rift basins located along the southern Australian margin. The combination of seismic reflection data and geochronological and geochemical data shows that this magmatic activity cannot be solely attributed to continental break-up and related decompressional melting processes or the presence of a hotspot or plume.

The 2.31 Ga mafic dykes in the Karelian Craton, eastern Fennoscandian shield: U–Pb age, source characteristics and implications for continental break-up processes

Major- and trace-element geochemistry, Sm–Nd isotopic and U–Pb geochronological (ID TIMS, baddeleyite) data are presented for Paleoproterozoic mafic dykes in the Karelian Craton, eastern Fennoscandian shield. Mafic dyke swarms recognized in the Lake Upper Kuito area in the Western Karelian terrane of the Karelian Craton include (i) NE-trending ca. 2450Ma gabbronorite dykes, (ii) NW-trending ca. 2310Ma dolerite dykes, and (iii) NNW-trending ca. 2130Ma continental MORB-type tholeiitic dykes. Each swarm belongs to a distinct intraplate igneous event and indicates specific mantle melting and extensional processes.The ca. 2.31Ga doleritic dykes have high-Fe, high-Ti tholeiitic series affinity and characterized by high concentrations of lithophile trace elements. The enrichment of LREE and unfractionated HREE patterns together with depletion of Nb on multi-element diagrams and initial ɛNd values of +0.5 to +0.8 are characteristic for ca. 2.31Ga dolerites. The geochemical modelling results suggest that these features could be originated via combination of fractional crystallization and crustal contamination processes, and do not indicative of a mantle source enrichment. It also indicates long-term storage of melts in an intracrustal magma chamber and a relatively slow ascent to the surface. The primary melts of ca. 2.31Ga dolerites possibly originated by partial melting of a DM-type mantle source in the spinel peridotite stability field.In contrast to previous ca. 2.45Ga event studied ca. 2.31Ga dolerites do not indicate high-temperature mantle-plume induced mantle melting. On the other hand, the ca. 2.31Ga dykes have some similarities with syn-breakup ca. 2.13Ga continental MORB-type tholeiites, although there is no evidence for the break-up of the continental lithosphere of the Karelian Craton at ca. 2.31Ga. Thus ca. 2.31Ga dykes could indicate the existence of relatively stable extensional tectonic environment and an unsuccessful attempt of break-up of the continent.The ca. 2.31Ga dykes together with previously studied Runkaus basalts and Taivalkoski dykes could be considered as a component of distinct relatively wide-spread Kuito-Taivalkoski igneous event within the ca. 2.2–2.4Ga “crustal age gap” period in the eastern Fennoscandian shield. This probably indicate that the number and intensity of endogenous events of age 2.4–2.2Ga are greatly undervalued. This also argues against a decrease of endogenic activity and magmatic “slowdown” during ca. 2.4–2.2Ga period.

Asymmetric three-dimensional topography over mantle plumes.

The role of mantle-lithosphere interactions in shaping surface topography has long been debated. In general, it is supposed that mantle plumes and vertical mantle flows result in axisymmetric, long-wavelength topography, which strongly differs from the generally asymmetric short-wavelength topography created by intraplate tectonic forces. However, identification of mantle-induced topography is difficult, especially in the continents. It can be argued therefore that complex brittle-ductile rheology and stratification of the continental lithosphere result in short-wavelength modulation and localization of deformation induced by mantle flow. This deformation should also be affected by far-field stresses and, hence, interplay with the 'tectonic' topography (for example, in the 'active/passive' rifting scenario). Testing these ideas requires fully coupled three-dimensional numerical modelling of mantle-lithosphere interactions, which so far has not been possible owing to the conceptual and technical limitations of earlier approaches. Here we present new, ultra-high-resolution, three-dimensional numerical experiments on topography over mantle plumes, incorporating a weakly pre-stressed (ultra-slow spreading), rheologically realistic lithosphere. The results show complex surface evolution, which is very different from the smooth, radially symmetric patterns usually assumed as the canonical surface signature of mantle upwellings. In particular, the topography exhibits strongly asymmetric, small-scale, three-dimensional features, which include narrow and wide rifts, flexural flank uplifts and fault structures. This suggests a dominant role for continental rheological structure and intra-plate stresses in controlling dynamic topography, mantle-lithosphere interactions, and continental break-up processes above mantle plumes.

The links between large igneous provinces, continental break-up and environmental change: evidence reviewed from Antarctica

ABSTRACTEarth history is punctuated by events during which large volumes of predominantly mafic magmas were generated and emplaced by processes that are generally accepted as being, unrelated to ‘normal’ sea-floor spreading and subduction processes. These events form large igneous provinces (LIPs) which are best preserved in the Mesozoic and Cenozoic where they occur as continental and ocean basin flood basalts, giant radiating dyke swarms, volcanic rifted margins, oceanic plateaus, submarine ridges, and seamount chains. The Mesozoic history of Antarctica is no exception in that a number of different igneous provinces were emplaced during the initial break-up and continued disintegration of Gondwana, leading to the isolation of Antarctica in a polar position. The link between the emplacement of the igneous rocks and continental break-up processes remains controversial. The environmental impact of large igneous province formation on the Earth System is equally debated. Large igneous province eruptions are coeval with, and may drive environmental and climatic effects including global warming, oceanic anoxia and/or increased oceanic fertilisation, calcification crises, mass extinction and release of gas hydrates.This review explores the links between the emplacement of large igneous provinces in Antarctica, the isolation of Antarctica from other Gondwana continents, and possibly related environmental and climatic changes during the Mesozoic and Cenozoic.

The A-type Pirrit Hills Granite, West Antarctica: an example of magmatism associated with the Mesozoic break-up of the Gondwana supercontinent

The Mesozoic geology of West Antarctica is largely related with the break-up of the Gondwana supercontinent and offers a good example for understanding magmatism associated with the continental break-up process. West Antarctica can be divided into five crustal blocks with relatively thin crust. The blocks are separated by deep rift zones and have moved during the Mesozoic break-up of Gondwana. The Pirrit Hills granite occurs as an isolated pluton in the Ellsworth-Whitmore Mountains block, which is the center of five blocks in the present configuration. The granite consists of quartz, perthitic alkali feldspar, and plagioclase with minor amounts of interstitial biotite and muscovite. The granite is a highly homogeneous, strongly fractionated, and mildly peraluminous granite and belongs to A-type granites with A2-type characteristics, suggesting its generation in an anorogenic environment. The strong enrichment of HREE and significant negative Eu anomalies suggest that the granitic magma was produced by a small degree of partial melting of a garnet granulitic source in the unusually hot lower crust. A weighted mean 206Pb/238U age of zircons is 164.5 ± 2.3 Ma (MSWD = 1.3), which is 8 to 9 Mys younger than a former Rb-Sr whole rock age (173 ± 3 Ma), and corresponds to the first rifting stage of the break-up of Gondwana (at 165 Ma). We suggest this age to be the emplacement age of the Pirrit Hills granite. The A-type Pirrit Hills granite was emplaced in the Middle Jurassic accompanying crustal thinning due to the break-up of Gondwana.

Making and Breaking of a Continent: Following the Scent of Geodynamic Imprints on the African Continent Using Electromagnetics

The African continent inherits a long history of continental accretion and breakup. The stage of “making” a continent goes back to the Archean, when the first continental masses formed cratons which mostly remained stable ever since. Subsequent collision of weaker continental masses was followed by several extension and compression episodes that resulted in the formation of super-continents. After the assemblage of Gondwana, a period of predominantly “breaking” , i.e., the breakup of super-continents, took over. The modern-day African continent exhibits different types of margins; continental rifting occurs side by side with recent collision. Since the late 1960s, magnetotelluric (MT) experiments have played an important role in studies of the electrical conductivity structure of Africa. The early results significantly shaped the MT community’s understanding of continental-scale conductivity belts and basic characteristics of cratons and mobile belts on both crustal and lithospheric mantle scales for some decades. Modern MT studies in Africa have generally supported earlier results with high resistivities observed on cratons and low resistivities observed across mobile belts. Advances in instrumentation, data processing and interpretation resulted in higher-resolution images of the lithosphere, which in consequence induce an improved understanding of tectonic processes and geological prerequisites for the occurrence of natural resources. The high electrical conductivity of mobile belts and their relation to reactivated fault and detachment zones were often interpreted to characterize mobile belts as tectonic weak zones, which can accommodate stress and constitute zones along which continents can break. Recent breaking of the African continent can be studied on land across the East African rift; however, the lack of amphibian MT experiments across today’s margins does not allow for good resolution of remnants of continental breakup processes. Naturally, the regions and the focus of the MT studies in Africa are diverse, but they all contribute to the story of making and breaking a continent.

Elastic thickness estimates at northeast passive margin of North America and its implications

Global estimates of the elastic thickness (Te) of the structure of passive continental margins show wide and varying results owing to the use of different methodologies. Earlier estimates of the elastic thickness of the North Atlantic passive continental margins that used flexural modelling yielded a Te value of ∼20–100 km. Here, we compare these estimates with the Te value obtained using orthonormalized Hermite multitaper recovered isostatic coherence functions. We discuss how Te is correlated with heat flow distribution and depth of necking. The E–W segment in the southern study region comprising Nova Scotia and the Southern Grand Banks show low Te values, while the zones comprising the NE–SW zones, viz., Western Greenland, Labrador, Orphan Basin and the Northern Grand Bank show comparatively high Te values. As expected, Te broadly reflects the depth of the 200–400 ◦ C isotherm below the weak surface sediment layer at the time of loading, and at the margins most of the loading occurred during rifting. We infer that these low Te measurements indicate Te frozen into the lithosphere. This could be due to the passive nature of the margin when the loads were emplaced during the continental break-up process at high temperature gradients.

Surface morphology of active normal faults in hard rock: Implications for the mechanics of the Asal Rift, Djibouti

Tectonic-stretching models have been previously proposed to explain the process of continental break-up through the example of the Asal Rift, Djibouti, one of the few places where the early stages of seafloor spreading can be observed. In these models, deformation is distributed starting at the base of a shallow seismogenic zone, in which sub-vertical normal faults are responsible for subsidence whereas cracks accommodate extension. Alternative models suggest that extension results from localised magma intrusion, with normal faults accommodating extension and subsidence only above the maximum reach of the magma column. In these magmatic rifting models, or so-called magmatic intrusion models, normal faults have dips of 45–55° and root into dikes. Vertical profiles of normal fault scarps from levelling campaign in the Asal Rift, where normal faults seem sub-vertical at surface level, have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks (basalt lava flows), using mechanical and kinematics concepts. We show that the studied normal fault planes actually have an average dip ranging between 45° and 65° and are characterised by an irregular stepped form. We suggest that these normal fault scarps correspond to sub-vertical en echelon structures, and that, at greater depth, these scarps combine and give birth to dipping normal faults. The results of our analysis are compatible with the magmatic intrusion models instead of tectonic-stretching models. The geometry of faulting between the Fieale volcano and Lake Asal in the Asal Rift can be simply related to the depth of diking, which in turn can be related to magma supply. This new view supports the magmatic intrusion model of early stages of continental breaking.