Rifting Mechanism Research Articles

Martian rifts: Structural geology and geophysics

Large extensional tectonic surface structures on Mars are investigated on the basis of image and topographic data. These features have lengths of up to 1400 km, typical widths of tens of kilometers, and depths of up to 3000 m. Their structural architecture is characterized by segmentation with changing style, the development of asymmetric basins with changing polarity, and accommodation zones between such basins. Most structures are associated with varying degrees of volcanism. Based on these observations and a comparison to terrestrial continental rifts, we interpret these series of basins or grabens as rift systems. Crater counts of rift-related geologic surface units indicate that rifts on Mars are ∼ 4.0 to 3.5 Ga old. Modelling of rift flank uplift yields heat flows ranging between 28 and 66 mW m − 2 , and a corresponding thickness of the elastic lithosphere in the order of ∼ 10 km at the time of rifting. The radial orientation of some rifts with respect to Tharsis is consistent with a rifting mechanism related to regional extensional stresses in the lithosphere. Rifts with other orientations might have formed in response to stress fields induced by horizontal gradients of the gravitational potential energy, modulated by magmatic zones of lithospheric weakness.

Major factors of the evolution of basins and faults in the Baikal Rift Zone: Tectonophysical analysis

In order to identify the major factors of the formation of the Baikal Rift Zone, tectonophysical analysis is carried out based on physical modeling with the application of similarity criteria. A single-layer model of elastoplastic clayey paste is superposed on two metal plates. One of the metal plates is displaced leftward according to the simple shear mechanism and its contact with the second metal plate has a bend morphologically similar to the Baikal segment of a marginal suture of the Siberian Craton. This scheme of loading corresponds, to a great extent, to the passive mechanism of rifting; i.e., the deformed layer is destructed due to strikeslip displacement of blocks and development of pull-apart structure in the model without any uplift and thermal impact related to the influence of mantle asthenolith in nature. The series of runs reproduces the major spatiotemporal trends in the evolution of the Baikal Rift Zone. Some experiments achieve for the first time a high degree of similarity in the morphology and mutual alignment of its main basins. This makes it possible to conclude that the evolution of basins and faults in the Baikal Rift Zone was governed by the following factors: (1) elastoplastic response of the substrate with a regular localization of strain; (2) left-lateral displacement of blocks; and (3) the presence of curved (in plan view) initiating structural heterogeneity.

The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay Basin, China: Constraints from volcanic records and tectono-stratigraphic sequences

The Bohai Bay Basin, located on the eastern Asian margin, is the second largest oil-production basin in China. It contains numerous depressions and sags, among which Nanpu Sag has become particularly important because of significant oil discoveries in recent years. Geologically and tectonically, however, the rifting mechanism and geodynamic evolution of the basin remain uncertain. This paper uses detailed volcanic and stratigraphic records obtained through extensive drilling and sampling in Nanpu Sag to interpret tectono-stratigraphic sequences and discuss basin dynamic evolution and crust-mantle interactions of the Bohai Bay Basin. Nanpu Sag contains voluminous volcanic rocks of dominantly alkaline basalts. Drill core samples, well logs, and seismic profiles reveal three volcanic cycles between the Eocene and Miocene Epochs. These basalts were created during a transition from garnet- to spinel-peridotite. Five tectono-stratigraphic sequences produced by episodic continental rifting have been identified, each composed of a basal coarse clastic sequence, a lower volcanic sequence, a middle deep-water clastic sequence, and an upper infilling sequence. Tectonically, Nanpu Sag experienced four evolutionary phases. The first three corresponded to three rifting events revealed by cyclic volcanism and sedimentary depositional features. The fourth phase was marked by extreme thermal subsidence and prograding fluvial deposition followed by tectonic inversion. A diapiric upper-mantle upwelling model is proposed to explain the dynamics that controlled the multiple rifting processes, the cyclic volcanism, and the periodic tectonic evolution of Nanpu Sag and the greater Bohai Bay Basin.

Gravity modeling of the ocean‐continent transition along the South Atlantic margins

Gravity, magnetic, and seismic data have been used to examine changes in crustal structure of conjugate portions of the South Atlantic volcanic margins south of the Walvis Ridge‐Rio Grande Rise hot spot tracks. We have constructed 18 seismically constrained crustal‐scale gravity models of the ocean‐continent transition: 12 across the African margin and 6 across the South American margin. We attribute changes in character of the gravity anomalies to variations in crustal thickness, sedimentation, and magmatic underplating. To investigate variations in the lower crust and upper mantle, we have removed the relatively well‐constrained contributions to the gravity field from shallow sources. The resulting residual anomalies delineate distinct crustal domains associated with crustal thinning and regions of underplating that correspond well with those independently identified from limited seismic refraction data. Near the hot spot tracks, we find underplated crust to be symmetrically distributed with a horizontal extent of ∼240 km for each margin. Further south, however, the underplated region is asymmetric with significantly more beneath the African than the corresponding South American margin. Residual gravity anomalies indicate that the width of underplated crust decreases from 240 km to ∼150 km at 33°S on the African side and to ∼100 km at 38°S on the South American side. The wider underplated region on the African side suggests an asymmetric rifting mechanism in which greater melts accumulate beneath an upper plate margin. Our results are consistent with recently proposed asymmetric rifting models that involve strain softening of a coupled, cold frictional upper lithosphere at low rifting velocities. Such models also predict greater symmetry for areas with more heat such as those associated with the hot spot.

Spatial and temporal variations in magma-assisted rifting, Taupo Volcanic Zone, New Zealand

Taupo Volcanic Zone (TVZ), New Zealand, is a NNE-trending rifting arc, active for ~ 2 Myr, with a 125-km-long central segment characterized by exceptionally voluminous rhyolite volcanism. The volcanic segmentation reflects along-axis variations in magmatism with implications for the thermal state of the crust and consequent rifting dynamics. Along the zone to the north and south of Central TVZ, the limbs of broad monoclines, disrupted to various degrees by normal faults, dip SE against major NW-facing fault zones. In these northern and southern segments, the loci of magmatism (shown by the position of volcanoes) and rifting (manifested by the distribution of seismicity and modern (< 61 ka) faulting in the Taupo Fault Belt (TFB)) coincide. Mantle-derived magmas are localized within the crust in a plexus of small bodies, dikes and sills, and dike-assisted rifting operates at times (but not always) as shown by the historic record. In contrast, throughout most of Central TVZ the loci of magmatism and tectonism (shown by the distribution of high-temperature geothermal systems and inferred from geophysical models and surface fault studies) are offset laterally and extensional strain appears to be partitioned accordingly. Geological, geophysical and geodetic studies indicate the following magma-assisted mechanisms of extension in Central TVZ: 1) mafic dike intrusion of length scale > 20 km and width > 1 m oriented perpendicular to the extension direction; 2) fault slips of < 2 m on structures along-strike from and coeval with silicic eruptions, some of which were triggered by mafic dike intrusion; 3) rifting episodes associated with regional-scale uplift, multi-fault rupture (slips < 2 m) and transient subsidence, arguably driven by changes in state at shallow depths. Volcanic studies of < 340 ka deposits demonstrate that an additional, but less frequent, mechanism involves temporally higher rates of fault slip with regional-scale collapse of rift basins in association with large-scale silicic eruptions. TVZ rifting mechanisms thus vary in space and time according to magmatic style and result in unpredictable fault behaviour over millennial time scales.

Lithospheric structure of the Rio Grande rift

A high-resolution, regional passive seismic experiment in the Rio Grande rift region of the southwestern United States has produced new images of upper-mantle velocity structure and crust-mantle topography. Synthesizing these results with geochemical and other geophysical evidence reveals highly symmetric lower-crustal and upper-mantle lithosphere extensional deformation, suggesting a pure-shear rifting mechanism for the Rio Grande rift. Extension in the lower crust is distributed over a region four times the width of the rift's surface expression. Here we propose that the laterally distributed, pure shear extension is a combined effect of low strain rate and a regionally elevated geotherm, possibly abetted by pre-existing lithospheric structures, at the time of rift initiation. Distributed extension in the lower crust and mantle has induced less concentrated vertical mantle upwelling and less vigorous small-scale convection than would have arisen from more localized deformation. This lack of highly focused mantle upwelling may explain a deficit of rift-related volcanics in the Rio Grande rift compared to other major rift systems such as the Kenya rift.

Deep structures and breakup along volcanic rifted margins: insights from integrated studies along the outer Vøring Basin (Norway)

Three-dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the Norwegian volcanic rifted margin. A surprising high-velocity lower crustal dome marked by a strong amplitude reflection (T Reflection) is described below the north Gjallar Ridge (NGR) near the volcanic plateau formed during the late Paleocene–Early Eocene breakup. Faulting along the NGR occurs during Early Campanian–Paleocene and is controlled by the crustal dome. The T Reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup. The influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup. Based on the structural relationship and the results of the modelling, it is therefore concluded that the T Reflection does not necessarily originate from an anomalous Tertiary magmatic event related to the breakup (underplated material), but could be also partly attributed to pre-breakup mafic material or/and inherited, high pressure granulite/eclogite rocks in the continental domain.

Typical basin-fill sequences and basin migration in Yanshan, North China

Basin-fill sequences of Mesozoic typical basins in the Yanshan area, North China may be divided into four phases, reflecting lithosphere tectonic evolution from flexure (T3), flexure with weak rifting (J1+2), tectonic transition (J3), and rifting (K). Except the first phase, the other three phases all start with lava and volcaniclastic rocks, and end with thick coarse clastic rocks and/or conglomerates, showing cyclic basin development rather than simple cyclic rift mechanism and disciplinary basin-stress change from extension to compression in each phase. Prototype basin analysis, based on basin-fill sequences, paleocurrent distribution and depositional systems, shows that single basin-strike and structural-line direction controlling basin development had evidently changed from east-west to northeast in Late Jurassic in the Yanshan area, although basin group still occurred in east-west zonal distribution. Till Early Cretaceous, main structural-line strike controlling basins just turned to northeast by north in the studied area.

Rifting between India and Madagascar – mechanism and isostasy

The nature of isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM) and their conjugate nature are examined. The first part of the study deals with the estimation of effective elastic thickness (Te) of the lithosphere for the two margins through cross-spectral analysis of gravity and bathymetry data. The results bring out comparable Te values of 8–15 km for the WCMI and 10–13 km for the ECMM, despite the WCMI having been traversed along strike by a hotspot trace. We have also compared topographic profiles across the two margins extending inland, to examine the rift-flank uplift experienced by the two landmasses abutting the margins, which bear a striking similarity. These results establish the symmetrical rifting mechanism resulting in the formation of the two margins, and indicate that despite the presence of a hotspot trace along one margin (i.e. the WCMI), the two margins have comparable isostatic compensation mechanisms with low Te values. In the second part of the study we have examined the possible mechanism for the rift-flank uplifts using a process-oriented approach of backstripping the sediments along a traverse across the WCMI and reconstructing the original rift flank topography by flexurally backstacking the amount of eroded material on the present-day topography of the Western Ghats. The backstack load configuration is taken from the amount of erosion that took place over the Western Ghats, obtained from AFTA (Apatite Fission Track Analysis) studies. The reconstructed rift time topography and bathymetry are used to get the Moho configuration, which was used to calculate the gravity anomaly, and is compared with the observed anomaly to derive a possible evolution model for the region. We have tried to examine the observed gravity anomaly through two different processes. In the first process, a Te=15 km for backstripping and backstacking, and a compensated underplating for the topography give a best fit model to the observed gravity. In the second process, a necking model with a deep level of necking (20 km) with Te=15 km and without any underplating explains the topography as well as the observed gravity. The nature and amount of the Western Ghat topography and cratonic age (Archaen) of the pre-rifted crust of western India and eastern Madagascar are more in favor of the rifting mechanism (necking model) rather than due to the passage of the Reunion hotspot (underplating model). This is evidenced by the observed increase in topography away from the influence of the Reunion hotspot trace as well as the presence of a similar topography in Madagascar.

Buttressing and fractional spreading of Tenerife, an experimental approach on the formation of rift zones

[1] The island of Tenerife is composed of three Miocene shields, which are centered by the Canadas volcano since the Pliocene. Tenerife sits on more than 2 km thick oceanic sediments. Quaternary volcanism of the Canadas series and giant landslides were principally confined to triaxial rift zones. The mechanism of triaxial rifting, however, has remained unclear. Physical analog models show that these rift zones may have formed by gravity-driven lateral escape of island segments, induced by loading of the deformable substratum. For experiments scaled to Tenerife, three adjacent sand cones were mounted onto viscous PDMS substratum. Gravitational spreading caused circumferential expansion of each cone, until a large edifice (Canadas) was constructed in their center. The older cones now acted each as a buttress; radial fractures were overprinted by fractional spreading of the Canadas edifice. This resulted in formation of three main extensional zones, resembling the triaxial rifting configuration of Tenerife.

Morphology of Europan bands at high resolution: A mid‐ocean ridge‐type rift mechanism

We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti‐Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid‐ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid‐ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor‐spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions.

Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P– T- composition lithospheric cross-sections in the region for the depth range of 35–80 km. Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of 143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle. Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P– T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.

Rifting along non-volcanic passive margins: stratigraphic and seismic evidence from the Mesozoic successions of the Alps and western Iberia

Abstract The paper examines aspects of the sedimentology and stratigraphy of rift basins that evolved in deep marine settings near the ocean-continent transition. It focuses on the applicability of a low-angle extensional detachment model developed in the Alps to the West Iberian margin, and on difficulties of objectively identifying syn-rift stratigraphic intervals in both areas. The paper examines evidence obtained from Ocean Drilling Program holes drilled in the Iberia Abyssal Plain. Despite the fact that all the holes were sited above highs in the acoustic basement and so did not penetrate a complete sedimentary record of rifting, they do provide some constraints on the age and mechanism of rifting. We suggest that published identifications of syn-rift intervals in distal basins off West Iberia and in the Southern Alps have not demonstrated, using objective criteria, the occurrence of syn-rift stratigraphic intervals. They have, therefore, probably overestimated the duration of rifting by as much as 20 Ma. The absence of syn-rift related stratal divergence towards fault footwalls may be due to resedimentation of syn-rift sediments towards basin centres, lack of significant hanging-wall rotation along flat detachment faults, or the syn-rift interval being too thin to resolve on seismic data. The syn-rift episode beneath the deep Galicia margin postdates Tithonian-Berriasian shallow-water carbonates, and predates Late Valanginian turbiditic sediments. Drilling results from the Iberia Abyssal Plain suggest a similar age because Tithonian siliciclastic mudrocks are overlain by Berriasian pelagic chalks. It seems likely that in both regions rifting lasted for &lt;5 Ma, probably from late Berriasian to early Valanginian. At Site 1068 in the Iberia Abyssal Plain, the interpretation from seismic reflection data, of a low-angle detachment dipping about 10° west, was confirmed by drilling, which revealed sedimentary and tectonic breccias containing clasts of lower-crustal rocks overlying a fault zone below which occurs serpentinized periodotite showing a downward decrease in deformation. At least 20.5 km of displacement is interpreted to have occurred along this fault, but it is not accompanied by large-amplitude, rift-related topography. This paradox is resolved if the detachment developed as a deepening-downwards, rolling-hinge fault.

40Ar/ 39Ar thermochronology of the Ethiopian and Yemeni basements: reheating related to the Afar plume?

In order to constrain the timing of mantle plume-related basement uplift and to discuss the active or passive mechanisms of the continental rifting in the Afar triple junction area, a 40Ar/ 39Ar thermochronological study has been carried out on the Panafrican basement in Ethiopia and Yemen. Muscovites, biotites and K-feldspars from granitoids and high-grade metamorphic samples have been analysed using the furnace step heating procedure. K-feldspar thermal histories have been numerically modelled, according to the multi-domain Ar diffusion theory. The cooling ages range from 462 Ma to 679 Ma for granitoids and from 495 Ma to 796 Ma for high-grade metamorphic rocks, which suggests that magmatic and metamorphic episodes belong to the same thermotectonic event. A gradual younging of cooling ages from north to south is consistent with the assumption that Panafrican continental collision began in Southern Ethiopia. Age gradients recorded by K-feldspars reflect Ar loss and are most likely related to the Cenozoic reheating of the basement. The estimated mean temperature over the last 50 Ma is between 138°C and 177°C at 2 km below the surface, which suggests a minimum mean thermal gradient of 69–88°C/km. The thermal anomaly is expected to reflect heat conduction from the mantle plume, magma underplating and melting of the lower crust, and heat advection related to the magma transfer through the crust. However, due to limited exhumation, it was not possible to constrain the timing of any mantle plume-related basement uplift. This study suggests that the 40Ar/ 39Ar thermochronology is generally unsuited to monitor denudation of a continental crust in an extensional setting. Furthermore, it highlights some of the thermal effects that continental flood basalts may have on the crust, which can lead to inconsistencies between K-feldspar and mica age data.

Geodynamical interpretation of crustal and upper mantle electrical conductivity anomalies in Sayan-Baikal province

There exists a number of models that account for the deep structure of the Sayan-Baikal province on the basis of different geophysical data. The models based on magnetotelluric soundings differ from those obtained from seismology with no regard to magnetotelluric soundings (MTS) results. To decide between the active and passive mechanisms of Baikal rifting, we propose to correlate the modeling results to the available seismological and petrological data, as well as to the model of asthenospheric diapirism put forward by Zorin et al. (1989, 1990, 1995). Inasmuch as the reality of MTS-based models depends on removal of static-shift effects, the reported three-dimensional modeling of the Baikal rift deep structure was preceded by normalization of the original MT curves by the global magnetovariation curve. The enhanced electrical conductivities of the crust and upper mantle of the region is interpreted in terms of geodynamics where the Cenozoic activity is suggested to be driven by convective mantle flow and triggered by the associated plate collisions. The origin and evolution of the Baikal rift can be explained in terms of reactivation of a pre-existing lithospheric inhomogeneity between the Siberian craton and the Sayan-Baikal fold area.

The Namib Rift: a rift system of possible Karoo age, offshore Namibia

Abstract This paper describes a hitherto poorly documented, deep-seated rift system offshore Namibia, here named the Namib Rift. The structural history and rift geometry of this system are determined from reflection seismic data. No wells penetrate the rift-fill succession but from its stratigraphic context, regional thickness relationships and by analogy with onshore Karoo stratigraphy, the age of the rift is interpreted as broadly equivalent to the Karoo Supergroup. The rift is divisible into two major arms, with rift segmentation evident in the north. An angular unconformity separates a rift and post-rift sequence. This is interpreted as a Mid-Triassic hiatus related to a regional compressional events. The basement comprises the Pan-African Fold Belt along the coast, with the Damaran arm extending inland. In the Cape Cross-Walvis Bay area, the Autseib Lineament separates the Kaoko and Damaran arms. This lineament, and the adjacent basement fabrics, are shown to have strongly influenced rift geometry in this area. South of the Autseib Lineament, Damaran basement fabrics largely determined fault trends within the Namib Rift. To the north, Kaoko basement fabrics strongly influenced basement extensional faulting. This area was also subjected to Mid-Triassic compression. South of the Autseib Lineament, large-scale uplift was distributed across large folds whereas to the north, uplift occurred close to pre-existing faults. North-northeast directed compression was common to both areas, but the Autseib Lineament partitioned the structural response. McKenzie-type pure shear extension is favoured as the rift mechanism, with Mid-Triassic uplift modifying thermal subsidence. The final compressive event of the Cape Orogeny, coeval with the regional Mid-Triassic hiatus, terminated rifting in offshore Namibia.

About this title

The evolution of oceanic plates is very well described by plate tectonic theory, but the study of continental tectonics is more complicated. Continental plates have a different rheology, a greater mean age, a higher heat productivity, and are more heterogeneous in composition. An understanding of continental tectonics requires, therefore, that these effects upon rigid plate behaviour be taken into account. This book brings together a series of papers which explore various aspects of the deformation of continental lithosphere, covering different tectonic settings from the Palaeozoic to the present day.These include the processes of terrane accretion and juxtaposition, the exhumation of high-pressure terrains, and mechanisms of crustal extension and rifting. The book will be of general interest to a broad audience of earth scientists concerned with global tectonics, continental growth processes, and the deformation of the continents during collision, exhumation and extension.

Tectonics of the Red Sea region reassessed

The brittle upper level of the continental crust had been rifted with or without ocean opening many times in many places during the geological past and the process is still happening. Since the advent of plate tectonic theory in the early 1960s, the formation of such rifts has been viewed in the context of plate tectonic processes that caused the repeated dispersal of supercontinents. Several researchers focused on the mechanisms of formation of continental rifts because some rifts, like the Red Sea and Gulf of Aden, are precursors to ocean basins and many hydrocarbons yet to be located which are either directly or indirectly related to rift structures. The East African Rift System and the Red Sea-Gulf of Aden young oceans have been considered as prime examples of the early stage of continental separation that has long been a testing ground for classical hypotheses of continental drift. The Red Sea separates the once contiguous Neoproterozoic Arabian–Nubian Shields and started opening about 25 Ma ago. Geophysics and geochronology of dredged basaltic rocks indicate that sea-floor spreading began at only about 4–5 Ma. Numerous multidisciplinary investigations have been carried out in this region. However, several questions remain unresolved. Examples pertain to the nature of the crust that underlies the shelves, the extent of the ocean floor, the interplay between sea-floor spreading, crustal extension and plutonic activity and mechanisms of rifting. Several mechanisms of rifting have been proposed for the formation of the Red Sea. Examples include extension by prolonged steep normal faulting (horst–graben terrain), early diffuse ductile extension followed by brittle deformation, low-angle lithospheric simple shear, low-angle shear and magmatic expansion, lithospheric thinning by faulting and dike injection, northeastward migration of asymmetric rifting over a fixed mantle plume and the formation of pull-apart basin(s) by transtension. The major differences between the various models center on the relative timing of updoming, rifting and magmatism and whether the rifting was active and driven by a mantle plume or passive and due to lateral extension of the lithosphere leading to reactive effects in the mantle. New geological field data from the western margin of the Southern Red Sea in Eritrea reveal two main stages of NE–SW extension history. The first semi-brittle stage (⩾30 Ma) was dominantly characterized by top-to-east low-angle detachments. The second brittle stage of extension (since ∼22 Ma) occurred on a new system of dominantly down-to-southwest planar normal faults and dikes with NW–SE strikes. The earlier semi-brittle stage of extension corresponds to the predicted low-angle simple shear zone through the lithosphere and the later gives some support to the models that invoke graben–horst formation along steep normal faults that ultimately soled out to detachments at intermediate crustal level or merge with the Moho.

Nouvelles données pétrographiques et structurales sur le magmatisme du Sud du bassin de Morondava (Sud-Ouest de Madagascar)

New petrological and geochemical data are presented on basic igneous rocks from the south of the Morondova basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magmatic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the different types of basaltic magmatism manifested in this area to be distinguished. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This geochemical evolution of basaltic magmatism can be linked to the initiation of rifting between Madagascar and Africa along the Mozambique channel and the length of the Davie ridge associated with the opening of the Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better understanding of the mechanism and evolution of Madagascan rifting.

Extensional processes in continental lithosphere

Since Vening‐Meinesz's realization that the East African Rift represented an extensional, not compressional, feature and since the widespread acceptance of plate tectonics two decades later, research on the nature and causes of extensional tectonism within continental lithosphere has intensified. Among the manifestations of extensional processes affecting continental lithosphere are passive margins (Atlantic margin), discrete intracontinental rift zones (East African Rift), diffuse rifts (Basin and Range Province), strike‐slip dominated rifts (Dead Sea Rift), and rifts in zones of regional compression (Tibetan grabens). Although no two rift zones are alike, continental rifts can generally be characterized by normal faulting with subsidiary strike‐slip faulting, lithospheric thinning which outpaces crustal stretching, varying amounts of alkaline magmatism, heat flow that is locally elevated near faults and magmatic centers, and crust that has experienced magmatic underplating and some amount of magmatic intrusion. Most aspects of rift related deformation can be explained in terms of three parameters: (1) lithospheric and (sometimes) asthenospheric thermal structure, (2) lithospheric (particularly crustal) rheology, and (3) temporal factors such as the absolute age, timing, and rate of extension. The interaction of these physical parameters determines the eventual outcome of rifting (failure or progression to complete continental breakup), the patterns of subsidence and uplift, and the mode of extensional deformation. Modes of rifting (the lithosphere's mechanical response to extensional stress) can be broadly divided into pure shear, simple shear, and lower crustal flow mechanisms. In a general sense, these categories of rift mechanisms can account for observations at rifted margins, in the Basin and Range Province, and at metamorphic core complexes respectively. The mechanisms of continental lithosphere rifting (the effects) are here distinguished from the processes which actually drive extension (the causes). Following the terminology of previous authors, the causal processes are categorized as either passive or active: Passive processes originate at plate boundaries or in response to convective drag on the base of the lithosphere, while active processes are seated in the sublithospheric mantle and typically involve the interaction of mantle plumes and the lithospheric plates. A peculiar set of factors, including the local stress state, the retreat of a subducting plate, and convective instability of thickened crustal roots, may drive extension in compressional settings like Tibet. In the future, rifting studies are likely to focus on further clarification of the active and passive rifting terminology, better constraints on deformation rates and lithospheric rheology and thermal structure, along‐axis segmentation of rifts, and the analogies and differences between rifting on Earth and nearby planets.