Main Rift Research Articles

Morphological and statistical characterisation of recent mafic volcanism on Tenerife (Canary Islands, Spain)

Tenerife is the largest island of the Canarian Archipelago and presents a complex volcanic history. The construction of a mafic shield and a phonolitic composite volcano represent the main features of the volcanic evolution of the island. Both volcanic complexes are still active, the first through two main rift zones and the second through the Teide–Pico Viejo central complex. Up to 297 mafic monogenetic volcanoes can be recognised on Tenerife, most of them corresponding to scoria cones that can been grouped into five geographical volcanic fields characterised by similar volcanological features. The large number of these edifices, compared to the other existing morphological volcano-types, indicates that they represent the most common eruptive events occurring during Tenerife's recent geological past and, therefore, the type with the shortest recurrence period and the most likely to occur in the near future. In this paper, the most frequent mafic monogenetic volcano is defined by means of the statistical analysis of its main volcano-morphological features (cone height, cone width ratio, crater width, crater depth, etc.). We have applied a simple methodology of our own design, based on statistical correlations and modal intervals of the morphological and morphometric parameters best defining the volcanoes' morphology. The most frequently identified mafic monogenetic volcano corresponds to a scoria cone with Strombolian to violent Strombolian dynamics, ≤ 100 high, and < 0.01 km 3 in volume, covering an area of < 0.2 km 2. By defining this most common mafic volcano or volcano-type we may provide key information on the nature of a potential volcanic event on Tenerife in the future.

Magma flow in dikes from rift zones of the basaltic shield of Tenerife, Canary Islands: Implications for the emplacement of buoyant magma

Dikes of rift zones in the Miocene basaltic shield of Tenerife have been studied along several profiles in order to record overall variations in lateral and vertical magma flow direction and sense. Dikes were sampled for Anisotropy of Magnetic Susceptibility (AMS) and other flow indicators and dike propagation indicators were also recorded at each sampling site. Bulk susceptibilities suggest a predominance of ferromagnetic minerals in the AMS fabrics and most of the sampled dikes show normal fabrics. Magnetic fabrics have been classified according to clustering and imbrication shown by the maximum susceptibility axis on each dike margin, a procedure that provides rapid information on the direction and sense of magma flow. The flow of magma tends to be upward and steep beneath the volcanic centers of the basaltic shield massifs. Along the axes of rift zones and in those rift zones located across the flanks of the volcanic edifices (roughly normal to the main rifts), magma is mainly emplaced laterally and downward. The flow of magma is mostly towards the external parts of the volcanic centers of the basaltic shield massifs and indicates that magma propagated toward regions of lower topography. This interpretation agrees with magma being emplaced above the level of neutral buoyancy and with buoyancy as the driving pressure gradient. The observed flow patterns can be explained in terms of density gradients within the volcanic edifices, in which magma flow trajectories are toward less dense regions.

Seismic facies and stratigraphy of the Cenozoic succession in McMurdo Sound, Antarctica: Implications for tectonic, climatic and glacial history

Integration of data from fully cored stratigraphic holes with an extensive grid of seismic reflection lines in McMurdo Sound, Antarctica, has allowed the formulation of a new model for the evolution of the Cenozoic Victoria Land Basin of the West Antarctic Rift. The Early Rift phase (Eocene to Early Oligocene) is recorded by wedges of strata confined by early extensional faults, and which contain seismic facies consistent with drainage via coarse-grained fans and deltas into discrete, actively subsiding grabens and half-grabens. The Main Rift phase (Early Oligocene to Early Miocene) is represented by a lens of strata that thickens symmetrically from the basin margins into a central depocenter, and in which stratal events pass continuously over the top of the Early Rift extensional topography. Internal seismic facies and lithofacies indicate a more organized, cyclical shallow marine succession, influenced increasingly upward by cycles of glacial advance and retreat into the basin. The Passive Thermal Subsidence phase (Early Middle Miocene) is recorded by an evenly distributed sheet of strata that thickens somewhat into the depocentre but is continuous across and over the earlier rift strata to the margins of the basin. Internally, it contains similar facies to the underlying Main Rift, but preserves more evidence for clinoform sets and large channels, and in core comprises many short, condensed and strongly top-truncated stratal cycles with continued, periodic glacial influence. These patterns are interpreted to record accumulation under similar environmental conditions but in a regime of slower subsidence. The Renewed Rifting phase (Middle Miocene to Recent, largely unsampled by coring thus far) is represented by intervals that thicken significantly into the basin depocentre and that are complicated by evidence of magmatic activity (McMurdo Volcanic Group). This succession is further divided into lower and upper intervals, separated by a major unconformity that displays increasing angular discordance towards the western basin margin and Transantarctic Mountain Front. The youngest part of the stratigraphy was accumulated under the influence of flexural loading imposed by the construction of large volcanic edifices, and was formed in an environment in which little sediment was supplied from the western basin margin, suggesting a change in environmental (glacial) conditions at possibly c. 2 Ma. The Cenozoic stratigraphy of the southern Victoria Land Basin preserves archives of both climate change and the complex rift history of the basin, and coincidences between key stratal surfaces in seismic data and evidence for environmental change in drillcores suggest that tectonic and climatic drivers may be causally linked.

Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr‐Donets rift zone, Ukraine

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat‐Dniepr‐Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide‐angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr‐Donets Basin. The seismic data are used for identification of large‐scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray‐trace‐based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high‐amplitude and low‐frequency arrivals from this high‐density body as well as from the Moho. We interpret the high‐amplitude, low‐frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (β = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back‐arc extension in relation to subduction of the Paleo‐Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.

Fault superimposition and linkage resulting from stress changes during rifting: Examples from 3D seismic data, Phitsanulok Basin, Thailand

The Phitsanulok basin, Thailand provides examples of changing fault displacement patterns with time associated with faults of different orientations. In the Northern Phitsanulok basin three main stress states have been identified associated with Late Oligocene–Recent fault development: (1) Late Oligocene–Late Miocene approximately E–W extension (N–S Shmax), ‘main rift’ stage, (2) Late Miocene–Pliocene transtension to tranpression (?) (E–W to NE–SW Shmax), ‘late rift’ stage, and (3) Pliocene–Recent very minor faulting, E–W extension, N–S Shmax, ‘post-rift’ stage. Syn-rift faults tend to strike N–S, but also follow NE–SW and NW–SE trends and are basement involved. The Late Miocene deformation produced a distinctly different type of fault population from the main rift fault set, characterized by numerous, small displacement (tens of metres), faults striking predominantly NE–SW. Most of these faults are convergent, conjugate sets aligned in discrete zones and nucleated within the sedimentary basin. Reactivation of main rift faults trends during the late rift stage favoured strike directions between 350° and 50°. The displacement characteristics of three large faults within the basin show variations depending upon fault orientation. The low-angle (23°-30° dip), Western Boundary Fault (∼7 km throw) displays little discernible difference in the distribution of displacement on fault zone during the different stress states other than increases and decreases in displacement amount. Smaller faults exhibit a more selective reactivation history than the Western Boundary fault and are more informative about fault response to a varying stress field. Activation of the (oblique) NE–SW striking NTM-1 initially produced a fault divided into three segments, splaying into N–S trends. Stress reorientation during the late rift stage finally linked NE–SW striking segments. The partial linkage of the fault zone at the time of oil migration resulted in the southwestern part of the NTM-1 fault sealing hydrocarbon bearing reservoirs, whilst late linkage areas along the northeastern part failed to seal hydrocarbons. The N–S striking PTO-1 Fault shows early isolated fault segments, linkage during syn-rift motion, then a more patchy distribution of displacement late in the fault history, perhaps due to non-optimal orientation of the fault to the regional stress field, when compared with the NTM-1 Fault. The fault characteristics described indicate that where strong pre-existing fabrics are present, and varying stress regimes have occurred, fault population characteristics and evolution of fault displacement can depart considerably from extensional fault populations associated with a single phase of extension.

Cretaceous extension of the Ganhang Tectonic Belt, southeastern China: constraints from geochemistry of volcanic rocks

New geochemical and isotopic data are reported from Cretaceous volcanic rocks of the Ganhang Tectonic Belt, southeast China. Two periods of basaltic eruptions (143 ± 1.1–139 ± 0.7 Ma and 105–98 Ma), each representing results of previous tectonic events, are recognized in the belt. Rock types associated with the eruptions are significantly different in mineral and chemical composition. For the 143 ± 1.1–139 ± 0.7 Ma basalts, 87Sr/ 86Sr ratio (0.7041–0.7048) and Sm–Nd isotopic compositions (ɛ Nd = +3.78 to +4.38) suggest that the magma came from the depleted mantle and had not mixed with crustal material. During the second episode of magmatic and tectonic activities (105–98 Ma), volcanism was accompanied by intense regional extension and the development of a series of “red basins”. The geochemical characteristics of these basalts exhibit both tholeiite and alkaline olivine-types, indicating inhomogeneous mixing of magma with crustal material. The tholeiites have a medium–high 87Sr/ 86Sr ratio (0.7054–0.7070) and a low ɛ Nd value (1.69–0.70), indicating that the magma of these basalts came from an unchanged mantle. The alkaline olivine basalts, on the other hand, have a relatively higher 87Sr/ 86Sr ratio (0.7076–0.7080) and a relatively lower value of ɛ Nd (−6.43 to −5.18), suggesting that these magmas came from mantle enriched by mixing with subducted crustal material. Based on comprehensive geological analyses, it is concluded that an extensional environment existed in the region from early in the Early Cretaceous, and the Ganhang Belt represents the main rift zone in southeast China during much of the Early Cretaceous.

Tectonic history and the biogeography of the freshwater fishes from the coastal drainages of eastern Brazil: an example of faunal evolution associated with a divergent continental margin

The eastern Brazilian coastal drainages are of great biogeographical significance, because of their highly endemic fish faunas. Phylogenetic patterns suggest a close biotic relationship between the rivers that flow into the Atlantic and those on the adjacent upland crystalline shield. However, little has been said on the dynamics of the geological processes causally related to the cladogenetic events between these areas. Distributional and phylogenetic patterns suggest a close association with the geological history of the passive continental margin of South America, from the Cretaceous to the present day. In this area megadome uplifts, rifting, vertical movements between rifted blocks and the erosive retreat of the South American eastern continental margin are hypothesized as the main geological forces controlling the distribution of freshwater fishes. The tectonic activity associated with the break-up of Gondwana and separation of South America and Africa formed six megadomes that control most of the current courses of the main crystalline shield river basins. Except for basins located at the edges of such megadomes, these river systems developed long, circuitous routes over the ancient Brazilian crystalline shield before emptying into the recently opened Atlantic Ocean. Initial cladogenetic events between upland crystalline drainages and Atlantic tributaries were probably associated with vicariant processes, and some ancient basal sister-groups of widespread inclusive taxa are found in these coastal hydrographic systems. Later, generalized erosive denudation resulted in an isostatic adjustment of the eastern margin of the platform. These, along with reactivations of ancient rifts led to vertical movements between rifted blocks and gave rise, in southeastern Brazil, to taphrogenic (rift related) basins. These basins, such as the Taubaté, São Paulo, Curitiba and Volta Redonda basins, among others, captured adjacent upland drainages and fauna. The fossil fishes from the Tremembé Formation (Eocene-Oligocene of Taubaté Basin) exemplify this process. Other taphrogenic systems of Tertiary age were also identified in other segments of the Atlantic continental margin, such as in Borborema province, in NE Brazil, with marked influence over drainage patterns. At the same time, erosive retreat of the eastern margin of the platform successively captured upland rivers, which became Atlantic tributaries evolving associated to main rift systems. The continued nature of these processes explains the mixed phylogenetic and distributional patterns between Atlantic tributaries and the upland crystalline shield areas, especially in the southeastern continental margin, represented by successively, less inclusive sister-groups associated with cladogenetic events from the Late Cretaceous to the present.

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

We present the results of a comprehensive major element, trace element and Sr–Nd–Pb–O isotopic study of post-glacial volcanic rocks from the Neovolcanic zones on Iceland. The rocks studied range in composition from picrites and tholeiites, which dominate in the main rift systems, to transitional and alkalic basalts confined to the off-rift and propagating rift systems. There are good correlations of rock types with geochemical enrichment parameters, such as La/Sm and La/Yb ratios, and with long-term radiogenic tracers, such as Sr–Nd–Pb isotope ratios, indicating a long-lived enrichment/ depletion history of the source region. Sr/Sr vs Nd/Nd defines a negative array. Pb isotopes define well-correlated positive arrays on both Pb/Pb vs Pb/Pb and Pb/Pb diagrams, indicating mixing of at least two major components: an enriched component represented by the alkali basalts and a depleted component represented by the picrites. In combined Sr–Nd–Pb isotopic space the individual rift systems define coherent mixing arrays with slightly different compositions. The enriched component has radiogenic Pb (Pb/Pb > 19 3) and very similar geochemistry to HIMU-type ocean island basalts (OIB). We ascribe this endmember to recycling of hydrothermally altered upper basaltic oceanic crust. The depleted component that is sampled by the picrites has unradiogenic Pb (Pb/Pb < 17 8), but geochemical signatures distinct from that of normal mid-ocean ridge basalt (N-MORB). Highly depleted tholeiites and picrites have positive anomalies in mantle-normalized trace element diagrams for Ba, Sr, and Eu (and in some cases also for K, Ti and P), negative anomalies for Hf and Zr, and low dOolivine values (4 6–5 0‰) below the normal mantle range. All of these features are internally correlated, and we, therefore, interpret them to reflect source characteristics and attribute them to recycled lower gabbroic oceanic crust. Regional compositional differences exist for the depleted component. In SW Iceland it has distinctly higher Nb/U ( 68) and more radiogenic Pb/Pb ratios (18 28–18 88) compared with the NE rift (Nb/U 47; Pb/Pb 1⁄4 18 07–18 47). These geochemical differences suggest that different packages of recycled oceanic lithosphere exist beneath each rift. A third and minor component with relatively high Sr/Sr and Pb/Pb is found in a single volcano in SE Iceland (Oraefajokull volcano), indicating the involvement of recycled sediments in the source locally. The three plume components form an integral part of ancient recycled oceanic lithosphere. The slope in the uranogenic Pb diagram indicates a recycling age of about 1 5 Ga with time-integrated Th/U ratios of 3 01. Surprisingly, there is little evidence for the involvement of North Atlantic N-MORB source mantle, as would be expected from the

Gravitational spreading and formation of new rift zones on overlapping volcanoes

AbstractLarge volcanic edifices are often shaped by the coalescence of adjacent volcanoes as well as intrusive rift zones and gravitational spreading. To better understand the structure of such volcanoes we designed analogue experiments simulating gravitational spreading of an edifice made by overlapping cones of different age, and examined the formation of rift zones. The results allow distinction of two main rift geometries. (i) Spreading edifices of similar age that partly overlap, tend to develop a rift zone approximately perpendicular to the boundary of both volcanoes. Such a rift zone causes two volcanoes to grow together and form an elongated topographic ridge. (ii) Partly overlapping volcanoes of different age are spreading at different rates and thus form a rift zone parallel to the boundary of both volcanoes. Such a rift zone causes two volcanoes to structurally separate. The results are widely applicable for large volcanoes subject to rifting and flank spreading, which we demonstrate for Réunion Island and for southern Hawaii.

The East African rift system

This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the ‘cradle of Mankind’ with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.

Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift

The lithospheric and sublithospheric processes associated with the transition from continental to oceanic magmatism during continental rifting are poorly understood, but may be investigated in the central Main Ethiopian Rift (MER) using Quaternary xenolith-bearing basalts. Explosive eruptions in the Debre Zeyit (Bishoftu) and Butajira regions, offset 20 km to the west of the contemporaneous main rift axis, host Al-augite, norite and lherzolite xenoliths, xenocrysts and megacrysts. Al-augite xenoliths and megacrysts derived from pressures up to 10 kb are the dominant inclusion in these recent basalts, which were generated as small degree partial melts of fertile peridotite between 15 and 25 kb. Neither the xenoliths nor the host basalts exhibit signs of carbonatitic or hydrous (amphibole + phlogopite) metasomatism, suggesting that infiltration of silicate melts resulting in pervasive Al-augite dyking/veining dominates the regional lithospheric mantle. Recent geophysical evidence has indicated that such veining/dyking is pervasive and segmented, supporting the connection of these Al-augite dykes/veins to the formation of a proto ridge axis. Al-augite xenoliths and megacrysts have been reported in other continental rift settings, suggesting that silicate melt metasomatism resulting in Al-augite dykes/veins is a fundamental processes attendant to continental rift development.

Structural control of volcanism and caldera development in the transtensional Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), New Zealand, comprises several segments with variable components of extension, and is characterised by longitudinal segmentation into a central zone dominated by rhyolitic calderas and extremities with only andesitic composite volcanoes. Modern (<300 ka) TVZ was investigated to define (a) the overall relationship between regional structures and volcanism and (b) the structural control on the evolution of calderas. New remote sensing and field structural data have been combined with previously published data for TVZ. The results show a general correlation between the amount of extension and the volume and style of eruption in each segment. The segments with the greatest extension coincide with the highly active Taupo and Okataina caldera complexes; conversely, the segments with the greatest dextral transtension correspond to volumetrically less active andesitic stratovolcanoes. Within Modern TVZ, two types of caldera are distinguished based on their structure and development. Calderas within the main zone of rifting (Taupo, Okataina) are multiple (two or more major eruptions) collapse structures with rectilinear margins, overprinted by younger volcanism and faulting. Their complexity is related to the proximity (and influence) of active faulting within TVZ. In contrast, calderas peripheral to the main rift (Rotorua, Reporoa) are sub-circular monogenetic collapse structures, with minor post-collapse volcanism. Their simpler evolution and structure is a attributable to their location away from the main rift zone. Thus, TVZ exemplifies the situation where firstly, magma is erupted as a function of the extension rate along the axial rift zone, and secondly, volcanism is manifest as distinctive caldera structures according to their proximity to the axial rift zone.

The role of evaporite mobility in modifying subsidence patterns during normal fault growth and linkage, Halten Terrace, Mid‐Norway

AbstractWell‐calibrated seismic interpretation in the Halten Terrace of Mid‐Norway demonstrates the important role that structural feedback between normal fault growth and evaporite mobility has for depocentre development during syn‐rift deposition of the Jurassic–Early Cretaceous Viking and Fangst Groups. While the main rift phase reactivated pre‐existing structural trends, and initiated new extensional structures, a Triassic evaporite interval decouples the supra‐salt cover strata from the underlying basement, causing the development of two separate fault populations, one in the cover and the other confined to the pre‐salt basement.Detailed displacement–length analyses of both cover and basement fault arrays, combined with mapping of the component parts of the syn‐rift interval, have been used to reveal the spatial and temporal evolution of normal fault segments and sediment depocentres within the Halten Terrace area. Significantly, the results highlight important differences with traditional models of normal fault‐controlled subsidence, including those from parts of the North Sea where salt is absent. It can now be shown that evaporite mobility is intimately linked to the along‐strike displacement variations of these cover and basement faults. The evaporites passively move beneath the cover in response to the extension, such that the evaporite thickness becomes greatest adjacent to regions of high fault displacement. The consequent evaporite swells can become large enough to have pronounced palaeobathymetric relief in hangingwall locations, associated with fault displacement maxima– the exact opposite situation to that predicted by traditional models of normal fault growth. Evaporite movement from previous extension also affects the displacement–length relationships of subsequently nucleated or reactivated faults. Evaporite withdrawal, on the other hand, tends to be a later‐stage feature associated with the high stress regions around the propagating tips of normal faults or their coeval hangingwall release faults. The results indicate the important effect of, and structural feedback caused by, syn‐rift evaporite mobility in heavily modifying subsidence patterns produced by normal fault array evolution. Despite their departure from published models, the results provide a new, generic framework within which to interpret extensional fault and depocentre development and evolution in areas in which mobile evaporites exist.

CHARACTERISATION OF THE LITHOSPHERE BENEATH THE NORTH-WESTERN ETHIOPIAN PLATEAU: SPINEL LHERZOLITES FROM INJIBARA (LAKE TANA PROVINCE)

We have investigated xenoliths of spinel lherzolites in a Quaternary basanitic flow near Injibara (Tana Lake Province) to characterise the mantle beneath the north-western Ethiopian Plateau. Previous studies of mantle rocks from the Main Rift (e.g. Mega, South Ethiopia; Bedini et al., 1997), have indicated that peridotites suffered modal (apatite) or cryptic (enrichment/depletion in LILE) metasomatism. The metasomatic agents were identified as silicate melts migrating through the lithospheric mantle. At Injibara, lherzolites [Ol (46-69), Opx (19-31), Cpx (9-22), and Spl (2-7) in vol.%] show porphyroclastic to equigranular textures. These rocks suffered a modal metasomatic event, testified by growth of Ti-pargasite, and enrichment in major (Mg,Na,Ti) and trace elements (LREE,Rb,Ba,Th,U,Pb). Equilibration temperatures are 907-1015 °C (median values 960-970°C), in agreement with previous data obtained from Injibara mantle xenoliths (954-985°C, Conticelli et al., 1999). Fluid inclusions (<1-40 _m) occur only in the most preserved rocks, along intragranular trails within porphyroclasts, and as rare primary inclusions in Opx neoblasts. At room temperature, monophase (L) and biphase (L+V) inclusions are observed with rare birefringent solids, recognised as magnesite by Raman spectroscopy. Microthermometric data reveal the presence of CO2 that melts (TmCO2) instantaneously at @ -56.6°C. Most of inclusions homogenise to the liquid (ThLCO2) between –39.2 and 31°C, and only rare homogenise to the vapour (ThVCO2; 28.3 - 30.9°C). For a temperature of 970°C, a minimum pressure of 0.95 GPa is obtained by the highest density (1.11 g/cm3) fluid isochore. In conclusion, our data indicate that the mantle under Injibara (North-Western Ethiopian Plateau) underwent a different evolution with respect to the mantle beneath the Rift. In mantle rocks beneath Injibara, growth of amphibole and the LILE enrichment are strictly associated to the migration of a CO2-rich fluid, whereas in rift peridotites, apatite growth and variable LILE contents are related to interaction with silicate melts.

The post-Variscan development of the British Isles within a regional transfer zone influenced by orogenesis

The break-up of Pangaea after the Variscan Orogeny included rifting extending southwards from the Barents Sea via the Norwegian–Greenland Rift and into the North Sea, and northwards from the Central Atlantic. These two major rift systems interacted to form an approximately 1200-km-wide transfer zone across the British Isles, where a complex network of basins developed during the Mesozoic. Fault patterns were commonly controlled by reactivation of Precambrian, Caledonian and Variscan structures. The two main rift systems were unable to breach this regional transfer zone, where the crust had been thickened by the Caledonian and Variscan orogenies, until the Eocene. Breaching did not occur down the North Sea and through the English Channel because of Alpine contraction in NW Europe. Instead, breaching occurred around the west of Ireland and NW Scotland, so the British Isles remained connected to Europe rather than to the North American Plate.

Sequence stratigraphy of the Jurassic of the Danish Central Graben

A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isochore maps. In the Lower Jurassic (Hettangian–Pliensbachian) succession, five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the uppermost Aalenian/lowermost Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped. On the basis of the sequence stratigraphic correlation and the construction of isochore maps for individual sequences, the Jurassic basin history of the Danish Central Graben can be subdivided into seven discrete phases: (1) Shallow marine and offshore sediments deposited in a prerift basin extending from the North Sea to the Fennoscandian Border Zone (Hettangian–Pliensbachian). (2) Uplift and erosion in association with a Toarcian–Aalenian North Sea doming event. A major hiatus represents this phase in the study area. (3) Terrestrial and marginal marine sedimentation during initial rifting (latest Aalenian/earliest Bajocian – Late Callovian). (4) Early Oxfordian – Early Kimmeridgian transgression during and after a rift pulse. The sedimentary environment changed from coastal plain and marginal marine to fully marine. (5) Regression associated with a cessation or slowing of subsidence during a structural rearrangement that took place in the Late Kimmeridgian during a break in the main rift climax. Shallow to marginal marine sandstones were deposited above an erosion surface of regional extent. (6) Deep-water mudstones deposited in a composite graben with high subsidence rates related to rift pulses (latest Late Kimmeridgian – middle Middle Volgian). (7) Deposition of organic-rich mudstones and turbidite sandstones during the late Middle Volgian – Early Ryazanian. The main basin shallowed, became more symmetrical and experienced a decreasing rate of subsidence, recording the onset of the post-rift stage. A relative sea-level curve is constructed for the Middle–Late Jurassic. It shows close similarity to published eustatic (global) and relative (North Atlantic area) sea-level curves in the latest Bathonian – late Early Kimmeridgian, but differs in the Late Kimmeridgian – Middle Volgian interval, probably due to the high rate of subsidence in the study area.

Middle Jurassic – Early Cretaceous rifting of the Danish Central Graben

During the Jurassic – Early Cretaceous, the Danish Central Graben developed as a N–S- to NNW– SSE-trending graben bounded by the Ringkøbing–Fyn High towards the east and the Mid North Sea High towards the west. The graben consists of a system of half-grabens and evolved by faultcontrolled subsidence; three main rift pulses have been recognised. The first pulse ranged from the Callovian to the Early Oxfordian, the second pulse was initiated in the latest Late Kimmeridgian and lasted for most of the Early Volgian, and the third and final pulse occurred within the Ryazanian in the Early Cretaceous. The first pulse was characterised by subsidence along N–S-trending faults. The most pronounced fault-controlled subsidence occurred in the east, especially along N–S-striking segments of the boundary fault to the Ringkøbing–Fyn High. During this period, minor salt movements occurred with the development of salt pillows. The activity along the N–S-trending faults ceased during the Oxfordian. During the second pulse, in Early Volgian times, subsidence was concentrated along new NNW–SSE-trending faults and the main depocentre shifted westward, being most marked within the Tail End Graben, the Arne–Elin Graben, and the Feda Graben. This tectonic event was accompanied by the accumulation of a relatively thick sediment load resulting in the development of salt diapirs, especially within the Salt Dome Province. The third tectonic pulse was essentially a reactivation of the NNW–SSE-trending structures and there is clear evidence of subsidence controlled by faulting and salt movements. Despite the overall extensional tectonic regime, local compressional tectonics resulted in thrusting. For instance, the Gert Ridge is interpreted to have formed by readjustment at the boundary fault between two subsiding blocks. The structural framework during graben evolution controlled, to some degree, the distribution of reservoir sandstones. Reservoir sandstones associated with periods of rotational tilt include Middle Jurassic deposits referred to the Bryne and Lulu Formations, and Upper Jurassic sandstones referred informally to the ‘Fife Sandstone Formation’. Sands deposited during tectonic relaxation are represented by the Heno Formation and Upper Jurassic turbidites interbedded in the Farsund Formation. Sea-level changes were probably most important during periods of tectonic relaxation, particularly with respect to the deposition of lowstand sandstones in basinal areas.

Upwelling and melting of the Iceland plume from radial variation of 238U– 230Th disequilibria in postglacial volcanic rocks

New 238U– 230Th disequilibria data by thermal ionisation mass spectrometry are presented for a comprehensive set of postglacial basaltic lavas from the neovolcanic zones in Iceland. The new data show a striking systematic decrease in 230Th excess towards central Iceland and the presumed centre of the Iceland plume. This finding would appear paradoxical if source composition was the main factor responsible for generating the 238U– 230Th disequilibria, because generally main rift lavas erupted proximal to the plume should be generated from a melting column that initiates deeper in the garnet stability field, compared to the marginal rift zones. Preferential crustal interaction in central Iceland, where the crust is thickest, involving either old (>350 kyr) Icelandic crust or lower crustal melts, may provide a viable explanation for only part of the data variation, namely the moderately low 238U– 230Th disequilibria found in the more evolved SE rift lavas. Moreover, there is no variation of 230Th excesses with degree of differentiation (Mg# or ppm Th) overall, or within individual rift systems, to indicate that crustal contamination causes the radial variation in 230Th excess. The 238U– 230Th disequilibria variation is therefore ascribed to variable dynamic parameters in the melting regime induced by interaction of the Iceland plume with the rift systems. The higher 230Th excesses in alkalic off-rift lavas (Snæfellsnes Peninsula) (24±3%) compared to the main rift lavas (15±3%) is consistent with more garnet-rich lithologies dominating the bulk melt compositions away from the main rifts and indicates small-scale source heterogeneity beneath Iceland. The data are reconciled within a model in which mantle upwelling rates in the centre of the plume are significantly faster than at the margins, consistent with fluid dynamic predictions for a plume head. The radial variation observed in ( 230Th/ 238U) provides independent support that the centre of the Iceland plume is located beneath SE Iceland, as has been proposed from seismic tomographic studies. For a reasonable range of mantle porosities ( Φ=0.05–0.2%) we can explain the Iceland data with a dynamic melting model, by relatively fast mantle upwelling rates in the centre (∼5–20 cm/yr), compared to those at the margins (∼1–4 cm/yr). The radial variation is also shown to be consistent with, though not requiring, a model of deep dehydration melting [Ito et al., Earth Planet. Sci. Lett. 165 (1999) 81–96]. In such a scenario, the generation of (moderately) low 238U– 230Th disequilibria will be confined to the lowermost part of the melting column, which is characterised by fast upwelling and low porosity. For Φ values down to 0.05% in the lower part of the hydrous melting zone, moderately low 230Th excesses (5–10%) are likely to result, whereas higher 230Th excesses may arise for lower values of Φ.

Holocene climate phases from buried soils in Tigray (northern Ethiopia): comparison with lake level fluctuations in the Main Ethiopian Rift

Stratigraphic analysis of alluvial/colluvial sequences and 14C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The 14C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 14C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale.

Analogue modelling of continental extension: a review focused on the relations between the patterns of deformation and the presence of magma

Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative processes, continental rifting may be explained by both active or passive mechanisms, which also differ in the volume of magmatic products and in the rheological properties and stratification of the extending lithosphere. Both numerical and analogue models have investigated the main parameters controlling the extension of a rheologically layered lithosphere. In particular, analogue models have highlighted that the style of deformation is mainly controlled by the competition between the total resistance of the lithosphere and the gravitational forces; this competition, in turn, is mainly controlled by boundary conditions, such as the applied strain rate and the rheological characteristics of the extending lithosphere. Magmatic bodies eventually present within the continental lithosphere may significantly affect the process of extension. Both the thermal and mechanical effects related to the presence of magma strongly weaken the lithosphere and localise strain; this effect may have important implications for the mode of continental extension. At a crustal scale, magmatic intrusions may affect significantly the local fault pattern also favouring the development of core complex structures. Results of analogue models, performed taking into account the presence of an initially underplated magma and reproducing various continental extensional settings, suggest a close interaction between deformation and magma emplacement during extension. Particularly, magmatic underplating influences deformation localising strain in correspondence to the low-viscosity body, while on the other hand, rift kinematics and associated deformation has a major control on the pattern of magma emplacement. In particular: (1) During orthogonal rifting, magma is passively squeezed from an axial position towards the footwall of the major boundary faults; emplacement occurs in a lateral position in correspondence to lower crust domes. This process accounts for the close association between magmatism and the development of core complex structures, as well as for the occurrence of off-axis volcanoes in continental rifts. (2) During oblique rifting, deformation causes magma to emplace within the main rift depression, giving rise to intrusions with oblique and en echelon patterns. In nature, these patterns are found in continental rifts and also in some oceanic ridges. (3) Polyphase first orthogonal–second oblique rifting models suggest lateral squeezing and off-axis emplacement in the first phase and oblique en echelon intrusions in the successive oblique rifting phase. This evolution matches the magmatic and tectonic history of the Main Ethiopian Rift. (4) Development of transfer zones between offset rift segments has a great influence on both magma migration and deformation. Particularly, magma accumulates in correspondence to the transfer zone, with a main flow pattern that is perpendicular to the extension direction. This pattern may explain the concentration of magmatism at transfer zones in continental rifts. Overall, analysis of centrifuge models and their comparison with nature suggest that deformation and magma emplacement in the continental crust are intimately related, and their interactions constitute a key factor in deciphering the evolution of both continental and oceanic rifts.