Rifting Cycle Research Articles

Tectonostratigraphic framework and provenance of a Mesoproterozoic rift succession: An example from the Espinhaço Supergroup, SE Brazil

During the Mesoproterozoic era, the amalgamation of the Rodinia supercontinent witnessed numerous intraplate rifting events far-field induced by “Grenvillian-type age” orogenic systems. The intimate connection between tectonics and sedimentation enables the reconstruction of these intraplate rifting cycles, linking them to accretionary events along craton margins. This is essential for understanding the paleotectonic history of continents during the Mesoproterozoic era. In southeastern Brazil, the Espinhaço Supergroup documents long-lasting superimposed rifting events that extended to the eastern border of the São Francisco paleocontinent from the Paleoproterozoic to the Mesoproterozoic era. Three unconformity-bound rift sequences (∼1.77–1.7 Ga; ∼1.57–1.5 Ga; and ∼ 1.19 Ga) have been identified within the Espinhaço Supergroup, terminated by sag-marine deposits of the Conselheiro Mata Group. In the southern extension of the southern Espinhaço range (Cipó range), a detailed sedimentological and stratigraphic study was conducted on a rift-related sequence with unknown stratigraphic positioning. This sequence is bounded at the base by eolian sediments of the Galho do Miguel Formation of the Southern Espinhaço Supergroup and at the top by Ediacaran-Cambrian carbonates and pelites of the Bambuí Group. Examination of the tectonically-induced depositional controls in the provenance signatures involved stratigraphic surveys coupled with U-Pb geochronological data of detrital zircons. The lower section of the rift-related sequence transitions eastward into marine sediments of the Conselheiro Mata Group within the Santa Rita Formation, now identified as the Lower Member. With a maximum depositional age of 1660 ± 32 Ma, this section directly overlies the Galho do Miguel Formation by an erosive unconformity, comprising syn-rift coarse-grained deposits from alluvial and fluvial systems within an inverted half-graben structure. The Lower Member of the Santa Rita Formation provides evidence of the Mesoproterozoic rifting event associated with the Conselheiro Mata Group. The marine sediments of the Undefined sequence have a maximum depositional age of 1462 ± 19 Ma, overlying the Lower Member of the Santa Rita Formation with an erosive unconformity. The stratigraphic position of this sequence remains speculative, possibly linked to a later extensional phase of the Mesoproterozoic Conselheiro Mata rifting event or one of the two continental rifting stages leading to the Neoproterozoic Macaúbas Group.

Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift

Two drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle.

Geometry and Kinematics of the Central Fault Zone, Fula Sag, Central Africa Shear Zone

The Central Africa Shear Zone (CASZ) harbors abundant hydrocarbon resources within its Central Fault Zones (CFZs). The studies of CASZ have dominantly focused on the evolution and superimposition processes of prototype basins in CASZ. Meanwhile, research on the geometry and segmental growth of main faults in CFZs remains poorly understudied, which limits hydrocarbon exploration. In this paper, we focus on the CFZ of the Fula sag as an example of CASZ and utilize the 3-D throw mapping technique along with the maximum throw subtraction method to investigate its geometric and growth processes. Results show faults in the northern and central parts of the CFZ form multiple Y-shaped combinations, and a system of sub-parallel faults in the south forms the bookshelf faults. Meanwhile, the divergent overlapping transfer zone is identified in the CFZ. Our investigation found abrupt changes in throw-distance diagrams of main faults in the CFZ, which indicate that the main faults, F1, F3 and F2, are laterally segmented into 4, 4, and 3 segments, respectively. As an intracontinental passive rift basin, the Fula sag has undergone three major rifting cycles since the Early Cretaceous, triggered by the segmental expansion of the Atlantic Ocean, the rapid opening of the Indian Ocean, and the separation of the Red Sea. Our analysis also reveals that the main faults in the CFZ were primarily active during the second rifting, with the fault segments undergoing isolated growth, soft linkage, and eventually forming fully grown faults during the third rifting. We observe a significant decrease in activity intensity during the transition between the second and third rifting cycles. Our findings provide insights into the growth and activity of the CFZ faults, which are applicable to other CFZs of similar origin in rift basins, and provide suggestions for hydrocarbon exploration and production.

Magmatic evolution during proto-oceanic rifting at Alu, Dalafilla and Borale Volcanoes (Afar) determined by trace element and Sr-Nd-Pb isotope geochemistry

Continental rifting and associated magmatism can eventually result in the formation of new ocean basins. However, the characteristics of magmatism in the latest stages of rifting are poorly understood. The Erta-Ale volcanic segment (EAVS) in the Danakil Depression of Afar, Ethiopia, provides a unique natural laboratory in which to investigate how magma generation evolves during the shift from continental rifting to oceanic spreading. Here we present new trace element data combined with Sr-Nd-Pb isotope ratios for three volcanoes, Alu, Dalafilla and Borale, in the north of the EAVS. These data shed light on the changes in melt production and storage that occur at this late stage in the rifting cycle. Elevated Ce/Pb and ΔNb (33–48, 0.25–0.47 respectively) of the basalts, alongside Sr-Nd-Pb isotope geochemistry indicate the presence of a HIMU component, supplied by the Afar plume, together with contamination by the crust. Melting conditions, estimated using the trace element ratios, Smn/Ybn, Dyn/Ybn and Cen/Smn, indicate that magmas were primarily derived from spinel lherzolite (85–90%) with minor garnet lherzolite (10–15%) with a melt fraction of ~4%. Melt-mantle equilibrium depths are estimated to be on the order of 64 to 83 km, shallower than that previously inferred within Afar. We suggest that this is likely a result of the increased plate thinning beneath the EAVS compared to other parts of Afar. Basaltic volcanics are found to exhibit heterogeneous Sr-Nd-Pb isotope compositions whilst more evolved rocks (i.e., SiO2 ≥52 wt%) exhibit consistent radiogenic compositions. This indicates that homogenisation of all melt compositions occurs prior to or during melt differentiation, with the latter process occurring rapidly in the upper crust with minimal crustal contamination. Overall whilst the Afar plume appears to be the dominant mantle component in the volcanic rocks, the melt characteristics and magmatic storage conditions beneath the EAVS shows variability that is likely controlled by a dynamic interplay between rifting and mantle processes.

Characteristics of sedimentary differential response for multi-episode rifting cycles in lacustrine rift basin: A case study of the Palaeogene Baxian sag in the Bohai Bay Basin

The evolution of a lacustrine rift basin is a multi-periodic and multi-cyclic procedure. In the present study, we interpreted and analysed a filling process in the Palaeogene Baxian sag of the Bohai Bay Basin (China). Further, we also investigated the evolution of the filling process. We identified the reason for the difference in lacustrine rift basins. Geological and geophysical data, including drilling data, well logging curves, and high-quality full-coverage 3D seismic volume survey data, were used to interpret and analyse the sedimentary system types and their spatial and temporal distribution. The filling evolution of the Baxian sag in the Palaeogene comprised three episodes and had stages where filling occurred. Four depositional systems, namely, alluvial-rivers, lake-deltas, lake beach-bars, and fluvial-floodplains, were detected. The findings of this study suggest that this basin has undergone stages of shallow–deep–shallow water. The entire rifting process could be separated into several multi-episodic sub-processes, with each sub-process undergoing four stages, i.e., initial rifting, strong rifting, stable rifting, and post-fault return. Further, the formation and evolution of faults, particularly boundary faults, control the stratigraphic structure and distribution of sedimentary systems in rift lacustrine basins. Differences in the category and distribution of the sedimentary system illustrate the relationship between fault activity intensity and provenance supply in such basins. Near-source sediments were deposited rapidly in the early stage; subsequently, the basin deepened rapidly. The deep-water deposits and far-source deposits were laid down slowly in the middle and late stages, respectively. Differential activities of boundary faults, influenced by mantle differential convection, controlled the formation and development of rift lacustrine deposits and the differential evolution of the basin. We further summarized the development process and general principles of typical rift lacustrine basins and determined the controlling factors of the corresponding differential evolution. This study provides a basis for investigating the genesis and evolution process of other similar basins.

Formation of the stratigraphic hydrocarbon accumulations of the Upper Yabus Formation in the Northern Sub-basin of the Melut Basin, South Sudan

The Melut Basin is an important petroliferous rift basin in Africa, and a large number of structural hydrocarbon accumulations of the Paleogene Yabus and Samma Formations have been discovered in it. However, there are few published research results and exploration discoveries on stratigraphic hydrocarbon accumulations in the Melut Basin. Based on information from the evaluation of the lithology assemblage, oil-source, migration pathway, preservation condition and the detailed analysis of typical exploration cases, the favorable exploratory interval, formation conditions and accumulation characteristics of the stratigraphic hydrocarbon accumulations have been studied in the northern Melut sub-basin. The sedimentary facies study indicated that the Upper Yabus Formation developed widespread delta front deposits with the lithology assemblage of “sand wrapped by mud”, which was conducive to forming depositional stratigraphic traps dominated by facies change or depositional pinchout. The Adar Formation is a set of mudstone deposition with a thickness of 200–600 m above the Yabus Formation and well sealed the Yabus and Samma hydrocarbon accumulations as the regional top seal. The parameters of organic matter, and the thickness and distribution of source rocks showed that the Al Renk Formation has good source rock quality with an average TOC value of 2.08% wt. and type II kerogen prone to oil generation. The thermal modelling revealed that the Al Renk source rocks continuously generated and expelled hydrocarbon since the Late Cretaceous to the present and provided sufficient oil source for the Upper Yabus stratigraphic traps. The tectonic evolution research showed that the multi-stage rifting cycles made the Melut Basin develop numerous normal faults, which provided essential vertical migration pathways for the hydrocarbon charging of the Paleogene stratigraphic traps. Among these faults, the basinal border faults and the intra-basinal late-rifting faults are the primary hydrocarbon conduit faults. The details of exploration cases showed that the hydrocarbon conduit faults are the key controlling factors for the formation of the Upper Yabus stratigraphic hydrocarbon accumulations, and the stratigraphic traps jointly dominated by lithology change (depositional pinchout) and hydrocarbon conduit faults have low hydrocarbon filling risk. The sandstones of the Upper Yabus Formation are shallow (700–2500 m) and have high porosity with a value of greater than 20% and high oil production of a few hundred to more than 1000 barrels per day. Therefore, the Upper Yabus Formation develops favorable petroleum geology conditions for the formation of stratigraphic hydrocarbon accumulations, and exploring such stratigraphic traps is of great significance to the increasing exploration in the mature northern Melut sub-basin.

Detrital zircon U–Pb geochronology and provenance of the Neoproterozoic Lower Roan Group, Chambishi Basin, North‐eastern Zambia: Implication for rift evolution of the Congo Craton

Zambia Copperbelt hosts numerous world‐class, high‐grade sediment‐hosted stratiform Cu deposits. The Lower Roan Group of the Katanga Supergroup is the main ore host and mainly comprises clastic rocks. In this study, we conducted laser ablation inductively coupled plasma mass spectrometry detrital zircon dating on the sedimentary rocks of the Lower Roan Group in the Chambishi Basin, in order to investigate the Neoproterozoic rift evolution of the Congo Craton. Our results reveal that the main zircon age population (97.1%) falls within 2.1–1.7 Ga (peaks at 1.95 and 1.88 Ga), and minor age populations fall into the Archaean (2.7–2.4 Ga), Palaeoproterozoic (2.3–2.1 Ga), and Mesoproterozoic (1.5–1.05 Ga). Detrital zircons from the Lower Roan Group have distinct oscillatory zoning in cathodoluminescence images, high Th/U ratios (mostly 0.5–2.0), and are sub‐angular, suggesting close detrital provenance. Provenance of the Lower Roan group may have been mainly the Lufubu Metamorphic Complex, Bangweulu Block, and Muva Supergroup, and minor the nearby granitoids of the Irumide Belt. The south‐eastern Congo Craton likely underwent two Neoproterozoic rifting events: the first rift cycle lasted from 880 to 820 Ma when the Lower Roan and Upper Roan groups were deposited in the Roan Basin. After an uplift, the Mwashia, Nguba, and Kundelungu groups of the Katanga Supergroup were likely deposited during the second rift basin development (765–635 Ma). The similar detrital zircon age spectra suggested that the Mwashia Group may have derived from the Lower Roan Group.

Initiation of a Proto‐transform Fault Prior to Seafloor Spreading

AbstractTransform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.

The synrift phase of the early Domeyko Basin (Triassic, northern Chile): Sedimentary, volcanic, and tectonic interplay in the evolution of an ancient subduction‐related rift basin

AbstractThe geodynamic setting along the SW Gondwana margin during its early breakup (Triassic) remains poorly understood. Recent models calling for an uninterrupted subduction since Late Palaeozoic only slightly consider the geotectonic significance of coeval basins. The Domeyko Basin initiated as a rift basin during the Triassic being filled by sedimentary and volcanic deposits. Stratigraphic, sedimentological, and geochronological analyses are presented in order to determine the tectonostratigraphic evolution of this basin and to propose a tectonic model suitable for other SW Gondwana‐margin rift basins. The Domeyko Basin recorded two synrift stages. The Synrift I (~240–225 Ma) initiated the Sierra Exploradora sub‐basin, whereas the Synrift II (~217–200 Ma) reactivated this sub‐basin and originated small depocentres grouped in the Sierra de Varas sub‐basin. During the rift evolution, the sedimentary systems developed were largely controlled by the interplay between tectonics and volcanism through the accommodation/sediment supply ratio (A/S). High‐volcaniclastic depocentres record a net dominance of the syn‐eruptive period lacking rift‐climax sequences, whereas low‐volcaniclastic depocentres of the Sierra de Varas sub‐basin developed a complete rift cycle during the Synrift II stage. The architecture of the Domeyko Basin suggests a transtensional kinematic where N‐S master faults interacted with ~NW‐SE basement structures producing highly asymmetric releasing bends. We suggest that the early Domeyko Basin was a continental subduction‐related rift basin likely developed under an oblique convergence in a back‐arc setting. Subduction would have acted as a primary driving mechanism for the extension along the Gondwanan margin, unlike inland rift basins. Slab‐induced dynamic can strongly influence the tectonostratigraphic evolution of subduction‐related rift basins through controls in the localization and style of magmatism and faulting, settling the interplay between tectonics, volcanism, and sedimentation during the rifting.

Tectono-sedimentary evolution and hydrocarbon reservoirs in the Early Cretaceous Tanan Depression, Tamtsag Basin, Mongolia

Abstract The responses of depositional patterns to extensional faulting during the rift cycle are discussed in this article, based on an integrated analysis of 3D seismic reflections, wire logs and cores from the Early Cretaceous Tanan Depression in the Tamtsag Basin, Mongolia. The synrift succession comprises four third-order sequences (SQ1-4). A four-stage fault evolution is defined, including early synrift SQ1 (initial isolate faults), early synrift SQ2 (rapid fault propagation), climax synrift SQ3 (maximum fault relief) and late synrift SQ4 (minor fault activity) stages. The initial isolated fault stage is defined by separated and small-sized half-grabens associated with short-dispersal alluvium. In contrast, the rapid fault propagation stage is characterised by much longer dispersal systems of fan-delta deposits. As rifting progressed, a considerable increase in the basin subsidence occurred and the maximum fault displacement led to a dustpan basin geometry, the coalescence of depocentres and extensive deep-water deposits. The observed both sediment starvation and balanced-filled patterns in SQ3 indicate that caution must be exercised when attempting to correlate reservoirs and source rocks since variations in basin-filled patterns would be enhanced by the displacement localisation on several major faults during the significant basin formation. During the early period of the climax synrift stage, footwall-sourced systems were prominent, but drainages in prior relay zone sites were mostly decayed or submerged. In response to the subsequent degradation of the footwall sources, axial-sourced braided deltaic systems became more important during the later period. An abrupt decrease of faulting at the end of the rift cycle gave a rise to an increased sediment supply and long-dispersal axially-sourced systems in the Tanan Depression. Petroleum exploration shows that fan-delta front sandstones in SQ2 are the most prospective hydrocarbon reservoirs, whereas the nearshore and offshore sub-lacustrine fan deposits in SQ3 have proved to be secondary potential oil-bearing reservoirs. This study indicates that sediment dispersal patterns would be significantly re-organised by the differential faulting displacement patterns and varied sediment potentials during the different synrift stages. The rapid fault propagation allowed the fault arrays to reach their nearly maximum length during the early period of the basin evolution, while the displacement remained almost constant. This fault growth pattern tends to establish large-scale topographic lows including overlapped transfer zones and hanging dip-slope. Hence, major drainage catchments and long-dispersal depositional systems of sand-rich deposits are facilitated during this early fault propagation stage, which may be a key aspect for the tectono-sedimentary evolution in rift basins. In addition, the fault and sedimentary relationships defined here may be used as a model when studying less data-rich rift basins elsewhere.

The role of pre‐existing structures during rifting, continental breakup and transform system development, offshore West Greenland

AbstractContinental breakup between Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, which are connected via the Davis Strait, a region mostly comprised of continental crust. This study contributes to the debate regarding the role of pre‐existing structures on rift development in this region using seismic reflection data from the Davis Strait data to produce a series of seismic surfaces, isochrons and a new offshore fault map from which three normal fault sets were identified as (i) NE‐SW, (ii) NNW‐SSE and (iii) NW‐SE. These results were then integrated with plate reconstructions and onshore structural data allowing us to build a two‐stage conceptual model for the offshore fault evolution in which basin formation was primarily controlled by rejuvenation of various types of pre‐existing structures. During the first phase of rifting between at least Chron 27 (ca. 62 Ma; Palaeocene), but potentially earlier, and Chron 24 (ca. 54 Ma; Eocene) faulting was primarily controlled by pre‐existing structures with oblique normal reactivation of both the NE‐SW and NW‐SE structural sets in addition to possible normal reactivation of the NNW‐SSE structural set. In the second rifting stage between Chron 24 (ca. 54 Ma; Eocene) and Chron 13 (ca. 35 Ma; Oligocene), the sinistral Ungava transform fault system developed due to the lateral offset between the Labrador Sea and Baffin Bay. This lateral offset was established in the first rift stage possibly due to the presence of the Nagssugtoqidian and Torngat terranes being less susceptible to rift propagation. Without the influence of pre‐existing structures the manifestation of deformation cannot be easily explained during either of the rifting phases. Although basement control diminished into the post‐rift, the syn‐rift basins from both rift stages continued to influence the location of sedimentation possibly due to differential compaction effects. Variable lithospheric strength through the rifting cycle may provide an explanation for the observed diminishing role of basement structures through time.

A multidisciplinary study of the final episode of the Manda Hararo dyke sequence, Ethiopia, and implications for trends in volcanism during the rifting cycle

Abstract The sequence of dyke intrusions between 2005 and 2010 in the Manda Hararo rift segment, Ethiopia, provided an opportunity to test conceptual models of continental rifting. Based on trends up to dyke 13 in the sequence, it was anticipated that, should magma supply continue, dykes would shorten in length and eruptions would increase in size and decrease in distance from the segment centre as extensional stress was progressively released. In this paper we revisit these predictions by presenting a comprehensive overview of the May 2010 dyke and fissure eruption, the 14th and last in the sequence, from InSAR, seismicity, satellite thermal data, ultraviolet SO 2 retrievals and multiple LiDAR surveys. We find the dyke is longer than other eruptive dykes in the sequence, propagating in two directions from the segment centre, but otherwise fairly typical in terms of opening, propagation speed and geodetic and seismic moment. However, though the eruption is located closer to the segment centre, it is much smaller than previous events. We interpret this as indicating that either the Manda Hararo rifting event was magma limited, or that extensional stress varies north and south of the segment centre.

Nature of and controls on volcanism in the ca. 2.32–2.06 Ga Pretoria Group, Transvaal Supergroup, Kaapvaal Craton, South Africa

The time of deposition of the Pretoria Group between 2.32 and 2.06Ga on South Africa's Kaapvaal Craton was characterized by the first major increase in atmospheric oxygen. It was accompanied by the extrusion of significant thicknesses of volcanic deposits, namely the Bushy Bend lavas of the Timeball Hill Formation, the Hekpoort Formation and the Machadodorp Volcanic Member of the Silverton Formation, marking the three major volcanic events within the Transvaal Supergroup which are thought to be precursors to the succeeding Bushveld Complex magmatism. The Bushy Bend Lava Member of the Timeball Hill Formation is characterized by subaqueous basaltic–andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the Transvaal Supergroup basin. The subaerial volcanism of the Hekpoort Formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material. Intermittent hiatuses in volcanism were marked by local lacustrine shale deposition. Finally, the Machadodorp Lava Member can be interpreted as the deposits of several seamounts aligned along a fissure, probably within an extensional environment.The geodynamic control inferred for the evolution of the Pretoria Group basin encompasses two cycles of prerift uplift, subsequent mechanical rifting and long lived thermal subsidence. The limited extent of the Bushy Bend lavas in the south of the Pretoria Group depository attests to the likelihood that volcanism accompanying the first rifting event was short-lived. In contrast, the second rifting cycle postulated for the Pretoria Group basin has a strong association with widespread and large scale volcanism of the Hekpoort–Ongeluk flood basalts, which may have been plume-related. The scale of the second cycle volcanism and its importance in influencing the upper part of the Pretoria Group basin-fill is underlined by the Machadodorp volcanism, probably related to hot spot volcanism as the Ongeluk–Hekpoort plume waned.

Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present

Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (∼65 km × 8 km) and intensity of crustal deformation (∼6 m), as well as the volume of intrusive and extrusive magmatism (>3 km3), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.

Recent rift-related volcanism in Afar, Ethiopia

Rift zones are the most common magmatic environment on Earth. However opportunities to observe active rifting are rare, and consequently the volcanological characteristics of rift systems are not well understood. An ongoing phase of magmatic rifting along a section of the Red Sea system in Afar, Ethiopia, presents an exceptional opportunity to constrain relationships between volcanism and crustal growth. Here, by integrating analyses of satellite images (i.e. MODIS, OMI, ASTER, and ALI) with field observations, we characterise two recent (August 2007 and June 2009) basaltic fissure eruptions in Afar and evaluate the role and significance of volcanism in the rifting process. Both events were brief (36–72 h) and erupted 4.4–18 × 10 6 m 3 of lava from a fissure system 4–6.5 km in length. Data from the spaceborne Ozone Monitoring Instrument (OMI) suggests total SO 2 emissions for each eruption of 26 ± 5 kt (2007) and 34 ± 7 kt (2009), consistent with complete degassing of the erupted magma volumes. Using geodetic models for the intrusive activity in Afar we estimate the partitioning of magma between intrusive and extrusive components, up to July 2009, to be ∼ 180:1. Comparing the first-order volcanic characteristics and the intrusive–extrusive volume balance for the Afar volcanism with data from the 1975–1984 Krafla rifting cycle (Iceland) suggests that the volcanic flux in Afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking, and subsequent dykes are able to propagate more easily to the surface. As a consequence, basaltic fissure eruptions in this section of the Afar rift will become of increasing large magnitude as the rifting event matures over the next 5–10 yr. Using available models of magmatic rifting we forecast the likely size and location of future eruptions in Afar.

Isotopic evidence for rapid continental growth in an extensional accretionary orogen: The Tasmanides, eastern Australia

The trace element signature of Earth's continental crust resembles that of arc lavas, but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra-subduction zone magmatism. We explore the role of coupled arc–back-arc accretionary processes in crust generation by considering the tectonic context of whole rock Nd isotope and zircon Hf–O isotope data from igneous rocks of the Australian Tasmanides (515–230 Ma), which is thought to have evolved by the repeated opening and closure of sediment-filled back-arc basins behind a long-lived subduction zone. The significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective. Granitic rocks in this area define striking secular ε Nd– ε Hf–δ 18O trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity. These trends reveal that juvenile magmatic input was enhanced during extensional, back-arc rifting episodes that immediately followed crustal thickening, suggesting a relationship between slab rollback and continental growth. Interaction between juvenile magma and sedimentary units deposited during a preceding back-arc rifting cycle was integral to the formation of stable continental material. This highlights the importance of back-arc environments for both the generation and differentiation of continental crust. The juvenile component within the Tasmanide igneous rocks increased from the Cambrian to the Triassic, consistent with a diminished input from craton-derived metasedimentary material as the subduction zone migrated outboard of the Gondwana margin. Subduction zone retreat formed large tracts of new crust in eastern Australia at comparable rates to crust generation at modern island arcs, providing a mechanism for rapid continental growth at convergent margins. Using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation.

Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar

SUMMARY We present a preliminary account of the near-field surface strain associated with a major magmatic rifting episode at a nascent slow spreading ridge in the Afar depression. Between 2005 September 14 and October 4, a volcanic eruption and 163 earthquakes (mb > 3.9), including seismic tremor, occurred within the ∼60-km-long Dabbahu magmatic segment. Results of the early response team demonstrated that ground deformation, derived from satellite radar data (InSAR), together with seismicity, is consistent with dyke-induced deformation along the entire length of the segment. We document the distribution of brittle strain associated with the early part of this rifting cycle to verify the predicted pattern of deformation and constrain a conceptual model for normal fault growth in Afar, with general application to other slow spreading divergent margins. Our field investigations concentrate on the northern half of the segment, which ruptured through to the surface over a length of >30 km and a width of ∼5 km, consistent with the pattern of microseismicity recorded using a network deployed ∼1 month after the initial onset of the rifting episode on September 14. Severe ground shaking during the event was more widespread; fresh rock fall is common across the entire magmatic segment, particularly at the intersections between faults. Recent ground breaks, in the form of reactivated or newly initiated normal faults and fissures, opened with horizontal displacements up to 3 m and vertical displacements locally up to 5 m, but commonly ∼2 m. These structures are generally subvertical and open along pre-existing cooling joints. Fault offset is greater than expected given the magnitude of earthquakes during the episode. The axial relief that developed consequent on fault and fissure initiation and reactivation during the 2005 Dabbahu episode is consistent with that of the entire magmatic segment. We therefore suggest that melt delivery is sufficiently frequent that favourable stress conditions for faulting are primarily achieved during dyke events.

Sandstone petrofacies in the northwestern sector of the Iberian Basin

During the most active rifting stages in the northwestern sector of the Iberian Basin (Cameros Basin and Aragonese Branch of the Iberian Range), thick sequences of continental clastic deposits were generated. Sandstone records from Rift cycle 1 (Permo-Triassic) and Rift cycle 2 (Late Jurassic-Early Cretaceous) show similarities in composition. Based on the most recent data, this paper describes sandstone petrofacies developed during both rifting periods. Six petrofacies can be distinguished: two associated with Rift cycle 1 (PT-1 and PT-2) and four with Rift cycle 2 (JC-1 to JC-4). All six petrofacies can be classifi ed as sedimentoclastic or plutoniclastic. Sedimentoclastic petrofacies developed during early rifting stages either through the recycling of pre-rift sediments or signifi cant palaeogeographical changes. These facies comprise a thin succession (<100 m) of clastic deposits with mature quartzose and quartzolithic sandstones containing sedimentary and metasedimentary rock fragments. Carbonate diagenesis is more common than clay mineral diagenesis. Sedimentoclastic petrofacies have been identifi ed in Rift cycle 1 (Saxonian facies, PT-1) and Rift cycle 2 (JC-1 and JC-3; Tithonian and Valanginian, respectively). In the absence of the pre-rift sedimentary cover, metasedimentoclastic petrofacies sometimes develop as a product of the erosion of the low- to medium-grade metamorphic substratum (Petrofacies JC-2, Tithonian-Berriasian). Plutoniclastic petrofacies were generated during periods of high tectonic activity and accompanied by substantial denudation and the erosion of plutonites. Forming thick stratigraphic successions (1000 to 4000 m), these feldspar-rich petrofacies show a rigid framework and clay mineral diagenesis. In Rift cycle 1, plutoniclastic petrofacies (PT-2) are associated with the Buntsandstein. This type of petrofacies also developed in Rift cycle 2 in the Cameros Basin (JC-4) from DS-5 to DS-8 (Hauterivian-Early Albian), and represents the main basin fi ll interval. Sedimentoclastic and plutoniclastic petrofacies can be grouped into three pairs of basic petrofacies. Each pair represents a ‘provenance cycle’ that records a complete clastic cycle within a rifting period. Petrofacies PT-1 and PT-2 represent the ‘provenance cycle’ during Rift-1. In the Cameros Basin, two provenance cycles may be discerned during Rift cycle 2, related both to the Tithonian-Berriasian and the Valanginian-Early Albian megasequences. Tectonics is the main factor controlling petrofacies. Other factors (e.g., maturation during transport, local supply) may modulate the compositional signatures of the petrofacies yet their main character persists and even outlines he hierarchy of the main bounding surfaces between depositional sequences in the intracontinental Iberian Rift Basin.

Modelling geochemical variations associated with rift relocations at Iceland

Rift relocations, where the onshore active spreading axes periodically jump eastwards relative to the offshore Reykjanes and Kolbeinsey ridges, to remain in alignment with the Iceland plume centre, have dominated the tectonic evolution of Iceland. During a rift relocation two adjacent spreading axes operate simultanaeously, and as a result there is a significant reduction in the spreading rate across an individual axis. Normal full spreading rates on Iceland are considered to be around 20mm/yr; this can decrease to around 10mm/yr across one of the spreading axes, during a rift jump [1]. It has been suggested that below a threshold of 20mm/yr, conductive cooling becomes a significant control on the extent of the melting region [2]. This study focusses on the Skagi region of Northern Iceland; the location of a palaeo-rift axis which died around 3Ma [1], when the transfer of extension to the presently active Northern Volcanic Zone (NVZ) was completed. There are 4 stages of the rift cycle represented at Iceland: birth (Eastern Volcanic Zone, EVZ), steady-state (NVZ), decline (Western Volcanic Zone, WVZ) and death (Skagi) of a spreading axis. New data shows that Skagi is geochemically distinct from the presently active NVZ and WVZ, but has similar characteristics to the EVZ; with high Fe-Ti and relative enrichment in the incompatible elements Nb, Zr and Y. Compositional variations are also associated with the current rift relocation from the WVZ to the EVZ. A numerical model, based on simple 2-D corner flow beneath a spreading ridge, is used to define the steady-state thermal structure for upwelling and melting mantle; and model the composition of ridge products. Models for the composition at each of the different rift cycle stages are then compared to the observed geochemistry. Future expansion of the numerical model to allow time-dependent variations in spreading rate will help constrain the rates of transfer of extension during a rift jump, and allow tracking of the resulting geochemical variations and melt production rates. The overall aim is to account for the compositional differences observed at Skagi and ultimately understand the dynamics of rift relocations at Iceland.

Numerical modelling of quaternary deformation and post-rifting displacement in the Asal–Ghoubbet rift (Djibouti, Africa)

Over the last three decades a host of information on rifting process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and rifting cycle displacement across the Asal–Ghoubbet rift has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust. The distributed slip throughout the inner rift is related to the closeness of magma chamber, leading to additional stress into the upper thinned crust. Assuming a constant Arabia–Somalia motion of 11 mm/year, the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm/year. These values, about equal to the regional opening rate, suggest that both volcanism and tectonic play an equivalent role in the rifting process. Our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis. Although part of the post-rifting deformation could be due to viscous relaxation, the high opening rate in the first years after the event and the abrupt velocity change in 1984–1986 argue for a large dyke inflation of 12 cm/year ending in 1985. The asymmetric and constant pattern of the GPS velocity since 1991 suggests that present post-rifting deformation is mainly controlled by fault creep and regional stretching. This study demonstrates the internal consistency of the data set, highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post-rifting process including magma injection, fault creep and regional stretching. D 2005 Elsevier B.V. All rights reserved.