Rift Depression Research Articles

Modern fault formation in the Earth’s crust of the Baikal rift system according to the data on the mechanisms of earthquake sources

The spatial characteristics of seismotectonic deformations and the most likely fracture planes in the earthquake sources of the Baikal rift system (BRS) are determined using the method of cataclastic analysis of fractures [1]. It is shown that extension conditions with a strike of modern fractures parallel to the rift-controlling faults are dominant in the central zone and in most of the NE flank of the BRS. The flat average dip of fractures in the earthquake sources of the main fault zones for some rift depressions allow a suggestion about the flattening of faults in the middle crust. The antithetic faults are steeper. The BRS flanks are characterized by dominant shear deformations and more diverse morphogenetic faults in the earthquake sources (strike-slip faults, reversed faults, and normal faults). The modern faults at the BRS flanks weakly inherit the neotectonic structure.

Structural architecture differences and petroleum exploration of passive continental margin basins in east Africa

Abstract Based on the plate tectonic theory, and by studying seismic, geologic data and related documents, this study restored the proto-type basins and lithofacies paleogeography of East African passive margin basins in the major geological periods, and carried out analysis on the basin architecture characteristics and sedimentary filling variance. Based on the dissection of fifteen reservoirs, three hydrocarbon accumulation models were identified to find out favorable plays and the further exploration direction in this region. The East African passive continental margin basins experienced three prototype stages, Late Carboniferous-Triassic Karoo intercontinental failed rifts, Jurassic intercontinental-intracontinental rifts, and passive continental margins since the Cretaceous. The rift sequences are developed in all the basins, forming the “rift type”, “rift depression type” and “delta reconstruction type” passive continental margin basins in line with the different thicknesses of the sediment fillings during the depression period. In the “rift type” basins, the sediment thickness during the depression period was less than 3 000 m, forming “single source – structure type” hydrocarbon accumulation model, where the exploration direction will mainly focus on giant structural traps on the top of the rift sequences. In the “fault-depression type” basins, the sediment thickness during the depression period was more than 5 000 m, forming “double sources – double combinations type” hydrocarbon accumulation model, where the exploration direction will mainly focus on giant slip-collapse-debris flow deposits on the top and middle of the slope. In the “delta-reconstruction type” sediment basins, the thickness is more than 6 000 m, forming constructional delta deposits with four structural belts from onshore to offshore, unique growth faults, mud diapirs, thrust faults and fore deep gentle slope, which are named as “three sources – multi-combinations type” hydrocarbon accumulation model, and all the four structural belts can form giant oil and gas fields.

Lithospheric-scale centrifuge models of pull-apart basins

We present here the results of the first lithospheric-scale centrifuge models of pull-apart basins. The experiments simulate relative displacement of two lithospheric blocks along two offset master faults, with the presence of a weak zone in the offset area localising deformation during strike–slip displacement. Reproducing the entire lithosphere–asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1g experiments and thus provide a better approximation of the dynamic evolution of natural pull-apart basins. Model results show that local extension in the pull-apart basins is accommodated through development of oblique–slip faulting at the basin margins and cross-basin faults obliquely cutting the rift depression. As observed in previous modelling studies, our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull-apart basins: the basins are lozenge shaped in the case of underlapping master faults, lazy-Z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults. Model cross sections show significant along-strike variations in basin morphology, with transition from narrow V- and U-shaped grabens to a more symmetric, boxlike geometry passing from the basin terminations to the basin centre; a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models. These geometries are also typical of 1g models and characterise several pull-apart basins worldwide. Our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone; a rise of the base of the lithosphere occurs beneath the basin, and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence (i.e., the basin depocentre).

The formation of graben morphology in the Dead Sea Fault, and its implications

AbstractThe Dead Sea Fault (DSF) is a 1000 km long continental transform. It forms a narrow and elongated valley with uplifted shoulders showing an east‐west asymmetry, which is not common in other continental transforms. This topography may have strongly affected the course of human history. Several papers addressed the geomorphology of the DSF, but there is still no consensus with respect to the dominant mechanism of its formation. Our thermomechanical modeling demonstrates that existence of a transform prior to the rifting predefined high strain softening on the faults in the strong upper crust and created a precursor weak zone localizing deformations in the subsequent transtensional period. Together with a slow rate of extension over the Arabian plate, they controlled a narrow asymmetric morphology of the fault. This rift pattern was enhanced by a fast deposition of evaporites from the Sedom Lagoon, which occupied the rift depression for a short time period.

The continent–ocean transition at the mid-northern margin of the South China Sea

The northern margin of the South China Sea (SCS) has particular structural and stratigraphic characteristics that are somewhat different from those described in typical passive margin models. The differences are attributable to poly-phase tectonic movements and magmatic activity resulting from the interaction among the Eurasian, Philippine Sea and Indo-Australian plates. Based on several crustal-scale multi-channel seismic reflection profiles and satellite gravity data across the northern SCS margin, this paper analyzes the structures, volcanoes and deep crust of the continent–ocean transition zone (COT) at the mid-northern margin of the SCS to study the patterns and model of extension there.The results indicate that the COT is limited landward by basin-bounding faults near Baiyun sag and is bounded by seaward-dipping normal faults near the oceanic basin in our seismic lines. The shallow anatomy of the COT is characterized by rift depression, structural highs with igneous rock and/or a volcanic zone or a zone of tilted fault blocks at the distal edge. Gravity modeling revealed that a high velocity layer (HVL) with a 0.8–6-km thickness is frequently present in the slope below the lower crust. Our study shows that the HVL is only located in the eastern portion of the northern SCS margin based on the available geophysical data. We infer from this that the presence of an HVL is not required in the COT at the northern SCS margin. The magmatic intrusions and HVL may be related to partial melting caused by the decompression of a passive, upwelling asthenosphere, which resulted primarily in post-rifting underplating and magmatic emplacement or modification of the crust. Based on this study, we propose that an intermediate mode of rifting was active in the mid-northern margin of the SCS with characteristics that are closer to those of the magma-poor margins than those of volcanic margins.

Sedimentary characteristics and tectonic setting of sublacustrine fans in a half-graben rift depression, Beibuwan Basin, South China Sea

Drilling has shown that there is a large petroleum reserve in the Liushagang Formation of the Fushan Depression, South China Sea from reservoirs of the sublacustrine fans. To reduce the risk associated with exploration, the sedimentary characteristics of the sublacustrine fans should be identified and described. Analysis of the drilling cores, well data and 3D seismic data was conducted to identify and interpret the sublacustrine fans which formed in a lacustrine succession as the low-stand fan of the 2nd member of Liushagang Formation (SQEls2). The reservoirs were commonly associated with pebbly conglomerates, mudstone shivers, muddy debris, various deformation structures including micro-faults, liquified deformation structure, ball and pillow structure, slump and stress-release deformation. Further contrastive analysis of sedimentary characteristics and palaeogeomorphology between the western and eastern sublacustrine fans indicates that they belong to two different types of sublacustrine system in continental basins. More specifically, the western fan, controlled by the multi-level step-fault belt, is a typical gravel-rich sublacustrine system which is characterised by structurally controlled, high gradient (2.8–3.3°), admixtures of muddy debris and pebbly conglomerates and poor reservoir quality, while the eastern fan, controlled by the flexure slope-break belt, is a sand-rich sublacustrine system dominated by relatively low gradient (2.0–2.5°), general fining-upwards trend with good reservoir quality. Therefore, the development of future exploration should be conducted at different levels in the Fushan Depression.

STRATIGRAPHY OF ОVRUCH FORMATION OF UKRAINIAN SHILD AS THE UNIT REALLY PALEOZOICUM IN AGE

The previous published stratigraphic charts of sedimentary deposits of Belokorovichi and Ovruch depressions were revised on the base of the vend-paleozoic age of the deposits (by microfossils) and on the hypothesis that the crystaline rocks among terrigenous Palaeozoicum were intruded into this deposits as result of tectonic movements. The new stratigraphic chart of Ovruch rift depression was proposed where Ovruch formation сomposed with Paleozoic suites has been restored and the new Vendian Dubrovskaya suit has been standed out.

Controls on magmatic cycles and development of rift topography of the Manda Hararo segment (Afar, Ethiopia): Insights from cosmogenic 3He investigation of landscape evolution

Crustal extension at mature continental rifts and oceanic ridges occurs by a combination of normal faulting and magma injection, which interact to create rift morphology. Quantifying the relative roles of faulting and melt intrusion in accommodating extension at magmatic rifts remains difficult and requires studies at sufficient spatial and temporal scales to resolve the interaction between these processes. In this study we provide new chronological constraints based on cosmogenic exposure dating for the ∼100kyr topographic evolution of a young and active magmatic rift segment in Afar, Ethiopia. We combine structural investigations, field mapping, geochemical analysis and cosmogenic 3He exposure dating of lava surfaces in order to investigate the interplay between volcanic activity and fault growth in the northern part of the axial depression, where the rift segment intersects a large stratovolcano. Our results allow us to determine the roles of the various magma reservoirs feeding this rift system and their interactions during accretion over the past 100kyr. New age data for key lava units allow several magmatic cycles to be distinguished. Each cycle lasts 20–40ka resulting in periods of high and low magma supply rate. The variations in magma supply rate at the segment extremity strongly affect the development of the rift depression, with the availability of melt controlling the morphological impact of faulting. Melts from different magma reservoirs feeding the segment are chemically distinct and geochemical analysis of lavas from the rift floor allows their respective contributions to maintaining magmatic accretion to be estimated. We propose that melts from the magma reservoir at the northern end of the segment contribute around one-third of the length of this portion of the segment, whereas the mid-segment reservoir is responsible for the remaining two-thirds of the segment accretion.

Sedimentogenesis and volcanosedimentary formations in the Mesozoic-Cenozoic continental rift depressions of the Baikal region and Southern Mongolia

Study of the Mesozoic and Cenozoic infill of rift depressions in the Baikal region (Transbaikal and Baikal regions) and Gobi Altai in southern Mongolia allowed us for the first time to consider regularities of sedimentogenesis in these structures and to establish peculiarities of their typical volcanosedimentary formations1. It is shown that continental rift depressions include plain fan and orogenic molasse formations, which were previously united into the orogenic molasse group. Abundance of the coarse-clastic material, which served as a basis for assigning the fan formations to the molasse group, was related to the destruction of near-wall benches (hereafter, escarps) of the grabens in ancient hillocky plains. The lithofacies and sedimentary environments of rift fan and molasse formations in the arid and humid zones are described. Specific features, which make it possible to discriminate the paragenetic associations, are presented. It was established that the riftogenic fan formations of the relief peneplanation epoch have no analogues beyond rift zones. In contrast, similar (in structure, composition, and sedimentary depositional environments) molasses were formed in various tectonic settings during the orogenic epochs.

The mid-Cretaceous transition from basement to cover within sedimentary rocks in eastern New Zealand: evidence from detrital zircon age patterns

AbstractDetrital zircon U-Pb ages for 30 Late Jurassic and Cretaceous sandstones from the Eastern Province of eastern New Zealand, combined with previously-published geochronological and palaeontological data, constrain the time of deposition in the Pahau and Waioeka terranes of the Cretaceous accretionary margin of Zealandia, and their adjacent cover strata. The zircon age patterns also constrain possible sediment source areas and mid-Cretaceous geodynamic models of the transition from basement accretionary wedge to passive-margin cover successions. Pahau Terrane deposition was mainly Barremian to Aptian but continued locally through to late Albian time, with major source areas in the adjacent Kaweka and Waipapa terranes and minor inputs from the inboard Median Batholith. Waioeka Terrane deposition was mainly Albian, with distinctive and exclusive sediment sources, principally from the Median Batholith but with minor inputs from the Western Province. Alternative tectonic models to deliver such exclusive Median Batholith and Western Province-derived sediment to the mid-Cretaceous Zealandia continental margin are: (1) the creation of a rift depression across Zealandia or (2) sinistral displacement of South Zealandia with respect to North Zealandia, to expose Western Province rocks directly at the Zealandia margin. Detrital zircon age patterns of Cretaceous cover successions of the Eastern Province of eastern New Zealand demonstrate purely local sources in the adjacent Kaweka and Waipapa terranes. Cretaceous zircon components show a decline in successions of late Early Cretaceous age and disappear by late Late Cretaceous time, suggesting the abandonment or loss of access to both the Median Batholith and Western Province as sediment sources.

Arshan palaeoseismic feature of the Tunka fault (Baikal rift zone, Russia)

The traditional concept of the rift development of flank depressions in the Baikal rift zone is now doubted in view of some indicators for compression deformations identified by the seismogeological and geodetic methods. Besides, the paleoseismological investigations revealed seismogenic strike-slips and reverse faults in the Tunka fault zone that is a major structure-controlling element of the Tunka rift depression. However, a detailed study of the upslope-facing scarp in the Arshan paleoseismogenic structure zone has shown that its formation might be due to rift mechanism of basin formation. Age estimation has been made for the previously unknown pre-historic earthquake whose epicentral area coincides with the western flank of the Arshan paleoseismogenic structure. Judging from previously determined ages of paleoearthquakes, the mean recurrence period for faulting events on the central Tunka fault is 2780–3440years.

Structural Architecture for Development of Marginal Extensional Sub-Basins in the Red Sea Active Rift Zone

This paper presents a robust kinematic model that describes northern Red Sea and Gulf of Suez rifting and the development of marginal extensional half-graben sub-basins (ESB). A combination of Landsat Enhanced Thematic Mapper Plus (ETM+) and structural data was used to provide model constraints on the development of rift segments and ESB in the active rift zones. Structural analysis shows rotation and change in strike of rift-bounding faults. The model describes the northern Red Sea region as a poly-phase rift system initiated by late Oligocene (30 - 24 Ma) orthogonal rifting and the development of marginal ESB (now inland ESB), followed by oblique rifting and flank uplift during the early Miocene (24 - 18 Ma). The oblique rifting fragmented the rift depression into segments separated by oblique-slip accommodation within reactivated Pan-African (ca. 600 Ma) fracture zones, resulting in the development of antithetic faults and an en-echelon distribution of inland ESB. The current phase of rifting was instigated by the development of the Dead Sea Transform in response to increased northeasterly extension during the middle Miocene (ca. 18 Ma). The model explains the widening of the Red Sea rift during the last phase more than the Gulf of Suez rift by developing more antithetic faults and formation of offshore ESB, and deepening the rift depression.

First 40Ar/39Ar age determinations on the Malkhan granite-pegmatite system: Geodynamic implications

The Malkhan granite-pegmatite system located in Central Transbaikalia, in the southwestern portion of the Malkhan-Yablonovy structure-formational zone of the Caledonian folding comprises two granite massifs (Bolsherechensk and Oreshny) and a miarolitic pegmatite field of the same name, which adjoins the Chikoi deep-seated fault and Lower Cretaceous Chikoi rift depression in the north. The first 40Ar/39Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, muscovite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granitepegmatite system is 123.8–127.6 Ma, which is consistent with the age of Lower Cretaceous rocks from the Chikoi depression. The intimate spatial relationship and isochronism between the Chikoi depression and the Malkhan granite-pegmatite system are strongly suggestive of a rift regime that affected its evolution, thus highlighting the need to regard the evolution of this system as being intimately related to depression development. Such a model can easily be realized within the framework of the concept of a metamorphic core complex, which was used to explain the nature of Transbaikal-type rift depressions and conjugate granite-gneiss swells.

Evolution, pattern, and partitioning of deformation during oblique continental rifting: Inferences from lithospheric‐scale centrifuge models

Oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone. During extension, this weakness localizes deformation, and different rift obliquity is obtained by varying its trend with respect to the stretching direction. Model results show that deformation is mostly controlled by the obliquity angle α (defined as the angle between the orthogonal to the rift trend and the extension direction). For low obliquity (α < 45°), rifting is initially characterized by activation of large, en echelon boundary faults bordering a subsiding rift depression, with no deformation affecting the rift floor. Increasing extension results in the abandonment of the boundary faults and the development of new faults within the rift depression. These faults are orthogonal to the direction of extension and arranged in two en echelon segments linked by a complex transfer zones, characterized by strike‐slip component of motion. In these models, a strong strain partitioning is observed between the rift margins, where the boundary fault systems have an oblique‐slip motion, and the valley floor that away from the transfer zones is affected by a pure extension. Moderate obliquity (α = 45°) still results in a two‐phase rift evolution, although boundary fault activity is strongly reduced, and deformation is soon transferred to the rift depression. The fault pattern is similar to that of low‐obliquity models, although internal faults become slightly oblique to the orthogonal to the direction of extension. Deformation partitioning between the rift margins and the valley floor is still observed but is less developed than for low‐obliquity rifting. For high obliquity (α > 45°), no boundary faults form, and the extensional deformation affects the rift depression since early stages of extension. Dominance of the strike‐slip motion over extension leads to the development of oblique‐slip and nearly pure strike‐slip faults, oblique to both the rift trend and the orthogonal to the extension direction, with no strain partitioning between the margins and the rift floor. These results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution, architecture, and strain partitioning, findings that have a significant relevance for natural oblique rifts.

Conditions of formation of ore-geochemical specialization of modern hydrotherms in the Baikal Rift Zone

Modern hydrotherms of the Baikal Rift Zone (BRZ) are divided according to contents of ore and rare elements and gas composition into: (1) carbonaceous, forming in carbonate rocks, (2) hydrosulfuric and H 2S-free nitric and methane, discharged in Cenozoic deposits, and (3) nitric, produced in granitoids. Carbonaceous hydrotherms are enriched in Sr, Ba, Y, Co, and Cr; nitric ones, in Li, Rb, Cs, Mo, W, and other granitophile elements; and hydrosulfuric hydrotherms are saturated with complex metals. High contents of U and associated elements are specific for oxidized nitric alkaline thermal waters and are atypical of carbonaceous ones, and high contents of Au are observed in methane hydrotherms. Separation of ore elements between organic and clayey matter takes place as early as the initial stage of sediment formation. During sediment accumulation, these elements pass into hydrothermal solution in the same separated associations. In oxidizing (Eh ranges from +50 to +260 mV) alkaline medium, U and, obviously, other elements of uranium association, present not only in granitoids but also in the organic matter of sediments, pass into the solution most actively. Alkaline medium (7 < pH < 10) favors the transition of Mo and W from igneous rocks and sediments into the solution. Methane hydrotherms (8 < pH < 9, Eh < –70 mV) can accumulate gold. In strongly reducing (Eh ranges from –200 to –360 mV) alkaline (pH > 8) hydrosulfuric anoxic medium, chalcophile complex metals of the clayey matter of sediments are the most active to pass into the solution. Thus, as Eh decreases and H 2S concentration increases, hydrothermal solution extracts various element associations from igneous and sedimentary deposits in the sequence U–Mo, W–Au–Sn, Cu, Pb. In the solution the differentiation of elements continues. Hence, the separation of ore-element associations at the initial stage of sediment formation, long before the hydrothermal activity, is the first track in a chain of processes leading to the differentiation of ore deposits according to formational types. Hydrotherms confined to deep-fault zones contain an endogenous fluid with mantle 3He/ 4He values close to those in methane and carbonaceous hydrotherms of the Tunka rift depression in the BRZ. According to results of thermodynamic modeling, the fluid consists of CH 4, H 2O, and CO 2 and obviously lacks mantle ore elements, as it was initially of hydrocarbon composition.

Structure of the Afar and Tanzania plumes based on the regional tomography using ISC data

Establishment of the correlation between deep dynamics and processes observed at the daytime surface is an important fundamental problem of modern geodynamics. The study of mantle plumes, which are responsible for volcanic processes and riftogenesis in different parts of the Earth, occupies a special place in this problem. The system of rift depressions in East Africa and the Afar triple junction is the most striking example of the influence of mantle processes on the near-surface tectonic activity. The correlation between mantle dynamics and riftogenesis in this region has attracted the attention of many scholars around the world (see, for example, [1]). Quantitative estimates of pull-apart forces in the lithosphere induced by an ascending plume are estimated in [2]. These works and a multitude of other observations demonstrate that rifting in East Africa is caused precisely by mantle processes. The Afar triple junction (ATJ) is a transition case from continental rifting to oceanic spreading (Fig. 1). Three branches of divergence of the lithosphere located at almost equal angles (120 ° ) with respect to each other are observed around the ATJ. Two branches (the Red Sea and the Gulf of Aden) are incipient oceans with prominent spreading zones. The third branch (the Ethiopian Rift) is the beginning of a large system of depressions extended along the African continent first to the southwest and then to the south. The total length of the East African Rift is more than 2500 km. At a distance of approximately 700 km from the ATJ, the rifting is attenuated and then resumes again along two branches around the Tanzania Craton (Western and Eastern rifts). The Eastern Rift is also called the Kenya Rift in the literature, while the Western Rift is called Tanganyika Rift (from the name of Lake Tanganyika). The riftogenesis along the entire length of the

Paleoproterozoic supercontinent: Origin and evolution of accretionary and collisional orogens exemplified in Northern cratons

The evolution of the North American, East European, and Siberian cratons is considered. The Paleoproterozoic juvenile associations concentrate largely within mobile belts of two types: (1) volcanic-sedimentary and volcanic-plutonic belts composed of low-grade metamorphic rocks of greenschist to low-temperature amphibolite facies and (2) granulite-gneiss belts with a predominance of high-grade metamorphic rocks of high-temperature amphibolite to ultrahigh-temperature granulite facies. The first kind of mobile belt includes paleosutures made up of not only oceanic and island-arc rock associations formed in the process of evolution of relatively short-lived oceans of the Red Sea type but also peripheral accretionary orogens consisting of oceanic, island-arc, and backarc terranes accreted to continental margins. The formation of the second kind of mobile belt was related to the activity of plumes expressed in vigorous heating of the continental crust; intraplate magmatism; formation of rift depressions filled with sediments, juvenile lavas, and deposits of pyroclastic flows; and metamorphism of lower and middle crustal complexes under conditions of granulite and high-temperature amphibolite facies that, in addition, spreads over the fill of rift depressions. The evolution of mobile belts pertaining to both types ended with thrusting in a collisional setting. Five periods are recognized in Paleoproterozoic history: (1) origin and development of a superplume in the mantle that underlay the Neoarchean supercontinent; this process resulted in separation and displacement of the Fennoscandian fragment of the supercontinent (2.51–2.44 Ga); (2) a period of relatively quiet intraplate evolution complicated by locally developed plume-and plate-tectonic processes (2.44–2.0 (2.11) Ga); (3) the origin of a new superplume in the subcontinental mantle (2.0–1.95 Ga); (4) the complex combination of intense global plume-and plate-tectonic processes that led to the partial breakup of the supercontinent, its subsequent renascence and the accompanying formation of collisional orogens in the inner domains of the renewed Paleoproterozoic supercontinent, and the emergence of accretionary orogens along some of its margins (1.95–1.75 (1.71) Ga); and (5) postorogenic and anorogenic magmatism and metamorphism (<1.75 Ga).

Middle Paleozoic basic magmatism of the northwestern Vilyui Rift: Composition, sources, and geodynamics

Middle Paleozoic magmatism at the eastern Siberian platform was related to riftogenic processes, which were most clearly expressed in the Vilyui Rift and led to the formation of rift depressions filled with sedimentary-volcanogenic rocks and extended basaltic dike belts in rift shoulders. Two fields of diamondiferous kimberlites were found along with basaltic dikes in the Vilyui-Markha dike belt surrounding rift in the northwest. Active subalkali basaltic magmatism predated the emplacement of kimberlite bodies, which occasionally (Nyurba pipe) are cut by dikes of potassium alkali basalts. Based on geochemical and Sr-Nd isotopic characteristics, deep-seated sources were determined for the intrusive and volcanic basalts of the northwestern shoulder of the Vilyui rift. The REE distribution patterns of the studied rocks normalized to the primitive mantle are close to that of OIB, except for somewhat higher HREE. In the diagrams of indicator ratios of trace and rare-earth elements, the basalts are also plotted in the OIB field, being located between the end member of plume composition (FOZO) and enriched mantle sources. The rocks have positive eSr (+3.5 and +28.6) and eNd (+1.3 and +5.3). In a diagram eNd(T)-eSr(T), two fields with distinct content of radiogenic Sr are distinguished, which can be regarded as derived by mixing of the moderately depleted PREMA-type mantle and a source enriched in radiogenic Sr. Available isotope-geochemical data confirm that OIB type basalts of the region were generated by plume activity. The geodynamic setting of Middle Paleozoic magmatism and rifting in the eastern part of the Siberian platform is considered in light of plume-lithosphere interaction. The sequence of tectonomagmatic events during evolution of the Vilyui rift is consistent with the model of plume-lithosphere interaction or the model of active rifting.

Sedimentological and geochemical evidence for palaeo-environmental change in the Makgadikgadi subbasin, in relation to the MOZ rift depression, Botswana

This work considers new evidence for palaeo-environmental change taking place during the Pleistocene in northern Botswana. Duricrusted strandlines along the northeastern margin of Sua Pan provide palaeo-environmental data pertaining to the Makgadikgadi subbasin (MSB) with inferences regarding the larger Makgadikgadi–Okavango–Zambezi (MOZ) rift depression. Field, XRD and geochemical data show that MSB strandlines comprise calcretes (LU1 type), MgO-rich calcretes with silica (LU2 type), sil-calcrete (LU3 type) and silcrete (LU4 type). Early freshwater episodes appear to have been followed by calcrete-dominated drying phases interspersed with repeated silcretisation. Calcretisation through pan littoral sediments may have been both biogenically and environmentally induced. Calcite precipitation was in part controlled by the Mg/Ca ratio of pore water in the pan littoral zone suggesting closed basin type evaporative conditions, which were followed by a major desiccation interval. Phases of silcrete precipitation appear to be related to periods when the geochemistry of the lake littoral more closely resembled present-day Na–CO 3–SO 4–Cl-type brines. Silica saturated acidic, moderately saline groundwater preceded Si precipitation which took place as the pH reduced. Si mobilisation occurred (inter alia) as a result of quartz grain dissolution enhanced by diatoms, bacteria and algal growth in the moist pan littoral. SiO 2-rich pore waters migrated through cracked and desiccated calcrete into areas of lower salinity and lower pH resulting in preferential calcite removal and silcrete precipitation. Approximate TL dates imply that exposed littoral sand underwent calcretisation during the drying phases of extensive palaeo-lakes which occurred prior to 110 ka, 80–90 ka and 41–43 ka. These wet periods compare fairly well with Vostok core chronologies for southern Africa.

Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: insights into the mechanics of continental rifting architecture

Centrifuge analogue experiments are used to model the reactivation of pre-existing crustal fabrics during extension. The models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones. The typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement. The transfer zone was also characterised by cross-basin faults with both trend and strike-slip component of movement opposite to that displayed by the master faults. Typically, different structural patterns were obtained by changing the offset angle φ between the rift segments, supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre-existing discontinuities with respect to the stretching vector. In the models, the aspect ratio (ratio of length vs. width) of the transfer zone shows a positive correlation with the offset angle (i.e., the more the inherited fabric is parallel to the extension direction, the longer and narrower the transfer zones). In case of staircase offset of the rift segments ( φ=90°), the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped. Prominent rise of the ductile lower crust was also observed at the transfer zone. Many of these geometrical features display striking similarities with natural rift systems. The results of the current experiments provide useful insights into the mechanics of continental rift architecture, supporting that rift propagation, width and along-axis segmentation may be strongly controlled by the reactivation of pre-existing pervasive crustal fabrics.