Petrological and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: Influence of the Tristan da Cunha mantle plume

Basalts from a section of the Mid-Atlantic Ridge close to the active volcanic island of Tristan da Cunha in the South Atlantic have been analysed to investigate the influence of the mantle plume on the geochemistry of basalts being erupted at the spreading center. Although petrographically the rocks show only limited variation, two basaltic types were determined to be erupting in this region based on their major, trace and REE compositions. One group shows depletion in the incompatible and LRE elements, and can be characterised as N-type mid-ocean ridge basalts. The second group shows “enriched” geochemical characteristics and is similar to T-type MORBs.Mixing hyperbolae for the incompatible element and REE ratios suggest that extensive mixing of an end-member, characteristic of a plume region with an end-member of normal depleted MORB, canaccount for the occurrence of the T-type MORBs in this region.Based on the nature and development of the Tristan da Cunha mantle plume over the past 100 Ma, a composite model of evolution is suggested,in which a ridge-centered hotspot progressed to a near ridge hotspot, and finally to a totally intraplate situation. The fact that Tristan da Cunha is highly alkalic now, but that an irregular geochemical anomalyis also present on the Mid-Atlantic Ridge at this latitude would suggest an intermediate stage between the near-ridge and totally intraplate situation. This model leads to the conclusion that, as the Mid-Atlantic Ridge migrated away from the Tristan hotspot, a preferential sublithospheric flow towards the Ridge was established. This discontinuous feature can explain the geochemical variations seen along the Mid-Atlantic Ridge by providing a mechanism for mixing of a depleted N-type MORB component with an enriched component originating through processes active at the Tristan da Cunha mantle plume.     

Origin of Tertiary to Recent EM- and subduction-like chemical and isotopic signatures in Auca Mahuida region (37°–38°S) and other Patagonian plateau lavas

The alkaline volcanic rocks of the 1.8–0.9 Ma Auca Mahuida and post-mid-Pliocene Rio Colorado backarc volcanic fields east of the Andean Southern Volcanic Zone at ~37°–38°S have pronounced intraplate-like chemical signatures with some striking similarities to oceanic DM-EM1-like lavas of the south Atlantic Tristan da Cunha type. These backarc lavas are considered to have formed as a series of mantle batches typified by 4–7 % melting, with decompression melting initiating in a garnet-bearing mantle above a steepening subduction zone, and final equilibration occurring near the base of a ~65- to 70-km-thick lithosphere at temperatures of ~1,350–1,380 °C. Evolved Auca Mahuida mugearite to trachytic samples are best explained by crystal fractionation with limited mixing of partial melts of recently underplated basalts, in line with isotopic signatures that preclude significant radiogenic contamination in a preexisting refractory crust. Higher Ba/La and subtly higher La/Ta ratios than in nearby ~24–20 Ma primitive basalts or oceanic (OIB) lavas are attributed to the residual effects of slab fluids introduced during a shallow subduction episode recorded in the arc-like chemistry of the adjacent 7–4 Ma Chachahuén volcanic complex. Positive Sr, K and Ba spikes on mantle-normalized patterns of both primitive Auca Mahuida and ~24–20 Ma basalts, like those in EM-like OIB basalts, are attributed to mixing of continental lithosphere into the asthenosphere. In Patagonia, this mixing is suggested to have peaked as the South America continent accommodated to major late Oligocene plate convergence changes, as similar Sr, K and Ba spikes and DM-EM1 signatures are absent in ~50–30 Ma backarc lavas north of 51°S, and all of those south of 51°S. Introduction of an EM1-like component associated with lateral mantle flow of a Tristan da Cunha source is largely precluded by its Cretaceous age and distance to Patagonia.     

The morphostructure of the South Atlantic Ridge in the Tristan da Cunha hot spot area

Investigation of the rift zone and flanks of the South Atlantic midoceanic ridge in the Tristan da Cunha hot spot area revealed that their morphostructure is atypical of slow spreading ridges. Feeding the spreading center with magmatic material, the hot spot changes the morphology of the axial zone and transform faults. It itself forms a relief ensemble that differs from the rift one as well. The continuous migration of the spreading center in the westerly direction stimulates the formation of a spacious volcanic highland, which is responsible for the regional asymmetry of ridge flanks in its extended segments (hundreds kilometers long).     

Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications

Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount (Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2) prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation and scale.     

Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge (SMAR) 19°S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridge-hotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni ​= ​239–594 ​ppm and Cr ​= ​456–1010 ​ppm) and low Fe/Mn (54–67) and Ce/Yb (0.65–1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (T_p) beneath the segment of 1321–1348 ​°C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of ~63 ​km and ceases melting at ~43 ​km with a final melting temperature of ~1265 ​°C. The extent of partial melting is up to 16%–17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al₂O₃) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.⁸⁷Sr/⁸⁶Sr (0.702398–0.702996), ¹⁴³Nd/¹⁴⁴Nd (0.513017–0.513177) and ²⁰⁶Pb/²⁰⁴Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased. However, the trace element and Sr-Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.     

Basalts from the Centre of the Assumed Icelandic Mantle Plume

Pleistocene to historic basalts in the northern part of theeastern volcanic zone in Iceland may have formed by partialmelting over an extended depth range in the centre of the assumedIcelandic mantle plume. Practically all basalt types found on the ocean ridges are representedin this volcanic rift zone. Volume relations are, however, infavour of low degree partial melting products. The basalts differ from ocean ridge basalts in being undepletedin large trace ions, indicating a primary mantle source. The prevalence of low degree partial melting products in Icelandmay explain the depleted nature of the astenosphere flowingaway from the plume and along the northern part of the Mid-AtlanticRidge. Volumes of lavas are found to correlate with degree of partialmelting. Exceptions from this correlation are found locallyand may be explained on the basis of volcano-tectonic implications. A simple model of thermal structure in the mantle plume-oceanridge system is suggested which may explain some aspects ofthe compositional variations in basalts within the system.     

Magmatic sources and geodynamics of the early Mesozoic Northern Mongolia-Western Transbaikalia rift zone

The Northern Mongolia-Western Transbaikalia rift zone is the largest Mesozoic riftogenic structure in eastern Asia and extends for a distance of more than 1200 km. The zone consists of depressions and grabens, which were formed between 233 and 188 Ma and are filled with basaltic and basalt-comendite (bimodal) volcanic associations accompanied by numerous peralkaline granite massifs. Geochemical and isotope (Sr, Nd, and Pb) studies showed that mantle and crustal sources contributed to the formation of the magmatic rocks of the rift zone. The basalts were formed from incompatible element-enriched mantle sources. Geochemical and isotope-geochemical data suggest that the peralkaline salic rocks (comendites and peralkaline grantoids) and basalts are genetically related and were formed by the fractionation of a common parental magma. In addition, the magmatic associations contain peralkaline granites and comendites whose isotope signatures indicate their formation through the crustal contamination of derivatives of basaltic melts. The rift zone has arisen during the formation of the Mongolia-Transbaikalia zoned magmatic area in a complex geodynamic setting, combining collision in the Mongolia-Okhotsk suture with a mantle plume impact. The rift zone occupies the northern periphery of the area, being controlled by the Northern Mongolia-Transbaikalia fault system, which marks the boundaries (sutures) of large terranes in the lithosphere. Asthenospheric traps beneath suture boundaries served as pathways for the penetration of a mantle plume into the upper lithosphere, thus playing an important role in the localization of the riftogenic processes.     

Chemical and Isotopic Characteristics of the Kuroko‐Forming Volcanism

The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back‐arc basin (23–17 Ma) followed by multiple stages of submarine rifting (19–13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back‐arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back‐arc basin spreading are geochemically similar to mid‐ocean ridge basalt, with a depleted Sr–Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post‐Kuroko quartz‐phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma.     

Volcanic highlands in the South Atlantic rift zone

The morphostructure of the segment between the Cardno and St. Helen transform fracture zones is studied in the rift zone of the South Atlantic slow-spreading mid-oceanic ridge (SAMOR). It was found that it is atypical of similar ridges because of the absence of an evolved rift valley. The rift zone in the transverse section is a cupola with flat slopes, whose surface is divided by volcanic massifs, plateau-like valleys, and unclear ridges and valleys. The entire morphostructure (a cupola-like regional pedestal and the listed relief forms of the second order) indicates its volcanic origin, and the rift zone in this segment is a volcanic high-land. This conclusion is supported by seismic and magnetic data. Because other (not all) SAMOR segments contain the rift valley, the results of this study indicate alternation of the tectonic and magmatic morphostructures along the entire rift zone and identification of its scales is the most important task of the morphostructural study of the SAMOR rift zone. Determination of geodynamic regimes on the basis of the results of morphostructural studies of the rift zone will arise from the solution of this task.     

Spatial and temporal variations in the compositions of Upper Miocene to Recent basic lavas in the northern main Ethiopian rift: Implications for the causes of Cenozoic magmatism in Ethiopia

The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.     

Recent tectonics in the northern part of the Knipovich Ridge,Atlantic ocean

The walls of the Knipovich Ridge are complicated by normal and reverse faults revealed by a high-frequency profilograph. The map of their spatial distribution shows that the faults are grouped into domains a few tens of kilometers in size and are a result of superposition of several inequivalent geodynamic factors: the shear zone oriented parallel to the Hornsunn Fault and superposed on the typical dynamics of the midocean ridge with offsets along transform fracture zones and rifting along short segments of the Mid-Atlantic Ridge (MAR). According to the anomalous magnetic field, the Knipovich Ridge as a segment of the MAR has formed since the Oligocene including several segments with normal direction of spreading separated by a multitransform system of fracture zones. In the Quaternary, the boundary of plate interaction along the tension crack has been straightened to form the contemporary Knipovich Ridge, which crosses the previously existing magmatic spreading substrate and sedimentary cover at an angle of about 45° relative to the direction of accretion. The sedimentary cover along the walls of the Knipovich is Paleogene in age and has subsided into the rift valley to a depth of 500–1000 m along the normal faults.     

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components

The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher level, interstratified high-Ti latites (three units)and quartz latites (five units), and one low-Ti quartz latite.Khumib basalts are enriched in high field strength elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic²⁰⁶Pb/²⁰⁴Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated ²⁰⁷Pb/²⁰⁴Pbratios, and Sr–Nd isotopic compositions plotting in thelower ¹⁴³Nd/¹⁴⁴Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr (     

Volcanic rocks and processes of the Mid-Atlantic Ridge rift valley near 36 ° 49′ N

Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters.The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%).In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills.FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel.The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present.The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.Contribution n ° 480 du Départment Scientifique, Centre Océanologique de Bretagne     

Conditions of formations of slightly enriched tholeiites in the northern Knipovich Ridge

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V “Akademik Nikolay Strakhov” in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.     

Ca-enrichment in olivines from volcanic rocks

Data are presented on the Ca-content of olivines in a range of volcanic rocks from a variety of suites. These include olivine basalts through to trachytes from Gough, St. Helena and Tristan da Cunha Islands; trachytes from central Victoria, Australia, and a leucitite and phonclitic-tephrite series from Bufumbira, Uganda. In olivine crystals from basaltic lavas the Ca- and Mn-contents are low and Fe shows the most significant zoning from core to rim. In Fe-rich olivines from trachytic differentiates Ca and Mn frequently show more significant variation than Fe. While the Mn-content is proportional to the Fe-content of these olivines, Ca-zoning, in many cases, is unrelated to Fe-content. The marked Ca-enrichment in olivines occurs with the absence of plagioclase in the host lavas. The Ca(Al  Na  K) ratio is shown to be related not only to the Ca-content of the olivines in the lava, but may be used to predict olivine stability in evolved compositions.     

Petrogenesis of volcanic rocks in the Sangxiu Formation,central segment of Tethyan Himalaya: A probable example of plume–lithosphere interaction

The ∼133 Ma volcanic rocks of Sangxiu Formation are distributed in the eastern part of the central Tethyan Himalaya and belong paleogeographically to the northeastern margin of Greater India. These volcanic rocks include alkaline basalts and felsic volcanic rocks. Major and trace element abundances and whole-rock isotopic data for selected samples of these volcanic rocks are used to infer their petrogenesis. Geochemically, the Sangxiu basalts are closely similar to the Emeishan high-Ti basalts. Major and trace element data and Sr–Nd isotopic compositions suggest that the Sangxiu basalts may have been derived from an OIB-type mantle source, with discernable contributions from subcontinental lithospheric mantle (SCLM). The basaltic magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere in a continental rift setting. The Sangxiu felsic volcanic rocks share most of the geochemical features of A-type granite, and have Sr–Nd isotopic compositions which differ considerably from the Sangxiu basalts, suggesting that they originated from the anatexis of ensialic continental crust. The Sangxiu volcanic rocks may be considered as the consequence of an interaction between the Kerguelen hotspot and the lithosphere of the northeastern margin of Greater India at ∼133 Ma, and may represent the initial stage of the separation of Greater India from southwestern Australia.     

Experimental modeling of structure-forming deformations in rift zones of mid-ocean ridges

The modern methods of physical modeling of structure-forming deformations in extension zones of oceanic lithosphere are discussed; the methods differ in their experimental equipment, model material, and experimental techniques. The simulation performed with an elastic-ductile model has demonstrated that extension of a brittle lithospheric layer results in disruption of its continuity and in formation of a rift valley according to the mechanism of running fracture propagation. The modeling results provide insights into qualitative pattern of faulting and fracturing within a rift zone, specific features of rift segmentation, and development of various structural elements (axis bends, echelons of fractures, nontransform offsets, small and large overlaps, etc.) under various geodynamic conditions of spreading. The modeling has shown that origination and evolution of structures of various types depend on the lithosphere’s thickness beneath the rift axis; the width of the lithosphere’s heating zone; the spreading orientation; and, to a lesser degree, on the spreading rate. A relatively rectilinear rift broken into particular segments bounded by small-amplitude offsets with or without minor overlaps arises in the case of both a small width of the heating zone, closely related to the axial magma chamber, and a small thickness of the lithosphere (fast-spreading conditions). In the case of a wide heating zone caused by ascent of an asthenospheric wedge or a mantle plume, offsets of rift are more pronounced and deformations embrace a wider region. If, as a result, the thickness of the lithosphere increases, the rift will be less linear and the structural heterogeneity will become more contrasting. In addition to the thickness of the lithosphere, the angle between the rift zone and the extension axis also controls the rift configuration: the greater the angle, the more conspicuous the en echelon arrangement of fractures. For any spreading type, the propagating front of linear microfractures that disrupt the upper brittle layer of the lithosphere predates the origin of mesoscopic fractures and predetermines a general trend of the rift zone. This indicates that the fractures of various sizes propagate simultaneously.     

Early Stage in the Evolution of the Paleoasian Ccean at the Western Margin of the Siberian Craton: Geochemical and Geochronological Evidence

The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.     

Topographic analysis of Devana Chasma, Venus: implications for rift system segmentation and propagation

Devana Chasma is a rift system on Venus formed in association with the Beta Regio and Phoebe Regio volcanic highlands, which are interpreted as mantle plumes. We present a new analysis of a 2500-km-long segment of Devana. Based on the rift topography, the horizontal extension across the rift boundary faults is 3–9 km. This is a lower bound on the total rift extension because the altimetry does not resolve the topographic relief across the numerous faults that are visible in radar images of the rift floor. The total extension across Devana is approximately 20 km, similar in magnitude to continental rift systems on Earth. Rift flank elevations are up to 3.1 km in the regions nearest the mantle plumes and decay strongly with increasing distance from the plumes, indicating a strong thermal component to the rift flank topography, unlike the situation usually reported for terrestrial rifts. As on Earth, there is also a flexural uplift component to the flank topography. Rift depths are up to 2.5 km below the surrounding plains, with considerable along-strike variability. There is a 600 km lateral offset along Devana Chasma near the mid-point between the two mantle plumes. Devana most likely formed as two distinct rifts due to the horizontal stresses created by outflow from the upwelling plumes. The offset zone formed as a result of the interaction between the two rift tips, which requires that upwelling at the two mantle plumes overlapped in time.     

The transition from rifting to sea-floor spreading within a magma-poor rifted margin: field and isotopic constraints

We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean–continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from gabbros and albitite yield concordant U–Pb ages of 161 ± 1 Ma; their initial ɛ_Hf (+ 14.4 to + 14.9) as well as bulk rock ɛ_Nd values of from gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle.