Recent Rift Research Articles

Rift obliquity in the Northern Volcanic Zone in Iceland using UAV-based structural data

Rift obliquity is known to be very frequent at divergent plate boundaries and shear components in the opening of fissures have been detected through seismicity in the monitoring of recent rifting events. However, the field structural evidence of obliquity in active rifts for Holocene events is still poorly studied, despite its relevance at both the lithospheric and upper-crust scales.We performed extensive high resolution UAV surveys on Holocene terrains in four areas along the active rift in the Northern Volcanic Zone of Iceland, to map structures and morphotectonic features in detail. We did a kinematic analysis and found an increase of the obliquity from N to S.We further discuss the origins of this obliquity and suggest that the combination of inherited structures reactivation, magmatic intrusions, and topography influence on the propagation of both fractures and magma intrusions can have an impact on the mismatch between rift opening direction and the far-field strain.We conclude that obliquity can be inferred for Holocene rifting events from surface structural data. It is thus fundamental to take into account the existing structural pattern and its (re)activation history in active rift zones to avoid missing out on possible preferential structure reactivation, magma propagation ways, and eruption sites during the monitoring of ongoing rifting events.

An Entomological Investigation during a Recent Rift Valley Fever Epizootic/Epidemic Reveals New Aspects of the Vectorial Transmission of the Virus in Madagascar.

A Rift Valley fever (RVF) outbreak occurred in at least five regions of Madagascar in 2021. The aim of this study was to provide an overview of the richness, abundance, ecology, and trophic preferences of mosquitoes in the Mananjary district and to investigate the distribution of mosquitoes that were RT-PCR-positive for RVFV. Three localities were prospected from 26 April to 4 May 2021, using light traps, BG-Sentinel traps baited with an artificial human odor, Muirhead-Thomson pit traps, and indoor pyrethroid spray catches. A total of 2806 mosquitoes belonging to at least 26 species were collected. Of 512 monospecific pools of mosquitoes tested with real-time RT-PCR, RVFV was detected in 37 pools representing 10 mosquito species. The RVFV-positive species were as follows: Aedes albopictus, Ae. argenteopunctatus, Anopheles coustani, An. gambiae s.l., An. mascarensis, An. squamosus/cydippis, Culex antennatus, Cx. decens, Cx. Tritaeniorhynchus, and Uranotaenia spp. Of the 450 tested engorged females, 78.7% had taken a blood meal on humans, 92.9% on cattle, and 71.6% had taken mixed (human-cattle) blood meals. This investigation suggests the potential role of mosquitoes in RVFV transmission within this epizootic/epidemic context and that the human populations at the three study sites were highly exposed to mosquitoes. Therefore, the use of impregnated mosquito nets as an appropriate prevention method is recommended.

Development of the Nyika Plateau, Malawi: A Long Lived Paleo‐Surface or a Contemporary Feature of the East African Rift?

AbstractNorthern Malawi's Nyika Plateau is a 3,700 km2 large, highly elevated (∼2,500 m) plateau located at the western margin of the Miocene‐Recent Malawi rift and the confluence of multiple Proterozoic orogenic belts. Neighboring asthenospheric upwelling in the Rungwe Volcanic Province, associated with the active East African Rift, has created similar topographic highs, leading some to speculate that the formation of Nyika could be related. Here, we present new low‐temperature data using apatite fission track, apatite (U‐Th‐Sm)/He and zircon (U‐Th)/He thermochronology to constrain the upper crustal thermal history of the Nyika region since the Devonian. The data suggest that Nyika was an isolated feature since at least the Permo‐Triassic, well before more recent rifting in Malawi, and may have developed as a horst between two large Karoo grabens, the Henga‐Ruhuhu and the North Rukuru to the southeast and northwest, respectively. Similarities between the thermal histories of Nyika and the currently separated Livingstone Plateau to the east allow for the possibility that these may have been connected in a contiguous highland prior to the formation of the intervening Neogene Malawi rift. Thermal history models for exposed Precambrian basement samples adjacent to Nyika, and once buried beneath the neighboring Karoo basins, indicate that up to 3.4 km of Permo‐Triassic section has since been eroded, with samples along the plateau not indicating burial of Karoo‐type sediment at this time. Most recent cooling histories suggest that the plateau surface continued to denude at varying degrees from the Cretaceous and reached near‐surface temperatures in the Late Paleogene‐Neogene.

Perceptions about Dual Citizenship and Diaspora Participation in Political, Economic, and Social Life in South Sudan

Dual citizens, especially dual citizens in the diaspora, are the subject of an important debate among South Sudanese residents. This article seeks to explore South Sudanese residents’ attitudes towards dual citizenship and the role dual-citizen individuals play in the political, economic and social life of the country. During recent Rift Valley Institute research projects, conducted for the Australian Government from 2017–2020, we interviewed residents in South Sudan and South Sudanese members of the diaspora in the UK, Australia, and Ethiopia to understand arguments about citizenship and diaspora participation in the political, economic, and social life of South Sudan. The South Sudanese diaspora are perceived within South Sudan to have dubious national identification and loyalty. However, the research also established that the South Sudanese diaspora have always been part of the political organization, economic activities and social fabric of South(ern) Sudan both directly and indirectly.

ANTASed – An Updated Sediment Model for Antarctica

We compile existing seismic, gravity, radar and magnetic data, together with the subglacial bedrock relief from the BEDMACHINE project, to build the most detailed sediment model for Antarctica. We interpolate these data according to a tectonic map of Antarctica using a statistical kriging method. Our results reveal significant sediment accumulation in Antarctica with several types of sedimentary basins: parts of the Beacon Supergroup and more recent rifting basins. The basement relief closely resembles major geological and tectonic structures. The thickness of sediments has significant variations around the continent, and depends on the degree of crustal extension. West Antarctica has wide sedimentary basins: the Ross basin (thickness 2–6 km), the Filchner-Ronne basin (2–12 km) with continuations into East Antarctica, the Bentley Subglacial Trench and the Byrd basin (2–4 km). The deepest Filchner-Ronne basin has a complex structure with multi-layered sediments. East Antarctica is characterized by vast sedimentary basins such as the Pensacola-Pole (1–2 km), Coats Land (1–3 km), Dronning Maud Land (1–2 km), Vostok (2–7 km), Aurora (1–3 km), Astrolabe (2–4 km), Adventure (2–4 km), and Wilkes (1–4 km) basins, along with narrow deep rifts filled by sediments: JutulStraumen (1–2 km), Lambert (2–5 km), Scott, Denman, Vanderford and Totten (2–4 km) rifts. The average thickness of sediments for the whole continent is about 0.77 km. The new model, ANTASed, represents a significant improvement over CRUST 1.0 for Antarctica, and reveals new sedimentary basins. Differences between ANTASed and CRUST 1.0 reach +12/−3 km.

Transatlantic partnership on artificial intelligence: realities, perceptions and future implications

ABSTRACT Recent confrontation between China and the US laid bare underlying factors that might accelerate decoupling in different realms. Technology is one of the main factors of recent rift. Specific to AI, democratic countries are striving for finding common ground to counter diffusion of authoritarian technologies. In this paper we argue that future AI alliance/partnership has to find a solution in three distinct political obstacles. Firstly, future alliance/partnership bear in mind the crumbling the US leadership. The US leadership is must, for the future of alliance/partnership due to its economic and technological prowess. Secondly, the EU's fragmentation in deploying Chinese technology and their dependency towards China not only technologically but also economically will be determinative factor for the success of alliance/partnership. Finally, democratic partners and allies should focus on creating mechanisms to offer AI-based technologies to both offset authoritarian counterparts and to become more competitive.

Mantle and Crustal Sources of Magmatic Activity of Klyuchevskoy and Surrounding Volcanoes in Kamchatka Inferred From Earthquake Tomography

AbstractKlyuchevskoy and surrounding volcanoes in central Kamchatka form the Northern Group of Volcanoes (NGV), which is an area of particularly diverse and intensive Pleistocene‐Holocene volcanism. In this study, we present a new seismic tomographic model of the crust and uppermost mantle beneath NGV based on local earthquake data recorded by several permanent and temporary seismic networks including a large‐scale experiment that was conducted in 2015–2016 by an international scientific consortium. Having an unprecedented resolution for this part of Kamchatka, the new model reveals many features associated with the present and past volcanic activity within the NGV. In the upper crust, we found several prominent high‐velocity anomalies interpreted as traces of large basaltic shield volcanoes, which were hidden by more recent volcanic structures and sediments. We interpret the mantle structure to reflect asthenospheric flow up through a slab window below the Kamchatka‐Aleutian junction that feeds the entire NGV. The interaction of the hot asthenospheric material with fluids released from the slab determines the particular volcanic activity within the NGV. We argue that the eastern branch of the Central Kamchatka Depression, which is associated with a prominent low‐velocity anomaly in the uppermost mantle, was formed as a recent rift zone separating the NGV from the Kamchatka Eastern Ranges.

Mechanism for Deep Crustal Seismicity: Insight From Modeling of Deformation Processes at the Main Ethiopian Rift

AbstractWe combine numerical modeling of lithospheric extension with analysis of seismic moment release and earthquake b‐value in order to elucidate the mechanism for deep crustal seismicity and seismic swarms in the Main Ethiopian Rift (MER). We run 2‐D numerical simulations of lithospheric deformation calibrated by appropriate rheology and extensional history of the MER to simulate migration of deformation from mid‐Miocene border faults to ∼30 km wide zone of Pliocene to recent rift floor faults. While currently the highest strain rate is localized in a narrow zone within the rift axis, brittle strain has been accumulated in a wide region of the rift. The magnitude of deviatoric stress shows strong variation with depth. The uppermost crust deforms with maximum stress of 80 MPa, at 8–14 km depth stress sharply decreases to 10 MPa and then increases to a maximum of 160 MPa at ∼18 km depth. These two peaks at which the crust deforms with maximum stress of 80 MPa or above correspond to peaks in the seismic moment release. Correspondingly, the drop in stress at 8–14 km correlates to a low in seismic moment release. At this depth range, the crust is weaker and deformation is mainly accommodated in a ductile manner. We therefore see a good correlation between depths at which the crust is strong and elevated seismic deformation, while regions where the crust is weaker deform more aseismically. Overall, the bimodal depth distribution of seismic moment release is best explained by the rheology of the deforming crust.

Jordan bid to the Gulf Cooperation Council and Implications for the Expended GCC

In 2011, the Gulf Cooperation council (GCC) invited Jordan and Morocco to join the council. This research is geared to evaluate the potential benefits of joining the GCC for the case of Jordan. It is argued that Jordan’s inclusion to the block would strengthen the economy of the member states, increase trade and investment opportunities and improve peace in the region. GCC Membership to Jordan might possibly support its economy and larger investment flows from the Gulf countries. On the flipside, Jordan accession carries several potential benefits to the GCC. Jordan’s strength is linked with its close proximity to the GCC, cordial relationships with the US and the EU and free trade agreements with major economies. Moreover, Jordan inherits professional military. If Jordan is part of the GCC, it might consolidate the military power and augment security of the GCC. Since the Arab Spring, the peace of the Middle East remained volatile. Recent rift between Saudi Arabia and Qatar, war in Syria and Yemen further destabilized the security of the region. This has also undermined the future of the GCC. In these circumstances, the expansion of the GCC is realistic and Jordan case caries some merit. Keywords : GCC, Jordan, Economic Integration, Peace and Security JEL : E61, F36, F50, F53, F59 DOI: 10.7176/EJBM/12-6-14 Publication date: February 29 th 2020

Influence of crustal fracturing on the thermal springs and earthquake swarms distribution in the north-east part of the Baikal rift system (Russia)

The Baikal rift system faults, having developed in the recent rift formation period, are characterized by hydrothermal and seismic activity. Especially in the northeastern part, the level of fracturing affects the localization of thermal outlets and the distribution of earthquake swarms. The specific features of the hydrothermal outputs and seismicity interposition, depending on the fracture heterogeneity and water saturation of the lithosphere layers, have not been previously evaluated. The results of the statistical analysis of the hydrothermal data, presented herein, show that most of the springs are distributed in areas of increased fault density. Multiple less hot hydrotherms are associated with zones of maximum density in the inter-block space. The spatio-temporal analysis of seismicity showed that migrations of weak and moderate seismic activity propagate from earthquake swarms through these zones. Swarms initiate the deformation front by propagating in the quasiplastic layer of the upper mantle at a speed of tens of kilometers per year, which can increase the aqueous fluid pore pressure in the lower earth’s crust, facilitate the movement of the fluid upwards along the section, and cause a process of successive stress relaxation in zones of high fracturing and concentration of hydrothermal springs. Earthquake swarms occur in areas of about average fracture density, associated with deep faults framing consolidated blocks of the earth’s crust. The hydrotherms with high or average temperature, and with probable deep source chemical composition components are related with such zones.

A Multiproxy provenance approach to uncovering the assembly of East Gondwana in Antarctica

East Gondwana is generally thought to have assembled through the amalgamation of Indo-Antarctica and Australo-Antarctica along the Ediacaran–Cambrian Kuunga orogen. The location of a boundary between Indo-Antarctica and Australo-Antarctica within this key Gondwana-forming orogen remains controversial because extensive ice cover in East Antarctica precludes traditional characterization of terranes. Here, we integrated Pb-isotope analysis of detrital feldspar grains with U-Pb dating of detrital monazite and zircon grains from offshore sediments to infer the location of the onshore boundary between Indo-Antarctica and Australo-Antarctica. New and compiled data from onshore basement exposures highlight the different age and Pb-isotope signatures of Indo-Antarctica and Australo-Antarctica. Holocene sediments offshore from Mirny Station (Queen Mary Land, East Antarctica) have detrital feldspar Pb-isotope signatures and detrital monazite and zircon U-Pb ages that reflect contributions from both Indo-Antarctica and Australo-Antarctica. The presence of both Indo-Antarctic and Australo-Antarctic crust beneath ice cover near Mirny Station implies proximity to a fundamental terrane boundary within the Kuunga orogen, which could coincide with a geophysical lineament at ∼94°E (Mirny fault). The geophysical expression of this boundary extends into the subglacial interior of East Antarctica, where, prior to more recent rifting, it may have connected with one or more previously inferred Gondwana-forming sutures. The revised geometry of the Kuunga orogen suggests that the assembly of East Gondwana involved dominantly strike-slip motion in the Mirny region coupled with high-angle convergence between Indo-Antarctica and Australo-Antarctica to the west.

Paleorift structure constrained by gravity and stratigraphic data: The Statherian Araí rift case

Gravimetric and stratigraphic data were used to investigate the Paleoproterozoic Araí Paleorift, a failed Statherian continental rift located in the western margin of the São Francisco Craton, where basement and cover were affected by the Neoproterozoic Brasiliano Orogeny. Euler deconvolution, tilt, total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift's main faults location, direction and depth, allowing to identify its main horsts, grabens, volcanic and plutonic centers. We found that rift faults occur to a maximum depth of ca. 38 km, but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km, probably the brittle-ductile transition zone at the time of rifting, practically disappearing at 20 km. Stratigraphic data and basement mapping were used in order to constraint gravimetric results. We classify the Araí Rift as a passive, three-armed failed rift, narrow to divergent type, that produced preferably anorogenic rapakivi-related magmas, most of it still lodged in the crust from surface down to ca. 19 km deep and subsidiary mafic magmatism. The results indicate the deep occurrence of low-density magmas beneath the rift's main axis, detected up to 20 km deep. Correlation to other global Statherian rifts show that the São Francisco Craton was strongly affected by taphrogenesis during the Statherian, together with Siberia, North America and North China cratons. Finally, by comparing our results to recent rifts we found that the Ethiopian rift's morphology is quite similar to the Araí. Surrounding the Tanzanian craton, the Cenozoic East Africa rift system morphology is compared to the Araí-Espinhaço rift system, which surrounds the São Francisco craton. The major contribution of this paper is the recognition of Araí Paleorift surface and subsurface morphology, up to now unknown, over an area of ca. 45.000 km2.

Recent rift formation and impact on the structural integrity of the Brunt Ice Shelf, East Antarctica

Abstract. We report on the recent reactivation of a large rift in the Brunt Ice Shelf, East Antarctica, in December 2012 and the formation of a 50 km long new rift in October 2016. Observations from a suite of ground-based and remote sensing instruments between January 2000 and July 2017 were used to track progress of both rifts in unprecedented detail. Results reveal a steady accelerating trend in their width, in combination with alternating episodes of fast ( > 600 m day−1) and slow propagation of the rift tip, controlled by the heterogeneous structure of the ice shelf. A numerical ice flow model and a simple propagation algorithm based on the stress distribution in the ice shelf were successfully used to hindcast the observed trajectories and to simulate future rift progression under different assumptions. Results show a high likelihood of ice loss at the McDonald Ice Rumples, the only pinning point of the ice shelf. The nascent iceberg calving and associated reduction in pinning of the Brunt Ice Shelf may provide a uniquely monitored natural experiment of ice shelf variability and provoke a deeper understanding of similar processes elsewhere in Antarctica.

Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analog models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression

AbstractInherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW‐SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present‐day N‐S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in subparallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along‐axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right‐lateral step over and avoiding a direct link to form a continuous N‐S depression. The models reveal five potential types of rift linkage across the preexisting basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.

Influence of regional tectonics and pre-existing structures on the formation of elliptical calderas in the Kenyan Rift

Abstract Calderas are formed by the collapse of large magma reservoirs and are commonly elliptical in map view. The orientation of elliptical calderas is often used as an indicator of the local stress regime; but, in some rift settings, pre-existing structural trends may also influence the orientation. We investigated whether either of these two mechanisms controls the orientation of calderas in the Kenyan Rift. Satellite-based mapping was used to identify the rift border faults, intra-rift faults and orientation of the calderas to measure the stress orientations and pre-existing structural trends and to determine the extensional regime at each volcano. We found that extension in northern Kenya is orthogonal, whereas that in southern Kenya is oblique. Elliptical calderas in northern Kenya are orientated NW–SE, aligned with pre-existing structures and perpendicular to recent rift faults. In southern Kenya, the calderas are aligned NE–SW and lie oblique to recent rift faults, but are aligned with pre-existing structures. We conclude that, in oblique continental rifts, pre-existing structures control the development of elongated magma reservoirs. Our results highlight the structural control of magmatism at different crustal levels, where pre-existing structures control the storage and orientation of deeper magma reservoirs and the local stress regime controls intra-rift faulting and shallow magmatism. Supplementary material: Details of the Standard Deviation Ellipse function and statistical methods are available at http://www.geolsoc.org.uk/SUP18849 .

ON SOME FLUVIAL LANDFORMS IN SOUTHERN AFRICA AND THEIR DEVELOPMENT UNDER CHANGING NATURAL ENVIRONMENT

Natural climatic and tectonic factors had a strong impact on the Mz-Kz development of fluvial relief of the region. During the dry seasons the chemical and physical weathering, different types of denudation, gravitational and aeolean processes are predominant, while fluvial processes play subsidiary role. Phased uprise of the Large Escarpment along the periphery of Southern Africa coming along with its axis displacement into the interior of the continent caused the formation of the deep incisions in the river valleys and strong erosion dissection of the neighbouring parts of the planation surface. The faults of the ancient and recent rifts have a significant impact on the morphology of the fluvial relief.

Crustal shear wave velocity and radial anisotropy beneath the Rio Grande rift from ambient noise tomography

AbstractShear wave velocity and radial anisotropy beneath New Mexico are obtained from ambient seismic noise tomography using data from the Transportable Array. Besides the distinct seismic structure imaged across the Rio Grande rift from the Colorado Plateau to the Great Plains, both velocity and anisotropy models also reveal significant variations along the rift. The rift at Albuquerque is characterized by remarkably low velocity in the shallow crust, high velocity and strong positive anisotropy in the middle and lower crust, and low velocity in the upper mantle. These observations can be interpreted as magma accumulation in the shallow crust and significant mafic underplating in the lower crust with abundant melt supply from the hot mantle. We propose that the Albuquerque region has recently been experiencing the most vigorous extensional deformation in the rift. Positive anisotropy with Vsh > Vsv appears in the central and southern rifts with a stronger anisotropy beneath younger volcanoes, reflecting layering of magma intrusion due to past and recent rifting activities. The low velocities in the uppermost mantle are observed under high‐elevation places, the Jemez Lineament, northern rift, and east rift boundary, implying that the buoyancy of hot mantle largely compensates the local high topography. Low mantle velocities appear at the boundary of the southern rift, corresponding to the large lithosphere thickness change, instead of the rift center, consistent with the prediction from the small‐scale, edge‐driven mantle convection model. We conclude that the edge‐driven upper mantle convection is probably the dominant mechanism for the recent and current rifting and uplift in the Rio Grande rift.

Generation and application of monoclonal antibodies against Rift Valley fever virus nucleocapsid protein NP and glycoproteins Gn and Gc

Rift Valley fever virus (RVFV) is a vector-borne virus that causes high neonatal mortality in livestock and deadly haemorrhagic fever in humans. In this paper, we describe the generation of monoclonal antibodies (mabs) against all three structural proteins of RVFV (glycoproteins Gn and Gc and nucleocapsid protein NP). After immunization of BALB/c mice with individual recombinant proteins, a total of 45 clones secreting ELISA-reactive monoclonal antibodies against NP, Gn and Gc epitopes were obtained. Twelve clones were directed to NP, 28 to Gn, and 5 to Gc. Western blot analysis revealed that most of the mabs were reactive to linearized epitopes on recombinant as well as native virus proteins. Six mabs against NP, 21 against Gn and all mabs against Gc also detected conformational epitopes, as shown by indirect immunofluorescence on RVFV-infected cells. All of the mabs were evaluated for their use in a competition enzyme-linked immunosorbent assay (ELISA) for the detection of a RVFV infection. Several mabs were identified that competed with polyclonal rabbit serum, and one of them - mab Gn123, raised against Gn protein - was selected for a proof-of-principle study with field sera from a recent Rift Valley fever outbreak. The novel Gn-based competition ELISA demonstrated high performance, offering a promising alternative and addition to serological assays based on nucleocapsid protein.

Constraints on the crustal structure of northern Vietnam based on analysis of teleseismic converted waves

article i nfo Here we report estimates of crustal thickness and Poisson's ratios for northern Vietnam, based on teleseismic receiver function analysis of observations from a dense broadband seismic array. The seismic array comprised of 24 stations distributed evenly over northern Vietnam, from whose records we examined 190 teleseismic events of Mw > 5.5 for the period 2006-2008. Using the radial receiver functions calculated from teleseismic records at individual stations, the optimum crustal thickness and Vp/Vs (where Vp and Vs are the velocities of P- and S-waves, respectively) ratio beneath each station were determined using the H-κ (where H is crustal thickness and κ is defined as the Vp/Vs ratio) stacking algorithm. Determined values of crustal thickness range from 26.5 km to 36.4 km, with an average of 31.0 ± 2.1 km. The simple pattern of variation of crustal thick- ness in the northeastern region of the study area, with a mean of ~31 km, suggests that the sector belongs to the craton of the South China block. A highly variable crustal thickness is found over the northwestern region of northern Vietnam, ranging from ~29.5 km to ~36.4 km, implying that complex tectonic processes have taken place in this region. The thinnest crust is found in the Red River Delta, where it ranges from 26.5 km to 30.4 km, which is suggestive of a recent rifting process. The determinations of crustal thickness show a good linear correlation with Bouguer gravity anomalies. Lower values of Poisson's ratio in the northeastern and Red River Delta sectors suggest a more felsic crust, and larger values in the northwestern sector suggest lithospheric extension in the Song Da depression. The findings enhance our understanding of the geotectonic architecture of the northern Vietnam region.

He, Ar, C and N isotopes in thermal springs of the Chukotka Peninsula: Geochemical evidence of the recent rifting in the north-eastern Asia

Thermal springs with water temperatures up to 97°C were sampled in the Chukotka Peninsula of the north-eastern Asia on the area of ~60,000km2. He and Ar isotope ratios in bubbling gases were measured in 30 samples of from 23 spring groups. 3Не/4Не=R values vary in the range of (20.2÷172)×10−8=(0.14÷1.23)Ratm. The over-atmospheric 40Ar/36Ar ratio values were found out in 18 groups. The compositions of gas phase were determined in 26 groups. In 16 of them N2 content exceeds 90vol.%. at δ15N values=(0.0÷+4.5) ‰. CO2 contents vary from <1 to 95vol.% and δ13CCO2 values determined in 19 samples vary from −20.4 to −4.4‰. In several samples δ13C values were also measured in methane and ethane.Geochemical specificity of gases was revealed in Kolyuchin–Mechigmen Zone (KMZ) of the Peninsula. The zone coincides with the trough of NW–SE strike that dissected the Precambrian basement of this part of the Chukotka–Seward microcontinent during the Triassic phase of tectonic activation. The latest re-activation of KMZ resulted in its subsidence during Quaternary, seismic activity and basic volcanism. The KMZ gases contain maximal contribution of light (mantle-derived) 3He and carbon dioxide whereas this CO2, and associated non-atmospheric N2 and Ar are all enriched by heavy isotopes (13C, 40Ar, and 15N, respectively). The waters of these springs are characterized by the highest reservoir temperatures estimated by solute geothermometers. These features are related to upwelling of mantle-derived melt supplying into the crust 3He-enriched helium and thermal energy stimulating rock metamorphism.