Rift Zone Research Articles

Structural analysis of fracture networks controlling geothermal activity in the northern part of the Malawi Rifted Zone from aeromagnetic and remote sensing data

We investigated the relationship between the geothermal activity and fracture networks that control storage and fluid pathways of geothermal systems in the northern part of the Malawi Rifted Zone (MRZ). It is suggested that low to medium temperature geothermal systems in the MRZ are mainly non-magmatic and structurally controlled. However, structural controls and favorable settings of geothermal activity are poorly understood in this region. We used an approach of remote sensing and aeromagnetic data analysis to 1) identify and characterize fracture networks to better understand the structural controls of geothermal activity in the northern part of the rift, 2) quantify and identify permeable areas using topological analysis of fracture intensity and connectivity frequency proxies, 3) understand the role that inherited structures play in the geothermal systems of the northern part of the rift, and 4) build preliminary geothermal conceptual models of selected geothermal systems. Our findings show that fracture networks in the Karonga and Nkhata regions comprise a varying degree of complexity along strike. This structural complexity occurs mainly in favorable structural settings such as 1) two fault segments coalescing to form hard and soft-linked relays, 2) two different oriented fracture segments intersecting each other, and 3) on the tips of major normal faults. The hot springs are located where one or more of these favorable settings occur. The remote sensing analysis shows that Quaternary normal faults are the primary controlling structures of thermal waters at shallow depths in the Karonga (NW-SE to N-S strike) and Nkhata (N-S strike) regions. The aeromagnetic data revealed that permeable NW-striking foliation planes of the Precambrian Mugesse Shear Zone and WNW to ENE-striking foliation planes of the Mwembeshi Shear Zone are important structural controls of geothermal fluids at greater depths in the Karonga and Nkhata region, respectively. The Mwankeja-Mwesia and Chiweta geothermal systems in the Karonga region show favorable structural settings for geothermal fluids related to NNW and NW-striking normal faults segments that coalesce to form hard and soft-linked relay ramps and NW, and NNE -striking faults intersecting each other and NW-striking foliation planes. The Mtondolo geothermal area in the Nkhata region shows that the intersection of N-striking normal faults and ENE-striking foliation planes is the favorable structural setting that controls the emergence of hot springs through the surface. We conclude that high fracture intensity and connectivity are related to the location of the hot springs at the surface and can be used to determine permeable zones and hidden geothermal fluids together with other methodologies. The aeromagnetic data analysis suggested that buried faults and inherited structures (e.g., foliation planes) are controlling the geothermal fluids at depth. Additionally, our aeromagnetic analysis of magnetic basement depth estimation suggests that some of the geothermal reservoirs in the northern part of the MRZ could have an estimated depth of ~ 500 to 1000 m.b.g.l. Finally, the low-cost methodology applied in this study can reduce the risk of drilling non-productive wells in the early exploration stages and become an exploration strategy for similar geothermal systems in countries of the Western Branch of the East African Rift System.

Digital Transformation of Hazardous Geological Processes Monitoring on the Example of the Baikal Natural Territory

Baikal Natural Territory (BNT) is the territory that adjacent to Lake Baikal, which is a unique natural object and, in accordance with the UNESCO Convention, a "World Natural Heritage". Baikal is in the central part of the Baikal Rift Zone (BRZ) – the most active seismic zone located in the middle of Russia. The development of the BRZ leads to the emergence of dangerous geological processes that can lead to a violation of the balance in the Lake Baikal ecological system and the surrounding area. In addition, these processes and phenomena pose a real threat to the smooth functioning of mainline communications, hydroelectric power plants and strategically important industries in the region, which, according to the classification of the Ministry of Emergency Situations of Russia, belongs to the first category of danger. To ensure systematic monitoring and forecasting of the environmental situation of the BNT, systematic observations are organized, as well as obtaining and analysing information about the activity of hazardous geological processes in digital form. The digital transformation of monitoring of hazardous geological processes, resulting from the digitalization of processes and the development of appropriate infrastructure, provides the possibility of using new models and methods, more flexible approaches to the analysis of ongoing processes and the prediction of possible extreme events. In this paper, a digital platform is proposed that provides support for the digital transformation of the monitoring of hazardous geological processes using the example of BNT. The platform under consideration may be used for ecological monitoring of BNT area.

Shrinking lakes of rift valley system in southern Tibet: Is it the climate?

In the context of global warming, the plateau lakes generally expand, but some lakes in the southern Tibetan Plateau appear to shrinkage, such as Duoqing Co Lake. We analyzed Duoqing Co which is located on the Yadong-Gulu Rift zone and two surrounding lakes using satellite and meteorological data. Optical and radar images were used to construct a time series of these lakes from 1988 to 2021. By comparing the area changes of surrounding lakes, it is found that Como Chamling Lake has shrunk, while Puma Yum Co Lake has shown an expansion trend. The interference deformation results show that both sides of the Yadong-Gulu Rift zone where the Duoqing Co Lake is located have experienced strong uplift and subsidence, sinking in the east and uplifting in the west. Under the northward compression of the Indian plate, the blocks on both sides of the Yadong-Gulu Rift zone have been relatively displaced. The disappearance of Duoqing Co Lake could be attributed to the existence of leakage channels in the Yadong-Gulu Rift zone. The north-south rift zones of the Tibetan Plateau pass through the Qiangtang Basin, and some lakes in this basin are shrinking, which could be related to the leakage of these rift zones. This work provides a new perspective for studying lake changes on the Tibetan Plateau and is a good reference for studying the lake water cycle on the plateau.

Tectonism of Late Noachian Mars: Surface Signatures from the Southern Highlands

Upwelling mantle plumes often instigate extensional stress within the continental crust of Earth. When stress exceeds crustal strength, extensional structures develop, reducing the effective stress and trigger magmatic processes at the crust–mantle boundary. However, such processes and their relationship to the formation of many surface structures remain poorly characterized on Mars. We identified a series of extensional structures in the southern highlands of Mars which collectively resemble continental rift zones on Earth. We further characterized these extensional structures and their surrounding region (area of ~1.8 M km2) by determining the surface mineralogy and bulk regional geochemistry of the terrain. In turn, this constrains their formation and yields a framework for their comparison with extensional structures on Earth. These terrains are notable for olivine and high-Ca pyroxene with a high abundance of potassium and calcium akin to alkali basalts. In the case of Mars, this Earth-like proto-plate tectonic scenario may be related to the plume-induced crustal stretching and considering their distribution and temporal relationship with the Hellas basin, we conclude that the plume is impact-induced. Overall, the findings of this work support the presence of mantle plume activity in the Noachian, as suggested by thermal evolution models of Mars.

Relationship of the Crustal Structure, Rheology, and Tectonic Dynamics Beneath the Lhasa‐Gangdese Terrane (Southern Tibet) Based on a 3‐D Electrical Model

AbstractThe tectonic dynamics of the Lhasa‐Gangdese terrane, southern Tibet, are still unclear and open questions persist regarding the structure, physical properties, and rheology of the lithosphere. A three‐dimensional electrical resistivity model was generated beneath the Lhasa terrane, 79°−94°E and 28°–33°N, an area of ∼1,500 by ∼600 km, using data from 537 magnetotelluric measurements. To overcome computational challenges, we present an approach in which multiple high‐resolution models from overlapping subregions are combined by means of an error‐weighted averaging scheme to construct a continuous electrical resistivity model. Based on the electrical structure, the rheology of the mid‐lower crust was investigated by constraining the melt fraction, in order to explore controls on dynamic mechanisms from a whole‐system perspective. The models shed light on the vertical and lateral migration of crustal materials, magma transportation, and continental deformation in the Lhasa terrane. The model shows resistive features in the south that may represent the Indian plate, and conductive features above this that may represent the migration of materials beneath the Lhasa terrane, both of which vary substantially along the Indus‐Yarlung Zangbo suture. In the mid‐lower crust, conductive features (with a likely corresponding decrease of the effective viscosity) may be related to underplating, magma reservoirs, mineralization sources, and vertical motion of buoyant materials. A weakened mid‐lower crust has consequences for surface deformation and may have contributed to the formation of (north‐south) rift zones. Furthermore, the inhomogeneous distribution of weakened areas in the mid‐lower crust has implications for the local lateral migration of materials.

Volcano‐Tectonic Evolution of the Christiana‐Santorini‐Kolumbo Rift Zone

AbstractLocated on the Hellenic Arc, the Christiana‐Santorini‐Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano‐tectonic systems in the Mediterranean. Although this rift zone has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high‐resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre‐volcanic NNE‐SSW‐oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE‐WNW‐oriented basins, which was overprinted by a NE‐SW‐oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small‐scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep‐seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large‐scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos‐Nisyros‐Yali volcanic field zone and Rhodos highlights concurrent regional volcano‐tectonic changes, suggesting a potential arc‐wide scale of the observed volcano‐tectonic interplay.

Stable isotopic study of carbonatites from the Pakkanadu Alkaline Complex, southern India: Constraints on carbonatite melt evolution and sub-solidus, high temperature fluid-rock interaction

Carbonatites of the Pakkanadu Alkaline Complex (PAC) of the Dharmapuri Rift Zone (DRZ), in the Northern Granulite Terrain (NGT) of southern India, were intruded as small concordant and discordant bodies into pyroxenites and syenites during the Neoproterozoic (∼759‐–750 Ma). Carbonatite samples from the PAC were grouped into three major groups for oxygen and carbon isotope analysis: (A) calcite-dominated (88–100%), (B) dolomite-dominated (83–97%) and (C) admixtures of calcite and dolomite. End-member isotopic compositions were ẟ 13 C = −5.7 ± 0.1‰ and ẟ 18 O = +8.4 ± 0.2‰ for calcite and ẟ 13 C = −6.0‰ and ẟ 18 O = +6.9‰ for dolomite. The carbonatite isotopic compositions lie within the range known for primary igneous carbonatites (PIC), indicating their origin from a mantle source. The higher ẟ 18 O of the calcite end-member than the dolomite end-member indicates oxygen isotopic disequilibrium in the carbonatites. This disequilibrium was the product of sub-solidus oxygen isotope exchange with fluids derived from carbonatite melt during the brine-melt stage (600–400 °C). The narrow range of ẟ 13 C of the carbonatites and its similarity to the ẟ 13 C of depleted upper mantle (−6 ± 2‰) suggests that very little CO 2 was associated with the carbonatite melt. We infer that this type of carbonatite melt evolved from the upper asthenosphere or the base of the lithosphere (> 3 GPa, i.e., >100 km). Similar observations were made based on compiled ẟ 13 C and ẟ 18 O data for carbonatites from Salem-Attur fault, Sevathur, and Hogenakkal regions of the NGT. The ẟ 13 C and ẟ 18 O compositions of carbonatites from Samalpatti, India, however, are distinctly different and suggest two sources for carbonatite melts, one containing a significant amount of CO 2 and possibly evolved from somewhere in the middle of the lithospheric mantle (∼ 2–3 GPa, i.e., 60–100 km) and the other associated with crustal sedimentary or metasedimentary carbonates, which were possibly incorporated during emplacement of carbonatite melts originating from the lithospheric mantle. This study shows that carbonatite melts in the NGT mostly evolved from different levels of the upper mantle and were emplaced to crustal levels, perhaps via a rifting-induced fracture network. • Carbonatites of the Pakkanadu Alkaline Complex from southern India were investigated. • ẟ 13 C and ẟ 18 O of the carbonatites indicate their origin from a mantle source. • O isotope exchange occurred between the carbonatite and carbonatite-derived fluids. • CO 2 production was insignificant during partial melting of the mantle source. • The carbonatite was emplaced to crustal levels via a rift-related fracture system.

Pre-eruptive timescales from the historical Hapaimamo eruption at Mauna Loa, Hawai‘i

The 240 yr B.P. Hapaimamo eruption occurred on the southwest rift zone (SWRZ) of Mauna Loa. A large tephra deposit and an extensive lava flow field were produced that extend from 1880 m above sea level down to the southwest coast. Detailed petrological study of olivine crystals from both deposit types are used to determine the timescales of crystal disaggregation from a cumulate mush zone. Over 70% of studied olivines in both, lava flow and tephra samples, exhibit normal zoning. Tephra and flow olivine core compositions are between Fo89–84 and Fo89–81. Olivine rims in the tephra have compositions of Fo86–81, whereas rims in the flow samples extend to lower values within the range Fo83–71. The remaining tephra olivines are unzoned and fall within the same core compositional range. Using FeMg interdiffusion chronometry, we calculate tephra olivine diffusion timescales from 13 to 1600 days (days to years), with over 70% of timescales <100 days, and lava flow olivine diffusion timescales from 25 to 1110 days with 65% of timescales >100 days. Zoning widths were defined as narrow (type I) or broad (type II) and together with the diversity of timescales indicate an ongoing process of crystal mush disaggregation into the erupted host melt.Comparison of tephra and flow diffusion timescales show consistently longer timescales across the lava flow population, as expected due to continued diffusive re-equilibration. Our results suggest that where textural evidence allows, and tephra timescales can be compared, olivines from lava flows can be used to provide temporal information for magmatic processes within the subsurface and the lava flow. Comparison of published elemental diffusion timescales between Mauna Loa and neighbouring Kīlauea suggest that magma mobilisation and transfer processes occur over similar timescales within the plumbing systems of both Mauna Loa and Kīlauea.

A relic thickened crustal root beneath the Cenozoic rift zone of the NW Ordos margin, North China, revealed by receiver functions

We investigate the crustal structure and Moho topography beneath the northwest Ordos rift zone by employing receiver functions analysis at dense broadband seismic stations. The results delineate a depressed Moho zone (by ∼5 km) beneath the Cenozoic rifts, which is unconventional for classical continental rift zones. We interpret this depressed Moho zone as a relict of the thickened crustal root formed during the intracontinental orogeny in late Mesozoic. The Cenozoic rift has not yet reequilibrated the crustal-mantle boundary possibly due to the lack of decompressional melting during the lithospheric extension.

Regional‐Scale Lithospheric Recycling on Venus Via Peel‐Back Delamination

AbstractWe currently have a limited understanding of the tectonic framework that governs Venus. Schubert and Sandwell (1995, https://doi.org/10.1006/icar.1995.1150) identified over 10,000 km of possible subduction sites at both coronae and chasmata rift zones. Previous numerical and experimental studies have shown the viability of regional‐scale lithospheric recycling via plume‐lithosphere interactions at coronae, yet little work has been done to study the possibility of resurfacing initiated at Venusian rift zones. We created 2D numerical models to test if and how regional‐scale resurfacing could be initiated at a lateral lithospheric discontinuity. We observed several instances of peel‐back delamination—a form of lithospheric recycling in which the dense lithospheric mantle decouples and peels away from the weak, initially 30 km‐thick crust, leaving behind a hot, thinned layer of crust at the surface. Delamination initiation is driven by the negative buoyancy of the lithospheric mantle and is resisted by the coupling of the plate across the Moho, the significant positive buoyancy of the crust arising from a range of crustal densities, and the viscous strength of the plate. Initial plate bending promotes yielding and weakening in the crust, which is crucial to allow decoupling of the crust and lithospheric mantle. When there is sufficient excess negative buoyancy in the lithospheric mantle, both positively and negatively buoyant plates may undergo delamination. Following a delamination event, the emplacement of hot, buoyant asthenosphere beneath the crust may have consequences for regional‐scale volcanism and local tectonic deformation on Venus within the context of the regional equilibrium resurfacing hypothesis.

Shear‐Velocity Structure and Dynamics Beneath the Sicily Channel and Surrounding Regions of the Central Mediterranean Inferred From Seismic Surface Waves

AbstractThe evolution of the Sicily Channel Rift Zone (SCRZ) is thought to accommodate the regional tectonic stresses of the Calabrian subduction system. Much of the observations we have today are either limited to the surface or to the upper crust or deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. It is unclear whether the rifting is passive from far‐field extensional stresses or active from mantle upwelling beneath. We measure Rayleigh‐and Love‐wave phase velocities from ambient seismic noise and invert for 3‐D shear‐velocity and radial anisotropic models. Variations in crustal S‐velocities coincide with topographic and tectonic features. The Tyrrhenian Sea has a ∼10 km thin crust, followed by the SCRZ (∼20 km). The thickest crust is beneath the Apennine‐Maghrebian Mountains (∼55 km). Areas experiencing extension and intraplate volcanism have positive crustal radial anisotropy (VSH &gt; VSV); areas experiencing compression and subduction‐related volcanism have negative anisotropy. The crustal anisotropy across the Channel shows the extent of the extension. Beneath the Tyrrhenian Sea, we find very low sub‐Moho S‐velocities. In contrast, the SCRZ has a thin mantle lithosphere underlain by a low‐velocity zone. The lithosphere‐asthenosphere boundary rises from 60 km depth beneath Tunisia to ∼33 km beneath the SCRZ. Negative radial anisotropy in the upper mantle beneath the SCRZ is consistent with vertical mantle flow. We hypothesize a more active mantle upwelling beneath the rift than previously thought from an interplay between poloidal and toroidal fluxes related to the Calabrian slab, which in turn produces uplift at the surface and induces volcanism.

STUDY, FORECAST AND CONTROLLED SEISMIC HAZARD REDUCTION IN THE IDENTIFIED SEGMENTS OF THE MAIN FAULTS BY CYCLIC INJECTION OF FLUID THROUGH DEEP MULTI-BRANCH DIRECTIONALLY INCLINED WELLS

The methods developed by the world community to date to withstand strong natural and induced destructive earthquakes do not effectively reduce material losses and the number of victims. The authors propose for discussion an integrated approach to solving the problem of ensuring seismic safety, based on the use of new important information about the geological conditions for earthquake generation. This involved the use of results of numerical and physical modeling, as well as physical full-scale experiments in the natural fault areas. The paper analyzes the petrophysical conditions of deep-seated frictional processes in coseismic faults, revealed through detailed studies of the fragments of paleoearthquake centers that became accessible after their exhumation from seismic-focal depths of the Earth’s crust. The collected information allowed the authors to clarify with a high degree of certainty the origin and occurrence of seismic motions. This paper presents briefly the results of the medium-term forecast of earthquakes with M≥5.0 as applied to the seismodynamic regime of the Baikal rift zone. The forecast emphasizes the detection of places for 1–11-year earthquake generation cycles.A comprehensive analysis of the collected information made it possible to substantiate the conclusion about an opportunity to prevent earthquake damage by using hydrodynamic damping of seismically hazardous fault segments. In the last section, consideration is being given to one of the most promising methods of such man-made impacts, which uses modern technological advances in drilling deep multil-branch and directionally inclined wells with horizontal deviation. The paper discusses the techniques that make it possible to prevent episodes of unexpected reactivation of fault segments in the form of excitation of earthquakes with M≥6.0. Attention is drawn to conducting tests at selected sites in order to improve the technology as part of the approach to earthquake damping.

Spatial distribution of 1.4-1.3 Ga LIPs and carbonatite-related REE deposits: Evidence for large-scale continental rifting in the Columbia (Nuna) supercontinent

Large-scale continental rifting and large igneous provinces (LIPs) have played important roles in the breakup and dispersal of supercontinents Pangea and Rodinia. Previous paleomagnetic and geological evidence suggests fragmentation of Columbia (or Nuna) supercontinent during 1.4-1.2 Ga. However, no large-scale continental rift system has been identified in Columbia during this period, although some continental rifting events between 1.6 Ga and 1.2 Ga have been reported. Here we provide a refined ca. 1.4 Ga paleogeographic reconstruction of Columbia based on previously published paleomagnetic data and geological evidence for connections between continental blocks, and consistent with spatial and temporal distributions of the 1.4-1.3 Ga LIPs and smaller intraplate mafic magmatic events interpreted as LIP fragments/remnants. Our results indicate a 1.4–1.3 Ga large-scale continental rift zone located along western Laurentia, western-northern Siberia, southeastern Baltica, western-northern West Africa, southwestern Amazonia, southern-eastern Congo/São Francisco, eastern Kalahari, northern North China and northern North Australia. This rift zone extends about 15,000 km across supercontinent Columbia, and is a main indicator and a proximal reason for its final breakup. The spatial distribution of the 1.4-1.3 Ga carbonatite-related REE deposits shows that this newly identified rift system also controlled Bayan Obo, the world's first and largest REE-Nb deposit in northern North China, and Mountain Pass, the world's second largest REE deposit in western North America. Given the scale of this rift system it can be expected to be the locus of additional carbonatite intrusions as well as other commodity types, and provides important constraints on mechanism and processes for final breakup and dispersal of Columbia supercontinent.

Geological structure and copper mineralization of the Tekturmas ophiolite belt and related structures of central Kazakhstan

The article presents an overview of the main stages of magmatism and ore mineralization of copper and other metals of the Tekturmas ophiolite belt and related structures of Central Kazakhstan. The age, composition and probable tectonic settings during the formation of Cu-Ni deposits in Central Kazakhstan associated with sulfide mineralization are discussed. The Tekturmas ophiolite belt includes formations of three lithological types: basaltic Karamurun and Kuzek formations, siliceous Tekturmas and Bazarbai formations and clastic Sarytau, Airtau and Yermek formations. The potential ore mineralization of the Tekturmas ophiolite belt is associated with plutonic mafic and ultramafic rocks of the ophiolite section, within the Karamurun and Kuzek formations of the Tekturmas and Bazarbai segments, respectively. The Uspenka rift zone, which is adjacent to the Tekturmas ophiolite belt, includes three volcanogenic-sedimentary associations of Early-Middle Silurian, Early-Middle Devonian and Late Devonian-Early Carboniferous ages. The Uspenska copper deposit is related to the youngest Late Devonian-Early Carboniferous association. The geological structure of the territory, which includes magmatic bodies with Cu-Ni ore mineralization, should serve as a basis for an integrated approach to studying oreforming systems. The most promising locations of copper mineralization within the Tekturmas ophiolite belt and Uspenska rift zone are the Kamkor and Uspenska-Bella deposits and the Urtynjal, Ordobasy and Shopa ore occurrences. All these deposits are associated with mafic and ultramafic intrusive complexes. The main regional and local geological characteristics of these deposits and ore occurrences, along with the types of ore mineralization, are discussed. It is argued that Central Kazakhstan has limited prospects for the discovery of large Cu deposits; however, the discovery of medium-size and small-size deposits of that type is quite possible. The discovery of new large Cu-porphyry deposit in the Tekturmas ophiolite belt is also unlikely; however, additional prospecting works aimed at identification of new ore bodies in the vicinity of already known deposits and ore occurrences presents research interest.

Crustal Structure of the Camamu‐Almada Margin Along the Northeastern Rift Segment of Brazil From an Integration of Deep‐Penetration Seismic Reflection Profiles, Refraction, and Gravity Modeling

AbstractThe Camamu‐Almada segment of the northeastern Brazil rifted‐passive margin covers an area of 22,000 km2 and includes: (a) thin to ultra‐thin metamorphic continental crust; (b) a transitional zone of unknown composition; and (c) Albian and younger oceanic crust. We investigate the structural‐magmatic evolution of the region by interpreting 19,000 line km of two‐dimensional reflection seismic data, 105 offshore well logs, and 15 seismic refraction stations. Interpretation of these data suggests that the 100 km‐wide rifted zone is structurally complex and composed of at least three discrete crustal blocks: (a) 10–40 km‐wide and 4–6 km‐thick zones of hyperextended continental and possibly unroofed lower continental crust with an average density of 2.88 g/cc; (b) two 20 km‐wide sub‐circular zones of exhumed mantle with an average density of 3.1 g/cc; and (c) 40–60 km‐wide and 6–7 km‐thick proto‐oceanic crust that formed in situ. These blocks are similar in thickness and composition to rifted zones described by previous workers along its conjugate margin in Gabon, West Africa, and on the rifted‐passive margin of the western Mediterranean.

Microgravity Change During the 2008–2018 Kı̄lauea Summit Eruption: Nearly a Decade of Subsurface Mass Accumulation

AbstractResults from nine microgravity campaigns from Kı̄lauea, Hawaiʻi, spanning most of the volcano's 2008–2018 summit eruption, indicate persistent mass accumulation at shallow levels. A weighted least squares approach is used to recover microgravity results from a network of benchmarks around Kı̄lauea's summit, eliminate instrumental drift, and restore suspected data tares. A total mass of 1.9 × 1011 kg was determined from these microgravity campaigns to have accumulated below Kı̄lauea Caldera during 2009–2015 at an estimated depth of 1.3 km below sea level. Only a fraction of this mass is reflected in surface deformation, and this is consistent with previously reported discrepancies between subsurface mass accumulation and observed surface deformation. The discrepancy, amongst other independent evidence from gas emissions, seismicity, and continuous gravimetry, indicate densification of magma in the reservoirs below the volcano summit. This densification may have been driven by degassing through the summit vent. It is hypothesized that during the final years of the summit eruption, magma densification resulted in a buildup of pressure in the reservoirs that may have contributed to the lower East Rift Zone outbreak of 2018. The observed mass accumulation beneath Kı̄lauea could not have been detected through other techniques and illustrates the importance of microgravity measurements in volcano monitoring.

Can Lava Flow Like Water? Assessing Applications of Critical Flow Theory to Channelized Basaltic Lava Flows

AbstractFlowing lava and water have dramatically different physical properties but can form similar hydraulic structures, including undular hydraulic jumps, or standing wave trains. In water flows, undular hydraulic jumps are evidence of critical flow (Froude number ∼1) and open‐channel hydraulic theory provides a powerful tool for estimating flow depth and velocity. Monitoring these parameters in an active lava channel is inherently challenging, but essential for calculating lava discharge (effusion rate), a primary control on the rate of flow front advance and ultimate flow runout distance. We analyze undular hydraulic jumps in both water and lava flows to assess the conditions under which they form and, by extension, the potential use of critical flow theory to estimate, in real time, lava flow velocity, depth, and discharge. Experimental data for water flows show that these structures mark the transition from supercritical to subcritical flow. Undular hydraulic jumps in the near‐vent lava channel of the 2018 lower East Rift Zone eruption of Kīlauea, Hawaiʻi also reflect critical flow conditions; their wavelengths scale with flow depth and velocity, consistent with hydraulic theory. Calculated lava effusion rates are similar to estimates made using more traditional approaches (Jeffreys', 1925, https://doi.org/10.1080/14786442508634662, equation based on lava viscosity, density, and channel slope) and with lava volumes derived from topographic‐change mapping. From this we conclude that critical flow phenomena show great potential to track flow dynamics and inform hazard assessment for a wide range of geophysical fluids.

Lava deltas, a key landform in oceanic volcanic islands: El Hierro, Canary Islands

Marine and subaerial erosion of volcanic ocean islands form coastal cliffs and shore platforms, particularly during stable sea levels. Posterosional lava flows can spill over these coastal cliffs and fill the platforms, leading to the progradation of lava deltas. This work aims to analyze this volcanic rocky coast setting at the island scale and to assess the volcanic constructional and erosive degradational effects on the coast at the scale of one volcanic edifice. El Hierro Island, Canary Islands, exemplifies a rocky coast with an active sea-cliff profile, reflecting its early evolutionary stage as a young ocean volcanic island with no fringing reef. The occurrence of a contemporary insular shelf formed during the Holocene sea-level highstand (<7 ka) allows constraining the ages of those eruptions forming lava deltas affecting this geomorphological landform. A detailed bathymetry around the island allowed us to distinguish 17 eruptions fulfilling this criterion. The Montaña del Tesoro, which occurred about 1050 years BP, is one of these eruptions and was selected as a case study for morphometric modeling integrating subaerial and submarine data at the scale of a volcanic edifice. This eruption was a Strombolian basaltic volcanic event that produced a scoria cone, pyroclastic fall deposits, and lava flows that reached the ocean in the eastern rift zone of El Hierro island. We combine field-based observations with topographic and bathymetric data analysis to reconstruct the pre- and post-eruption Digital Elevation Models (DEMs) and, comparing with present-day DEM, to analyze morphometrically the influence of volcanism on the coastal landscape's development. The resulting landform complexity required the discretization of the lava field according to the coastline evolution and lava front sectors, and the subaerial or submarine lava placement. The pyroclastic materials' total erupted bulk volume (12,829,578 m3) corresponds to a volcanic eruption index (VEI) of 3. This event was primarily effusive. From a dense rock equivalent (DRE) volume of 25,615,424 m3, 87 % flowed as lava, 10 % formed the cinder cone, and 3 % the tephra fall deposits. We quantitatively demonstrate that dominant degradation occurs in the lava field, mainly disturbed by marine erosion. Marine erosion removed 9 % of the erupted volume of lava flows against 1 % by fluvial erosion. This work provides methods and results of great interest with different implications in oceanic volcanic islands, among which we can mention coastal planning (e.g., rock coast evolution) and volcanic risk assessment (e.g., the importance of Holocene sea-level rise on the development of shore platforms facilitating the progradation of lava deltas).

Test of P-wave receiver functions for a seismic velocity and gravity model across the Baikal Rift Zone

SUMMARY The seismic receiver function (RF) technique is widely used as an economic method to image earth's deep interior in a large number of seismic experiments. P-wave receiver functions (RFs) constrain crustal thickness and average Vp/Vs in the crust by analysis of the Ps phase and multiples (reflected/converted waves) from the Moho. Regional studies often show significant differences between the Moho depth constrained by RF and by reflection/refraction methods. We compare the results from RF and controlled source seismology for the Baikal Rift Zone by calculating 1480 synthetic RFs for a seismic refraction/reflection velocity model and processing them with two common RF techniques [H–κ and Common Conversion Point (CCP) stacking]. We compare the resulting synthetic RF structure with the velocity model, a density model (derived from gravity and the velocity model), and with observed RFs. Our results demonstrate that the use of different frequency filters, the presence of complex phases from sediments and gradual changes in the properties of crustal layers can lead to erroneous interpretation of RFs and incorrect geological interpretations. We suggest that the interpretation of RFs should be combined with other geophysical methods, in particular in complex tectonic regions and that the long-wavelength Bouguer gravity anomaly signal may provide effective calibration for the determination of the correct Moho depth from RF results. We propose and validate a new automated, efficient method for this calibration.

Recycled Crustal Components of the Iceland Plume Centre: Ultra-Dehydrated Crust and Oxidised Water-Bearing Fertile Mantle

AbstractPrimitive basalts erupted mainly above the proposed centre of the Iceland plume have been analysed geochemically. The data shed light on the distribution of plume heterogeneities and constrain their nature and origin. Low-Ca (down to 1420 ppm) olivine macrocrysts and chromite inclusions in Snæfell rocks of the eastern flank zone indicate that the magmas were hydrous (c. 2 wt% H2O) and relatively oxidised (fO2 = c. ΔQMF+1). Al-in-olivine crystallisation temperatures indicate that magmas from central Iceland magmas are not hotter than from other parts of Iceland. Some Gæsafjöll rocks with high Nb/U = 110–120 from the northern rift zone (NRZ) represent magmas widespread in Iceland that are also characterised by low Ba/La and high La/U, La/Th, Nb/Th and Ce/Pb. They reflect a mantle component with Nb/U ≈ 200 present in many basalts of all rift zones. This mantle apparently consists of peridotite refertilised by ultra-dehydrated recycled oceanic crust (ROC). This component is in clear contrast to the main source of the Snæfell magmas, which is modelled to be peridotite strongly refertilised by enriched ROC more hydrous than Mid-Ocean Ridge Basalt. In contrast to more-depleted rift zone melts, which are extracted from depths just below the thin rifted lithosphere, some rift zone basalts are extracted deeper and produced by intermediate degrees of melting of the two types of refertilised mantle. Snæfell-type melts are common in the NRZ and eastern rift zones (ERZ) and most strongly present at Upptyppingar. Mixing of magmas is indicated at crustal levels. Mantle melting may be modelled for homogeneous sources of peridotite or olivine websterite. Double-spike Pb isotope analyses suggest that the depleted mantle sources of the ERZ basalts together with Snæfell and high-Nb/U sources, both with 206Pb/204Pb = 18.3–18.6, Δ207Pb ≈ 0 and Δ208Pb = 20–30, explain both ERZ and NRZ magmas, except for Theistareykir. They are distinct from magmas of the western rift zone, which is characterised by melt contributions from a differently enriched mantle with more radiogenic lead, negative Δ207Pb, Δ208Pb ≈ 0. This difference in type of enriched mantle suggests that the Iceland mantle plume is laterally zoned.