Low-velocity Zone Research Articles

Geomechanical modeling of fault-propagation folds: A comparative analysis of finite-element and the trishear kinematic model

Fault-propagation folds are common structures within fold and thrust belts. The trishear kinematic model has been widely used to understand the kinematics and geometry of these folds, effectively reproducing various characteristics. However, the resulting geometry of natural prototypes may diverge from the predictions of the trishear model depending on the rheological properties involved in the deformation. In order to address this limitation, finite element viscoplastic numerical models were implemented. The analysis revealed that in models with a 15° fault angle, these simulations develop a mechanically weaker discontinuity, which is defined as the low viscosity zone (LVZ). The LVZ induces faulting and absorbs slip, causing deviations of velocity vectors from parallel alignment with the main reverse ramp. In models with fault angles set at 25° or 35°, the kinematic vectors of the hanging wall aligned parallel to the ramp, and a zone of progressive rotation of the velocity vectors was observed in the forelimb, resembling the theoretical trishear zone. In these scenarios, the resulting folds exhibited greater symmetry. However, in cover layers with a viscosity equal to 1020 Pa s, the forelimb exhibits the highest velocities, which is attributed to material flow toward the footwall.

Thermal-hydraulic performance enhancement of modified hexagonal micro-Pin fin heat sinks using rotational configurations

Nowadays, micro-pin fin heat sinks (MPFHSs) have shown promising performance in removing high heat fluxes in small dimensions and are widely used in electronic cooling applications. Among various techniques for thermal–hydraulic enhancement of MPFHSs, geometric design modification has been emphasized in this study, which focuses on investigating the orientation of the micro-pin fins (MPFs) as an overlooked but crucial parameter. To this end, rotational configurations of modified hexagonal MPFs, including co-rotation, alternating rotation, and counter-rotation have been investigated at rotation angles ranging from 0 to 60 degrees. In the numerical study, the copper MPFHS with a heat flux of 50 W/cm2 is cooled using water within the Reynolds number (Re) range of 400 to 1000. The results revealed that the rotation angle of MPFs governs the flow behavior around them. In general, counter-rotation configurations exhibited better thermal performance by providing the possibility of increased fluid mixing and contraction of low-velocity recirculation zones. However, the considerable increase in pressure drop in this configuration may limit its usability in higher Re regimes. By considering the compromise between pressure drop and heat transfer, the counter-rotation configuration with a 30-degree angle was selected as the optimal design. The optimized design results in an increase of approximately 50 % in Nusselt number (Nu) and up to 26.7 % in performance evaluation criteria (PEC). Furthermore, a supplementary experimental study confirmed the numerical results. It demonstrated the remarkable performance of the proposed design in removing heat fluxes ranging from 125 to 250 W/cm2, within the range of Re under investigation.

Ambient noise tomography of El Hierro island (Canary Islands)

El Hierro island is one of the most active islands in the Canary Islands from a volcanological point of view. This is the reason why the imaging of the internal crustal structure is of huge importance. The geophysical exploration methods employed on El Hierro Island, such as gravimetry and seismic tomography, allowed obtaining the high-resolution characterization of the crust’s deep part. However, these methods did not yield significant information about the surface and the shallower part of the crust. To gain a deeper insight into the shallow geological structure of El Hierro island, we employed Ambient Noise Tomography to construct a 3D S-wave velocity model. Our investigation revealed the presence of seven significant seismic velocity anomalies, partly identified by previous studies. We identified two high-velocity anomalies located in the eastern and western parts of the island at a depth between 0 and 3 km below sea level (b.s.l.). We interpreted these anomalies as dense intrusive complexes of dikes, possibly linked to the Tanganasoga volcano and the formation of the Tiñor edifice. Additionally, we observed two high-velocity anomalies in the northern and southern parts of the island at a depth between 3 and 4 km b.s.l., which we related to the accumulation of solidified igneous rocks. On the other hand, a low-velocity anomaly was observed in the Golfo valley, between 0 and 0.5 km b.s.l., and we interpreted it as megalandslide deposits. This anomaly was evidenced for the first time in the present study. Finally, two low-velocity anomalies were observed in the southern part of the island at different depths, between 0–0.5 km b.s.l. and 0–2 km b.s.l. These were interpreted as fractures generated by Quaternary volcanism along the SSE Rift. Also, one of them was evidenced for the first time in this study, corresponding to the zone of the fractures produced during the Quaternary volcanism. This study has allowed us to gain a more detailed understanding of the shallow geological structure of the island. Even if most of the anomalies had been evidenced previously, we could observe the existence of two low-velocity zones in the shallow crust that have not been observed before.

Three-dimensional fine crust-mantle structure imaging and structural characteristics of Bohai Sea

This study used the consistency-constrained double-difference tomography method to invert 3D fine structure models of Vp, Vs, Vp/Vs at depths above 60 km, and precise relocation parameters of earthquakes in the Bohai Sea. According to the results, the velocity structures of P-wave and S-wave in the Bohai Sea area were highly similar and demonstrated noticeable lateral non-uniformity. The crust beneath the Tanlu Fault displayed a clear stratification structure, with a continuous velocity transition in the middle of the crust. The intricate crustal structure beneath the Zhangpeng Fault displayed high-velocity bodies in the crust and low-velocity anomalous zones connected to the top of the mantle in the lower section of the crust. The structural pattern in the deep crust of the Bohai Sea controls the occurrence characteristics of the Zhangpeng Fault and the Tanlu Fault. The earthquakes in the Bohai Sea area were concentrated mainly in the southern part near the Zhangpeng Fault. There is a good correspondence between the relocated earthquakes and velocity structure. There are many significant differences in crustal structure between the north and south of the Bohai Strait, and there are obvious velocity anomalies in the middle and upper crust. The distribution shape of high Vp/Vs value indicates that mantle material migration has occurred at the bottom of the crust. This paper provided important reference for further research on the relationship between deep tectonic features and tectonic activity in the North China Craton.

Relationships among the fish passage efficiency, fish swimming behavior, and hydraulic properties in a vertical‐slot fishway

AbstractThree‐dimensional hydrodynamic numerical simulation and laboratory fish‐passage experiment with juvenile grass carp (Ctenopharyngodon idella) were used to systematically estimate hydrodynamic parameters, fish passage efficiency, fish swimming behavior, and fish swimming trajectories in a vertical‐slot fishway (VSF) under five different flow discharges. The spatial‐mean time‐averaged velocity magnitude along the slot section determined passage success rate. The absolute value of the time‐averaged Reynolds‐shear‐stress component in the X–Y direction played a decisive role in selecting the swimming path. In addition, fish energy expenditure along the mean‐50% swimming route indicated that higher mainstream velocity did not necessarily signify larger energy consumption, because successfully‐migrating fish selected the relatively lower velocity zone during the migration process. Our findings for fish swimming behavior in response to velocity and turbulence can be used to develop high‐efficiency fishways for grass carps or other species with similar traits.

Improvement in energy performance from the construction of inlet guide vane and diffuser vane geometries in an axial-flow pump

Advanced inlet guide vane (IGV) and diffuser vane (DV) geometries were constructed in an effort to increase the energy performance of an axial-flow pump at the best efficiency point (BEP). DV setting angles were also investigated to increase energy performance at the off-design points. By integrating the advantages of an adjustable IGV, combinations of adjustable IGV and DV geometries were constructed and thoroughly analyzed by way of energy loss. The asymmetrical geometry of the IGV, upgraded through the use of a hydrofoil profile, resulted in higher hydraulic performance compared to that of the reference model. The efficiency and total head at the BEP increased significantly with the implementation of the new DV, by 1.456% and 5.756% over those of the reference model, respectively. Using the new DV reduced the unsteady turbulent flow behind the trailing edge of the DV under all flow rate conditions, resulting in a reduction in vibration and noise. The positive setting angles of the DV increased the energy performance in the high-flow-rate region, whereas the negative DV setting angles produced a good performance in the low-flow-rate region. Combining an adjustable IGV with an adjustable DV model resulted in a significant increase in the total head, with more optimal energy performance provided by the positive IGV setting angles. At the BEP and under high-flow-rate conditions, the low-velocity zone is closely related to high entropy generation. Furthermore, these high-entropy generation regions follow the trajectory of the low-velocity zones. However, the low-velocity zone is not strongly associated with the high-entropy generation region when operating under low-flow-rate conditions.

Towards a better comprehension of reactive transport coupling experimental and numerical approaches

In this work we focus on further understanding reactive transport in carbonate rocks, in particular limestones characterized by a bimodal pore size distribution. To this end, we performed injection experiments with CO2-saturated water on a sample of Euville limestone and monitored the experiments with a medical CT scanner. Microscanner imaging was performed before and after alteration. Experiments showed that permeability increased by nearly two decades due to the alteration process. This increase could be attributed to the formation of a preferential dissolution path visualized on the CT images. Microscanner images show that preferential dissolution areas are characterized by the presence of numerous enlarged macropores. The preferential dissolution path created therefore retains a porous structure and does not correspond to a wormhole-type channel. To provide further knowledge of the small-scale physics of reactive transport, we performed Lattice-Boltzmann simulations of flow in a numerically generated model 2D porous medium having geometrical and topological features designed to approach Euville limestone. We showed that the fluid velocity increased in nearly percolating paths of macropores. Considering the experiments, this means that the CO2-saturated water starts to enter high-velocity zones earlier than low-velocity zones, inducing an earlier onset of the alteration process and a more pronounced local dissolution. However, numerical results showed that the alteration of non-connected macropores leads to an increase of permeability much smaller than the experimentally observed one. To explain this fact we used effective medium modelling that permits predicting the variation in permeability as a function of the fraction of macropores and consequently as a function of alteration. It proved that as long as there is no alteration-induced percolating path consisting of macropores, the increase in permeability is relatively low as shown by the Lattice-Boltzmann simulations. An increase in permeability of several orders of magnitude is only observed when the macroporosity is close to the percolation threshold. This fact is in accordance with the experimentally observed results.

Lithosphere Removal in the Sierra Nevada de Santa Marta, Colombia

AbstractThe Sierra Nevada de Santa Marta (SNSM) in northwestern Colombia is one of the world's highest coastal mountains, with an elevation above 5 km. Gravity measurements show that the SNSM has a positive Bouguer anomaly (>+130 mGal), indicating that the mountain lacks a crustal root. In this work, we test the hypothesis that these observations can be explained by gravitational removal of the dense lower lithosphere. We use 2D numerical models to examine the dynamics of lithosphere removal and its effect on surface elevation and gravity anomaly. The models consist of continental lithosphere that includes a pre‐thickened crustal region, representing the SNSM. In our preferred model, the dense mantle lithosphere and crustal root are gravitationally unstable and undergo removal as local drips within ∼10 Ma from the onset of foundering. This creates an area of thinned crust (∼38 km) underlain by a buoyant sublithospheric mantle where melting and low seismic velocities are predicted. Subsequent non‐isostatic forces maintain a topography of 3.3 km with a 2D Bouguer gravity anomaly of +103 mGal. Parameter tests show that a strong lower‐crustal rheology provides greater support for the high topography and that a weak mantle lithosphere rheology produces faster removal. The models demonstrate that local lithosphere dynamics can explain the first‐order observations in the SNSM. We propose that lithosphere removal could have occurred recently (∼2 Ma), explaining the localized low seismic velocity zone below the SNSM, or at 56–40 Ma, inducing anomalous short‐lived Eocene magmatism.

Self-supervised learning waveform inversion for seismic forward prospecting in tunnels: A case study in Pearl River Delta Water Resources Allocation Project in China

Tunnel and underground engineering construction often encounter unfavorable geology, leading to disasters such as water and mud inrushes and landslides. To prevent geologic hazards, it is important to look ahead and predict the location and distribution of adverse geology ahead of the tunnel face. This process is known as seismic forward prospecting in tunnels, and it typically requires an accurate calculation of velocity. Seismic waveform inversion methods based on deep learning have demonstrated potential in estimating velocity from synthetic seismic data. However, the superiority of these methods over traditional ones on field data is still an area of active research. Here, the Pearl River Delta Water Resources Allocation Project in China is used as an example to develop a self-supervised learning waveform inversion method for building a reliable velocity distribution in front of the tunnel. By introducing the background velocity as large-scale information and implementing multiscale loss functions, the previous self-supervised learning inversion method on synthetic data is improved. In addition, the corresponding network-based workflow for field data is developed. To demonstrate the effectiveness of our method, a comparison is conducted with practical tunneling exposure, for which the low-velocity zone corresponds with the fault-fractured zones and the water-flowing zones. This indicates that the results obtained from our method can be used as geologic guidance for safe tunneling practices. In the end, the applicability and disadvantages of our deep-learning inversion method for seismic forward prospecting in tunnels are discussed.

A Simulation and Experiment on the Optimization Design of an Air Outlet Structure for an Air-Assisted Sprayer

In response to the issues of low-velocity zones and non-uniform jet velocity distribution in the airflow field of traditional air-assisted orchard sprayers, an arc-shaped air outlet suitable for axial-flow air-assisted systems is designed. This article employs the method of CFD numerical simulation and experimental verification to compare and analyze the internal flow field of the air-assisted system and validates the reliability of the numerical simulation results through calculation error and chi-square test. The wind speed of the cross-section is measured at different distances from the outlet, and the distribution characteristics of the outflow field wind speed before and after the structural optimization of the air-assisted system are compared. The horizontal distribution of fog droplets is collected using a fog collection chamber. The experimental results show that the design of the arc-shaped outlet increases the average wind speed of the annular outlet from 14.95 m/s to 18.20 m/s and reduces the proportion of low-speed area from 20.83% to 0.71%. When the rounded corner radius of the air outlet is 50 mm, optimal parameters are attained. The maximum error between the simulated and experimental values is 9.52%. At a significance level of 0.05, the χ2 value is 0.252, indicating that the simulated values follow the distribution of the actual measurement values. On the cross-sections located at distances of 0.5, 0.75, 1, 1.25, and 1.5 m from the air outlet, the wind speed distribution with no arc-shaped air outlets exhibits a “low left and high right” type, tending to shift towards the right as a whole. Fog droplets also display a drift tendency towards the right side. The wind speed distribution with arc-shaped air outlets shows a symmetric “high in the middle and low on the sides” type. Fog droplets concentrate in the central position. The optimized air-assisted system can reduce the air field’s low-flow area, increase the airflow distribution uniformity, improve the average wind speed at the outlet, and decrease fog droplet drift. This provides a reference for the structural design of air-assisted systems in current orchard sprayers of the same type.

A three-dimensional Moho depth model beneath the Yemeni highlands and rifted volcanic margins of the Red Sea and Gulf of Aden, Southwest Arabia

Knowing Moho discontinuity undulation is fundamental to understanding mechanisms of lithosphere-asthenosphere interaction, extensional tectonism and crustal deformation in volcanic passive margins such as the study area, which is located in the southwestern corner of the Arabian Peninsula bounded by the Red Sea and the Gulf of Aden. In this work, a 3D Moho depth model of the study area is constructed for the first time by inverting gravity data from the Earth Gravitational Model (EGM2008) using the Parker-Oldenburg algorithm. This model indicates the shallow zone is situated at depths of 20 km to 24 km beneath coastal plains, whereas the deep zone is located below the plateau at depths of 30 km to 35 km and its deepest part coincides mainly with the Dhamar-Rada’a Quaternary volcanic field. The results also indicate two channels of hot magmatic materials joining both the Sana’a-Amran Quaternary volcanic field and the Late Miocene Jabal An Nar volcanic area with the Dhamar-Rada’a volcanic field. This conclusion is supported by the widespread geothermal activity (of mantle origin) distributed along these channels, isotopic data, and the upper mantle low velocity zones indicated by earlier studies.©2023 China Geology Editorial Office.

Imaging high-resolution 3D shallow crustal structure by ambient noise method in the Shiziyang Waterway area, Guangzhou, South China

The Shiziyang Waterway, located in the northern continental margin of the South China Sea, has developed a complex fault system owing to the coupling action of the oceanic crust and continental crust. However, the geological structure in the shallow crust beneath the Shiziyang Waterway and the relationship between faults and waterway development are still unclear, resulting in difficulty in assessing the earthquake risk to the surrounding cities. In this study, based on a high-density short-period seismic array that was recently deployed around the Shiziyang Waterway, a high-resolution shear wave velocity model is inverted to as shallow as 4 km by the ambient noise tomography method. The results showed that at depths ranging from 0.2 to 4 km, three sets of low-velocity zones (LVZs), trending east–west, northeast, and northwest, are revealed, which are speculated to be the Shougouling fault (SGLF), Xinhui-Shiqiao fault (XSF), and Shiziyang fault bundle (SZYFb), respectively. The shear-wave velocity anomalies illustrate that the SGLF stopped extending east at the north endpoint of the Wenchong-Zhujiangkou fault (WZF). The two-stage differential vertical movements of the WZF, Nangang-Humen fault (NHF), and Hualong-Haioudao fault (HHF) dominated the opening of the Shiziyang Waterway. This study provides new seismological evidence of the opening process for the Shiziyang Waterway and reveals that internal fault activity dominated the development and evolution of the Pearl River Delta.

Constraints and insights into the tectonics of the Bayan Obo deposit from the shallow velocity structure

The Bayan Obo deposit in Inner Mongolia is a crucial source of rare earth elements-niobium-iron, with significant economic importance for China. The ore-bearing dolomite is believed to have originated from mantle-derived magma, indicating high prospectivity in the deeper sections. However, the deep distribution of the ore-bearing dolomite remains poorly understood. Here, we develop a 3D S-wave velocity ([Formula: see text]) structure down to 2.5 km depth beneath the Bayan Obo using ambient noise tomography techniques, which used 25 days of continuous waveform data recorded by 312 short-period seismometers. Combined with previous geological, geochemical, and geophysical studies, our results reveal the possible 3D spatial distribution of the ore-bearing dolomite beneath the Bayan Obo. The results indicate that the [Formula: see text] structures beneath the main, east, and west pits change from relatively high-velocity (high-V) zones (with [Formula: see text] of approximately 2.8–3.2 km/s) at shallow depths (depth to approximately 1.5–1.8 km) to significant low-velocity (low-V) zones (with [Formula: see text] < 2.6 km/s) at deeper depths (depth below approximately 1.8 km). We suggest that the content of ore-bearing dolomite may decrease significantly at deeper depths (depth below approximately 1.8 km) beneath these pits, which possibly indicates a diminished prospecting potential there. Our results also indicate a significant high-V zone (with [Formula: see text] > 3.4 km/s) at depths of approximately 1–2.5 km between the main and west pits. Its lithology remains unclear and is proposed to be a key scientific issue in the future to improve our understanding of the ore-controlling structure associated with the tectonic events in the Bayan Obo deposit.

Experimental and numerical studies on capture of SO3 aerosols from flue gas using finned tube condenser with vortex generators

A heterogeneous condensation device of finned tube condenser equipped with vortex generators has been proposed for capturing SO3 aerosols and moisture from saturated flue gas. An original numerical model for simulating the growth and capture behavior of SO3 aerosols was proposed and verified. The simulation results show that the mean diameter and capture efficiency all increase gradually along the mainstream direction. The VGs can induce a large number of low-velocity recirculation zones on the fin surface, thereby promoting the agglomeration and deposition of SO3 aerosols. The experimental results show that the maximum number and mass capture efficiencies reached 81.1% and 40.1% respectively at SO3 concentration of 30 mg/m3. Besides, the rear end of the condenser has the smallest condensate volume and the lowest condensate pH value, which makes this part more susceptible to corrosion. Moreover, the effects of main parameters were also explored to provide references for engineering applications.

Seismic characterisation of multiple BSRs in the Eastern Black Sea Basin

Long offset seismic reflection data reveal the presence of four Bottom Simulating Reflectors (BSR0-3) within folded sediments of the Tuapse Trough, along the NE margin of the Eastern Black Sea Basin (EBSB). Multiple BSRs are observed in other sites worldwide, however, their origin and formation mechanisms are still debated. Here, we investigate the formation mechanisms of the EBSB multiple BSRs based on their seismic character and on their physical properties derived from reflected and refracted arrival seismic velocities. Seismic reflection data are downward continued to enhance refracted arrivals. A 2D travel-time velocity model of the sub-seabed, using combined travel-times from non-downward-continued reflected and downward-continued refracted signals, shows variations in the physical properties at the BSRs and nearby sediments. The P-wave velocity (VP) increase of 1.55–1.72 km/s between the seafloor and BSR0 (258 mbsf) reflects normal compaction trends in sediments, whereas the VP of 1.75–1.83 km/s between BSR0 and BSR1 (360 mbsf) is higher than that expected for sediments at that depth. Beneath BSR1, a VP decrease from 1.83 km/s to 1.61 km/s occurs within a 70-80 m-thick layer including BSR2 (395 mbsf) and extending to BSR3 (438 mbsf). Beneath BSR3, VP increases. Based on an analytical model linking seismic velocity to physical properties, these VP trends can be explained by a gas hydrate saturation from 0 to 2% between the seafloor and BSR0, reaching 4 ± 2% just above BSR1. A free gas saturation of up to 20–25% is estimated within the low-velocity zone between BSR1 and BSR3. BSR1 likely represents the present-day base of the gas hydrate stability zone (BGHSZ), which aligns with the theoretical BGHSZ assuming a geothermal gradient of 26–30 °C/km. Based on seismic polarities and results from travel-time analysis and rock physics modelling, we suggest that hydrate dissociation and recycling processes may explain the negative polarity of BSR2 and BSR3, which are still visible due to the presence of relict gas, and inferred higher gas hydrate saturations close to the present-day base of the stability zone at BSR1. Also, structural and stratigraphic controls seem to have favoured focused free gas flow and hydrate formation at the top of an anticlinal structure, thus likely controlling multiple BSR generation in the EBSB.

Extent of the lower crustal flow constrained by the rheological structure at the eastern boundary of the Chuan-Dian Rhombus Block

The Chuan-Dian Rhombus Block is the main channel for materials escaping southeastward from the Tibetan Plateau. Verifying the extent of the lower crustal flow and its impact on the lithospheric deformation, isostatic state, and seismicity in the Chuan-Dian Rhombus Block is of vital importance. Based on data from the China Seismic Experimental Site, the current study analyzes the rheological structure of the eastern boundary of the Chuan-Dian Rhombus Block (i.e., the Xianshuihe–Xiaojiang Fault System). The results reveal a low-viscosity zone of 1020 Pa∙s, with a thickness of approximately 20 km in the middle–lower crust beneath the Xianshuihe fault. The low-viscosity zone extends southward and gradually thins until it basically disappears at the Xiaojiang fault. Our results indicate that the extent of the lower crustal flow is restricted in the Xianshuihe fault and the northernmost part of the Anninghe–Zemuhe fault and does not extend to the Xiaojiang fault. The crust of the Xianshuihe fault is notably thickened and in an isostatic disequilibrium state. The Xiaojiang fault maintains a relatively thick seismogenic layer and isostatic equilibrium.

Effects of rotating inlet distortion on the stall mechanism of an axial flow compressor

To study the influence of rotating inlet distortion on a single-stage axial compressor, both experiment and full-annulus unsteady simulation methods are carried out to reveal the stall mechanism under different inflows. First, the experimental results show that compared with static distortion, rotating distortion reduces the compressor stall margin more seriously. The wavelet analysis of dynamic pressure data shows that although the addition of inlet distortions has changed the stalling process, the compressor still stalls through spike-type. The full-annulus unsteady simulations indicate that the low-velocity zone first appears at the hub under uniform inflow condition. When the compressor is subjected to rotating distortion, the low-velocity zone first appears at the tip region. Time-averaged solutions of unsteady simulations are further studied to reveal the underlying mechanisms. Compared with the flow field under uniform inflow, the addition of rotating distortion can produce co-swirl, but it also causes the change of axial velocity distribution along the radial direction. Under the combined effect of both, the blade loading of rotor tip increases significantly, altering the position where the low-velocity zone first appears.

Hydrothermal activity and metallogenic environment revealed by seismic wave tomography in Yunnan Province, China

S-wave velocity structures are revealed using double-difference tomography, which shows large-scale low-velocity anomalies around the Xiaojiang and Lijiang−Xiaojinhe fault zones at 10−25 km depth. In contrast, an obvioushigh-speed anomaly exists in the center of Emeishan area, which is consistent with the EmeishanLarge Igneousprovince. Combined with the geological and geophysical data, the low-velocity anomaly might relate to the lower crustal flowof central Tibetan Plateau. The high-velocity anomaly associated with mantle plume in Emeishan would obstruct the crustal flow and limit it around the large deep faults. Meanwhile, the results also show that many mineral resources are mainly concentrated on the major faults and low-velocity zones, indicating that the lower crustal flow and the large deep faults provide conditions for mineral fluids and minerals deposit.

IMPRINTS OF PLUME PROCESSES IN THE MODERN LITHOSPHERIC VELOCITY STRUCTURE OF THE KHIBINO-LOVOZERO REGION

The paper presents new deep velocity of the earth’s crust and upper mantle in the Khibiny-Lovozero tectonic region located on the Kola Peninsula. The artifacts of plume-lithospheric interactions that occurred in the investigated area from the Proterozoic to the Paleozoic time are discussed. A detailed comparison of the velocity structure of the Imandra-Varzug rift, the Khibiny and Lovozero plutons, and the adjacent Archean part of the Fennoscandinavian Shield has been carried out. A significant heterogeneity of the structure of the crust, upper mantle, as well as the structure and depth of the crust-mantle transition is shown. The presence of a zone of low seismic velocities in the upper mantle associated with the mid-lithospheric discontinuity (MLD) has been revealed. Structural features of the Moho zone and the upper mantle in the region of the Khibiny and Lovozero plutons were interpreted as artifacts of the plume process.

The composition and redox state of hydrous partial melts generated at the top of the lower mantle

Seismic low-velocity zones, that are proposed to form through hydrous partial melting, have been observed at the top of the lower mantle. The buoyancy of such melts will depend significantly on their iron content, but little is known about how iron partitions into melts under these conditions. In this study, ferrous and ferric iron partitioning between bridgmanite and melt has been studied at 25 GPa and 1973 K. A model, based on these data, indicates that increasing oxygen fugacity and the Al content of bridgmanite can change the mineral–melt partitioning of iron from incompatible to compatible. Hydrous partial melts at the top of a pyrolitic lower mantle are calculated to have an Fe3+/∑Fe ratio of 0.26–0.5 and a total iron content of 5.9–7.8 wt. % in the fO2 range between IW+2 and IW+4. Such melts would be less dense than the surrounding lower mantle and would migrate upwards into the transition zone. This could facilitate the formation of superdeep diamonds and crystallize an assemblage that also contains ferropericlase as the melts lose water to the surrounding ringwoodite.