Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.     

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.     

深地震测深揭示的华南地区地壳结构及其动力学意义

20世纪70年代以来,我国实施与华南地区有关的深地震测深剖面达57条.本文收集该区的深地震测深研究成果,利用保真能力强的三维克里金插值技术构建了100°E~125°E,18°N~34°N区域内的三维地壳速度模型.基于三维地壳结构模型,分别探讨了华南地区不同构造单元地壳厚度空间变化特征、地壳属性、上地幔顶部地震波速变化以及太平洋向欧亚板块的俯冲方向等.研究结果表明:(1)扬子地块平均地壳厚度为40 km左右,地壳平均速度为6.30 km/s;华夏地块平均地壳厚度32 km左右,地壳平均速度6.24 km/s.(2)扬子地块的四川盆地与全球地台区具有相似的地壳速度-深度变化特征,华夏地块与全球伸展区结构相似,台湾造山带具有较典型全球大陆弧的特点.(3)华夏地块上地幔顶部Pn波平均速度为8.02 km/s;台湾造山带Pn波平均速度为7.98 km/s;扬子地块的Pn波速度为7.94 km/s,包含四川盆地(Pn速度达8.02 km/s)和川滇地区(Pn速度最低为7.75 km/s).川滇地区和松潘—甘孜褶皱带东部构造活动性较强,四川盆地和华夏地块相对较稳定.(4)推测太平洋板块向欧亚板块俯冲的方向为NW-SE方向.     

Hydrothermal system beneath Aso volcano as inferred from self-potential mapping and resistivity structure

We conducted self-potential (SP) surveys sequentially from part to part over the central cones of Aso volcano since August 1998 by December 2001. The compiled SP map revealed large SP anomalies on the central cones. The main feature of the SP map is a ‘W-shaped’ profile along the NS-transect over the central cones. It is probable that this characteristic SP profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect. A positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect.To evaluate this SP anomaly, we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three-dimensional resistivity structure. A layered structure obtained from a time-domain electromagnetic (TDEM) field experiment was used for the resistivity model. The estimated current source is 300 A, being located in a conductive layer around the sea level. Meanwhile, sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer.Water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of Nakadake. This value was used to estimate the equivalent current in either case of electro-kinetic (EK) [Mizutani, H., Ishido, T., 1976. A new interpretation of magnetic field variation associated with the Matsushiro earthquakes, J. Geomag. Geoelectr., 28, 179–188.] or rapid fluid disruption (RFD) process [Johnston, M.J.S., Byerlee, J.D., Lockner, D., 2001. Rapid fluid disruption: A source for self-potential anomalies on volcanoes, J. Geophys. Res. 106(B3), 4327-4335.]. This comparison suggests that the former process is preferable to explain the observed SP anomaly. From these results we infer a large-scale hydrothermal system beneath the central cones of Aso volcano, in which the fluid flow initiates from the surrounding area, converging to the central vent to transport the heat and materials up to the crater lake of Nakadake through a vapor-filled conduit.     

远震剪切波分裂研究华南西南部上地幔各向异性特征

华南板块由扬子克拉通和华夏地块在新元古代碰撞拼合形成, 其西南部的缝合带位置存在强烈争议.本文基于一条南北向宽频带流动台阵天然地震数据, 利用SKS波分裂研究华南西南部上地幔各向异性特征的南北向变化.分裂测量结果显示, 26°N以南的右江盆地快波偏振方向为E-W或NEE-SWW方向, 分裂时差为0.5~2.5 s, 变化范围较大; 川东褶皱带存在较多空解, 快波偏振方向以近东西向为主, 分裂时差在0.5~1.5 s范围内变化; 在右江盆地北缘和川东褶皱带的过渡区域(约26°N附近)各向异性明显较弱.结合前人的上地幔速度成像结果, 我们认为川东褶皱带与右江盆地过渡带是扬子克拉通的南边界, 在其下方北侧的川东褶皱带巨厚的岩石圈向南转变成较薄的岩石圈, 陡变的岩石圈边界引起了软流圈上涌, 软流圈的垂向运动导致其水平方位各向异性较弱.

     

Magma supply and storage at Kilauea volcano, Hawaii, 1956–1983

Shallow crustal magma reservoirs beneath the summit of Kilauea Volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano's summit. Our model builds on the well-documented pattern of summit inflation as magma accumulates in a shallow summit reservoir, followed by deflation as magma is discharged to the surface or into the rift zones. Magma supply to the summit reservoir is thus proportional to summit uplift, and supply to the rift zones is proportional to summit subsidence; the average proportionality constant is 0.33 × 10⁶ m³/γrad. This model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones.Calculations suggest that magma was supplied to Kilauea during July 1956– April 1983 at a minimum average rate of 7.2 × 10⁶ m3/month. Roughly 35% of the net supply was extruded; the rest remains stored within the volcano's east rift zone (55%) and southwest rift zone (10%). Periods of relatively rapid supply were associated with the large Kapoho eruption in 1960 and the sustained Mauna Ulu eruptions in 1969–1971 and 1972–1974. Bursts of harmonic tremor from the mantle beneath Kilauea were also unusually energetic during 1968–1975, suggesting a close link between Kilauea's deep magma supply region and shallow storage reservoirs. It remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply, or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source.     

Two overlapping plates subducting beneath central Japan as revealed by strontium isotope data

In central Japan, the Pacific plate subducts westward beneath the Eurasian plate and the Philippine Sea plate subducts northwestward into the mantle wedge between the Eurasian plate and the subducted Pacific slab. There, the Northeast Japan arc is joined to the Izu-Ogasawara arc. We determined ⁸⁷Sr/⁸⁶Sr ratios and Rb and Sr contents for 47 volcanic rock samples from 15 Quaternary volcanoes in central Japan and summarized the geographical distribution of the ratios. The general trend of slowly increasing ⁸⁷Sr/⁸⁶Sr ratio from the back-arc side toward the volcanic front in the Northeast Japan arc is broken by a marked high ratio (above 0.7060) centered around Akagi volcano located at the southernmost region of the arc. Elsewhere, the ratio along the volcanic front in this arc varies within the range 0.7038 to 0.7045. The marked high ⁸⁷Sr/⁸⁶Sr ratio is considered to be due to the addition of slab-derived components transported by the Philippine Sea plate to the magma-generating region in the mantle wedge beneath central Japan. Therefore, the geographical distribution of the high ratio may correspond to that of the Philippine Sea slab-derived components in the mantle wedge and we may draw the underground outline of the Philippine Sea plate. This outline implies that an aseismic portion of the Philippine Sea plate continues a few tens km ahead of the seismic one. A belt of low ⁸⁷Sr/⁸⁶Sr ratios from the Izu Peninsula northwestward along the northern end of the Izu-Ogasawara arc coincides with the zone where the subducting Philippine Sea plate is not observed seismologically, while it is detected seismologically on both sides of the belt.     

Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers

The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR) derived from soil thicknesses between ¹⁴C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K₂O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous. The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene, each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber, represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and relatively low eruption frequency.     

Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan)

Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943–1945: Showa-Shinzan and 1977–1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60±10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).     

Magma convection and mixing dynamics as a source of Ultra-Long-Period oscillations

Many volcanic eruptions are shortly preceded by injection of new magma into a pre-existing, shallow (<10 km) magma chamber, causing convection and mixing between the incoming and resident magmas. These processes may trigger dyke propagation and further magma rise, inducing long-term (days to months) volcano deformation, seismic swarms, gravity anomalies, and changes in the composition of volcanic plumes and fumaroles, eventually culminating in an eruption. Although new magma injection into shallow magma chambers can lead to hazardous event, such injection is still not systematically detected and recognized. Here, we present the results of numerical simulations of magma convection and mixing in geometrically complex magmatic systems, and describe the multiparametric dynamics associated with buoyant magma injection. Our results reveal unexpected pressure trends and pressure oscillations in the Ultra-Long-Period (ULP) range of minutes, related to the generation of discrete plumes of rising magma. Very long pressure oscillation wavelengths translate into comparably ULP ground displacements with amplitudes of order 10⁻⁴–10⁻² m. Thus, new magma injection into magma chambers beneath volcanoes can be revealed by ULP ground displacement measured at the surface.     

Magma storage conditions beneath Dabbahu Volcano (Ethiopia) constrained by petrology, seismicity and satellite geodesy

A variety of methods exist to constrain sub-volcanic storage conditions of magmas. Petrological, seismological and satellite geodetic methods are integrated to determine storage conditions of peralkaline magmas beneath Dabbahu Volcano, Afar, Ethiopia. Secondary ion mass spectrometry (SIMS) analysis of volatile contents in melt inclusions trapped within phenocrysts of alkali feldspar, clinopyroxene and olivine from pantellerite obsidians representing the youngest eruptive phase (<8?ka) show H₂O contents ≤5.8?wt.% and CO₂ contents generally below 500?ppm, although rarely as high as 1,500 ppm. Volatile saturation pressures (at 679–835°C) are in the range 43–207?MPa, consistent with published experimental data for similar pantellerites, which show that the phenocryst assemblage of alkali feldspar + cpx + aenigmatite ± ilmenite is stable at 100 to 150?MPa. Inferred magma storage depths for these historic eruptions are ~1–5?km below sea-level, consistent with the depths of earthquakes, associated with magma chamber deflation following a dyke intrusion in the period Oct 2005–Apr 2006. Interferometric synthetic aperture radar (InSAR) data for the same period reveal a broad ~20?km diameter area of uplift. Modelling of different geometries reveals that a series of stacked sills over a 1–5?km depth range best matches the InSAR data. The consistency of depth estimates based on petrological study of ancient eruptions and the seismicity, inflation and deflation of Dabbahu observed in relation to the dyking event of 2005, suggest a small but vertically extensive and potentially long-lived magma storage region.     

A preparation zone for volcanic explosions beneath Naka-dake crater,Aso volcano,as inferred from magnetotelluric surveys

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.     

用转换函数方法研究腾冲—临沧地区地壳结构

根据流动数字地震台网提供的三分量地震波形记录资料,应用转换函数及快速模拟退火算法对腾冲－临沧地区30个地震台站下的地壳横波速度结构进行了反演.反演结果说明,研究区壳幔边界清晰、莫霍界面附近速度跳跃明显,由此得出该区地壳厚度在40 km左右、并具有从南向北增厚趋势.一个普遍的现象是,在腾冲-宝山地块下地壳存在明显的低速带,低速带的厚度在10～20 km间.研究结果进一步表明各台站下方上地幔速度结构复杂.这些结果为探讨青藏高原东南缘下地壳的侧向黏性流动、碰撞板块边界处壳幔物质交换等均提供了重要的地球物理证据,为探讨印-藏汇聚过程中青藏高原东构造结岩石圈变形、高原隆升及其深部动力学有一定理论意义.     

Fine S wave velocity structure beneath Iwate volcano, northeastern Japan, as derived from receiver functions and travel times

A genetic algorithm inversion of receiver functions derived from a dense seismic network around Iwate volcano, northeastern Japan, provides the fine S wave velocity structure of the crust and uppermost mantle. Since receiver functions are insensitive to an absolute velocity, travel times of P and S waves propagating vertically from earthquakes in the subducting slab beneath the volcano are involved in the inversion. The distribution of velocity perturbations in relation to the hypocenters of the low-frequency (LF) earthquakes helps our understanding of deep magmatism beneath Iwate volcano. A high-velocity region (dV_S/V_S=10%) exists around the volcano at depths of 2–15 km, with the bottom depth decreasing to 11 km beneath the volcano’s summit. Just beneath the thinning high-velocity region, a low-velocity region (dV_S/V_S=−10%) exists at depths of 11–20 km. Intermediate-depth LF (ILF) events are distributed vertically in the high-velocity region down to the top of the low-velocity region. This distribution suggests that a magma reservoir situated in the low-velocity region supplies magma to a narrow conduit that is detectable by the hypocenters of LF earthquakes. Another broad low-velocity region (dV_S/V_S=−5 to −10%) occurs at depths of 17–35 km. Additional clusters of deep LF (DLF) events exist at depths of 32–37 km in the broad low-velocity zone. The DLF and ILF events are the manifestations of magma movement near the Moho discontinuity and in the conduit just beneath the volcano, respectively.     

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano,Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4–0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H₂O contents of >∼0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H₂O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H₂O content of 3.1 wt% establishes the maximum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H₂O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO₂ (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open-system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressurization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.     

Discovery of a huge sector collapse at the Nisyros volcano,Greece, by on-land and offshore geological-structural data

This study uses on-land and offshore geological and structural data to demonstrate that a huge lateral collapse involved the SE flank of Nisyros volcano. The collapse beheaded the summit part of the volcano and also involved the submarine portion of the slope, producing a large debris avalanche deposit with a volume of about 1 km³ which has been recognized on the sea floor. On-land, stratigraphic and structural data indicate that a thick succession of lava flows (Nikia lavas) was emplaced in a huge horseshoe-shaped depression open seaward and extending below the sea. The magma-feeding system in the volcano, pre-dating and following the collapse, was structurally influenced by a dominant NE–SW direction, which is perpendicular to the newly-recognised sector collapse. The NE–SW structural trend is consistent with the regional tectonic structures found offshore around Nisyros and with the related NW–SE extension direction. We suggest that the lateral magma pressure produced by repeated magma injections along tectonic discontinuities contributed to destabilise the volcano flank. The occurrence of a pyroclastic deposit that mantled the scar left by the collapse suggests that a magma batch might have been injected inside the volcano and triggered the collapse. The lavas of the pre-collapse edifice have been deposited in alternating submarine and subaerial environments, suggesting that vertical movements might also be a major triggering mechanism for large lateral collapses. Recognition of this phenomenon is particularly important in recent/active island or coastal volcanoes, as it can trigger tsunamis.     

Internal sedimentary architecture and coastal dynamics as revealed by ground penetrating radar, Kachchh coast, western India

The coastline constitutes a very sensitive geomorphic domain which is constantly subjected to dynamic coastal processes and stores vital information regarding past sea level fluctuations. A ground-penetrating radar (GPR) survey was carried out along the northern coast of the Gulf of Kachchh which is one of the largest macrotidal inlets of the Arabian Sea, Western India. Our studies have delineated several radar surfaces and radar facies which reflect the internal architecture and sediment body geometry, which can be related to the processes acting along this coastline. Various radar facies, namely, beach ridge (Br), washover (Wo), coastal dune (Cd), swale (Sw), berm plain (Bp), and sandsheet facies (Ss) have been identified. The GPR studies successfully documented the subsurface presence of ancient beach ridge system towards the sea, and the coastal dunes towards the land side. The results are suggestive of signatures of changes in sea level and the coastline being prone to high energy events in the recent past. The GPR has been found to be an important non-invasive geophysical tool in the study of past coastal dynamics.     

Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana

To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west.