High-Speed Observations of Sprite Streamers

Sprites are optical emissions in the mesosphere mainly at altitudes 50–90 km. They are caused by the sudden re-distribution of charge due to lightning in the troposphere which can produce electric fields in the mesosphere in excess of the local breakdown field. The resulting optical displays can be spectacular and this has led to research into the physics and chemistry involved. Imaging at faster than 5,000 frames per second has revealed streamer discharges to be an important and very dynamic part of sprites, and this paper will review high-speed observations of sprite streamers. Streamers are initiated in the 65–85 km altitude range and observed to propagate both down and up at velocities normally in the 10⁶–5 × 10⁷ m/s range. Sprite streamer heads are small, typically less than a few hundreds of meters, but very bright and appear in images much like stars with signals up to that expected of a magnitude ?6 star. Many details of streamer formation have been modeled and successfully compared with observations. Streamers frequently split into multiple sub-streamers. The splitting is very fast. To resolve details will require framing rates higher than the maximum 32,000 fps used so far. Sprite streamers are similar to streamers observed in the laboratory and, although many features appear to obey simple scaling laws, recent work indicates that there are limits to the scaling.     

Initiation of CMEs: A review

Solar coronal mass ejections (CMEs) are a striking manifestation of solar activity seen in the solar corona, which bring out coronal plasma as well as magnetic flux into the interplanetary space and may cause strong interplanetary disturbances and geomagnetic storms. Understanding the initiation of CMEs and forecasting them are an important topic in both solar physics and geophysics. In this paper, we review recent progresses in research on the initiation of CMEs. Several initiation mechanisms and models are discussed. No single model/simulation is able to explain all the observations available to date, even for a single event.     

Intricately Braided River Channels of Lowland Rivers: Formation Conditions,Morphology, and Deformation

The types of braided channels of major rivers that show the most complex morphology and rearrangement are considered. The formation conditions of these channels are analyzed, and a relationship accounting for both the hydrological and morphological properties is proposed. The parameter of this relationship is demonstrated to distinguish each type of braiding.     

Morphology and sedimentology of a microtidal beach with beachrocks: Vatera,Lesbos, NE Mediterranean

The present contribution considers the dynamics of beaches occupied by outcropping/buried beachrocks, i.e. hard coastal formations consisting of beach material lithified by in situ precipitated carbonate cements. The dynamics of a Greek microtidal beach with beachrocks (Vatera, Lesbos) are examined through the collection and analysis of morphological and sedimentary field data, a 2-D nearshore hydrodynamic model and a specially constructed 1-D morphodynamic model. The results showed that the beachrock-occupied part of the beach is characterised by distinctive morphodynamics as: (i) its beachface is associated with large slopes; (ii) there is a good spatial correlation between the sub-aerial and shallow submerged mean beach profile and the buried/outcropping upper beachrock surface; and (iii) the seaward margins of the submerged beachrock outcrops are always associated with a ‘scour step’ i.e. a submerged cliff. The results also showed that beachrock outcrops can bias cross-shore sediment exchanges by impeding onshore transport due to the presence of the scour step. In this sense, beachrock outcrops may be considered as offshore transport ‘conduits’ for the beach sediments. A conceptual model of beach sediment transport, based on the field data and the hydrodynamic modelling is proposed. According to this model, fresh beach material from adjacent terrestrial sources is transported alongshore, towards the central part of the embayment, where a littoral transport convergence zone occurs under most wave conditions. There, the laterally supplied sediments are lost offshore.     

Initiation and evolution of the South China Sea: an overview

Different models have been proposed for the formation and tectonic evolution of the South China Sea(SCS), including extrusion of the Indochina Peninsula,backarc extension, two-stage opening, proto-SCS dragging,extension induced by a mantle plume, and integrated models that combine diverse factors. Among these, the extrusion model has gained the most attention. Based on simpli?ed physical experiments, this model proposes that collision between the Indian and Eurasian Plates resulted in extrusion of the Indochina Peninsula, which in turn led to opening of the SCS. The extrusion of the Indochina Peninsula, however, should have led to preferential opening in the west side of the SCS, which is contrary to observations. Extensional models propose that the SCS was a backarc basin, rifted off the South China Block. Most of the backarc extension models, however, are not compatible with observations in terms of either age or subduction direction. The two-stage extension model is based on extensional basins surrounding the SCS. Recent dating results indeed show two-stage opening in the SCS, but the Southwest Subbasin of the SCS is much younger, which contradicts the two-stage extension model. Here we propose a re?ned backarc extension model. There was a wide Neotethys Ocean between the Australian and Eurasian Plates before the Indian-Eurasian collision. The ocean ?oor started to subduct northward at *125 Ma, causing backarc extension along the southern margin of the Eurasian Plate and the formation of the proto-SCS. The Neotethys subduction regime changed due to ridge subduction in the Late Cretaceous, resulting in fold-belts, uplifting, erosion, and widespread unconformities. It may also have led to the subduction of the proto-SCS. Flat subduction of the ridge may have reached further north and resulted in another backarc extension that formed the SCS. The rollback of the?at subducting slab might have occurred *90 Ma ago; the second backarc extension may have initiated between 50 and 45 Ma. The opening of the Southwest Subbasin is roughly simultaneous with a ridge jump in the East Subbasin, which implies major tectonic changes in the surrounding regions, likely related to major changes in the extrusion of the Indochina Peninsula.     

Crude Oil at the Bemidji Site: 25 Years of Monitoring,Modeling, and Understanding

The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump‐and‐skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore‐scale capillary pressure‐saturation hysteresis and the presence of fine‐grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound‐specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field‐scale reaction rates and hydrocarbon mass balance. Long‐term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap‐shot study of a hydrocarbon plume may not provide information that is of relevance to the long‐term behavior of the plume during natural attenuation.     

Asymmetry of river channel cross-sections: Part I. Quantitative indices

Most river cross-sections display some degree of asymmetry for which there is no adequate quantitative definition. Indices of asymmetry are derived using two principles based on areal differences and maximum depth displacement relative to the channel centreline. The indices are assessed firstly by comparing values for a set of constructed channel shapes which are ranked in order of increasing visual asymmetry, and secondly by analysing a series of 50 cross-sections from a small mountain stream. Two indices, A* and A₂, are probably the most valuable, and could be used to test and generate hypotheses regarding the related changes to cross-sectional and planimetric form which occur during the transition from a straight to a meandering channel, particularly if parallel measures of flow asymmetry are defined. Part II of this paper, concerned with ‘Mode of Development and Local Variation’ will be published in a subsequent issue of this journal.     

The Mid-Rivera-Transform Discordance: Morphology and Tectonic Development

To better define the morphotectonic elements and tectonic development of the Mid-Rivera-Transform Discordance, multibeam bathymetric, seafloor backscatter, multichannel seismic reflection and total field marine magnetic data were collected along the entire Rivera Transform west of 107°W during the BART and FAMEX campaigns of the N.O. L??Atalante conducted in 2002. These data show that, although the transform tectonized zone of the Rivera Transform west of 107°30??W is a single continuous morphologic basin, this basin consists of two distinct morphotectonic domains: an eastern domain which morphologically is a deep rhombochasm within which organized seafloor spreading has occurred, and a western ??leaky transform?? domain. These new data, in conjunction with the results of previous studies, support the idea that the Rivera-Pacific Euler pole is migrating southward towards the eastern half of the Rivera Transform, and further indicate a recent (<0.14?Ma), and most likely ongoing, clockwise reorganization of the principle transform displacement zones of the Rivera Transform west of 108°W. We propose that the Mid-Rivera-Transform Discordance owes its origin to this eastward progressing, clockwise reorganization of the transform segments that is occurring in response to recent changes in Rivera-Pacific relative plate motion.     

Asymmetry of river channel cross-sections: Part II. mode of development and local variation

A five-stage model of the development of cross-sectional asymmetry is proposed in which alternating bar deposition and bed scour are important elements. Quasi-periodic reversals in the sense of asymmetry and a progressive increase in the magnitude of asymmetry at sections which develop an asymmetry initially are major features of the model. Application of Bridge's (1977) model confirms several features of the model and suggests that the degree of asymmetry is directly related to bend curvature. After the initial stages, subsequent changes to the cross-sectional and planimetric form of the channel may be intensified by the developing asymmetry through a system of positive feedbacks in which the levels of form and flow distortion are interrelated. Asymmetry appears to provide an important link between cross-sectional and plan form adjustment. Spatial series of asymmetry obtained for three lengths along a mountain stream contain features predicted by the model. In particular, the dominant peak in spectral density functions occurs at a frequency with a corresponding wavelength close to 4πw which has significance in terms of the meander wavelength-width relationship (Richards, 1976a). This consistency is remarkable considering local differences. In addition, sections in curved reaches tend to be more asymmetric, especially when sited at pools. However, contrary to expectation, channel width is not significantly correlated with asymmetry.     

Initiation of a multiple peat slide on Cuilcagh Mountain,Northern Ireland

In October 1998 a multiple peat slide occurred on the northern slopes of Cuilcagh Mountain, Co. Fermanagh, in response to a high‐magnitude rainfall event. Few peat slides have been recorded in Ireland, and a detailed field survey and investigation of the failure was undertaken within four weeks of the event. The morphological evidence indicated a distinct sequence of events which appeared to begin with the failure of a small segment of slope above a degraded transverse drainage ditch which was cut less than ten years previously. This segment of slope was no more than 42 m wide and 25 m long, with 0·7 m of peat overlying up to 0·5 m of a pale coloured clay, the latter containing small pipes and resting on the surface of a darker coloured loamy material. The failure surface was located at or near the base of the pale clay layer. Finite element software was used to model hydrological conditions within the upper segment of slope and to calculate factors of safety for different slope configurations including the presence or absence of a drain or a subsurface pipe. Using the peak shear strength of the pale clay, as determined in the laboratory, both the drain and subsurface pipes were required to obtain a factor of safety of 1·0 or less. Allowing for the uncertainties associated with the hydrological modelling of the pipes, it is suggested that the cutting of the drain and the hydrological impacts of its subsequent degradation are ultimately responsible for the failure. Copyright © 2001 John Wiley & Sons, Ltd.     

Morphology and emplacement of flows from the Deccan Volcanic Province,India

The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.Most of the studies relating to the emplacement of Continental Flood Basalt (CFB) lavas have relied on observations of flows from the CRB. Much of the current controversy surrounding the emplacement of CFB flows centers around the comparison of Hawaiian lava flows to those from the CRB. We demonstrate that the DVP displays a variety of lava features that are similar to those from the CRB as well as those from Hawaii. This suggests that there may have been more than one mechanism or style for the emplacement of CFB flows. These need to be taken into account before arriving at any general model for flood basalt emplacement.Editorial responsibility: T. Druitt     

Initiation of earthquake-induced slope failure: influence of topographical and other site specific amplification effects

Increased structural damage caused byearthquakes on hilltops and along ridgeshas often been related to amplification ofground motion due to the presence oftopography. However, comparison betweenobservations and numerical modeling hasshown that amplification is only partlydependent on the prominent surfacemorphology. Strong effects are also inducedby soft layers, such as weathered rockmaterial or colluvium, covering thetopographies.Numerous seismically triggered landslidesare reported to occur in the same materialsthat are likely to amplify ground motions.Therefore, it can be suspected that groundmotion dynamics significantly contribute tothe observed slopes failures. Thispotential relationship is the subject ofthe present case study, the Ananevorockslide in the northeastern Tien Shanmountains. The survey included geophysicalprospecting, earthquake recordings andstructural analyses of the rock fabric. Onthe basis of the field data, observedamplification effects could be related tothe local geological conditions andparticularly to the surface morphology andto the presence of low-velocity layers –deeply weathered rocks – on the top of thebedrock. Surface layer- andtopography-dependent amplification has alsobeen studied numerically by 2D and 3Dfinite element modeling of ground motiondynamics. The present paper focuses onadditional effects that may be induced bythe presence of a fault zone and of thelandslide scarp. Further, observed andcomputed ground motion dynamics areconnected with slope failuresusceptibility: 2D numerical simulationsreveal that strain localization is closelyrelated to wave amplification in surficiallow-velocity layers, particularly belowconvex surface morphologies.     

Morphology of Red Creek,Wyoming, an arid-region anastomosing channel system

Red Creek, in the Red Desert area of the Great Divide Basin, Wyoming, is an arid-region anastomosing stream. The narrow, deep, and sinuous main channel is flanked by anastomosing flood channels, or anabranches. Most anabranches are initiated at meander bends. The primary mechanism of anabranch initiation is avulsion during overbank floods. Anabranch enlargement occurs by headward erosion. Anabranches act as distributary channels during floods, when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches. During periods of low discharges, the anabranches act as tributaries to the main channel, transporting runoff from the floodplain and surrounding hillslopes to the main channel of Red Creek. Aggradation is occurring in the main channel and on the floodplain throughout the study reach. Infilling of the main channel occurs primarily by lateral accretion, while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches. Infilling of the main channel may cause avulsion of the main channel into an anabranch. The abandoned main channel segment may then fill completely or act as an anabranch. Because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment, periodic avulsion is the primary form of lateral mobility in the system.     

Initiation of subduction by post-collision foreland thrusting and back-thrusting

While postulated causes of initial subduction and trench formation include underthrusting, controls on its location and age have not been determined. Consideration of the age of subduction zones bordering five collisional orogens suggests that subduction may have been initiated by foreland thrusts and back-thrusts. Foreland thrusts develop within a continental foreland on the subducting plate mostly within 50 my of collision with an arc system; where the foreland is narrow the thrusts may intersect the continent-ocean crust boundary. Back-thrusts develop in the fore-arc or back-arc area on the overriding plate within 10 to 20 my of collision, and can result in tectonic burial of the magmatic arc; where the arc system is oceanic the back-thrusts may intersect the arc-ocean crust boundary. Possible examples of subduction initiated by foreland thrusts are the start of subduction in the late Jurassic beneath the northern Sunda Arc, and at the end-Miocene in the Negros Trench. Examples of back-thrusts which have initiated or may initiate subduction are the late Cenozoic eastward translation of Taiwan over the Philippine Sea plate, the incipient southward subduction of the Banda Sea beneath Timor, and the W-dipping back-thrust comprising the Highland Boundary Fault zone and postulated early Ordovician thrusts to the SE in Scotland. The suggested relationship of subduction to collision helps to explain the persistence of Wilson cycles in the still-active late Mesozoic to Cenozoic orogenic belts and implies that orogeny will cease only with collision between major continents.     

Badland Morphology and Evolution: Interpretation Using a Simulation Model

A drainage basin simulation model is used to interpret the morphometry and historical evolution of Mancos Shale badlands in Utah. High relief slopes in these badlands feature narrow divides and linear profiles due to threshold mass-wasting. Threshold slopes become longer in proportion to erosion rate, implying lower drainage density and higher relief. By contrast, in slowly eroding areas of low relief, both model results and observations indicate that drainage density increases with relief, suggesting control by critical shear stress. Field relationships and simulation modelling indicate that the badlands have resulted from rapid downcutting of the master drainage below an Early Wisconsin terrace to the present river level, followed by base level stability. As a result, Early Wisconsin alluvial surfaces on the shale have been dissected up to 62 m into steep badlands, and a Holocene alluvial surface is gradually replacing the badland slopes which are erocing by parallel retreat. © 1997 by John Wiley & Sons, Ltd.     

Understanding historical coastal spit evolution: A case study from Spurn,East Yorkshire,UK

Globally sandy coastlines are threatened by erosion driven by climatic changes and increased storminess. Understanding how they have responded to past storms is key to help manage future coastal changes. Coastal spits around the world are particularly dynamic and therefore potentially vulnerable coastal features. Therefore, how they have evolved over the last few centuries is of great importance. To illustrate this, this study focuses on the historical evolution of a spit at Spurn on the east coast of the UK, which currently provides critical protection to settlements within the Humber estuary. Through the combination of digitized historical mapping and luminescence dating, this study shows that Spurn has been a consistent coastal feature over at least the past 440 years. No significant westward migration was observed for the last 200 years. Results show a long-term extension of the spit and a decrease in its overall area, particularly in the last 50 years. Breaches of the neck cause temporary sediment pathway changes enabling westward extension of the head. Use of digitized historical maps in GIS combined with OSL dating has allowed a more complete understanding of long-term spit evolution and sediment transport modes at Spurn. In doing so it helps inform future possible changes linked to pressures, such as increases in storm events and sea-level rise. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Morphology and magnetic survey of the Rivera-Cocos plate boundary of Colima,Mexico

The propagation of the Pacific-Cocos Segment of the East Pacific Rise (EPR-PCS) has significantly altered the plate configuration at the north end of the Middle America Trench. This ridge propagation, the collision of the EPR-PCS with the Middle America Trench, the separation of the Rivera and Cocos plates and the formation of the Rivera Transform have produced a complex arrangement of morphotectonic elements in the area of Rivera-Cocos plate boundary, atypical of an oceanic transform boundary. Existing marine magnetic and bathymetric data has proved inadequate to unravel this complexity, thus, a dense grid of total field magnetic data were collected during campaigns MARTIC-04 and MARTIC-05 of the B/O EL PUMA in 2004 and 2006. These data have greatly clarified the magnetic lineation pattern adjacent to the Middle America trench, and have revealed an interesting en echelon, NE-SW oriented magnetic high offshore of the Manzanillo Graben. We interpret these new data to indicate that the EPR-PCS ridge segment reached the latitude (~18.3°N) of the present day Rivera Transform at about Chron 2A₃ (~3.5Ma) and propagated further northward, intersecting the Middle America Trench at about 1.7 Ma (Chron 2). At 1.5 Ma spreading ceased along the EPR north of 18.3°N and the EPR-PCS has since retreated southward in association with a southward propagation of the Moctezuma Spreading Segment. North of 18.3°N the seafloor near the trench has been broken into small, uplifted blocks, perhaps due to the subduction of the young lithosphere generated by the EPR-PCS.     

Initiation and evolution of intra-oceanic subduction in the Uralides: Geochemical and isotopic constraints from Devonian oceanic rocks of the Southern Urals, Russia

Abstract The Southern Uralides are a collisional orogen generated in the Late Devonian–Early Carboniferous by the collision of the Magnitogorsk island arc (MA) generated in the Early to Middle Devonian by intra‐oceanic convergence opposite to the continental margin, and the continental margin of the East European craton. A suture zone of the arc to the continental margin, the Main Uralian Fault (MUF), is marked by ophiolites and exhumed high‐pressure–low‐temperature metamorphic rocks of continental origin. The pre‐orogenic events of the Southern Urals and their geodynamic setting are traced by means of fluid‐immobile incompatible trace elements (rare earth elements and high field strength elements) and Sr–Nd–Pb isotope geochemistry of the MA suites, in particular the protoarc suite with boninites and probably ankaramites, and the mature arc comprised of island arc tholeiitic (IAT) suites, transitional IAT to calc‐alkaline (CA), and CA suites. The MA volcanics result in genetically distinct magmatic source components. In particular, depleted normal‐mid‐oceanic ridge basalt‐type mantle sources with various enrichments in a slab‐derived aqueous fluid component are evident. The enriched component is not involved in significant amounts, as testified by the rather radiogenic Nd isotopes and unradiogenic Pb isotopes. Further information on the pre‐orogenic events is provided by the Mindyak Massif metagabbros derived from diverse gabbroic protoliths that were affected by oceanic rodingitization, and subsequently by a high‐temperature (HT) metamorphism related to the development of a metamorphic sole. The HT metamorphism has the same age as the protoarc volcanism, and constrains the initiation of subduction at approximately 410 Ma. Consequently, the maximum timespan between initial intra‐oceanic convergence and final collision is approximately 31 my, a duration consistent with that of present‐day ongoing collisions in the western Pacific. The characteristics of early volcanism and the traces of a metamorphic sole provide useful criteria to attribute most MUF ophiolites to the Tethyan type with a complex pre‐orogenic evolution.     

Cyclic Variations,Magnetic Morphology,and Complexity of Active Regions in Solar Cycles 23 and 24

Geomagnetism and Aeronomy - In this paper, 2046 active regions of solar cycle 23 and 1507 active regions of solar cycle 24 observed during the period from May 1996 to December 2018 have been...     

Morphology of the Wadati-Benioff zone,andesitic volcanism,and active fracture zones in Central America

Summary The morphology of the Wadati-Benioff zone in the region of Central America, based on the distribution of 1377 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Cocos plate. Four deep seismically active fracture zones, genetically connected with the process of subduction, and three fracture zones manifesting the possible boundary between the Americas and Caribbean plates were identified in the continental wedge.