Lava Field Research Articles

High-level landscapes along the margin of southern East Greenland—A record of tectonic uplift and incision after breakup in the NE Atlantic

Elevated plateaux and deeply incised valleys characterise the large-scale landscapes along the East Greenland margin as in many elevated, passive continental margins around the world. The absence of syn- or post-rift rocks in, for example, the mountains of Norway, hampers the assessment of the age of these landscapes and of the present-day elevation. The mountains of southern East Greenland (68–71°N), however, expose thick basalts that were extruded onto a largely horizontal lava plain near sea level during breakup of the NE Atlantic at the Paleocene–Eocene transition. We take advantage of these favourable geological conditions to investigate the uplift history after continental breakup. In particular, it is clear that present-day elevations of these basalts up to 3.7km above sea level (a.s.l.) were reached after breakup. We have mapped regional erosion surfaces and integrated the information about the landscape with the stratigraphic record (i.e. stratigraphic landscape analysis). The analysis led to the following relative denudation chronology for southern East Greenland: At breakup, the margin subsided and underwent km-scale burial. Around the Eocene–Oligocene transition, the first phase of uplift, tilting and subsequent erosion led to the formation of an extensive, low-relief erosion surface (the Upper Planation Surface, UPS) that was graded towards the base level of the adjacent ocean before the eruption of Miocene lavas onto that surface. A second uplift that most likely occurred after the Miocene produced a new erosion surface (the Lower Planation Surface, LPS) by incision below the UPS. Finally, a third event in the late Cenozoic lifted the UPS and the LPS to their present elevations of up to 3 and 2kma.s.l., respectively and shaped the present-day valleys and fjords by incision of rivers and glaciers below the LPS. The general picture of landscape development is highly similar to West Greenland and the common characteristics between the stepped landscapes in East Greenland and those on the conjugate margin in Scandinavia lead us to conclude that the mountains of Norway also formed after the North Atlantic breakup.

Deep long‐period earthquakes west of the volcanic arc in Oregon: Evidence of serpentine dehydration in the fore‐arc mantle wedge

AbstractHere we report on deep long‐period earthquakes (DLPs) newly observed in four places in western Oregon. The DLPs are noteworthy for their location within the subduction fore arc: 40–80 km west of the volcanic arc, well above the slab, and near the Moho. These “offset DLPs” occur near the top of the inferred stagnant mantle wedge, which is likely to be serpentinized and cold. The lack of fore‐arc DLPs elsewhere along the arc suggests that localized heating may be dehydrating the serpentinized mantle wedge at these latitudes and causing DLPs by dehydration embrittlement. Higher heat flow in this region could be introduced by anomalously hot mantle, associated with the western migration of volcanism across the High Lava Plains of eastern Oregon, entrained in the corner flow proximal to the mantle wedge. Alternatively, fluids rising from the subducting slab through the mantle wedge may be the source of offset DLPs. As far as we know, these are among the first DLPs to be observed in the fore arc of a subduction‐zone system.

The lithosphere–asthenosphere boundary and the tectonic and magmatic history of the northwestern United States

This study explores the properties of the lithosphere–asthenosphere boundary (LAB) and other shallow mantle discontinuities across the diverse geologic provinces of the northwest United States. Sp phases were used to image three-dimensional discontinuity structure by common conversion point stacking with data from 804 temporary and permanent broadband stations, including 247 from EarthScope's USArray Transportable Array. Substantial variations in mantle discontinuity structure are apparent over a variety of spatial and temporal scales. To the west of the Sevier Thrust Belt, a coherent Sp negative phase coincides with the LAB depth range inferred from tomographic models. To the east, within the stable craton, multiple negative phases typically occur in the high velocity lithospheric layer, although in places a single mid-lithospheric discontinuity is imaged. Sub-cratonic LAB phases are often absent, indicating an LAB velocity gradient that is distributed over >50 km and that is consistent with effects of temperature alone. Where weak and intermittent LAB phases appear, they suggest more vertically localized velocity gradients produced by other factors such as bulk composition, volatile content, or contrasts in grain size or melt. In the tectonically active west, a positive Sp phase at depths consistent with the base of the asthenospheric low velocity zone in tomography models is intermittently observed. Beneath magmatic provinces in the west, the LAB Sp discontinuity deepens by ∼10 km from the High Lava Plains, where magmatism has occurred from 0–10.5 Ma, to the northern region of the Columbia River Basalts, which has been magmatically quiet since 15 Ma. Here we suggest a model in which the negative LAB velocity gradient is created by a layer of partial melt ponding beneath a solidus-defined boundary. This model predicts that higher temperatures associated with more recent magmatism would result in a shallowing of the intersection of the geotherm with the solidus. Beneath the Yellowstone Caldera, the absence of an LAB Sp phase suggests that the contrast in seismic velocity between the lithosphere and asthenosphere has been erased by intrusion of partial melt and heat into the lithosphere.

Is the ‘Azores Hotspot’ a Wetspot? Insights from the Geochemistry of Fluid and Melt Inclusions in Olivine of Pico Basalts

The concept of an 'Azores mantle plume' has been widely debated, and the existence of an Azores hotspot questioned. In an effort to shed new light on this controversy, we present He isotope and major, trace and volatile element compositions for basaltic scoriae from five monogenetic cones emplaced along the fissure zone of Pico Island, the youngest island of the Azores archipelago. The bulk scoriae and lavas are moderately alkaline basalts, and their He isotope ratios, determined on olivine crystals, vary between 10*2 and 11*1 ± 0*1 Ra. In contrast, melt inclusions hosted in olivine (Fo76-83*5) span a large range of compositions (K2O = 0*7-1*7 wt %; Ce = 32-65 ppm; Nb = 21-94 ppm), which extends the compositional field of lavas erupted along the Pico fissure zone. This chemical evolution is predominantly controlled by polybaric fractional crystallization. Most melt inclusions share similar enrichments in large ion lithophile and light rare earth elements, and trace element ratios (La/Sm, La/Yb, Sr/Nd, Ta/Th, Zr/Y) with their bulk-rocks. Only a few of them differ in their lower contents of incompatible elements and La/Sm, Li/Ta and Na/K ratios, a feature that is ascribed to distinct conditions of melting. As a whole, the melt inclusions preserve high and variable volatile contents, and contain up to 1*8-2*0 wt % of H2O and 0*4 wt % of CO2. The total fluid pressures, retrieved from the dissolved CO2 and H2O concentrations, and the PCO2 from fluid inclusions, indicate magma ponding and crystallization at the crust-mantle boundary (ca. 18 km deep). The H2O/Cl and H2O/Ce ratios in the inferred parental undegassed basalts of the Pico fissure zone average 0*036 ± 0*006 and 259 ± 21, respectively. The latter value is significantly higher than that reported for typical mid-ocean ridge basalts from the southern Mid-Atlantic Ridge, but is similar to published ratios for submarine undegassed basalts from the Azores platform. Combining the calculated compositions of Pico primary magmas formed by low degrees of melting with recent geophysical data for the Azores, we propose a model for Azores magma generation involving the decompression melting of a water-enriched mantle domain (H2O = 680-570 ppm) with an estimated temperature excess of ≤120°C with respect to the Mid-Atlantic Ridge.

フィリピン諸島火山の地形発達と分類

The evolution of 84 volcanoes in the Philippines is outlined geomorphologically on the basis of interpretations of volcanic landforms using aerial photographs, topographic maps, satellite images, and geomorphological and geological field surveys. Fifty-six stratovolcanoes, three caldera, three lava domes, four scoria cones, four lava fields, and 10 shield volcanoes are identified. Large basaltic volcanoes such as lava fields and shield volcanoes were found unexpectedly in subduction zones. No lava field or shield volcano has been discovered on the Japanese Islands. Among the 34 subduction zones in the world, 19 do not have lava fields or shield volcanoes. Two oblique subduction zones form the Philippine Fault Zone. The fault zone mostly coincides with the eastern volcanic zone. At the southwestern part of Mindanao Island, a volcanic chain, consisting of Bulibu, Basilan, and Cagayan Sulu lava fields, Balatukan, Mangabon, Katangrad, Kalatungan, Makaturing, Bacolod, and Pagayawan shield volcanoes, and Pagadian lava domes (monogenetic volcanoes) trends northeast-southwest, in parallel with the trenches and main arcs. The association of the volcanoes with the trenches and arcs suggests that the volcanic chain is a marginal sea ridge between Sulu Sea and Celebes Sea and that the back-arc basin ridge might have extended under Mindanao Island from Moro Gulf to the northern part of Mindanao Island.

Richard Long’s Passage as Line: Measuring Toward the Horizon

In 1967, artist Richard Long became famous for completing his revolutionary^ Line Made From Walking (Figure 1)-a simple black and white photograph of a straight line etched into the ground. Long's mark managed to underscore the perspectival space of the field as it receded into the distance, while it simultaneously functioned formally, akin to a Barnett Newman zip down the center of the canvas. This was not Long's first attempt to carve his mark onto the surface of the earth, but his earlier experimental work from 1966, Turf Circle, a large circle of grass sliced from its soil and reconstructed into a recessed position, was initially overshadowed by the sheer visual power of the line. Today, it is clear that both Turf Circle and A Line Made From Walking have become the basis for Long's life-long art practice. After decades of trekking in both straight and circular paths, the artist has progressively undertaken more ambitious walks, as is represented by a 1981 piece entitled A Line and Tracks in Bolivia: An Eleven Day Walk Crossing and Re-crossing a Lava Plain. Despite the increased scope, the premise remains the same, with Long etching a line into the land through his repeated movement back and forth over the surface of the earth. The performance of the walk, a ritual wandering, is only later documented through a photograph, marked map, or text work. Though the art object is created as a permanent remembrance of the walk, the traces are always ephemeral, sometimes drawn in the snow, or sometimes signified by upturned stones, and will disappear through the passing of time and exposure to the elements.By making his own human contact with the earth the central focus of the artistic performance, Long marks his own presence and withdrawal from the site (Boetzkes 18). Because his physical body is almost never visible in the documenting photograph, the line alone reveals the artist's existence in the landscape but simultaneously announces his absence from the scene. This seemingly paradoxical insight extends the concept of passage beyond Long's traceable movements from point A to B, with each coming and going gesturing toward the existential. In a 1982 artist's statement, Words After the Fact, published in conjunction with a solo exhibition at the Amolfini Art Gallery in Bristol, Long announced to the world: My work has become a simple metaphor of life. With this highly charged statement, the artist offered up his oeuvre for thoughtful reflection; his artworks are not to be appreciated exclusively on an aesthetic level, but rather with a certain ontological attention.It is significant that Long's artistic breakthrough came during his time at St. Martin's College (1966-1968) as a student in the recently developed Advanced Course. This unique program offered a rather unconventional approach to traditional artistic techniques, and former instructor Peter Atkins recalled of his own curriculum that it was specifically engineered to expose young artists to a larger philosophic, psychological and social context {Heaven and Earth 38). It would seem that the atmosphere was intellectually inspiring for Long, and, in conversation with contemporary curator Clarrie Wallis, he fondly remembered the many hours spent poring over a copy of Bertrand Russell's 1945 History of Western Philosophy (38). It is reasonable to speculate then, that, given his keen interest in both path-making and philosophy, Long might have taken a special interest in the teachings of Martin Heidegger, a twentieth-century German philosopher who has written at least two texts devoted solely to the consideration of paths: Der Feldweg or The Pathway and Holzwege, literally translated as wooded paths but known to an English audience as Off the Beaten Track. Heidegger placed himself within a long trajectory of great thinkers-Hegel, Kant, Rousseau-who recognized the simple act of walking as a form of mediation (Roelstraete 11), asserting further that [t]he pathway remains as close to the step of the thinker as to that of the farmer walking out to the mowing in early morning {The Pathway 33). …

Modeling propagation of infrasound signals observed by a dense seismic network

The long-range propagation of infrasound from a surface explosion with an explosive yield of about 17.6 t TNT that occurred on June 16, 2008 at the Utah Test and Training Range (UTTR) in the western United States is simulated using an atmospheric model that includes fine-scale layered structure of the wind velocity and temperature fields. Synthetic signal parameters (waveforms, amplitudes, and travel times) are calculated using parabolic equation and ray-tracing methods for a number of ranges between 100 and 800 km from the source. The simulation shows the evolution of several branches of stratospheric and thermospheric signals with increasing range from the source. Infrasound signals calculated using a G2S (ground-to-space) atmospheric model perturbed by small-scale layered wind velocity and temperature fluctuations are shown to agree well with recordings made by the dense High Lava Plains seismic network located at an azimuth of 300° from UTTR. The waveforms of calculated infrasound arrivals are compared with those of seismic recordings. This study illustrates the utility of dense seismic networks for mapping an infrasound field with high spatial resolution. The parabolic equation calculations capture both the effect of scattering of infrasound into geometric acoustic shadow zones and significant temporal broadening of the arrivals.

Evolution of a subglacial basaltic lava flow field: Tennena volcanic center, Mount Edziza volcanic complex, British Columbia, Canada

Studies of terrestrial glaciovolcanic deposits have elucidated the utility of these deposits as tools to constrain ice conditions at the time of their emplacement. Very few studies, however, have documented the emplacement of effusion-dominated, basaltic glaciovolcanic eruptions. The Tennena volcanic center (TVC), located at 57° 40′ 57.705″ N 130° 39′ 43.138″ W on the western flank of Mount Edziza volcanic complex (MEVC) in northwestern British Columbia, Canada, preserves a detailed record of interactions between coherent basaltic lavas and a local/regional ice sheet inferred to be associated with the Last Glacial Maximum (LGM). Here we describe the field characteristics of five primary volcanic lithofacies and five associated glaciogenic lithofacies, and discuss their spatial distribution within the volcanic center. We find that 1) the distribution of primary hydrovolcanic clastic deposits (tuff breccias and lapilli tuffs) is confined to Tennena Cone, 2) pillow lavas are present throughout the extent of the surrounding lava field with morphologies that include distinctive structures comprising vertically-oriented, distended pillows, 3) multiple lobes of massive (non-pillowed) lavas that represent high initial magma discharge rates are confined to medial distances from the cone, and 4) associated glaciogenic facies that underlie or onlap the TVC lavas indicate a variable sediment/water ratio during subglacial meltwater drainage at the time of the eruption or shortly afterwards. Analyses of H2O/CO2 in pillow rim samples give broad constraints for emplacement pressures equivalent to 500–1400m of overlying ice. No subaerial lava morphologies are found on the cone or in the proximal, medial and distal lithofacies, and the sequence is interpreted as documenting an eruption of basaltic lava flows beneath either the LGM Cordilleran ice sheet or a Younger Dryas expansion of the still-extant Edziza ice cap. The TVC lavas, especially medial and distal ones, display excellent examples of textures and morphologies that can be used to: 1) help identify other terrestrial and Martian basaltic lava flows emplaced beneath ice and 2) further our understanding on how sub-ice lava flows are emplaced.

A controlled‐source seismic and gravity study of the High Lava Plains (HLP) of Eastern Oregon

This study employs data collected during the High Lava Plains (HLP) controlled‐source seismic experiment conducted in September 2008. In this experiment, 2612 short‐period seismic recorders and 120 3‐component recorders were deployed across eastern Oregon and adjacent parts of Nevada and Idaho to record 15 seismic sources. Seismic and gravity data were integrated to create 2‐D crustal scale P‐wave velocity and density models for the ∼400 km long NW‐SE and N‐S profiles to provide a better understanding of the crustal and upper mantle structure and ultimately the magmatic and tectonic evolution of the region. These models are the first high‐resolution images beneath the path of migratory, bi‐modal volcanism that dotted the High Lava Plains beginning at 16 Ma. Our models show that the crustal structure across the HLP region is similar to that of the northern Basin and Range which has experienced extension since ∼35 Ma but with moderate magmatic modification of the crust. A thick layer (5–7 km) of sediments and volcanics extends over most of the area and is thickest in the Harney Basin area. We interpret denser/faster material in the lower to middle crust under the southern Harney Basin area to be mafic intraplating. We have also identified a region of denser/faster material in the upper crust in the vicinity of Oregon‐Idaho border. The crust thickens from 34 to 37 km, and the lower crust increases in density (2.8–2.85 g/cm3), from west to east across eastern Oregon in close proximity to the interpreted position of the 0.706 Sr isotope line, suggesting a decrease in extension. In the lowermost crust below the southeastern HLP, there is relatively high velocity (7.2–7.4 km/s) and density (2.95 g/cm3) that suggests the presence of underplating. The HLP region has undergone moderate extension, and the average crustal velocity is somewhat higher than in the adjacent Basin and Range, suggesting some magmatic modification in the lower crust, but not as much as might be expected given the voluminous surface volcanism.

Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

We compute high‐resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of ∼45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre‐2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere‐asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present‐day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E‐W seismic anisotropy reported across this region may reduce the effective S‐wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back‐arc and the close casual link between subduction‐related processes and the origin of HLP volcanism.

Late Holocene lava flow morphotypes of northern Harrat Rahat, Kingdom of Saudi Arabia: Implications for the description of continental lava fields

A “lava morphotype” refers to the recognizable and distinctive characteristics of the surface morphology of a lava flow after solidification, used in a similar way to a sedimentary facies. This classification method is explored on an example volcanic field in the Kingdom of Saudi Arabia, where copious lava outpourings may represent an important transition between monogenetic and flood basalt fields. Here, young and well-preserved mafic lava fields display a wide range of surface morphologies. We focussed on four post-4500yrs. BP lava flow fields in northern Harrat Rahat (<10Ma) and propose a framework for describing systematic changes in morphotypes down-flow. The morphotypes give insight into intrinsic and extrinsic parameters of emplacement, rheology and dominant flow behavior, as well as the occurrence and character of other lava structures. The Harrat Rahat lava flow fields studied extend up to 23km from the source, and vary between 1–2m and 12m in thickness. Areas of the lava flow fields are between ∼32 and ∼61km2, with individual flow field volumes estimated between ∼0.085 and ∼0.29km3. They exhibit Shelly-, Slabby-, and Rubbly-pahoehoe, Platy-, Cauliflower-, and Rubbly-a’a, and Blocky morphotypes. Morphotypes reflect the intrinsic parameters of: composition, temperature, crystallinity and volatile-content/vesicularity; along with external influences, such as: emission mechanism, effusion rate, topography and slope control of flow velocity. One morphotype can transition to another in individual flow-units or lobes and they may dominate zones. Not all morphotypes were found in a single lava flow field. Pahoehoe morphotypes are related to the simple mechanical disaggregation of the crust, whereas a’a morphotypes are related to the transitional emergence and subsequent transitional disappearance of clinker. Blocky morphotypes result from fracturing and auto-brecciation. A’a morphotypes (i.e. platy-, cauliflower-, rubbly-a’a) dominate the lava flow field surfaces in northern Harrat Rahat, which suggests that core-dominated flows were predominant during flow movement. Lava structures are well-developed and well-preserved and some may be related to some morphotypes. Down-flow changes exhibit key illustrative and easy recognizable features in the lava flow fields and might provide insights into real-time monitoring of future flows in this region.

Single and Multi-Date Landsat Classifications of Basalt to Support Soil Survey Efforts

Basalt outcrops are significant features in the Western United States and consistently present challenges to Natural Resources Conservation Service (NRCS) soil mapping efforts. Current soil survey methods to estimate basalt outcrops involve field transects and are impractical for mapping regionally extensive areas. The purpose of this research was to investigate remote sensing methods to effectively determine the presence of basalt rock outcrops. Five Landsat 5 TM scenes (path 39, row 29) over the year 2007 growing season were processed and analyzed to detect and quantify basalt outcrops across the Clark Area Soil Survey, ID, USA (4,570 km2). The Robust Classification Method (RCM) using the Spectral Angle Mapper (SAM) method and Random Forest (RF) classifications was applied to individual scenes and to a multitemporal stack of the five images. The highest performing RCM basalt classification was obtained using the 18 July scene, which yielded an overall accuracy of 60.45%. The RF classifications applied to the same datasets yielded slightly better overall classification rates when using the multitemporal stack (72.35%) than when using the 18 July scene (71.13%) and the same rate of successfully predicting basalt (61.76%) using out-of-bag sampling. For optimal RCM and RF classifications, uncertainty tended to be lowest in irrigated areas; however, the RCM uncertainty map included more extensive areas of low uncertainty that also encompassed forested hillslopes and riparian areas. RCM uncertainty was sensitive to the influence of bright soil reflectance, while RF uncertainty was sensitive to the influence of shadows. Quantification of basalt requires continued investigation to reduce the influence of vegetation, lichen and loess on basalt detection. With further development, remote sensing tools have the potential to support soil survey mapping of lava fields covering expansive areas in the Western United States and other regions of the world with similar soilscapes.

Distinctive upper mantle anisotropy beneath the High Lava Plains and Eastern Snake River Plain, Pacific Northwest, USA

The Pacific Northwest (PNW) has experienced voluminous intraplate volcanism over the past ∼17 Ma, beginning with the Steens/Columbia River flood basalts and continuing with the still‐ongoing volcanism in the High Lava Plains (HLP) and eastern Snake River Plain (SRP). Here we present two complementary datasets (SKS splitting and Rayleigh wave phase velocity anisotropy) that place constraints on the anisotropic structure of the upper mantle beneath the HLP and SRP regions. Beneath the HLP, SKS phases reveal dominantly E‐W fast splitting directions and large (up to ∼2.7 s) delay times, with pronounced lateral variations in δt. Lateral and depth variability in the strength of anisotropy beneath the HLP is also evident from Rayleigh wave dispersion. Beneath the SRP, SKS splitting delay times are much smaller (∼0.5 s), and surface wave observations suggest a region of upper mantle anisotropy (∼50–150 km depth) with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the SRP. Beneath the HLP, the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle, resulting in constructive interference and large SKS splitting delay times. Beneath the SRP, the geometry of the anomalous anisotropic region in the shallow mantle is different, resulting in destructive interference and reduced SKS splitting delay times. We discuss several possible explanations for these observations, including variations in olivine lattice‐preferred orientation (LPO) strength, transitions in olivine fabric type, and a contribution from aligned partial melt.

Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

Una nueva especie del géneroEsenbeckia(Rutaceae) en México

Esenbeckia vazquezii, a new species of Esenbeckia Kunth (Rutaceae) with round fruit is described. It was found in the Spa “El Texcal” located in the Chichinautzin lava beds near Cuernavaca, Morelos. Because it is growing in an urbanised area, probably it is endangered.

Investigating the P wave velocity structure beneath Harrat Lunayyir, northwestern Saudi Arabia, using double‐difference tomography and earthquakes from the 2009 seismic swarm

AbstractIn 2009, a swarm of more than 30,000 earthquakes occurred beneath the Harrat Lunayyir lava field in northwest Saudi Arabia. This event was just one of several seismic swarms to occur in this region over the past decade. Surface deformation associated with the seismicity, modeled in previous studies using Interferometric Synthetic Aperture Radar (InSAR) data, is best attributed to the intrusion of a 10 km long dyke. However, little is known about the velocity structure beneath Harrat Lunayyir, making assessment of future seismic and volcanic hazards difficult. In this study, we use local double‐difference tomography to generate a P wave velocity model beneath Harrat Lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm. A pronounced fast velocity anomaly, centered at ~15 km depth with a shallower extension to the N‐NW, is interpreted as an area of repeated magmatic intrusion. The crust surrounding the fast intrusion is slower than that suggested by broader‐scale models for the Arabian Shield. The largest magnitude events occurred early in the swarm, concentrated at shallow depths (~2–8 km) beneath northern Harrat Lunayyir, and these events are associated with the dyke intrusion. Later, deep earthquakes (~15 km) beneath the southern end of the study region as well as a group of intermediate‐depth events connecting the shallow and deep regions of seismicity occurred. These later events likely represent responses to the local stress conditions following the intrusion. Our results are unique since harrat magma systems are rarely imaged, and our observations, coupled with the seismic history in this region, suggest that future volcanic intrusions beneath Harrat Lunayyir are likely.

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 &gt;72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age‐progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern‐most Basin and Range (NWBR) rhyolite volcanism into a single age‐progressive trend. The spatial‐temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment‐filled basins within the more volcanically active HLP. The west‐northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi‐stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.

Development of intra-basaltic lava-field drainage systems within the Faroe–Shetland Basin

The Faroe–Shetland Basin, located on the NW continental shelf of the UK, represents arguably the last frontier area of hydrocarbon exploration of the UK territorial waters. The basin contains an aerially extensive lava sequence, which forms part of the Palaeogene-aged North Atlantic Igneous Province (NAIP). In 2004, a major oil and gas discovery was made within the Palaeocene–Eocene lavas in the form of the Rosebank Field. Unusually, the reservoir intervals are a series of intra-basaltic fluvial clastic sequences separated by basalt lava flows, hyaloclastites and volcaniclastic sediments, giving rise to a new hydrocarbon play concept. The discovery has led to an increased emphasis on trying to understand the detailed stratigraphy of the offshore lava fields within the Faroe–Shetland Basin, as well as the nature of the regional intra-basaltic drainage systems and sediments that form the reservoir intervals in Rosebank. In this paper we show a regional integrated three-dimensional (3D) seismic and well analysis of the lava sequences within the area of the Corona Ridge, exploring the age relationships and spatial distribution of the sequence T40 and T45 Palaeocene lava field. In particular, we note the occurrence of an intra-basaltic (sequence T40, Colsay Member) drainage network that appears to represent a major drainage pathway within the Faroe–Shetland Basin lava fields. Few published studies have dealt with drainage system development on lava but our preliminary analysis appears to indicate that lava flow-field morphology plays a dominant role in controlling the development and evolution of a drainage system.

A Palaeocene intracanyon-style lava emplaced during the early shield-building stage of the Cuillin Volcano, Isle of Skye, NW Scotland

ABSTRACTThe twin summits of Preshal More and Preshal Beg, near Talisker, Isle of Skye, comprise the erosional remnants of a thick (at least 120 m) compound olivine tholeiite lava, or flow field, that ponded in palaeo-valleys within the Palaeocene lava field of west-central Skye. This unique flow field constitutes the Talisker Formation and is the youngest preserved extrusive unit of the Skye Lava Field. The lava inundated a complex of palaeo-valleys incised into the higher stratigraphical levels of the existing lava field, and remnants of the original sedimentary fill of these valleys still exist, the Preshal Beg Conglomerate Formation. The lava displays spectacularly well-developed two-tier (colonnade-entablature) columnar joint sets that formed as a consequence of slow, uninterrupted cooling through its base and sidewalls, aided by groundwater circulation and water ingress (from displaced drainage) directed into the lava's interior by master-joint systems. Intrusive phenomena developed at both the base and the top of the lava and there is evidence for the existence of subsurface feeder tubes. The tholeiitic composition of the Talisker Formation lava contrasts with the transitional, mildly alkaline characteristics of the remainder of the (older) lavas of Skye Lava Field. In broad terms, the Talisker Formation lava is compositionally very similar to the suite of cone-sheets emplaced into the oldest of the four intrusive centres that comprise the Skye Central Complex – the Cuillin Intrusive Centre – together with a high proportion of the Skye regional dyke swarm. The stratigraphical position, field relationships and compositional characteristics of the lava indicate that it was erupted and emplaced as an intracanyon-style flow field during the early shield-building stage in the growth of the (tholeiitic) Cuillin Volcano, which post-dates the main Skye ‘plateau’ Lava Field. Although the remnant outcrops are detached from their likely source area through erosion, this tholeiitic lava provides the first direct evidence linking the central complexes of the British Palaeogene Igneous Province and their eruptive products.

Surface dispersion of uranium in the Al-Hayt area, NE Arabian Shield

Al-Hayt area, although located on the Cenozoic basaltic plateau of Harrat Khaybar, is threatened by uranium contamination emanating mainly from the rhyolite hills of Asfar Thwelil just outside the eastern boundary of the lava field. This outcrop is an exhumed volcanic dome belonging to the Shammar Group of Late Proterozoic age. The rock units in this dome are porphyritic and flow rhyolites with some ignimbrites. The flow rhyolite is especially enriched in uranium and much of this is concentrated in thin vertical veins of uraninite and uranophane. Radiation measurements were carried out in the field using a portable gamma ray differential spectrometer that was capable of counting radiation at 5 spectral intervals representing uranium, thorium, potassium and total radiation (TC1 & TC2). Four traverses containing 67 points were made across the hills of Asfar Thwelil and nearby plains, and some measurements were also carried out in various parts of the Al-Hayt Town itself. Radiation measurements were also carried out in several parts of the eastern and northern shield for the sake of comparison. Soil samples for chemical analysis were also collected along the traverses and from the town and its main quarry. The samples were sieved to 3 sizes (coarse, medium and fine) and analyzed for 34 elements using the Neutron Activation technique. Results of the chemical analyses reveal the concentration of uranium and thorium in the fine soil fraction; although this may indicate adsorption onto clay minerals, a similar trend by immobile elements such as Zr, Hf, Ta, REE and Co rules out this possibility and points to heavy mineral concentration in the fine fraction as the main cause. Radiation was measured in 5 exploration trenches, and the upper layers (alluvium) were found to be the least radioactive, while layers containing salt and calcrete were highly enriched in radioactive elements. Correlation coefficients from soil samples are strongly in favor of the concentration of U & Th with other incompatible elements in heavy minerals such as zircon. Factor analysis of the same samples provides a number of underlying causes that control their chemistry; most important of all is the heavy mineral content followed by the influence of basaltic detritus form the Harrah.