Basaltic Lava Flows Research Articles

Emplacing a Cooling-Limited Rhyolite Lava Flow: Similarities with Basaltic Lava Flows

Accurate forecasts of lava flow length rely on estimates of eruption and magma properties and, potentially more challengingly, an understanding of the relative influence of characteristics such as the apparent viscosity, the yield strength of the flow core, or the strength of the surface crust. Consequently, even the most straightforward models of lava advance involve sufficient parameters that constraints can be relatively easily fitted within the uncertainties involved, at the expense of gaining insight. Here, for the first time, we incorporate morphological observations from during and after flow field evolution to improve model constraints and reduce uncertainties. After demonstrating the approach on a basaltic lava flow (Mt. Etna, 2001), we apply it to the 2011-12 Cordon Caulle rhyolite flow, where unprecedented observations and syn-emplacement satellite imagery of an advancing silica-rich lava flow have indicated an important crustal influence on flow emplacement. Our results show that an initial phase of viscosity-controlled advance at Cordon Caulle was followed by later crustal control, accompanied by formation of flow surface folds and large-scale crustal fractures. Where the lava was unconstrained by topography, the cooled crust ultimately halted advance of the main flow and led to the formation of breakouts from the flow front and margins, influencing the footprint of the lava, its advance rate, and the duration of flow advance. Highly similar behaviour occurred in the 2001 Etna basaltic lava flow. The processes controlling the advance of crystal-poor rhyolite and basaltic lava flow therefore appear similar, indicating common controlling mechanisms that transcend profound rheological and compositional differences.

Subsurface architecture of Las Bombas volcano circular structure (Southern Mendoza, Argentina) from geophysical studies

The Plio-Pleistocene Llancanelo volcanic field is located in the south-eastern region of the province of Mendoza, Argentina. This wide back-arc lava plateau, with hundreds of monogenetic pyroclastic cones, covers a large area behind the active Andean volcanic arc. Here we focus on the northern Llancanelo volcanic field, particularly in Las Bombas volcano. Las Bombas volcano is an eroded, but still recognizable, scoria cone located in a circular depression surrounded by a basaltic lava flow, suggesting that Las Bombas volcano was there when the lava flow field formed and, therefore, the lava flow engulfed it completely. While this explanation seems reasonable, the common presence of similar landforms in this part of the field justifies the need to establish correctly the stratigraphic relationship between lava flow fields and these circular depressions. The main purpose of this research is to investigate Las Bombas volcano 3D subsurface architecture by means of geophysical methods. We carried out a paleomagnetic study and detailed topographic, magnetic and gravimetric land surveys. Magnetic anomalies of normal and reverse polarity and paleomagnetic results point to the occurrence of two different volcanic episodes. A circular low Bouguer anomaly was detected beneath Las Bombas scoria cone indicating the existence of a mass deficit. A 3D forward gravity model was constructed, which suggests that the mass deficit would be related to the presence of fracture zones below Las Bombas volcano cone, due to sudden degassing of younger magma beneath it, or to a single phreatomagmatic explosion. Our results provide new and detailed information about Las Bombas volcano subsurface architecture.

Prehispanic (Guanches) mummies and natrium salts in burial caves of Las Cañadas del Teide (Tenerife).

The prehispanic population of Tenerife (Guanches) buried their dead in caves. Many corpses are totally or partially mummified. Embalming procedures are largely unknown, besides some information recorded by chroniclers and the general idea that environmental conditions in some areas may favour natural mummification. Detailed observation of burial caves containing mummified corpses revealed the presence of a white mineral in form of crusts or powder that was not present in the vast majority of non-burial caves, especially in basaltic and trachybasaltic lava flows. We analysed these crusts and they consist of natrium carbonate salts. Therefore, we conducted a search visiting 30 caves, 3 of them harbouring mummified corpses, and made analyses if there were natrium salts in these caves and their amount. One burial cave was located in trachyphonolitic lava flow, and two further caves in trachybasaltic and basaltic lava flows. There was a significant association between presence of natrium salts and use of the cave as a burial site (χ2 = 9.37; p = 0.0259). The semiquantitatively assessed amount of natrium salts was also by far higher in burial caves than in non-burial ones (Z = 2.58, p = 0.01). There are no reports that support the use of natrium salts in the embalming process among the Guanches, but we found a clear-cut relationship between the presence and/or amount of natrium salts in caves and their use as burial sites by the prehispanic population buried in the central highlands, suggesting an intentional use of caves containing this mineral as burial caves, based perhaps on the observation of the ability of natrium salts to preserve corpses.

Primary vesicles, vesicle-rich segregation structures and recognition of primary and secondary porosities in lava flows from the Paraná igneous province, southern Brazil

This study focuses on a volcanic succession of pāhoehoe to rubbly lavas of the Parana–Etendeka Province exposed in a single road profile in southernmost Brazil. This work provides an integrated approach for examining primary vesicles and vesicle-rich segregation structures at the mesoscopic scale. In addition, this study provides a quantitative analysis of pore types in thin section. We documented distinct distribution patterns of vesicle and vesicle-rich segregation structures according to lava thickness. In compound pāhoehoe lavas, the cooling allows only vesicles (<1 cm size) and pipe vesicles to be frozen into place. In inflated pāhoehoe lavas, vesicles of different sizes are common, including pipe vesicles, and also segregation structures such as proto-cylinders, cylinders, cylinder sheets, vesicle sheets, and pods. In rubbly lavas, only vesicles of varying sizes occur. Gas release from melt caused the formation of primary porosity, while hydrothermal alteration and tectonic fracturing are the main processes that generated secondary porosity. Although several forms of porosity were created in the basaltic lava flows, the precipitation of secondary minerals within the pores has tended to reduce the original porosities. Late-stage fractures could create efficient channel networks for possible hydrocarbon/groundwater migration and entrapment owing to their ability to connect single pores. Quantitative permeability data should be gathered in future studies to confirm the potential of these lavas for store hydrocarbons or groundwater.

Marco geológico y tectónico de la Isla del Coco y la región marítima circunvecina, Costa Rica

La Isla del Coco es la única isla oceánica y el único afloramiento subaéreo de la Cordillera Volcánica del Coco, el rasgo geográfico y geológico más extenso en aguas territoriales costarricenses. Desde el punto de vista geológico, está conformada por rocas volcánicas, predominantemente coladas de lavas basálticas y traquíticas en menor cantidad, con rocas piroclásticas y epiclásticas subordinadas. Posee suelos, coluvios y depósitos de playas (arenas y cantos) superficiales; su topografía es muy variable pero predomina el relieve quebrado y rugoso. La Isla del Coco es la parte emergida de un volcán submarino de evolución compleja, desarrollado a partir y durante el Pleistoceno Inferior (entre 2.2 y 1.5 millones de años, Ma), producto de una anomalía térmica en el manto a través de varias fisuras, que originaron varios alineamientos de volcanes submarinos. La Isla del Coco se encuentra en un estadio de erosión activo, y sus arcos y plataformas sumergidas (90-110m y 183m) son probablemente el producto de una erosión subaérea durante las dos últimas máximas glaciaciones, combinados quizás con la subsidencia debida al enfriamiento del escudo volcánico y de la corteza oceánica. Al ser la isla geológicamente joven, posee importantes implicaciones para la comprensión de la evolución y el endemismo de su biodiversidad. Las amenazas en la geodinámica principales identificadas son los deslizamientos, los tsunamis y, en menor grado, la sismicidad, poco frecuente y con magnitudes moderadas (≤ 5.8 Mw hasta la fecha) dentro de un radio de alrededor de 300km, predominantemente asociada con fallas dextrales de rumbo N-S. Los pocos datos obtenidos hasta el momento indican que hay un cierto grado de actividad sísmica en los alrededores de la Isla del Coco, relacionados con las fallas locales. Algunos pequeños tsunamis históricos y prehistóricos han afectado a la Isla del Coco y los sismos así como la precipitación pluvial elevada han generado deslizamientos. La licuefacción está restringida a las dos playas arenosas (bahías Chatham y Wafer).

Continental intraplate volcanism in the Sinai subplate: The Oligo-Miocene basalts of the Gulf of Suez rift

Three major Oligo-Miocene basaltic occurrences in Sinai, along the eastern side of the Gulf of Suez rift (Wadi Tayiba, Wadi Matulla and Gabal Abu Durba), have been chosen to reveal the geochemistry and petrogenesis of the magmatism associated with this newly-formed continental rift. They include rift-related basaltic/doleritic dikes and sills and isolated basaltic lava flows, belonging to a prolonged persistent magmatic event occurred around 20 Ma, that crosscut older Phanerozoic strata and Precambrian basement. The Gulf of Suez rift (GSR) volcanics show confined range of SiO2 contents (47.51–52.25 wt%) and Mg# values (31–48) and consist of alkaline basalts, hawaiites, mugearites and transitional tholeiitic basalts. All of the investigated basaltic samples possess explicit Fe-enrichment and sodic nature and are silica-oversaturated (quartz-normative) and metaluminous (diopside-normative and have NK/A<1 and A/CNK <1). The compatible transition elements Ni (9–69 ppm), Cr (14–123 ppm), V(237–453 ppm), Sc (19–43 ppm) are generally depleted relative to primitive lavas, consistent with the evolved nature of these basalts (e.g. low Mg# (<68) and silica-oversaturation). Conversely, these basalts show strong enrichment of most of the incompatible LIL and HFS elements relative to primitive mantle, analogous to ocean island basalts. The GSR basalts have suffered minimal degree of crustal contamination, which was likely a minor contribution from the lower crust that superimposed on crystal fractionation. The Nb/Ta and Zr/Hf ratios of the studied basalts are comparable to those of OIB and enriched MORB and are suggestive of their derivation from mantle source. The GSR basalts were most probably derived via partial melting of quite deep asthenosphere (>70 km) amphibole-bearing garnet peridotite mantle source.

Analyze the spatial distribution of lava flows in Al-Ays Volcanic Area, Saudi Arabia, using remote sensing

The Al-Ays Volcanic Area (AAVA) experienced a massive earthquake swarm in 2009 under the Harrat Lunayyir (HL). To examine the space distribution of basaltic lava flows in AAVA, we present here an analysis of satellite images. This has helped to identify three generations of lava flows within AAVA for the first time, their spatial extent and relationship to geological features. Various remote sensing treatments were applied to the Landsat 8 image: (a) color composites, (b) band ratioing, (c) principal component analysis, and (d) a mathematical index. Spectral analysis of the maps assists with charting the spatial configuration of the lava flows. Total area occupied by basalts is ~677 km2, of which the oldest lava flows occupy almost 52% (354 km2) and the lavas of intermediate age share 43% (295 km2) while the recent lava flows merely share 5% (28 km2), including 6.8 km2 referring to the volcanic cones. It should be noted that the generation of lava flows outlines a rather small area, of ≤20 km radius, circumventing the swarm epicenter zone. These maps may prove useful in volcanic hazard mapping for the rugged terrain.

40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry of the Neogene bimodal volcanism in the Yükselen area, NW Konya (Central Anatolia, Turkey)

Bimodal volcanic suites occur in both orogenic and anorogenic geotectonic settings. Although their formation can be attributed to either fractional crystallization from basaltic parents to felsic derivatives or partial melting of different sources, the origin of bimodal suites is still unclear. By reporting mineral chemistry, 40Ar/39Ar geochronology, elemental and Sr-Nd-Pb isotope geochemistry data, this study aims to investigate the genesis of bimodal basalt-dacite association from the Yükselen area located on the northern end of the Sulutas Volcanic Complex (Konya, Central Anatolia).The Yükselen area volcanic rocks are represented by basaltic lava flows, and dacitic dome with enclaves and pyroclastics. Basaltic flows and pyroclastic rocks are interlayered with the Neogene fluvio-lacustrine sedimentary units, while dacitic rocks cut the pre-Neogene basement in the area. A biotite separation from dacites yielded 40Ar/39Ar plateau age of 16.11 ± 0.18 Ma. On the other hand, a whole rock sample from basalts gave two plateau ages of 16.45 ± 0.76 Ma and 22.37 ± 0.65 Ma for the first steps and next steps, respectively. The investigated basalts are sodic alkaline, and characterized by ocean island basalt (OIB)-like anorogenic geochemical signatures. However, dacites are calc-alkaline and metaluminous, and carry geochemical signatures of orogenic adakites. Sr-Nd-Pb isotopic systematics suggest that the basalts were derived from an asthenospheric mantle source enriched by recycled crustal rocks. The dacites show more enriched Sr and Pb ratios and more depleted Nd ones relative to the basalts, which at the first glance might be attributed to crustal contamination of the associated basalts. However, trace element features of the dacites rule out cogenetic relationship between the two rock types, and point to an origin by melting of lower crust. On the other hand, enclaves share several elemental and isotopic characteristics with the dacites, and appear to be fragments of sub-volcanic intrusions closely related to the dacitic host magma. Based on the obtained geochemical data combined with the published geological and geophysical data, the investigated bimodal volcanic activity can be explained by slab break-off process in the convergence system between the African and Anatolian plates.

Chapter 12 Inner-arc volcanism: Barren and Narcondam islands

Abstract Barren and Narcondam are active and now extinct, respectively, volcanic islands that belong to the inner-arc volcanic belt that extends from Java in the east to Burma in the north. Modern eruptions on Barren Island have been exclusively scoria cone-forming Strombolian-type activity in contrast to historic (1787–32) eruptions that witnessed a switch from scoria cone activity to abundant basaltic lava flows. Prehistoric (late Pleistocene) eruptions were alternating effusive and explosive events. The prehistoric events formed a mafic stratovolcano (island) and the associated large eruptions produced a c. 2.0 km diameter crater at the centre of the stratocone. Barren Island lavas are mainly basalts and basaltic andesites, whereas Narcondam rocks are dacite-andesite-rhyolite and contain evidence of magma mixing of both mantle and crustal material. The types of volcanoes and volcanism associated with Barren and Narcondam have been attributed to differences in tectonic setting and the nature of the basement below the volcanoes. It is likely that continental or transitional crust exists below Narcondam, in contrast to oceanic lithosphere below Barren Island.

Geochemistry and petrogenesis of Soltan Maidan basalts (E Alborz, Iran): Implications for asthenosphere-lithosphere interaction and rifting along the N margin of Gondwana

Soltan Maidan Basaltic Complex with thickness up to about 1300m is located in the eastern Alborz zone, north of Iran. This complex is dominantly composed of transitional to mildly alkaline basaltic lava flows, agglomerates and tuffs, together with a few thin sedimentary interlayers. Field geological evidence and study of palynomorph assemblages in the shale interlayer show Late Ordovician to Early Late Silurian ages. Chondrite- and primitive-mantle normalized multi-element patterns of Soltan Maidan basalts demonstrate enrichment in highly incompatible elements relative to less incompatible ones and their patterns are most similar to OIB. Trace elemental and Sr-Nd isotopic compositions indicate interaction and mixing of asthenospheric mantle source (OIB-type) with enriched subcontinental lithospheric mantle components (EM1-type). This asthenosphere-lithosphere interaction occurred in an extensional continental setting, which resulted in opening of the Paleotethys Ocean in the north of Gondwana during the Late Silurian to Middle Devonian.

Role of Fracturing in the Organization of the Karst Features of Azrou Plateau (Middle Atlas, Morroco) Studied by Remote Sensing Imagery

Karst formation geometry can be controlled by fractures and faults, and by other lithologies. Here we show the organization of kastic collapse features related to structures and to extensive basaltic lava flows in the Middle Atlas of Morocco. A lineament map of major faults and fractures has been created for the Middle Atlas using Landsat 7 ETM+ satellite images. This shows a dominant NE–SW regional direction and less prominent NNW–SSE and ENE–WSE directions. All these directions coincide with the alignments of karstic depressions that have formed in the Liassic limestones. The basaltic flows covering these formations on the Middle Atlas limestone plateau, have allowed the generation of cryptokarst, geometrically organized a long these major lineament directions. Karst landforms probabaly existed before the eruption of the lavas, but there were partly invaded by intrusions and volcanism. The extensive basaltic flows allowed for increased infiltration, and subsurface water flow, increasing the rate of kast formation after eruptions. Some basins show evidence of increased subsidence after lava emplacement (Aguelmam Sidi Ali Lake) and some maar-like craters also have subsided after eruption, by karts formation. We lay out the structural and lithological controls on Karstic formation in an intraplate volcanic field based on limestones and evaporites.

富士山の地質と地下水流動

Mt. Fuji, at 3,776m, is a stratovolcano located on the Pacific coast of central Japan. It started to form at ca. 100,000 yr. B.P. and continued eruptions ejected huge quantities of basaltic lavas and volcanic ashes. Its activities are divided into two stages: before and after ca. 11,000 yr. B.P. The old stage, or the Ko-Fuji stage, is characterized by frequent volcanic mudflows and ashes, and the new stage, or the Shin-Fuji stage, is characterized by continuous effusions of enormous basaltic lava flows at its initial phase. Basaltic lava flows ejected at the initial phase of the Shin-Fuji stage flew extended to cover most of the foot of the volcano. There are many springs at the foot of Mt. Fuji. Among these, large springs such as the Kakidagawa and Wakuike are located at the termini of the lava flows of the Shin-Fuji stage. Groundwater recharged at the upper parts of Mt. Fuji flows down the flanks under confined conditions through clincker zones formed above and below the massive central cores of basaltic lava flows. At the termini of the lava flows, confined groundwater gushes out of the clinker zones under pressure, forming many large springs. Hydrologic studies indicate that the total recharge to these confined aquifers in Mt. Fuji is ca. 4,700,000 m3/day.

Volcano-sedimentary rocks of the Sortavala Group: A habitat of ancient microorganisms

The assemblages of fossilized microorganisms from volcano-sedimentary material sandwiched between basaltic lava flows in Early Proterozoic (2.0 Ga ago) volcanics of the Sortavala Group of the Sortavala Uplift (northern Lake Ladoga region, near the village of Kharlu) are studied. It is confirmed that volcanogenic and volcano-sedimentary rocks are of significance for bacterial paleontology.

Hydrogeological delineation of groundwater vulnerability to droughts in semi-arid areas of western Ahmednagar district

Groundwater, a renewable and finite natural resource, is a vital source of sustenance for humans and different ecosystems in the semi-arid regions. Rapid population growth in the last three decades has caused a rise in water demand which has inadvertently posed a stress on its availability. Occurrence of groundwater in the Deccan Volcanic Province is governed by the subsurface hydrogeological heterogeneity of basaltic lava flows and by the presence of geological structures like dykes, sills and fractures that influence spatial & vertical groundwater flow. The main objective of this paper is to map and assess areas that are naturally most susceptible to groundwater scarcity and at risk of depletion due to over extraction. The current study involves a field hydrogeological mapping that was integrated with remote sensing and GIS to delineate areas. This technique was based on using different thematic layers viz. lithology, slope, land-use and land cover, lineament, drainage, soil type, depth to groundwater and annual rainfall. Additionally, pumping tests were carried out to classify the study area into different hydrogeological typologies to help delineate communities that are most vulnerable to subsurface heterogeneity. This paper attempts to underline the groundwater scarcity zones based on different influencing thematic layers and provide a robust methodology to prioritize areas vulnerable to groundwater unavailability, by categorizing the study area into different vulnerable class types – extreme, high, moderate and low.

Characterising extrusive and intrusive magmatism at the Kipper Field, Gippsland Basin, using 3D seismic data

Increased hydrocarbon exploration along rifted continental margins indicates the need of a better understanding of the impact and influence of igneous rocks on hydrocarbon systems as they are often present in these tectonic settings. The southern Australian margin contains several petroliferous sedimentary basins which contain Cretaceous-Cenozoic igneous rocks and is therefore an ideal study area to investigate both the positive and negative effects on hydrocarbon systems. In particular, the Kipper Field in the offshore Gippsland Basin forms an excellent example of a volcanic play, as it holds a 328 m gas column and 14 m oil leg sealed by a >100 m thick basaltic lava flow juxtaposed against a sealing fault. The basin also contains a number of cross-cutting and layer-parallel type intrusions, although their impact on the petroleum system is as-yet unclear. Seismic interpretation techniques such as spectral decomposition and opacity rendering combined with electrical log signatures allowed us to identify the lava flow and intrusions down dip from the fault. Whether this fault has acted as a conduit for the magma responsible for the lava flow is still unclear. Future work will aim to further delineate and constrain flow paths of the intrusive and extrusive rocks near the Kipper Field and their influence on the Kipper play. This study highlights the importance of volcanic rocks in hydrocarbon basins and the possible effect they can have on hydrocarbon systems.

Late Permian basalts in the Yanghe area, eastern Sichuan Province, SW China: Implications for the geodynamics of the Emeishan flood basalt province and Permian global mass extinction

We report the finding of a ∼20m thick sequence of massive pyroxene–plagioclase–phyric basalt lava flows in the Yanghe area of the northeastern Sichuan Basin, within the Yangtze craton of SW China, which were previously considered to be located outside the Emeishan flood basalt province. This basaltic sequence above the middle Permian Maokou Formation (Fm.) is overlain by the late Permian Longtan Fm. Thus, the Yanghe basalts should be stratigraphically correlated with the Emeishan flood basalts. The Yanghe basalts show typical oceanic island basalt (OIB) affinity, and geochemically resemble Emeishan basalts, especially in the case of high-Ti (HT) basalts from the eastern domain of the Emeishan flood basalt province. The rocks have low age-corrected (87Sr/86Sr)t (t=260Ma) ratios (0.704158–0.704929) and Pb isotopic ratios [206Pb/204Pb(t) (18.264–18.524), 207Pb/204Pb(t) (15.543–15.58), and 208Pb/204Pb(t) (38.147–38.519)], and positive εNd(t) values (+3.15 to +3.61), suggesting that the lavas have not undergone any significant crustal contamination. The crystallization temperature of clinopyroxene is estimated to be 1368–1420°C, suggesting anomalously thermal inputs from a mantle source and a possible plume-head origin. The fractionation of middle rare earth elements (MREE) to heavy REE (HREE) suggests that these rocks were produced by small degrees of partial melting of mantle peridotite within the garnet–spinel transition region. The stratigraphic relationships and similar geochemical signatures with the Emeishan flood basalts suggest that the Yanghe basalts are part of the Emeishan flood basalt province and can be considered as the northeastern limit of the Emeishan flood basalt province. Our finding extends the diameter of the Emeishan flood basalt province to ∼1200–1400km, covering an area of up to ∼7×105km2, two times more than previously estimated. The larger areal extent and giant eruption volume, incorporating the Sichuan Basin, lend support to the correlation between the end-Guadalupian mass extinction and the Emeishan flood basalt province.

Evidence of the deformation of dykes from the Central Deccan Volcanic Province, Aurangabad, Maharashtra, India

Abstract The Deccan Volcanic Province (DVP) covers an area of more than 500 000 km 2 . There are three major dyke swarms in this province, viz. the Narmada-Tapi, the Western Coast and the Pune-Nasik. In addition, the Nandurbar-Dhule and Pachmari dykes have also been studied. However, the Central Deccan Volcanic Province (CDVP), including parts of Aurangabad (Maharashtra, India), has received less attention than the SW, northern and NE parts of the Deccan Trap. We report on deformed/offset dykes in and around Aurangabad, in the CDVP. These dykes intrude the stratified host basaltic lava flows composed of weathered porphyritic and amygdaloidal basalt. This rheological contrast locally affected the emplacement of dykes, producing offset, the formation of sills, the curvature of dyke walls and finger-like intrusions. The offset of dykes is presumably caused by variations in local stress. The dyke swarm from the CDVP is reported for the first time. The range in the length and width of the dykes is 2.4–26.2 and 1–27 m, respectively. The dykes trend NE, which matches that of the Narmada-Tapi dyke swarm. The present study indicates that the feeder dyke(s) may have fed the basaltic lava flows of the Aurangabad region. The bent pipe vesicles indicate that the lava flowed towards the SE.

Middle Triassic back-arc basalts from the blocks in the Mersin Mélange, southern Turkey: Implications for the geodynamic evolution of the Northern Neotethys

The Mersin Mélange is a tectonostratigraphic unit within the allochthonous Mersin Ophiolitic Complex in the Taurides, southern Turkey. This chaotic structure consists of blocks and tectonic slices of diverse origins and ages set in a clastic matrix of Upper Cretaceous age. In this study, we examine two blocks at two different sections characterized by basaltic lava flows alternating with radiolarian-bearing pelagic sediments. The radiolarian assemblage extracted from the mudstone–chert alternation overlying the lavas yields an upper Anisian age (Middle Triassic). The immobile element geochemistry suggests that the lava flows are predominantly characterized by sub-alkaline basalts. All lavas display pronounced negative Nb anomalies largely coupled with normal mid-ocean basalt (N-MORB)-like high field strength element (HFSE) patterns. On the basis of geochemical modelling, the basalts appear to have dominantly derived from spinel–peridotite and pre-depleted spinel-peridotite sources, while some enriched compositions can be explained by contribution of garnet-facies melts from enriched domains. The overall geochemical characteristics suggest generation of these Middle Triassic lavas at an intra-oceanic back-arc basin within the northern branch of Neotethys. This finding is of significant importance, since these rocks may represent the presence of the oldest subduction zone found thus far from the Neotethyan branches. This, in turn, suggests that the rupturing of the Gondwanan lithosphere responsible for the opening of the northern branch of Neotethys should have occurred during the Lower Triassic or earlier.

The effects of 10 to &gt;160 GPa shock on the magnetic properties of basalt and diabase

AbstractHypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and &gt;160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo‐single‐domain (titano)magnetite. Our experiments reveal shock‐induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum‐heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First‐order reversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters.

High northern geomagnetic field behavior and new constraints on the Gilsá event: Paleomagnetic and 40Ar/39Ar results of ∼0.5–3.1 Ma basalts from Jökuldalur, Iceland

Recent paleomagnetic results of extrusive rocks from high southern latitudes (>60°S) and high northern latitudes (>60°N) have been suggested to reflect a hemispheric asymmetry of the geomagnetic field on time-scales of 105 to 106 yrs, with higher and more stable fields in the north. This interpretation, however, is based on only a few modern-standard paleodirectional data sets and on high northern stable field paleointensity data of rocks that are mainly younger than 100 kyr. The sparsity of modern-standard data questions the validity (and age range) of this potential geomagnetic asymmetry. In 2013 and 2014, we sampled basaltic lava flows in Jökuldalur, north-eastern Iceland, to obtain high-standard paleodirectional and paleointensity data at relatively high-northern latitudes (65.2°N). On average, we sampled >15 cores per site at 51 sites of predominantly Matuyama age. Complete demagnetization was carried out on all samples using AF or thermal demagnetization. We present 45 distinct paleomagnetic directions based on overall N>10 ChRMs per site and α95<3.5°. We obtain a mean direction of D=355.7°, I=76.3°, and α95=3.2 for N=45 sites that is not significantly different from a GAD field. The resulting 45 VGPs distribute around the North Pole, and the global mean paleomagnetic pole (λ¯=87.8°, ϕ¯=224.3°) is coincident with the North Pole within the α95 confidence limit. We calculate a VGP dispersion SB(Mat)=20.523.317.8 and an average inclination anomaly ΔI=−0.91°2.94−4.88 for our 38 Matuyama age data. The dispersion SB overall supports the interpretation of a dependence of SB on latitude during the Matuyama, while the negligible ΔI suggests little deviation from a GAD field. Based on relatively strict cut-off criteria we also present six new field strength estimates from the time interval ∼1.2–1.83 Ma, thus filling a large data gap of the high-northern stable field behavior. We obtain a median VADM of 57±3ZAm2 (VDM of 60±5Am2), which is higher than the median VADM of 16 intensity estimates from Antarctica (39±7 ZAm2) from the same period. A higher northern field is also found when using less strict cut-off criteria resulting in 14 field estimates from Jökuldalur, i.e. we find support for higher field strength in the northern hemisphere as compared to the southern hemisphere during the Matuyama. Finally, we deliver a revised magneto-chronostratigraphic model of Jökuldalur and conduct an investigation of the type sections of the so-called Gilsá normal polarity event around 1.62 Ma. Our revised model is based on 11 new 40Ar/39Ar ages. No evidence is found of the existence of the Gilsá event in Jökuldalur. Instead we find that the normal polarity intervals in the type sections can both be correlated to Olduvai subchron.