Rifting Kinematics Produced by Magmatic and Tectonic Stresses in the North Volcanic Zone of Iceland

In the North Volcanic Zone of Iceland, we studied with the greatest possible detail the complete structural architecture and kinematics of the whole Theistareykir Fissure Swarm (ThFS), an N-S-trending, 70 km long active rift. We made about 7500 measurements along 6124 post-Late Glacial Maximum (LGM) extension fractures and faults, and 685 pre-LGM structures. We have collected the data over the last 6 years, through extensive field surveys and with the aid of drone mapping with centimetric resolution. In the southern sector of the study area, extension fractures and faults strike mainly N10°-20°, the opening direction is about N110°, and the dilation amount is in the range 0.1–10 m. In the central sector, faults and extension fractures strike mainly N00-10°, the opening direction is N90-100°, and the dilation amount is 0.1–9 m. In the northern sector, extension fractures and faults strike N30-40°, the opening direction is about N125°, and the dilation amount is 0.1–8 m. The variations in strike are attributable to two processes: the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and Grímsey Oblique Rift (Grímsey lineament), and the structural inheritance of older NNE- to NE-striking normal faults. Most extension fractures show a minor strike-slip component: a systematic right-lateral component can be accounted for by the interaction with the WNW-ESE-striking fault zones and the regional, oblique opening of the rift. We regard dyke propagation as a possible cause for the more complex strike-slip components measured at several other fractures. Cumulated dilation and fracture frequency decrease along the rift with distance away from the Theistareykir volcano, situated in the central sector of the ThFS. This is interpreted as a decrease in the number of dykes that are capable of reaching great distances after being injected from the magma chamber

Evolution of deformation and stress changes during the caldera collapse and dyking at Bárdarbunga, 2014–2015: Implication for triggering of seismicity at nearby Tungnafellsjökull volcano

Stress transfer associated with an earthquake, which may result in the seismic triggering of aftershocks (earthquake–earthquake interactions) and/or increased volcanic activity (earthquake–volcano interactions), is a well-documented phenomenon. However limited studies have been undertaken concerning volcanic triggering of activity at neighbouring volcanoes (volcano–volcano interactions). Here we present new deformation and stress modelling results utilising a wealth of diverse geodetic observations acquired during the 2014–2015 unrest and eruption within the Bárdarbunga volcanic system. These comprise a combination of InSAR, GPS, LiDAR, radar profiling and optical satellite measurements. We find a strong correlation between the locations of increased seismicity at nearby Tungnafellsjökull volcano and regions of increased tensile and Coulomb stress changes. Our results suggest that stress transfer during this major event has resulted in earthquake triggering at the neighbouring Tungnafellsjökull volcano by unclamping faults within the associated fissure swarm. This work has immediate application to volcano monitoring; to distinguish the difference between stress transfer and new intrusive activity

Sprungusveimar Norðurgosbeltisins á Íslandi

The Northern Volcanic Rift Zone, Iceland, is a ~200 km long segment of the Mid-Atlantic plate boundary, where the North American and the Eurasian plates are diverging. The rift zone consists of about 5-6 volcanic systems with central volcanoes and fissure swarms, in addition to the Tungnafellsjökull Volcanic System at the border of the rift zone. The volcanic systems are the locus of eruptive activity in the Northern Volcanic Rift Zone. The central volcanoes consist of elevated massifs, high temperature geothermal areas, calderas and silicic formations. Fissure swarms with eruptive fissures and high density of fractures extend in opposite directions from the central volcanoes. In the Northern Volcanic Rift Zone, the fissure swarms are between 0.5 and 15 km wide and between 30 and ~125 km long. In this study, fractures and eruptive fissures within the fissure swarms of the Northern Volcanic Rift Zone were mapped in detail from aerial photographs. The results of the study indicate that eruptions are less common at the distal parts of the fissure swarms than closer to the central volcanoes. The proximal parts of the fissure swarms also generally show higher fracture density, even when the effect of the age of the lava flows has been taken into account. Older lava flows in the Krafla and Askja Fissure Swarms have usually higher fracture densities, suggesting repeated dike intrusions into the same parts of the fissure swarms during Postglacial times. Fractures in the fissure swarms of the Northern Volcanic Rift Zone are characteristically oriented towards north or NNE, i.e. more or less perpendicular to the spreading direction. However, deviations from this pattern occur in certain areas. These areas include the caldera volcanoes in the Northern Volcanic Rift Zone, Krafla and Askja, where some fractures and eruptive fissures are concentric to, or radiate from the calderas. Second example involves east-west oriented fractures and eruptive fissures near the Vatnajökull glacier, and third example eroded WNW-oriented fractures that can be found intermittently, cutting across the Northern Volcanic Rift Zone, from the north end of the Kverkfjöll Fissure Swarm to the south end of the Krafla Fissure Swarm. Other examples involve the previously known WNW oriented transform zones north of Iceland that connect with the Northern Volcanic Rift Zone. The transform zones influence the fissure swarms, although surface fractures that belong to them are not visible, except in the Þeistareykir Fissure Swarm. The number of fractures peaks and a graben widens to the north at the intersection of the Húsavík Transform Zone and the Krafla Fissure Swarm, indicating a buried continuation of the transform zone beneath the fissure swarm. Several fractures at the intersection of the Grímsey Oblique Rift and the Fremrinámar Fissure Swarm are WNW-oriented, as opposed to the general N to NNE oriented fissure swarms, suggesting an onshore continuation of the transform zone.Norðurgosbeltið er hluti af flekaskilum Atlantshafshryggjarins, þar sem Evrasíuflekann og Norður-Ameríkuflekann rekur í sundur. Gosbeltið samanstendur af 5-6 eldstöðvakerfum, sem hvert inniheldur megineldstöð og sprungusveima. Auk þeirra liggur eldstöðvakerfi Tungnafellsjökuls í jaðri gosbeltisins. Eldvirkni í Norðurgosbeltinu á sér stað innan eldstöðvakerfanna. Þar sem megineldstöðvar finnast liggur landslag yfirleitt hátt, ásamt því að þar finnast háhitasvæði, öskjur og/eða súrar myndanir. Innan sprungusveima má finna gossprungur, og þéttleiki sprungna er þar mikill. Sprungusveimarnir liggja í gagnstæða átt út frá megineldstöðvunum. Innan Norðurgosbeltisins eru sprungusveimarnir á milli 0,5 og 15 km breiðir, og milli 30 og ~125 km langir. Í þessari rannsókn voru sprungur og gossprungur innan Norðurgosbeltisins kortlagðar með mikilli nákvæmni eftir loftmyndum. Niðurstöður rannsóknarinnar gefa til kynna að eldgos innan sprungusveima Norðurgosbeltisins séu algengari nær megineldstöðvunum heldur en fjær. Þéttleiki sprungna innan sprungusveima er yfirleitt mestur næst megineldstöðvunum, jafnvel þegar tekið hefur verið tillit til aldurs yfirborðshraunlaganna sem sprungurnar liggja í. Þéttleiki sprungna í eldri hraunum innan sprungusveima Kröflu og Öskju er yfirleitt meiri en í yngri hraunum, sem gefur til kynna að gangainnskot sem átt hafa sér stað eftir að ísaldarjökla leysti hafi ítrekað farið í sömu hluta sprungusveimanna. Sprungur innan sprungusveima Norðurgosbeltisins stefna jafnan til norðurs eða til norð-norðausturs, meira og minna hornrétt á flekarekið. Þó má finna undantekningar á vissum svæðum. Sumar sprungur og gossprungur nærri öskjum Norðurgosbeltisins (í Öskju og Kröflu) hringa sig um öskjurnar eða eru geislalægar út frá þeim í stað þess að fylgja hefðbundinni sprungustefnu Norðurgosbeltisins. Þá má finna sprungur og gossprungur nærri Vatnajökli sem stefna í austur-vestur, og einnig má sjá ummerki um rofnar sprungur með vest-norðvestur stefnu á belti sem teygir sig frá nyrsta hluta sprungusveims Kverkfjalla til syðsta hluta Kröflusprungusveimsins. Auk þessara dæma má finna sprungur innan Norðurgosbeltisins með vest-norðvestlæga stefnu þar sem Norðurgosbeltið og þverbrotabeltin fyrir norðan land mætast, en slík þverbrotabelti geta einnig haft önnur áhrif á sprungusveimana. Til að mynda er hámarksþéttleiki sprungna í sprungusveim Kröflu á móts við Húsavíkurmisgengið, og þar víkkar einnig sigdalurinn til norðurs í sprungusveimnum. Þetta bendir til að Húsavíkurmisgengið nái að sprungusveim Kröflu, þótt yfirborðssprungur misgengisins sjáist ekki. Sömuleiðis má finna ummerki um Grímseyjarbrotabeltið á landi, þar sem sprungur með VNV stefnu finnast í framhaldi þess, innan þess hluta sprungusveims Fremrináma sem liggur í Öxarfirði. Stefna þessi stangast á við hina hefðbundnu norður til norð-norðaustur sprungustefnu í sprungusveimunum.Icelandic Research Fund Eimskip University Fun

The fissure swarm of the Askja central volcano

The Askja volcanic system forms one of the 5-6 volcanic systems of the Northern Volcanic Zone, that divides the North-American and the Eurasian plates. Historical eruptions have occurred both within the central volcano and in its fissure swarm. As an example, repeated fissure eruptions occurred in the fissure swarm, and a Plinian eruption occurred within the volcano itself in 1875. This led to the formation of the youngest caldera in Iceland, which now houses the Lake Öskjuvatn. Six eruptions occurred in the 1920„s and one in 1961 in Askja. No historical accounts have, however, been found of eruptive activity of Askja before 1875, likely due to its remote location. To improve the knowledge of historic and prehistoric activity of Askja, we mapped volcanic fissures and tectonic fractures within and north of the Askja central volcano. The 1800 km2 area included as an example Mt. Herðubreið, Mt. Upptyppingar and the Kollóttadyngja lava shield, as well as Askja. The results indicate that the activity of different subswarms of the Askja central volcano alternates with time, as the NE subswarm ends suddenly at a 3500-4500 BP lava flow. This may possibly occur due to different locations of inflation centers in Askja. If, as an example, the inflation center is easterly in Askja, a dike might propagate from this inflation center into the eastern part of the fissure swarm. Volcanic fissures are most common close to Askja, but the number of tectonic fractures increases with distance from the volcano. This may indicate a higher magma pressure in dikes close to Askja, than farther away. The number of fractures decreases with altitude in Kollóttadyngja, which may indicate more depth to the top of the dikes under the center of Kollóttadyngja, than beneath its slopes, due to altitude. Shallow eartquakes are mostly originated at non-fractured areas, like the ones that occur near Mt. Herðubreið, where fault-plane solutions have indicated the formation of strike-slip faults. In only about 4 km distance from Herðubreið, dilatational fractures, aged 4500-10.000 BP can be found. This may indicate that the maximum stress axis may have rotated since the formation of the dilatational fractures took place. The latest dike intrusions into either the Askja or the Kverkfjöll fissure swarms may have caused this rotation. Volcanic fissures are either oriented away from, or circle around the calderas in Askja. The volcanic fissures that are oriented away from the calderas may have formed after an inflation in a caldera, and the ones that circle around the calderas may have formed shortly after an inflation started in a previously deflating caldera. The first four eruptions that took place in the 1920s, and occurred around the newly formed caldera, may therefore indicate that an inflation had started in the caldera. The irregular orientation of fissures and fractures close to Askja suggests that it has a local stress field. The 1.7 km long pit crater chain in the Kollóttadyngja lava shield lies in and parallel with the Askja fissure swarm. We suggest that the topography of Kollóttadyngja caused a horizontal component of magma flow in an underlying dike. Increased magma flow in the upper part of the dike caused lower magma pressure, causing even more magma to flow into this part of the dike. A pipe was eventually formed which later collapsed, and formed the pit crater chain

The fissure swarm of the Askja central volcano

The Askja volcanic system forms one of the 5-6 volcanic systems of the Northern Volcanic Zone, that divides the North-American and the Eurasian plates. Historical eruptions have occurred both within the central volcano and in its fissure swarm. As an example, repeated fissure eruptions occurred in the fissure swarm, and a Plinian eruption occurred within the volcano itself in 1875. This led to the formation of the youngest caldera in Iceland, which now houses the Lake Öskjuvatn. Six eruptions occurred in the 1920„s and one in 1961 in Askja. No historical accounts have, however, been found of eruptive activity of Askja before 1875, likely due to its remote location. To improve the knowledge of historic and prehistoric activity of Askja, we mapped volcanic fissures and tectonic fractures within and north of the Askja central volcano. The 1800 km2 area included as an example Mt. Herðubreið, Mt. Upptyppingar and the Kollóttadyngja lava shield, as well as Askja. The results indicate that the activity of different subswarms of the Askja central volcano alternates with time, as the NE subswarm ends suddenly at a 3500-4500 BP lava flow. This may possibly occur due to different locations of inflation centers in Askja. If, as an example, the inflation center is easterly in Askja, a dike might propagate from this inflation center into the eastern part of the fissure swarm. Volcanic fissures are most common close to Askja, but the number of tectonic fractures increases with distance from the volcano. This may indicate a higher magma pressure in dikes close to Askja, than farther away. The number of fractures decreases with altitude in Kollóttadyngja, which may indicate more depth to the top of the dikes under the center of Kollóttadyngja, than beneath its slopes, due to altitude. Shallow eartquakes are mostly originated at non-fractured areas, like the ones that occur near Mt. Herðubreið, where fault-plane solutions have indicated the formation of strike-slip faults. In only about 4 km distance from Herðubreið, dilatational fractures, aged 4500-10.000 BP can be found. This may indicate that the maximum stress axis may have rotated since the formation of the dilatational fractures took place. The latest dike intrusions into either the Askja or the Kverkfjöll fissure swarms may have caused this rotation. Volcanic fissures are either oriented away from, or circle around the calderas in Askja. The volcanic fissures that are oriented away from the calderas may have formed after an inflation in a caldera, and the ones that circle around the calderas may have formed shortly after an inflation started in a previously deflating caldera. The first four eruptions that took place in the 1920s, and occurred around the newly formed caldera, may therefore indicate that an inflation had started in the caldera. The irregular orientation of fissures and fractures close to Askja suggests that it has a local stress field. The 1.7 km long pit crater chain in the Kollóttadyngja lava shield lies in and parallel with the Askja fissure swarm. We suggest that the topography of Kollóttadyngja caused a horizontal component of magma flow in an underlying dike. Increased magma flow in the upper part of the dike caused lower magma pressure, causing even more magma to flow into this part of the dike. A pipe was eventually formed which later collapsed, and formed the pit crater chain

Fracture Kinematics and Holocene Stress Field at the Krafla Rift, Northern Iceland

In the Northern Volcanic Zone of Iceland, the geometry, kinematics and offset amount of the structures that form the active Krafla Rift were studied. This rift is composed of a central volcano and a swarm of extension fractures, normal faults and eruptive fissures, which were mapped and analysed through remote sensing and field techniques. In three areas, across the northern, central and southern part of the rift, detailed measurements were collected by extensive field surveys along the post-Late Glacial Maximum (LGM) extension fractures and normal faults, to reconstruct their strike, opening direction and dilation amount. The geometry and the distribution of all the studied structures suggest a northward propagation of the rift, and an interaction with the Húsavík–Flatey Fault. Although the opening direction at the extension fractures is mostly normal to the general N–S rift orientation (average value N99.5° E), a systematic occurrence of subordinate transcurrent components of motion is noticed. From the measured throw at each normal fault, the heave was calculated, and it was summed together with the net dilation measured at the extension fractures; this has allowed us to assess the stretch ratio of the rift, obtaining a value of 1.003 in the central sector, and 1.001 and 1.002 in the northern and southern part, respectively

Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real‐time photogrammetric monitoring

International audienc

International database of Glacially Induced Faults

We provide a GIS data inventory of confirmed and proposed glacially-induced faults. Stresses, perturbated as a response to the advance and retreat of continental ice sheets and glaciers, can reactivate pre-existing faults. Previously referred to as "PostGlacial Faults" (PGFs), these faults are now called "Glacially-Induced Faults" (GIFs). More than a dozen kilometre-long and several metre-high fault-scarps have been identified in northern Fennoscandia since extensive investigations started in the 1960s and 1970s. Similar faults, but by far not of such dimensions, have also been described in eastern Canada. In other formerly glaciated areas in Europe, e.g., the southern parts of Sweden, Norway and Finland, the southern Baltic Sea, Denmark, northern Germany and Poland, and the Baltic countries, GIFs have rarely been observed and discussed in the literature. However, the number of studies with reliable field evidence for proposing such faults has increased considerably in recent years. The estimated fault movements are of minor magnitude, though, as compared with those in northern Fennoscandia. The database contains the confirmed GIFs in northern Fennoscandia including north-western Russia. The geological surveys in Norway, Sweden and Finland analysed recent LiDAR (Light Detection And Ranging) data from their countries, which helped uncover new faults and revise the geometry of the existing ones. In addition, we include several proposed GIFs outside this area, e.g., in southern Sweden, Denmark and Germany. Ongoing work suggests the occurrence of GIFs in Iceland, Canada and Antarctica. The database will be continually updated, considering new results. A summarized description of the GIF in this database is given in: Steffen, H., Olesen, O., and Sutinen, R. (2021). Glacially-Triggered Faulting. Cambridge University Press, Cambridge, UK, ca. 450 pp., expected publication February 2021

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland

Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long1. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens2, 3, 4, or magma flowing vertically into dykes from an underlying source5, 6, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries