Northern Volcanic Zone Of Iceland Research Articles

Rift obliquity in the Northern Volcanic Zone in Iceland using UAV-based structural data

Rift obliquity is known to be very frequent at divergent plate boundaries and shear components in the opening of fissures have been detected through seismicity in the monitoring of recent rifting events. However, the field structural evidence of obliquity in active rifts for Holocene events is still poorly studied, despite its relevance at both the lithospheric and upper-crust scales.We performed extensive high resolution UAV surveys on Holocene terrains in four areas along the active rift in the Northern Volcanic Zone of Iceland, to map structures and morphotectonic features in detail. We did a kinematic analysis and found an increase of the obliquity from N to S.We further discuss the origins of this obliquity and suggest that the combination of inherited structures reactivation, magmatic intrusions, and topography influence on the propagation of both fractures and magma intrusions can have an impact on the mismatch between rift opening direction and the far-field strain.We conclude that obliquity can be inferred for Holocene rifting events from surface structural data. It is thus fundamental to take into account the existing structural pattern and its (re)activation history in active rift zones to avoid missing out on possible preferential structure reactivation, magma propagation ways, and eruption sites during the monitoring of ongoing rifting events.

A step forward to understanding the development of volcanotectonic rifts: the structure of the Fremrinamar Fissure Swarm (Iceland)

We analysed all the Holocene structures defining the Fremrinamar Fissure Swarm (FFS), in the Northern Volcanic Zone of Iceland, through the interpretation of aerial photos, orthomosaics and Digital Surface Models (DSMs), and field surveys. We measured the strike, dip, length and kinematics of 761 normal faults and reconstructed the slip profile of 76 main faults (length >2 km), with the purpose of evaluating the overall direction of along-axis rift propagation. We also measured the strike of 146 eruptive fissures and 1,128 extension fractures. A total of 421 faults dip towards the east and 340 dip towards the west, mainly striking N0°-10°E. Maximum fault length is 14.2 km, and W-dipping faults are longer than E-dipping faults. The majority of eruptive fissures strike N10°-20°E, and are concentrated in the southern part of the FFS, around the Fremrinamar central volcano. Extension fractures mainly strike N0°-10°E, with a maximum length of 2,508 m. We evaluated the variation of strike, fracture density and spacing along the FFS, and observed a change of its trend from NNE-SSW in the central-southern part, to NNW-SSE in the northern part. We interpret this evidence as the effect of the intersection with the Grimsey Lineament. The tapering of fault slip profiles indicates a main northward propagation of the rift, and thus of the deformation, interpreted as the effect of lateral propagation of dykes from the magma chamber below the central volcano towards the north. Such interpretation is also supported by the distribution of normal faults, vertical offset and dilation values, and also by the rift width, which tend to decrease towards the north.

The behaviour of metals in deep fluids of NE Iceland

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.

Seismicity of the Northern Volcanic Zone of Iceland

A half century of monitoring of the Northern Volcanic Zone of Iceland, a branch of the North America—Eurasia plate boundary, shows that the seismicity is very unevenly distributed, both in time and space. The four central volcanoes at the boundary, Þeistareykir, Krafla, Fremrinámar, and Askja, show persistent but very low-level seismicity, spatially coinciding with their high-temperature geothermal systems. On their rift structures, on the other hand, seismicity is almost absent, except during rifting episodes. Krafla went through a rifting episode in 1975–1984 with inflation, interrupted by 20 diking events with extensive rifting, eruptive activity, and intense seismicity along an 80 km long section of the rift. During inflation periods, the seismicity was contained within the caldera of the volcano, reflecting the inflation level of the magma chamber. Diking events were marked by seismicity propagating away from the volcano into the fissure swarms to the south or north of the volcano, accompanied by rapid deflation of the caldera magma chamber. These events lasted from 1 day to 3 months, and the dike length varied between 1 and 60 km. The area around the Askja volcano is the only section of the Northern Volcanic Zone that shows persistent moderate seismicity. The largest events are located between fissure swarms of adjacent volcanic systems. Detailed relative locations of hypocenters reveal a system of vertical strike-slip faults, forming a conjugate system consistent with minimum principal stress in the direction of spreading across the plate boundary. A diking event into the lower crust was identified in the adjacent fissure swarm at Upptyppingar in 2007–2008. Four nests of anomalously deep earthquakes (10–34 km) have been identified in the Askja region, apparently associated with the movements of magma well below the brittle-ductile transition. Several processes have been pointed out as possible causes of earthquakes in the deformation zone around the plate boundary. These include inflation and deflation of central volcanoes, intrusion of propagating dikes, both laterally and vertically, strike-slip faulting on conjugate fault systems between overlapping fissure swarms, migration of magma in the lower, ductile crust, and geothermal heat mining.

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.

UAV-based surveying in volcano-tectonics: An example from the Iceland rift

In the present work, we applied the use of an Unmanned Aerial Vehicle (UAV) - a quadcopter - and the Aerial Structure from Motion digital photogrammetry image processing technique (ASfM) to study volcano-tectonics and tectonic features in an active Icelandic rift. Data have been collected in order to evaluate the Holocene deformation in the Northern Volcanic Zone of Iceland. We mapped 397 structures, mainly related to extension fractures and subordinately normal faults in the Theistareykir Fissure Swarm, obtaining 1098 and 21 structural data, respectively. This allowed to reconstruct an overall spreading direction of N108° during Holocene times, and to calculate a stretch of 1.013 regarding 8–10 ka old lava units. Deformation in the area is related to both dyke intrusions and extensional tectonics. Furthermore, detailed geological-structural field and UAV surveys were also performed in two test areas in order to determine data accuracy and the associated reliability of this approach. In addition to the above, different flight heights were tested, suggesting that photo collection with a 12.4 MPx camera at 100 m is efficient to study fracture dilation and kinematics.

Deformation in the Northern Volcanic Zone of Iceland 2008–2014: An interplay of tectonic, magmatic, and glacial isostatic deformation

AbstractGPS measurements spanning 2008 to 2014 are used to derive the surface velocity field across the Northern Volcanic Zone (NVZ) of Iceland, a subaerial part of the divergent boundary between the North American and Eurasian plates. No volcanic activity nor magmatic intrusions were detected in the zone during this time period. We infer an extensional rate of 17.4 mm/yr in direction 292.0 °, consistent with the results of previous studies and current plate motion models including MORVEL2010 and GEODVEL2010. The horizontal velocity field reveals about 50 km wide stretching zone caused by the divergent plate movements. Glacial isostatic adjustment (GIA) induces uplift of over 20 mm/yr at the northern edge of Vatnajökull ice cap and 3–4 mm/yr horizontal motion directed away from the ice cap. Deformation in the NVZ between 2008 and 2014 can be reproduced by a combination of models relating to several different processes: (i) Mogi sources for volcanic and geothermal deformation at the Askja and Krafla volcanoes, (ii) scaled version of a velocity field derived from a glacial isostatic model, and (iii) simple arctangent‐based model for secular plate spreading. We find the approximate location of the plate boundary spreading axis as well as its locking depth. The spreading axis lies through the Krafla, Fremrinámar, and Askja central volcanoes, the most active ones in the NVZ. It does not appear to follow the general direction of each fissure swarm but rather to change direction at the central volcanoes. The locking depth is on average within the 7–9 km range.

The temperature of the Icelandic mantle from olivine‐spinel aluminum exchange thermometry

AbstractNew crystallization temperatures for four eruptions from the Northern Volcanic Zone of Iceland are determined using olivine‐spinel aluminum exchange thermometry. Differences in the olivine crystallization temperatures between these eruptions are consistent with variable extents of cooling during fractional crystallization. However, the crystallization temperatures for Iceland are systematically offset to higher temperatures than equivalent olivine‐spinel aluminum exchange crystallization temperatures published for MORB, an effect that cannot be explained by fractional crystallization. The highest observed crystallization temperature in Iceland is 1399 ± 20°C. In order to convert crystallization temperatures to mantle potential temperature, we developed a model of multilithology mantle melting that tracks the thermal evolution of the mantle during isentropic decompression melting. With this model, we explore the controls on the temperature at which primary melts begin to crystallize, as a function of source composition and the depth from which the magmas are derived. Large differences (200°C) in crystallization temperature can be generated by variations in mantle lithology, a magma's inferred depth of origin, and its thermal history. Combining this model with independent constraints on the magma volume flux and the effect of lithological heterogeneity on melt production, restricted regions of potential temperature‐lithology space can be identified as consistent with the observed crystallization temperatures. Mantle potential temperature is constrained to be °C for Iceland and °C for MORB.

Origin of the compositional diversity in the basalt-to-dacite series erupted along the Heiðarsporður ridge, NE Iceland

The Heiðarsporður ridge, located in the Northern Volcanic Zone of Iceland, was formed approximately 9000years ago by a volcanic episode known as Lúdent Fires. The episode produced a broad spectrum of different magma types, forming approximately 50 small scoria cones and two larger craters (Lúdent and Hraunbunga). The bulk compositions cluster in five distinct groups: (1) olivine basalts, (2) Fe-Ti basalts, (3) basaltic icelandites, (4) icelandites, and (5) dacites. Major and trace element trends, together with mineral chemistry and isotopic ratios, suggest that the dominant process involved in generating the evolved magmas was crystal fractionation occurring at variable depth. An origin by polybaric differentiation is confirmed by MELTS modeling. Magma mixing played a dominant role in the formation of the basaltic icelandites. Additionally, the Fe-Ti basalts, which erupted shortly after the dacites and used approximately the same vent area, display unusually high concentrations of Fe, Ti, P, and Sr. Their composition is best explained by some pyroxene-dominated fractionation (prior to Fe-Ti oxide stability), and by entrainment of some crystal cumulate material at shallow depth, mostly left over from the silicic differentiation stage. Textural and chemical features of the minerals (e.g., presence of glomerocrysts, two populations of plagioclase in these basalts) support this interpretation of evolved cumulate remobilization. Fe-Ti basalts with the same field, compositional and textural characteristics have also been erupted in the nearby but magmatically independent Krafla Volcanic System, suggesting that a similar differentiation trend occurs also in this larger central volcano.

Tomographic image of melt storage beneath Askja Volcano, Iceland using local microseismicity

We use P wave and S wave arrivals from microseismic earthquakes to construct 3‐D tomographic Vp and Vs images of the magma storage region beneath Askja's central volcano in the Northern Volcanic Zone of Iceland. A distinctive ellipsoidal low‐velocity anomaly, with both Vp and Vsvelocities 8‐12% below the background, is imaged at 6‐11 km depth beneath the caldera. The presence of a shallow magma chamber is corroborated by geodetic and gravity studies. The small Vp/Vs anomaly suggests a lack of pervasive melt. We interpret this anomaly as a region of multiple sills, some frozen but hot, others containing partial melt. A second, smaller low‐velocity anomaly beneath the main magma storage region may represent a magma migration pathway. This interpretation is supported by the close proximity to the anomaly of clusters of deep, magmatically induced earthquakes. However, the location and shape of this deep anomaly are poorly constrained by the current data set.

From feeder dykes to scoria cones: the tectonically controlled plumbing system of the Rauðhólar volcanic chain, Northern Volcanic Zone, Iceland

The Rauðholar volcanic chain, located in the Northern Volcanic Zone of Iceland, has been variably eroded such that, in the northern part, the original scoria cones are preserved, while the central and southern parts expose their shallow feeders. The chain thus offers insight into the inner workings of the near-surface feeder system of scoria cones. The volcanic chain was mapped in 3D using GPS. The en echelon-arranged volcanic chain can be divided into three parts: The southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi-tiered lava flows. The central part combines partially eroded scoria cones, (feeder) dyke intersections, and welded scoria interbedded within rootless and clastogenic lava flows; the welded scoria is composed of different kinds of lithics and bombs. The northern part preserves almost intact, overlapping scoria cones with voluminous lapilli-sized scoriaceous deposits. The overall dyke trend is orthogonal but shows radial patterns in individual cone complexes. Feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20–50 m. The exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental, repeated intrusions. We interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the Rauðholar volcanic chain is located within a major relay structure between volcanoes on the eastern Fremrinamur rift arm and a rift extension with grabens on the western periphery.

Crustal accretion under northern Iceland

Successful models of oceanic crustal accretion should be consistent with both geochemical and geophysical observations from active mid-ocean ridges as well as those from ophiolite sections. The major element concentrations of a set of basalt and picrite samples from the Krafla and Theistareykir volcanic systems of the Northern Volcanic Zone of Iceland show two distinct trends. The compositional variation in samples with MgO>9.5 wt% can be explained by addition/removal of a wehrlitic cumulate, while the major element variability in samples with 5–9.5 wt% MgO is dominated by gabbro removal. The results of thermobarometry based on the composition of the samples and the crystals found within them show that crystallisation took place at a range of temperatures (1160–1350°C) and pressures (<0.3–0.9 GPa) in the crust and uppermost mantle under Krafla and Theistareykir. The geochemical results are consistent with crustal accretion models where crystallisation takes place over a range of depths in the crust and uppermost mantle (<10–30 km). The geochemical observations allow estimates of the composition, mineralogy, pressure and temperature of material in the crust and shallow mantle under northern Iceland to be made and these estimates can be used to predict the seismic velocity of the material at the ridge axis. These P-wave velocity estimates are in agreement with the results of a seismic survey of the ridge axis at Krafla. The presence of temperatures of over 1000°C and magma chambers at depths greater than 10 km in the Icelandic crust cannot be ruled out using the available geophysical data from the Icelandic rift zones. Therefore both the geochemical and geophysical observations are consistent with models where crustal accretion takes place at a range of depths under northern Iceland.

Melt Generation and Movement beneath Theistareykir, NE Iceland

A detailed study of the volume and composition of all the lavas from the Theistareykir segment of the Northern Volcanic Zone of Iceland was designed to study basaltic melt generation and movement beneath a spreading ridge. The trace element compositions of the lavas are variable, and those of melt inclusions in olivine, clinopyroxene and plagioclase phenocrysts even more so. We show that this variability can be produced by mixing instantaneous melts produced by isentropic decompression of mantle whose initial potential temperature is 1480°C, and that the calculated volume and composition of the average melt is consistent with geophysical and petrological observations. Pressure and temperature estimates suggest that the phenocrysts form in the upper mantle, at depths of 30–40 km, and trap melts formed at greater depths. Some mixing of the instantaneous melts occurs before the melt is trapped, and more mixing occurs before the lavas are erupted. A similar model can account for the composition of melt inclusions from the FAMOUS area of the Mid-Atlantic Ridge, and from the Gorda and Juan de Fuca Ridges.

Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (&lt;800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Thin low‐velocity zone within the krafla caldera, ne‐Iceland attributed to a small magma chamber

We examine seismograms from microearthquakes recorded at Krafla central volcano, in the Northern Volcanic Zone of Iceland, during a short inflation period in 1988. We produce record sections for propagation both to the north and to the south of the center of the caldera by combining seismograms from many earthquakes. The northern record section contains clear evidence of a low velocity zone (LVZ), including a low amplitude diffraction of the P wave as it impinges on top of the LVZ and high amplitude reflections from beneath the LVZ. The absence of clear shear waves indicates that the LVZ is at least partially molten and should be interpretated as a magma chamber. The magma chamber is less than 1 km thick, with a top approximately 3 km beneath the surface. We find no evidence for a LVZ in the southern part of the caldera.