Rift Zone Of Iceland Research Articles

МОРФОЛОГИЯ ОБЛАСТИ СОЧЛЕНЕНИЯ ХРЕБТА РЕЙКЬЯНЕС С РИФТОВЫМИ ЗОНАМИ ИСЛАНДИИ

The joint zone between subaquatic Reykjanes Ridge and Icelandic rift zones known as Reykjanes Rift Zone is a unique transform zone formed in conditions of interaction between the mantle plume and the mid-oceanic ridge. Its morphostructures include a row of en-echelon volcanic systems with oblique extension. The modern tectonic and volcanic relief was formed almost completely during Holocene: the previous landforms were reworked or destroyed by erosion of the Late Pleistocene glaciation. Morphology of the Reykjanes Rift Zone is transitional between the Reykjanes Ridge and the adjacent Western Rift Zone from the northeast. Unlike the Reykjanes Ridge, volcanic activity is not concentrated with in axial volcanic ridges, but is confined by eruptive centers represented by fissure volcanoes. Central volcanoes, in contrast to the Western Rift Zone, are not expressed in topography. Oppositely, tectonic activity increases with distance from the Reykjanes Ridge. Fault scarps reach their largest size within the Western Rift Zone, where Thingvellir graben is located. At the same time, the tectonic and volcanic landforms of the Reykjanes rift zone are in paragenetic association, appearing sequentially during the episodes of riftogenesis.

Pre-existing structural control on the recent Holuhraun eruptions along the Bárðarbunga spreading center, Iceland

The active rift zones in Iceland provide unique insight into the geodynamic processes of divergent plate boundaries. The geodynamics of Iceland are studied intensively, particularly, by geophysical methods sensitive to active and/or visible structures such as earthquake seismic and Synthetic Aperture Radar observations or aerial photographs. However, older and less active structures, that may exert a strong control on the presently active geodynamics, are often buried beneath recent volcanic or sedimentary deposits and are—due to their passive mode—overseen by the typical geophysical investigations. Aeromagnetic surveys provide spatial information about subsurface magnetization contrasts relating to both active and inactive structures. However, the aeromagnetic data in Iceland were collected in the 1970-80s and are relevant only to large-scale regional rift studies. With the availability of reliable drones and light-weight atomic scalar sensors, high-quality drone magnetic surveys can provide an unprecedented spatial resolution of both active and passive structures of rift systems as compared to conventional airborne surveys. Here, we present the results of a drone-towed magnetic scalar field and scalar gradiometry study of the north-northeast trending Bárðarbunga spreading center to the north of the Vatnajökull ice cap, Iceland. Our results provide new information about the structural complexity of rift zones with evidence of densely-spaced, conjugate and oblique faults throughout the area. Evidence is shown of a hitherto unknown and prominent east-northeast trending fault structure that coincides with the northern tip of the main eruption edifice of the 1797 and 2014-15 Holuhraun volcanic events. We suggest that this pre-existing structure controlled the locus of vertical magma migration during the two Holuhraun events.

STRUCTURE AND DYNAMICS OF ENDOGENIC RELIEF OF THE TJÖRNES TRANSFORM ZONE (NORTHERN ICELAND)

The Tjörnes Transform Zone connects the Kolbeinsey spreading ridge and the Northern rift zone of Iceland. It includes two overlapping rift segments resulting from the rift zones propagating. Nowadays it includes several morphostructures that vary in their organization and dynamics, such as magmatic and amagmatic rifts, oblique fault zones, or tectonic-volcanic rises. They often overlap with each other promoting the development of block morphostructures of different scale. Using the morphometric analysis of present-day fault scraps we revealed the areas of different recent tectonic activity and faulting intensity, and explained geodynamic reasons of these differences. For instance, the most active morphostructures are spreading and rift segments of the western branch of the transform zone. The eastern branch has significantly weaker tectonic activity which is mainly due to the intensive volcanic processes. Nowadays the eastern branch of the transform zone continues its development that is expressed through the formation of block rises between the overlapping rift segments. The western branch has less complicated structure and gradually fading tectonic-magmatic activity. We also showed the functional changes of amagmatic rifts: at the early stages of transform zone development, they probably evolved as a continuation of adjacent spreading centers. Meanwhile now they have their own independent extension centers. All explored morphostructures influence each other in the process of overlapping that significantly impacts the endogenic relief formation, especially faulting, at different sections of the morphostructures.

Conditions of the Formation and Evolution of the Tjörnes Transform Zone on the Basis of Physical Modelling

The Tjörnes Transform Zone (TZ) is on the northern margin of Iceland, mostly in the shelf zone. It connects the Kolbeinsey spreading ridge and the Northern Rift Zone (RZ) of Iceland. The zone is complexly structured and comprises several heterochronous sections that evolved in various geodynamic conditions. The geodynamic conditions of the formation and evolution of the Tjörnes TZ were reconstructed by means of physical modelling. The models measured the thickness of the model lithosphere, displacement, and the overlapping between the spreading segments. Results of the modelling show that the Tjörnes TZ formed successively, in two stages. The first stage was the formation of the general configuration of the area of interaction of the extension centres of the Kolbeinsey Ridge and the Northern RZ of Iceland in the form of a small overlap of the spreading centres with a rotating block between them. At the second stage, one of the formed transtensional fault zones between the spreading centres became the feeder channel for a magma pulse of the Iceland Plume, which led to the formation of the Grímsey Oblique Rift (OR). Standing alone is the Húsavík-Flatey fault zone that might have formed under the influence of two at a time spreading segments, with the domination of the more magmatically active Northern Rift Zone.

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Abstract The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.

Seismic Volcanostratigraphy: The Key to Resolving the Jan Mayen Microcontinent and Iceland Plateau Rift Evolution

AbstractVolcanostratigraphic and igneous province mapping of the Jan Mayen microcontinent (JMMC) and Iceland Plateau Rift (IPR) region have provided new insight into the development of rift systems during breakup processes. The microcontinent's formation involved two breakup events associated with seven distinct tectono‐magmatic phases (∼63–21 Ma), resulting in a fan‐shaped JMMC‐IPR igneous domain. Primary structural trends and anomalous magmatic activity guided initial opening (∼63–56 Ma) along a SE‐NW trend from the European margin and along a WNW‐ESE trend from East Greenland. The eastern margin of the microcontinent formed during the first breakup (∼55–53 Ma), with voluminous subaerial volcanism and emplacement of multiple sets of SSW–NNE‐aligned seaward‐dipping reflector sequences. The more gradual, second breakup (∼52–23 Ma) consisted of four northwestward migrating IPR (I–IV) rift zones along the microcontinent's southern and western margins. IPR I and II (∼52–36 Ma) migrated obliquely into East Greenland, interlinked via segments of the Iceland‐Faroe Fracture Zone, in overlapping sub‐aerial and sub‐surface igneous formations. IPR III and IV (∼35–23 Ma) formed a wide igneous domain south and west of the microcontinent, accompanied by uplift, regional tilting, and erosion as the area moved closer to the Iceland hotspot. The proto‐Kolbeinsey Ridge formed at ∼22–21 Ma and connected to the Reykjanes Ridge via the Northwest Iceland Rift Zone, near the center of the hotspot. Eastward rift transfers, toward the proto‐Iceland hotspot, commenced at ∼15 Ma, marking the initiation of segmented rift zones comparable to present‐day Iceland.

Olivine chemistry reveals compositional source heterogeneities within a tilted mantle plume beneath Iceland

High-Fo olivine (Fo = Mg/(Mg+Fe) mol%) is an ideal proxy for establishing the compositions of primary melts and their mantle sources. This has been exploited in establishing lithological variations in the mantle source regions of oceanic basalts, including in Iceland. However, previous studies on Icelandic olivine lack spatial and temporal coverage. We present high-precision in-situ major, minor and trace element analyses of Fo-rich olivine from a suite of 53 primitive basalts erupted in the neovolcanic rift and flank zones of Iceland, as well as in older regions of Quaternary and Tertiary crust. Most of these samples have previously been analysed for 3He/4He, which ranges from 6.7 to 47.8 RA, the largest span reported for any oceanic island. By combining trace elemental variability with 3He/4He, we assess the extent of lithological variability in the Icelandic mantle plume. Trace-element ratios that are likely to preserve information about mantle source regions (e.g., Mn/Fe, Ni/(Mg/Fe), Ga/Sc, Zn/Fe and Mn/Zn) suggest a peridotitic mantle source in all rift-related volcanic regions, as well as in the off-rift flank zones of Öræfajökull and Snæfellsnes. However, a signal of a more pyroxenitic mantle lithology is clearly visible in olivine from the South Iceland Volcanic Zone, which represents the southward propagation of the Eastern Rift Zone, while olivine from Tertiary lavas suggests a mixed peridotite-pyroxenite source composition. We are able to identify four components present in the Icelandic mantle: a lithologically heterogeneous plume component with 3He/4He >MORB; a depleted MORB-like peridotite; an isotopically enriched MORB-like peridotite; and a peridotitic component with 3He/4He <MORB, sampled in the off-rift flank zone at Öræfajökull. The spatial distribution of these four components can be explained by a northward tilted mantle plume. This has previously been proposed by geophysical and geochemical studies of both Hawaii and Iceland suggesting that the plume geometry could be a contributing factor in controlling some of the spatial variation in source lithology beneath ocean islands.

Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data

Abstract. Normal faults in basalts develop massive dilatancy in the upper few hundred meters below the Earth's surface with corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDFs) are widespread in Iceland and the East African Rift, but the details of their geometry are not well documented, despite their importance for fluid flow in the subsurface, geohazard assessment and geothermal energy. We present a large set of digital elevation models (DEMs) of the surface geometries of MDFs with 5–15 cm resolution, acquired along the Icelandic rift zone using unmanned aerial vehicles (UAVs). Our data present a representative set of outcrops of MDFs in Iceland, formed in basaltic sequences linked to the mid-ocean ridge. UAVs provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, bridging the gap between outcrop-scale observations and remote sensing. We acquired photosets of overlapping images along about 20 km of MDFs and processed these using photogrammetry to create high-resolution DEMs and orthorectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects commonly cause a higher-than-expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDFs: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TBs) and (iii) opening-mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled with recent sediment. TB-dominated normal faults tend to have the largest ratio of opening width and vertical offset. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset shows that there is a continuous transition between the endmembers. We conclude that for these endmembers, the ratio between opening width and vertical offset R can be reliably used to predict fault structures at depth. However, fractures associated with MDFs belong to one larger continuum and, consequently, where different endmembers coexist, a clear identification of structures solely via the determination of R is impossible.

Countrywide Observations of Plate Spreading and Glacial Isostatic Adjustment in Iceland Inferred by Sentinel‐1 Radar Interferometry, 2015–2018

AbstractEast‐west and vertical ground velocities for 2015–2018 are retrieved over 81% of Iceland from Sentinel‐1 radar interferometry, using satellite images from six different tracks. Only summertime images are considered, to avoid snow cover. Average line‐of‐sight velocity fields for 2015–2018 for each track are estimated using a simple approach: single master interferometry time series together with a linear component estimation for each pixel. The line‐of‐sight velocity fields are combined and their signal is decomposed to extract approximate east (near‐East) and approximate vertical (near‐Up) velocities. Only pixels passing a coherence and outlier criteria are considered, resulting in 81% coverage of Iceland. The 19% of missing coverage is mostly glaciers and farmland. We find a general agreement between the near‐East velocity field and a revised plate spreading model, and the near‐Up velocity field and a glacial isostatic adjustment model. Models and their residuals suggest a difference in rheology between the rift zones in Iceland.

Magmatic-tectonic control on the generation of silicic magmas in Iceland: Constraints from Hafnarfjall-Skarðsheiði volcano

Hafnarfjall-Skarðsheiði (H-S) central volcano, located at the edge of the Western rift zone in Iceland, provides a snapshot into silicic magma generation that occurred soon after establishment of a rift. We present in situ zircon U-Pb ages and oxygen and hafnium isotope compositions, complemented by whole-rock major and trace element and Pb, Nd, and Hf whole rock isotope data, from the dominant silicic units erupted throughout H-S's eruptive history. Zircon U-Pb ages (ca. 5.4 to 3.9 Ma) and field relationships indicate silicic magmatism was episodic. However, relatively low (for Iceland) whole rock εHf (+11.9 to 13.3) and εNd (+7.2 to 7.6), in addition to Pb isotope data from basalt and rhyolite units indicate that the same mantle-derived source is dominantly responsible for the geochemical characteristics observed in both magma types, which are more similar to those from magmas from a propagating rift than an established one. This observation is consistent with a role for fractional crystallization of mantle melts in addition to contributions of partially melted altered crust to explain the low δ18Ozrc values (1.5 to 4.6‰) observed. This study highlights the importance of the evolutionary state of the rift, crustal history, and mantle heterogeneity all as contributing factors to the isotopic composition of silicic Icelandic magmas. We invoke a petrogenetic model where the timescales of rift drift explain the long-lived, episodic silicic magmatism produced during rift propagation at Hafnarfjall-Skarðsheiði volcano.

Seismicity of the Reykjanes Peninsula 1971–1976

The Reykjanes Peninsula Oblique Rift, a segment of the mid-Atlantic plate boundary, connects the Reykjanes Ridge to the rift zones of Iceland. The axis of the deformation zone of the plate boundary can be traced as a rather narrow epicentral zone of earthquakes that is highly oblique to the spreading direction of the North America Plate with respect to the Eurasia Plate. The seismicity of the area is episodic. Active periods with a duration of about a decade are separated by more quiet periods of one to two decades. Our observation period spans a good part of an episode that began in 1967 and ended in 1975. Nine earthquake swarms illuminate a 50 km long segment of the plate boundary. The epicentral zone cuts obliquely across the fissure swarms of the volcanic systems of the Peninsula. Geothermal areas are preferentially located at the intersection of the fissure swarms with the seismic zone. Outside the intersections with the seismic zone the fissure swarms show very low seismicity. The tectonic characteristics gradually change along the plate boundary segment, from being similar to those of a spreading segment in the western part to being more like a transform zone in the eastern part. This is seen in the type of earthquake sequences, maximum magnitude, and type of faulting.

No direct contribution of recycled crust in Icelandic basalts

Using Melt-PX to model the decompression melting of a heterogeneous mantle, I investigated the role of major-element composition of the lithologies present in the source on magmatic productivity, and trace element and isotopic melt compositions, independently of the bulk mantle composition. My calculations demonstrate that the volume of magma produced is not significantly affected by the nature of the lithological heterogeneity, but depends on the bulk mantle composition. However, an isochemical bulk mantle can produce contrasting trace element and isotopic melt compositions depending on the major-element compositions of the lithologies present in the source. Results show that the observed crust thickness of the Icelandic rift zones is consistent with about 10 % of recycled crust in the source, but also demonstrate there is no need to involve the contribution of melts derived from a recycled basalt component to explain the compositional variability of the Icelandic basalts in rift zones, and rather advance the contribution of olivine-bearing hybrid lithologies formed by solid-state reactions between the recycled crust and the peridotite.

A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones

Pressure is one of the key variables that controls magmatic phase equilibria. However, estimating magma storage pressures from erupted products can be challenging. Various barometers have been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and extend into the mantle. However, tests performed on commonly used clinopyroxene-liquid barometers with data from experiments on H 2 O-poor tholeiites in the 1 atm to 10 kbar range reveal substantial pressure-dependent inaccuracies, with some models overestimating pressures of experimental products equilibrated at 1 atm by up to 3 kbar. The pressures of closed-capsule experiments in the 1–5 kbar range are also overestimated, and such errors cannot be attributed to Na loss, as is the case in open furnace experiments. The following barometer was calibrated from experimental data in the 1 atm to 20 kbar range to improve the accuracy of Jd-in-clinopyroxene barometry at pressures relevant to magma storage in the crust: P ( kbar ) = − 26 . 27 + 39 . 16 T ( K ) 10 4 ln [ X Jd Cpx X NaO 0 . 5 liq X AlO 1 . 5 liq ( X SiO 2 liq ) 2 ] − 4 . 22 ln ( X DiHd Cpx ) + 78 . 43 X AlO 1 . 5 liq + 393 . 81 ( X NaO 0 . 5 liq X KO 0 . 5 liq ) 2 This new barometer accurately reproduces its calibration data with a standard error of estimate (SEE) of ±1.4 kbar, and is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition, but should be used with caution below 1100 °C and at oxygen fugacities greater than one log unit above the QFM buffer. Tests performed using with data from experiments on H 2 O-poor tholeiites reveal that 1 atm runs were overestimated by less than the model precision (1.2 kbar); the new calibration is significantly more accurate than previous formulations. Many current estimates of magma storage pressures may therefore need to be reassessed. To this end, the new barometer was applied to numerous published clinopyroxene analyses from Icelandic rift zone tholeiites that were filtered to exclude compositions affected by poor analytical precision or collected from disequilibrium sector zones. Pressures and temperatures were then calculated using the new barometer in concert with Equation 33 from Putirka (2008). Putative equilibrium liquids were selected from a large database of Icelandic glass and whole-rock compositions using an iterative scheme because most clinopyroxene analyses were too primitive to be in equilibrium with their host glasses. High-Mg# clinopyroxenes from the highly primitive Borgarhraun eruption in north Iceland record a mean storage pressure in the lower crust (5.7 kbar). All other eruptions considered record mean pressures in the mid-crust, with primitive clinopyroxene populations recording slightly higher pressures (3.1–3.6 kbar) than evolved populations (2.6–2.8 kbar). Thus, while some magma processing takes place in the shallow crust immediately beneath Iceland’s central volcanoes, magma evolution under the island’s neovolcanic rift zones is dominated by mid-crustal processes.

Structure and tectonic position of the Eyjafjallajökull volcano, S-Iceland

The Eyjafjallajökull volcano, one of the oldest active volcanoes in Iceland, is located in the volcanic flank zone of South Iceland, a few tens of kilometers south of the nearest branch of the mid-Atlantic plate boundary. It is an elongated, broad cone of about 1650 m height. A 100–200 m thick glacier covers the upper part of the volcano and its elliptical 2.5–km-wide summit crater or caldera. An E–W trending rift zone transects the volcano, but a few radial fissures are observed around the summit area. Eruptive fissures on the west flank are curved and tend to be aligned along the maximum gradient of the topography. The E–W orientation of the rift zone and the apparent correlation with the topography suggests strong influence of gravity. Dikes in the older parts of the volcano strike north-easterly and indicate a change in the stress orientation during the last 0.78 My. This change may be related to a southward propagation of the Eastern Volcanic Rift Zone of Iceland and the transfer of spreading from the Western to the Eastern Volcanic Rift Zone. We suggest that the anomalous orientation of the Eyjafjallajökull volcanic system is the result of preexisting topography and gravitational stresses when the volcanic edifice was built up unconformably on old oceanic crust. All known episodes of activity in Eyjafjallajökull have been accompanied by activity in the neighbouring volcano Katla. The most recent examples are the two thermal events, possibly subglacial eruptions, of 1999 and 2011 at Katla following the 1999 sill intrusion and 2010 eruption of Eyjafjallajökull. The coupling mechanism between the volcanoes remains enigmatic. One volcano may be triggered by the other by direct dike or sill injection. Furthermore, pressure perturbation in the mantle may affect the magma sources of both volcanoes.

Fracturing and earthquake activity within the Prestahnúkur fissure swarm in the Western Volcanic Rift Zone of Iceland

AbstractThe Prestahnúkur fissure swarm is located within the ultraslowly spreading Western Volcanic Zone in Iceland. The fissure swarm is characterized by normal faults, open fractures, and evidence of subglacial fissure eruptions (tindars). In this study, fractures and faults within the Prestahnúkur fissure swarm were mapped in detail from aerial photographs to determine the extent and activity of the fissure swarm. Earthquakes during the last ~23 years were relocated to map the subsurface fault planes that they delineate. The Prestahnúkur fissure swarm is 40–80 km long and up to ~20 km wide. Most of the areas of the fissure swarm have been glacially eroded, although a part of it is covered by postglacial lava flows. The fissure swarm includes numerous faults with tens of meters vertical offset within the older glacially eroded part, whereas open fractures are found within postglacial lava flows. Comparison of relocated earthquakes and surface fractures indicates that some of the surface fractures have been activated at depth during the last ~23 years, although no dike intrusions have been ongoing. The existence of tindars nevertheless indicates that dike intrusions and rifting events do occur within the Prestahnúkur fissure swarm. The low‐fracture density within postglacial lava flows and low density of postglacial eruptive fissures indicate that rifting episodes occur less often than in the faster spreading Northern Volcanic Zone.

The interaction of fissure swarms and monogenetic lava shields in the rift zones of Iceland

The divergent plate boundaries of Iceland are marked by rift zones that consist of fissure swarms, i.e. areas with a high density of fractures, faults and eruptive fissures, generally considered to be the surface expressions of dikes. Monogenetic lava shields are common within the rift zones. They are gently sloping, circular cones created by eruptions from a single vent. Many of the shields were emplaced in the early Holocene in response to increased mantle melting when the Pleistocene glacier load on the crust was reduced. In our studies of Icelandic fissure swarms we have established that in most cases where a fissure swarm transects a lava shield there is an inverse correlation between surface elevation and the density of fractures. The summit area of a shield is generally less fractured than the flanks. Fracture densities are also highest on one site of the lava shield, but much lower and even approaching zero values on the other side. This phenomenon is most clearly seen within early-Holocene lava shields (i.e. between ~4500 and ~13,600yearsBP of age). Lava shields younger than 4500yearsBP of age have low fracture densities or are not fractured at all. The lower fracture densities within Holocene lava shields suggest that dike intrusions stop or remain deeper in the crust when they approach a lava shield. Various mechanisms may explain this. The topographic load of the lava shield adds to the lithostatic stress, thus increasing the strength, and may affect the depth of a propagating dike. A long-lived eruption of a lava shield may also affect the stress field in the surrounding crust, increase the horizontal compressive stress and thus create a stress barrier for propagating dikes for some time period following the eruption.

Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes.

As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process.

Sill and lava geochemistry of the mid‐Norway and NE Greenland conjugate margins

This paper presents major, trace‐elements, and Sr‐Nd isotopes for two prominent sills formed during the opening of the North Atlantic, sampled by the Utgard borehole (6607/5‐2) in the Vøring Plateau. The Utgard sills are compared to opening‐related lavas recovered from ODP Leg 104 Hole 642E farther west on the Vøring Plateau and on the NE Greenland conjugate margin. The Utgard sills (3.6–5.9 wt % MgO) are enriched in strongly relative to moderately incompatible trace elements and have 87Sr/86Sr and 143Nd/144Nd ratios of 0.70380–70387 and 0.51292–0.51293, respectively, in the Upper Utgard Sill, and 0.70303–0.70306 and 0.51297–0.51299 in the Lower Sill. Alteration is minor. The Utgard melts originated by partial melting of an asthenospheric, depleted mantle source (DMM or Iceland Rift Zone, IRZ, type) with chemical characteristics similar to the source that gave rise to NE Greenland lavas. The Utgard magmas underwent extensive fractional crystallization in the lower crust (Upper Sill: &gt;70%; Lower Sill: &gt;55%) with removal mainly of olivine and pyroxenes, accompanied by ≤1% assimilation of crustal melts. This crystallization formed significant masses of dense cumulates (∼3.25 g/cm3) (underplating). Assuming an areal extent similar to that of the two sills, we estimate a composite layer of ultramafic cumulates mixed with less dense country rocks to be &gt;320 m thick beneath the two Utgard sills and &gt;8.8 km beneath the thickest part of the Vøring Plateau lavas. Opening‐related cumulates may thus account for a significant part of the lower crustal high‐velocity, high‐density bodies (average density 3.1 g/cm3) along the Norwegian margin.

Stable isotopes of hydrothermal minerals as tracers for geothermal fluids in Iceland

The Reykjanes and Krafla geothermal systems, located within the active rift zone of Iceland, are both potential venues for exploitation of deep supercritical fluids by the Iceland Deep Drilling Project (IDDP). An essential aspect of properly characterizing geochemical and hydrologic processes occurring at supercritical depths is establishing the source, composition and evolution of geothermal fluids. Traditionally, hydrogen isotopes of thermal fluids are used to determine their source. We show that for these, and likely many other Icelandic geothermal systems, analyzing fluid dD is not sufficient alone. Rather, d18O and dD of hydrothermal minerals in conjunction with geochemical characteristics of extant geothermal fluids are necessary to characterize the source and geologic evolution of geothermal reservoir fluids. Here we review results from existing drill holes in the Reykjanes and Krafla geothermal systems to depths of =3km, and explore the utility of using stable isotopes in alteration minerals such as epidote to assess the hydrogeology of extinct volcano-hydrothermal systems by presenting new data from the Geitafell fossil hydrothermal system in southeast Iceland.

Implications of fracturing mechanisms and fluid pressure on earthquakes and fault slip data in the east Iceland rift zone

Comparison of the eroded off-rift zone left inactive by plate motion with the inner active seismic rift zone allows us to constrain the fracturing mechanisms. In eastern off-rift zone, we measured 423 fault slips (including normal and strike-slip faults). Inversion of fault slip data reveals the parallelism of the minimum stress (σ3) computed for the normal and for the strike-slip faulting, and consistency with the direction of plate divergence. North of the Vatnajökull, in the active rift zone, we analysed 14,250 earthquakes recorded by the Icelandic Meteorological Office (IMO) between 2004 and 2009, especially northeast of the Askja volcano. Two main average focal depths were determined at 5km (Herðubreið table mountain) and 15km (Upptyppingar hyaloclastite ridge). The double couple focal mechanisms determined by IMO revealed that more than half of the mechanisms are strike-slip. Faulting type both in active and off-rift zone reveals an unusual importance of strike-slip regime in such an extensional tectonic context. This can be explained by stress permutations (σ1/σ2). Similarities in terms of stress orientations and type of faulting are observed both in the old and present-day rift zones. We assume that the seismic events may be generated by rapid deep magma intrusion, also associated with shallower hydrothermal activities. We thus propose that the presence of fluids, shear failure (double couple focal mechanism) and stress permutations in both active and off-rift zones are closely linked. Assuming a Drucker–Prager failure criterion, we evaluated analytically that a state near of lithostatic pore pressure is a necessary condition for shear failure at shallow (5km) and deep (15km) depth, in a simple context of crustal extension that allows for stress permutations. However, processes favouring stress permutations cannot be further constrained from our observations since the dynamics of fluid, materiel heterogeneity and post-glacial rebound can also play a significant role.