Rift Zone Of Iceland Research Articles

Effective tension-shear relationships in extensional fissure swarms, axial rift zone of northeastern Iceland

The geometry of fracture systems in selected areas of the active Krafla fissure swarm, mid-Atlantic ridge, northeastern Iceland, is analysed. Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the deformation of this initially horizontal surface can be reconstructed. Extensional deformation is localised at all scales and block tilting, though present, remains minor. Using simple models of the surface expression of normal faults, the geometrical characteristics of the topographic features related to active deformation during tectonic-volcanic events are quantitatively analysed. At crustal depths of about 1 km, normal faults are present and have an average 70 ° dip. Comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at palaeodepths of 1–2 km, indicates that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin, newly formed brittle crust of the rift zone. In the axial oceanic rift zone of NE Iceland, the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense, their relative importance depending on depth. Absolute tension dominates in the uppermost several hundred metres of the crust, resulting in the development of fissure swarms. Effective tension plays an important role at a deeper level (2–5 km), because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections. At crustal depths of about 1 km, normal shear prevails along fault planes which dip 60 °–75 °. This importance of normal shear at moderate depth, between upper and lower crustal levels where tension prevails, is pointed out. Within the extensional context of rifting, these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure.

Gravity and magnetic studies of the subglacial Grímsvötn volcano, Iceland: Implications for crustal and thermal structure

The highly active, subglacial, Grímsvötn volcano is associated with a 20‐mGal Bouguer gravity high and a broad magnetic low. The magnetic low roughly coincides with the gravity high, but short‐wavelength magnetic highs are superimposed on the low, especially along caldera rims. Forward models of the gravity and magnetic fields show variable magnetization (0–12 A m−1) in the uppermost 1–2 km of the crust, probably reflecting differences in the proportions of crystalline rocks and hyaloclastites and variations in hydrothermal alteration. The models are consistent with, but do not require, a 10‐km3 molten magma chamber in the shallow crust beneath the main caldera. Between 1.5 and 4 km depth, a dense body (ρ = 2.95–3.0 Mg m−3) with a volume of 400 km3 is located, probably composed of gabbro. Modeling suggests that this intrusive body is nonmagnetic, which may imply that its temperature is at or above the Curie point. The distribution of geothermal activity at the glacier bed is consistent with heat extraction from this dense, hot intrusive body; it may be an important source for the presently observed geothermal heat output of 2000–4000 MW at Grímsvötn. This high heat transfer rate is difficult to reconcile with estimates of long‐term heat production in an Icelandic rift zone volcano. The heat flux may be a transient phenomenon, caused by enhanced vertical permeability due to a recent caldera collapse and subsequent heat mining of the intrusive body.

A new model for the evolution of table mountains: volcanological and petrological evidence from herdubreid and Herdubreidartögl volcanoes (Iceland)

We present a new model for the evolution of volcanic table mountains, based on volcanological and petrological studies of Herdubreid/Herdubreidartogl, an upper Pleistocene volcanic complex within the active Icelandic rift zone. The evolution of these table mountains is highly complex and influenced substantially by different eruptive environments as well as changing magma sources and volcanic and magmatic processes. Whereas current models imply entirely subglacial conditions and continuous compositional (“monogenetic”) evolution for these volcanoes, we subdivide their evolution into four eruptive periods characterized by different environments: (a) Subaerial lava flows erupted during an interglacial period. (b) Lavas and voluminous hydroclastic deposits formed in a lake during a waning period of the last glaciation. (c) Subglacial eruptions during thickening of the ice sheet as a result of a climatic deterioration built pillow lava piles overlain by steep-sided complexes of hydroclastic deposits. These deposits differ from those of the second eruptive period in structure, texture, and degree of alteration. Subaerial lava flows and agglutinates covered these deposits after buildup above the ice sheet. (d) Subaerial lava flows and fallout deposits during a postglacial period. The detailed analysis of volcanic table mountains appears to be a very sensitive indicator of climatic fluctuations. Although most deposits of the studied volcanoes were formed during the waning period of the last glaciation, the subglacial deposits in the upper part of the volcanic complex reflect a temporary, but major, thickening of the ice sheet.

Ocean-ridge discontinuities in Iceland

Abstract The Tjörnes fracture zone in North Iceland and the South Iceland seismic zone are ocean-ridge discontinuities that are partly exposed on land. These are zones of high seismicity, but contrasting style of crustal deformation. Detailed field studies and numerical modelling suggest that the Tjörnes Fracture Zone has all the characteristics of an oceanic transform fault, is 7–9 Ma old, and developed along the zone of maximum shear stress between the nearby tips of the rift zones of the Kolbeinsey Ridge and North Iceland. Conversely, the results indicate that the South Iceland seismic zone developed within an area of shear-stress concentration between the overlapping rift zones in South Iceland. The development of the South Iceland seismic zone is a consequence of the propagation of the eastern rift zone to the southwest during the past 3 Ma.

Generation and equilibration of olivine tholeiites in the northern rift zone of Iceland. A petrogenetic study of the Bláfjall table mountain

The Bláfjall table mountain is located in the active northern rift zone of Iceland. It is part of an olivine tholeiite complex, erupted mostly during Weichselian times (75-10 ka). The compositions and volumes of the magmas, as well as the time span and spatial proximity of the eruptive units, imply the contemporaneous existence of large magma reservoirs at different upper mantle/lower crustal levels beneath the area. Three distinct geochemical and petrographical tholeiite types can be identified: (1) A small unit of ca. 0.1 km 3 consists of some of the most primitive tholeiites reported from Iceland. The rocks are nearly aphyric with olivine as the only minor phenocryst phase. (2) The bulk of the complex consists of large (up to 6 km 3) units of average icelandic tholeiites with olivine and occasionally plagioclase phenocrysts. (3) The youngest unit amounts to ca. 3 km 3, forming the top of the Bláfjall table mountain. This is the only unit that has crystallized a pseudo-invariant assemblage of olivine, plagioclase and clinopyroxene. Whole-rock compositions of all units indicate equilibrium with mantle phases at 7–10 kbar where the largest extent of melting takes place. Liquids derived by melting over a wide range of pressure in the upper mantle accumulated and mixed in a primary reservoir located at around 4 kbar. This reservoir subsequently fed most of the eruptions in the area. Glass geochemistry implies that a secondary magma reservoir developed at 2–3 kbar, crystallizing a gabbroic three-silicate-phase assemblage of crustal affinity. Extraction from this magma reservoir was triggered by a very large eruption (6 km 3) from the primary reservoir.

Oxidation state of iron in mantle-derived magmas of the Icelandic rift zone

Olivine tholeiites are mantle-derived magmas that are formed by partial melting of their deep sources and which have equilibrated with mineral assemblages at slightly different subcrustal pressure-temperature conditions prior to eruption. The minimum depth of the pre-eruptive reservoirs of these magmas is in the order of 10–15 km and their liquidus temperatures fall within the range of 1180–1240 ‡C. Three types of primitive olivine tholeiites are exposed along the rift zones in Iceland. In the present study, the ferric/ferrous ratios of natural glasses (pillow crusts) of the three types of olivine tholeiites were obtained by Mossbauer spectrometry. This technique is particularly well suited for the analysis of high-Mg glasses since it resolves microcrystallites of olivine which contribute to ferrous iron in chemical analysis. All results fall within 10–15% Fe(III). At the liquidus temperature of these glasses, this ferric/ferrous ratio corresponds to fugacity close to the fayalite-magnetite-quartz-oxygen (FMQ) buffer with an uncertainty of less than one log unit in fO2. This result confirms that there is no significant difference in the oxidation state of the three magma types.

Fissure‐fill and tunnel‐fill sediments: expressions of permafrost and increased hydrostatic pressure

AbstractA spectacular network of fissure fillings and pipes (tunnels) cuts Quaternary gravelly delta deposits northeast of Myvatn, precisely on the spreading axis of the North Icelandic rift zone. The delta was formed in an ice‐contact lake during deglaciation towards the end of the last glaciation. Subsequently the lake was drained and permafrost conditions developed in these poorly sorted gravel deposits. Hydrostatic pressure was transmitted from the adjacent glacier to the non‐frozen core of the delta beneath the discontinuous permafrost crust and the seasonally frozen active layer. Owing to increased hydrostatic pressure, a network of subhorizontal to vertical fissures was opened along the taliks. In these fissures free ground‐water flow and sediment transport were established. Tunnel erosion and probably also seepage erosion were associated with these fissures. Subsequently, the fissures and tunnels were filled by laminated fine sediments interbedded with poorly sorted material resulting in the formation of fissure‐fill sediments and tunnel‐fill sediments.

Effect of glacier loading/deloading on volcanism: postglacial volcanic production rate of the Dyngjufj�ll area, central Iceland

Tephrochronological dating of postglacial volcanism in the Dyngjufjoll volcanic complex, a major spreading center in the Icelandic Rift Zone, indicates a high production rate in the millennia following deglaciation as compared to the present low productivity. The visible and implied evidence indicates that lava production in the period 10 000–4500 bp was at least 20 to 30 times higher than that in the period after 2900 bp but the results are biased towards lower values for lava volumes during the earlier age periods since multiple lava layers are buried beneath younger flows. The higher production rate during the earlier period coincides with the disappearance of glaciers of the last glaciation. Decreasing lithostatic pressure as the glacier melts and vigorous crustal movements caused by rapid isostatic rebound may trigger intense volcanism until a new pressure equilibrium has been established.

Structure and development of the Sveinagja graben, Northeast Iceland

The Sveinagja graben strikes N8° E, is 34 km long and as much as 2.5 km wide, and is located in the Holocene basaltic (mostly pahoehoe) lava flows of the rift zone in Northeast Iceland. During a rifting episode in 1872–1875, the slip on the boundary faults of the northern part of the graben was as much as 3–6 m, but this part was mostly buried under a lava formed in the 1875 Sveinagja eruption. The southern part of the graben, referred to as Sveinagja, is itself an exceptionally well-defined graben, 9 km long and 1–1.7 km wide. Over one hundred accurate field measurements of the width and throw were made at intervals of 100 m along the length of the boundary faults of this part. These faults are normal faults, continuous for 5.2–5.8 km, as much as 10–13 m wide and with throws of up to 16–17 m. The arithmetic mean width at all points of measurement is 4.2 m. It is proposed that the boundary faults of Sveinagja developed from (mostly columnar) joints in the lava pile. The faults were generated either directly from sets of inclined joints, when the lava pile had become tilted, or from large-scale tension fractures which in turn developed from joints. In the rift zone of Iceland, where the in situ tensile strength ranges from 1 MPa to 6 MPa, large-scale tension fractures rarely reach crustal depths in excess of 0.5–0.8 km; at greater depths they usually change into normal faults. Theoretical considerations suggest that, whether they developed from sets of inclined joints or from large-scale tension fractures, the boundary faults of Sveinagja initiated at crustal depths of 0.5–1.5 km. Using continuum crack models in combination with the aspect ratios of the boundary faults and the appropriate elastic moduli, the driving tensile and shear stresses associated with the fault development are estimated as 4–5 MPa and 2–5 MPa, respectively. From field data, historical accounts and theoretical considerations, it is concluded that the Sveinagja graben was mostly, or completely, generated during the Holocene.

Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, South Iceland

A Pleistocene subaqueous, volcanic sequence in South Iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units. Emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of Iceland. The higher sea level was caused by eustatic sea-level change, probably towards the end of a glaciation. This sequence, nearly 700 m thick, rests unconformably on eroded flatlying lavas and sedimentary rocks of likely Tertiary age. A Standard Depositional Unit, describing the flows of hyaloclastite, starts with compact columnar-jointed basalt overlain by cubejointed basalt, and/or pillow lava. This in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes, and bedded hyaloclastite at the top. A similar lithofacies succession is valid for proximal to distal locations. The flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the Eastern Rift Zone of Iceland. The fissures are assumed to lie in the same general area as the 1783 Laki fissure which produced 12 km3 of basaltic lava. Due to very high extrusion rates, the effective water/melt ratio was low, preventing optimal fragmentation of the melt. The result was a heterogeneous mass of hyaloclastite and fluid melt which moved “en masse” downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above. The upper and distal parts of the flow moved as low-concentration turbulent suspensions that deposited bedded hyaloclastite.

Gas geochemistry of geothermal fluids, the Hengill area, southwest rift zone of Iceland

A systematic study of the chemical and isotopic (C, S, N, He) composition of fumarole and geothermal well gases has been undertaken along a 15 km section of a subaerial spreading center (Hengill area, southwest Iceland). The highest geochemical (H 2) temperatures and highest helium contents are found along the rift axis and tend to decrease with distance from this axis, as a result of cooling and gas loss from underground fluids. In terms of components, these gases appear to be derived mostly from the mantle, with minor addition from the atmosphere. Chemical ratios between species do not support mineral control of CO 2 fugacity in this kind of geothermal reservoir. δ 15N values range from −10.4‰ to −0.2‰ (versus ATM) which may result either from kinetic fractionation of atmospheric nitrogen or from the occurrence of a deep component. Values of δ 34S close to 0‰ (versus CDT) and constant helium isotope ratios, typical of hot spot helium ( R R A = 14.4 ), confirm the deep origin of the gases, δ 13C values varying from −9‰ to −1.6‰ (versus PDB) are attributed to aqueous phase changes in geothermal aquifers. When corrected for fractionation, a δ 13C mean of −4.1‰ may represent the magmatic signature in the rift area. Heat 3 He ratios (1.3–8.4) × 10 16 J/mol are intermediate between Loihi Seamount (Hawaii) and mid-ocean ridge values. High enthalpy geothermal systems in rift zones of Iceland provide a means for studying volatile transfers at spreading centers and of supplementing data derived from oceanographic studies.

Seismic crustal structure in Iceland and surrounding area

The basaltic crust of Iceland and surrounding areas can be divided into two parts, the upper crust and the lower crust. The upper crust is characterized by increasing P-wave velocity with depth. At the surface of the basaltic lava pile, the velocity can be as low as 2.0–3.0 km/s in fresh highly porous lava in the volcanic rift zones of Iceland and at the crest of the adjacent Mid-Atlantic Ridge (MAR). Elsewhere, the P-wave velocity at the surface is in the range of 3.0–5.0 km/s, mainly depending on the degree of secondary mineralization. From the surface the velocity of the upper crust increases more or less continuously down to the top of the lower crust where the velocity is nearly constant at 6.3–6.7 km/s. The continuous increase in velocity with depth in the upper crust is considered to be due to closure of fractures caused by lithostatic pressure and to an increasing content of secondary minerals which reduces the porosity. In addition to the general increase in velocity with depth, thin high-velocity layers are observed within the upper crust. These are interpreted as relatively dense parts of the lava pile with a low content of scoriaceous material. The thickness of the upper crust is in the range 3–6 km in Iceland, except in southeastern Iceland where it exceeds 10 km. On the Greenland-Iceland-Faeroe Rise it is in the range of 3–8 km but on the Reykjanes Ridge it is considerably less, close to 2 km. This implies that the structure of the Icelandic upper crust is more similar to that observed on the Greenland-Iceland-Faeroe Rise than that on the MAR. The lower crust is very homogeneous in terms of P-wave velocity. It is most likely also composed of basaltic lava and intrusive rocks of almost zero porosity and with considerable amounts of heavy secondary minerals such as epidote In some cases there seems to be a low velocity layer at the base of the lower crust indicating partially melted material. There are no signs of any sudden changes in the velocity structure of the crust between Iceland and the adjacent oceans, but the crustal layers in Iceland are generally thicker than on the Reykjanes Ridge. The crustal thickness in Iceland is rather poorly mapped but seems to be of the order of 10–15 km. This thickness reaches as much as 30–35 km on the Iceland-Faeroe Rise and probably 25 km on the Greenland-Iceland Rise. The subcrustal material has a P-wave velocity of 7.0–7.4 km/s below Iceland and at the crest of the MAR, 7.8–7.9 km/s below the Iceland-Faeroe Rise and 8.4 km/s below the flank of the Reykjanes Ridge. There seems to be a sharp change in the subcrustal velocity structure at the edge of the Icelandic shelf. Multichannel reflection surveys in the Tjörnes Fracture Zone north of Iceland reveal a sedimentary basin with up to 4 km of sediments. The western part of the basin is a prominent graben with numerous listric normal faults. This graben is an apparent continuation of the spreading axis on the Kolbeinsey Ridge.

Crustal spreading due to dikes and faults in southwest Iceland

A 30 km long profile, extending from 3 million year-old Pleistocene rocks to the Holocene rocks of the rift zone in southwest Iceland, is dissected by 443 dikes and 156 normal faults. Nearly all the dikes are basaltic, with an average strike of N26°E, which is subparallel to the trend of fractures in the nearby rift zone. Their average dip is 69° and average thickness 1.37 m, both values much lower than averages for regional Tertiary dike swarms in Iceland. Many dikes are seen to change into sills. The great majority of the faults are pure dip-slip, with an average throw of 10.2 m. Their average strike, N37°E, is different from the average strike of the dikes in this area and the fractures in the nearby rift zone. Their average dip is 75.2°, which is somewhat steeper than in the Tertiary areas, but the dips are less in the interbedded sedimentary layers than in the lava flows. Growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10,000 years. In subsections, crustal dilation due to dikes is 0.4–3.7%, that due to faults is 0.6–5.3%, and that due to both dikes and faults is 1.04–6.44%. In these subsections, average fault spacing is one fault per 170–563 m. Spacing decreases toward the active rift zone. It is concluded that in typical fissure swarms of the rift zone, the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults. In the depth range of a couple of hundred metres to perhaps 1 km, the non-vertical faults contribute most to the dilation. At deeper crustal levels, however, dilation due to dikes becomes dominant.

High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Experimental results

Melting experiments have been performed on a primitive, mildly alkalic glassy lava (10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fo 81) crystallizes from 1240°C, followed by plagioclase (An 70) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite are ferrobasaltic enriched in FeO (13.6–14.2 wt.%) and TiO 2 (4.0–4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in Al 2O 3 and Na 2O and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in Al and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite, but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivine-plagioclase-diopside normative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopside normative divide.

High and low pressure phase equilibria of a mildly alkalic lava from the 1965 Surtsey eruption: Implications for the evolution of mildly alkalic and transitional basalts in the south-eastern propagating rift zone of Iceland.

The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al 2O 3 and Na 2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2–3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3–8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions. In contrast to the Vestmannaeyjar lavas, the lavas behind the propagator often contain augite as a phenocryst. It is concluded that the chemical variation observed along the south-eastern propagating rift of Iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator.

Earthquake swarms on the Mid‐Atlantic Ridge: Products of magmatism or extensional tectonics?

By analogy with the microearthquake swarms which accompany terrestrial volcanic eruptions, teleseismically observed earthquake swarms along mid‐ocean ridges are commonly thought to be indicators of volcanism, particularly dike emplacement. We test this assumption by investigating the spatial and temporal patterns and other characteristics of earthquakes in 34 swarms on the Mid‐Atlantic Ridge. Resolution of the relative locations of swarm events is improved by the use of a multiple‐event relocation technique. Most swarms extend 5 to 15 km across the ridge axis and 5 to 20 km along it. None of the swarms displays the large length‐to‐width ratio and temporal migration of epicenters along axis which are characteristic of earthquake swarms in the well‐studied rift zones of Hawaii and Iceland. The relative locations, magnitudes, and source mechanisms of swarm events are most easily explained if teleseismically observable swarms are expressions of extension across a substantial portion of the median valley and perhaps into the rift mountains. The possibility that some of these swarms include events directly associated with magmatic intrusion cannot be excluded, however. Like their terrestrial counterparts, purely volcanic earthquakes on the mid‐ocean ridges probably occur dominantly at magnitudes below the teleseismic detection threshold of global seismic networks (mb < 4.5). The largest normal‐faulting earthquakes on mid‐ocean ridges and many of the teleseismically observed swarms likely occur on the faults marking the inner rift wall, where the crust first experiences significant faulting and is broken into numerous closely‐spaced normal faults. We speculate that the high degree of order in the dimensions, spacing, throws, and mechanical properties of these faults, combined with stress concentrations arising naturally from the mechanical behavior of the lithosphere at rifted, slow‐spreading ridges, may account for the common occurrence of earthquake swarms.

A comparison of rift-zone tectonics in Iceland and Hawaii

Observations of eroded volcanic rift zones indicate that dikes in Iceland are typically several times thicker than those in Hawaii. Geodetic and seismic observations of active rifts, however, suggest that dike heights in the two regions are similar. Provided the elastic properties of the rift zones are the same, this implies that dikes are intruded with higher driving pressures (magma pressure minus compressive stress perpendicular to the dike plane) in Iceland than Hawaii. A second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in Iceland. From this it can be infered that a lower percentage of dikes breach the surface in Iceland than in Hawaii. Thus, although dikes in Iceland are intruded with higher driving pressures, they possess lower absolute magma pressures than in Hawaii. These differences can be interpreted in terms of the tectonic settings in the two regions. In Iceland, a steady remote extension reduces the horizontal stress perpendicular to the rift zone, allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available. In Hawaii, a more continuous magma supply on the timescale over which the dike-induced stresses are relaxed, and perhaps a greater role for intrusions in driving long-term rift extension, ensure that the rift-compressive stress is not relaxed significantly before the next dike is intruded. Thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur. If the mechanism for relaxing the rift-compressive stress were less efficient still, then an even higher percentage of dikes would erupt, and at times the rift zone trend could become an unfavorable orientation for dike intrusion. Such might be the case at Mauna Loa, which lacks large rift-zone faults and fissures and possesses numerous radial vents outside its two main rift zones.

Thorium, strontium and oxygen isotopic geochemistry in recent tholeiites from Iceland: crustal influence on mantle-derived magmas

We present new Sr, Th, and O isotopic results on recent olivine tholeiites and quartz tholeiites from the active rift zones in Iceland. Our data show a negative correlation between 87Sr/ 86Sr ratios and δ 18O values, and a positive correlation between ( 230Th/ 232Th) ratios and δ 18O. These covariations of isotopic ratios strongly support the model proposed by Oskarsson et al. (1982) [1]; which involves a mixing of mantle-derived magmas with crustal silicic magmas produced by melting of old hydrothermally altered rocks under central volcanoes. The hybrid magmas have quartz tholeiite compositions. Thus a large part of the Iceland geochemical anomaly is probably inherited from interaction of the primitive magmas (P-type tholeiites) with the Icelandic crust, and the mantle isotopic heterogeneities in the Icelandic “mantle plume” seem to be rather small.

Dike-induced faulting in rift zones of Iceland and Afar

Geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the Earth9s surface. Elastic modeling of the vertical ground-surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth. A mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike. Therefore, slip extending into or beyond the dike cavity must occur either (1) on faults that intersect the dike near its top, above the zone of dike-induced compression, or (2) on faults that slip ahead of the dike as it propagates laterally. Data from Iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike.

Dynamics of crustal rifting in NE Iceland

Magnetotelluric measurements have revealed a crustal thickness of 8–10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20–30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north with increasing distance from the hot spot. The crust is underlain by a diapiric updoming asthenosphere over large parts of the country. At the crust asthenosphere interface there is a partially molten basaltic layer a few kilometers thick. This layer partially decouples the crust from the anomalous mantle. Its low viscosity may explain the spatial and temporal instability of the rift axis and the high mobility of the crust during technically active periods. A major volcano‐tectonic rifting episode started in the Krafla area in 1975 involving an 80‐km‐long segment of the plate boundary. Several meters of opening accompanied by subsidence was observed in the center of the axial fissure swarm and contraction and uplift occurred on the sides. The rifting occurred in several short events. Magma flowed horizontally from an upper crustal magma chamber and mixed with magma ascending directly from the semifluid layer at the base of the crust. Crustal accretion was mainly engineered by dike intrusions. Only a minor part of the magma transported reached the surface. During technically quiet epochs, plate‐driving forces, caused by the upwelling asthenosphere, result in long‐term stretching and accumulation of tensile stress in the thinnest part of the crust. The vertical and horizontal displacements during rifting episodes can be explained by elastic rebound, where fissure opening accompanied by magma flow causes the release of tension.