Northern Volcanic Zone Research Articles

Regional vertical motion in Iceland 1987–1992, determined using GPS surveying

Abstract Vertical motion in Iceland may arise from tectonic sources, e.g. large earthquakes and spreading episodes, isostatic movements owing to the change in mass of icecaps, and secular sinking of the volcanic pile under its own weight. Regional Global Positioning System (GPS) surveys covering the eastern half of Iceland have revealed the vertical motion for the period 1987–1992. Although the errors associated with the vertical component are large, exceptionally good agreement between independent data processing and agreement with isolated terrestrial measurements lends credence to the somewhat surprising results. These are dominated by relative uplift of c. 12 cm in the northern part of the Northern Volcanic Zone and subsidence of up to c. 12 cm elsewhere. This motion can be partially explained by recent tectonic and glacio-isostatic events. The most significant of these are the 1975–1984 Krafla spreading episode and isostatic uplift around the currently melting Vatnajökull icecap of c. 1 cm/year. However, a substantial amount of the observed motion cannot be explained by any known processes. It is concluded that hitherto unknown, largescale processes associated with the hotspot and plume, possibly continuing at depths of c. 75 km, may be responsible. Behaviour of this kind has been observed associated with the Yellowstone hotspot, which is broadly comparable with the Iceland hotspot. Future remeasurements of the GPS network in Iceland will be valuable for refining these observations.

Compressional and shear velocity structure of the lithosphere in northern Iceland

AbstractSeismic data from the B96 array in northern Iceland are used to constrain the compressional and shear velocity structure of the crust and uppermost mantle in northern Iceland, at the western flank of the northern volcanic zone (NVZ) (the mid-Atlantic plate boundary in northern Iceland). Travel times from P and S waves from shots and microearthquakes north and south of the array are used. Two dome structures, with velocities 6% faster than average, are detected at depths down to 7 km. They may be the fossil roots of extinct central volcanoes. P, S, PmP, and SmS wave travel times indicate that the compressional-to-shear-wave velocity ratio in the crust is 1.75 to 1.76, with no significant variation detected between the mid and lower crust. Near-solidus lower crustal temperatures, previously predicted on the basis of high surface heat flow, are ruled out. The crust is 25 to 31 km thick, with the southward thickening occurring in an abrupt step. The relatively high Pn and Sn wave apparent velocities of 8.00 ± 0.1 km/sec and 4.31 ± 0.04 km/sec, respectively, from an earthquake in southern Iceland, are consistent with a mantle lid, that is, a layer of subsolidus mantle separating the Moho from a deeper, partial melt zone. Travel times from P waves crossing the NVZ from a fan shot in eastern Iceland cross the previously identified Krafla high-velocity dome at a depth of 8 to 10 km. They provide new support for the existence of this mid-crustal feature and indicate that its base extends 40 to 50 km along the strike of the NVZ. The neighboring Þeistareykir central volcano has no dome at 8 to 10 km depth, suggesting that it has had a more subsidiary role in the formation of the lower crust.

Deglaciation effects on mantle melting under Iceland: results from the northern volcanic zone

A striking feature of Icelandic volcanism is the effect that the last ice age had on volcanic activity. After the final retreat of ice ∼11 kyr BP, the average eruption rate is estimated to have been 20–30 times greater than it is today. This increase has been attributed to the release of pooled magma through differential tectonic movements during the unloading of ice. However recent work has shown that deglaciation can account for the increase in mantle melting by decreasing the pressure in the upper mantle. We present geochemical data and volume estimates of erupted magmas from Iceland's northern neovolcanic zone which show that the average composition of magmas erupted during the last glacial period in Iceland are significantly more enriched in incompatible trace elements than postglacial and interglacial lavas. The difference in light rare earth element concentrations cannot be accounted for by liquid–crystal fractionation. Averaging the compositions of glacial and postglacial magmas also eliminates the likelihood that the compositional change is due to variations in source composition. An increase in mantle melting from deglaciation can account for both the magma eruption rate and observed changes in trace element concentrations. Finite transport times for magma to travel from the source region to the surface can be estimated from the delay in timing of the increased eruption rates and the end of the last glacial period. This gives transport times of about 1–3 kyr and is consistent with estimates from ( 226Ra/ 230Th) activity ratios measured in ocean island and mid-ocean ridge basalts.

Komatiite Flooding of A Rifted Archean Rhyolitic Arc Complex: Geochemical Signature and Tectonic Significance of the Stoughton‐Roquemaure Group, Abitibi Greenstone Belt, Canada

The 0.2‐2 km thick, Archean Stoughton‐Roquemaure Group (SRG) in the Northern Volcanic Zone of the Abitibi greenstone belt (Quebec, Canada) is composed of tholeiitic basalt, komatiitic basalt and komatiite. The mafic and ultramafic rocks are pillowed, brecciated, and massive columnar‐jointed flows. The SRG conformably overlies the 2730 Ma Hunter Mine Group, a volcanic complex dominated by calc‐alkaline felsic rocks. The tholeiitic basalts of the SRG resemble MORB. The komatiitic basalts and komatiites have positive ϵNd values, overlapping those of the tholeiitic basalts. Komatiitic basalts, with low Al2O3/TiO2 ratios (∼10) and fractionated heavy REE patterns, are similar to Al‐depleted komatiites. In contrast, the komatiites have high Al2O3/TiO2 (∼20), unfractionated heavy REE patterns and resemble Al‐undepleted Munro‐type komatiites. The Al‐depleted komatiitic basalts occur at the base of the SRG, whereas the Al‐undepleted komatiites are prevalent higher up in the stratigraphy. The association of calcalka...

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 (<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.

Petrography and geochemistry of Plio-Quaternary calc-alkaline volcanoes of Southwestern Colombia

The six studied southwestern Colombian Plio-Quaternary volcanoes (Puracé, Doña Juana, Galeras, Azufral, Cumbal and Chiles) are located in the central part of the Northern Volcanic Zone (NVZ) of the Andes. The lavas are predominantly andesites and dacites, with lesser amount of basalt, and range 52% to 70% SiO 2. The two-pyroxenes basic and acid andesites exhibit plagioclase phenocrysts with core composition of An 50–80 and An 40–70 respectively. Hornblende and biotite occur in dacites in association with plagioclase cores ranging An 30–50. The lavas belong to medium- to high-potassic calc-alkaline series, typical of active continental margins. Two types of lavas can be distinguished in the studied area: 1. 1) Northeastern-type lavas which show high TiO 2, K-group elements, LREE, and Ce Yb ratio, low HREE content and a low Ba La ratio 2. 2) Southwestern-type lavas are characterized by low TiO 2, K-group elements, LREE, low Ce Yb , an enrichment in HREE content and high Ba La ratio. The northward increase in incompatible elements is attributed to a higher contribution of the siliceous Pre-Mesozoic crust in the north of the studied area. The younger (Post-Cretaceous) oceanic basement underlying the southwestern volcanoes cannot be responsible for an enrichment in incompatible elements only by contamination processes. Geochemical evidences suggest that the NVZ calc-alkaline lavas were derived from a primitive mantle which was enriched in Ba and partially in other K-group elements by dehydration and/or partial melting of the oceanic slab within the asthenosphere. This primitive melt, induced in response of the hydration processes within the mantle, suffered various stages of assimilation and fractional crystallization at lower to intermediate crustal levels, as revealed by the regional depletion in HREE of the northeastern-type lavas. At an upper crustal level, within the magma chamber, magma mixing between “primitive” up-rising basalt melt and andesitic-dacitic differentiated magma accompanied by fractional crystallization and crustal assimilation processes produced the andesitic series of each volcanoes.

A case of no-wind plinian fallout at Pululagua caldera (Ecuador): implications for models of clast dispersal

The caldera of Pululagua is an eruptive centre of the Northern Volcanic Zone of the South American volcanic arc, located about 15 km north of Quito, Ecuador. Activity leading to formation of the caldera occurred about 2450 b.p. as a series of volcanic episodes during which an estimated 5–6 km3 (DRE) of hornblende-bearing dacitic magma was erupted. A basal pumice-fall deposit covers more than 2.2x104 km2 with a volume of about 1.1 km3 and represents the principal and best-preserved plinian layer. Circular patterns of isopachs and pumice, lithic and Md isopleths of the Basal Fallout (BF) around the caldera indicate emplacement in wind-free conditions. Absence of wind is confirmed by an ubiquitous, normally graded, thin ash bed at the top of the lapilli layer which originated from slow settling of fines after cessation of the plinian column (co-plinian ash). The unusual atmospheric conditions during deposition make the BF deposit particularly suitable for the application and evaluation of pyroclast dispersal models. Application of the Carey and Sparks' (1986) model shows that whereas the 3.2-, 1.6-, and 0.8-cm lithic isopleths predict a model column height of about 36 km, the 6.4-cm isopleth yields and estimate of only 21 km. The 4.9- and 6.4-cm isopleths yield a column height of 28 km using the model of Wilson and Walker (1987). The two models give the same mass discharge rate of 2x108 kg s-1. A simple exponential decrease of thickness with distance, as proposed by Pyle (1989) for plinian falls, fits well with the BF. Exponential decrease of size with distance is followed by clasts less than about 3 cm, suggesting, in agreement with Wilson and Walker (1987), that only a small proportion of large clasts reach the top of the column. Variations with distance in clast distribution patterns imply that, in order to obtain column heights by clast dispersal models, the distribution should be known from both proximal and distal zones. Knowledge of only a few isopleths, irrespective of their distance from the vent, is not sufficient as seemed justified by the method of Pyle (1989).

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.

Tectonic evolution of the Northern Volcanic Zone, Abitibi belt, Quebec

The Archean Abitibi Subprovince has been divided formally into a Northern Volcanic Zone (NVZ), including the entire northern part of the subprovince, and a Southern Volcanic Zone (SVZ) on the basis of distinct volcano-sedimentary successions, related plutonic suites, and precise U–Pb age determinations. The NVZ has been further formally subdivided into (i) a Monocyclic Volcanic Segment (MVS) composed of an extensive subaqueous basalt plain with scattered felsic volcanic complexes (2730–2725 Ma), interstratified with or overlain by linear volcaniclastic sedimentary basins; and (ii) a Polycyclic Volcanic Segment (PVS) comprising a second mafic–felsic volcanic cycle (2722–2711 Ma) and a sedimentary assemblage with local shoshonitic volcanic rocks.A sequence of deformational events (D1–D6) over a period of 25 Ma in the NVZ is consistent with a major compressional event. North–south shortening was first accommodated by near-vertical east-trending folds and, with continued deformation, was concentrated along major east-trending fault zones and contact-strain aureoles around synvolcanic intrusions, both with a downdip movement. Subsequent dextral strike-slip movement occurred on southeast-trending faults and major east-trending faults which controlled the emplacement of syntectonic plutons (2703–2690 Ma).This study suggests that the NVZ, which is a coherent geotectonic unit, initially formed as a diffuse volcanic arc, represented by the MVZ, in which the northern part, represented by the PVS, evolved into a mature arc as documented by a second volcanic and sedimentary cycle associated with major plutonic accretion. Volcano-sedimentary evolution and associated plutonism, as well as structural evolution, are best explained by a plate-tectonic model involving oblique convergence.

Development of sedimentary basins in the Archean Abitibi belt, Canada: an overview

Sedimentation in the Archean Abitibi greenstone belt occurred during four depositional episodes: (i) sedimentary cycle 1, 2730–2720 Ma; (ii) sedimentary cycle 2, 2715–2705 Ma; (iii) sedimentary cycle 3, 2700–2687 Ma; and (iv) sedimentary cycle 4, 2685–2675 Ma. Records of the first two sedimentary cycles are preserved in basins within the northern volcanic zone, whereas basins formed during the latter two sedimentary cycles are located within the southern volcanic zone of the Abitibi belt. Sedimentary cycles 1 and 3 represent deep-water facies, as indicated by turbidites, resedimented conglomerates, pelagic sediments, and ubiquitous iron-formations; subaerial deposits have not been identified. In contrast, sedimentary cycles 2 and 4 show a prevalence of fluvial to shallow-water marine and (or) lacustrine deposits. Tectono-magmatic influence on sedimentation during cycles 2 and 4 is documented by (i) the presence of numerous unconformities underlain by plutonic and volcanic rocks; (ii) locally voluminous shoshonitic and calc-alkaline volcanic rocks; (iii) abundance of plutonic detritus; (iv) rapid vertical and lateral facies changes; and (v) repetition of successions of large-scale (50–250 m thick) alluvial and shallow-water deposits. Sedimentary cycle 1 represents incipient arc basins dominated by volcaniclastic debris, whereas cycle 2 reflects unroofing of arc volcanoes down to the plutonic roots. The sedimentary basins of cycle 3 have been tentatively interpreted as basins connecting arc terranes, within which small extensional cycle 4 basins of the successor or pull-apart type developed. The sedimentary facies associations, the tectono-magmatic influence on sedimentation, the chronological basin evolution, and overall southward younging of the basins invite comparison with modern island arcs formed by plate-tectonic processes.

The Dolodau dykes, Canada: An example of an archean carbonatite

The Archean Dolodau carbonatite dykes occur near a late tectonic syenite stock located in the Northern Volcanic Zone of the Abitibi greenstone belt. The biotite sovite and amphibole-biotite silicocarbonatite dykes produced fenitization of the host rocks. The Dolodau carbonatites compare favourably with Phanerozoic carbonatites in petrography, mineralogy and geochemistry. An Archean age is suggested by field geology and the isotope data available. The similarities suggest that Archean carbonatite petrogenetic processes were similar to modern day processes.

The tectonic evolution of the Abitibi greenstone belt of Canada

AbstractBased on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics.The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma.The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.

Regional O-, Sr-, and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean Cordillera

Quaternary-Recent volcanism in the Andes has occurred in three regions: 45–33°S, 28–16°S and 2°S–5°N, each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics. Isotope ratios of O and Sr are lowest and those of Pb least variable in the southern volcanic zone (SVZ) where medium-K lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent. The SVZ lavas were probably derived from an asthenospheric mantle source above a shallow Benioff zone. Parental basaltic magmas rose largely unmodified through thin continental crust, where differentiation occurred by low-pressure crystal fractionation without concurrent modification of isotopic composition. The slight enrichment of 206 Pb in the northern volcanic zone (NVZ), where the crust is thin and the Benioff zone deep, suggests a greater subduction-zone component for parental medium-K magmas. The slight 18 O enrichment suggests a small amount of lower crustal interaction. Isotope ratios of O and Sr are highest and those of Pb most variable in the central volcanic zone (CVZ). Parental magmas in the CVZ were probably generated within the same mantle source region as those in the SVZ and NVZ. Subsequently, during transit through the exceptionally thick continental crust of the central Andes, high-K magmas were produced by a combination of bulk contamination in the lower crust and later low-pressure fractional crystallization-assimilation processes in the upper crust which altered both chemical and isotopic compositions.