Axial Rift Zone Research Articles

Spatial distribution and geochemical characterization of Icelandic mantle end-members: Implications for plume geometry and melting processes

We compile a new geochemical database (Icelandic Volcanic rocks Isotopic Database, IVID) which includes previously reported 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 187Os/188Os and 3He/4He data and major and trace element concentrations measured in Icelandic volcanic rocks. Isotopic compositions were evaluated and filtered to identify the highest quality data and data most likely to represent mantle-derived compositions. The carefully filtered, comprehensive geochemical database is an important contribution to the geochemical community and can be used to put further constraints on the generation of geochemical heterogeneity in Iceland. We use the compiled database to examine the spatial distribution of geochemical components in the Icelandic mantle, and test whether melting processes control how source heterogeneity from the deep mantle is extracted on the surface. The PRIMELT software (which combines an inverse model for crystallization in the crust with a forward model of mantle melting to estimate the primary mantle-derived magma composition) is used to estimate mantle potential temperatures and melt fractions in Iceland. The results of these calculations and forward melting models demonstrate that mantle potential temperatures have a largely insignificant role, and that lithospheric thickness plays a key role in generating geochemical heterogeneity in Icelandic volcanic rocks. Volcanic rocks from the off-rift volcanic zones are generally produced by lower melt fractions, consistent with preferential sampling of geochemically enriched and fertile “blobs” at lower degrees of melting, where the lithosphere is thicker; more geochemically depleted volcanic rocks from the axial rift zones, where the lithosphere is thinner, show higher degrees of melting, consistent with dilution of enriched blob melts by melts of more depleted and refractory mantle.

Across and along-strike crustal structure variations of the western Afar margin and adjacent plateau: Insights from receiver functions analysis

We used teleseismic receiver function analysis to image the crustal structure beneath 24 broadband seismic stations densely deployed along two profiles traversing different structural units across the western Afar margin. Our high-resolution receiver function results image pronounced spatial variations in the crustal structure along the profiles and provide improved insights to understand how strain is partitioned in the crust during rifting. Beneath the western plateau next to northern Afar, the crust is likely felsic-to-intermediate in composition (average Vp/Vs 1.74), with a step like thinning of the crust from an average of 38 km beneath the western plateau to an average of 22 km beneath the marginal graben. Consistently thicker crust is observed beneath the southern profile (central Afar), showing four distinct regions of uniform crustal thickness: 1) an average crustal thickness of 42 km beneath the western plateau; 2) 34 km beneath the foothills area; 3) 28 km beneath the marginal graben and the wide extensional basin and 4) 21 km beneath the central rift axis. We use crustal thickness results to estimate a stretching factor β of 2.2 and 2.7 for central Afar and northern Afar respectively. Our estimated values are lower than β > 3.0 predicted from plate reconstructions, and we interpret that the variations are best explained by 2–5 km magmatic addition into the crust. The crustal composition beneath the southern profile is more complex with elevated Vp/Vs ratios ranging between 1.79 and 1.85 beneath the western plateau and marginal graben. This is consistent with a greater mafic component and best explained by crust altered by intrusions due to significant pre and syn-rift magmatic activity. Abnormally high Vp/Vs ratios of more than 1.90 are observed beneath the axial rift zone of central Afar, which most likely suggests the localization of partial melt within the crust.

Evidence of an active rift zone in the northern Okinawa Trough

The character of continental rifting processes in the northern part of the Okinawa Trough remains largely unknown due to the absence of an obvious axial rift zone or recent rift-related volcanism in the region. This paper describes the discovery of a tectonically and magmatically active rift zone in the northern trough, and discusses the nature of the rift. Ship-based bathymetry identified a group of NE–SW-trending surface lineaments on the western margin of the northern trough that form a small rift structure. The newly discovered rift, named the Uji Rift, is ~30 km long, ~10 km wide and ~180 m deep. A seismic reflection profile near the rift confirmed the existence of well-developed normal faults with large displacements, suggesting that the rift was formed by back-arc rifting. A previously unknown volcanic edifice, named West Uji Knoll, which consists of multiple cones with pronounced summits and high acoustic backscatter characteristics, also exists within the rift. High-resolution mapping with an autonomous underwater vehicle at one of the cones confirmed the existence of lava flow and eruptive fissure morphologies. Lava flows cover the pre-existing lineaments that form the Uji Rift. The flows are undissected by any lineaments, suggesting that West Uji Knoll is a young submarine volcano and the most recent eruptions likely occurred after rifting activity in the early or late Pleistocene. Active magmatism is also inferred from evidence of hydrothermal venting, such as the existence of an acoustic plume and sharp temperature increases of up to ~0.06 °C, even at a depth of ~100 m above the seafloor. In addition, three bubble plumes were detected near the cones by the shipboard multibeam sonar, suggesting emission of magmatic gases. Our volcano-tectonic observations show that the Uji Rift developed at an unusual location, ~170 km behind the arc volcanic front. Continental rifting in the northern Okinawa Trough is considered to extend onshore at the Beppu-Shimabara Rift Zone in SW Japan. However, due to the small number of submarine studies in this area, where and how the rifting is developing in the trough and has extended from the ocean to the continent is poorly understood. As an active rift, the Uji Rift provides an important constraint on the continuity of rifting in the Okinawa Trough, as well as new insights into the propagation and migration processes of continental rifting in subduction systems.

Field evidence of the interplay between rift and transform structures in the Krafla geothermal area, N-Iceland

The Krafla volcano-geothermal area is located in Northeast Iceland, within the active Northern Volcanic Zone sector of the axial rift and has experienced two eruptions in historical times. The Krafla system displays NNE-SSW oriented fissure swarm, a central volcano and associated geothermal manifestations (i.e. steam and gas emissions, hydrothermal alterations). However, during its exploration and exploitation, several geophysical indications have suggested the occurrence of WNW-ESE trending structures, i.e. near orthogonal with respect to the faults delimiting the axial rift. This paper describes the results of a structural survey carried out in a wide area in the NE-sector of Iceland along the Húsavík-Flatey shear zone and in the Krafla area, measuring fractures and alignments of geothermal manifestations and fissure swarms. The dataset highlights the occurrence of structures oriented in two main trends, i.e. NNW-SSE to NNE-SSW and WNW-ESE. At their intersection, geothermal manifestations and silica sinter deposits commonly occur. Both of these trends have been documented in the Krafla geothermal area and compared with borehole and geophysical data. Our conclusions support that the location of the high-temperature Krafla geothermal area is mainly controlled by the enhanced permeability at the intersection between structures of the axial rift zone and incipient transform faults crossing the Krafla area.

On time of the triassic rifts system origin in West Siberia

It is commonly supposed that a very substantial volume of early basalt magmatism effused synchronously on Siberia platform and West Siberia in a very short time interval at 249.4 ± 0.5 Ma (Reichow et al., 2002, etc.). This magmatism and induced climate change are considered as a main reason of the most catastrophic in the Earth history extinction at the border of Permian and Triassic time. But these conclusions were based on incomplete and unrepresentative data on West Siberia. We have obtained by analysis of pyroxenes monofraction from kainotype basalts of Guslinskaya P-430 well Ar-Ar age 268.4 ± 7.5 Ma. In Taurovskaya 503 well this age is 268.1 ± 7.5 Ma. Hence, volcanism in axial rift zones of the basement of West Siberia plate began earlier than that considered before and significantly earlier than on Siberia platform.

Geological structure of the Pre-jurassic basement of the Yugansk-Koltogorsk zone of the West Siberia

Subject of the study.We have carried out an additional exploration on petrography, petrochemistry, geochemistry, geochronology and biostratigraphy of rock complexes of the Yugansk-Koltogorsk (central part of West Siberia).Materials and methods.The materials are core samples from wells that penetrated the Pre-Jurassic basement rock complexes The used methods are as follows: X-ray spectrum microanalysis (Cameca SX 100, JEOL-733 Superprobe); the silicate analysis of the rocks (СРМ-18 and EDX-100); mass-spectroscopy with inductively coupled plasma (ICP-MS) (ELAN-9000 and Element2); zircons were analysed with a high resolution ionic microprobe SHRIMP-II (U-Pb); Ar-Ar dating was carried out by Micromass 5400; biostratigraphy, petrochemestry.Results.A geological map of Yugan-Koltogor zone of the central part of West-Siberian plate in a scale 1:500 000 was created. The map is a set of informational geological, geophysical and other layers. It was established that age of granites is Early Permian, the basalts of the rift zone began to form 268.4 ± 7.5 Ma ago (Ar-Ar). Conclusions. Volcanism in the axial rift zones of the West-Siberian megabasin basement began earlier than it is supposed before our study and significantly earlier of that of trap basalts of Siberian platform.

Distributed Deformation in an Oceanic Transform System: Applying Statistical Tools to Structural and Paleomagnetic Data Near the Húsavík‐Flatey Fault, Northern Iceland

AbstractThe right‐lateral Húsavík‐Flatey fault is part of the Tjörnes Fracture Zone, which links the offshore and onshore rift axes in northern Iceland. There has been debate about whether rocks near this fault have accommodated distributed off‐fault deformation, which is testable using paleomagnetic data. Recent studies from Flateyjarskagi show clockwise declination deflections that are largest near the fault. We augment these data with new structural and paleomagnetic measurements from 106 lava flows across three peninsulas—Flateyjarskagi, Tröllaskagi, and Tjörnes—also finding clockwise deflections that vary with distance from the fault. To test whether the deflections could be caused by off‐fault deformation, we combine our measurements with other regional data sets, applying several statistical tools including regressions of structural or paleomagnetic directions versus fault‐normal distance. To evaluate the significance and uncertainties of the regressions, we use permutation tests and bootstrapping. For Flateyjarskagi, our analysis suggests that lavas and dikes were deformed together; the regression results predict 4°–6° of rotation per kilometer about a steep, but not vertical, axis. Rocks on Tröllaskagi hint at similar spatial patterns with fault distance, but the data quality precludes a full analysis. Rocks on Tjörnes show no spatial patterns, but they preserve a temporal history, where rotation seems to have ceased after deposition of the Pliocene‐age Tjörnes beds. Using constraints from our statistical analyses, geochronology, and comparisons with the transform system in southern Iceland, we propose several modifications to models for the evolution of axial rift zones in northern Iceland.

Seismic hazard assessment of the Kivu rift segment based on a new seismotectonic zonation model (western branch, East African Rift system)

In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed for the Kivu rift segment in the central part of the western branch of the East African rift system. As the geological and tectonic setting of this region is incompletely known, especially the part lying in the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic maps.The seismic risk assessment is based on a new earthquake catalogue based on the ISC reviewed earthquake catalogue and supplemented by other local catalogues and new macroseismic epicenter data spanning 126 years, with 1068 events. The magnitudes have been homogenized to Mw and aftershocks removed. The final catalogue used for the seismic hazard assessment spans 60 years, from 1955 to 2015, with 359 events and a magnitude of completeness of 4.4. The seismotectonic zonation into 7 seismic source areas was done on the basis of the regional geological structure, neotectonic fault systems, basin architecture and distribution of thermal springs and earthquake epicenters.The Gutenberg-Richter seismic hazard parameters were determined by the least square linear fit and the maximum likelihood method. Seismic hazard maps have been computed using existing attenuation laws with the Crisis 2012 software. We obtained higher PGA values (475 years return period) for the Kivu rift region than the previous estimates. They also vary laterally in function of the tectonic setting, with the lowest value in the volcanically active Virunga – Rutshuru zone, highest in the currently non-volcanic parts of Lake Kivu, Rusizi valley and North Tanganyika rift zone, and intermediate in the regions flanking the axial rift zone.

Sulfide geochronology along the Northern Equatorial Mid-Atlantic Ridge

Hydrothermal processes and seafloor massive sulfide (SMS) deposits have different characteristics at fast and slow spreading mid-ocean ridges. One such parameter is the age of a SMS deposit, which differs by 1–2 orders of magnitude between the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR). The large collection of SMS samples dated from the 18 hydrothermal fields of the northern equatorial part of the Mid-Atlantic Ridge (194 samples) demonstrates a relatively old average age of hydrothermal fields here (~66ka) with the oldest one estimated as ca. 223ka (Peterburgskoye field). Based on geochronological data it was confirmed that hydrothermal discharge has an episodic character: active and inactive periods of the SMS formation alternate. The distribution of events at all hydrothermal fields demonstrates that maximum activity occurred at 38–35, 30–20, and 8–2ka and increased with time. Based on statistical analyses, dating variations can be explained as a superposition of several periods of activity with the duration of ~15, 10 and 5ka.Relationship between the age and distance from the axial rift zone as well as between the age and aerial distribution is different for SMS deposits hosted by basalts and by gabbro-peridotites depending on their geological setting on the particular MAR segment. This difference can be explained by a variety of hydrothermal processes determined by “tectonic” or “magmatic” segment evolution and symmetrical or asymmetrical mode of accretion (Escartin et al., 2008).

Carbon isotope and abundance systematics of Icelandic geothermal gases, fluids and subglacial basalts with implications for mantle plume-related CO2 fluxes

We report new carbon dioxide (CO2) abundance and isotope data for 71 geothermal gases and fluids from both high-temperature (HT>150°C at 1km depth) and low-temperature (LT<150°C at 1km depth) geothermal systems located within neovolcanic zones and older segments of the Icelandic crust, respectively. These data are supplemented by CO2 data obtained by stepped heating of 47 subglacial basaltic glasses collected from the neovolcanic zones. The sample suite has been characterized previously for He–Ne (geothermal) and He–Ne–Ar (basalt) systematics (Füri et al., 2010), allowing elemental ratios to be calculated for individual samples. Geothermal fluids are characterized by a wide range in carbon isotope ratios (δ13C), from −18.8‰ to +4.6‰ (vs. VPDB), and CO2/3He values that span eight orders of magnitude, from 1×104 to 2×1012. Extreme geothermal values suggest that original source compositions have been extensively modified by hydrothermal processes such as degassing and/or calcite precipitation. Basaltic glasses are also characterized by a wide range in δ13C values, from −27.2‰ to −3.6‰, whereas CO2/3He values span a narrower range, from 1×108 to 1×1012. The combination of both low δ13C values and low CO2 contents in basalts indicates that magmas are extensively and variably degassed. Using an equilibrium degassing model, we estimate that pre-eruptive basaltic melts beneath Iceland contain ∼531±64ppm CO2 with δ13C values of −2.5±1.1‰, in good agreement with estimates from olivine-hosted melt inclusions (Metrich et al., 1991) and depleted MORB mantle (DMM) CO2 source estimates (Marty, 2012). In addition, pre-eruptive CO2 compositions are estimated for individual segments of the Icelandic axial rift zones, and show a marked decrease from north to south (Northern Rift Zone=550±66ppm; Eastern Rift Zone=371±45ppm; Western Rift Zone=206±24ppm). Notably, these results are model dependent, and selection of a lower δ13C fractionation factor will result in lower source estimates and larger uncertainties associated with the initial δ13C estimate. Degassing can adequately explain low CO2 contents in basalts; however, degassing alone is unlikely to generate the entire spectrum of observed δ13C variations, and we suggest that melt–crust interaction, involving a low δ13C component, may also contribute to observed signatures. Using representative samples, the CO2 flux from Iceland is estimated using three independent methods: (1) combining measured CO2/3He values (in gases and basalts) with 3He flux estimates (Hilton et al., 1990), (2) merging basaltic emplacement rates of Iceland with pre-eruptive magma source estimates of ∼531±64ppm CO2, and (3) combining fluid CO2 contents with estimated regional fluid discharge rates. These methods yield CO2 flux estimates from of 0.2–23×1010 mol a−1, which represent ∼0.1–10% of the estimated global ridge flux (2.2×1012 mol a−1; Marty and Tolstikhin, 1998).

Seismicity observed under Snæfellsjökull volcano

The Snæfellsnes peninsula in western Iceland is characterized by Pleistocene volcanism dominated by alkalic magmatism across an extinct (&gt;6 Ma) axial rift zone of the Mid-Atlantic Ridge. Although Iceland has an extensive seismic coverage, no systematic seismic monitoring is in place on the Snæfellsnes peninsula. In this paper, we present results of a three months field campaign in the Snæfellsnes peninsula and show for the first time that the region is (micro) seismically active in the depth range of 8–15 km. We identified and located a total of 29 seismic events that occurred in close proximity of the Snæfellsjökull volcano, with most events clustering beneath the volcano in swarm sequences. We propose that seismicity is associated with fluid-induced fracturing related to one or more magmatic reservoir(s), which may be elucidated in future seismic campaigns. Our observations are the first step towards the understanding of the plumbing system of the Snæfellsjökull volcano.

Compositional Characteristics and Spatial Distribution of Enriched Icelandic Mantle Components

We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three flank-zone regions in Iceland (Vestmannaeyjar in the south, Snaefell in the east, and Snaefellsnes in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This study integrates He isotope data with radiogenic isotope data (Sr–Nd–Pb–Hf), stable isotope data (δ18O), and trace element data to characterize the compositional features of the trace element enriched components of the Icelandic mantle. We also present high-precision Pb isotope data on an additional 57 lava samples from the flank zones (including Oraefajokull in the SE) and the Northern and Eastern rift zones. Most Icelandic lavas have negative Δ207Pb (–4 to –1), with higher values (–1 to +4) found only in samples from Oraefajokull, Snaefell, and parts of the Reykjanes Peninsula. At Snaefell, this EM1-type component is characterized by a low δ18Oolivine signature (+4·1‰ to +4·6‰), moderate 206Pb/204Pb values (18·4–18·6) and mid-ocean ridge basalt (MORB)-like 3He/4He (6·9–7·5 R/RA). Samples from Vestmannaeyjar and Snaefellsnes have mantle-like δ18Oolivine (+4·9‰ to +5·0‰), and radiogenic 206Pb/204Pb values (18·9–19·3) that fall on the Northern Hemisphere Reference Line for 208Pb/204Pb (Δ208Pb –5 to +5). Compared with the Vestmannaeyjar lavas, Snaefellsnes lavas have higher La/YbN (5–11 vs 3–5), lower eNd (5·5–6·5 vs 6·8–7·6) and lower 3He/4He (6·3–8·6 R/RA vs 11·4–13·5 R/RA). Therefore, the most trace element enriched components in the Icelandic mantle are not the carriers of the high 3He/4He values (>15 R/RA) found in some lavas on Iceland and the adjacent ridges, and instead are consistent with degassed, recycled components. Even after excluding the EM1-type high Δ207Pb samples, high-precision Pb isotope data produce a kinked array on a 206Pb/204Pb vs 208Pb/204Pb plot, which is not consistent with simple binary mixing between two end-members. This requires significant lateral heterogeneity within the Icelandic mantle and the presence of more than just two compositionally distinct local mixing end-member components. Samples from each of the main axial rift zones define different trends. Despite the tectonic continuity between the Northern Volcanic Zone and the Eastern Volcanic Zone, lavas from these two rift zones define separate sub-parallel linear arrays. Lavas from the adjacent Western Volcanic Zone and the Eastern Volcanic Zone define oblique linear arrays that converge on a common local end-member that is not involved in the magmatism of the Northern Volcanic Zone. Therefore, there is a distinct NE–SW compositional heterogeneity within the Icelandic mantle.

Pillow lavas of the Sierra Leone test site, Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics, geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the 87Sr/86Sr ratio between the basalts and their chilled glasses and less significant difference in ɛNd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.

Temperature of the Icelandic crust: Inferred from electrical conductivity, temperature surface gradient, and maximum depth of earthquakes

Two different models of the structure of the Icelandic crust have been presented. One is the thin-crust model with a 10–15 km thick crust beneath the axial rift zones, with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up-domed asthenosphere. The thick-crust model assumes a 40 km thick and relatively cold crust beneath central Iceland. The most important and crucial parameter to distinguish between these different models is the temperature distribution with depth. Three methods are used to estimate the temperature distribution with depth. First, the surface temperature gradient measured in shallow wells drilled outside geothermal areas. Second, the thickness of the seismogenic zone which is associated with a 750 °C isothermal surface. Third, the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 °C at the base of the crust. Combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust. These results are strongly in favour of the thin-crust model. The scattered deep seismic reflectors interpreted as Moho in the thick-crust model could be caused by phase transitions or reflections from melt pockets in the mantle.

Physical models of rifting and transform faulting, due to ridge push in a wedge-shaped oceanic lithosphere

By scaled physical modelling, we have investigated the mechanical response to gravitational forces in an oceanic lithosphere, overlying a less dense asthenosphere. In the models, an upper wedge-shaped layer of sand represented an oceanic lithosphere (0–35 Ma old, with a half-spreading velocity of 3 cm/yr), and a lower layer of polydimethylsiloxane (PDMS), mixed with dense wolframite powder, represented the asthenosphere. In the models, as in nature, isostatic compensation resulted in uplift of ridges and subsidence on their flanks. The resulting relief was responsible for ridge push. We tested two main configurations: straight ridges and offset ridges. In all the models, ridge push was sufficient to cause plate motion, underlying advection, and symmetrical rifting at the ridge axis. There was no need to impose plate motions through external pistons and motors. In models of straight ridges, the style of normal faults in the axial rift zone depended on the local thickness of the brittle sand layer. For thick layers, normal faults rafted out from the active zone of rifting, creating a fossil topography of tilted blocks, between faults dipping toward the ridge. In a model of an offset ridge, with thin lithosphere at the ridge crest and no embedded weakness, ridge push was responsible for a short transform fault, linking en-échelon rifts. In a similar model, but with thick lithosphere, an oblique rift formed at about 20° to the offset trace. We conclude that ridge push was not adequate to create an ideal transform fault. In a model of an offset ridge, with an embedded thin vertical layer of pure PDMS at 90° to the ridge, transform motion concentrated along this weak layer, and the resulting structural style was very similar to that in nature. On the basis of these results, we infer that, in nature, (1) ridge push can indeed drive plate motion, and (2) ridge push can drive strike-slip motion on transform faults, provided that these are weaker than the adjacent oceanic lithosphere and that they form early in the history of spreading.

2480 Ma mafic magmatism in the northern Black Hills, South Dakota: a new link connecting the Wyoming and Superior cratons

The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ~2560–2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the northern Black Hills, a 1 km thick, layered sill (the Blue Draw metagabbro) that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift-basin sedimentation. Ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207Pb/206Pb upper-intercept age of 2480 ± 6 Ma (all age errors ±2σ), comparable to two-point 207Pb/206Pb errorchron ages obtained by Pb stepwise leaching of the same titanites. Nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 ± 16 to 2284 ± 20 Ma (i.e., differentially reset along U–Pb concordia), and hornblende from an associated metadiorite yields a partially reset date with oldest apparent-age increments ranging between 2076 ± 16 and 2010 ± 8 Ma. We interpret these data as indicating that an episode of gabbroic magmatism occurred at 2480 Ma, in response to earlier rifting of the eastern edge of the Wyoming craton. Layered mafic intrusions of similar thickness and identical age occur along a rifted belt in the southern Superior craton (Sudbury region, Ontario). Moreover, these mafic intrusions are spatially aligned using previous supercontinent restorations of the Wyoming and Superior cratons (Kenorland–Superia configurations). This new "piercing point" augments one previously inferred by spatial–temporal correlation of the Paleoproterozoic Huronian (southern Ontario) and Snowy Pass (southeastern Wyoming) supergroups. We propose that layered mafic intrusions extending from Nemo, South Dakota, to Sudbury, Ontario, delineate an axial rift zone along which Wyoming began to separate from Superior during initial fragmentation of the Neoarchean supercontinent at ≥2480 Ma.

Correlation of Plio–Pleistocene Tephra in Ethiopian and Kenyan rift basins: Temporal calibration of geological features and hominid fossil records

The 200-m-thick fossiliferous Konso Formation and overlying terrace deposits, which crop out at the end of the southern sector of the Main Ethiopian Rift (MER), contain more than 30 distal tephra layers. Local and regional tephra correlations of more than 20 tephra units were established using major and trace element data of discrete and purified bulk glass samples within the Konso study area. Eleven correlative marker tuffs were recognized in stratigraphic sections of both the Konso Formation and the Omo-Turkana Basin sediments in southern Ethiopia and northern Kenya. The Turoha, Hope, Ivory, Bright White, and Boleshe Tuffs in the Konso Formation, and the Upper White Tuff in the overlying terrace deposits are securely correlated with the KBS, Akait, Lokapetamoi, Chari, Lower Nariokotome, and Silbo Tuffs of the Omo-Turkana Basin, using least mobile major elements (CaO, Fe 2O 3*, and TiO 2) and geochronology. Preliminary correlations are also suggested between the Konso Formation distal tephra and proximal units of the Quaternary caldera-forming silicic centers in the central sector of the MER. The strongly peralkaline tuffs of the Konso Formation are compositionally similar to proximal eruptions mostly located along the Quaternary axial rift zone of the southern, central, and northern sectors of the MER. The tephra correlation provides information about the temporal and spatial features of the volcanic and tectonic processes recorded in the evolving basins. Thickness and sedimentation rate were determined for both the Konso Formation and the Omo-Turkana Basin sections, measured between the Turoha (=KBS; 1.91 ± 0.03 Ma) and the Bright White (=Chari; 1.40 ± 0.02 Ma) Tuffs. Although the lithostratigraphic sequence at the Konso study area is younger, sedimentation rate within the Konso Formation was comparable to that of the Koobi Fora Formation, higher in the Nachukui Formation, and lower in the Shungura Formation. Local and regional differences in thickness and sedimentation rates within the Plio–Pleistocene deposits of the Konso, Shungura, Nachukui, and Koobi Fora Formations may be attributed to erosion associated with increased topographic gradients maintained by continued subsidence mostly along axial fault zones of basins and uplift along the margins of the broadly rifted zone of the Omo-Turkana Basin and the southern sector of the MER.

Structural control of volcanism and caldera development in the transtensional Taupo Volcanic Zone, New Zealand

The Taupo Volcanic Zone (TVZ), New Zealand, comprises several segments with variable components of extension, and is characterised by longitudinal segmentation into a central zone dominated by rhyolitic calderas and extremities with only andesitic composite volcanoes. Modern (<300 ka) TVZ was investigated to define (a) the overall relationship between regional structures and volcanism and (b) the structural control on the evolution of calderas. New remote sensing and field structural data have been combined with previously published data for TVZ. The results show a general correlation between the amount of extension and the volume and style of eruption in each segment. The segments with the greatest extension coincide with the highly active Taupo and Okataina caldera complexes; conversely, the segments with the greatest dextral transtension correspond to volumetrically less active andesitic stratovolcanoes. Within Modern TVZ, two types of caldera are distinguished based on their structure and development. Calderas within the main zone of rifting (Taupo, Okataina) are multiple (two or more major eruptions) collapse structures with rectilinear margins, overprinted by younger volcanism and faulting. Their complexity is related to the proximity (and influence) of active faulting within TVZ. In contrast, calderas peripheral to the main rift (Rotorua, Reporoa) are sub-circular monogenetic collapse structures, with minor post-collapse volcanism. Their simpler evolution and structure is a attributable to their location away from the main rift zone. Thus, TVZ exemplifies the situation where firstly, magma is erupted as a function of the extension rate along the axial rift zone, and secondly, volcanism is manifest as distinctive caldera structures according to their proximity to the axial rift zone.

The magmatic evolution of the Snæfell volcanic centre; an example of volcanism during incipient rifting in Iceland

The Snæfell volcanic centre is situated in central-east Iceland, at the northern end of a short flank volcanic zone. Its products represent a typical suite of Icelandic volcanics and comprise a bimodal suite of alkalic lavas: a series from basaltic to mugearitic compositions and a small cluster of peralkaline rhyolites. Compositional variations across the whole series can be broadly explained by fractional crystallisation of a family of related parental/primary magmas in a magma chamber at mid-crustal (∼13 km) levels, that has been subject to periodic replenishment and periodic tapping, with sufficient repose times for extreme differentiation. Interaction with the crust appears minimal, although some crustal input into the rhyolites is indicated by their isotopic characteristics. The Snæfell volcanics therefore represent largely new additions to the Icelandic crust. The apparent depth of the magma chamber appears significant and sets Snæfell apart from the axial rift zones at centres such as Krafla, where seismic studies (e.g. Brandsdóttir, B., Menke, W., Einarsson, P., White, R.S., Staples, R.K., 1997. Faeroe–Iceland ridge experiment. 2. Crustal structure of Krafla central volcano. J. Geophys. Res. 102 (B4), 7867–7886) have detected the presence of magma bodies at depths of around 3 km beneath centres in the northern volcanic zone. Comparing Snæfell with volcanic centres in the propagating eastern volcanic zone suggests that, in terms of its stage of evolution, Snæfell is approximately equivalent to Torfajökull. Thus, the Öræfajökull–Snæfell volcanic zone may have represented a site of incipient rifting.

Postrifting seismicity and a model for the 1993 diking event on the Coaxial segment, Juan de Fuca Ridge

Microearthquake data from two deployments of ocean bottom seismographs made shortly after the June/July 1993 rifting episode on the CoAxial segment of the Juan de Fuca Ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field. The first deployment, consisting of five instruments in a small array (aperture of ∼2 km) at the eruption (Flow) site (46°30′N), was made 3 months after the episode. The second deployment, consisting of two larger arrays (apertures of ∼9 km) of five instruments each, one at the eruption site and one at a site of vigorous venting of bacterial flocculent material (Floe) site (46°18′N), was made 11 months after the eruption. Epicenters from nine posteruption earthquakes near the Floe site at the south end of the segment are confined to a narrow zone on the rift axis. Epicenters from 61 earthquakes observed near the Flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows. We combine these posteruption earthquakes with T wave data recorded by SOSUS hydrophones during the rifting episode to develop a model for the diking event. In this model, magma propagated at depth along the axial rift zone until reaching the eruption site near 46°31′N. During the next few days the dike swelled as magma accumulated behind the arrested tip, elevating local stress levels and generating a swarm of seismic activity. About 3 days after the magma reached the north end of the segment, the top of the dike penetrated the seafloor surface, resulting in eruptions along the rift zone at the Flow site.