Active Rift Zone Research Articles

Radon in Icelandic Cold Groundwater and Low-Temperature Geothermal Water

Samples of hot and cold water for radon analysis were collected from boreholes and springs in Iceland in 2014 and 2015. The majority of the samples was collected from municipal district heating services or potable water utilities. The total number of samples was 142, covering most towns and villages in Iceland. Radon activity is generally rather low, in most cases less than 5 Bq/L. Only 12 samples had a measured radon activity higher than 5 Bq/L, with a maximum activity of 10.8 Bq/L. The hot water samples generally have a higher radon activity than cold water samples, but samples from boiling borholes have a lower radon activity as radon fractionates into the vapour phase. The geographical distribution of the samples indicates that radon activity is generally lower within the active rift zones. This is most likely due to the very low uranium content of the tholeiites typically erupted within the rift zones. Higher radon values (> 5 Bq/L) are in most cases close to extinct central volcanoes and thus it seems plausible that the water sampled has been in contact with felsic rocks.

Spatiotemporal evolution of the completeness magnitude of the Icelandic earthquake catalogue from 1991 to 2013

In 1991, a digital seismic monitoring network was installed in Iceland with a digital seismic system and automatic operation. After 20 years of operation, we explore for the first time its nationwide performance by analysing the spatiotemporal variations of the completeness magnitude. We use the Bayesian magnitude of completeness (BMC) method that combines local completeness magnitude observations with prior information based on the density of seismic stations. Additionally, we test the impact of earthquake location uncertainties on the BMC results, by filtering the catalogue using a multivariate analysis that identifies outliers in the hypocentre error distribution. We find that the entire North-to-South active rift zone shows a relatively low magnitude of completeness Mc in the range 0.5–1.0, highlighting the ability of the Icelandic network to detect small earthquakes. This work also demonstrates the influence of earthquake location uncertainties on the spatiotemporal magnitude of completeness analysis.

A pulse of mid-Pleistocene rift volcanism in Ethiopia at the dawn of modern humans.

The Ethiopian Rift Valley hosts the longest record of human co-existence with volcanoes on Earth, however, current understanding of the magnitude and timing of large explosive eruptions in this region is poor. Detailed records of volcanism are essential for interpreting the palaeoenvironments occupied by our hominin ancestors; and also for evaluating the volcanic hazards posed to the 10 million people currently living within this active rift zone. Here we use new geochronological evidence to suggest that a 200 km-long segment of rift experienced a major pulse of explosive volcanic activity between 320 and 170 ka. During this period, at least four distinct volcanic centres underwent large-volume (>10 km3) caldera-forming eruptions, and eruptive fluxes were elevated five times above the average eruption rate for the past 700 ka. We propose that such pulses of episodic silicic volcanism would have drastically remodelled landscapes and ecosystems occupied by early hominin populations.

CO2 Storage Potential of Basaltic Rocks Offshore Iceland

Injection of CO2 into basaltic formations provides significant benefits including permanent storage by mineralisation and large storage volume. The largest geological storage potential lies offshore and in the case of basalt, along the mid-oceanic ridges where CO2 could be stored as carbonate minerals for thousands of years. Most of the bedrock, both on land and offshore Iceland consists of basalt that could theoretically be used for injection of CO2, fully dissolved in water. The most feasible formations are the youngest formations located within the active rift zone. It is estimated that up to 7000 GtCO2 could be stored offshore Iceland within the Exclusive Economic Zone. Site specific geological research and pilot studies are required for refining the concept and offshore pilot scale projects should be considered as the next steps in evolving the method.

Magmatic Evidence for Carbonate Metasomatism in the Lithospheric Mantle underneath the Ohře (Eger) Rift

Magmas erupted in intracontinental rifts typically form from melting of variable proportions of asthenospheric or lithospheric mantle sources and ascend through thick continental lithosphere. This ascent of magma is accompanied by differentiation and assimilation processes. Understanding the composition of rift-related intracontinental volcanism is important, particularly in densely populated active rift zones such as the Ohře (Eger) Rift in Central Europe. We have sampled and analysed nephelinites from Železna hůrka (Eisenbuhl), the youngest (<300 ka) Quaternary volcano related to the Ohře Rift where frequent earthquake swarms indicate continuing magmatic activity in the crust. This nephelinite volcano is part of a larger eruptive centre (Mýtina Maar) representing a single locality of recurrent volcanism in the Ohře Rift. We present a detailed petrographic, mineralogical and geochemical study (major and trace elements and Sr–Nd–Hf–O isotopes) of Železna hůrka to further resolve the magmatic history and mantle source of the erupted melt. We find evidence for a highly complex evolution of the nephelinitic melts during their ascent to the surface. Most importantly, mixing of melts derived from different sources and of strong chemical contrast controls the composition of the erupted volcanic products. These diverse parental melts originate from a highly metasomatized subcontinental lithospheric mantle (SCLM) source. We use a combined approach based on mineral, glass and whole-rock compositions to show that the mantle underneath the western Ohře Rift is metasomatized dominantly by carbonatitic melts. The nephelinites of Železna hůrka formed by interaction between a carbonatitic melt and residual mantle peridotite, partial crystallization in the lithospheric mantle and minor assimilation of upper continental crust. Thermobarometric estimates indicate that the stagnation levels of the youngest volcanism in this part of the Ohře Rift were deeper than the focal depths of recent earthquake swarms, indicating that those are not directly linked to magma ascent. Furthermore, close mineralogical and geochemical similarities between the Železna hůrka nephelinite and fresh kimberlites may point towards a genetic link between kimberlites, melilitites and nephelinites.

The collapse of Bárðarbunga caldera, Iceland

Lying below Vatnajokull ice cap in Iceland, Barðarbunga stratovolcano began experiencing wholesale caldera collapse in 2014 August 16, one of the largest such events recorded in the modern instrumental era. Simultaneous with this collapse is the initiation of a plate boundary rifting episode north of the caldera. Observations using the international constellation of radar satellites indicate rapid 50 cm d^(−1) subsidence of the glacier surface overlying the collapsing caldera and metre-scale crustal deformation in the active rift zone. Anomalous earthquakes around the rim of the caldera with highly nondouble-couple focal mechanisms provide a mechanical link to the dynamics of the collapsing magma chamber. A model of the collapse consistent with available geodetic and seismic observations suggests that the majority of the observed subsidence occurs aseismically via a deflating sill-like magma chamber.

CO2 storage potential of basaltic rocks in Iceland and the oceanic ridges

Abstract Iceland is the largest landmass found above sea level at the mid-ocean ridges, about 103,000 km2 mostly made of basaltic rocks (∼90%). Theoretically much of Iceland could be used for injection of CO2, fully dissolved in water. Most of the pore space in the older rocks is filled with secondary minerals, thus the young and porous basaltic formations, found within the active rift zone and covering about one third of Iceland, are the most feasible for carbon storage onshore. Studies on mineral storage of CO2 in basaltic rocks are still at an early stage. Therefore, natural analogues are important for gaining a better understanding of CO2 fixation in basaltic rocks. Volcanic geothermal systems serve as an applicable analogue since the systems receive considerable amounts of CO2 from magma in the roots of the systems. Wiese et al. [1] quantified the amount and spatial distribution of CO2 stored as calcite within the bedrock of three active geothermal systems in Iceland. The results from this study reveal a large storage potential of basaltic rocks and can be used as a guideline for the theoretical potential of CO2 storage in basaltic formations. The largest storage potential lies offshore, with long-term advantages for safe and secure CO2 storage in the mid-ocean ridges. The theoretical mineral CO2 storage capacity of the ocean ridges, using the Icelandic analogue, is orders of magnitude larger than the anticipated release of CO2 caused by burning of all fossil fuel on Earth.

Defining a deep fault network for Australia, using 3D “worming”

Defining a deep fault network for Australia, using 3D 'worming'. The original work on multi-scale edge detection ('worming') done by Hornby et al. in 1999 has become increasingly popular as a starting point for rapid interpretation using potential field data. Both gravity and magnetic grids are used. Several improvements have been made in the following years such that it is routine to capture points, 'worms' and linear features in a form suitable for overlay in GIS packages. There are important extensions which have recently been completed: (a). support for gravity gradiometry using the measured gradients directly (b). creation of interface and foliation data implying 3D surfaces that are geolocated. (c). adaption of Euler deconvolution techniques to both full tensor gravity gradiometry (FTG) and a best located method for scalar measures. This paper reports on these new extensions applied to the Australian gravity grid and on issues arising from these efforts to improve/generalise the worming technology and get 3D contacts that can be interpreted. This large scale gravity dataset, with a high density of near surface features, contains far too many gradients interpreted as faults, so a thinning strategy is required to find the significant deep crustal faults. These deep crustal faults require proper attribution of depth, dip direction and dip, and also extent constraints. For the FTG case, the largest 3-survey merge attempted to date has occurred with data acquired in Kenya. The new and improved 3D worming has been applied in the context of rapidly defining the horst/graben layouts in an active rift zone. Some discussion and illustration of the present problems of how to build, merge and manage a 3D fault network, where the issues of faults terminating on other faults and which fault cuts which, will be shown and discussed.

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

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

Magmagenesis within the Hunter Ridge Rift Zone resolved from olivine-hosted melt inclusions and geochemical modelling with insights from geodynamic models

The Vanuatu region of the southwest Pacific comprises a complex 3D plate boundary system that has developed within the last 20 Ma owing to the opening of the North Fiji Basin and asymmetric rollback along the New Hebrides Trench. The southern Vanuatu region is characterised by a modern triple junction between the New Hebrides Trench, the backarc-spreading centre of the North Fiji Basin, and the Hunter Fracture Zone. At this intersection, the Hunter Ridge (a former magmatic arc) is being pulled apart by the southernmost propagating tip of the backarc Central Spreading Ridge of the North Fiji Basin. Extension of the Hunter Ridge lithosphere is being accommodated by the formation of a magmatically active rift zone referred to as the Hunter Ridge Rift Zone. Three distinct subduction-related melt types were identified within the olivine-hosted melt inclusions from a primitive olivine- and clinopyroxene-phyric volcanic rock from the Hunter Ridge Rift Zone, one of which has adakitic affinities. The primary nature of these melts indicates that we may have sampled the initial stages of mantle partial melting within a young and anomalously hot subduction environment. The preservation of geochemical diversity of the trapped melts suggests complex melting regimes influenced by different proportions of three enriched subduction- and mantle-derived silicate melts of possible slab, sediment and backarc origins, under tectonic conditions that facilitate the rapid ascent of newly formed melts. The three-dimensional geodynamics of this triple junction may play an important role in the generation and preservation of the unique and rare types of primitive magmas at the Hunter Ridge Rift Zone, by facilitating the interaction between backarc and arc magmatic processes, due to complex mantle flow including toroidal flow around the New Hebrides slab edge, upwelling at the slab edge associated with the return flow, and upwelling from backarc spreading at the Central Spreading Ridge.

Architecture and development of (Pliocene to Holocene) faults and fissures in the East Volcanic Zone of Iceland

In Southeast Iceland, comparison between the deeply eroded off-rift and flank zones, on the one hand, and the active rift zone, on the other hand, allows us to characterize the geometry and mechanisms of faulting and fracturing since the Upper Pliocene to Holocene. We used different approaches based on the inversion of fault-slip data and of focal mechanisms of earthquakes, as well as on study of aerial photographs by means of photogrammetry. Our study reveals that about half of the past and current stress states are strike-slip whereas surface deformations are large normal faults and extensional fractures. The parallelism between the directions of extension (σ3) in both extensional and strike-slip regimes, implies a σ1/σ2 stress permutations probably due to uniaxial extensional stress (σ1=σ2>σ3) and fluid overpressure. In detail, the deviation of σ3 trajectories, probably relates to the presence of central volcanoes, which may locally induce perturbations of the general stress field.

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

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

Structural Architecture for Development of Marginal Extensional Sub-Basins in the Red Sea Active Rift Zone

This paper presents a robust kinematic model that describes northern Red Sea and Gulf of Suez rifting and the development of marginal extensional half-graben sub-basins (ESB). A combination of Landsat Enhanced Thematic Mapper Plus (ETM+) and structural data was used to provide model constraints on the development of rift segments and ESB in the active rift zones. Structural analysis shows rotation and change in strike of rift-bounding faults. The model describes the northern Red Sea region as a poly-phase rift system initiated by late Oligocene (30 - 24 Ma) orthogonal rifting and the development of marginal ESB (now inland ESB), followed by oblique rifting and flank uplift during the early Miocene (24 - 18 Ma). The oblique rifting fragmented the rift depression into segments separated by oblique-slip accommodation within reactivated Pan-African (ca. 600 Ma) fracture zones, resulting in the development of antithetic faults and an en-echelon distribution of inland ESB. The current phase of rifting was instigated by the development of the Dead Sea Transform in response to increased northeasterly extension during the middle Miocene (ca. 18 Ma). The model explains the widening of the Red Sea rift during the last phase more than the Gulf of Suez rift by developing more antithetic faults and formation of offshore ESB, and deepening the rift depression.

Geodynamics of rift–plume interaction in Iceland as constrained by new 40Ar/ 39Ar and in situ U–Pb zircon ages

The interaction between a rift zone and a mantle plume leads to exceptional situations in Iceland where the island is 1.5 wider than expected, given the North-Atlantic spreading rate. In order to give a better idea of the timeframe of this evolution, we present 32 new 40Ar/ 39Ar and in-situ U–Pb dating on zircon from 16 volcanic systems located from the west to east coasts of Iceland. The North Iceland Rift Zone (NIRZ) was initiated at least 12 Ma ago. Furthermore, during these last 12 Ma, the NIRZ half spreading rate was between 0.7 and 1.2 cm/yr and it propagated to the south at a rate of 1.0–1.2 cm/yr. The excess width of Iceland can thus not be explained by faster spreading rate in the past. Here we discuss a model that explains the ~ 200 km ‘excess’ of crust, taking into account the eastward relocation of the rift zone and corresponding older crustal capture over the course of Iceland's geological history. The most recent rift relocation is dated at approximately 6 Ma at Snæfellsnes Peninsula in the west, whereas the oldest volcanic systems (15–13 Ma) from the extreme north east of Iceland were most likely generated at the Kolbeinsey ridge north of Iceland rather than in the NIRZ itself. The need for rift relocations and crustal capture to explain the width of Iceland strongly suggests that during rift–plume interaction the mantle plume plays an active role. It forces the active rift zone to be frequently relocated by rift jumps above its center leaving inactive rift zones as older synclines in the geological record. This result in an eastward position of the rift zone in Iceland relative to the North Atlantic ridge, and it can be predicted that in a few tens of millions of years the Mid-Atlantic ridge and the Icelandic plume may become decoupled.

New insights into Mediterranean cold seep ecosystems: editorial comment on the feature article by Ritt et al

Tectonically active areas of the ocean Xoor harbor diverse benthic ecosystems that are essentially independent of photosynthesis and are largely driven by chemosynthetic bacterial processes. This includes symbiotic relationships between invertebrates and chemosynthetic bacteria oxidizing, e.g., methane (CH4) or sulWde (H2S). In contrast to the relatively ephemeral hydrothermal vent systems that Xourish along very active rift zones and other hot spots of volcanic activity, cold seeps are more long-term stable systems found in less active areas around Wssure zones in the sea Xoor, where reduced gases (H2S, CH4) and hydrocarbonrich Xuids emerge over larger areas (Levin 2005). Cold seeps were Wrst discovered in the Gulf of Mexico (Paull et al. 1984) and have since been found worldwide in many other ocean regions including polar seas and the Mediterranean. Cold seeps have been subject for intense interdisciplinary studies in recent years. They represent extremely heterogeneous habitats harboring a multitude of microbial, meiofaunal and macrofaunal communities that respond to changes in structural complexity, habitat geochemistry, nutrient sources, and interspeciWc interactions (Cordes et al. 2010). Understanding cold seep community composition and biogeography is a central topic of such studies, yet quantitative information is still scarce (Vanreusel et al. 2010). The study by Ritt et al. (2011) now presents the hitherto most detailed study of the macroand meio-fauna communities associated with Mediterranean cold seeps along with geochemical data on their habitats. The authors compare faunal communities of Mud volcanoes and pockmarks to the surrounding oligotrophic deep-sea habitats and conclude that cold seep communities harbor both larger biomass and biodiversity. However, the results do also highlight a tremendous heterogeneity in structure and composition of animal communities associated with cold seep ecosystems, which in part are linked to diVerent reduced microhabitats and substrate types in carbonate crusts and sediments. In conclusion, this article gives new important insights to the animal community ecology of cold seeps and points out directions for future studies of these relatively recently discovered habitats.

Active folding along a rift-flank: The Catania region case history (SE Sicily)

Abstract In this paper a new kinematic and dynamic model on the Recent and active contractional deformation of the Catania region, eastern Sicily, is discussed. The study area represents one of the most seismically active region of the Mediterranean, located at the intersection between the front of the Sicily collision belt and the seismogenic Siculo-Calabrian Rift-Zone. The analysed contractional tectonics form an active triangle zone that originated from the tectonic inversion of a Lower-Middle Pleistocene extensional basin, which was located at the northern edge of the African foreland. The triangle zone consists of two antithetic ENE–WSW oriented thrust-ramps that show evidence of motion during the Holocene and bound a folded belt that involves alluvial deposits as young as 40 ka. These contractional structures represent the final product of the positive tectonic inversion of extensional features located, in the Hyblean Plateau in SE Sicily, along the flank of the active rift zone. The Late Quaternary motions along the inverted structures was accommodated to the west by a major N–S oriented left-lateral fault zone, which separates the active contractional domains from the adjacent sectors of the African margin. As a whole, the Late Quaternary contractional tectonics of SE Sicily have been related to a NW-verging crustal stacking, related to a Mantle intrusion beneath the Hyblean Plateau that developed as effect of the rift-flank deformation. The crustal lineaments, which compose the new kinematic model, represents potentially active seismogenic sources that might be considered in the frame of the seismotectonic picture of the Catania region.

In situ stress variations at the Variscan deformation front — Results from the deep Aachen geothermal well

In 2004 the 2544 m deep RWTH-1 well was drilled in the city centre of Aachen to supply geothermal heat for the heating and cooling of the new student service centre “SuperC” of RWTH Aachen University. Aachen is located in a complex geologic and tectonic position at the northern margin of the Variscan deformation front at the borders between the Brabant Massif, the Hohes Venn/Eifel areas and the presently active rift zone of the Lower Rhine Embayment, where existing data on in situ stress show complex changes over short distances. The borehole offers a unique opportunity to study varying stress regimes in this area of complex geodynamic evolution. This study of the in situ stresses is based on the observation of compressive borehole breakouts and drilling-induced tensile fractures in electrical and acoustic image logs. The borehole failure analysis shows that the maximum horizontal stress trends SE–NW which is in accordance with the general West European stress trend. Stress magnitudes modelled in accordance to the Mohr–Coulomb Theory of Sliding Friction indicate minimum and maximum horizontal stress gradients of 0.019 MPa/m and 0.038 MPa/m, respectively. The occurrence of drilling-induced tensile failure and the calculated in situ stress magnitudes are consistent with a model of strike-slip deformation. The observed strike-slip faulting regime supports the extension of the Brabant Shear Zone proposed by Ahorner (1975) into the Aachen city area, where it joins the major normal faulting set of the Roer Valley Graben zone. This intersection of the inherited Variscan deformation grain and the Cenozoic deformation resulting in recent strike-slip and normal faulting activity proves the tectonically different deformation responses over a short distance between the long-lived Brabant Massif and the Cenozoic Rhine Rift System.

Geodetic evidence for en echelon dike emplacement and concurrent slow slip during the June 2007 intrusion and eruption at Kīlauea volcano, Hawaii

A series of complex events at Kīlauea Volcano, Hawaii, 17 June to 19 June 2007, began with an intrusion in the upper east rift zone (ERZ) and culminated with a small eruption (1500 m3). Surface deformation due to the intrusion was recorded in unprecedented detail by Global Positioning System (GPS) and tilt networks as well as interferometric synthetic aperture radar (InSAR) data acquired by the ENVISAT and ALOS satellites. A joint nonlinear inversion of GPS, tilt, and InSAR data yields a deflationary source beneath the summit caldera and an ENE‐striking uniform‐opening dislocation with ∼2 m opening, a dip of ∼80° to the south, and extending from the surface to ∼2 km depth. This simple model reasonably fits the overall pattern of deformation but significantly misfits data near the western end of an inferred dike‐like source. Three more complex dike models are tested that allow for distributed opening including (1) a dike that follows the surface trace of the active rift zone, (2) a dike that follows the symmetry axis of InSAR deformation, and (3) two en echelon dike segments beneath mapped surface cracks and newly formed steaming areas. The en echelon dike model best fits near‐field GPS and tilt data. Maximum opening of 2.4 m occurred on the eastern segment beneath the eruptive vent. Although this model represents the best fit to the ERZ data, it still fails to explain data from a coastal tiltmeter and GPS sites on Kīlauea's southwestern flank. The southwest flank GPS sites and the coastal tiltmeter exhibit deformation consistent with observations of previous slow slip events beneath Kīlauea's south flank, but inconsistent with observations of previous intrusions. Slow slip events at Kīlauea and elsewhere are thought to occur in a transition zone between locked and stably sliding zones of a fault. An inversion including slip on a basal decollement improves fit to these data and suggests a maximum of ∼15 cm of seaward fault motion, comparable to previous slow‐slip events.

Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia

Research Article| September 01, 2010 Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia Valerio Acocella Valerio Acocella † Dipartimento Scienze Geologiche Università Roma Tre Largo San Leonardo Murialdo, 1, Roma, 00146, Italy †E-mail: acocella@uniroma3.it Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (9-10): 1717–1728. https://doi.org/10.1130/B30105.1 Article history received: 08 Jun 2009 rev-recd: 25 Oct 2009 accepted: 27 Oct 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Valerio Acocella; Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia. GSA Bulletin 2010;; 122 (9-10): 1717–1728. doi: https://doi.org/10.1130/B30105.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Magma along divergent plate boundaries is erupted from fissures or vents from central volcanoes, with limited impact on rift architecture. Here I summarize the geological and structural features accompanying the eruption of part of a km-thick volcanic sequence (“stratoids”) along the Red Sea divergent boundary in central Afar, in the area of Tendaho and Dobi grabens. More than 4700 km3/Ma (per 100 km of rift length) of magma have been produced by repeated fissure eruptions from within Tendaho graben. The graben sides show distinctive structural features, as steep topographic gradients, coinciding with inward-tilted blocks forming dominoes coeval to the emplacement of the km-thick volcanic sequence. Similar features are observed also in the Dobi graben. This allows proposing an original mechanism, where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence. This portion of Afar shows how rift architecture is shaped by voluminous fissure eruptions, forming collapsed rifts. These occur in continental domains, during the breakup stage (central Afar) and in oceanic domains, where rifts narrow (East Pacific Rise). Collapsed rifts represent an end-member type of volcano-tectonic activity, where the width of the erupting reservoir balances that of the active rift zone. Along divergent boundaries, the width of the reservoir influences the style of surface deformation: a progressively higher ratio of the width of the reservoir emptied (Re) to that of the active rift zone (Ri) generates, in sequence, axial grabens, calderas, and collapsed rifts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rayleigh-wave phase-velocity maps and three-dimensional shear velocity structure of the western US from local non-plane surface wave tomography

SUMMARY We utilize two-and-three-quarter years of vertical-component recordings made by the Transportable Array (TA) component of Earthscope to constrain three-dimensional (3-D) seismic shear wave velocity structure in the upper 200 km of the western United States. Single-taper spectral estimation is used to compile measurements of complex spectral amplitudes from 44 317 seismograms generated by 123 teleseismic events. In the first step employed to determine the Rayleigh-wave phase-velocity structure, we implement a new tomographic method, which is simpler and more robust than scattering-based methods (e.g. multi-plane surface wave tomography). The TA is effectively implemented as a large number of local arrays by defining a horizontal Gaussian smoothing distance that weights observations near a given target point. The complex spectral-amplitude measurements are interpreted with the spherical Helmholtz equation using local observations about a succession of target points, resulting in Rayleigh-wave phase-velocity maps at periods over the range of 18–125 s. The derived maps depend on the form of local fits to the Helmholtz equation, which generally involve the non-plane-wave solutions of Friederich et al. In a second step, the phase-velocity maps are used to derive 3-D shear velocity structure. The 3-D velocity images confirm details witnessed in prior body-wave and surface-wave studies and reveal new structures, including a deep (>100 km deep) high-velocity lineament, of width ∼200 km, stretching from the southern Great Valley to northern Utah that may be a relic of plate subduction or, alternatively, either a remnant of the Mojave Precambrian Province or a mantle downwelling. Mantle seismic velocity is highly correlated with heat flow, Holocene volcanism, elastic plate thickness and seismicity. This suggests that shallow mantle structure provides the heat source for associated magmatism, as well as thinning of the thermal lithosphere, leading to relatively high stress concentration. Our images also confirm the presence of high-velocity mantle at ≳100 km depth beneath areas of suspected mantle delamination (southern Sierra Nevada; Grande Ronde uplift), low velocity mantle underlying active rift zones, and high velocity mantle associated with the subducting Juan de Fuca plate. Structure established during the Proterozoic appears to exert a lasting influence on subsequent volcanism and tectonism up to the Present.