Vertical linear feeder to elliptical igneous saucer-shaped sills: evidences from structural observations, geochemistry and experimental modeling

NASA Astrophysics Data System (ADS)

Galerne, C. Y.; Galland, O.; Neumann, E. R.; Planke, S.

2009-12-01

The structural relationships between sills and their feeders are poorly documented because they are rarely observed in the field and difficult to image on seismic data. For instance, it is unclear whether sills are fed by pipes, dikes or other sills. Nevertheless, the geometrical relationships between sills and their feeders provide first-order constraints on magma emplacement mechanisms. Here, we investigate the structural and geochemical relationships between sills and potential feeder dikes in a remarkably well-preserved and exposed sill complex, the Golden Valley Sill Complex (GVSC), Karoo Basin, South Africa. The GVSC consists of five major saucer-shaped sills and six dikes. The Golden Valley sill itself is an elliptical saucer, with a N-S trend. A one meter thick dike (D4) crops out underneath the southern tip of the Golden Valley sill. The strike of this dike is parallel to the long axis of the Golden Valley sill. Detailed sampling and geochemical analyses of the GVSC show that each sill and dike exhibits a specific geochemical signature. The Golden Valley sill and its underlying dike D4 have identical signatures. Although there is no clear structural evidence, the consistent geometrical and geochemical relationships between the Golden Valley sill and the D4 dike suggest that this vertical linear structure is the feeder of the overlying saucer-shaped sill. In order to investigate the relationships between sills and feeders, we resorted to scaled laboratory experiments. The experiments consisted of a low-viscosity vegetable oil representing magma and a cohesive fine-grained silica flour representing brittle rocks. We placed a horizontal weak layer into the silica flour, just above the top of the inlet, to simulate strata. Such a weak layer controlled the formation of horizontal sill that subsequently turned into a transgressive sheet leading to the formation of a saucer geometry. We ran experiments with varying inlet shapes: 1) a point inlet representing a pipe-like feeder and 2) a linear feeder representing a dike-like feeder. In the experiments with point inlet, circular saucer-shaped sills formed. In the experiments with linear feeder, elliptical saucer-shaped sills formed. In the latter experiments, the long axes of the saucers were parallel to, and located directly above, the linear feeder. The experiments show that the feeder geometry has an important influence on the geometry of the emplaced sills. There are close similarities between the geometry of the Golden Valley sill and the intrusions formed in the experiments. The elliptical shape of the Golden Valley sill suggests that it was fed by an elongated feeder, probably the D4 dike. In general, our results show that the three-dimensional geometry of saucer-shaped sills observed in sedimentary basins, may constrain the shape of their feeders, i.e. their emplacement mechanisms.

Development and Psychometric Testing of the Strategy Inventory for Language Learning (SILL). Appendix

DTIC Science & Technology

1986-11-01

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Physical controls and depth of emplacement of igneous bodies: A review

NASA Astrophysics Data System (ADS)

Menand, Thierry

2011-03-01

The formation and growth of magma bodies are now recognised as involving the amalgamation of successive, discrete pulses such as sills. Sills would thus represent the building blocks of larger plutons ( sensu lato). Mechanical and thermal considerations on the incremental development of these plutons raise the issue of the crustal levels at which magma can stall and accumulate as sills. Reviewing the mechanisms that could a priori explain sill formation, it is shown that principal physical controls include: rigidity contrast, where sills form at the interface between soft strata overlaid by comparatively stiffer strata; rheology anisotropy, where sills form within the weakest ductile zones; and rotation of deviatoric stress, where sills form when the minimum compressive stress becomes vertical. Comparatively, the concept of neutral buoyancy is unlikely to play a leading control in the emplacement of sills, although it could assist their formation. These different controls on sill formation, however, do not necessarily operate on the same length scale. The length scale associated with the presence of interfaces separating upper stiffer layers from lower softer ones determines the depth at which rigidity-controlled sills will form. On another hand, the emplacement depths for rheology-controlled sills are likely to be determined by the distribution of the weakest ductile zones. Whereas the emplacement depth of stress-controlled sills is determined by a balance between the horizontal maximum compressive stress, which favours sill formation, and the buoyancy of their feeder dykes, which drives magma vertically. Ultimately, the depth at which a sill forms depends on whether crustal anisotropy or stress rotation is the dominant control, i.e. which of these processes operates at the smallest length scale. Using dimensional analysis, it is shown that sill formation controlled by remote stress rotation would occur on length scales of hundreds of meters or greater. This therefore suggests that crustal heterogeneities and their associated anisotropy are likely to play a larger role than remote stress rotation in controlling sill emplacement, unless these heterogeneities are several hundred meters or more apart. This also reinforces the role of local stress barriers, owing to interactions between deviatoric stress and crustal heterogeneities, in the formation of sills.

Geology of the saucer-shaped sill near Mahad, western Deccan Traps, India, and its significance to the Flood Basalt Model

NASA Astrophysics Data System (ADS)

Duraiswami, Raymond A.; Shaikh, Tahira N.

2013-07-01

An ˜22-m-thick saucer-shaped sill occurs near Mahad and is exposed as a curvilinear, miniature ridge within the Deccan Traps. The sill has variable dips (42-55°). It has a 7.1-km long axis and 5.3 km short axis (aspect ratio of 1.4) and is larger than the MV sill of the Golden Valley sill complex, South Africa and the Panton sill, Australia. The sill has distinct glassy upper and lower chilled margins with a coarse-grained highly jointed core. The samples from the margin are invariably fractured and iron stained because of deuteric alteration. The rock from the sill is plagioclase-phyric basalt. At least three thick sill-like apophyses emanate from the base of the main sill. The apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet. We interpret such features as `nascent saucer-shaped sills' that did not inflate to form nested sills. Geochemically, the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range. Critical trace element ratios and primitive mantle normalised trace and REE patterns indicate that the sills have geochemical affinities to the Poladpur chemical type and that the pahoehoe flow they intrude belongs to the Bushe Formation. Calculated magmatic overpressures during sill emplacement range from 8.4 to 11.3 MPa (for Young's modulus E = 5 GPa) and 16.7 to 22.5 MPa (for E=10 GPa) and depth to magma chamber ranges from 8.5 to 11.5 km ( E = 5 GPa) and 17.1 to 22.9 km ( E = 10 GPa), consistent with petrological and gravity modelling. The volume of the Mahad sill is approximately 276 km3 and is constant irrespective of the variations in the values of host-rock Young's modulus. In 1980, Cox (J Petrol 21:629-650, 1980) proposed a conceptual model of the crust-mantle section beneath the Karoo CFB which is considered as the fundamental model for flood basalt volcanism. Our paper confirms the presence of a sill plus the inferred substructure beneath Mahad that are compatible with predictions of that model. In LIPS, saucer-shaped sills are formed in areas experiencing extensional tectonics where processes such as the Cook-Gordon delamination and Dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills. A similar process is envisaged for the formation of the Mahad sill.

Lavas and Sills in the Ferrar Large Igneous Province: Field and Geochemical Evidence for the Order of Emplacement.

NASA Astrophysics Data System (ADS)

Elliot, D. H.; Fleming, T. H.

2005-12-01

Many large igneous provinces, particularly those associated with Gondwana break-up, include major sill complexes as well as flood basalt fields. In the Ferrar province, radiometric dates of lavas and sills are indistinguishable. Nevertheless, in north Victoria Land (NVL) field evidence suggests the lavas had to have been erupted first in order to create the overburden needed for emplacement at shallow depths of thick sills, lacking vesicles, in a thin (100 m) Upper Triassic sedimentary sequence overlying basement. Elsewhere in the Transantarctic Mountains sills occur almost exclusively in a thick (2-2.5 km) Devonian-Triassic sedimentary sequence (Beacon Supergroup) that was possibly capped by 500+m of lavas before sill emplacement. For south Victoria Land (SVL), Marsh (2004) proposed that the most evolved rocks were erupted first as lavas, and sills were emplaced at progressively greater depth as increasingly more magnesian magmas and crystal mushes were injected into supracrustal and finally basement rocks. In NVL most lavas have MgO between 6-8% with a few as low as 4.5% MgO, whereas analyzed chilled margins of sills range from 3.7-5.6% MgO. In the Prince Albert Mountains (PAM), SVL, lava and sill compositions overlap (3.9-7.3% MgO). In the greater Dry Valleys region (SVL) lavas at Carapace Nunatak range from 3.6-6.7% MgO; chilled margins of Dry Valleys sills range from about 4.2 to 7.2% MgO. In the Queen Alexandra Range, central Transantarctic Mountains (CTM), lavas are predominantly 2.6-5.7% MgO; sills in the region range from 4.5% to 10.7% MgO. In the Otway Massif region (head of the Shackleton Glacier, CTM) most lavas are strongly evolved (2.7-3.4% MgO); sills in the Shackleton Glacier region range from 4.3-7.3% MgO. Nowhere do lavas show unequivocal systematic temporal change in MgO, and notably in CTM the initial flows are the most mafic (7.5-8.0% MgO). Olivine dolerite sills (chilled margins: -9% MgO) tend to occur low in the stratigraphic section. Except for NVL where Beacon strata exposures are limited, sills are thicker (100-200 m) and more regular in lower stratigraphic levels. Sills with orthopyroxene crystal-mush tongues are not known outside the Dry Valleys except perhaps the Warren Range (SVL). No province-wide systematic relationship is apparent between compositions of lavas and sill chilled margins. Nevertheless, in CTM most lavas are significantly more evolved than the sills; within the sills there is no clear relationship between MgO and stratigraphy, and some less evolved compositions occur at relatively high stratigraphic levels. In SVL compositional overlap is almost complete; locally, cross-cutting relations show more mafic sills and sheets cutting less mafic compositions. In NVL the chemical relations between lavas and sills are opposite from those that have been advocated for SVL. Interpretation is compounded by sills that exchange stratigraphic position or climb stratigraphically. Factors affecting magma emplacement include magma density, lithostatic pressure, overpressures required for lateral emplacement, and rock physical properties; when and where the evolving source was tapped may play an equal role in the emplacement order. Further, detailed work on the sills will show whether crystallization might have yielded lower density residual liquids that could have migrated and formed distal fingers of sills or migrated to higher stratigraphic levels.

Igneous sills record far-field and near-field stress interactions during volcano construction: Isle of Mull, Scotland

NASA Astrophysics Data System (ADS)

Stephens, T. L.; Walker, R. J.; Healy, D.; Bubeck, A.; England, R. W.; McCaffrey, K. J. W.

2017-11-01

Sill emplacement is typically associated with horizontally mechanically layered host rocks in a near-hydrostatic far-field stress state, where contrasting mechanical properties across the layers promote transitions from dykes, or inclined sheets, to sills. We used detailed field observations from the Loch Scridain Sill Complex (Isle of Mull, UK), and mechanical models to show that layering is not always the dominant control on sill emplacement. The studied sills have consistently shallow dips (1°-25°) and cut vertically bedded and foliated metamorphic basement rocks, and horizontally bedded cover sedimentary rocks and lavas. Horizontal and shallowly-dipping fractures in the host rock were intruded with vertical opening in all cases, whilst steeply-dipping discontinuities within the sequence (i.e. vertical fractures and foliation in the basement, and vertical polygonal joints in the lavas) were not intruded during sill emplacement. Mechanical models of slip tendency, dilation tendency, and fracture susceptibility for local and overall sill geometry data, support a radial horizontal compression during sill emplacement. Our models show that dykes and sills across Mull were emplaced during NW-SE horizontal shortening, related to a far-field tectonic stress state. The dykes generally accommodated phases of NE-SW horizontal tectonic extension, whereas the sills record the superposition of the far-field stress with a near-field stress state, imposed by emplacement of the Mull Central Volcano. We show that through detailed geometric characterisation coupled with mechanical modelling, sills may be used as an indication of fluctuations in the paleostress state.

The effects of solidification on sill propagation dynamics and morphology

NASA Astrophysics Data System (ADS)

Chanceaux, L.; Menand, T.

2016-05-01

Sills are an integral part of the formation and development of larger plutons and magma reservoirs. Thus sills are essential for both the transport and the storage of magma in the Earth's crust. However, although cooling and solidification are central to magmatism, their effects on sills have been so far poorly studied. Here, the effects of solidification on sill propagation dynamics and morphology are studied by means of analogue laboratory experiments. Hot fluid vegetable oil (magma analogue), that solidifies during its propagation, is injected as a sill in a colder layered gelatine solid (elastic host rock analogue). The injection flux and temperature are maintained constant during an experiment and systematically varied between each experiment, in order to vary and quantify the amount of solidification between each experiments. The oil is injected directly at the interface between the two gelatine layers. When solidification effects are small (high injection temperatures and fluxes), the propagation is continuous and the sill has a regular and smooth surface. Inversely, when solidification effects are important (low injection temperatures and fluxes), sill propagation is discontinuous and occurs by steps of surface-area creation interspersed with periods of momentary arrest. The morphology of these sills displays folds, ropy structures on their surface, and lobes with imprints of the leading fronts that correspond to each step of area creation. These experiments show that for a given, constant injected volume, as solidification effects increase, the area of the sills decreases, their thickness increases, and the number of propagation steps increases. These results have various geological and geophysical implications. The morphology of sills, such as lobate structures (interpretation of 3D seismic studies in sedimentary basin) and ropy flow structures (field observations) can be related to solidification during emplacement. Moreover, a non-continuous morphology as observed in the field does not necessarily involve multiple injections, but could instead reflect a continuous, yet complex morphology induced by solidification effects during emplacement. Also, a discontinuous sill propagation induced by solidification effects should be associated with bursts of seismic activity. Finally, our study shows that once a sill has initiated, the dimensionless flux influences the sill thermal state, and in turn its propagation, and final extent and thickness. In restricting the lateral extent of sills, magma cooling and solidification are likely to impact directly the size of plutons constructed by amalgamated sills.

49 CFR 231.8 - Tank cars without side sills and tank cars with short side sills and end platforms.

Code of Federal Regulations, 2013 CFR

2013-10-01

... 49 Transportation 4 2013-10-01 2013-10-01 false Tank cars without side sills and tank cars with... APPLIANCE STANDARDS § 231.8 Tank cars without side sills and tank cars with short side sills and end platforms. (a) Hand brakes—(1) Number. Same as specified for “Box and other house cars” (see § 231.1(a)(1...

49 CFR 231.8 - Tank cars without side sills and tank cars with short side sills and end platforms.

Code of Federal Regulations, 2011 CFR

2011-10-01

... 49 Transportation 4 2011-10-01 2011-10-01 false Tank cars without side sills and tank cars with... APPLIANCE STANDARDS § 231.8 Tank cars without side sills and tank cars with short side sills and end platforms. (a) Hand brakes—(1) Number. Same as specified for “Box and other house cars” (see § 231.1(a)(1...

49 CFR 231.8 - Tank cars without side sills and tank cars with short side sills and end platforms.

Code of Federal Regulations, 2010 CFR

2010-10-01

... 49 Transportation 4 2010-10-01 2010-10-01 false Tank cars without side sills and tank cars with... APPLIANCE STANDARDS § 231.8 Tank cars without side sills and tank cars with short side sills and end platforms. (a) Hand brakes—(1) Number. Same as specified for “Box and other house cars” (see § 231.1(a)(1...

49 CFR 231.8 - Tank cars without side sills and tank cars with short side sills and end platforms.

Code of Federal Regulations, 2012 CFR

2012-10-01

... 49 Transportation 4 2012-10-01 2012-10-01 false Tank cars without side sills and tank cars with... APPLIANCE STANDARDS § 231.8 Tank cars without side sills and tank cars with short side sills and end platforms. (a) Hand brakes—(1) Number. Same as specified for “Box and other house cars” (see § 231.1(a)(1...

49 CFR 231.8 - Tank cars without side sills and tank cars with short side sills and end platforms.

Code of Federal Regulations, 2014 CFR

2014-10-01

... 49 Transportation 4 2014-10-01 2014-10-01 false Tank cars without side sills and tank cars with... APPLIANCE STANDARDS § 231.8 Tank cars without side sills and tank cars with short side sills and end platforms. (a) Hand brakes—(1) Number. Same as specified for “Box and other house cars” (see § 231.1(a)(1...

Growth of Transgressive Sills in Mechanically Layered Media: Faroe Islands, NE Atlantic Margin

NASA Astrophysics Data System (ADS)

Walker, R. J.

2014-12-01

Igneous sills represent an important contribution to upper crustal magma transport, acting as magma conduits and stores (i.e. as sill networks, or as nascent magma chambers). Complex sill-network intrusion in basin settings can have significant impact on subsurface fluid flow (e.g., water aquifer and hydrocarbon systems), geothermal systems, the maturation of hydrocarbons, and methane release. Models for these effects are critically dependent on the models for sill emplacement. This study focuses on staircase-geometry sills in the Faroe Islands, on the European Atlantic Margin, which are hosted in mechanically layered lavas (1-20 m thick) and basaltic volcaniclastic units (1-30 m thick). The sills range from 20-50 m thick, with each covering ~17 km2, and transgressing a vertical range of ~480 m. Steps in the sills are elliptical in cross section, and discontinuous laterally, forming smooth transgressive ramps, hence are interpreted as representing initial stages of sill propagation as magma fingers, which inflate through time to create a through-going sheet. Although steps correspond to the position of some host rock layer interfaces and volcaniclastic horizons, most interfaces are bypassed. The overall geometry of the sills is consistent with ENE-WSW compression, and NNW-SSE extension, and stress anisotropy-induced transgression. Local morphology indicates that mechanical layering suppressed tensile stress ahead of the crack tip, leading to a switch in minimum and intermediate stress axes, facilitating lateral sill propagation as fingers, and resulting in a stepped transgressive geometry.

Solidification effects on sill formation: An experimental approach

NASA Astrophysics Data System (ADS)

Chanceaux, L.; Menand, T.

2014-10-01

Sills represent a major mechanism for constructing continental Earth's crust because these intrusions can amalgamate and form magma reservoirs and plutons. As a result, numerous field, laboratory and numerical studies have investigated the conditions that lead to sill emplacement. However, all previous studies have neglected the potential effect magma solidification could have on sill formation. The effects of solidification on the formation of sills are studied and quantified with scaled analogue laboratory experiments. The experiments presented here involved the injection of hot vegetable oil (a magma analogue) which solidified during its propagation as a dyke in a colder and layered solid of gelatine (a host rock analogue). The gelatine solid had two layers of different stiffness, to create a priori favourable conditions to form sills. Several behaviours were observed depending on the injection temperature and the injection rate: no intrusions (extreme solidification effects), dykes stopping at the interface (high solidification effects), sills (moderate solidification effects), and dykes passing through the interface (low solidification effects). All these results can be explained quantitatively as a function of a dimensionless temperature θ, which describes the experimental thermal conditions, and a dimensionless flux ϕ, which describes their dynamical conditions. The experiments reveal that sills can only form within a restricted domain of the (θ , ϕ) parameter space. These experiments demonstrate that contrary to isothermal experiments where cooling could not affect sill formation, the presence of an interface that would be a priori mechanically favourable is not a sufficient condition for sill formation; solidification effects restrict sill formation. The results are consistent with field observations and provide a means to explain why some dykes form sills when others do not under seemingly similar geological conditions.

Igneous Sheet Intrusions as a Record of Paleostress States

NASA Astrophysics Data System (ADS)

Stephens, T. L.; Walker, R. J.; Healy, D.; Bubeck, A.; England, R. W.; McCaffrey, K. J. W.

2017-12-01

The architecture of igneous sheet intrusion networks provides useful constraints on paleostress during emplacement. Several models for sill emplacement have used the close spatial relationships between sills and dikes in layered (sedimentary) host rocks to propose that dike-sill transitions are driven by layering. Such models require a stress rotation - from horizontal extension for dikes, to horizontal compression for sills - which is assumed to reflect a near-hydrostatic stress state, facilitating the dilation and intrusion of pre-existing structures (e.g. faults, joints, and bedding). Here, we present case examples of sills for which layering is not the main control on emplacement: Isle of Mull (UK), Faroe Islands (European Atlantic margin) and the San Rafael Subvolcanic Field (Utah, USA). In each case, dikes cut, or are cut by, sills; indicating that dikes were not the feeders to sills in the same section. The sills consist of linked, flat and shallowly-dipping segments that always show near-vertical opening directions. Sills cut bedding and formation contacts with consistent low-angle dips, and cut or abut against vertical faults, fractures, and tectonic foliations. From this, we infer that magma pressure during emplacement did not exceed the horizontal stress. To constrain the stress state during emplacement we present a novel approach that combines analysis of local and overall sill geometry data with mechanical models for slip tendency, dilation tendency, and fracture susceptibility. We also present a new depth-independent mechanical model, which estimates paleostress ratio and driving fluid pressure ratio using the opening angles of dilated fluid-filled fractures. Our results show that the studied sills record previously unrecognised local fluctuations in the far-field stress state, during magmatic supply. Sills, therefore, present an important tool for determining paleostress in areas where few brittle deformation structures (e.g. faults), other than intrusions, are present.

Forced folding and complex overburden deformation associated with magmatic intrusion in the Vøring Basin, offshore Norway

NASA Astrophysics Data System (ADS)

Omosanya, Kamaldeen Olakunle; Johansen, Ståle E.; Eruteya, Ovie Emmanuel; Waldmann, Nicolas

2017-06-01

In this study, three-dimensional seismic reflection and borehole data from the Vøring Basin, offshore Norway have been used to characterize a supra-sill related forced fold to understand its evolution and relevance in the context of regional tectonics. Magmatic sills were recognised to be positive high-amplitude anomalies with similar polarity to the seabed reflection. The seismic dataset reveals two groups of sills in the study area comprising interconnected sills beneath the regional forced fold, and those intruded into the overburden. Magmatic sills forming the interconnected sill complex are emplaced at a depth of about 5.5 s TWTT below the modern seafloor. Aspect ratio (length/width), A for the sills ranges from 1.63-6.90. The regional forced fold is interpreted based on its bathymetric and seismic-stratigraphic expression on horizon H7, which is part of the Palaeocene to Eocene Tang Formation. Amplitude of the accommodation fold is about 780 km2. Hydrothermal vent complexes and fluid-flow conduits in the study area develop above the sill edges and on the flanks of the interconnected sill complex extending from the lower part of the Tang Formation to the uppermost section of the Brygge Formation evidencing vertically focussed fluid flow in the study area. The overlying overburden is in turn deformed and structurally compartmentalized through forced folding and Late Cenozoic tectonics. We demonstrate that accommodation folding is formed in response to the emplacement of several interconnected sills during the opening of the Norwegian-Greenland Seas. Sill emplacement in the study area causes uplift of the Cretaceous to Palaeocene depocentre prior to further restructuration during Cenozoic tectonic inversion. Magmatic intrusions documented in this study have wider implications for understanding supra-sill deformations along volcanic margins with well-developed emplaced sills at depth and likewise hydrocarbon prospectivity in the study area.

Evidence of a modern deep water magmatic hydrothermal system in the Canary Basin (eastern central Atlantic Ocean)

NASA Astrophysics Data System (ADS)

Medialdea, T.; Somoza, L.; González, F. J.; Vázquez, J. T.; de Ignacio, C.; Sumino, H.; Sánchez-Guillamón, O.; Orihashi, Y.; León, R.; Palomino, D.

2017-08-01

New seismic profiles, bathymetric data, and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel, or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4 and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78 ± 0.05 and 1.61 ± 0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep water oceanic basins related to intraplate hotspot activity.