Volcanic Construction Research Articles

The nakhlite meteorites: Augite-rich igneous rocks from Mars

The seven nakhlite meteorites are augite-rich igneous rocks that formed in flows or shallow intrusions of basaltic magma on Mars. They consist of euhedral to subhedral crystals of augite and olivine (to 1cm long) in fine-grained mesostases. The augite crystals have homogeneous cores of Mg′=63% and rims that are normally zoned to iron enrichment. The core–rim zoning is cut by iron-enriched zones along fractures and is replaced locally by ferroan low-Ca pyroxene. The core compositions of the olivines vary inversely with the steepness of their rim zoning – sharp rim zoning goes with the most magnesian cores (Mg′=42%), homogeneous olivines are the most ferroan. The olivine and augite crystals contain multiphase inclusions representing trapped magma. Among the olivine and augite crystals is mesostasis, composed principally of plagioclase and/or glass, with euhedra of titanomagnetite and many minor minerals. Olivine and mesostasis glass are partially replaced by veinlets and patches of iddingsite, a mixture of smectite clays, iron oxy-hydroxides and carbonate minerals. In the mesostasis are rare patches of a salt alteration assemblage: halite, siderite, and anhydrite/gypsum. The nakhlites are little shocked, but have been affected chemically and biologically by their residence on Earth.Differences among the chemical compositions of the nakhlites can be ascribed mostly to different proportions of augite, olivine, and mesostasis. Compared to common basalts, they are rich in Ca, strongly depleted in Al, and enriched in magmaphile (incompatible) elements, including the LREE. Nakhlites contain little pre-terrestrial organic matter. Oxygen isotope ratios are not terrestrial, and are different in anhydrous silicates and in iddingsite. The alteration assemblages all have heavy oxygen and heavy carbon, while D/H values are extreme and scattered. Igneous sulfur had a solar-system isotopic ratio, but in most minerals was altered to higher and lower values. High precision analyses show mass-independent fractionations of S isotopes. Nitrogen and noble gases are complex and represent three components: two mantle sources (Chas-E and Chas-S), and fractionated Martian atmosphere.The nakhlites are igneous cumulate rocks, formed from basaltic magma at ∼1.3Ga, containing excess crystals over what would form from pure magma. After accumulation of their augite and olivine crystals, they were affected (to various degrees) by crystallization of the magma, element diffusion among minerals and magma, chemical reactions among minerals and magma, magma movement among the crystals, and post-igneous chemical equilibration. The extent of these modifications varies, from least to greatest, in the order: MIL03346, NWA817, Y000593, Nakhla=Governador Valadares, Lafayette, and NWA998.Chemical, isotopic, and chronologic data confirm that the nakhlites formed on Mars, most likely in thick lava flows or shallow intrusions. Their crystallization ages, referenced to crater count chronologies for Mars, suggest that the nakhlites formed on the large volcanic constructs of Tharsis, Elysium, or Syrtis Major. The nakhlites were suffused with liquid water, probably at ∼620ma. This water dissolved olivine and mesostasis glass, and deposited iddingsite and salt minerals in their places. The nakhlites were ejected from Mars at ∼10.75Ma by an asteroid impact and fell to Earth within the last 10,000 years.Although the nakhlites are enriched in incompatible elements, their source mantle was strongly depleted. This depletion event was ancient, as the nakhlites’ source mantle was fractionated while short-lived radionuclides (e.g., t1/2=9my) were still active. This differentiation event may have been core formation coupled with a magma ocean, as is inferred for the moon.

Evidence for a high‐Th, evolved lithology on the Moon at Hansteen Alpha

We have revisited a previous interpretation of lunar Th abundances at Hansteen Alpha, which is thought to be a nonmare volcanic construct. Previous studies concluded that while Th abundances were higher than the surrounding mare basalts, Th abundances at Hansteen Alpha were not high enough to be indicative of known, evolved rock types. However, a forward modeling analysis of the Lunar Prospector gamma‐ray data provides evidence that Hansteen Alpha may have Th abundances up to 25 μg/g. This indicates Hansteen Alpha may be composed of a rhyolitic, volcanic lithology. We propose that a possible formation process for Hansteen Alpha is underplating where basaltic magma intrudes into and partially melts the overlying crust. The resulting partial melt is compositionally and thermally buoyant and can be erupted to produce rhyolite. Based on Earth‐analogues, underplating could explain both the composition and the relatively large volume (∼100 km3) of material represented by Hansteen Alpha.

Cassini Radar Views the Surface of Titan

The Cassini Titan Radar Mapper imaged about 1% of Titan's surface at a resolution of approximately 0.5 kilometer, and larger areas of the globe in lower resolution modes. The images reveal a complex surface, with areas of low relief and a variety of geologic features suggestive of dome-like volcanic constructs, flows, and sinuous channels. The surface appears to be young, with few impact craters. Scattering and dielectric properties are consistent with porous ice or organics. Dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons.

Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery

The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh‐looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along‐axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid‐Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra‐slow spreading ridges. The correlation between the distribution of fresh‐looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along‐axis variations of the gravity‐derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh‐looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra‐slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.

A numerical study of a mantle plume beneath the Tharsis Rise: Reconciling dynamic uplift and lithospheric support models

The Tharsis Rise is an area of extensive volcanism containing the most significant long‐wavelength topographic and areoid anomalies on Mars. The mechanism for supporting this large topographic expression and correlating areoid remains controversial. The two main competing ideas are dynamic support by a deep, mantle plume and a volcanically constructed lithosphere followed by lithospheric flexure. While both of these models, separately, can account for specific features of Tharsis, neither explain its entirety. Most of the support of the Tharsis topographic anomaly is best explained by a volcanic structure modifying the lithosphere. However, this raises the important question of the origin of the heat source for volcanic construction, leading to possible small‐scale convection. This demonstrates that the remaining topography and areoid not accounted for in lower end‐members of volcanically constructed surface loads is consistent with a deeper, mantle plume source. Our results show we can produce about 15% and 18% of the total Tharsis long‐wavelength areoid and topography anomaly with a plume forming in a strong, temperature‐dependent rheology. Additionally, our plume model has a 165 km thick rheological lithosphere (within the range for present‐day estimates on Mars) and generates about 1% partial melting, more than sufficient to provide a heat source for recent volcanism. This implies both a model with dynamic support from a plume and a volcanically constructed lithosphere not only can coexist but offer an explanation for both the surface anomalies and heat needed for the minor amount of recent volcanic activity at Tharsis Rise.

A Compilation of Geophysical Data from the Lazarev Sea and the Riiser-Larsen Sea, Antarctica

On the basis of new geophysical data acquired by the Federal Institute of Geosciences and Natural Resources (BGR) and the Polar Marine Geological Research Expedition (PMGRE) as well as existing data new geophysical maps were compiled for the Lazarev Sea and the Riiser-Larsen Sea between 10°W and 25°E. The new results are: – The drastic change in the strike direction of the volcanic Explora Wedge between longitudes 10°W and 5°W is accompanied with a gradual change from one major wedge, i.e. the Explora Wedge, into at least two wedge-shaped volcanic constructions, each manifested by a sequence of seaward-dipping reflectors in the seismic records. – The southern Lazarev Sea is best described as a continental margin affected by multiple rifting episodes accompanied with transient volcanism. – A distinct N80°E striking basement depression separates the volcanic-prone continental margin of the southern Lazarev Sea from oceanic crust upon which the Maud Rise rests. The southern scarp of the narrow depression was presumably aligned with the eastern scarp of the Mozambique Ridge during the Early Cretaceous. – The Astrid Ridge proper occupies the transition from the volcanic-prone continental margin of the Lazarev Sea to old oceanic crust of the Riiser -Larsen Sea, and it rests upon a large volcanic apron which covers the basement of the southwestern Riiser-Larsen Sea. – No evidence was found that prolific volcanism has affected the early opening of the Riiser-Larsen Sea. – The Lazarev Sea is a sediment-starved region.

Crustal structure of Mars from gravity and topography

Mars Orbiter Laser Altimeter (MOLA) topography and gravity models from 5 years of Mars Global Surveyor (MGS) spacecraft tracking provide a window into the structure of the Martian crust and upper mantle. We apply a finite‐amplitude terrain correction assuming uniform crustal density and additional corrections for the anomalous densities of the polar caps, the major volcanos, and the hydrostatic flattening of the core. A nonlinear inversion for Moho relief yields a crustal thickness model that obeys a plausible power law and resolves features as small as 300 km wavelength. On the basis of petrological and geophysical constraints, we invoke a mantle density contrast of 600 kg m−3; with this assumption, the Isidis and Hellas gravity anomalies constrain the global mean crustal thickness to be >45 km. The crust is characterized by a degree 1 structure that is several times larger than any higher degree harmonic component, representing the geophysical manifestation of the planet's hemispheric dichotomy. It corresponds to a distinction between modal crustal thicknesses of 32 km and 58 km in the northern and southern hemispheres, respectively. The Tharsis rise and Hellas annulus represent the strongest components in the degree 2 crustal thickness structure. A uniform highland crustal thickness suggests a single mechanism for its formation, with subsequent modification by the Hellas impact, erosion, and the volcanic construction of Tharsis. The largest surviving lowland impact, Utopia, postdated formation of the crustal dichotomy. Its crustal structure is preserved, making it unlikely that the northern crust was subsequently thinned by internal processes.

COMPRESSIONAL STRUCTURES ON MARS

▪ Abstract Wrinkle ridges accommodate very low amounts of shortening strain around immense volcanic constructs and within impact basins on Mars. They originate from stresses distributed uniformly throughout the brittle lithosphere and are consistently located in stratified deposits, including lava flows and sediment. Most recent models interpret wrinkle ridges as the surface manifestation of folding above underlying blind thrusts that accommodate similarly low strain and likely penetrate tens of kilometers into the brittle crust. Alternative models suggest shortening accommodated by some wrinkle ridges is confined to only the upper few kilometers of the crust. The interpretation of the geometry of blind thrusts on Mars appears to be quite varied and remains controversial, although some models suggest they may ultimately flatten into the brittle-ductile transition in the middle to lower crust. Small-scale crenulations superposed on ridges are interpreted as produced by high-level back thrusts nucleating at a weak layer in the upper crust, or by flexural slip faults that facilitate bending of layered materials. Wrinkle ridges are related to their structural cousins, lobate scarps that accommodate shortening in older Noachian cratered highlands as surface fault ruptures. Wrinkle ridges thus form when displacement across upwardly propagating blind thrusts is consumed by folding of layered material near the surface. Conversely, lobate scarps are formed by blind thrusts that are not impeded by folding of overlying layered deposits. Broad, low-amplitude arches associated with ridges on the Tharsis rise also accommodate shortening, but their relationship to adjacent or superposed ridges remains enigmatic. The evenly spaced nature of wrinkle ridges that appears to vary systematically between the ridged plains and northern lowlands may be related to the depth of the brittle-ductile transition, which is respectively located in the middle crust and upper mantle in these regions.

Transient hydrogeologic models for submarine flow in volcanic seamounts: 1. The Hawaiian Islands

Seamounts and volcanic islands are regions of the seafloor where pressure gradients, caused by temperature variations, are large enough to drive fluid flow. Compaction in surrounding sedimentary aprons, formed during mass wasting of these features, drives fluid flow as well. In seamount environments a complex flow pattern evolves as effects of compaction and free convection interact. We use transient finite element modeling to explore fluid flow and pore pressure in a volcanic edifice and its surrounding sedimentary apron to determine the hydrogeologic and pore pressure regime. Models are based on geophysical cross sections of the Hawaiian Islands and include the volcanic edifice, sedimentary apron, and underlying oceanic crust. Models simulate growth of the volcanic island, flexure of the oceanic crust due to loading, and formation of the sedimentary apron. High sedimentation rates of up to 1 mm/yr during volcanic building cause compaction in the sedimentary apron, and the weight of the edifice compacts the pelagic sediment beneath it; compaction‐driven flow dominates flow patterns. Excess pore pressures are highest during the beginning of volcanic construction and decline through the remainder of the simulation. As sedimentation decreases, compaction‐driven and buoyancy‐driven flow both control flow patterns. When sedimentation stops, pore pressure dissipates, and buoyancy‐driven flow dominates the flow regime. Pore pressure ratios indicate that under certain parameter conditions, pore pressure may exceed lithostatic, leading to unstable edifice conditions. Although fluid velocities are small (q < 0.5 cm/yr), the hydrologic regime has important implications for slope stability, volcanic spreading, sedimentation processes, and geochemical transport.

Aberrant youth: Chemical and isotopic constraints on the origin of off‐axis lavas from the East Pacific Rise, 9°–10°N

We report measurements of U‐series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and major and trace element abundances in a suite of well‐located, off‐axis MORBs that span the East Pacific Rise (EPR) ridge crest from 9°48′–52′N and across it for ∼4 km on either side. The geological context of the samples are well constrained as they were collected by submersible in an area that has been extensively imaged by remote sensing techniques. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off‐axis N‐MORB are identical to the axial lavas from this same region, suggesting that their sources are similar and that melting processes are the dominant influence in establishing the U‐Th‐Ra disequilibria and trace element fractionations. A majority of off‐axis samples have U‐Th and Th‐Ra disequilibria that are larger, and model ages that are younger, than would be predicted from their off‐axis distance and the time‐integrated spreading rate. There are, however, a few off‐axis samples with U‐Th ages that are consistent with their spreading rate ages. It is likely that these samples erupted within or close to the axial summit trough (AST) and aged at a rate proportional to the spreading rate. The anomalously young ages determined for most of the off‐axis lavas suggest that volcanic construction along this region is occurring over a zone that is wider (at least 4 km) than the AST (10s to 100s of meters). The combined observational, chemical and isotopic data support a model for the 9°0′N area that includes a significant component of crustal accumulation resulting from lavas that breach the AST and flow down the flanks of the EPR ridge crest. However, these data also require a minor component of off‐axis eruptions that occur on distinct pillow mounds and ridges. This suggests that MOR construction involves several volcanic and tectonic processes acting in concert to form a complex patchwork of lava ages and compositions along, and across, this fast spreading ridge crest.

Influence of enhanced melt supply on upper crustal structure at a mid‐ocean ridge discontinuity: A three‐dimensional seismic tomographic study of 9°N East Pacific Rise

We present a three‐dimensional upper crustal model of the 9°03′N overlapping spreading center (OSC) on the East Pacific Rise that assists in understanding the relationship between melt sills and upper crustal structure at a ridge discontinuity with enhanced melt supply at crustal levels. Our P wave velocity model obtained from tomographic inversion of ∼70,000 crustal first arrival travel times suggests that the geometry of extrusive emplacement are significantly different beneath the overlapping spreading limbs. Extrusive volcanic rocks above the western melt sill are inferred to be thin (∼250 m). More extensive accumulation of extrusives is inferred to the west than to the east of the western melt sill. The extrusive layer inferred above the eastern melt sill thickens from ∼350 (at the neovolcanic axis) to 550 m (to the west of the melt sill). Volcanic construction is likely to be significant in the formation of ridge crest morphology at the OSC, particularly at the tip of the eastern limb. On the basis of our interpretation of the velocity model, we propose that enhanced magma supply at crustal levels at the OSC may provide an effective mechanism for the migration of ridge discontinuities. This “dynamic magma supply model” may explain the commonly observed nonsteady migration pattern of ridge discontinuities by attributing this to the temporal fluctuations in melt availability to the overlapping spreading limbs.

On the support of the Tharsis Rise on Mars

The most significant long-wavelength geoid and topographic anomalies on Mars are associated with the Tharsis Rise [Smith et al., Science 286 (1999) 94–97; Smith et al., Science 284 (1999) 1495–1503]. However, the origin of the elevated geoid and topography in the Tharsis region remains unresolved between two competing models. In the first model the Tharsis Rise is dynamically supported by a thermal plume in the mantle [Kiefer and Hager, LPI Tech. Rep. 89-04 (1989) 48–50; Harder and Christensen, Nature 380 (1996) 507–509; Breuer et al., J. Geophys. Res. 101 (1996) 7531–7542], while the second model attributes the Tharsis anomalies to volcanic construction and its associated lithospheric flexural effects [Turcotte et al., J. Geophys. Res. 86 (1981) 3951–3959; Phillips et al., Science 291 (2001) 2587–2591]. Here we resolve this ambiguity by modeling the ratio of gravity and topography at long wavelengths with a new loading formulation that simultaneously considers plume buoyancy and surface loads. We demonstrate that a thermal plume contributes no more than 15% to the geoid and 25% to the topography for the Tharsis Rise for the range of lithospheric thicknesses that are constrained by observations. This indicates that the Tharsis Rise is best explained by volcanic construction on lithosphere.

Melt supply variations to a magma‐poor ultra‐slow spreading ridge (Southwest Indian Ridge 61° to 69°E)

The Southwest Indian Ridge (SWIR) to the east of the Melville Fracture zone receives anomalously low volumes of melt on average. However, a small number of ridge segments appear to receive more melt than this regional average. We use off‐axis bathymetry, gravity, and magnetic data to show that this melt distribution pattern, quite distinct from what is observed at the Mid‐Atlantic Ridge (MAR), has been a characteristic of the easternmost SWIR for at least the past 10 myr. We also show that segments of the easternmost SWIR are substantially shorter lived than most segments of the MAR. Melt distribution in our SWIR study area is therefore both more focused and more variable in time than at the MAR. We tentatively propose a mechanism by which strong and transient melt‐focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere, causing thermal thinning of this lithosphere and along‐axis melt migration. These two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments. Rapid melt extraction by dikes that feed large volcanic constructions on the seafloor, followed by tectonic disruption of these volcanic constructions by deep‐reaching faults, may then cool the axial lithosphere back to its original thickness and end the melt‐focusing events. The easternmost SWIR is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks. At the faster spreading MAR, similar characteristics are found near the ends of ridge segments. We propose that spreading at the ultra‐slow SWIR during periods when the melt supply is low (i.e., most of the time for the easternmost SWIR) is dominated by large offset asymmetric normal faulting, with significant flexural uplift of the footwalls. Faults face either north or south, and changes in fault polarity are frequent, both along axis and along flow lines (i.e., with time). Producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick, a predictable characteristic for this ultra‐slow ridge, which has an anomalously low regionally averaged melt supply.

Hansteen Alpha: A volcanic construct in the lunar highlands

We have used data from the Clementine and Lunar Prospector spacecraft in conjunction with near‐IR reflectance spectra collected with Earth‐based telescopes to study the composition and origin of Hansteen Alpha, an arrowhead‐shaped highlands feature located on the southern margin of Oceanus Procellarum. Hansteen Alpha is a member of a class of lunar spectral anomalies (Red Spots) that are characterized by a relatively high albedo and a strong absorption in the UV. It has been suggested that at least some of these spectral anomalies were produced by nonmare volcanism. The stratigraphic relationships among the geologic and compositional units in the region show that Hansteen Alpha was emplaced by extrusive igneous processes. The Imbrian‐aged craters Hansteen and Billy emplaced relatively FeO‐ and TiO2‐rich ejecta. Hansteen Alpha exhibits much lower FeO and TiO2 values. If Hansteen Alpha was present prior to the Billy and Hansteen impact events, it should have been covered with FeO‐ and TiO2‐rich ejecta because it is located within one crater diameter of the rim crest of each crater. Since it is not, Hansteen Alpha was superposed on these ejecta units. Nonmare volcanism is the only viable process for the formation of Hansteen Alpha. The morphology and surface texture of Hansteen Alpha is similar to many terrestrial features of dacitic and rhyolitic composition formed by extrusions of relatively viscous lavas. Such highly evolved compositions should be very enriched in Th, but Hansteen Alpha exhibits Th abundances of ∼6 ppm. Hence Hansteen Alpha is not composed of a highly evolved highlands lithology.

Magmato‐tectonic cyclicity at the ultra‐slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On‐axis deep tow side scan sonar data are used together with off‐axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra‐slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off‐axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato‐tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato‐tectonic cycle is a pseudoperiodic process that lasts ∼0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long‐lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

Age and composition of the Amanay Seamount, Canary Islands

A number of samples have been dredged from the upper parts of Amanay and El Banquete Seamounts, yet volcanic materials have been collected only on Amanay Seamount. Based on textural features and the presence or absence of kaersutite, two main types of olivine pyroxene basaltic rocks have been identified. The rocks are basanites with high enrichment in the most incompatible elements, similar to that displayed by Ocean Island Basalts. Samples from Amanay Seamount formed due to a low degree of melting of an enriched mantle, very similar to that which probably caused the Miocene volcanic activity of Fuerteventura. The age of Amanay volcanic rocks, 15.3 ± 0.4 and 13.1 ± 0.3 Ma, is similar to those of the older volcanic units exposed in the nearby islands (Gran Canaria, Fuerteventura and Lanzarote). This proves the formation of a separate submarine volcanic edifice coeval with the other edifices of the Eastern Canarian Volcanic Ridge. Volcanic activity on the submarine edifice is thought to have ceased at about 13 Ma, simultaneous with the adjacent main volcanic construction.

Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system

Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian–Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85–100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.

Effects of lithosphere on the long‐wavelength gravity anomalies and their implications for the formation of the Tharsis rise on Mars

The Tharsis rise represents the most significant long‐wavelength gravity and topography anomalies on Mars. Two competing models have been proposed to explain the origin of these anomalies. In the first model the Tharsis rise is attributed to loading of volcanic construction on the lithosphere, while in the second model these anomalies are explained as dynamic effects of a one‐plume thermal structure below the Tharsis rise. In this study we seek to distinguish between these two models by formulating a generalized viscoelastic loading model that determines responses to loads at different depths. With this new formulation we found that the instantaneous viscous flow loading formulation used to compute responses from a plume may significantly overestimate the geoid anomalies if the elastic thickness of the Tharsis lithosphere is ∼150 km as inferred from gravity and topography data. We suggest that the one‐plume structure if currently existing below the Tharsis rise may be responsible for only a small fraction (<10%) of the Tharsis geoid anomalies and that other processes, including surface loading of volcanic construction, may be the primary cause of the geoid anomalies. The plume structure is inefficient in producing geoid anomalies because the long‐wavelength components of the plume structure are confined to a ∼300 km layer below the lithosphere in which the geoid response is minimal. Future studies on gravity and topography at different wavelengths may put constraints on possible contribution of a plume to the elevated Tharsis topography.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

Age constraints and characteristics of subaqueous volcanic construction, the Archean Hunter Mine Group, Abitibi greenstone belt

The Archean Hunter Mine Group (HMG) is a 6–7 km-thick south-facing volcanic sequence at the southern limit of the Northern Volcanic Zone, Abitibi greenstone belt. The HMG is composed of (1) felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits; (2) pyroclastic fire-fountain products and high- to low-concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron-formation; (3) mafic pillowed and brecciated flows; (4) a prominent felsic dyke swarm; and (5) abundant mafic intrusions. The stratigraphy is complex, as inferred by detailed volcano-sedimentary facies mapping. A 5–7 km-wide, N-trending felsic dyke swarm, coupled with abundant synvolcanic and synsedimentary faults, chaotic breccias and mineralization suggest a central volcanic depression, which is consistent with a caldera structure. Voluminous effusive activity and explosive fire-fountaining eruptions contributed to caldera development. Rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills. The lithofacies and overlying komatiites flows are consistent with a deep-water marine setting. U-Pb age determinations help constrain the HMG caldera formation to ≈6 million years. The sampled quartz–feldspar–phyric dykes yielded U-Pb zircon ages of 2728.3+4/−3.4 and 2728.9±0.8 Ma and together with a previously published dyke age of 2729.6±1.4 Ma from the same dyke swarm, a case is made that the swarm evolved over 1 million years (2728.6–2729.6 Ma). A lobate flow and flow breccia revealed similar ages of 2724.6+4.6/−1.7 and 2727.6+4.2/−2.0 Ma, whereas the up to 1 km-thick, gabbro-diorite Roquemaure sill precedes both felsic dykes and flows, as indicated by the 2731.8+2.2/−2.0 Ma age. The 2706.4+5.7/−4.3 Ma feldspar–quartz–phyric dykes occurred late in the history of the HMG and were not associated with subaqueous volcanic construction. The 207Pb/ 206Pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the HMG complex. Collectively, these age determinations and inheritance, as well as stratigraphy, support a tripartite division into a 2734–2730 Ma lower formational stage, a 2732 Ma middle formational stage and a 2730–2728 Ma upper formational stage. The study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice.