Submarine Rift Research Articles

An exceptionally well-preserved Jurassic plateau-top to marginal escarpment in the Northern Apennines (Central Italy): sedimentological, palaeontological and palaeostructural features

Submarine palaeo-escarpment tracts at the basin margins commonly border isolated drowned carbonate platforms in the Jurassic rifted margins of the Western Tethys and bear fundamental structural and palaeogeographic significance. These tectonically generated escarpments, rooted in late Hettangian-early Sinemurian master faults, provide key information on the regional architecture of the rift, and on the nature and timespan of activity of the faults which define them. An isolated drowned carbonate platform (the Sabina Plateau) in the Northern Apennines of Central Italy exhibits a wealth of peculiar details on the Jurassic submarine topography. The deposits covering this escarpment host multiple unconformities related to a margin-failure episode, documented by breccias resting on a submarine-scar surface, perched with respect to the basin bottom and draped discontinuously by condensed ammonite-rich pelagites. Thin lenses of graded and laminated oolitic limestones in the condensed pelagic succession of the Sabina Plateau are interpreted as likely produced by the overbanking of turbidity currents, which were shed into the basin by a neighbouring productive carbonate platform. The final leveling of the submarine rift bathymetry occurred in the earliest Cretaceous, as documented by the onlap of the aggrading basinal succession against the palaeo-escarpment, and the burial of the plateau-top condensed succession. This complex onlap surface exhibits a distinctive overprint, such as diffuse silicification driven by the transit of silica-rich diagenetic fluids sourced by the radiolarian-rich basinal units.

Hydrogen emissions from hydrothermal fields in Iceland and comparison with the Mid-Atlantic Ridge

Hydrogen emissions from geothermal systems are well known, for example associated with mafic and ultramafic rock formations along mid-ocean ridges (MOR). Here we report concentrations and emissions for H 2 from geothermal fields in Iceland that provides the only MOR on Earth that exposes a largely submarine rift on land and compare them with submarine vent systems along the mid-Atlantic Ridge (MAR). The concentrations of H 2 in Icelandic hydrothermal fluids are 0.022–20.5 mmol/kg comparable to H 2 concentrations in MAR hydrothermal fluids of 0.07–26.5 mmol/kg. According to geochemical modeling, the H 2 is considered to form in these settings associated with chemical reactions of primary mineral leaching, Fe +II to Fe +III oxidation and secondary mineral precipitation and reduction of water to hydrogen. The main factors influencing these reactions are rock type and source water composition. Geothermal power plants in Iceland emit in total ∼1.2 kt H 2 per year (2019–20) or on average ∼0.7 g/MWh. The H 2 is today emitted to the atmosphere but could in principle be utilized given technical aspects of such extraction are implemented. • Large quantities of H 2 are present in the fumaroles and the steam of the power plants of Iceland. • The higher values are found along the active volcanic zone. • The salinity and the type of water influence the H 2 generation from the mafic and ultra-mafic rocks. • If we consider a price of H 2 of 2€/kg, the native H 2 emitted by the existing power plants corresponds to 2.3 million €/yr.

Geological mapping reveals the role of Early Jurassic rift architecture in the dispersal of calciturbidites: New insights from the Central and Northern Apennines

AbstractThe role of sea‐bottom topography in the dispersal of shallow water‐derived calciturbidites across a submarine rift, as determined by the local extensional architecture, is under‐investigated, namely with pelagic settings along ancient passive continental margins. A comparison with modern carbonate platform/basin analogues, or with siliciclastic systems, is not always feasible, as ancient carbonate systems were commonly home to anachronistic environments (e.g. the Western Tethyan Mesozoic). Our study focuses on: (a) a reconstruction of the palaeotectonic architecture of the Umbria‐Marche Basin in the Jurassic and on (b) how this architecture produced a submarine topography which governed the dispersal of sediment, shed by the neighbouring Lazio‐Abruzzo Carbonate Platform, for >40 million years. A geological mapping project was performed in the Apennines of Central Italy, a region which in the Jurassic displayed a pattern of intrabasinal highs (pelagic carbonate platforms) and intervening basins which was exceedingly complex due to the high density, and oddly variable trends, of faults rooted in a shallow detachment layer corresponding to thick Triassic salt. A map pairing the occurrences of these resedimented beds with an updated palaeogeography becomes the natural descriptor of the itineraries followed by sediment gravity flows. This qualitative method represents a companion, or even alternative, approach to the one strictly based on physical stratigraphy, and it greatly improves our knowledge of regional geology and rift‐basin analysis. Our case history potentially represents the analogue of hydrocarbon fields both inland and in the offshore. Geological mapping shows that the marginal palaeoescarpments of pelagic carbonate platforms formed obstacles to the gravity flows as sediment load was discharged at their toes. While turbidity flows were locally vigorous enough to climb the escarpments, leaving overbank deposits on the pelagic carbonate platform‐tops, a ‘shelter’ effect is evidenced by the resediment‐free nature of those basins lying downflow, which were shielded by the highs.

Correlated Changes Between Volcanic Structures and Magma Composition in the Faial Volcanic System, Azores

The interaction between magmatic and tectonic processes in ocean intraplate volcanism yields insights into the ascent and transport of magmas. Many oceanic intraplate studies lack a temporal component and do not consider changes in tectonic regime and/or magmatic processes during the evolution of magmatic systems. The eastern part of the Azores archipelago formed under the influence of both an intraplate melting anomaly and a system of ultraslow spreading rift axes. The majority of recent submarine and subaerial eruptions in the Azores occur along volcanic rift zones and thus melt transport and volcanic processes are largely controlled by tectonic processes, and are therefore ideally suited to study volcano-tectonic interactions. Here, we investigate how variable the magmatic and tectonic processes are in space and time and how they interact. We present new bathymetric, geophysical, geochemical and Sr-Nd-Pb isotope data from Faial Island and the surrounding seafloor, providing insights into the interaction between the asthenospheric melting anomaly and extensional lithospheric stresses. The bathymetry reveals large submarine volcanic rift zones on the western flank of Faial, including that of the 1957-1958 Capelinhos eruption. Based on absolute ages and seismic imagery, we develop a relative chronology of the magmatic evolution of the submarine rift zones. Their preferred WNW-ESE orientation implies that the stress field has not changed within the last ~1 Ma. We can however show that melt productivity progressively decreased with time. Compositionally different magma batches fed distinct volcanic rift zones and edifices, suggesting that changes in the melting regime occur on a small spatial scale and that the distribution of compositionally similar lavas is tectonically controlled. As melt supply decreases, the tectonic influence on volcanism increases with a stronger localisation of melts along tectonically controlled lineaments. The youngest mafic and intermediate melts (<10 ka) on Faial are exclusively erupted along single rift zones and cover a smaller area, whereas the older volcanism was more widespread.

Ka'ena Volcano--A precursor volcano of the island of O'ahu, Hawai'i

Ka‘ena and Wai‘alu Ridges form prominent submarine ridges NW of the island of O‘ahu, Hawai‘i. We evaluate whether or not either one of these ridges represents a submarine extension of Wai‘anae Volcano on O‘ahu using new bottom observations, geophysical surveys, and geochemical data acquired on new samples from the region. Wai‘alu Ridge has the morphology of a submarine rift zone but is too shallow for its distance from the O‘ahu shoreline; Ka‘ena Ridge also is unusually shallow and is surmounted by two topographic shields. Ka‘ena and Wai‘alu Ridges have similar magmatic and volcanic evolutionary histories, beginning ca. 5 Ma with a submarine, shield phase of volcanism that produced high-SiO 2 , low-FeO* tholeiites with higher 208 Pb/ 204 Pb than in the adjacent Wai‘anae Volcano. Late-shield volcanism included transitional and alkalic rock types, with lower SiO 2 and enrichment in incompatible elements, especially P 2 O 5 , Nb, Zr, Ti, and light rare earth elements. The transition from shield to late-shield stage occurred as the edifice was beginning to emerge from the sea. Geological observations and K/Ar ages indicate that Ka‘ena emerged above sea level ca. 3.5 Ma, reaching a maximum height of ∼4000 m above the abyssal ocean floor and 1000 m above sea level. Relatively weak gravity anomalies, topographic lineaments, and the orientation of dike complexes indicate a volcanic structure that is independent of Wai‘anae Volcano. Thus, volcanic structure, geochemistry, and age all indicate a precursor volcano to the island of O‘ahu, which we call Ka‘ena Volcano. After emergence, Ka‘ena Volcano tilted ∼2° to the south. We estimate a total volume of 20–27 × 10 3 km 3 for Ka‘ena Volcano, taking into account overlapping geometry of concurrently active volcanoes. Sample compositions from the Ka‘ena landslide deposit are entirely consistent with derivation from Ka‘ena, whereas most samples from the Wai‘anae slump are likely derived from Wai‘anae Volcano. Uniformly oriented dikes in the Wai‘anae NW rift zone likely reflect buttressing by a preexisting Ka‘ena Volcano. Unusual isotopic compositions of some Wai‘anae samples, including unique hydrous silicic lavas, probably reflect interaction with underlying Ka‘ena crust. A newly recognized lava flow field on the southern flank of Ka‘ena Ridge extends the previously known distribution of secondary volcanism in the Kaua‘i Channel. Putative submarine volcanic activity in the region in 1956 cannot have built a large edifice and is unlikely to have produced pumice that was found on O‘ahu shores. This eruptive activity therefore remains unconfirmed.

Seismic precursors and magma ascent before the April 2011 eruption at Axial Seamount

For volcanoes at submarine rift zones, a direct link between seismicity, seafloor deformation and magma intrusion has not been demonstrated. Recordings from ocean-bottom hydrophones and bottom-pressure recorders map an increasing rate of seismicity at Axial Seamount, northeast Pacific, over several years before its eruption in April 2011.

Early growth of Kohala volcano and formation of long Hawaiian rift zones

AbstractTransitional-composition pillow basalts from the toe of the Hilo Ridge, collected from outcrop by submersible, have yielded the oldest ages known from the Island of Hawaii: 1138 ± 34 to 1159 ± 33 ka. Hilo Ridge has long been interpreted as a submarine rift zone of Mauna Kea, but the new ages validate proposals that it is the distal east rift zone of Kohala, the oldest subaerial volcano on the island. These ages constrain the inception of tholeiitic volcanism at Kohala, provide the first measured duration of tholeiitic shield building (≥870 k.y.) for any Hawaiian volcano, and show that this 125-km-long rift zone developed to near-total length during early growth of Kohala. Long eastern-trending rift zones of Hawaiian volcanoes may follow fractures in oceanic crust activated by arching of the Hawaiian Swell in front of the propagating hotspot.

Geochemistry, geochronology and tectonic implications of Late Silurian – Early Devonian volcanic successions, Central Lachlan Orogen, New South Wales

The Ural Volcanics and Mt Hope Volcanics in the Central Lachlan Orogen, New South Wales, overlie non-volcanic, sedimentary, below-wave-base submarine facies in two intracontinental rift basins, the Rast and Mt Hope Troughs. The Ural Volcanics and Mt Hope Volcanics consist primarily of felsic (rhyolite to dacite), coherent facies and associated felsic monomictic breccia facies. These volcanic facies are interpreted to represent submarine lava-sill complexes, which define intrabasinal, effusive, volcanic and shallow intrusive centres. Small felsic intrusions and rare intermediate (monzodiorite) to mafic (gabbro) dykes also occur. The rocks experienced chlorite to biotite greenschist-facies metamorphism. LA-ICPMS U/Pb zircon dating indicates the Ural Volcanics and the Mt Hope Volcanics were broadly contemporaneous, but with mean ages of 421 ± 2 and 411 ± 5 Ma, respectively, the Ural Volcanics are slightly older. Given the differences in the degree of metamorphism, the different ages might represent greater Pb loss for the Mt Hope Volcanics. Major- and trace-element data (variably high K+, Zr, Nb, and Y, among others) is most consistent with A-type, within-plate origins for the igneous rocks, although some loss of immobile elements apparently accompanied metamorphism. The geologic setting, based on mapping, indicates that the Ural Volcanics and Mt Hope Volcanics were erupted and deposited in an intrabasinal environment, best described as an elongate, intracontinental submarine rift. This setting, and apparent absence of a proximal contemporaneous arc, supports contentions that A-type magmas are produced in rift settings.

Magnetic imaging of the feeding system of oceanic volcanic islands: El Hierro (Canary Islands)

SUMMARY El Hierro is the youngest of the Canary Islands, a volcanic archipelago in the central Atlantic, near the African coast. The subaerial part of the island shows the characteristic shape of three convergent ridges that has been interpreted as a triple-arm rift system. At least four giant landslides formed wide, horseshoe embayments that separate these ridges. Recent studies based on high-resolution bathymetry, however, showed that the submarine rift structure is much more complex. We analysed an aeromagnetic anomaly data set acquired in 1993 by the Spanish National Geographic Institute in order to obtain a structural model of the island from a magnetic point of view. A digital elevation model of the volcanic edifice was divided into a mesh of prismatic cells, each of them with a top corresponding to the topographic height (or bathymetric depth in the marine area) and a bottom at a constant depth of 4000 m below sea level. A three-dimensional (3-D) inversion algorithm and forward modelling along representative profiles provided us with a magnetization distribution containing valuable information about the inner structure of the island. The magnetic model cast new light on the rift structure of El Hierro. In particular, high magnetization values have been mainly interpreted as intrusive complexes on which rift zones are rooted. Their location confirms the hypothesis of a complex rift structure in the marine area. The inverse magnetization that characterizes the NE submarine rift area implies that this part of the volcanic edifice formed during the Matuyama and, therefore, predates the NW submarine rift zone, which is normally magnetized. The N‐S rift zone extending southwards from the island seems to be shifted to the west with respect to the bathymetric high in this area. This result suggests that the original rift zone was located in the area where the highest magnetizations presently occur so that the present morphology may reflect the westward collapse of the original ridge. In addition, very low magnetizations characterize the areas affected by giant landslides, indicating that magnetic anomalies can provide important constraints on the distribution of these catastrophic events.

Submarine volcanic morphology of the western Galápagos based on EM300 bathymetry and MR1 side‐scan sonar

A compilation of high‐resolution EM300 multibeam bathymetric and existing MR1 side‐scan sonar data was used to investigate the volcanic morphology of the flanks of the western Galápagos Islands. The data portray an assortment of constructional volcanic features on the shallow to deep submarine flanks of Fernandina, Isabela, and Santiago Islands, including rift zones and groups of cones that are considered to be the primary elements in constructing the archipelagic apron. Ten submarine rift zones were mapped, ranging in length from 5 to 20 km, comparable in length to western Canary Island rift zones but significantly shorter than Hawaiian submarine rift zones. A detailed analysis of the northwestern Fernandina submarine rift, including calculated magnetization from a surface‐towed magnetic study, suggests that the most recent volcanism has focused at the shallow end of the rift. Small submarine volcanic cones with various morphologies (e.g., pointed, cratered, and occasionally breached) are common in the submarine western Galápagos both on rift zones and on the island flanks where no rifts are present. At depths greater than ∼3000 m, large lava flow fields in regions of low bathymetric relief have been previously identified as a common seafloor feature in the western Galápagos by Geist et al. (2006); however, their source(s) remained enigmatic. The new EM300 data show that a number of the deep lava flows originate from small cones along the mid‐lower portion of the NW submarine rift of Fernandina, suggesting that the deep flows owe their origin, at least in part, to submarine rift zone volcanism.

Submarine Fernandina: Magmatism at the leading edge of the Galápagos hot spot

New multibeam and side‐scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos volcanoes. Submarine Fernandina has three well‐developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandina's submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal‐series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine‐gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High‐K” lavas were recovered from the southwest rift and are attributed to hybridization between normal‐series basalt and evolved‐series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galápagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep‐water lava flows, which over time construct a broad submarine platform. The deep‐water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.

Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiian island chain, extending from Maui 140 km to the ESE, has a complex morphology compared with other Hawaiian rift zones. A total of 108 rock specimens have been collected from the submarine Hana Ridge by six submersible dives. All of the rocks (76 bulk rocks analyzed) are tholeiitic basalts or picrites. Their major element compositions, together with distinctively low Zr/Nb, Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast, the lavas forming the subaerial Honomanu shield are intermediate in composition between those of Kilauea and Mauna Loa. The compositional characteristics of the lavas imply that clinopyroxene and garnet were important residual phases during partial melting. The compositions of olivine and glass ( formerly melt) inclusions imply that regardless of textural type (euhedral, subhedral–undeformed, deformed) olivine crystallized from host magmas. Using the most forsteritic olivine (Fo90 6) and partition coefficients K melt DFe Mg and D ol melt CaO , the primary magma composition is constrained to have 16 7% MgO and 8 4 wt % CaO. Modeling calculations using MELTS show that olivine first crystallized at 1380–1390 C and 0 1–0 3 GPa, under slightly hydrous conditions (0 5–1 wt % water).

Genovesa Submarine Ridge: A manifestation of plume‐ridge interaction in the northern Galápagos Islands

Despite its circular coastline and calderas, Genovesa Island, located between the central Galapagos Platform and the Galapagos Spreading Center, is crosscut by both eruptive and noneruptive fissures trending NE‐SW. The 075° bearing of the fissures parallels that of Genovesa Ridge, a 55 km long volcanic rift zone that is the most prominent submarine rift in the Galapagos and constitutes the majority of the volume of the Genovesa magmatic complex. Genovesa Ridge was the focus of detailed multibeam and side‐scan sonar surveys during the Revelle/Drift04 cruise in 2001. The ridge consists of three left stepping en echelon segments; the abundances of lava flows, volcanic terraces, and eruptive cones are all consistent with constructive volcanic processes. The nonlinear arrangement of eruptive vents and the ridge's en echelon structure indicate that it did not form over a single dike. Major and trace element compositions of Genovesa Ridge glasses are modeled by fractional crystallization along the same liquid line of descent as the island lavas, but some of the glasses exhibit higher Mg # than material sampled from the island. Most of the submarine and the subaerial lavas have accumulated plagioclase. Incompatible trace element abundances of dredged Genovesa Ridge rocks are lower than the island's lavas, but ratios of the elements are similar in the two settings, which suggests that the island and ridge lavas are derived from nearly identical mantle sources. Glass inclusions in plagioclase phenocrysts from the ridge are compositionally diverse, with both higher and lower MgO than the matrix glass, indicative of homogenization at shallow levels. The structural and geochemical observations are best reconciled if Genovesa Ridge did not form in response to injection of magma laterally from a hot spot‐supplied central volcano, like Kilauea's Puna Ridge. Instead, Genovesa Ridge and its western extension are the result of passive upwelling directed by far‐field tectonic stresses that are generated by tension across the 91°W transform. The proximity of the plume causes magmatism in the extensional zones where it would not ordinarily occur.

Volatiles in glasses from Mauna Loa Volcano, Hawai'i: implications for magma degassing and contamination, and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H2O and CO2 by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115–1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO2 but variable H2O/K2O and S/K2O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Lō'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Lō'ihi magmas have comparable, but lower H2O than those from Kilauea. Thus, Kilauea's source may be more H2O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.

Formation of submarine flat-topped volcanic cones in Hawai'i

High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea), Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep (∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m. The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine flat-topped cones but formed under different cooling conditions.

Sonar Studies of Submarine Mass Wasting and Volcanic structures off Savaii Island, Samoa

—Sidescan sonar observations show that mass wasting plays an important role in the geologic development of the Savaii Island edifice. Observations on the south and west flanks indicate that debris movement on the submarine slopes between rift zones is characterized by large sheets of unchannelized debris. Farther downslope these sheets have slumped into folded although still relatively coherent slump sheets. Closer to the rift zones, more chaotic slumps are found. The presence of large detached landslide blocks, without obvious upslope headwall scarps, suggests that earlier slumps are covered by subsequent veneers of debris moving downslope.¶In contrast, on Stearns Bank west of the island of Savaii most of the features are of constructional origin, formed during the building of this volcanic edifice of unknown age. Two prominentsubmarine platforms are evident, the shallower one with a summit cone. Sea cliffs and subdued terraces record platforms cut by sea-level oscillations late in the history of the volcanic edifice. Fractures and fissures are present on the bank, however there is little evidence of landslides in this area. The absence of landslides may reflect differing ages of the bank and the island or the edifice could have remained submarine during its construction with few or no subaerially derived ashes and clays present to facilitate mass wasting.¶We conclude that mass wasting is an important influence on the evolution of the Savaii volcanic edifice. It appears that sediment and debris cover most of the slope outside the submarine rift zones. The sonar images indicate that mass wasting is a common process in the submarine environment. Unlike the giant landslides documented by GLORIA imagery around the Hawaiian Islands, the southern margin of Samoa is characterized by numerous small slumps and slides. Although we have little information at present regarding the recurrence interval for submarine landslides, their ubiquitous presence in these sidescan sonar records indicates that they are an important component of the geologic record of the Samoan Islands.

Hydrothermal mineralization along submarine rift zones, Hawaii

This is the first article to describe mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and bimessite. Thin Fe‐Mn crusts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of δ‐MnO2. The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material...

Major element, volatile, and stable isotope geochemistry of Hawaiian submarine tholeiitic glasses

Tholeiitic glasses were dredged from the submarine rift zones of the five volcanoes comprising the island of Hawaii and Loihi Seamount. The major element composition of the glasses follows a systematic trend that is related to the stage of evolution of the volcano. Glasses from Loihi Seamount (the youngest Hawaiian volcano) are enriched in Fe, Ca, Ti, Na, and K and depleted in Si and Al relative to the glasses from the other, older volcanoes. Kilauea is intermediate in age and its glasses are intermediate in composition between those from Loihi and Mauna Loa, the largest and oldest of the active Hawaiian tholeiitic volcanoes. The volatile contents (H20, CO2, S, F, Cl) of the glasses from these volcanoes follow the same trend (highest in Loihi; lowest in Mauna Loa). Glasses from Hualalai Volcano are similar in composition to those from Mauna Loa; those from Kohala Volcano are similar to Kilauea; Mauna Kea glasses range from Mauna Loa‐like to Kilauea‐like. The observed systematic variation in composition of Hawaiian tholeiites may be related to the progressive melting and depletion of the source of these volcanoes during their growth. Oxygen and hydrogen isotope analyses were made on many of the glasses from each volcano. The δ18O values of Hawaiian tholeiites are distinctly lower than those of mid‐ocean ridge basalt (MORB) (averages: 5.1 versus 5.7). These low values are probably a distinct feature of hot spot lavas. The δD values for these glasses (−88 to −61) are typical of mantle and MORB values. Thus the H2O in the Hawaiian glasses is probably of magmatic origin. Previous isotopic and trace element data indicate that the source of Hawaiian tholeiites contains two distinct source components. Based on the results of this study, the plume component in the source for Hawaiian tholeiites is characterized by moderate 87Sr/86Sr (0.7035–0.7037) and 206Pb/204Pb ratios (18.6–18.7), a low δ18O value (∼5.0), and greater contents of volatiles, Fe, Ca, Ti, Na and K relative to the MORB source.

87:6324 The geomorphic and structural development of Hawaiian submarine rift zones: Fornari, D.J., 1987. Prof. Pap. U.S. geol. Surv., 1350:125–132

87:6324 The geomorphic and structural development of Hawaiian submarine rift zones: Fornari, D.J., 1987. Prof. Pap. U.S. geol. Surv., 1350:125–132

Pantelleria Rift — crustal extension in a convergent intraplate setting

The Pantelleria Rift system is a wide zone of post-Miocene northwest-trending grabens and horsts beneath the Sicily Strait. The central grabens host volcanics of predominantly alkalic composition which are exposed on the islands of Pantelleria and Linosa. On the Maltese Islands, along the northeastern shoulder of the rift, an Oligocene-Miocene carbonate succession exposed above sea level allows structural analysis and determination of shallow crustal stresses within the otherwise largely submarine rift system. An older northeast-trending set of normal faults is probably the expression of an Oligocene-Miocene crustal extension event which produced continental rifts in western Europe and led to passive margin formation in the western Mediterranean. Younger northwest-trending grabens of the Pantelleria Rift system cut the older faults almost at right angles and define a zone of lithospheric stretching between Tunisia and Sicily. The northwest-trending grabens which subsided dramatically since the beginning of Pliocene time appear to be connected by east-trending dextral and, more rarely, north-trending sinistral transforms. Displacement along the transforms is probably in the order of a few kilometres. In-situ stress measurements carried out on the Maltese Islands show maximum horizontal compression (SH) parallel to the rift. This suggests that in general σ1 (vertical) and σ2 (horizontal and parallel to the rift) are of about the same magnitude; both exceed σ3 (Sh) which trends northeasterly. Slight intraplate convergence in a NW-SE direction seems to be more than balanced by extension in a NE-SW direction. Neotectonics of the region possibly reflects an asthenospheric flow pattern which became established during the Messinian salinity crisis. The mechanism of recent intraplate deformation of the Pelagian shelf has relevance for the understanding of more anciently subsided platforms of the Apulian Plate.