Volcanic Rift Research Articles

How do volcanic rift zones relate to flank instability? Evidence from collapsing rifts at Etna

Volcanic rift zones, characterized by repeated dike emplacements, are expected to delimit the upper portion of unstable flanks at basaltic edifices. We use nearly two decades of InSAR observations excluding wintertime acquisitions, to analyze the relationships between rift zones, dike emplacement and flank instability at Etna. The results highlight a general eastward shift of the volcano summit, including the northeast and south rifts. This steady‐state eastward movement (1–2 cm/yr) is interrupted or even reversed during transient dike injections. Detailed analysis of the northeast rift shows that only during phases of dike injection, as in 2002, does the rift transiently becomes the upper border of the unstable flank. The flank's steady‐state eastward movement is inferred to result from the interplay between magmatic activity, asymmetric topographic unbuttressing, and east‐dipping detachment geometry at its base. This study documents the first evidence of steady‐state volcano rift instability interrupted by transient dike injection at basaltic edifices.

Crustal versus source processes recorded in dykes from the Northeast volcanic rift zone of Tenerife, Canary Islands

The Miocene–Pliocene Northeast Rift Zone (NERZ) on Tenerife is a well exposed example of a feeder system to a major ocean island volcanic rift. We present elemental and O–Sr–Nd–Pb isotope data for dykes of the NERZ with the aim of unravelling the petrological evolution of the rift and ultimately defining the mantle source contributions. Fractional crystallisation is found to be the principal control on major and trace element variability in the dykes. Differing degrees of low temperature alteration and assimilation of hydrothermally altered island edifice and pre-island siliciclastic sediment elevated the δ18O and the 87Sr/86Sr ratio of many of the dykes, but had little to no discernible effect on Nd and Pb isotopes. Once the data are screened for alteration and shallow level contamination, the underlying source variations of the NERZ essentially reflect derivation from a young High-μ (HIMU, where μ=238U/204Pb)-type mantle component mixed with depleted mid-ocean ridge-type mantle (DMM). The Pb isotope data of the NERZ rocks (206Pb/204Pb and 207Pb/204Pb range from 19.591 to 19.838 and 15.603 to 15.635, respectively) support a model of initiation and growth of the rift from the Central Shield volcano (Roque del Conde), consistent with latest geochronology results. The similar isotope signature of the NERZ to both the Miocene Central Shield and the Pliocene Las Cañadas central volcano suggests that the central part of Tenerife Island was supplied from a mantle source that remained of similar composition through the Miocene to the Pliocene. This can be explained by the presence of a discrete column of young HIMU-like plume material, ≤100km in vertical extent, occupying the melting zone beneath central Tenerife throughout this period. The most recent central magmatism on Tenerife appears to reflect greater entrainment of DMM material, perhaps due to waning of the HIMU-like “blob” with time.

The connection between iron ore formations and “mud-shrimp” colonizations around sunken wood debris and hydrothermal sediments in a Lower Cretaceous continental rift basin, Mecsek Mts., Hungary

In the Early Cretaceous, the continental rift basin of the Mecsek Mts. (Hungary), was situated on the southern edge of the European plate. The opening of the North Atlantic Ocean created a dilatational regime that expanded to the southern edge of the European plate, where several extensional basins and submarine volcanoes were formed during the Early Cretaceous epoch. Permanent seaquake activity caused high swell events during which a large amount of terrestrial wood fragments entered into submarine canyons from rivers or suspended woods which had sunk into the deep seafloor. These fragments created extended wood-fall deposits which contributed large-scale flourishing of numerous burrowing thalassinid crustaceans. Twelve different thalassinid coprolite ichnospecies can be found in the Berriasian–Hauterivian volcano-sedimentary formations.According to the seladonitic crustacean burrows which associated with framboidal pyrite containing Zoophycos and Chondrites ichnofossils (i.e. a “fodinichnia” trace fossil association), the bottom water was aerobic and the pore water was anaerobic; in the latter sulfate reduction occurred. The preservation of wood fragments around thalassinid burrows can be explained by rapid sedimentation related to turbidity currents.Due to the low temperature hydrothermal circulations of seawater, large amounts of iron were released from intrusive, pillowed basaltic sills; these sills intruded into soft, water-saturated sediments containing large amounts of thalassinid excrement. In the coprolites can be found idiomorphic mineral particles originating from the basalts, and coprolites can often be found in peperitic interpillow sediments. This indicates that the life-activity of the decapoda crustaceans in many Lower Cretaceous occurrences initially preceded the first magmatic eruptions. The paroxysm of the rift volcanism took place during the Valanginian age, when some submarine volcanoes emerged above sea level, reaching a maximum height of 300m (above sea level); from these volcanoes further terrestrial plant debris got into the basin. Hydrothermal vents, which periodically occurred around basaltic bodies until the Hauterivian, could have contributed to the creation of favourable temperature or nutritional conditions for some decapoda crustaceans – e.g the recently described new callianassid (Nihonotrypaea thermophila), which is known only from hydrothermally infuenced habitats.Around the intrusive pillow basalts, hydrothermal circulation of oxygenated seawater occured and thick seladonitic and goethitic fills formed along the cracks and cavities of pillowed basalts. When oxidized, sulfate-rich fluids passed into the crustacean coprolite-rich, reductive and anaerobic interpillow sediments, these fluids underwent an intensive sulfate reduction. This was primarily due to termophil sulfate reducers which as proved by the negative sulfur isotope values (−35.9‰ and −28.0‰ δ 34S) of sulfidic hydrothermal chimneys which contain framboidal pyrite and which were formed between the pillow basalts. The largest chimney structure reached a height of 1m, with a mass of about 150kg. The sulfide phase is characterized by Mo enrichments up to 511ppm. The fluid inclusion measurements from the calcitic precipitations of the sulfide chimneys indicate low temperature (~129°C) hydrothermal activity, and the salinity of the primary fluid inclusions proves the seawater origin of the hydrothermal fluids. In some thalassinid crustacean coprolite rich interpillow sediments and in the cracks of some hydrothermal calcite, there is the presence of black, lustrous bitumine (gilsonite) which is the distillation product of hydrothermal petroleum formed mainly by the coprolites. Hydrothermal circulations of oxygenated seawater caused subsequent oxidation of the sulfidic, interpillow sediments and chimneys; these were altered to form goethite. Due to the short-period of the hydrothermal activity among the intrusive pillowed basalts, sulfidized interpillow sediments could not be oxidized completely.The texture of the goethitic iron ore (as an interpillow sediment) is network-like and dentritic, which is very similar to the iron-oxidic and microbial textured sediments of the Juan de Fuca Ridge. The dendritic iron-oxide-hydroxide particles which were involved in this study are not hollow and exceed the size-domain characteristic for bacterial products. However, in some cases hollow- and tube-like particles having a diameter of 1.2–1.5μm can refer to the activity of the Sphaerotilus–Leptothrix iron-oxidizer bacterial group.

The Ngorongoro Volcanic Highland and its relationships to volcanic deposits at Olduvai Gorge and East African Rift volcanism

The Ngorongoro Volcanic Highland (NVH), situated adjacent and to the east of Olduvai Gorge in northern Tanzania, is the source of the immense quantities of lava, ignimbrite, air fall ash, and volcaniclastic debris that occur interbedded in the Plio-Pleistocene sedimentary deposits in the Laetoli and Olduvai areas. These volcanics have proven crucial to unraveling stratigraphic correlations, the age of these successions, the archaeological and paleontological remains, as well as the source materials from which the bulk of the stone tools were manufactured. The NVH towers some 2,000 m above the Olduvai and Laetoli landscapes, affecting local climate, run-off, and providing varying elevation – climate controlled ecosystem, habitats, and riparian corridors extending into the Olduvai and Laetoli lowlands. The NVH also plays a crucial role in addressing the genesis and history of East African Rift (EAR) magmatism in northern Tanzania.In this contribution, we provide age and petrochemical compositions of the major NVH centers: Lemagurut, basalt to benmorite, 2.4–2.2 Ma; Satiman, tephrite to phonolite, 4.6–3.5 Ma; Oldeani, basalt to trachyandesite, 1.6–1.5 Ma; Ngorongoro, basalt to rhyolite, 2.3–2.0 Ma; Olmoti, basalt to trachyte, 2.0–1.8 Ma; Embagai, nephelinite to phonolite, 1.2–0.6 Ma; and Engelosin, phonolite, 3–2.7 Ma. We then discuss how these correlate in time and composition with volcanics preserved at Olduvai Gorge. Finally, we place this into context with our current understanding as to the eruptive history of the NVH and relationship to East African Rift volcanism.

Early Paleoproterozoic rift volcanism in the eastern Fennoscandian Shield related to the breakup of the Kenorland supercontinent

The widespread early Paleoproterozoic (ca. 2.5–2.2Ga) volcanic and associated sedimentary rocks represent an episode of initial rifting of the late Archean craton in the eastern Fennoscandian Shield. These continental margin rocks cover cratonized Archean basement of the Fennoscandian Shield, in eastern-northern Finland and northwest Russia, and represent a typical Sumian-Sariolian volcanic-sedimentary succession. Physical volcanology and major element geochemistry of the subaerially erupted calc-alkaline basaltic to andesitic lavas of the Matinvaara Formation in north-eastern Finland, provide globally important characteristics for this craton-wide event. The lavas are generally massive or amygdaloidal, but flow-top breccias and variolitic lavas are also found. Laterally, different lava flows of this formation can be followed up to several hundred meters. Diagnostic pseudoforms after primary minerals and volcanic glass in the lavas are used in identifying crystallizing phases. Additionally, the primary structures of lava flows give indication of accumulation and settling processes. Specific internal mineral textures also relate to cooling history of these lavas. Flow inflation is suggested by accumulation of multiple vesicle-rich horizons. Thick massive flows may be analogues of tube pahoehoe flows produced by subaerial flow inflation. The vesicular units are, however, directly evidence for devolatilization of lava, whereas varioles, in the Matinvaara Formation, represent a form of plagioclase spherulite. Varioles consist of radiating crystals of plagioclase and pyroxene, presently identified as albite-actinolite pseudoforms. Under near equilibrium crystal growth conditions, kinetics favored euhedral, compositionally homogenous crystals. Continued undercooling facilitated the formation of compositionally zoned crystals and under greater undercooling anisotropic, skeletal and dentritic crystals of plagioclase and pyroxene rapidly developed. Tectono-magmatic evolution of the Matinvaara Formation suggests subalkaline magmatism in a intracratonic setting, which was related to the breakup of the Kenorland supercontinent ca. 2.5–2.1Ga. Geochemistry indicates that the volcanism was affected by felsic continental crustal contamination with plume-driven rifting. This volcanism likely occurred in distinct pulses with total duration of tens of millions of years.

A new interpretation of deformation rates in the Snake River Plain and adjacent basin and range regions based on GPS measurements

We evaluate horizontal Global Positioning System (GPS) velocities together with geologic, volcanic, and seismic data to interpret extension, shear, and contraction within the Snake River Plain and the Northern Basin and Range Province, U.S.A. We estimate horizontal surface velocities using GPS data collected at 385 sites from 1994 to 2009 and present an updated velocity field within the Stable North American Reference Frame (SNARF). Our results show an ENE-oriented extensional strain rate of 5.9 {+-} 0.7 x 10{sup -9} yr{sup -1} in the Centennial Tectonic belt and an E-oriented extensional strain rate of 6.2 {+-} 0.3 x 10{sup -9} yr{sup -1} in the Intermountain Seismic belt combined with the northern Great Basin. These extensional strain rates contrast with the regional north-south contraction of -2.6 {+-} 1.1 x 10{sup -9} yr{sup -1} calculated in the Snake River Plain and Owyhee-Oregon Plateau over a 125 x 650 km region. Tests that include dike-opening reveal that rapid extension by dike intrusion in volcanic rift zones does not occur in the Snake River Plain at present. This slow internal deformation in the Snake River Plain is in contrast to the rapidly-extending adjacent Basin and Range provinces and implies shear along boundaries of the Snakemore » River Plain. We estimate right-lateral shear with slip rates of 0.5-1.5 mm/yr along the northwestern boundary adjacent to the Centennial Tectonic belt and left-lateral oblique extension with slip rates of 10{sup 6} yrs, the low-strain field in the Snake River Plain and Owyhee-Oregon Plateau would extend through the Quaternary.« less

Petrogenetic evolution of the felsic and mafic volcanic suite in the Siang window of Eastern Himalaya, Northeast India

The Abor volcanics outcroping in the core of the Siang window in the Eastern Himalaya comprise voluminous mafic volcanics (47%–56% w(SiO2)), with subordinate felsic volcanics (67%–75% w(SiO2)). The felsic volcanics are dacitic to rhyolitic in composition and are typically enriched in LREE (La/SmN = 3.09–3.90) with high REE contents (256–588 ppm), moderately fractionated REE patterns (CeN/YbN = 6.54–9.52) and pronounced negative Eu anomalies (Eu/Eu* = 0.55–0.72). Wide variations in Rb/Zr, K/Rb and La/Sm ratios suggest that they were derived from magmas which were randomly contaminated with crustal material. Chemical characteristics and petrogenetic modelling indicate that the dacites were generated by ∼15% partial melting of a mafic source leaving a residue with 55% plagioclase, 14% orthoclase, 18% clinopyroxene, 5% orthopyroxene, 8% hornblende. The silica-rich rhyodacites and rhyolites were derived from a dacite magma source by a higher degree (>45%) fractional crystallization of an assemblage consisting of 70% plagioclase, 12% clinopyroxene, 7% amphibole and 11% magnetite. The associated LREE-LILE enrichment and pronounced negative anomalies for HFSE (Nb, P, and Ti) exhibited by these felsic volcanics are characteristic of continental rift volcanism, implying that they were emplaced during lithospheric extension.

Tectonostratigraphic analysis of the Late Triassic-Early Jurassic syn-rift sequence of the Neuquen Basin in the Sanico depocentre, Neuquen Province, Argentina

The initial syn-rift infill (Upper Triassic-Lower Jurassic) of the Neuquen Basin involved a complex stacking of lava, pyroclastic and sedimentary units, in which volcanic rocks compose up to 72% of the infill. The high-quality outcrops of the southern Sanico depocentre, which was uplifted and exposed during the Andean orogeny, were studied in order to determine the lithofacies arrangements, the tectonostratigraphic framework and the main controls over the syn-rift succession. The tectonostratigraphic study made it possible to determine three sections characterized by the predominance of effusive, pyroclastic or sedimentary deposits. These sections indicate three different evolutionary stages of the volcanic rifting. The onset of this rift stage was dominated by effusive rocks with minor ignimbrite and alluvial volcaniclastic units, which indicates the occurrence of composite volcanoes at the boundaries of the depocentre. The mid-rift stage was characterized by the development of fault-controlled, large ignimbrite deposits intebedded with volcaniclastic alluvial deposits and lava units, which suggests a volcanotectonic depression related to asymmetrical and incremental subsidence. The end of the volcanic rifting event was marked by a decrease in the volume of the volcanic rocks and the occurrence of low-relief effusive aggradational volcanic edifices and lava fields in close relation with alluvial, deltaic and lacustrine units. The analysis of the synrift sequence in the Sanico depocentre showed the complex relationships between tectonic and volcanic controls which occurred in different magnitudes and times during the rifting evolution.

The impact of conjugate margins analysis on play fairway evaluation – an analysis of the hydrocarbon potential of Nova Scotia

An analysis of the rifting history of the Central Atlantic focused on Nova Scotia and Morocco reveals new plays potentially charged from an Early Jurassic source system. The revised rift model is based on newly acquired seismic refraction data, reprocessing of existing refraction and multi-channel seismic data, and integration of potential fields data from the conjugate margins. We infer from the geophysical evidence that there was volcanic rifting from south-west Nova Scotia to the Newfoundland fracture zone during the Early Jurassic, with a restricted, shallow marine environment for sediment deposition. We have integrated plate tectonics, geochemistry, and sequence stratigraphic interpretation with the aim of identifying new plays. Analysis and integration of geochemical evidence has led to a new paradigm for the exploration potential offshore Nova Scotia. In particular, there is now good evidence for deposition of a source rock in restricted marine conditions during the Early Jurassic. This source rock is capable of providing charge to an oil play in the south-western half of the Scotian margin, which is a new concept for this area.

Late Cretaceous–Palaeocene continental rifting in the High Arctic: U–Pb geochronology of the Kap Washington Group volcanic sequence, North Greenland

Abstract: The Kap Washington Group volcanic sequence is exposed on the north coast of Greenland. The sequence was erupted in a continental rift setting during the opening of the Arctic Ocean and provides important geological constraints on the timing of this event. In this study we present the first isotope dilution thermal ionization mass spectrometry U–Pb zircon ages from 10 samples of trachytic to rhyolitic composition. Samples from Lockwood Island ( n = 4) gave concordant ages between 67.2 ± 0.5 and 61.0 ± 0.3 Ma (2σ). Lavas from Kap Kane ( n = 5) yielded concordant ages between 70.5 ± 0.3 and 69.7 ± 0.2 Ma. An ignimbrite from the Kap Washington peninsula yielded a concordant age of 68.5 ± 0.3 Ma. The Kap Washington Group sequence and similar occurrences on Ellesmere Island record a prolonged period of continental rift volcanism lasting from 92 to 58 Ma. We propose that this volcanism occurred along the margins of a nascent Eurasia Basin undergoing east–west extension linked to rifting in the Labrador Sea–Baffin Bay system.

Deep 3D structure of the Sydney Basin using gravity modelling

A detailed deep 3D geological model is an important basis for many types of exploration and resource modelling. Renewed interest in the structure of the Sydney Basin, driven primarily by sequestration studies, geothermal studies and coal seam gas exploration, has highlighted the need for a model of deep basin geology, structure and thermal state. Here, we combine gravity modelling, seismic reflection surveys, borehole drilling results and other relevant information to develop a deep 3D geological model of the Sydney Basin. The structure of the Sydney Basin is characteristic of a typical intracontinental rift basin, with a deep north–south orientated channel in the Lachlan Fold Belt basement, filled with up to 4 km of rift volcanics, and overlain with Permo-Triassic sediments up to 4 km thick. The deep regional architecture presented in this study will form the framework for more detailed geological, hydrological and geothermal models.

The Kverkfjöll fissure swarm and the eastern boundary of the Northern Volcanic Rift Zone, Iceland

Rift zones at the divergent plate boundary in Iceland consist of central volcanoes with swarms of fractures and fissures extending away from them. Fissure swarms can display different characteristics, in accordance with their locations within the ∼50-km-wide rift zones. To better discern the characteristics of fissure swarms, we mapped tectonic fractures and volcanic fissures within the Kverkfjoll volcanic system, which is located in the easternmost part of the Northern Volcanic Rift Zone (NVZ). To do this, we used aerial photographs and satellite images. We find that rifting structures such as tectonic fractures, Holocene volcanic fissures, and hyaloclastite ridges are unevenly distributed in the easternmost part of the NVZ. The Kverkfjoll fissure swarm extends 60 km north of the Kverkfjoll central volcano. Holocene volcanic fissures are only found within 20 km from the volcano. The Fjallgarðar area, extending north of the Kverkfjoll fissure swarm, is characterized by narrow hyaloclastite ridges indicating subglacial volcanism. We suggest that the lack of fractures and Holocene volcanic fissures there indicates decreasing activity towards the north in the easternmost part of the NVZ, due to increasing distance from the long-term spreading axis. We argue that arcuate hyaloclastite ridges at the eastern boundary of the Northern Volcanic Rift Zone are mainly formed during deglaciations, when three conditions may occur; firstly, eruption rate increases due to decompression of the mantle. Secondly, the high tensile stresses accumulated during glaciations due to lack of magma supply may be relieved as magma supply increases during deglaciations. Thirdly, faulting may occur during unloading due to differential movements between the thinner and younger Northern Volcanic Rift Zone crust and the thicker and older crust to the east of it.

Dynamics of dyke intrusion in the mid-crust of Iceland

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke moving upward from a sill at 18 km depth in the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth's crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing before it erupts of molten rock generated within the underlying mantle. Here we show seismicity accompanying melt intrusion from 17.5 to 13.5 km depth along a dyke dipping at 50° in the mid-crust of the Icelandic rift zone. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location, but breaking along fault planes with the same orientations. We suggest several possible reasons for the flipping fault mechanisms: the breakage of solidified plugs of basalt within the dyke itself as more melt intrudes; intrusion along sub-parallel fractures or dykelet fingers into the local stress field created near the tip of a propagating dyke; or movement on small jogs or offsets between adjacent en echelon dykes. Although the faulting is caused ultimately by melt movement, there is no resolvable volumetric component in the moment tensor solutions. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. Melt injection occurs in bursts propagating at 2–3 m/min along channels c. 0.2 m thick, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours.

Sands of West Gondwana: An archive of secular magmatism and plate interactions — A case study from the Cambro-Ordovician section of the Tassili Ouan Ahaggar (Algerian Sahara) using U–Pb–LA-ICP-MS detrital zircon ages

Enormous masses of highly mature quartz sands were deposited in Western Gondwana during the Cambrian–Ordovician time, and provide a wide range of information concerning magmatic events through time, provenance, paleoclimate, and basin history. We present a provenance study based on 630 U–Pb (LA-ICP-MS) ages of detrital zircon from the latest Cambrian to Ordovician siliciclastic rocks of the Tassili Ouan Ahaggar basin situated in the Algerian Sahara. Most authors suggest local sources only for the sandstones. Instead, we demonstrate that the detritus is derived from different cratons and terranes which contributed to the deposition of a Cambrian–Ordovician overstep sequence covering western and northern Africa. Most zircon ages (61.0%) fall in the range of ~ 540 to 740 Ma and are interpreted to have been derived from Pan-African orogenic belts such as the Trans-Saharan Belt of NW Africa and previously from the Brazila belt of South America. Other potential sources for this zircon population are terranes of Cadomian affinity situated marginal to West Africa. The second-largest zircon population (20.2%) is 2.0 to 2.2 Ga, and is attributed to sources in the West African craton, such as the Birimian basement and the Eburnean orogenic belt, with possible partial input from the Amazonian craton. A zircon population of 7.1% yields Mesoproterozoic and early Paleoproterozoic ages in the range of ~ 1.3 to ~ 1.8 Ga and was probably derived from source rocks outside of the West African basement, the Tuareg shield and other adjoining areas. The Amazonian craton is a potential source region. A population of 6.7% of all zircon ages scatter from ~ 750 Ma to ~ 980 Ma and may reflect input from latest stages of the formation of Rodinia and its subsequent dispersal. A smaller population (3.2%) of zircon ages lie between ~ 2.3 and 2.65 Ga, and may be derived from late Paleoproterozoic to early Archaean rocks from the West African craton and possibly from Amazonia. Less than 1% of all zircons are Meso- to Paleoarchaean ones and provide evidence for the input of very old cratonic basement, most likely from cratonic inliers of the West African craton (Leonian, Liberian). Because of the potential input of detrital zircon from the Amazonian craton, which is reflected in the Mesoproterozoic and late Paleoproterozoic grains, we speculate that some of the Paleoproterozoic to Neoarchean (2.0 Ga to 2.6 Ga) zircons were also derived from Amazonia. Due to the total lack of 1.0–1.2 Ga old zircon, our data set excludes all crustal domains situated in the Arabian-Nubian shield and the East African belt, as well as the Sunsás belt of Amazonia (“Sunsás-Grenvillian”) as potential sediment sources. Sedimentation in the Tassili Ouan Ahaggar basin started in uppermost Cambrian to Ordovician time due to the opening of the Rheic Ocean. This event led to subsidence related to the rift and drift of Avalonia and related terranes from the northwestern Gondwanan margin. The basal Early Tassili quartzite has detrital zircon populations that suggest a local provenance either from West African or from a related terrane in the Tuareg shield. A dramatic change occurs in the deltaic to shallow marine strata of the Lower Ordovician Ajjers Formation and in the overlying marine sandstones of the Middle Ordovician d'In Azaoua Formation. Our data for both formations indicate the Pan-African orogen, and very likely Cadomian terranes as the main source for the detritus. During this time, the region was affected by rift tectonics due to the opening of the Rheic Ocean and therefore amenable to erosion at rift shoulders and escarpments. Our data also indicate that glacial erosion in Upper Ordovician (Hirnantian) time must have affected larger areas of old cratonic surfaces as the populations of Paleoproterozoic to Archaean zircons are significantly higher than in other age clusters. Large parts of highly mature sands of the Cambro-Ordovician section in the Tassili Ouan Ahaggar basin were derived from a peneplain in the interior of Gondwana, that formed during Cambrian times. This peneplain was formed under warm-humid climate on a vegetation-free land surface and in an extreme corrosive environment that was influenced by high atmospheric pCO 2 caused by Pan-African and Avalonian–Cadomian volcanism, volcanic activity related to the opening of the Iapetus, and Late Cambrian–Early Ordovician rift volcanism as well.

The continent-ocean transition at the southeastern margin of the South China Sea

The South China Sea formed by magma-poor, or intermediate volcanic rifting in the Paleogene. We investigate the structure of the continent-ocean transition (COT) at its southern margin, off NW Palawan between the continental blocks of Reed Bank and the islands of Palawan and Calamian. Several surveys, recorded by the BGR from 1979 to 2008, established a comprehensive database of regional seismic lines, accompanied with magnetic and gravity profiles. We interpret two major rifted basins, extending in the NE direction across the shelf and slope, separated by a structural high of non volcanic origin. The continent-ocean transition is interpreted at the seaward limit of the continental crust, when magnetic spreading anomalies terminate some 80–100 km farther north. The area in between displays extensive volcanism - as manifest by extrusions that occasionally reach and cut the seafloor, by dykes, and by presumed basaltic lava flows - occurring after break-up. The COT is highly variable along the NW Palawan slope: One type shows a distinct outer ridge at the COT with a steep modern seafloor relief. The other type is characterised by rotated fault blocks, bounded by listric normal faults ramping down to a common detachment surface. Half-grabens developed above a strongly eroded pre-rift basement. The seafloor relief is smooth across this other type of COT. We suggest the pre-rift lithospheric configuration had major influence on the formation of the COT, besides transfer zones. Volcanic domains, confined to the north of competent crustal blocks correlate with the style of the COT. Gravity modelling revealed an extremely thinned crust across the shelf. We propose a depth-dependent extension model with crust being decoupled from mantle lithosphere, explaining the discrepancy of subsidence observed across the South China Sea region.

Chapter 64 Neoproterozoic glacial deposits of Tasmania

Abstract In Tasmania, Neoproterozoic glaciogenic deposits were laid down in one or more epicratonic basins, probably situated at the eastern margin of the Australian–Antarctic craton. Rifting and volcanism took place in the late Cryogenian to early Ediacaran. On King Island, north of Tasmania, the Cottons Breccia consists of 50–200 m of diamictite, conglomerate and sandstone. Limestone and dolostone clasts are abundant in the diamictite, although carbonate is unknown in the underlying successions. The Cottons Breccia is overlain by 10 m of laminated dolostone and limestone with a negative, upward-decreasing δ 13 C profile. Rift volcanics and shallow intrusives higher in the sequence are dated at c. 575 Ma. In NW Tasmania, two diamictite units are found in the Togari Group. The Julius River Member, 200 m thick, contains dominantly dolostone clasts and overlies a shallow-marine dolostone unit with vase-shaped microfossils and C-isotopes consistent with a mid-Cryogenian age. Some clasts in the Julius River Member contain a stromatolite ( Baicalia cf. B. burra ) very similar to a form that is abundant in the middle part of the Burra Group, Adelaide rift basin. The Julius River Member is immediately overlain by black shale and impure carbonate dated by Re–Os at 641±5 Ma. The younger diamictite in the Togari Group is the Croles Hill Diamictite, 70 m thick, with predominantly volcanic clasts, underlain by a shale and mafic-volcaniclastic succession and overlain by thin mudstone followed by thick rift tholeiites. At one locality this diamictite is underlain by a rhyodacite flow dated at 582±4 Ma. In southern Tasmania, diamictites are found in the Wedge River Beds and in the Cotcase Creek Formation (Fm.) (Weld River Group). Laminated siltstone with dropstones is associated with the diamictites in the Cotcase Creek Fm. The southern Tasmanian deposits are poorly constrained in age. Chronometric and other evidence suggests correlation of the Julius River Member, Cottons Breccia and Croles Hill Diamictite with the Sturt, Elatina and Gaskiers glacial phases, respectively. However, a glacial origin for the Julius River Member and Croles Hill Diamictite remains uncertain.

The Kerlingar fault, Northeast Iceland: A Holocene normal fault east of the divergent plate boundary

The Kerlingar fault is a ∼30 km long fault located at the boundary between the Northern Volcanic Rift Zone, and the Tertiary Eastern Fjords Block in Iceland. The fault has a throw of 2–9 m down to the east and is most likely a normal fault. It probably ruptured in several earthquakes over extended time, but assuming it ruptured in one event it would have a magnitude of about $M_w$ = 6.7. The Kerlingar fault forms a sharp offset in a flat moraine, showing that the fault was active in the Holocene. Several characteristics of the fault are different from that of the presently active fissure swarms of the NVZ. It is unusually long, straight and continuous, and it is parallel with the boundary between the NVZ and EFB not perpendicular to the plate spreading. We consider three possible explanations for the existence of the fault. It may be formed in a rifting event, by stress transfer in relation to the Húsavík transform, or by a stress field caused by rapid crustal unloading during the last deglaciation. We favour the third explanation but note that the other two cannot be excluded. Differential movements at the NVZ–EFB boundary during deglaciations can occur as the two crustal blocks have different density, Young’s modulus, thickness, and subcrustal viscosity. They therefore respond differently to the unloading. This may explain why the fault is parallel with the NVZ–EFB boundary and not with the Holocene fissure swarms in the NVZ. Other faults at the NVZ–EFB boundary may be formed in a similar manner. Magma may have intruded some of them to form the distinct arcuate pattern of hyaloclastite ridges at the boundary between the NVZ and the EFB. Future model calculations could constrain better the effects this process has on the formation of faults.

Rift zone volcanism and associated cinder cone field in Utopia Planitia, Mars

We analyzed a small cone field in SW Utopia Planitia that shows striking similarities to volcanic rift zones on Earth. This is of particular interest as the study area lies off any of the volcanic centers of Mars in the northern hemisphere lowlands and is surrounded by material of the Amazonian aged Vastitas Borealis Formation (VBF). We believe that the study area could be the first direct evidence for volcanic activity in Utopia Planitia prior to the resurfacing by VBF material. The most striking characteristics that we observed are (1) a set of broad eruptive fissures showing signs of ongoing extension during cone formation, (2) parallel dike swarms, (3) magmatic intrusions, and (4) a number of lava flows that have erupted from the fissures and pitted cones that surround a larger cone in the center of the study area and that are often associated with, or aligned along, the fissures and dikes. Cratering model ages indicate that the study area is of Late Noachian to Early Hesperian age and is thus distinctly older than the surrounding VBF, falling into the time frame of the main phases of activity in the volcanic centers of Syrtis Major, Elysium Planitia, and the southern highlands.

Three‐dimensional structure of the submarine flanks of La Réunion inferred from geophysical data

La Réunion (Indian Ocean) constitutes a huge volcanic oceanic system of which most of the volume is submerged. We present a study of its submarine part based on the interpretation of magnetic and gravity data compiled from old and recent surveys. A model of the submarine internal structure is derived from 3‐D and 2‐D models using constraints from previous geological and geophysical studies. Two large‐scale, previously unknown, buried volcanic construction zones are discovered in continuation of the island's construction. To the east, the Alizés submarine zone is interpreted as the remnants of Les Alizés volcano eastward flank whose center is marked by a large hypovolcanic intrusion complex. To the southwest, the Etang Salé submarine zone is interpreted as an extension of Piton des Neiges, probably fed by a volcanic rift zone over a large extent. They were predominantly built during the Matuyama period and thus probably belong to early volcanism. A correlation exists between their top and seismic horizons recognized in previous studies and interpreted as the base of the volcanic edifice. Their morphology suggested a lithospheric bulging beneath La Réunion, not required to explain our data, since the seismic interfaces match the top of our volcanic constructions. The coastal shelf coincides with a negative Bouguer anomaly belt, often associated with magnetic anomalies, suggesting a shelf built by hyaloclastites. A detailed analysis of the offshore continuation of La Montagne Massif to the north confirms this hypothesis. The gravity analysis confirms that the bathymetric bulges, forming the northern, eastern, southern, and western submarine flanks, are predominantly built by debris avalanche deposits at the surface.

Spatial distribution of dissolved constituents in Icelandic river waters

Summary In this study we map the spatial distribution of selected dissolved constituents in Icelandic river waters using GIS methods to study and interpret the connection between river chemistry, bedrock, hydrology, vegetation and aquatic ecology. Five parameters were selected: alkalinity, SiO2, Mo, F and the dissolved inorganic nitrogen and dissolved inorganic phosphorus mole ratio (DIN/DIP). The highest concentrations were found in rivers draining young rocks within the volcanic rift zone and especially those draining active central volcanoes. However, several catchments on the margins of the rift zone also had high values for these parameters, due to geothermal influence or wetlands within their catchment area. The DIN/DIP mole ratio was higher than 16 in rivers draining old rocks, but lowest in rivers within the volcanic rift zone. Thus primary production in the rivers is limited by fixed dissolved nitrogen within the rift zone, but dissolved phosphorus in the old Tertiary catchments. Nitrogen fixation within the rift zone can be enhanced by high dissolved molybdenum concentrations in the vicinity of volcanoes. The river catchments in this study were subdivided into several hydrological categories. Importantly, the variation in the hydrology of the catchments cannot alone explain the variation in dissolved constituents. The presence or absence of central volcanoes, young reactive rocks, geothermal systems and wetlands is important for the chemistry of the river waters. We used too many categories within several of the river catchments to be able to determine a statistically significant connection between the chemistry of the river waters and the hydrological categories. More data are needed from rivers draining one single hydrological category. The spatial dissolved constituent distribution clearly revealed the difference between the two extremes, the young rocks of the volcanic rift zone and the old Tertiary terrain.