Magnetometer Array Study in North-Northeast Brazil: Conductivity Image Building and Functional Induction Modes

—Magnetovariational fields recorded by an array of magnetometers in the equatorial region of north-northeast Brazil are analyzed to infer the configuration of internal induced currents in and around the extensive intracratonic Parnaíba basin. Only nighttime magnetovariational fields were used because of the prevailing uniform source field conditions. For periods exceeding 40 min. the vertical fields at all inland sites are dominated by the effects of electric currents originating in the northeast, in the deep Atlantic Ocean. Below this period, although best developed in the 12–15 min. period range, the anomalous signatures are principally controlled by two distinct continental current paths. The first is associated with a N60°E trending graben-like structure in the southeastern part of the basin (named the Parnaíba Basin Conductivity Anomaly—PBCA) and the second appears as a subsurface sedimentary channel, from the NW corner of the array to the central part of the basin. This is named the LINK anomaly, as it connects the northwestern Marajó basin with the Parnaíba basin. While the PBCA is shown to highlight the importance of basement tectonics in the geological evolution of the Parnaíba basin, the LINK anomaly provides strong geophysical evidence of the direction of the sea intrusion into the region of the basin and possibly indicates the connectivity of the Parnaíba basin to the adjoining Amazon basin through the Marajó basin. Frequency and polarization dependence suggest that the induction response of individual structures is not determined by the local conductivity alone but also by their interconnectivity as well as by their linkage to the continental shelf and deep oceanic region.     

Review of Carboniferous-Permian volcanicity in Scotland

The main Carboniferous outcrop in Scotland is in the Midland Valley rift, though rocks of that age also occur farther south along the English Border. The cyclic sedimentary sequence comprises up to 3,000 m. of Dinantian (mainly Viséan) and 2,000 m. of Silesian beds which at one locality or another include lavas or tuffs almost at every level. The distribution of these volcanic rocks is known in exceptional detail because of mining and exploratory borings in the Viséan oil shale field and in the various Namurian and Westphalian coalfields.The lavas and tuffs form part of an alkaline (sodic) magma series. Their outpourings were greatest in volume during the Dinantian, with the formation of the extensive lava piles which now form the Clyde Plateau (maximum thickness c. 900 m.) in the west, the Garleton Hills (c. 600 m.) and Burntisland Anticline (c. 400 m.) in the east, and the Kelso Traps of the Tweed Basin (c. 120 m.), the Birrenswark Lavas of Dumfriesshire (c. 90 m.), the Kershopefoot Basalt of Liddesdale (c. 60 m.) and the Glencartholm Volcanic Beds (mostly tuffs) of Eskdale (c. 180 m.) to the south. Most of the lavas are varieties of olivine-basalt, but subordinate trachybasalts, trachytes and rhyolites are included in the upper parts of the Clyde Plateau and Garleton Hills successions. The areas covered by individual lava fields are difficult to assess because they varied from time to time and occasionally overlapped.By the end of the Dinantian this form of activity had ceased everywhere except in West Lothian, where it continued into early Namurian (E₁). Thereafter volcanicity continued periodically from scattered centres and gave rise to relatively short-lived ash-cones. They formed in Ayrshire (E₁ age) but were most abundant in Fife, occurring there almost throughout the Namurian succession. The highly explosive nature of this phase of volcanicity is apparent from the rarity of associated lavas (though minor basaltic intrusions occur in the necks) and from the presence within the surrounding sediments of thin layers of tuff representing fine ash carried for distances of up to 32 km from source. These layers, some of which are kaolinized and are a variety of tonstein, are of local use in correlation.The explosive phase is now known to have continued throughout Westphalian A and possibly into Westphalian B in Fife, but in Ayrshire volcanicity was more intermittent and is represented by two outpourings of basalt lavas, one late Namurian to early Westphalian in age and up to 150 m. thick, the other Stephanian in age and 90 to 237 m. thick. The Stephanian lavas and associated necks in Ayrshire, like some of the necks exposed on the classic Fife coastal section on the Firth of Forth, were until recently believed to be Permian, but there is now no positive evidence of Permian eruption in Scotland.Two groups of intrusions cut the Carboniferous rocks. The most extensive is of alkaline dolerites which are co-magmatic with the basaltic lavas and tuffs, and are of similar geographic distribution. They form sills up to 120 m. thick and are probably of various ages up to Stephanian. It is possible that some of them were high-level reservoirs from which diatremes issued. The other group is of quartz-dolerite or tholeiite which forms a series of east-west dykes and a sill-complex extending over an area of about 1500 km² and having a maximum thickness of 127 m. Their age is late Carboniferous.The distribution of the Carboniferous volcanic rocks is not apparently related to the rift-valley, but is clearly linked in places to the earlier Caledonoid structures which determined the pattern of sedimentation. The lavas and, to a lesser extent, the tuffs, influenced the sedimentary processes by partially enclosing basins, by causing local anomalies in cyclic sequences and by providing sources of atypical sediment.

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Zusammenfassung Im Midland Valley (Schottland) war der permokarbonische Alkali-(Na)-Vulkanismus besonders im Dinantium aktiv, wobei bis 900 m mächtige Laven (Olivinbasalte, untergeordnet Trachytandesite, Trachyte und Rhyolite) gefördert wurden. Gegen Ende des Dinantium hörte diese Art der Aktivität fast ganz auf, dafür setzte eine explosive, periodische Tätigkeit ein, die zu kurzlebigen Aschenkegeln führte. Ein Beweis für das früher angenommene permische Alter dieser Necks läßt sich nicht erbringen.Zwei Gruppen von Intrusiva sind in die karbonischen Sedimente eingedrungen. Die größere der beiden wird durch Alkali-Dolerite (Sills) gebildet, die co-magmatisch zu den Laven und Tuffen sind. Die andere besteht aus Quarz-Doleriten und Tholeyiten (vorwiegend Dykes), die spätkarbonisch bis frühpermischen Alters sind.Die Verbreitung der karbonischen Vulkanite ist nicht an das Midland Valley, sondern an frühere caledonische Strukturen gebunden und für den Sedimentationsprozeß von Bedeutung.

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Résumé Le principal affleurement carbonifère en Ecosse se trouve dans le graben du Midland Valley, quoiqu'on rencontre les roches de cet âge le long de la frontière anglaise. La succession du cycle sédimentaire comprend jusqu'à 3,000 m. de roches de l'âge Dinantien (principalement Viséen) et à 2,000 m. des couches Silésiennes qui dans une localité quelconque comprennent des laves ou des tufs à presque tous les niveaux. La distribution de ces roches volcaniques est bien connu dans tous les détails à cause des mines et des forages.Les laves et les tufs font partie d'une série alkaline (sodique) magma. Leurs éruptions atteignirent leur maximum en volume pendant Ie Dinantien, avec la formation du Plateau de Clyde (épaisseur maximum 900 m. env.) à l'ouest, les Collines de Garleton (600 m. env.) et l'anticlinal de Burntisland (400 m. env.) a l'est, et les Kelso Traps du bassin du Tweed (120 m. env.), les Birrenswark Lavas de Dumfriesshire (90 m. env.), le Kershopefoot Basait de Liddesdale (60 m. env.) et les Glencartholm Volcanic Beds (des tufs pour la plupart) d'Eskdale (180 m. env.) au sud. La plupart des laves sont des variétés de basalte-olivine, mais des quantié peu considérables de trachybasalte, trachytes et rhyolites sont comprises dans les parties supérieures dans les successions du Plateau de Clyde et des Collines de Garleton.A la fin du Dinantien, cette forme d'activité avait cessé partout excepté dans l'Ouest Lothian, où elle continuait jusqu'au commencement du Namurien. Après cela le volcanisme continua périodiquement des centres dispersés, et donna naissance à des cônes de cendre de relativement petite durée. Ceux-ci se formèrent en Ayrshire (age E₁), mais on les trouvait beaucoup plus souvent en Fife, et on les y rencontrait presque pendant toute la succession namurienne. La nature explosive de cette phase de volcanisme est apparente par la rareté des laves et par la présence dans le sédiment qui se trouve dans les couches étroites de tuf, résultat du cendre fin porté jusqu'à 32 km. de sa source. Ces couches-ci dont quelques-unes furent transformées en kaolin, sont une variété de tonstein qui s'emploie beaucoup en corrélation. La distribution des quelques cheminées volcaniques coïncide aux structures nord—est d'un âge précarbonifère.La phase explosive est maintenant connue d'avoir continué à travers toute la Westphalie «Äs» et possiblement d'avoir pénétré dans la Westphalie «B» en Fife, mais en Ayrshire le volcanisme était plus intermittent et est représenté par deux éruptions de lave basaltique, l'une pendant la fin du Namurien jusqu'au début du Westphalien et d'une épaisseur de 150 m., l'autre du Stéphanien et d'une épaisseur de 90 à 237 m. Les laves du Stéphanien et les cheminées associées étaient classées auparavant comme Permien, mais maintenant il n'y a aucune évidence positive d'éruption permianique en Ecosse.Deux groupes d'intrusions scindèrent les roches carbonifères. Le groupe le plus étendu est fait de dolérites alkalines qui sont co-magmatiques avec les laves basaltiques et les tufs, et sont de pareille distribution géographique. Elles forment des silles d'une épaisseur jusqu; à 120 m. et elles sont probablement d'âges variés jusqu'au Stéphanien. Il est possible que quelques-une d'entre elles furent des réservoirs de haut-niveau desquels des diatrèmes sortirent. L'autre groupe est fait de quartz-dolérite ou tholéiite qui forme une série de dikes est—ouest et une sille-complexe couvrant une étendue de 1,500 km² et ayant une épaisseur maximale de 127 km. Leur áge est de la fin du Carbonifère ou du début du Permien.

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Published by permission of the Director, Institute of Geological Sciences.     

Lithium isotope systematics of lavas from the Cook–Austral Islands: Constraints on the origin of HIMU mantle

Radiogenic isotope variations in lavas from the Cook–Austral volcanic chain have delineated three distinct mantle sources: a HIMU component, a depleted component (DM), and an enriched component (EM). To better constrain the mantle sources for South Pacific hot spot volcanism, we determined lithium isotopic compositions of lavas from Raivavae, Rapa, Mangaia and Tubuai of the volcanic chain. The study includes whole rock and mineral analyses. In general, δ⁷Li of most olivines resemble bulk rock composition whereas clinopyroxenes are variably lighter. This implies that clinopyroxene is more susceptible to diffusion-induced fractionation, in agreement with previous studies. Olivine δ⁷Li values span a narrower range than whole rock values, and do not extend to the very heavy compositions previously reported in HIMU bulk lavas. This discrepancy likely reflects alteration of bulk lavas, and suggests that Li-isotope analyses of bulk lavas should be interpreted with caution. Olivines from HIMU lavas have heavy δ⁷Li values (up to 6.2‰), and extend beyond the range reported for fresh MORB. Correlations between Li-isotopes and radiogenic isotopes suggest that the heavy δ⁷Li values in HIMU olivines are a source characteristic and not the result of post-magmatic alteration. Although the Li-isotope composition of recycled, dehydrated oceanic crust is currently under debate, our results suggest that HIMU lavas derive from a source containing recycled dehydrated oceanic crust, and that the “heavy” Li-isotope signature of altered oceanic crust is partially preserved during passage through the subduction factory.     

Seismic profile analysis of the Kangra and Dehradun re-entrant of NW Himalayan Foreland thrust belt,India: A new approach to delineate subsurface geometry

In the NW Sub-Himalayan frontal thrust belt in India, seismic interpretation of subsurface geometry of the Kangra and Dehradun re-entrant mismatch with the previously proposed models. These procedures lack direct quantitative measurement on the seismic profile required for subsurface structural architecture. Here we use a predictive angular function for establishing quantitative geometric relationships between fault and fold shapes with ‘Distance–displacement method’ (D–d method). It is a prognostic straightforward mechanism to probe the possible structural network from a seismic profile. Two seismic profiles Kangra-2 and Kangra-4 of Kangra re-entrant, Himachal Pradesh (India), are investigated for the fault-related folds associated with the Balh and Paror anticlines. For Paror anticline, the final cut-off angle \(\beta =35{^{\circ }}\) was obtained by transforming the seismic time profile into depth profile to corroborate the interpreted structures. Also, the estimated shortening along the Jawalamukhi Thrust and Jhor Fault, lying between the Himalayan Frontal Thrust (HFT) and the Main Boundary Thrust (MBT) in the frontal fold-thrust belt, were found to be 6.06 and 0.25 km, respectively. Lastly, the geometric method of fold-fault relationship has been exercised to document the existence of a fault-bend fold above the Himalayan Frontal Thrust (HFT). Measurement of shortening along the fault plane is employed as an ancillary tool to prove the multi-bending geometry of the blind thrust of the Dehradun re-entrant.     

Experimental study on the instability mechanism of the major-defect fractured rock

Based on calculations of rock fracture surface and angle, incremental-load creep experiments were conducted on two groups of major-defect fractured rock specimens in an RLW-2000 rheology test system. The research investigated the fracture type and the creep properties of major-defect fractured rock and analyzed the relationships between failure load and horizontal or vertical projection distance, and between each of theme and fracture area or angle. The results showed that rock fracture was divided into three types according to the distribution, including I, II, and III types. I, II, and III types were respectively an internal fracture running through neither the upper nor lower end, one through the upper or lower end and one through both upper and lower ends, and a III type was further sub-divided into III_I and III_II types. The instantaneous strain was larger than the creep strain under the same creep loading stage. As the creep loading increased on two groups of major-defect fractured rock, the instantaneous strain decreased abruptly and then increased abruptly, while the creep strain decreased rapidly at first and later increased near-linearly. When the failure angle was larger than the friction angle, failure load was positively correlated with failure angle yet was negatively correlated with vertical projection distance. Vertical projection distance and fracture angle, which decided fracture type, controlled rock failure load. Failure load increased in turn from I type to III type, and low-type fracture determined mainly failure load in multiply-fracture specimen.     

Wavefield analysis of crosswell seismic data

Despite the numerous advantages of crosswell seismic data over surface seismic data, crosswell seismic geophysics is still underutilized and underdeveloped. The factors limiting the full utilization of crosswell data include the lack of standardized methods for processing and imaging the data. This is because crosswell data is not completely understood. To improve the understanding of crosswell data, we performed acoustic and elastic modeling of a west Texas carbonate oilfield data using finite difference methods and crosswell geometry. To account for the different wave modes in the field data, we decomposed the full data into its constituent wave modes. Results of the forward modeling show that elastic synthetic data is a better representation of crosswell field data than the popular acoustic synthetic data. Wavefield decomposition gave insight into the time-space kinematics behavior of the different wave modes that constitute the full data. Overall, the study improved our understanding of crosswell field data. The learning from this study has been utilized to perform data-driven reflection enhancement processing where the discerned characteristic of different seismic arrival is utilize to suppress unwanted and enhanced the desired wave modes. The processing reduced the complex data to only up-going P-P reflections that can be imaged to reveal the subtle geological structures of the oilfield.     

Towards a Digital Earth: using archetypes to enable knowledge interoperability within geo-observational sensor systems design

Earth System Science (ESS) observational data are often inadequately semantically enriched by geo-observational information systems to capture the true meaning of the associated data sets. Data models underpinning these information systems are often too rigid in their data representation to allow for the ever-changing and evolving nature of ESS domain concepts. This impoverished approach to observational data representation reduces the ability of multi-disciplinary practitioners to share information in a computable way. Object oriented techniques that are typically employed to model data in a complex domain (with evolving domain concepts) can unnecessarily exclude domain specialists from the design process, invariably leading to a mismatch between the needs of the domain specialists, and how the concepts are modelled. In many cases, an over simplification of the domain concept is captured by the computer scientist. This paper proposes that two-level modelling methodologies developed by health informaticians to tackle problems of domain specific use-case knowledge modelling can be re-used within ESS informatics. A translational approach to enable a two-level modelling process within geo-observational sensor systems design is described. We show how the Open Geospatial Consortium’s (OGC) Observations & Measurements (O&M) standard can act as a pragmatic solution for a stable reference-model (necessary for two-level modelling), and upon which more volatile domain specific concepts can be defined and managed using archetypes. A rudimentary use-case is presented, followed by a worked example showing the implementation methodology and considerations leading to an O&M based, two-level modelling design approach, to realise semantically rich and interoperable Earth System Science based geo-observational sensor systems.