bluuebunny/crossref_metadata_2025 huggingface dataset:
Collage tectonics in the northeasternmost part of the Variscan Belt: the Sudetes, Bohemian Massif
Authors: Aleksandrowski P. , Mazur S.
Published: April 2008

A synthesis of published and new data is used to interpret the Sudetic segment of the Variscan belt as having formed by the accretion of four major and two or three minor terranes. From west to east the major terranes are (1) Lusatia-Izera Terrane, exposing Armorican continental basement reworked by Ordovician plutonism and Late Devonian-Carboniferous collision, showing Saxothuringian affinities; (2) composite Góry Sowie-Kłodzko Terrane characterized by multistage evolution (Silurian subduction, mid- to late Devonian collision, exhumation and extension, Carboniferous deformational overprint), with analogues elsewhere in the Bohemian Massif, Massif Central and Armorica; (3) Moldanubian (Gföhl) Terrane comprising the Orlica-Śnieżnik and Kamieniec massifs, affected by Early Carboniferous high-grade metamorphism and exhumation and (4) Brunovistulian Terrane in the East Sudetes, set up on Avalonian crust and affected by Devonian to late Carboniferous sedimentation, magmatism and tectonism. The main terranes are separated by two smaller ones squeezed along their boundaries: (1) Moravian Terrane, between the Moldanubian and Brunovistulian, deformed during Early Carboniferous collision, and (2) SE Karkonosze Terrane of affinities to the Saxothuringian oceanic realm, sandwiched betwen the Lusatia-Izera and Góry Sowie-Kłodzko (together with Teplá-Barrandian) terranes, subjected to high pressure-metamorphism and tectonized during Late Devonian-Early Carboniferous convergence. The Kaczawa Terrane in the NW, of oceanic accretionary prism features, metamorphosed and deformed during latest Devonian-Early Carboniferous times, may either be a distinct unit unrelated to closure of the Saxothuringian Ocean or represent a continuation of the SE Karkonosze Terrane.
A compressional wedge model for the Lower Palaeozoic Anglo-Brabant Belt (Belgium) based on potential field data
Authors: Sintubin Manuel , Everaerts Michel
Published: April 2008

Structural field observations only allow kinematic inferences to be made for the southern extremity of the predominantly concealed Lower Palaeozoic Anglo-Brabant Belt. On the other hand potential field data (aeromagnetic and Bouguer anomaly maps), enable the field of vision to be enlarged. They not only corroborate the kinematics derived from structural field observations; they also reveal possible key players in the late Silurian to early Devonian deformation event causing the observed orogenic architecture.

This integrated approach has led to a compressional wedge model for the Brabant Massif. It is also proposed that crystalline basement blocks, most probably of Precambrian age, controlled the kinematics. Finally, it is suggested that the development of rift or pull-apart basins in a transtensional intracontinental setting during the Cambrian may have been crucial in the subsequent deformation history of the Anglo-Brabant Belt.
Tectonic controls on magmatic-hydrothermal gold mineralization in the magmatic arcs of SE Asia
Authors: Barley M. E. , Rak P. , Wyman D.
Published: April 2008

The magmatic arcs of SE Asia contain some of the world’s major gold deposits. These are mainly related to magmatic-hydrothermal activity and include epithermal, porphyry Cu-Au and skarn deposits. Most gold deposits in SE Asian arcs formed during three intervals of tectonic reorganization rather than during periods of normal or steady-state subduction. These plate reorganizations and periods of gold mineralization were caused initially by the collision of the Australian craton with the Philippine Sea plate arc at 25 Ma. A second Mid-Miocene period of mineralization accompanied plate reorganization following maximum rotation or extrusion of Indochina and the cessation of spreading of the South China Sea at 17 Ma. However, the vast majority and largest deposits formed since 5 Ma in a broad belt from Taiwan to the Solomon Islands during an important period of tectonic reorganization. This tectonic reorganization accompanied a postulated change in the relative motion between the Indian-Australian and Pacific plates between 5 and 3.5 Ma following collision of the Philippine arc and the Eurasian plate in Taiwan. Arc-related magmatism in unusual tectonic settings produced the most abundant and largest deposits with many deposits associated with relatively rare high-K calc-alkaline, shoshonite, adakite and alkaline magmatism. In particular peak mineralization appears related to melting of mantle that had been previously modified by subduction. During large-scale plate reorganization this can occur at the end of a period of normal subduction, following the cessation of subduction, following arc collision or accompanying subduction reversal at approximately the same time in different parts of a complex system of magmatic arcs such as those in SE Asia. Such tectonic controls may be impossible to recognize in older orogenic belts where recognition of a major change in tectonic style and unusual magma types may be the best guides to mineralization.
Tectonics, geodynamics and gold mineralization of the eastern margin of the North Asia craton
Authors: Fridovsky Valeriy Yu. , Prokopiev Andrei V.
Published: April 2008

The North Asia craton is a crustal block including the Siberian platform and marginal fold-and-thrust belts. On the eastern margin of the North Asia craton there is the Verkhoyansk fold-and-thrust belt making up the western part of the Verkhoyansk-Chersky collisional orogenic belt extending for 2000 km from the Laptev Sea in the north to the Sea of Okhotsk in the south. A system of frontal thrusts separates the belt from the platform structures. The frontal part of the belt is mainly made of Carboniferous and Permian terrigenous rocks of palaeodeltas and submarine fans which grade eastward into Triassic and Jurassic sediments of the continental slope. The front of the belt is characterized by thrusting and strike-slip faulting with large horizontal displacements. The largest anticlinoria at the front of the belt have a duplex structure. Formation of the major gold deposits and fold and fault structures, as well as igneous activity in the region, are related to the collision of the North Asia craton with the Kolyma-Omolon superterrane and the Okhotsk terrane in the Late Jurassic-Neocomian. The collision occurred in two stages: the early Neocomian frontal collision and the late Neocomian oblique collision.
Supercontinents, superplumes and continental growth: the Neoproterozoic record
Authors: Condie Kent C.
Published: April 2008

Between 1300 and 500 Ma the Neoproterozoic supercontinent Rodinia aggregated (1300–950 Ma), broke up (850–600 Ma) and a new supercontinent, Pannotia-Gondwana, formed (680–550 Ma). Only
c.
11% of the preserved continental crust was produced during this 800 Ma time interval and most of this crust formed as arcs, chiefly continental margin arcs. At least 50% of juvenile continental crust produced between 750 and 550 Ma is in the Arabian-Nubian Shield and in other terranes that formed along the northern border of Amazonia and West Africa. An additional 20% occurs in Pan-African orogens within Amazonia, and
c.
16% in the Adamastor and West African orogens. The growth rate of continental crust between 1350 and 500 Ma was similar or less than the average rate of continental growth during the Phanerozoic of 1 km
3
/a, and this low rate characterizes both formation and breakup stages of the supercontinents.

The low rates of continental growth during the Neoproterozoic may be due to the absence of a superplume event associated with either Rodinia or Pannotia-Gondwana. If supercontinent breakup is required to produce a superplume event, perhaps by initiating catastrophic collapse of lithospheric slabs at the 660 km seismic discontinuity, the absence of a Meso-proterozoic-Neoproterozoic superplume event may mean that a Palaeoproterozoic supercontinent did not fully breakup prior to aggregation of Rodinia.
Sedimentary response to tectonics in extensional basins: the Pechelbronn Beds (Late Eocene to early Oligocene) in the northern Upper Rhine Graben, Germany
Authors: Derer C. , Kosinowski M. , Luterbacher H. P. , Schäfer A. , Süß M. P.
Published: April 2008

The deposition of the late Eocene to early Oligocene Pechelbronn Beds in the northern Upper Rhine Graben was controlled by changes in accommodation space, sediment supply and basin physiography, imposed by the syn-rift tectonic framework. Base-level cycles, defined by variations of the ratio of accommodation space to sediment supply (A/S ratio), allow untangling of the depositional history in this complex structural setting. A transfer zone divided the northern part of the Upper Rhine Graben into a southern and a northern sub-basin and created major depositional gradients. The low A/S ratio in the transfer zone led to sediment bypassing and cannibalisation. Only asymmetric cycles of fluvial and alluvial fan deposits developed, as the sediment was transported to the sub-basins. The higher A/S ratio on the major gradient of the southern sub-basin, which increased from the transfer zone to the south, allowed the formation of symmetric delta/shoreface and lacustrine cycles. At times starvation occurred in the transfer-zone-distal parts of the sub-basin. On subordinate scale, within the southern sub-basin, tilt-blocks bounded by growth faults created halfgrabens with inferior depositional gradients. On the footwall crest, due to low A/S ratio, bypassing and erosion occurred. Here asymmetric cycles of coarse-grained channel fill deposits were preserved. On the hangingwall, close to the normal fault, high A/S conditions were present and symmetric cycles developed. The creation of accommodation space kept pace and even outpaced the footwall-derived sediment supply, which created thick shallow water deposits.
Subsidence, stress regime and rotation(s) of a tectonically active sedimentary basin within the western Alpine Orogen: the Tertiary Piedmont Basin (Alpine domain, NW Italy)
Authors: Carrapa B. , Bertotti G. , Krijgsman W.
Published: April 2008

The Oligocene to Miocene Tertiary Piedmont Basin (TPB) is located in the NW part of Italy at the junction between the Apennine and the Alpine thrust belts. The position of the TPB on top of the Alpine/Apennine Orogen poses fundamental questions as to the tectonics of the basin subsidence. Having undergone little deformation, the TPB sediments provide an insight into the stress regime and rotations in the kinematically very complex area surrounding the basin itself. In this study we integrate subsidence and structural analysis with measurements of magnetic susceptibility anisotropy (AMS) and natural remanent magnetization (NRM) in order to better constrain the tectonic kinematics of the basin evolution. A major important period of subsidence occurred in the Middle Miocene involving the whole basin. During this period the TPB experienced NE-SW-directed compression and limited shortening. Some NW-SE-directed compressional features have been identified and they were probably active during post Tortonian times. Structures associated with north-south tension are quite common, but the amount of strain that they accommodate is minor. In addition this research provides new preliminary data suggesting counterclockwise rotation in the TPB by
c
. 20° which has taken place during Middle Miocene time.
Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′-48°30′S)
Authors: Augustsson C. , Bahlburg H.
Published: April 2008

Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The ɛ
Nd
(T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.
Effects of rate and nature of synkinematic sedimentation on the growth of compressive structures constrained by analogue models and field examples
Authors: Nalpas T. , Gapais D. , Verges J. , Barrier L. , Gestain V. , Leroux G. , Rouby D. , Kermarrec J.-J.
Published: April 2008

Compressive basins show various geometries of growth structures. In this paper, we examine the effects of the rate and nature of synkinematic sedimentation by comparing analogue models and field examples. We performed different types of experiments in order to simulate various natural conditions, the parameters tested including the rate of synkinematic sedimentation and the presence of a potential décollement layer deposited during deformation. Modelling techniques are similar to those usually used for experiments on brittle-ductile systems made of sand and silicone putty.

To study the influence of the synkinematic sedimentation rate, we used sets of experiments with different velocity ratios
R
between the rate of sedimentation (
V
s
) and the rate of uplift of the top of the structure (
V
u
). In standard experiments, we deposited only sand during deformation. In all experiments, growth folds and growth faults developed. The geometry of growth folds changes from a steep anticline for
R
=1, to a rounded broken anticline when
R
exceeds 2. The geometry of growth reverse faults changes: from segments which initiate with a dip angle of about 30°, but then flatten or steepen, for
R
=1/2 and for
R
=1, respectively. For some sets of experiments, a thin silicone layer deposited instead of sand at some stage of sedimentation simulated the introduction of potential décollement layers.

When a change in the nature of synkinematic sedimentation occurs between the two flanks of a fold, a fault develops in the flank where brittle material dominates. The direction of growth folds forming during compression in front of a ramp and above a décollement layer is parallel to the ramp and oblique to the direction of bulk shortening.

Good correlations are observed between experimental geometries and field examples from the southern Pyrenees (Eastern Ebro Basin and Jaca Basin, Spain) and the Apennines (Italy).
The role of spectral curvature mapping in characterizing subsurface strain distributions
Authors: Wynn T. J. , Stewart S. A.
Published: April 2008

The curvature of structured geological surfaces can be used to assess the degree of strain they have undergone. In many hydrocarbon reservoirs, this strain is expressed as brittle fracturing that may significantly impact reservoir performance. Here we describe the development of an algorithm for measuring the curvature of gridded surfaces derived from seismic data. For any grid node, the algorithm calculates the magnitude and orientations of the two principal curvatures,
K
1
and
K
2
, from which other curvature measurements can be derived, such as Gaussian curvature and summed absolute curvature (
K
1
+
K
2
). The algorithm has also been used to generate plots of summed absolute curvature as a function of grid node separation (
k
versus λ). These ‘spectral’ or
k
λ plots can be generated for each grid node and allow the definition of short-wavelength, high-amplitude noise cut-off lengths. They also deliver intermediate wavelength features such as fault drag or buckle folding and the identification of long-wavelength (basin-scale) curvatures. Portions of these data can be collapsed into single values by calculating the integral of the
k
λ curve. Further filters designed to screen the effects of background tectonic, or non-tectonic, curvatures can be applied to the
k
λ integral.

This algorithm has been tested using data from several North Sea chalk fields. A range of alternative types of curvature and curvature spectra are compared with other approaches to curvature calculation and other factors relevant to the calibration of such techniques in terms of the distribution of brittle fractures in sedimentary rocks.

The
k
λ integral provides a relatively simple approach to calculating the degree of multi-wavelength strain present at a particular grid node. Freeing algorithms from the restriction of the ‘arbitrarily’ selected minimum grid node spacing is a key step towards calibrating measured curvature against strain mechanisms. However, care must be taken to separate intrinsic and tectonic curvatures when generating and interpreting
k
λ plots and their integrals.
Reservoir characterization for the management of stress-sensitivity
Authors: Smart Brian G. D. , Somerville Jim McL. , Jin Min , Koutsabeloulis Nicos C.
Published: April 2008

Reservoir stress-sensitivity operates at the near-wellbore and the reservoir scale. While the near-wellbore effects have been recognized and managed for some time, awareness of the significance of the reservoir scale effects is only now developing, aided by extreme examples such as Ekofisk, HP/HT reservoirs and unconsolidated sands. Fractured reservoirs must also be considered to be stresssensitive, with this possibly being masked in the past by high productivities and abundant reserves. It is proposed that the appropriate conceptual model for all reservoirs is that of intact blocks of rocks bounded by discontinuities (fractures and bedding-parallel), loaded by an anisotropic stress state. This model should be used to drive the collection of stress-sensitive reservoir characterization data, knowing that the new generation of coupled simulators can usefully accommodate this data set. The data set listed enables all aspects of reservoir stress-sensitivity to be examined.
Geological evidence of patterns of fluid flow through fracture networks: examination using random realizations and connectivity analysis
Authors: Jolly Richard J. H. , Cosgrove John W.
Published: April 2008

Fluid flow through fractured media is dependent upon a variety of parameters including fracture length, orientation and density, and also on the relative magnitude of the lithostatic stress and the fluid pressure. Because of this it is often difficult to determine the fluid pathway through a particular fracture system even when the geometry of the network is known. Although many fluids (e.g. gas, water etc.) leave little or no evidence of their passage through the rock others such as magmas and fluidized sediments preserve the pathways they follow by forming dykes and sills.

It is found that the pathways preserved by the two types of fluids are different i.e. the spacing of the clastic dykes follows a power-law distribution and that of the igneous dykes a log-normal distribution. It is suggested that this in part might reflect the different properties of the dyke material (specifically its permeability) which determines whether or not the fracture containing the dyke can continue to act as a channel of easy fluid migration once the dyke has been emplaced.
Cenozoic strike-slip faulting from the eastern margin of the Wilkes Subglacial Basin to the western margin of the Ross Sea Rift: an aeromagnetic connection
Authors: Ferraccioli Fausto , Bozzo Emanuele
Published: April 2008

Tectonic modelling of regional aeromagnetic anomaly patterns suggests Cenozoic right-lateral strike-slip faulting along an inherited fault system of the Transantarctic Mountains and adjacent hinterland. We name it here the Prince Albert Fault System. The Reeves Fault and David Fault are Cenozoic right-lateral strike-slip faults and form part of the NW-SE-striking segment of this complex fault system, extending to the eastern margin of the Wilkes Subglacial Basin. Our aeromagnetic interpretation suggests therefore that the Wilkes Subglacial Basin may be connected to the Cenozoic strike-slip kinematic framework of the Transantarctic Mountains and western Ross Sea Rift. The southernmost segment of the Prince Albert Fault System parallels the N-S-striking McMurdo Sound Fault Zone and, together with it, defines a transtensional western Ross Sea Rift margin. High-resolution aeromagnetic images define the Cape Roberts pull-apart basin and suggest that Cenozoic magmatism may have focused along the transtensional western Ross Sea Rift margin itself.
Late Palaeozoic formation and development of the St Marys Basin, mainland Nova Scotia, Canada: a prolonged record of intracontinental strike-slip deformation during the assembly of Pangaea
Authors: Murphy J. B.
Published: April 2008

The Late Palaeozoic evolution of the St Marys Basin, mainland Nova Scotia, preserves evidence of protracted dextral shear along an intracontinental fault zone during collisional orogenesis and the assembly of Pangaea. The St Marys Basin formed within the E-W-trending Minas Fault Zone (MFZ) along the boundary between the Avalon and Meguma terranes and contains latest Devonian-Tournaisian continental clastic rocks that are 3000–4000 m in thickness.

The origin and evolution of the basin is attributed to either discrete or progressive dextral strike-slip tectonics along the MFZ between the Late Devonian and Late Carboniferous. Evidence for the Late Devonian origin of the basin is recorded along its southern flank by the fabrics of the deformed
c.
370 Ma granites, the overall sedimentary facies distribution, and some syndepositional features within the clastic rocks. The most intense deformation within the basin is concentrated in a relatively narrow ENE-trending zone, in which predominantly fine-grained clastic rocks are deformed into periclinal folds and related reverse faults. The orientation of this zone relative to the MFZ is consistent with dextral shear. At least some of this deformation occurred after the deposition of the overlying Visean Windsor Group. The style of deformation along the present northern margin of the basin (the Chedabucto Fault) is also consistent with regional dextral shear.

The St Marys Basin is an example of basin development and evolution adjacent to an intracontinental fault zone associated with oblique convergence during orogenesis. Its evolution provides constraints on the potential relationship between the termination of the mid-Palaeozoic Acadian orogeny, subsequent basin development, and the ongoing interactions between the Avalon and Meguma terranes, and between Laurentia and Gondwana during the assembly of Pangaea. More generally, because the relationship between fabric development and motion along intracontinental strike-slip faults in continental zones is difficult to interpret, the sedimentology and structural geology in basins developed along these fault zones may preserve a less ambiguous record of the main tectonic events.
Geometry, kinematics and scaling properties of faults and fractures as tools for modelling geofluid reservoirs: examples from the Apennines, Italy
Authors: Cello G. , Tondi E. , Van Dijk J. P. , Mattioni L. , Micarelli L. , Pinti S.
Published: April 2008

The study of some major fault zones in the Apennines was mostly focused on the acquisition of quantitative data
in situ
aimed at deriving input parameters for modelling faulted rock volumes. Structural data collected in the Monte Alpi area (southern Italy) and in the central Apennines allowed us to: (i) estimate the fractal dimension characterizing the geometric complexity and size distribution of different fault and fracture patterns; (ii) assess the appropriate parameters defining the overall architecture, anisotropy and related permeability structure of the mapped fault zones; and (iii) constrain the scaling properties of some of the attributes (i.e. length, spacing, map pattern, fracture density, etc.) of both fault-related and regional fracture sets.

The results of our work suggest that the above data are appropriate for extrapolating field based information at different scales, and for producing 3D models of fault and fracture networks.
3D finite element model of major tectonic processes in the Eastern Mediterranean
Authors: Heidbach O. , Drewes H.
Published: April 2008

A three-dimensional finite element model of the Eastern Mediterranean was developed in order to investigate the tectonic processes and model parameters which are mainly responsible for the observed surface deformation. The main faults are modelled as surfaces with Coulomb friction. Boundary conditions are slab pull at the Hellenic arc and the displacement for the African and Arabian plate taken from the rigid plate model NUVEL-1A. The rheology is elasto-visco-plastic. By varying the model properties a set of parameters can be determined which leads to a minimal mean deviation comparing the velocity field of the models with the space geodetic observations at 42 sites from global positioning system (GPS) and satellite laser ranging (SLR). The best result was attained with a low friction coefficient between 0.2 and 0.45 along the subduction zones and the main faults, a slab pull stress of 150 MPa and different cohesive strengths
C
in areas with compressional (
C
= 460 MPa) and extensional tectonic regimes (
C
= 40 MPa). The velocity field of the models shows the overall pattern of a westward escape of Anatolia and the SSW movement of the Aegean region.
3D discrete kinematic modelling of sedimentary basin deformation
Authors: Cornu T. , Schneider F. , Gratier J. -P.
Published: April 2008

3D coupled backward and forward deformation of geological layers is a new step in basin modelling. This problem can be treated with a mechanical or a kinematic approach. Because of the difficulties met with the mechanical approach, the kinematic approach is more often used. The kinematic model described here allows a geologically acceptable path to be built, which takes into account an incremental evolution of time. To obtain a better description of 3D geometries, the model uses a full hexaedric discretization. The discrete neutral surface of each layer is used to perform the flexural slip deformation.
Water and halogens in volcanic clasts: tracers of degassing processes during Plinian and dome-building eruptions
Authors: Villemant B. , Boudon G. , Nougrigat S. , Poteaux S. , Michel A.
Published: April 2008

Magma degassing may occur either with no significant gas escape from the magma column, which corresponds with typical Plinian type eruptions, or with gas loss, which corresponds with typical effusive (dome-building) eruptions. Magma degassing may also lead to melt crystallization, which modifies the residual melt composition and, in turn, may significantly affect the degassing conditions. We propose a method for modelling these processes for H
2
O-rich rhyolitic melts through measurements of volatiles (H
2
O, Cl, Br) in the microcrystalline matrix and glass of erupted volcanic clasts (pumice and dome clasts). This method is applied to two Plinian and dome-building eruptions at Mount Pelée (Martinique) and Santa Maria (Guatemala) volcanoes. Extreme magma degassing and crystallization during dome-building eruptions may explain the contrasts in halogen and H
2
O contents of residual melts of dense volcanic clasts: they display very large ranges of Cl contents (few ppm to thousands of ppm), whereas the ranges of H
2
O contents are much narrower, and lower than 1%. This method allows prediction of the evolution of volcanic gas chemistry (as HCl content or HCl/HF ratio) as a function of the degassing style of magma at shallow depth.
Magma extrusion dynamics revealed by high-frequency gas monitoring at Soufrière Hills volcano, Montserrat
Authors: Young S. R. , Voight B. , Duffell H. J.
Published: April 2008

Andesitic to dacitic dome-building volcanoes often present a problem for eruption forecasting because signs of impending activity can be minimal or ambiguous. Gas monitoring is one of a number of techniques used to assist in eruption forecasting. However, a variety of explanations have been offered for the large variations in gas release that are commonly reported from erupting volcanoes. Difficulties in interpretation can arise because gas-flux measurements are generally acquired at lower sampling rates than other geophysical observations. Here, we report SO
2
flux measurements, by correlation spectroscopy, recorded semi-continuously during December 1999 to January 2000 at the Soufrière Hills Volcano, Montserrat. We compare these data to continuously recorded seismic records, and interpret the results in terms of conduit dynamics. We demonstrate two- to six-fold variations in gas flux over a few hours, and show that these variations can be systematic and directly correlated with long-period swarm seismicity. For the period of study, we find that the gas-flux peak lags several tens of minutes behind the peak in seismic energy release. These features are consistent with models of oscillating magma flow, where magma viscosity is dependent on melt volatile content. We propose that seismicity reflects conduit pressurization, and find that gas flux directly reflects magma flow rate. Although other volcanoes might behave differently, our results suggest that it can be possible to use continuous gas measurements to monitor conduit behaviour, perhaps providing short-term warnings of impending eruptions.
The tropospheric sulphur cycle and the role of volcanic SO
2
Authors: Stevenson D. S. , Johnson C. E. , Collins W. J. , Derwent R. G.
Published: April 2008

A global three-dimensional chemistry-transport model has been applied to study the tropospheric sulphur cycle, and in particular the volcanic component. The model is in general agreement with previous studies of the global S budget. We find that volcanic emissions constitute 10% of the present-day global SO
2
source to the atmosphere, but form 26% of the SO
2
burden, and 14% of the sulphate aerosol burden. Two previous modelling studies suggested that the volcanic fraction of sulphate was 18% and 35%, from sources representing 7% and 14%, respectively, of the global total SO
2
emission. The results are dependent upon various assumptions about volcanic emissions (magnitude, geographical location, altitude), the global distribution of oxidants, and the physical processes of dry and wet deposition. Because of this dependence upon poorly constrained parameters, it is unclear which modelling study is closest to the truth.
Compositional variation in tropospheric volcanic gas plumes: evidence from ground-based remote sensing
Authors: Horrocks L. A. , Oppenheimer C. , Burton M. R. , Duffell H. J.
Published: April 2008

Remotely sensed measurements of volcanic plumes have been undertaken for 30 years with instruments such as the correlation spectrometer, and more recently, open-path Fourier transform infrared (OP-FTIR) spectrometers. Observations are typically made several kilometres from the source, by which time chemical reactions may have occurred in the plume, overprinting the source composition and flux. Volcanological interpretations of such data therefore demand an understanding of the atmospheric processes initiated as gases leave the volcanic vent. Ground-based remote sensing techniques offer the temporal resolution, repeatability and quantitative analysis necessary for investigation of these processes. Here we report OP-FTIR spectroscopic measurements of gas emissions from Masaya Volcano, Nicaragua, between 1998 and 2001, and examine the influence of atmospheric processes on its tropospheric plume. Comparisons of observations made at the summit and down-wind, and in different measurement modes confirm that tropospheric processes and local meteorology have only minor impact on gas composition after the plume has left the crater. This study demonstrates that plume monitoring downwind provides a reliable proxy for at-crater sampling, and that volcanological information content is not obscured by the intervening transport. From February 1998 to May 2000, Masaya’s plume composition was strikingly stable and characterized by SO
2
/HCl and HCl/HF molar ratios of 1.6 and 5.0, respectively. Departures from this stable background composition are likely to signify changes in the volcanic system or degassing regime, as identified in April–May 2001.
Correlations between eruption magnitude, SO
2
yield, and surface cooling
Authors: Blake S.
Published: April 2008

Sulphurous gases from explosive eruptions have the potential to form stratospheric aerosols and so produce surface cooling on a hemispheric to global scale. However, testing for any correlation between SO
2
yield and surface cooling is hampered by instrumental SO
2
and temperature measurements being available for time periods that include only a few large eruptions. To overcome this, published dendroclimatological data, satellite (Total Ozone Mapping Spectrometer) data on SO
2
emissions, stratospheric optical depth data, and volcanological observations are integrated, revealing several relevant new correlations. First, the efficient conversion of SO
2
into stratospheric aerosols occurs when the ratio of plume height to tropopause height is greater than about 1.5. Second, the mass of emitted SO
2
correlates well with the mass of erupted magma. The SO
2
yield is 0.1 to 1% by mass of magma, irrespective of composition. The best-fit power law (
r
2
=0.67) is mass of SO
2
in Mt=1.77(mass of magma in Gt)
0.64
. Third, of the eruption clouds that are believed to have entered the stratosphere in the period 1400–1994, those with masses <5 Gt magma (DRE <2 km
3
) appear to have had insignificant effects on Northern Hemisphere summer temperature. The scattered data for eruptions of >10 Gt (>4 km
3
) magma suggest a mean cooling effect of about 0.35 °C.
Coupled mineral-fluid evolution of a basin and high: kaolinization in the SW England granites in relation to the development of the Plymouth Basin
Authors: Psyrillos Agamemnon , Burley Stuart D. , Manning David A. C. , Fallick Anthony E.
Published: April 2008

A new genetic model is proposed for the formation of the St Austell kaolin deposits, incorporating geological, isotopic, paragenetic and microthermometric evidence from kaolin-quartz veins combined with a reconstruction of the thermal evolution of the Cornubian pluton during the Mesozoic. Fluid inclusions in quartz, paragenetically associated with kaolin, document that the kaolinization took place at temperatures between 50°C and 100°C, indicating that the kaolinization is a low-temperature hydrothermal event coincident with the oil generation window.

Kaolinization occurred prior to the unroofing of the pluton, during the Late Jurassic to Early Cretaceous. The kaolinization is thus contemporary with the major Early Cretaceous uplift that affected the Cornubian massif as a consequence of rifting in the offshore Western Approaches. Geological, isotopic and geochemical considerations argue strongly against the involvement of unmodified meteoric waters in the kaolinization process. The most plausible fluid types for the kaolinization are either basinal brines expelled from Permo-Triassic sediments of the adjacent offshore Plymouth Basin, or highly evolved meteoric waters that circulated through the sediments enclosing the pluton. The kaolinization process converted large volumes of fractured granite to a porous quartz-kaolin rock matrix.
Applications of high-resolution X-ray computed tomography in petrology, meteoritics and palaeontology
Authors: Carlson W. D. , Rowe T. , Ketcham R. A. , Colbert M. W.
Published: April 2008

High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.
Porosity measurements of sedimentary rocks by means of microfocus X-ray computed tomography (μCT)
Authors: Van Geet M. , Lagrou D. , Swennen R.
Published: April 2008

Porosity of reservoir rocks is an important petrophysical characteristic, used as a basic parameter in simulation studies for predicting reservoir quality. An extensive debate continues about the techniques that are available for porosity measurements and visualization. One aspect is the fact that petrophysical measurements are performed on volumetric samples, whereas classical geological petrography using a petrographical microscope is restricted to 2D analysis. This leads to a discrepancy between petrographical and petrophysical studies. This paper aims to evaluate microfocus X-ray computed tomography (μCT) as a technique that can link petrography and petrophysics. A short overview of the μCT technique is given, together with a discussion of its limitations, mainly due to artefacts. Optimization of image quality and procedures for quantification are outlined. μCT results for porosity measurements of a limestone and a sandstone are compared with results obtained by other techniques.
Geostatistics and the representative elementary volume of gamma ray tomography attenuation in rock cores
Authors: Vogel J. R. , Brown G. O.
Published: April 2008

Semivariograms of samples of Culebra Dolomite have been determined at two different resolutions for gamma ray computed tomography images. By fitting models to semivariograms, small-scale and large-scale correlation lengths are determined for four samples. Different semivariogram parameters were found for adjacent cores at both resolutions. Relative elementary volume (REV) concepts are related to the stationarity of the sample. A scale disparity factor is defined and is used to determine sample size required for ergodic stationarity with a specified correlation length. This allows for comparison of geostatistical measures and representative elementary volumes. The modifiable areal unit problem is also addressed and used to determine resolution effects on correlation lengths. By changing resolution, a range of correlation lengths can be determined for the same sample. Comparison of voxel volume to the best-fit model correlation length of a single sample at different resolutions reveals a linear scaling effect. Using this relationship, the range of the point value semivariogram is determined. This is the range approached as the voxel size goes to zero. Finally, these results are compared to the regularization theory of point variables for borehole cores and are found to be a better fit for predicting the volume-averaged range.
3D soil image characterization applied to hydraulic properties computation
Authors: Delerue J. F. , Perrier E. , Timmerman A. , Swennen R.
Published: April 2008

We propose a novel method to characterize the fluid-filled (usually air or water) space in images of porous media at the pore scale. First, an aperture map is created based on a skeleton process, to describe all local sizes in the pore space. Then the pore space is segmented in pores, defined as elementary objects that compose the pore space. Using this segmented image, a pore network is created, which is a graphic representation of the pore space that includes local sizes and direct information about connectivity at the pore scale. As an application of this method for pore space modelling, the equivalent hydraulic conductivity or permeability for a soil sample is computed.
Numerical modelling of reverse-density structures in soft non-Newtonian sediments
Authors: Harrison Philip , Maltman Alex J.
Published: April 2008

A numerical code has been used to simulate the flow patterns in geological soft sediments that are driven by buoyancy forces resulting from reverse-density stratification. The aim was to provide a clearer understanding of the different roles of initiating conditions, inertia and rheological behaviour on the morphologies and timing of formation of natural features such as load casts and flame structures. Particular attention was paid to the cuspate form of rising intrusions that is commonly seen in nature but that has proved elusive in most earlier experiments. The numerical results demonstrate that large localised initiating perturbations and inertial influence during flow both tend to cause a decrease in the wavelength of the resulting flow pattern and can, under certain circumstances, serve to promote a cuspate morphology. The use of a relatively low viscosity Newtonian fluid as an approximation of the coarse-grained upper layer coupled with, critically, power-law behaviour in the underlying clayey layer was also found to promote a cuspate form in the rising intrusion.
Near-surface sediment mobilization and methane venting in relation to hydrate destabilization in Southern Lake Baikal, Siberia
Authors: Van Rensbergen Pieter , Poort Jeffrey , Kipfer Rolf , de Batist Marc , Vanneste Maarten , Klerkx Jean , Granin Nick , Khlystov Oleg , Krinitsky Petr
Published: April 2008

Four seeps and mud extrusion features at the lake floor were discovered in August 1999 in the gas hydrate area in Lake Baikal’s South Basin. This paper describes these features in detail using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, selected seismic profiles and heat flow data calculated from the depth of the hydrate layer as well as obtained from
in situ
thermoprobe measurements. The interpretation of these data is integrated with published geochemical data from shallow cores. The seeps are identified as methane seeps and appear as mud cones (maximum 24 m high, 800 m in diameter) or low-relief craters (maximum 8 m deep, 500 m in diameter) at the lake floor. Mud cones (estimated to be approximately 50–100 ka old) appear to be older than the craters and have a different structural setting. Mud cones occur at the crest of rollover structures, in the footwall of a secondary normal fault, while the craters occur at fault splays. The seeps are found in an area of high heat flow where the base of the gas hydrate layer shallows rapidly towards the vent sites from about 400 m to about 160 m below the lake floor. At the site of the seep, a vertical seismic chimney disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. Integration of these results leads to the interpretation that focused destabilization of gas hydrate caused massive methane release and forced mud extrusion at the lake floor and that the gas seeps and mud diapirs in Lake Baikal do not have a deep origin. This is the first time that methane seeps and/or mud volcanoes associated with gas hydrate decomposition have been observed in a sub-lacustrine setting. The finding suggests that gas hydrate destabilization can create large pore fluid overpressures in the shallow subsurface (<500 m subsurface) and cause mud extrusion at the sediment surface.
Pore pressure/stress coupling and its implications for rock failure
Authors: Hillis Richard R.
Published: April 2008

Clastic dykes and sills witness that subsurface sediment mobilization is often controlled by the brittle failure of units ‘sealing’ overpressured and liquidized sediments. Brittle failure also imposes a limit on the buoyancy pressure that can be exerted by hydrocarbon columns. Conventional understanding of brittle failure induced by increasing pore pressure (P
p
) assumes that total minimum horizontal stress (σ
h
) is unaffected by changes in pore pressure. However, total minimum horizontal stress increases from shallow, normally pressured sequences to deeper, overpressured sequences. Data from the Canadian Scotian Shelf, the North Sea and the Australian North West Shelf demonstrate such P
p
/σ
h
coupling, with the minimum horizontal stress increasing at approximately 60–80% of the rate of pore pressure (i.e., Δσ
h
/ΔP
p
= 0.6–0.8). Hence, a greater increase in pore pressure can be sustained prior to brittle failure of units sealing overpressured compartments than would be predicted by conventional, uncoupled failure models. Furthermore, because total vertical stress is not similarly coupled to pore pressure, differential stress (σ
1
–σ
3
) reduces as pore pressure increases in normal fault regime basins. Thus, the mode of rock failure can not be inferred from differential stress in the stable state and P
p
/σ
h
coupling promotes tensile over shear failure.
New morphological observations on Triassic pterosaurs
Authors: Dalla Vecchia Fabio M.
Published: April 2008

Since 1973, about 20 specimens of Triassic pterosaurs have been found in northern Italy, Austria and Greenland, belonging to
Eudimorphodon, Peteinosaurus, Preondactylus
and
Austriadactylus
. Their age is middle to late Norian and
Eudimorphodon
is the most common genus. The restudy of the specimens shows that
Peteinosaurus
presents trimorphodonty in the lower jaw and a fibula unreduced in length, distally expanded and fused to the tibiotarsus above the lateral condyle. Specimen MCSNB 3359 does not show diagnostic features of
Peteinosaurus
and is referred to it with doubt, whereas MCSNB 3496 is not
Eudimorphodon
but
Peteinosaurus. Preondactylus, Peteinosaurus, ?Peteinosaurus
and
Dimorphodon
could form a monophyletic group.

The tarsus of Triassic pterosaurs consists of two proximal tarsals, which fuse to the tibia during ontogeny, forming a tibiotarsus, and two distal tarsals. The larger of the two proximal tarsals was probably the calcaneum. The lateral condyle of the tibiotarsus is larger or more well formed than the medial one. The shape of the distal tarsals is similar to that of the distal tarsals in
Dimorphodon
. The metatarsals did not spread and the foot was ectaxonic; metacarpal length increases from metacarpal I to IV. This suggests that footprints of Triassic pterosaurs were different from
Pteraichnus
-like footprints.

Some features are unique to Triassic pterosaurs. and
Austriadactylus
have a multicuspid dentition,
Peteinosaurus
has cuspules in the distal teeth of the lower jaw and
Preondactylus
has serrated maxillary teeth. This could be a convergent feature or symplesiomorphic for their clade. and
Austriadactylus
have very large maxillary teeth below the ascending process of the maxilla. and possibly
Preondactylus
do not have the bundles of elongated caudal zygapophyseal and haemapophyseal processes which are present in
Peteinosaurus
and in the Jurassic long-tailed pterosaurs.
A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur
Authors: Frey Eberhard , Martill David M. , Buchy Marie-Céline
Published: April 2008

An exceptionally well-preserved cranium and mandible of a new species of pterodactyloid pterosaur from the Nova Olinda Member of the Crato Formation (Aptian, Early Cretaceious) of the Araripe Basin, northeastern Brazil, is described. The new taxon is characterized by the presence of a caudally directed parietal crest similar to that seen in pteranodontids, but is referred to the Ornithocheiridae of the Ornithocheiroidea. The specimen is referred to a new genus within the Ornithocheiridae, as it lacks the diagnostic rostral crest and instead possesses this parietal crest oriented. A lanceolate leaf with frayed distal end wedged between the mandibular rami suggests the cause of death for the specimen.