bluuebunny/crossref_metadata_2025 huggingface dataset:
Partitioning pre-, syn- and post-Variscan deformation in the Holy Cross Mountains, eastern Variscan foreland
Authors: Lamarche J. , Lewandowski M. , Mansy J.-L. , Szulczewski M.
Published: April 2008

In this study we demonstrate how a combined structural, sedimentological and palaeomagnetic approach provides a new perspective on the tectonic evolution of the Holy Cross Mountains. In the field, we performed a structural and sedimentological analysis of Palaeozoic rocks. Our analysis was complemented by a palaeomagnetic study and by the restoration of balanced cross sections in Palaeozoic and Mesozoic rocks. Different steps of deformation were restored for a
c.
350 Ma period. (1) The extensional tectonics of the Devonian basin was unravelled: the resulting normal fault system constituted the fundamental structural control for the later Variscan tectonic inversion and Alpine deformations. (2) The style of Variscan folding is characterized and quantified by way of a cross section across the Holy Cross Mountains. (3) The role of the reactivation of Variscan faults during the Permo-Triassic initiation of the Polish Basin was examined. (4) The localized Alpine compressive deformation was quantified and shown to contribute only to a minor degree to the present-day state of deformation in the Holy Cross Mountains. The Holy Cross Fault zone is the product of the interplay of changing transtensional and transpressional settings during the Variscan diastrophic cycle, with the final effect of the Variscan evolution being the flower-like structure of the Holy Cross Fault zone.
Oligocene-Early Miocene tectonic evolution of the northern Apennines (northwestern Italy) traced through provenance of piggy-back basin fill successions
Authors: Cibin U. , Di Giulio A. , Martelli L.
Published: April 2008

The provenance history of sediments deposited in the piggy-back basins of the Northern Apennines has been drawn by means of a petrographic study of nearly 200 sandstone samples collected over 250 km of the belt; it allows the evolution of the eroded part of the belt in Oligocene-Early Miocene times to be determined in detail, with special emphasis on the age of the exhumation and the onset of erosion of the high-pressure/low-temperature Pennine metamorphic units of the Ligurian Alps and Corsica that form the innermost part of the chain.

Five petrofacies were distinguished, representing three sources that were active separately (three ‘pure’ petrofacies) or together (two ‘mixed’ petrofacies). The resultion sandstone composition reflects the erosion of different source units, changing through time and space along the belt.

The stratigraphic distribution of petrofacies records a change in the main clastic source from Ligurian calcareous units to Penninic units. This change occurred over most of the study area, reflecting the complete exhumation of the Penninic metamorphic units within the innermost part of the belt. It occurred at different times along the chain, migrating from northwest to southeast from Late Rupelian to Aquitanian. This time shift is interpreted to be related to the obliquity of the Northern Apennines convergent system.
Silurian sedimentation in East Siberia: evidence for variations in the rate of tectonic subsidence occurring without any significant sea-level changes
Authors: Artyushkov E. V. , Chekhovich P. A.
Published: April 2008

It is widely accepted that major variations of sea-level have occurred in the Phanerozoic. Third-order cycles, 1–10 Ma long with amplitudes of 20–100 m, are of special interest for geochronology and petroleum geology. The amplitude of sea-level changes in the Silurian was estimated based on highly detailed data on the East Siberian Basin, which was 2×106km2in size. Fischer plots were compiled based on the thickness of 54 chronostratigraphic units — chronozones, each corresponding to a time intervalc.0.5 Ma long. The synchronicity of the chronozones ensures reliable comparison of the changes occurring with time in accommodation space in different regions. The subhorizontal Fischer plots derived for several regions indicate that sea-level changes were very small in the Silurian (<-5–10 m). A mathematical analysis of relative sea-level changes, which takes into account the finite rate of crustal subsidence and different possible forms of eustatic fluctuation, shows that from the observed structure of numerous Silurian successions in East Siberia, eustatic third-order sealevel changes could not have exceeded 6–20 m. In several regions of East Siberia, the rate of crustal subsidence varied as much as several hundred per cent at different times. These variations showed good similarity in form, but their amplitudes were different at different places in the basin. Most probably they were caused by variations in the rate of phase transformations in mafic rocks in the lower crust. Based on the example of the East Baltic, the absence of large-scale third-order cycles in eustatic changes of sea-level has been proven earlier for the Cambrian and earliest Ordovician. Probably, a similar situation was characteristic of many other epochs, when no large glaciations occurred, while many rapid changes of water depth in cratonic areas actually resulted from vertical crustal movements.
Fracture and in-situ stress characterization of hydrocarbon reservoirs: definitions and introduction
Authors: Ameen Mohammed S.
Published: April 2008

Fracture and in-situ stress characterization is fast-evolving as an essential part of characterizing hydrocarbon reserviors. In this book, the Geological Society presents a selection of sixteen chapters that demonstrate the tools, methods, analysis, interpretation and application of this subject. These are of great interest to researchers and scientists in the industry and academia. This chapter includes definitions pertinent to the subject of the book and gives an overview of the papers, classified into seven major themes according to the nature of the studies.
Fault and stress magnitude controls on variations in the orientation of
in situ
stress
Authors: Yale David P.
Published: April 2008

There has been a great deal of interest in mapping regional trends in stress orientation and their relationships to crustal deformation processes over the last quarter century. Less emphasis has been placed on explaining local variations in the stress field. However, it is variations in the local stress field which are most critical to characterizing hydrocarbon reservoirs. Fracture orientation, well stability, well orientation, and permeability anisotropy are all strongly affected by variations in the local stress field.

Using stress orientation data from a number of fields in different tectonic environments, we have tried to determine some of the tectonic and geological controls on variations in in situ stress orientation. We find that nearness to faults, fault structure, and magnitude of tectonic stress play primary roles in determining whether the regional stress field will be perturbed in a given reservoir and whether small-scale variations in the stress field can be expected.

We find that high tectonic stress environments (large horizontal differential stress) lead to a much more consistent local stress field than more tectonically quiescent areas. Faults can play a large role in rotating the local stress field. The smaller the difference between the maximum and minimum horizontal stress magnitude (i.e. lower tectonic stress), the larger a fault’s zone of influence is. Large-scale faulting with segmentation of the reservoir into discrete fault blocks can lead to significant stress orientation variations across the reservoir even in areas of large differential stress.
3D fracture network dynamics in reservoirs, faults and salt tectonic systems
Authors: Tuncay K. , Park A. , Payne D. , Ortoleva P.
Published: April 2008

A unique 3D computer simulator is used to predict natural fracture network characteristics in the subsurface. The model is based on the numerical solution of rock deformation processes coupled to the myriad of other basin reaction, transport and mechanical (RTM) processes. The model integrates seismic, well log and surface geological data to arrive at a quantitative picture of the distribution of fractures, stress, petroleum and porosity, grain size and other textural information.

An important component of the model is an incremental stress rheology that accounts for poroelasticity, non-linear viscosity with yield/faulting, pressure solution and fracturing. It couples mechanics to multi-phase flow and diagenesis (through their influence on effective stress and rock rheological properties, respectively). The model is fully 3D in terms of the full range of fracture orientations and the tensorial nature of stress, deformation and permeability. All rock properties (rheologic, multi-phase fluid transport, grain shape, etc.) are coevolved with the other variables. Examples illustrate the relative importance of various overpressuring mechanisms, lithology and flexure on the location and characteristics of a fracture network.
Difficulties and uncertainty in mathematical/numerical modelling of fluid flow in fractured media
Authors: Ewing Richard E. , Spagnuolo Anna M.
Published: April 2008

The ability to numerically model single-phase and multiphase flow of fluids in porous or fractured media is extremely important in developing an understanding of the complex phenomena governing the flow. The flow is complicated by the presence of heterogeneities in the reservoir at many different length scales by special flow features such as fractures or faults and by phenomena such as diffusion and dispersion. These effects must be effectively modelled by terms in coupled systems of non-linear partial differential equations which form the basis of the simulator. The simulator must be able to model both single and multiphase flows and the transition regimes between the two in unsaturated flow applications. A discussion of some of the aspects of modelling unsaturated and multiphase flows in the presence of heterogeneities and severe channelling is presented along with directions for future work. Simulators are severely hampered by the lack of knowledge of reservoir properties, heterogeneities, fracture dimension and orientation, and relevant length scales and other important mechanisms. Simulations can be performed either deterministically, to predict the outcome of a single realization of reservoir and flow properties, or via stochastic techniques to incorporate uncertainties of flow directly. Due to the extreme difficulties in using stochastic differential equation models for non-linear multiphase flows, we will concentrate on the potential of deterministic models. Recent developments have been made in homogenization, scaled averaging, and the use of the simulator as an experimental tool to develop methods to model the interrelations between localized and larger-scale media effects. Monte Carlo techniques using simulators with effective parameters can generate statistics for multiphase flow.
Fault development and interaction in distributed strike-slip shear zones: an experimental approach
Authors: Schreurs G.
Published: April 2008

Analogue model experiments using both brittle and viscous materials were performed to investigate the development and interaction of strike-slip faults in zones of distributed shear deformation. At low strain, bulk dextral shear deformation of an initial rectangular model is dominantly accommodated by left-stepping, en echelon strike-slip faults (Riedel shears, R) that form in response to the regional (bulk) stress field. Push-up zones form in the area of interaction between adjacent left-stepping Riedel shears. In cross sections, faults bounding push-up zones have an arcuate shape or merge at depth. Adjacent left-stepping R shears merge by sideways propagation or link by short synthetic shears that strike subparallel to the bulk shear direction. Coalescence of en echelon R shears results in major, through-going faults zones (master faults). Several parallel master faults develop due to the distributed nature of deformation. Spacing between master faults is related to the thickness of the brittle layers overlying the basal viscous layer. Master faults control to a large extent the subsequent fault pattern. With increasing strain, relatively short antithetic and synthetic faults develop mostly between old, but still active master faults. The orientation and evolution of the new faults indicate local modifications of the stress field. In experiments lacking lateral borders, closely spaced parallel antithetic faults (cross faults) define blocks that undergo clockwise rotation about a vertical axis with continuing deformation. Fault development and fault interaction at different stages of shear strain in our models show similarities with natural examples that have undergone distributed shear.
Recent strike-slip deformation of the northern Tien Shan
Authors: Buslov M. M. , Klerkx J. , Abdrakhmatov K. , Delvaux D. , Batalev V. Yu. , Kuchai O. A. , Dehandschutter B. , Muraliev A.
Published: April 2008

The paper presents a geodynamic interpretation of the deep structure and active tectonics of the northern Tien Shan, with particular emphasis on strike-slip motions, which produced a pull-apart in the centre of the Issyk-Kul basin. The study is based on a detailed interpretation of satellite imagery, fault plane solutions of earthquakes, seismic, and geodetic data.

Seismic and magnetotelluric studies show tectonic layering of the Tien Shan lithosphere, with several nearly horizontal viscoelastic layers and the lower layer underthrust northward in the northern Tien Shan. This active process may be responsible for the intricate present-day tectonic framework of the northern Tien Shan.

The recent tectonics of the northern Tien Shan inherits the earlier structure: The lens-shaped Issyk-Kul microcontinent comprising Precambrian-Palaeozoic metamorphic and magmatic rocks is surrounded by thick shear zones which have been involved in the activity over most of the Cenozoic. In the Quaternary the strain propagated as far as the central part of the Issyk-Kul basin.
Implications of late Pan-African shearing in western Dronning Maud Land, Antarctica
Authors: Perritt S. H. , Watkeys M. K.
Published: April 2008

The Mesoproterozoic cover rocks of the Archaean Grunehogna Province in western Dronning Maud Land, Antarctica, are gently folded and intensely fractured by a progressive ductile to brittle deformation event. The cause was dominantly NNW-SSE transpression related to a regionally extensive intracontinental sinistral strike-slip event at
c.
520 Ma. This reactivated the boundary between the Archaean Grunehogna Province and the high grade Mesoproterozic Maudheim Province, and resulted in the formation and deformation of the Cambrian Urfjell pull-apart basin. In a Gondwana refit, this strike-slip system can be linked to similar zones in southeast Africa which post-date the 750–650 Ma early Pan-African collisional events of that region. The NNW movement of the Maudheim Province along this strike-slip zone was responsible for the development of thrusts in the Lurio Belt of northern Mozambique. The driving force for this movement is considered to be related to the late Pan-African collisional events elsewhere in the growing supercontinent.
The Salina del Fraile pull-apart basin, northwest Argentina
Authors: Reijs J. , McClay K.
Published: April 2008

The Salina del Fraile in northwest Argentina is a Pliocene to Recent pull-apart basin developed at a releasing stepover along the NNE-SSW-trending El Fraile sinistral strike-slip fault. The basin is 35 km long and 12 km wide with a characteristic rhomboidal shape and is starved of synkinematic sediments thus providing unique 3D exposures. Prominent basin side-wall fault systems form scarps 700 m high and large tilted fault blocks form a terraced system along the southwest basin sidewall. A short-cut, basin-floor fault transects the pull-apart basin connecting the northwest strand of the El Fraile Fault to the southeast strand. An anticlinal positive flower structure in the northwest of the basin is a relict of early, segmented, fault growth typical of strike-slip fault evolution. Extension faults in the basin floor indicate a NE-SW intrabasinal extension direction during the Pliocene to Recent. The pull-apart basin accommodates an estimated 7.7 km of sinistral displacement along the El Fraile fault system. The morphology and fault architecture of the Salina del Fraile pull-apart can be directly compared to scaled sandbox models of strike-slip pull-apart basins. This first detailed analysis of the Salina del Fraile pull-apart basin provides a model for 3D architecture and evolution of similar pull-apart basins, and may serve as a template for the interpretation of other pull-apart systems.
Building and unfaulting fault-horizon networks
Authors: Rutten K. W. , Verschuren M. A. J.
Published: April 2008

A fault-horizon network is a set of horizon and fault surfaces that is geologically consistent and topologically complete. It is the missing link between seismic interpretation and reservoir modelling. Building the fault-horizon network from a seismic interpretation can be a fast and automatic process. The fault slip vector field embedded in the network is used to construct tip points and fault branches, and as a powerful tool to validate network quality.

A novel, very fast method is introduced to unfault the network, i.e. to remove fault-related strain. It allows further validation of the geological consistency of the network, and it delivers an unfaulted framework for reservoir modelling. The unfaulting method approximates the three-dimensional kinematics related to faulting with the simplest possible analytical description that can be calculated from the geometry of the fault-horizon network and its fault slip vector field.

The construction of ‘fault swallows’ at branching faults decomposes discontinuous slip and strain at branch lines into continuous segments. With this decomposition, the unfaulting method can handle complex branching fault situations.

The methods in this paper have been developed for normal faults in deltaic areas, but they can easily be extended to other geological settings.
A four-year study of shear-wave splitting in Iceland: 2. Temporal changes before earthquakes and volcanic eruptions
Authors: Volti T. , Crampin S.
Published: April 2008

This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a build-up of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between
M
3.5 and
M
5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a
M
5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of
in situ
rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.
Segment linkage during evolution of intracontinental rift systems: insights from analogue modelling
Authors: Tentler T. , Temperley S.
Published: April 2008

On the basis of the effective scale-independence of brittle structures, from microcracks to regional fault systems, we have used analogue centrifuge models to provide insights into the initiation and evolution of continental active rifts, with a single dilational fracture segment representing a prototype rift segment. In the models, which are physically and dynamically scaled, a semi-brittle compound material, with flexural rigidity, was devised to simulate the lithosphere as a single layer. This is justified by the fact that, at the largest scale, the lithosphere behaves as a single viscoelastic, flexurally rigid unit. Silicon polymers of two different viscosities and densities represent the asthenosphere. A parallelepiped of lower viscosity and lower density material is embedded within and near the base of the second polymer that fills up most of the model box. The former is activated as a plume-like diapir that rises, spreads laterally and ultimately generates extensional stress in the semi-brittle layer on top. In terms of model fracture distributions, both narrow and wide failure modes were achieved, analogous to narrow and wide modes of rifting. The processes of fracture initiation, propagation and coalescence are the same for each mode. During the early stages of model runs, fracture initiation is more important than the propagation of existing segments in relieving stress, although the latter dominates during later stages. The key parameters of overlap, offset, obliquity and propagation angle, defined and illustrated, are used to distinguish different ways in which pairs of fractures coalesce. We recognize three distinct types of coalescence (type 1, type 2 and type 3) involving both tip-to-tip and tip-to-sidewall linkages. These are described and both graphically and statistically discriminated. Whether narrow or wide mode, an intracontinental rift system consists of a number of discrete fault-bound rift segments. Similar types of interactions to those in the models have been identified between pairs of rift segments from the Cenozoic Baikal and East African rift systems, and the Mesozoic to early Tertiary Central African rift system. The factors that control the type of coalescence between natural rift segments would appear to be precisely those that govern linkages of fracture segments in the models.
Erosional forcing of basin dynamics: new aspects of syn- and post-rift evolution
Authors: Burov E. , Poliakov A.
Published: April 2008

We revisit a number of important topics associated with the problem of interactions between surface and subsurface processes during syn- and post-rift evolution. To demonstrate the importance of these interactions and to verify a number of earlier ideas on rift evolution, we use a fully coupled three-fold mechanical behaviour, surface processes, heat transport numerical model, which combines brittle-elastic-ductile rheology, fault localization, erosion and sedimentation mechanisms. The model simulates fault formation causing brittle strain localization. Fault distribution and evolution are thus outputs of the model, allowing for new, geologically sensible constraints on the results. The numerical algorithm accounts for ‘true’ surface erosion/sedimentation, that is, the numerical elements are eliminated (eroded) and created (sedimented) with respective changes in properties. The results show that sedimentation in the basin and erosion on the rift flanks strongly control the mode of extension. In particular, active erosion/sedimentation on the synrift phase results in more pronounced thinning and widening of the basin, so that the apparent coefficients of extension increase by a factor of 1.5–2. Surface loading/unloading results in lithospheric flexure. Flexural stresses in places of maximum bending exceed lithospheric strength and create zones of localized weakening that partly or completely compensate strengthening due to cooling in the post-rift phase, when the subsidence rates also accelerate. Erosional unloading on the rift shoulders has the opposite effect, producing local strengthening and flexural rebound. Pressure gradients induced by subsidence/rebound result in lower crustal flow that controls 20–30% of subsidence rates, stability of the rift shoulders and drives some post-rift extension or compression. By taking account for the intermediate and lower crustal rheology, new explanations for some synrift phase effects such as polyphase subsidence of the basement provoked by crustal flow and ‘switching’ of the level of necking from one competent lithospheric level to another are suggested. Syn- and post-rift stagnation, upward and downward accelerations find a natural explanation within our model without the necessity to invoke external mechanisms.
Modelling and processing of 3D seismic data collected over the overlapping spreading centre on the East Pacific Rise at 9° 03′ N
Authors: Hobbs R. , Tong C. H. , Pye J.
Published: April 2008

Oceanic ridges are important as the locus for the generation of new crust associated with the movement of plates on the Earth’s surface. The geometry of the axial magma chamber (AMC) under these ridges is poorly constrained. This is especially true for the AMC under an overlapping spreading centre (OSC) where the ridge undergoes a small lateral offset, typically less than 8 km. One such OSC exists on the East Pacific Rise at 9° 03′ N. Both 2D and more recently 3D seismic reflection surveys have been conducted over the area in an attempt to establish the relationship of the AMC with the two overlapping limbs of the axial rise. A major problem for imaging the AMC is caused by the seafloor, which has a large velocity contrast and has rapid changes in topography. This presents a major challenge for the processing of data. This paper aims to show that the understanding the effects of acquisition and processing can be facilitated by accurate 3D modelling of the seismic wavefield. In this paper we use a modelling method based on complex-screens, which is both robust and fast for large 3D models with deep water and diffractive interfaces. We compare synthetics with real data and explore the sensitivity of processing 3D data from shot gather to final 3D depth-migrated image. We show that the combination of severe seabed topography and 3D velocity variation within the oceanic crust causes distortion and complex focusing of the reflected energy. Also, for data with only sparse reflectivity it is not possible to construct an adequate velocity model for depth imaging from the reflection data alone. This problem may be overcome if there is an alternative method to determine velocity, e.g. refraction data, but care must be exercised during interpretation given the different resolutions of the two datasets. Lessons from the modelling exercise can be used to aid the interpretation of the real data and examples are shown for comparison; however, the geological implications of the results are outside the scope of this paper.
Integrated 3D geomechanical modelling for deep subsurface deformation: a case study of tectonic and human-induced deformation in the eastern Netherlands
Authors: Van Wees J. D. , Orlic B. , Van Eijs R. , Zijl W. , Jongerius P. , Schreppers G. J. , Hendriks M. , Cornu T.
Published: April 2008

Current advances in finite element computer codes and increase in computer power theoretically allow quantitative modelling of the geomechanical effects of hydrocarbon depletion from reservoirs. Here we show that it is technically possible to incorporate the full complexity of the 3D geological structure of a reservoir including faults into geomechanical models. In the workflow GOCAD is used for the structural modelling, integrated with DIANA for the geomechanical calculations.

A case study on the Roswinkel gas field in the eastern Netherlands illustrates the working methodology. The case study clearly shows the strong dependency of gas depletion deformation effects on the prevailing tectonic stress field. Our models for gas depletion predict a stabilization of the stress field (further away from failure) for reservoirs in compressive and strike-slip regimes. On the other hand extensional stress regimes will result in failure of the reservoir, in agreement with observed earthquakes, provided that (a) the reservoir material or existing faults are weak and (b) the state of stress is close to failure of the material.

The Roswinkel field, which is marked by a high abundance of earthquakes occurring after gas depletion started, is therefore most likely marked by an extensional tectonic regime and by geomechanically weak rock or pre-existing faults. According to base Tertiary fault displacements and the
World Stress Map
, the extension (minimum horizontal principal stress) is most likely NE-SW oriented.
Dynamics of magma degassing
Authors: Sparks R. S. J.
Published: April 2008

Gas exsolution and segregation are fundamental controls on eruption dynamics and magma genesis. Basaltic magma loses gas relatively easily because of its low viscosity. However, bubbles grown by decompression and diffusion during magma ascent are too small to segregate. Coalescence, however, can create bubbles big enough for gas to escape from the rising basalt magma. In evolved magmas, such as andesite and rhyolite, high viscosity prevents bubbles rising independently through the magma. The original gas content of magma erupted as lava is commonly the same as that erupted explosively, so that a gas separation mechanism is required. A permeable magma foam can form to allow gas escape once bubbles become interconnected. Magma permeabilities can be much higher than wall-rock permeabilities, and so vertical gas loss can be an important escape path, in addition to gas loss through the conduit walls. This inference is consistent with observations from the Soufrière Hills Volcano, Montserrat, where gas escapes directly from the dome, and particularly along shear zones (faults) related to the conduit wall. Dynamical models of magma ascent have been developed which incorporate gas escape. The magma ascent rate is sensitive to gas escape, as the volume proportion of gas affects density, magma compressibility and rheology, resulting in both horizontal and vertical pressure gradients in the magma column to allow gas escape. Slight changes in gas loss can make the difference between explosive and effusive eruption, and multiple steady-state flow states can exist. In certain circumstances, there can be abrupt jumps between effusive and explosive activity. Overpressures develop in the ascending magma, caused primarily by the rheological stiffening of magma as gas exsolves and crystals grow. A maximum overpressure develops in the upper parts of volcanic conduits. The overpressure is typically several MPa and increases as permeability decreases. Thus, the possibility of reaching conditions for explosions increases as permeability decreases, both due to overpressure increase and the retention of more gas. Models of magma ascent from an elastic magma chamber, combined with concepts of permeability and overpressure linked to degassing, provide an explanation for the periodic patterns of dome growth with short-lived explosive activity, as in the 1980–1986 activity of Mount St Helens. Degassing of magma in conduits can also cause strong convective circulation between deep magma reservoirs and the Earth’s surface. Such circulation not only allows degassing to occur from deep reservoirs, but may also be a significant driving force for crystal differentiation.
Water diffusion in natural potassic melts
Authors: Freda C. , Baker D. R. , Romano C. , Scarlato P.
Published: April 2008

Water diffusion experiments were performed on a trachytic melt from the Agnano-Monte Spina explosive eruption (Phlegrean Fields, South Italy). Experiments were run in a piston cylinder apparatus at 1 GPa pressure, at temperatures from 1373 to 1673 K and for durations of 0 to 255 s, using the diffusion-couple technique. Water concentration profiles were measured by Fourier transform infrared spectrometry. Water diffusion coefficients at different temperatures and water concentrations were calculated from the total water profiles, using the Boltzmann-Matano technique.

Over the investigated range of temperatures and water concentrations, the diffusivity of water in potassic melts (
D
water
), m
2
/s can be described by Arrhenius equations that can be generalized for water concentrations between 0.25 and 2 wt% as follows:

D
water

=
exp
(
−
11.924
−
1.003
ln

C

H
2

O

)
exp
(

−
(
exp
(
11.836
−
0.139
ln

C

H
2

O

)
)

RT

)

where
C

H
2
O

is the water concentration in wt%,
R
is 8.3145 (J K
−1
mol.
−1
) and
T
is the temperature in Kelvin. Water diffusivities in trachytic melts were compared with water diffusivities in rhyolitic and basaltic melts. The activation energies for water diffusivity in trachyte and basalt are comparable, and higher than the haplogranitic melt. This results in a convergence of water diffusion coefficients in all melts at lower (magmatic) temperatures.
A model for the saturation of C-O-H-S fluids in silicate melts
Authors: Moretti R. , Papale P. , Ottonello G.
Published: April 2008

The behaviour of volatile components in magmas is crucial for magmatic and volcanic processes, from the deep regions of magma generation and storage to the shallow regions of magma eruption and emplacement. Water, carbon dioxide, and sulphur compounds are the main volatile components in natural magmas, generally comprising more than 99% of the volcanic gases released before, during, and after eruption. We have set up a method to calculate the chemical equilibrium between a fluid phase in the C-O-H-S system and a silicate liquid with a composition defined by ten major oxides. The method is based on previous models for the saturation of H
2
O-CO
2
fluids (Papale 1997, 1999) and sulphur solubility (Moretti 2002) in silicate liquids, and for the fugacities of components in fluids with complex composition (Belonoshko
et al.
1992). The model calculations provide estimates of the partitioning of H
2
O, CO
2
and S between the silicate liquid and the coexisting fluid, and the composition of the fluid phase in terms of H
2
O, CO
2
, SO
2
, and H
2
S, as a function of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and total amount of each volatile component in the system. Model calculations are presented for silicate liquids with tholeiitic and rhyolitic composition, oxygen fugacities in the NNO ± 2 range, and pressures from a few hundred MPa to atmospheric, with the simplifying assumption that no reduced or oxidized sulphur-bearing solid or liquid phases nucleate or separate from the liquid-gas system. The results are in good agreement with the bulk of experimental data from the literature, and show the well-known minima in sulphur saturation contents as a function of oxygen fugacity, the mutual effects of volatiles on their saturation contents, and the complex relationships between the saturation surface of multi-component fluids and the liquid composition, volatile abundance, and pressure-temperature-oxygen fugacity conditions. The new model therefore represents a powerful tool for the prediction of multi-component gas-liquid equilibria in natural magmatic systems, for the simulation of magmatic and volcanic processes, and for the interpretation of data on the degassing of magma bodies and composition of volcanic plumes, provided that the assumptions on which the model rests, first of all that no separation of additional S-bearing phases occurs, are satisfied.
Particles from the plume of Popocatépetl volcano, Mexico — the FESEM/EDS approach
Authors: Obenholzner J. H. , Schroettner H. , Golob P. , Delgado H.
Published: April 2008

Magma-wall-rock interaction contributes gases to evolving magmatic systems, and removes volatiles into the country rock. These processes happen at depth, far away from direct observation. Micro-analysis of particles collected from volcanic plumes can provide information about these processes. For Popocatépetl volcano, scanning electron microscope (SEM) and field emission gun SEM (FESEM/EDS) analysis of contact-metamorphosed particles from fallout ash reveal the presence of wollastonite, hercynite and glass of non-volcanic, contact-metamorphic origin. Condensates from the passively degassing plume show a wide variety of chemical elements and are rich in phosphorus, indicating a possible non-magmatic source for this element.
Elevation effects in volcano applications of the COSPEC
Authors: Gerlach T. M.
Published: April 2008

Volcano applications commonly involve sizeable departures from the reference pressure and temperature of COSPEC calibration cells. Analysis shows that COSPEC SO
2
column abundances and derived mass emission rates are independent of pressure and temperature, and thus unaffected by elevation effects related to deviations from calibration cell reference state. However, path-length concentrations are pressure and temperature dependent. Since COSPEC path-length concentration data assume the reference pressure and temperature of calibration cells, they can lead to large errors when used to calculate SO
2
mixing ratios of volcanic plumes. Correction factors for COSPEC path-length concentrations become significant (
c
. 10%) at elevations of about 1 km (e.g. Kilauea volcano) and rise rapidly to
c
. 80% at 6 km (e.g. Cotopaxi volcano). Calculating SO
2
mixing ratios for volcanic plumes directly from COSPEC path-length concentrations always gives low results. Corrections can substantially increase mixing ratios; for example, corrections increase SO
2
ppm concentrations reported for the Mount St Helens, Colima, and Erebus plumes by 25–50%. Several arguments suggest it would be advantageous to calibrate COSPEC measurements in column abundance units rather than path-length concentration units.
Carbon dioxide emissions from fumarolic ice towers, Mount Erebus volcano, Antarctica
Authors: Wardell L. J. , Kyle P. R. , Campbell A. R.
Published: April 2008

Degassing at Mount Erebus occurs as a plume from a persistent convecting anorthoclase phonolite lava lake, and by flank degassing through warm ground and fumarolic ice towers within the summit caldera. The fumarolic ice towers offer a unique and simple approach to quantifying the flank CO
2
emissions. Carbon dioxide effluxes were determined at openings in the ice towers by measuring the CO
2
concentration, air-flow velocity, and size of the exit orifice. Fluxes ranged from <0.0001 to 0.034 kg s
−1
at 43 actively degassing ice towers. Small patches of steaming warm ground contributed 0.010 kg s
−1
. The δ
13
C isotopic compositions of the CO
2
samples ranged from −2.1 to −4.7‰, suggesting a magmatic origin for the CO
2
. Fumarolic ice towers allow diffuse degassing to be visually identified, providing a strong advantage in determining the total flux rate of these passive emissions. The estimated output of flank CO
2
degassing is 0.46 kg s
−1
(40 Mg d
−1
). Compared with direct airborne measurements of the volcanic plume, passive flank emissions constitute less than 2% of the total volcanic CO
2
budget emitted from Mount Erebus.
FTIR remote sensing of fractional magma degassing at Mount Etna, Sicily
Authors: Burton M. , Allard P. , Murè F. , Oppenheimer C.
Published: April 2008

The chemical composition of volcanic gas emissions from each of the four summit craters of Mount Etna was measured remotely in May 2001, using a Fourier transform infrared (FTIR) spectrometer operated on the upper flanks of the volcano. The results reveal constant HCl/HF ratio but distinct SO
2
/HCl and SO
2
/HF ratios in the emitted gases, which, in the light of melt inclusion data for Etna basalts, can be interpreted in terms of escape of gases from partially, and variably, degassed magma at different depths beneath the summit. Gases released from the three main summit craters (Bocca Nuova, Voragine, and Northeast) had an identical composition, controlled by bulk degassing of a single magma body that had previously lost
c.
25% of its original sulphur. The similar gas composition at all three main craters suggests that these are connected to a central conduit system that branches at relatively shallow depth. Measurements of the bulk volcanic plume on the same day,
c.
7 km downwind, show that degassing from these craters dominated the total gas output of the volcano, and that no significant chemical evolution occurred within the plume over a time-scale of
c.
12 min. Weaker gas emissions from the Southeast crater were comparatively depleted in SO
2
(SO
2
/HCl and SO
2
/HF ratios a factor of two lower), implying that this crater is fed either by a separate conduit or by a branch of the central conduit whose geometry favours solubility-controlled volatile fractionation. Still lower SO
2
/HCl and SO
2
/HF ratios measured for residual degassing of a lava flow erupted from the Southeast crater verify the lower solubility and earlier escape of sulphur compared to halogens at Etna. Fractional magma degassing is also implied by strong chemical contrasts between the bulk volcanic plume and fissure gas emissions measured during the July-August 2001 flank eruption. These results highlight the ability of FTIR spectrometry to detect fine spatial and temporal variations in magma degassing processes, and thereby constrain models of shallow plumbing systems.
Changes in stratospheric composition, chemistry, radiation and climate caused by volcanic eruptions
Authors: Grainger R. G. , Highwood E. J.
Published: April 2008

The primary effect of a volcanic eruption is to alter the composition of the stratosphere by the direct injection of ash and gases. On average, there is a stratospherically significant volcanic eruption about every 5.5 years. The principal effect of such an eruption is the enhancement of stratospheric sulphuric acid aerosol through the oxidation and condensation of the oxidation product H
2
SO
4
. Following the formation of the enhanced aerosol layer, observations have shown a reduction in the amount of direct radiation reaching the ground and a concomitant increase in diffuse radiation. This is associated with an increase in stratospheric temperature and a decrease in global mean surface temperature (although the spatial pattern of temperature changes is complex). In addition, the enhanced aerosol layer increases heterogeneous processing, and this reduces the levels of active nitrogen in the lower stratosphere. This in turn gives rise to either a decrease or an increase in stratospheric ozone levels, depending on the level of chlorine loading.
Environmental impacts of tropospheric volcanic gas plumes
Authors: Delmelle P.
Published: April 2008

Recent studies suggest that the environmental effects of volcanic gas emissions in the lower troposphere have been underestimated. This chapter first briefly summarizes the techniques available for characterizing tropospheric volcanic gas plumes, including the composition and fluxes of emitted gases and aerosols, as well as their atmospheric dispersion. The second part documents the contribution of gas emissions from degassing craters to the composition of the atmosphere, including effects from dry and wet deposition chemistry. The third section deals with the detrimental impacts on vegetation, soils, and groundwater in relation to passive degassing activity. Improved understanding of the impacts of volcanic degassing on the atmospheric and terrestrial environment will require: (1) systematic two-dimensional and three-dimensional measurements of tropospheric volcanic plumes, (2) development of general physical and chemical models to describe the fate of volcanic gases and aerosols during transport in the troposphere, and (3) investigation of the response of diverse ecosystems to volcanogenic air pollution.
Occurrences of hydrocarbons in and around igneous rocks
Authors: Schutter Stephen R.
Published: April 2008

Data on the occurrence of hydrocarbons in and around igneous rocks show them to be global in extent, occurring in over 100 countries worldwide. While this list is not exhaustive, it is possibly the first of its kind to be published, and will serve as a useful source of reference for those wishing to embark on further study.
Fracture formation and evolution in crystalline rocks: Insights from attribute analysis
Authors: McCaffrey K. J. W. , Sleight J. M. , Pugliese S. , Holdsworth R. E.
Published: April 2008

Fractures are ubiquitous in crystalline rocks and control the strength and geophysical and fluid transport characteristics of the Earth’s upper crust. A quantitative description of fracture attributes may constrain models of fracture formation and evolution. In this study, fracture attributes collected from one-dimensional samples across exposures of typical crystalline rocks show comparable variability in fracture size and spacing to sedimentary rocks. Vein thickness and fracture aperture data show predominately power-law distributions. Vein and fracture spacing data are best described by exponential distributions with negative slopes and appear to vary with composition in intrusive rocks. The fracture systems exhibit a range of anti-clustered to clustered patterns, and densities are an order of magnitude higher for joints compared to veins. Fracture clustering data can be used in conjunction with the spatial distributions to provide information on the controlling processes of fracture spacing. We suggest that exponential spacing distribution is produced as a sampling effect for both periodic-spaced and clustered fracture sets. In the examples given here, thermal stress-related joint patterns are distinguishable from tectonic-related fractures in plutonic rocks and fracture density and clustering is increased towards a major reactivated basement fault.
Quantitative characterization of fracture apertures using microfocus computed tomography
Authors: Vandersteen K. , Busselen B. , Van Den Abeele K. , Carmeliet J.
Published: April 2008

Microfocus X-ray computed tomography (μCT) was used as a tool to determine the apertures of a fracture in a cylindrical sample of crinoidal limestone. After scanning, artefacts were removed from the images. Phantom objects were used to establish a calibration relationship between real fracture apertures and fracture aperture measurements on the μCT images. The performance of different procedures for quantitative fracture determination was examined. The calibration relationship was then used to determine the fracture apertures in a naturally fractured sample. A comparison of the μCT technique and a microscope technique shows a good agreement between their results.
Direct imaging of fluid flow in fault-related rocks by X-ray CT
Authors: Hirono T. , Takahashi M. , Nakashima S.
Published: April 2008

Faults profoundly affect patterns and rates of fluid flow and solute transport in the geological environment. They may act as conduits, barriers or combined conduit-barrier systems. In order to elucidate the relationship between fluid flow properties and deformation mechanisms of fault-related rocks, we applied X-ray CT during laboratory permeameter measurements for direct imaging of fluid flow during permeability testing. A KI solution, which has high X-ray attenuation values, was used as a contrast medium for the advection imaging. Three-dimensional fluid flow distributions were measured for the studied fault-related rocks. Fault zones characterized by independent particulate flow as deformation mechanism act as conduits for fluid flow, whereas cataclastic fault zones act as barriers.