bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theoretical aspects of particle movement in flowing magma: implications for the anisotropy of magnetic susceptibility of dykes
Authors: Cañón-Tapia Edgardo , Chávez-Álvarez J. 
Published: April 2008

Abstract

Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that the axes of maximum susceptibility (
k
max
) should define an opposed imbrication pointing along the direction of magma flow, and that this orientation should be preserved along the dyke. This assumption is partially based on a model predicting the orientation of ellipsoidal particles floating in a moving liquid although the model actually predicts a cyclic movement of the particles that has been overlooked in msot AMS studies without further justification. The consequences of considering the full rotation of the ellipsoidal particles, as actually predicted by the theory, in the expected AMS of dykes are examined in this work. The complete version of the motion of ellipsoidal particles is then incorporated in a model of magma movement that takes into consideration the distribution of shear deformation within the dyke as predicted from the velocity gradient of the moving magma. Results of this model show that both particle elongation and the amount of shear that is sampled will affect the quality of the AMS results. By paying attention to the systematic variations of the AMS predicted by the theory, however, it is possible to devise sampling schemes that can be used to add more confidence to the interpretation of the AMS results. Although based on an idealized scenario of magma movement within a dyke, the model developed here explains satisfactorily the sometimes observed variation of AMS along flow direction in one dyke, and provides a simple explanation for many of the ‘abnormal’ magnetic fabrics that have been reported in dyke swarms around the world.
Phyllosilicate preferred orientation as a control of magnetic fabric: evidence from neutron texture goniometry and low and high-field magnetic anisotropy (SE Rhenohercynian Zone of Bohemian Massif)
Authors: Chadima Martin , Hansen Anke , Hirt Ann M. , Hrouda František , Siemes Heinrich 
Published: April 2008

Abstract
The low- and high-field magnetic anisotropy (AMS, HFA) of the Rhenohercynian mudstones and greywackes is compared to the theoretical anisotropy calculated from neutron texture goniometry measurements. The magnetic anisotropy is predominantly carried by the paramagnetic phyllosilicates in the form of chlorite/mica stacks and the ferromagnetic contribution is insignificant. The respective principal directions of the theoretical anisotropy and the AMS and HFA are sub-parallel; magnetic foliation reflects the orientation of the maximal concentration of phyllosilicate basal planes, magnetic lineation is subparallel to the intersection axis of those planes. For the purpose of quantitative comparison, the infrequently used standard deviatoric susceptibility as a measure of the HFA degree is employed. A very good linear correlation of the degree of theoretical anisotropy and the measured AMS and HFA is found. The prolate and oblate shapes of the respective fabric ellipsoids are reasonably well correlated. Neutron texture goniometry justifies the use of the conventional magnetic anisotropy technique for the assessment of the mineral fabric of studied rocks. When compared with other works relating the magnetic anisotropy to the mineral preferred orientation (examined by e.g. U-stage or X-ray texture goniometry) neutron texture goniometry seems to be a preferable and very precise method fabric analysis.
Sub-fabric identification by standardization of AMS: an example of inferred neotectonic structures from Cyprus
Authors: Hamilton Thomas D. , Borradaile Graham J. , Lagroix France 
Published: April 2008

Abstract

Calcite petrofabrics are sensitive to weak strains, possibly being the most sensitive classical petrofabric indicator. Thus, calcareous sediments may reveal stress trajectories in neotectonic environments. Calcite aligns by crystal-plastic deformation and pressure solution produce corresponding alignments in accessory clay minerals and magnetite (possibly fossil-bacterial). Their alignments are rapidly and precisely detected by anisotropy of low field magnetic susceptibility (AMS) with net magnetic fabrics, which blend diamagnetic contributions from matrix calcite (diamagnetic bulk susceptibility κ ∼ −14 μSI), accessory clay minerals (κ = 100 to 500 μSI) and sometimes trace magnetite (κ > 2 SI). Their relative abundances and different anisotropies must be considered in interpreting AMS orientations, nevertheless our study reveals orientation distributions of AMS axes in sub-areas and regions that are sensibly interpreted as palaeostress trajectories in Neogene and Quaternary strata. The AMS axes may be correlated with the orientation of faults, plate-motion vectors and seismic solutions. Large samples (1090 specimens from 419 sites) are treated by different statistical approaches (‘standardization’) to emphasize or suppress the contribution of subfabrics with anomalous mean susceptibility. A sub-sample of 254 specimens from 219 sites, from different sub-areas was investigated by anisotropy of anhysteretic remanence (AARM), which isolates the orientation distributions of magnetite. Magnetic fabrics are mostly of the L-S kind with the magnetic lineations compatible with gravitational stretching of the sedimentary cover away from the Troodos massif and orthogonal to the principal faults and graben. The L-direction (
k
max
) shows a smooth variation in orientation, through the sub-areas, directed radially from the Troodos massif and the S-components of the magnetic fabrics are inclined gently to the bedding, compatible with vergence toward the Cyprean Arc to the S and SW of Cyprus.
Optimal elicitation of probabilistic information from experts
Authors: Curtis Andrew , Wood Rachel 
Published: April 2008

Abstract
It is often desirable to describe information derived from the cumulative experience of human experts in a quantitative and probabilistic form. Pertinent examples include assessing the reliability of alternative models or methods of data analysis, estimating the reliability of data in cases where this cannot be measured, and estimating ranges and probable distributions of rock properties and architectures in complex geological settings.
This paper presents a method to design an optimized process of elicitation (interrogation of experts for information) in real time, using all available information elicited previously to help in designing future elicitation trials. The method maximizes expected information during each trial using experimental design theory. We demonstrate this method in a simple experiment in which the conditional probability distribution or relative likelihood of a suite of nine possible 3-D models of fluvial-deltaic geologies was elicited from a geographically remote expert. Although a geological example is used, the method is general and can be applied in any situation in which estimates of expected probabilities of occurrence of a set of discrete models are desired.
Interactive inverse methodology applied to stratigraphic forward modelling
Authors: Wijns C. , Poulet T. , Boschetti F. , Dyt C. , Griffiths C. M. 
Published: April 2008

Abstract
An effective inverse scheme that can be applied to complex 3-D hydrodynamic forward models has so far proved elusive. In this paper we investigate an interactive inverse methodology that may offer a possible way forward. The scheme builds on previous work in linking expert review of alternate output to rapid modification of input variables. This was tested using the SEDSIM 3-D stratigraphic forward-modelling program, varying nine input variables in a synthetic example. Ten SEDSIM simulations were generated, with subtle differences in input, and five dip sections (fences) were displayed for each simulation. A geoscientist ranked the lithological distribution in order of similarity to the true sections (the true input values were not disclosed during the experiment). The two or three highest ranked simulations then acted as seed for the next round of ten simulations, which were compared in turn. After 90 simulations a satisfactory match between the target and the model was found and the experiment was terminated. Subsequent analysis showed that the estimated input values were ‘close’ to the true values.
Glacier-permafrost interaction in Arctic and alpine mountain environments with examples from southern Norway and Svalbard
Authors: Etzelmüller Bernd , Hagen Jon Ove 
Published: April 2008

Abstract
The interaction between glaciers and permafrost was long ago addressed for glaciers in Arctic regions. Analogies from modern environments have been used to understand landform development at the margins of Pleistocene ice sheets. During more recent decades many systematic measurements of permafrost in boreholes, geophysical soundings and temperature monitoring have revealed permafrost to be more abundant in many more high-mountain areas than previously thought. This suggests that permafrost may be a governing factor not only for periglacial landform evolution in these areas, but also, given the potential for glacier-permafrost interaction, for glacial landform generation. This paper presents and discusses observation and study results on the geomorphological significance of the interrelationship between glaciers and permafrost, in relation to geomorphological processes, landform generation and response of the system to climate fluctuations.
Glacier-permafrost interactions and glaciotectonic landform generation at the margin of the Leverett Glacier, West Greenland
Authors: Waller Richard I. , Tuckwell George W. 
Published: April 2008

Abstract

This paper describes the key characteristics of a proglacial moraine complex at the Leverett Glacier, western Greenland. The presence of a large stream-cut exposure allowed the examination of its internal structure, as well as its surface geomorphology. It is composed of a variety of ice and sediment facies, including debris-poor ice, ice-rich diamicton and ice-rich gravel. These units are glaciotectonized, with the exposure featuring a major fault and associated drag fold, a planar, erosional unconformity, and a variety of small-scale folds. Various interpretations are considered, including the possibility that the sequence represents a buried basal ice layer. However, it is argued that the structural characteristics are best explained by a two-phase model involving ice advance and proglacial or ice-marginal compression, followed by overriding and subglacial deformation and erosion, tentatively related to ice advance after the Holocene Hypsithermal (
c.
4900–3000 calendar years
bp
). The polygenetic origin of this ice-marginal, glaciotectonic landform contrasts with the majority of Arctic push-moraines, which are largely considered the result of proglacial deformation and the stacking of imbricate thrust sheets of frozen sediment. This contrast may reflect differences in the thickness and spatial continuity of permafrost within the glacier foreland, and adds to the range of ice-marginal landforms associated with glacier-permafrost interactions.
Experimental simulation of ice-wedge casting: processes, products and palaeoenvironmental significance
Authors: Harris Charles , Murton Julian B. 
Published: April 2008

Abstract

In six experiments, model ice wedges 150 mm high and ≤50 mm wide were thawed from the surface downward at 30
g
in a geotechnical centrifuge, simulating ice-wedge casting during progressive active-layer deepening through permafrost. The frozen host soils ranged from sand to silt to clayey silt. Resulting ice-wedge pseudomorphs indicated that the degree of deformation during casting was determined by factors that control thaw consolidation. Hence, deformation increased as the host sediments became finer-grained and more ice-rich. Thaw of host sand at 15 and 20% gravimetric ice content and host silt at 20 and 40% ice content resulted in the formation of partially-developed ice-wedge pseudomorphs with an upper ‘plug’ of sediment derived largely from the cover soil, a central tunnel and a basal plug of sediment. Thaw of host clayey silt at 30 and 60% ice content resulted in the formation of fully developed pseudomorphs that were significantly narrower and shorter than the initial ice wedges. The experiments support the hypothesis that ice-wedge pseudomorphs tend to be better preserved in coarser-grained sediments that are not ice-rich and therefore deform little during thaw of constituent ice wedges. This selective preservation has probably led to underestimation of ice-wedge pseudomorphs in fine-grained soils that were originally ice-rich and therefore a bias towards reconstructing cold temperatures.
Fracture and vein patterns as indicators of deformation history: a numerical study
Authors: Koehn Daniel , Arnold Jochen , Passchier Cees W. 
Published: April 2008

Abstract
Fracture and vein patterns in the brittle crust of the Earth contain information on the stress and strain field during deformation. Natural examples of fracture and vein patterns can have complex geometries including combinations of extension and conjugate shear fractures. Examples are conjugate joint systems that are oriented with a small angle to the principal stress axis and veins that show an oblique opening direction. We developed a discrete numerical model within the modelling environment ‘Elle’ to study the progressive development of fractures in two dimensions. Results show that pure shear deformation alone can produce complex patterns with combinations of extension and shear fractures. These patterns change in geometry and spacing depending on the Young’s modulus of the deforming aggregate and the initial noise in the system. A complex deformation history, including primary uniaxial loading of the aggregate that is followed by tectonic strain, leads to conjugate shear fractures. During progressive deformation these conjugate shear fractures may accommodate extensional strain or may be followed by a secondary set of extension fractures. The numerical patterns are consistent with joint, fault and vein geometries found in natural examples. The study suggests that fracture patterns can record complex deformation histories that include primary uniaxial loading due to an overlying rock sequence followed by tectonic strain.
Contrasting microstructures and deformation mechanisms in metagabbro mylonites contemporaneously deformed under different temperatures (
c.
650 °C and
c.
750 °C)
Authors: Baratoux L. , Schulmann K. , Ulrich S. , Lexa O. 
Published: April 2008

Abstract

Deformation mechanisms of amphibole and plagioclase were investigated in two metagabbroic sheets (the eastern and western metagabbros from the Staré Město belt, eastern Bohemian Massif), using petrology, quantitative microstructural and electron back-scattered diffraction methods. After the gabbroic pyroxene was replaced by amphibole, both gabbroic bodies became progressively deformed. The eastern metagabbros were deformed under temperature of
c.
650 °C and the western metagabbros under
c.
750 °C. Subgrain rotation and dislocation creep, characterized by strong crystallographic and shape preferred orientations, operated in plagioclase of the eastern belt during the early stages of deformation. Subsequent randomizing of plagioclase crystallographic preferred orientation is interpreted to be due to grain boundary sliding in the mylonitic stage. Large (50–150 μm) grain sizes during the mylonitic stages are interpreted to be due to low strain rates. Amphibole is stronger and deforms cataclastically, leading to important grain size reduction when the bulk rock strength drops substantially. In the western belt, plagioclase deformed by dislocation creep accompanied by grain boundary migration (possibly chemically induced) while heterogeneous nucleation and syndeformational grain growth in conjunction with dislocation creep were typical for amphiboles.
Dynamic recrystallization and strain softening of olivine aggregates in the laboratory and the lithosphere
Authors: Drury Martyn R. 
Published: April 2008

Abstract
The effects of dynamic recrystallization on the deformation mechanisms and rheology of olivine aggregates in the laboratory and the lithosphere are reviewed in this paper. The low-strain rheology of olivine is well documented; however, deformation in the lithosphere often involves large strains. Large strain experiments show that recrystallization can result in both hardening and softening during deformation. Moderate strain softening in experimental shear and torsion can be explained by the operation of dislocation-accommodated grain boundary sliding in bands of fine recrystallized grains.
Data on the temperature dependence of recrystallized grain size are needed to extrapolate the effects of dynamic recrystallization to the lithosphere. Theories of dynamic recrystallization suggest that grain size is strongly stress dependent and moderately temperature dependent. A re-analysis of experimental grain size data indicates that the recrystallized grain size is temperature independent for olivine aggregates with low water content (<300 ppm H/Si).
Rheological regime maps have been constructed for the lithospheric mantle. The maps suggest that grain size sensitive power law creep, involving both grain boundary sliding and dislocation creep, will produce strong strain softening, greater than found so far in experimental studies, in dry and wet lithosphere shear zones.
Continental subduction and exhumation: an example from the Ulten Unit, Tonale Nappe, Eastern Austroalpine
Authors: Ranalli Giorgio , Martin Silvana , Mahatsente Rezene 
Published: April 2008

Abstract

Some exhumed complexes in collisional belts consist of continental basement containing slivers of mafic and ultramafic material showing evidence of UHP metamorphism (
P c.
3 GPa). Their
PTt
history can be interpreted in terms of subduction of continental material to depths ≥ 100 km and subsequent exhumation.

This type of tectonic history is illustrated by the Late Palaeozoic evolution of the Ulten Unit, Tonale Nappe, Eastern Austroalpine. The upper crustal felsic component (
c.
80% by volume) incorporated mafic material at the trench, and peridotitic material at deeper levels in the subduction zone. The peridotites show evidence of a
P
-increasing,
T
-decreasing path before incorporation in the felsic material, compatible with flow in the mantle wedge above the subducting slab. After emplacement of the peridotites, which occurred at or near peak metamorphic conditions (
P
≥ 2.7 GPa,
T
≥ 850 °C), the complex underwent a two-stage pre-Alpine exhumation path: a first, fast stage (
c.
0.1–1 cm a
−1
), lasting
c.
30 Ma and bringing rocks from depths ≥ 100 km to approximately 25 km; and a second, slow stage (
c.
0.01–0.1 cm a
−1
), lasting
c.
100 Ma and bringing rocks to depths <20 km.

The subduction of felsic material to the required depths can be modelled by analysing the time-evolution of negative buoyancy, which confirms that relatively light continental upper crust can be subducted to depths > 200 km if attached to a mature oceanic slab that does not break-off during the early stages of continental subduction. The first exhumation stage can be accounted for by buoyancy-driven tectonic extrusion of continental slices along the subduction channel during continuing subduction. A force balance analysis shows that such a mechanism is compatible with the rheology of felsic and intermediate rocks at high temperature. The second exhumation stage is compatible with isostatic rebound and tectonic denudation following slab break-off.
The conclusion that fast exhumation occurs during continuing subduction and before slab break-off is in accordance with the observed rates, which show fast movement of the rising slices with respect to the surrounding material. Slab break-off, on the other hand, generates a long-wavelength gentle upwarping of the overlying region, which is more compatible with later and slower exhumation rates.
Synergistic effects of melting and deformation: an example from the Variscan belt, western France
Authors: Brown Michael 
Published: April 2008

Abstract

In the Vannes and St. Nazaire regions of the central part of the Domaine Sud-Armoricain, Variscan belt, western France, the lowermost tectonic unit is exposed as structural culminations composed of supracrustal migmatites that were deformed and metamorphosed along a multistep clockwise
P-T
path during Carboniferous time; peak
P-T
was around 800 °C at 9 kbar. The strain field that emerged under subsolidus conditions during prograde metamorphism controlled the initial distribution of granite melt produced by suprasolidus mica breakdown; the limited retrograde reaction of peritectic garnet indicates that melt loss occurred around the metamorphic peak. A second episode of melt production occurred during the retrograde evolution due to a decompression event that led to interconnection of melt in a mesoscale network of deformation bands and formation of ductile opening-mode fractures, as evidenced by layer-parallel and transverse leucosomes linked with petrographic continuity to granite in dykes. The preservation of peritectic cordierite with only limited associated leucosome and the occurrence of pucker structures without leucosome both indicate that melt loss occurred during the second event. Dykes vary from centimetric (common) to hundreds of metres in width (rare), and exhibit scale-invariance over a limited range of measurements; larger dykes are inferred to have fed upper crustal plutons. Melt extraction may have been a self-organized critical phenomenon, but this remains to be demonstrated satisfactorily in nature. Fugitive melt was trapped in the vicinity of the brittle—ductile transition zone and emplaced laterally along horizons reactivated as extensional detachments. A feedback relation is postulated between dextral transtensive deformation, decompression melting and lower crustal doming, and between dome amplification, melt extraction and emplacement in developing extensional detachments and core complex formation.
High-resolution borehole image analysis in a slope fan setting: examples from the late Miocene Mt Messenger Formation, New Zealand
Authors: Johansson M. 
Published: April 2008

Abstract
High-resolution resistivity image surveys and core were acquired from two wells 150 m apart within the late Miocene Mt Messenger Formation. The wells were located at Pukearuhe Beach, Taranaki, New Zealand and were situated along strike of the palaeoslope. The formation is interpreted as a slope fan setting and the interval logged was dominated by a major channel that incised into older channel-levee and overbank fan facies. The scour surface and the distinctive channel-fill enabled the two wells, Pukearuhe Central and Pukearuhe North, to be correlated with the local outcrop. The sedimentary deposits within the studied section were divided into three facies associations: channel-fill, channel levee and overbank deposits. The channel deposits exhibited similar facies in both wells. The dip data from the image logs indicate that the original channel was in-filled by sedimentation dipping towards the NW, the same orientation as the regional palaeoslope. In contrast, analysis of the levee and overbank deposits indicated little correlatability between the two wells, with beds dipping, locally up to 10°, in directions ranging from up-slope (SE) to oblique (NE) to the palaeoslope. The variation in dip directions in the levee and overbank deposits is attributed to scouring and infilling of irregular topography at and beyond the channel margin. As a reservoir prospect the thicker channel sands would be a preferential play to the laterally discontinuous thin beds.