bluuebunny/crossref_metadata_2025_split huggingface dataset:
Iceberg scours in the geological record: examples from glacial Lake Agassiz
Authors: Woodworth-Lynas C. M. T. , Guigné J. Y. 
Published: April 2008

Abstract
Observation of sub-surface deformation structures beneath modern oceanic iceberg scours is impossible, but Quaternary scours exposed above sea or lake level on land provide this opportunity. Well developed iceberg scours which are clearly visible on the former bed of glacial Lake Agassiz in southeastern Manitoba were chosen for detailed studies of sub-scour deformation.
Partial burial of the scour troughs and the absence of bioturbation of the clay sediments have preserved the surface morphology and sub-scour deformation structures in pristine condition. Shallow-angle faults and sub-horizontal thrust faults are developed below the scour trough, the former extending to at least 3 m beneath the deepest part of the scour incision surface. These faults formed in response to vertical and horizontal loading of the lake bed by the encroaching keel. High-angle normal faults developed outside the scour margin represent gravity collapse of sediment blocks that were piled up on the scour berms as a result of lateral displacement of material away from the axis of the scour. Definition of deeply penetrating faults has significant engineering implications for the burial depths required to protect oil and gas production pipelines in areas of active scouring today.
Remnant pods and lenses of tightly folded laminated sediments, in places carrying a well developed fracture cleavage, are all that remain of original primary layering in the severely reworked sub-scour sediments. Based on our findings, criteria for the recognition of scours and of ice keel turbates exposed in argillaceous sediments are proposed.
The Pleistocene—Holocene transition in Southwestern Sweden and the recognition of deglaciation effects in adjacent seas
Authors: Lord A. R. 
Published: April 2008

Abstract

Detailed investigation of Late Quaternary sequences in southwest Sweden via shallow cored boreholes provides an excellent record of sedimentological patterns and the biological history of the area during regional deglaciation. Climatic amelioration and consequent deglaciation led to the Baltic region being dominated by the Baltic Ice Lake, a major physiographic feature which is estimated to have become undammed at
c.
10 200 years BP, discharging some 10
4
km
3
of freshwater south and west into the Skagerrak and North Sea Basin. Such an event should be easily traced in the marine environment, but away from the immediate onshore area convincing records have yet to be found. The freshwater influx affected the upper part of the water column in the North Sea Basin and should be detectable via an isotopic signature, but good quality cores with planktonic organisms have not yet provided such a record. The effects of Baltic Ice Lake discharge into the North Sea Basin may well be overprinted by contemporary major oceanographic changes in the northeast Atlantic, especially the initiation of the Norwegian Current, and by meltwater input from adjacent land areas.
Sedimentation patterns and facies geometries on a temperate glacially-influenced continental shelf: the Yakataga Formation, Middleton Island, Alaska
Authors: Eyles Carolyn H. , Lagoe Martin B. 
Published: April 2008

Abstract
The 5 km thick Yakataga Formation of the Gulf of Alaska provides the longest and most accessible record of late Cenozoic glaciation in the world. The formation consists of interbedded marine and glacimarine sediments of late Miocene to Holocene age and is well exposed on offshore islands and in the coastal mountain ranges as a result of the convergent margin setting of the Gulf. About 1.3 km of early Pleistocene Yakataga Formation sediments are exposed on extensive intertidal platforms around Middleton Island which is sited close to the modern continental shelf edge. Variably graded gravels and sands at the base of the Middleton Island succession were deposited by sediment gravity flows and represent the fill of a large submarine channel. The remainder of the succession is dominated by thick (up to 100 m) planar tabular units of fossiliferous diamict containing a series of striated boulder pavements and shell-rich beds (coquinas). Diamict facies formed predominantly by the settling out of fines from suspended sediment plumes together with ice-rafting of coarser debris; foraminiferal biofacies data indicate palaeobathymetries of between 20 and 250 m. Boulder pavements are lag surfaces which developed in relatively shallow water and were subsequently faceted and striated by a grounding ice sheet extending to the continental shelf edge; coquinas record episodes of current winnowing in shallow water (20–50 m) but also contain ice-rafted debris. Extensive mud units formed in relatively deep water during periods of reduced ice volume and are similar to ‘mud blankets’ accumulating on the modern shelf. The Yakataga Formation on Middleton Island contains a rich record of changing palaeobathymetry and ice volumes in the Gulf of Alaska.
Control of fluids on deformation of rocks
Authors: Carter N. L. , Kronenberg A. K. , Ross J. V. , Wiltschko D. V. 
Published: April 2008

Abstract
Fluids of many compositions, concentrations and pressures, are ubiquitous throughout the continental lithosphere, exerting strong control on the deformation properties and processes of rocks both by mechanical means and by complex chemical rock-fluid interactions. Fluids of meteoric and juvenile origin, released by compaction, dehydration reactions, melting, and degassing, commonly during large-scale tectonic events, flow by means of thermal convection, advection (infiltration), and surface and intracrystalline diffusion. These fluids transport mass for distances ranging from the grain scale to hundreds of kilometres; fracture zones provide favourable conduits for flow. Abnormal pore pressures, recorded at all metamorphic grades, develop intermittently during syntectonic deformation, enhancing fluid infiltration by promoting increased porosity and permeability, hydraulic fracturing and severe grain size reductions. The infiltrating fluids enhance hydrolytic weakening, several grain boundary mechanisms, and reaction kinetics in a feedback manner so that strain is commonly localized into semibrittle and ductile shear zones. Large-scale detachments may take place along these shear zones at virtually any depth below the uppermost few kilometres, which, when combined with softening resulting from depth-dependent petrological and geochemical segregations, form a rheological stratigraphy. The rheology of the lithosphere through time has been governed by a combination of bulk rock flow and localized deformation is shear zones, both of which have been aided or controlled by pervasive dynamic rock-fluid interactions.
Conditions for fault-valve behaviour
Authors: Sibson Richard H. 
Published: April 2008

Abstract

Evidence for fluid pressures well in excess of hydrostatic during crustal deformation is provided by direct measurements of pressure levels in active tectonic regions and by hydrothermal vein systems in ancient orogenic belts. Faults that act as impermeable seals except immediately postfailure, when they become highly permeable channelways for fluid discharge, may behave as fluid-pressure-activated
valves
wherever they transect a suprahydrostatic fluid pressure gradient. Such
fault-valve
behaviour, causing abrupt fluctuations in fluid pressure linked to the earthquake cycle, is particularly likely for faults that remain active while unfavourably oriented for frictional reactivation in prevailing stress fields. The most extreme fault-valve action is likely to be associated with high-angle reverse faults. Valve activity may be especially prevalent near the base of the seismogenic zone when unstable frictional faulting gives way to aseismic shearing with increasing depth. At such structural levels, cyclic variations in the style of deformation may accompany fluid pressure cycling.

Fault-valve activity plays an important role in the development of much fault-hosted mineralization, and may also assist the migration of hydrocarbon fluids in some structural settings.
Textures, deformation mechanisms, and the role of fluids in the cataclastic deformation of granitic rocks
Authors: Evans James P. 
Published: April 2008

Abstract
Optical and scanning electron microscopy were used to examine the textures and infer deformation mechanisms in rocks deformed at upper crustal conditions in the Geesaman oblique-slip fault, southeastern Arizona, and the Washakie thrust fault, northwestern Wyoming. Feldspar in slightly and moderately deformed rocks in both faults deformed by formation of extension and shear fractures along cleavage planes, and quartz grains deformed by brittle fracture. Fracture density is greater in feldspar than in quartz. Intergranular fractures and faults developed by the linking of intragranular fractured feldspars, which in turn resulted in the development of foliated cataclasites in the Washakie fault and phyllonites in the Geesaman fault. Clays and mica developed by the syntectonic alteration of feldspar, and with increasing mica or clay content quartz grains deformed by fracture in a progressively softer matrix. Fracturing and healing of fractures in quartz resulted in irregularly-shaped, elongate pods of quartz in a mica matrix in the Geesaman fault and in a clay and feldspar fragment matrix in the Washakie fault. The syntectonic alteration of feldspars, the presence of iron oxides in the faults, and late-stage quartz veins attest to the flow of water during deformation. The fault zones developed by early fracture in feldspar which gave way to cataclastic flow in feldspars and subsequent development of clay-rich cataclasites at low temperature and of phyllonites at high temperatures. Most of the slip in mature cataclasites was localized by slip and cataclasis of clays. Slip in the phyllonites was localized by plastic deformation of micas. The brittle to ductile transition in the rocks was the result of fracture and subsequent cataclasitic flow in feldspars, followed by deformation of clay or mica, and for the most part excluded the plastic deformation of quartz.
The calculation of bulk rheologies of structured materials and its application to brittle failure in shear
Authors: Casey Martin , Wust Gilles 
Published: April 2008

Abstract
A finite element method is presented which allows the calculation of the bulk rheological properties of a material consisting of a regular array of heterogeneous sub-regions. The method is used to calculate the elastic properties of rocks with aligned cracks containing air or fluid. It is found that the cracked material has anisotropic properties. A compressible fluid filling makes the bulk rock less stiff. The stiffness perpendicular to the long axis of the cracks is reduced more strongly than that parallel to the cracks. It is shown that a concentration of aligned cracks in an inclined planar zone could result in an instability leading to shear failure.
Damage development during rupture of heterogeneous brittle materials: a numerical study
Authors: Cox S. J. D. , Paterson L. 
Published: April 2008

Abstract
We have used a finite element method with an iterative algorithm to simulate rupture development in complex brittle materials such as rocks. A simple set of continuous distributions of properties has been used, and only tensile failure of elements allowed. Wider strength distribution leads to more damage and less abrupt mechanical breakdown characteristics. Variations in strength are found to have a greater effect on the geometry of breakdown than variations in elastic properties. The material behaviour simulated here gives a basic explanation for the extent to which natural fractures in the earth, such as joints and dykes, have a non-planar form.
Influence of fractal flaw distributions on rock deformation in the brittle field
Authors: Main Ian G. , Meredith Philip G. , Sammonds Peter R. , Jones Colin 
Published: April 2008

Abstract

The geometrical distribution of flaws plays a crucial role in the physical behaviour of geological materials under stress. Flaws are present in the earth on all scales, from microcracks to plate-rupturing faults. They may be distributed on one characteristic length scale (e.g. joints, ‘characteristic’ earthquakes), or more commonly exhibit scale-invariance over a specified range of sizes. Scale-invariance implies that the discrete length distribution in a finite range is a power law of negative exponent
D
, where 1 ≤
D
< 3. Fault systems where motion is concentrated on a dominant fault (e.g. San Andreas) have
D
≈ 1, but more diffuse fault systems have
D
near 2.
D
is one of the fractal dimensions of the fracture system. The length distribution of faults or microcracks may be inferred from the slope
b
of the log-linear frequency—magnitude distribution of earthquakes, or laboratory-scale acoustic emissions, since it can be shown that
D
= 3
b/c
. The scaling factor
c
depends on the relative time constants of the seismic event and the recording instrument, and is usually equal to 3/2.
b
is found experimentally to be negatively correlated with the stress intensity on the dominant flaw, which depends in turn on the applied stress and the flaw length. Thus a fracture mechanics model of rock failure which includes a range of flaw sizes can be tested by seismic monitoring.

We describe a fracture mechanics model of rock failure for a variety of styles of deformation, ranging from elastic failure to quasi-static cataclastic flow, and predict the time-dependence of
D
and the seismic
b
-value at different times up to and including failure. Critical coalescence of microcracks during dynamic failure (e.g. earthquake foreshocks) occurs when
D
= 1 (
b
= 0.5); random processes (e.g. cataclastic flow, background seismicity) are associated with
D
= 2 (
b
= 1); positive feedback in the concentration of stress on the dominant flaw (e.g. during strain softening and shear localisation) occurs when
D
< 2 (
b
< 1); negative feedback in stress concentration (e.g. during the early stages of dilatancy), and where a highly diffuse fracture system is produced, occurs at low stress intensities and is associated with
D
> 2 (
b
> 1).

It has long been a goal of structural geologists to measure stress on rocks, since most geometrical signatures of deformation are strain-related. We show that stress is not usually as significant in rock fracture as stress intensity, and furthermore that the geometric signature of the length distribution of microcracks is well-correlated with the stress intensity.
Mechanical compaction and the brittle—ductile transition in porous sandstones
Authors: Wong Teng-Fong 
Published: April 2008

Abstract
Grain scale brittle fracture is an important mechanism for porosity reduction and tectonic deformation in rocks under diagenetic and low grade metamorphic conditions. Microstructural observations and acoustic emission measurements were used to elucidate the micromechanical processes involved (1) in pressure-induced grain crushing, and (2) in the brittle-ductile transition in porous sandstones. Experimental observations show that the critical effective pressure for the onset of grain crushing decreases as a function of increasing grain size and of increasing porosity. The correlation among grain crushing pressure, porosity and grain size can be interpreted using a Hertzian fracture mechanics model. Experimental observations show that both porosity and effective pressure have profound influence on the inelasticity and failure mode. Based on such observations, a brittle-ductile transition map for porous rocks can be constructed in the effective pressure-porosity space. The theoretical predictions of a fracture mechanics model of a pore-emanated crack are in qualitative agreement with experimental observations on the effect of pressure and porosity on inelastic behaviour and failure mode. However, such a model overestimates the crack stabilization effect due to confining stresses since it neglects grain rotation and pore collapse which are important deformation mechanisms in porous rocks. Implications of the experimental and theoretical results on depth — porosity relation, aseismicity at the shallow region of subduction zones, and faulting in sandstones are discussed.
Shear bands in a plastic layer at yield under combined shortening and shear: a model for the fault array in a duplex
Authors: Davies Russell K. , Fletcher Raymond C. 
Published: April 2008

Abstract
To model the regular spacing of the thrust faults in duplexes we subject a single layer embedded in an overlying viscous fluid, and an underlying rigid medium, to combined layer-parallel shortening and shear. The layer is a rigid-plastic solid at yield. A cusp introduced as a small perturbation on the upper layer interface perturbs the flow. The lower layer interface is weak and remains flat throughout the deformation, whereas the upper interface is deformable and can assimilate both weak and strong conditions. If the upper interface is strong it folds into a broad open fold. If the interface is weak the flow localizes into narrow shear bands that cut into the layer from the cusp, and reflect from the layer interfaces at a constant spacing. Material is displaced across the shear bands in a fault-like manner. If the layer is subjected solely to layer-parallel shortening these shear bands are symmetric. Superposing a shear component on the layer interface reorients the shear bands. The shear band dipping opposite to the sense of shear is more strongly developed and intersects the interface at a lower angle than the other. This is in agreement with the observations of duplex configurations.
The failure mechanism for deep-focus earthquakes
Authors: Green H. W. , Burnley P. C. 
Published: April 2008

Abstract

Experimental deformation of Mg
2
GeO
4
olivine at pressures between 1 and 2 GPa in the spinel stability field has led to discovery of a faulting instability that develops at the kinetically-controlled threshold of transformation. Very fine-grained olivine and spinel are found in fault zones. Deformation at lower temperatures is ductile; transformation is inhibited and specimens are very strong. Deformation at higher temperatures also is ductile but transformation is rapid and specimens are much weaker. Detailed examination of the microstructures of specimens deformed in the faulting regime lead to an anticrack theory of faulting that explains the experimental data and provides a fundamentally new mechanism for deep-focus earthquakes. The new mechanism is analogous to the Griffith theory of fracture; nucleation and growth of spinel under stress produces spinel-filled microanticracks normal to the maximum compressive stress that link up to produce faulting. The friction paradox for deep earthquakes is resolved because this faulting process provides a fine-grained, superplastic, ‘lubricant’ for faults. The temperature distribution within subducting slabs of lithosphere requires that the conditions of instability are reached as a natural consequence of subduction; metastable olivine in the interior of deep slabs warms to a critical temperature where faulting ensues in the presence of a shear stress.
Mechanical controls on dilatant shear zones
Authors: Ord A. 
Published: April 2008

Abstract
The finite difference code FLAC is used to examine the distribution of regions of high and low mean normal stress (or pressure) and of maximum dilation around deforming, periodic shear zones. The assumption is made here that the fluid pressure is equal to the mean normal stress. Fluid flow is favoured by large pressure gradients, and is enabled by regions of dilatancy. It is commonly assumed that regions of high dilation are necessarily associated with regions of low pressure. However, it is shown here that this need not be the situation. Cases in which maximum dilation is associated with the maximum pressure may be useful for understanding the presence of periodic melt segregations whereas cases in which maximum dilation is associated with minimum pressure may be useful for understanding metamorphic differentiation during crenulation cleavage development.
Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution
Authors: Spiers C. J. , Schutjens P. M. T. M. , Brzesowsky R. H. , Peach C. J. , Liezenberg J. L. , Zwart H. J. 
Published: April 2008

Abstract

Theoretical models for compaction creep of porous aggregates, and for conventional creep of dense aggregates, by grain boundary diffusion controlled pressure solution are examined. In both models, the absolute rate of creep is determined by the phenomenological coefficient
Z
* =
Z
0
exp (−Δ
H/RT
), a thermally activated term representing effective diffusivity along grain boundaries. With the aim of determining
Z
0
, Δ
H
and hence
Z
* for pressure solution creep in rocksalt, compaction creep experiments have been performed on wet granular salt. Compaction experiments were chosen since theory indicates that pressure solution creep is accelerated in this mode. The tests were performed on brine-saturated NaCl powder (grainsize 100–275 μm) at temperatures of 20–90°C and applied stresses of 0.5–2.2 MPa. The mechanical data obtained show excellent agreement with the theoretical equation for compaction creep. In addition, all samples exhibited well-developed indentation, truncation and overgrowth microstructures. We infer that compaction did indeed occur by diffusion controlled pressure solution, and best fitting of our data to the theoretical equation yields
Z
0
= (2.79 ± 1.40) × 10
−15
m
3
s
−1
, Δ
H
= 24.53 kJ mol
−1
. Insertion of these values into the theoretical model for conventional creep by pressure solution leads to a preliminary constitutive law for pressure solution in dense salt. Incorporation of this creep law into a deformation map suggests that flow of rocksalt in nature will tend to occur in the transition between the dislocation-dominated and pressure solution fields.
Phenomenological superplasticity in rocks
Authors: Gilotti Jane A. , Hull Joseph M. 
Published: April 2008

Abstract
Superplastic deformation of rocks can be defined as: homogeneous deformation without loss of continuity to very large strains. This phenomenological definition differs from current geological usage that equates superplasticity with either grain boundary sliding, a deformation mechanism, or with a specific set of material parameters. Two examples of natural, phenomenological superplasticity in rocks are presented. A diabase dyke deformed continuously in progressive simple shear to shear strains of at least γ = 100, despite the low grade of metamorphism and relatively coarse grain size of the resultant tectonite. Very coarse-grained pegmatite dykes show continuous deformation to high strains under predominantly pure flattening; intracrystalline plasticity was an important micromechanism of flow. A much wider range of material and mechanical parameters and physical conditions give rise to phenomenological superplasticity in rocks than predicted by experiments on metals. By inference and example, more deformation mechanisms will be associated with superplasticity in nature. The profound differences between the metallurgical experiments and the geological environment prohibit simple rheological interpretations of natural superplastic behaviour.