bluuebunny/crossref_metadata_2025_split huggingface dataset:
High-strain zones: laboratory perspectives on strain softening during ductile deformation
Authors: Burlini L. , Bruhn D. 
Published: April 2008

Abstract
Deformation in the Earth’s outer shell is mostly localized into narrow high-strain zones. Because they can have displacements up to several hundreds or thousands of kilometres, they can affect the entire lithosphere. The properties of high-strain zones control the kinematics and dynamics of our planet, and are therefore of key importance for an understanding of plate tectonics, stress accumulation and release (e.g. earthquakes), mountain building, etc.

One of the requirements of shear zone formation in ductile rocks is localized strain softening (Hobbs
et al.
1990). In this paper we review the strain softening mechanisms that were identified and proposed 25 years ago and analyse their relevance in light of recent experimental results conducted to large strains. For this purpose, some of the newer developments in experimental deformation techniques that permit high strain in torsion are summarized and recent results are reviewed. Using these results we discuss mechanisms, processes and conditions that lead to localization.
Shear strain localization from the upper mantle to the middle crust of the Kohistan Arc (Pakistan)
Authors: Burg J.-P. , Arbaret L. , Chaudhry N. M. , Dawood H. , Hussain S. , Zeilinger G. 
Published: April 2008

Abstract
Shear structures from mantle to middle crust levels of the Kohistan palaeo-island arc, in Pakistan, are described. Pre-Himalayan ductile shear zones show a wide variety in size and shape, and developed from gabbro subsolidus to amphibolite facies conditions. Their lithological context and geological history give insights into mechanisms that initiate shear strain localization, factors that control stabilization of deformation in shear zones and flow properties at the mantle-crust transition. Shear strain localization began within compositional gradients. Gabbros were more prone to localization into anastomosing patterns than diorites and granites, which show more homogeneous strain. Shear strain localization during cooling led to less numerous but longer and thicker shear zones. Viscous heating within shear zones resulted in melt production and segregation in deformation structures, and seems to have taken part in the plutonic history of the arc. Using Kohistan as an example, we suggest that the plutonic, lower crust of arcs is strongly affected by subhorizontal, synmagmatic shear zones, probably consistent with the bulk flow direction of the subduction zone. These features can obviously be preserved in collision orogens and may be mistaken for structures documenting the continental collision.
Geometric aspects of synkinematic granite intrusion into a ductile shear zone — an example from the Yunmengshan core complex, northern China
Authors: Passchier C. W. , Zhang J. S. , Konopásek J. 
Published: April 2008

Abstract
The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile décollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due to this rotation, but also to foliation-parallel injection of younger syntectonic pegmatite veins. Several small-scale structures have been recognized that allow distinction of solid-state deformation of veins. Granite veins do not extend much above the ductile shear zone that seems to act as a lid and an effective depository to intruding granite veins from the underlying batholith. There was considerable volume increase in the footwall and lower part of the shear zone by vein intrusion.
Initiation of basement thrust detachments by fault-zone reaction weakening
Authors: Wibberley Christopher A. J. 
Published: April 2008

Abstract
This paper examines how crystalline basement thrust sheets can detach in foreland thrust belts, in terms of the deformation mechanisms and rheological evolution of the detachment fault zones. Basement thrust fault zones of the Moine Thrust Belt and the external Western Alps show relatively narrow thrust zones considering the large displacements accommodated. Microscopic examination of fault rocks from these high strain thrust zones show that syntectonic alteration of fractured feldspars to white mica of strong preferred orientation generated ultramylonites deforming by diffusion creep and other viscous deformation mechanisms, similar to documented basement thrust zones in North America. Motivated by these observations coupled with other published examinations of foreland basement thrust zones, and recent developments in crustal hydrology, a conceptual model is proposed to explain basement detachment formation and evolution. Meteoric fluid that percolated into a previously fractured upper crust is drawn into developing fault zones by dilatancy pumping during the early stages of thrust-related deformation. The generation of cataclastic fault rocks with fresh fracture surfaces by microfracturing enhances the rate of fluid-rock interaction. Syntectonic alteration causes a deformation-mechanism transition to phyllosilicate-dominated ductile fault-rock rheologies, resulting in a large ductility contrast between host rock and fault zone that inhibits growth of the zone into the wall rock and weakens the thrust. Deformation becomes focused into these weakened early thrust zones so that they become zones of high strain, preventing the development of other newer fault zones elsewhere. This model explains the detachment and continued sliding of basement thrust sheets on narrow mica-rich zones of high strain in foreland thrust belts, and suggests that reaction weakening of the basement is important in decreasing the strength of the foreland crust in orogenic wedge evolution.
The Tasmanides of eastern Australia
Authors: Glen R. A. 
Published: April 2008

Abstract
The Tasmanides of eastern Australia record the break-up of Rodinia, followed by the growth of orogenic belts along the eastern margin of Gondwana. Spatially, the Tasmanides comprise five orogenic belts, with an internal Permian-Triassic rift-foreland basin system. Temporally, the Tasmanides comprise three (super)cycles, each encompassing relatively long periods of sedimentation and igneous activity, terminated by short deformational events. The Neoproterozoic-earliest Ordovician Delamerian cycle began by rifting, followed by convergent margin tectonism and accretion of island-arc forearc crust and ?island arcs in the Middle-Late Cambrian. The Ordovician-Carboniferous convergent margin Lachlan supercycle consists of three separate cycles, each ending in major deformation. The Ordovician Benambran cycle includes convergent (island-arc) and transform margin activity terminated by terrane accretion in the latest Ordovician-earliest Silurian. The Silurian-Middle Devonian Tabberabberan cycle reflects development of a large back-arc basin system, marked by rift basins and granite batholiths, behind intra-oceanic arcs and an Ordovician-Early Devonian terrane that were accreted in the Middle Devonian. The Middle Devonian to Carboniferous Kanimblan cycle began by rifting, followed by continental sedimentation inboard of a major convergent margin system that forms the early part of the Late Devonian-Traissic Hunter-Bowen supercycle. This supercycle comprises a Late Devonian-Carboniferous continental arc, forearc basin and outboard accreted terranes and subduction complexes intruded by the roots of a Permian-Triassic continental margin arc. Complex deformation ended with accretion of an intra-oceanic arc in the Early Triassic. Key features of the Tasmanides are: continuity of cycles across and along its length, precluding growth by simple eastwards accretion; development of a segmented plate margin in the Late Cambrian, reflected by major rollback of the proto-Pacific plate opposite the southern part of the Tasmanides; rifting of parts of the Delamerian margin oceanwards, to form substrate to outboard parts of the Tasmanides; the presence of five major Ordovician terranes in the Lachlan Orogen; and the generation of deformations either by the accretion of arcs, the largely orogen-parallel ‘transpressive’ accretion of Ordovician turbidite terranes (in the Lachlan Orogen), or by changes in plate coupling.
New Zealand tectonostratigraphy and implications from conglomeratic rocks for the configuration of the SW Pacific margin of Gondwana
Authors: Wandres A. M. , Bradshaw J. D. 
Published: April 2008

Abstract
The active margin of Gondwana is presently preserved in the southwest Pacific region in the formerly continuous Gondwana fragments of Australia, Antarctica and New Zealand. The Phanerozoic tectonic history of New Zealand is interpreted in terms of progressive Pacific-ward growth by accretion of arc-trench systems and the basement rocks are described in terms of a number of volcano-sedimentary accreted terranes, suites and batholiths that intrude the terranes. The age of these basement rocks ranges from Early Cambrian to late Early Cretaceous. The origin of the magmatic and sedimentary rocks and the time of accretion of the New Zealand terranes to the Gondwana margin are important for the understanding of Phanerozoic Pacific tectonics. Geochronological research over the last decade on igneous rocks and conglomeratic units shows that the Tutoko Complex/Amundsen Province plutons are major contributors of detritus to the Pahau depositional basin and that the Antarctic sector of the Panthalassan Gondwana margin has to be (re)considered as the likely source for the Permo-Triassic Rakaia sediments. Igneous clast data have greatly improved understanding of the evolution of the New Zealand microcontinent and have put tighter constraints on its Mesozoic tectonic setting within the southwest Pacific margin of Gondwana.
Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia
Authors: Rapela C. W. , Pankhurst R. J. , Fanning C. M. , Hervé F. 
Published: April 2008

Abstract
The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.
Northern Victoria Land terranes, Antarctica: far-travelled or local products?
Authors: Tessensohn Franz , Henjes-Kunst Friedhelm 
Published: April 2008

Abstract
The early Palaeozoic Ross Orogen in northern Victoria Land, Antarctica, consists of three major fault-bounded tectonostratigraphic terranes. Their true nature, fartravelled or local part of the accretionary collage, is under discussion. The inboard Wilson terrane (WT) consists mainly of high-grade metamorphic rocks intruded by the calcalkaline magmatic arc of the Ross Orogen. The terrane nature of the WT is doubtful, as it appears more like the leading edge of the East Antarctic craton. The Bowers terrane (BT) comprises a mixed sedimentary-volcanic succession, beginning with volcanic rocks of island-arc character, followed by turbidites, mudstones, conglomerates and fossiliferous Middle Cambrian shallow-water sediments. The whole sequence is capped by a fluvial to deltaic quartizitic series several kilometres thick, with strong continental affinity. The combination of primitive forearc to back-arc volcanics at the bottom and mature continental sediments at the top poses a problem. The outer Robertson Bay terrane (RBT) is made up of a thick turbidite succession which, in one area, contains allochthonous blocks of fossiliferous Tremadocian limestones. All terrane boundaries appear to be distinct fault zones. The WT/BT boundary forms a deep-reaching continent-ocean suture associated with strongly sheared rock units, ultramafic lenses and high-pressure rocks. Coesite in eclogites of the Lanterman Range indicates a depth of burial of around 90 km. A greenschist-facies schist belt marks the BT/RBT boundary. The terranes contain evidence for subduction at an active margin setting as well as for accretion processes along major faults. The present changes of the Cambrian time-scale, such as younging of the base of the Upper Cambrian by about 30 Ma since the 1980s, allow separation of arc formation and later terrane accretion events.
Archaeocyathan limestone blocks of likely Antarctic origin in Gondwanan tillite from the Falkland Islands
Authors: Stone P. , Thomson M. R. A. 
Published: April 2008

Abstract
Cambrian limestone clasts containing a rich, well-preserved archaeocyathan fauna have been recovered from the late Carboniferous Fitzroy Tillite Formation of the Falkland Islands. Since neither Cambrian strata nor limestone are present anywhere in the indigenous rock succession, the clasts are regarded as exotic erratics introduced during the Permo-Carboniferous Gondwana-wide glaciation. Most recent reconstructions of Gondwana rotate the Falklands into proximity with the Eastern Cape, South Africa and the Ellsworth Mountains, Antarctica. In both of these areas, Permo-Carboniferous diamictites correlated with the Fitzroy Tillite Formation also contain rare, exotic clasts of archaeocyathan limestone. The Transantarctic Mountains seem the most likely source for all of these unusual erratics. This interpretation sustains the requirement for substantial rotation of the Falklands microplate into Gondwana reconstructions and illustrates the extent of the late Carboniferous ice sheet. Apparent differences in the tillite clast assemblages between East and West Falkland suggest variable provenance within the regional ice-flow regime.
Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina
Authors: Zimmermann Udo 
Published: April 2008

Abstract

A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Q
t
60–80, F15–35, L5–20, P/F 0.2–0.4, L
v
/L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.
Distinguishing biological from hydrothermal signatures via sulphur and carbon isotopes in Archaean mineralizations at 3.8 and 2.7 Ga
Authors: Grassineau N. V. , Appel P. W. U. , Fowler C. M. R. , Nisbet E. G. 
Published: April 2008

Abstract

Carbon and sulphur isotopes have been analysed in mineralization from two Archaean greenstone belts at 3.8 Ga and 2.7 Ga, with the aim of distinguishing between the inorganic and organic processes that occurred. Despite an obvious overprinting by metamorphism (in the early belt), or hydrothermal fluids, there are convincing differences between the values of carbon and sulphur in inorganic formations and those formed by biological processes. An attempt is made to estimate the changes that occurred in the early life activities over this 1 Ga year period. Life in the Isua Greenstone Belt (3.8 Ga) was most likely present in transitory settings, probably under high temperature conditions. This was very different from the life at 2.7 Ga in the Belingwe Greenstone Belt, indicated by ranges of 38‰ for δ
34
S and 37‰ for δ
13
C
red
. By this time, the biological sulphur and carbon cycles seem to have reached almost full operation, with the presence of well-established photosynthetic microbial mat communities.
Diamond mega-placers: southern Africa and the Kaapvaal craton in a global context
Authors: Bluck B. J. , Ward J. D. , De Wit M. C. J. 
Published: April 2008

Abstract
Diamond mega-placers, defined as ≥ 50 million carats at ≥ 95% gem quality, are known only from along the coast of southwestern Africa, fringing the Kaapvaal craton, where two are recognized. One is associated with the Orange-Vaal dispersal, the other, to the south, has an uncertain origin. Placers are residual when left on the craton, transient when being eroded into the exit drainage, and terminal. Terminal placers, the final depositories of diamonds, have the greatest probability of being a mega-placer. There are four main groups of controls leading to the development of a mega-placer: the craton, the drainage, the nature of the environment at the terminus and the timing.
Cratons, being buoyant, have a tendency to leak diamonds into surrounding basins; however, being incompressible they may have orogens converge onto them resulting in some lost sediment being returned as foreland basin fills. The craton size, its diamond-fertility and the retention of successive kimberlite intrusions that remain available to the final drainage, are significant to mega-placer development.
Maximum potential recovery is achieved when the drainage delivering diamonds to the mega-placer is efficient, not preceded by older major drainages and focuses the supply to a limited area of the terminal placer. There should be sufficient energy in the terminal placer regime to ensure that sediment accompanying the diamonds is removed to areas away from the placer site. All conditions should be near contemporaneous and most were satisfied in the Orange-Vaal Rivers-Kaapvaal system and mega-placers were consequently generated.
The NERC Micro to Macro Programme: implications for fluid resource management
Authors: Heffer K. J. 
Published: April 2008

Abstract
The Micro to Macro (μ2M) Programme has been focused on developing understanding of subsurface fluid flows within geological heterogeneities spanning wide ranges of spatial and temporal scales. This paper highlights the opportunities for industries to incorporate recent observations and emerging theories in this field towards improved fluid resource management. The background to, and objectives of, the μ2M Programme are reviewed. Selected results from the projects in the programme are discussed and, where possible, compared with evidence from industrial field data. Some conclusions and recommendations for future practice in reservoir characterization are made. For example, there is currently very little recognition of modern theories that point to the likelihood of prevailing criticality in the mechanical state of the Earth’s crust and its implication for coherent large-scale collective behaviour emerging from small-scale interactions. Also associated with criticality are long-range spatial correlations and the likelihood that flow properties change during the life of commercial developments: such changes, for example, to absolute permeability, should be looked for and analysed for spatial and temporal patterns. Allied with these features is the importance of coupled processes, principally geomechanics, fluid flow, heat flow and chemistry. Knowing that local faults and fractures play a strong role in fluid flow mechanisms in a potentially time-varying, rather than just a static, fashion, gives even more motivation for acquiring detailed information on micro- and macro-structure over a range of scales.
MOPOD: a generic model of porosity development
Authors: Bloomfield J. P. , Barker J. A. 
Published: April 2008

Abstract

A code, MOPOD, has been developed to investigate general relationships between simple porosity growth laws and pore growth phenomena. MOPOD has been formulated as an ‘initial value problem’ and to date, investigations have focused on a very simple porosity growth law of the form

da
i
(t)/dt

=

v
e
i

, where
e
is the aperture growth rate exponent. A range of qualitatively distinct evolved geometries have been described for porosity growth on 2D and 3D arrays of varying geometries and connectivities as a function of the exponent,
e
, of the aperture growth-rate law, and the width of the initial aperture distribution, σ

z

. At low growth-rate exponents and moderate values of σ

z

over time there is a homogenization of apertures oriented sub-parallel to the head gradient. At moderate growth-rate exponents these apertures become increasingly heterogeneous in evolved arrays, with planar heterogeneities developing sub-parallel to the head gradient for low values of σ

z

while anastomosing structures develop at higher values of σ

z

. For larger growth-rate exponents preferentially enlarged array-spanning paths develop. No self-organization phenomena have been observed because periodic or cyclic behaviour is not inherent in the simple growth laws investigated to date.