bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Avaatech XRF Core Scanner: technical description and applications to NE Atlantic sediments
Authors: Richter Thomas O. , van der Gaast Sjerry , Koster Bob , Vaars Aad , Gieles Rineke , de Stigter Henko C. , De Haas Henk , van Weering Tjeerd C. E. 
Published: April 2008

Abstract
X-ray fluorescence (XRF) core scanning provides rapid high-resolution (down to 1 mm) records of chemical composition on split sediment cores. The measurements are non-destructive and require very limited sample preparation. The new Avaatech XRF Core Scanner, operational since 2002, covers the atomic mass range from Al to U. Instrument parameters, especially tube voltage, can be adjusted to provide optimum settings for selected elements or sets thereof. Owing to the nature of the surface of split sediment cores, particularly effects resulting from sample inhomogeneity and surface roughness, results are semiquantitative, yet provide reliable records of the relative variability in elemental composition downcore. Selected case studies from diverse sedimentary settings in the NE Atlantic Ocean illustrate a range of applications of XRF logging data. These include preliminary stratigraphic interpretations (glacial-interglacial cycles), provenance studies of the terrigenous sediment fraction, lithological characterization, early diagenetic processes and distinction between carbonate phases (aragonite v. calcite).
Turbidite emplacement on the southern Balearic Abyssal Plain (western Mediterranean Sea) during Marine Isotope Stages 1–3: an application of ITRAX XRF scanning of sediment cores to lithostratigraphic analysis
Authors: Rothwell R. Guy , Hoogakker Babette , Thomson John , Croudace Ian W. , Frenz Michael 
Published: April 2008

Abstract
The upper part (0–20 m) of a long piston core from the SE Balearic Abyssal Plain — spanning the past 50 ka — has been studied using the ITRAX micro-XRF core scanner to obtain downcore elemental profiles. The Ca/Fe ratio was found to be an effective parameter to distinguish between turbidites and pelagites, because turbidites generally have higher Fe contents and lower Ca contents compared with pelagic intervals. Beds that were obscure when visually logged could be identified as turbidites or pelagites on their geochemical characteristics, allowing more complete subdivision of the sequence into genetic units. The ITRAX XRF data also provide useful information on textural grading, bioturbative mixing, identification of geochemically distinctive marker beds, indications of differences in provenance, and confirm or query the presence of early arrivals during turbidite emplacement. A chronostratigraphic framework for the core based on accelerator mass spectrometry (AMS) radiocarbon dating and correlation with oxygen isotope stages of pelagic intervals in other cores (using calcium carbonate stratigraphy) was also established. This shows that turbidite emplacement on this part of the Balearic Abyssal Plain has been modulated strongly by climate and sea-level change, with turbidite emplacement most frequent during the early Holocene when the rate of post-glacial sea-level rise was greatest. Deposition of the coarsest (i.e. sand and silt-based) turbidites at the core site was restricted to the full and Late Glacial (11–25 ka). Turbidite emplacement during Oxygen Isotope Stage 3 was rare. Most of the turbidites at the site are distal, but some coarse-grained-based turbidites are characterized by higher Sr/Ca ratios (possibly indicating a higher aragonite content), higher Ca and lower Fe contents compared to other turbidites, and are interpreted as having a more proximal shelf source. Such turbidites are generally rare, however, and restricted to full Glacial and Younger Dryas time. There is little evidence for large-scale seismogenic turbidites (expected to be seen as randomly timed emplacement, seemingly independent of eustatic control) at the core site, despite proximity to the seismically active Algerian margin 100 km to the south. This suggests that seismogenic turbidites must largely bypass this part of the plain. Although the ITRAX core scanner provides a rapid and non-destructive means of characterizing downcore geochemical distributions in great detail, interpretation of the data requires caution and assessment from an informed standpoint. Analytical artefacts such as those caused by water or organic content, degree of compaction, grain-size and mineral effects, unevenness of the cut core surface and poor discrimination of closely spaced element XRF peaks need identification and elimination.
Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction
Authors: Godin L. , Grujic D. , Law R. D. , Searle M. P. 
Published: April 2008

Abstract
The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya-Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya-Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya-Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented.
A synthesis of the Channel Flow-Extrusion hypothesis as developed for the Himalayan-Tibetan orogenic system
Authors: Hodges K. V. 
Published: April 2008

Abstract
Surface and subsurface geological features of the Himalayan-Tibetan orogenic system may be explained by three sets of processes: those related to plate convergence, those related to the gravitational spreading of a fluid middle crust beneath the Tibetan Plateau, and those related to aggressive erosion along the southern margin of the plateau. In this paper, the possible relationships among the last two of these—and their tectonic manifestations—are presented in the form of a ‘Channel Flow-Extrusion’ hypothesis. This hypothesis, deriving from a series of ideas advanced by many geologists and geophysicists over the past two decades, suggests the definition of three phases in the Early Miocene-Recent history of the orogenic system. During Phase I (Early Miocene), the crust of southern Tibet was sufficiently hot and thick to enable lateral flow of a weak middle crust. To the north and east, this flow resulted in the expansion of the Tibetan Plateau. To the south, erosion at the Himalayan front permitted the mid-crustal channel to breach the surface; this process is recorded in the deformational history of the Himalayan metamorphic core and the Main Central and South Tibetan fault systems that bound it. While the lateral expansion of the plateau by mid-crustal flow has continued throughout Neogene time, some evidence suggests that extrusion across the Himalayan front waned substantially during the Middle Miocene-Early Pliocene interval (Phase II). In Middle Miocene time, large magnitude extension of the decoupled upper crust of southern Tibet led to the development of a subsidiary channel; its extrusion explains the existence of the North Himalayan gneiss domes. Phase III (Late Pliocene-Recent) has involved renewed southward extrusion of the main channel due to climatically induced increases in the erosion rate at the Himalayan range front.
Kinematic dilatancy effects on orogenic extrusion
Authors: Grasemann B. , Edwards M. A. , Wiesmayr G. 
Published: April 2008

Abstract
We undertake kinematic modelling to explore the role of volume increase in a slab extruding froman orogenic wedge with constant or decreasing slab width. Using a dilatancy term, we modify the velocity gradient tensor dependent on the stretching-rate factor, kinematic dilatancy and vorticity number. We use this to explore the previously largely ignored role of volume change in kinematic evolution of extrusive flow, considering area change for non-isochoric flow types with no deformation in the intermediate direction. By keeping individual parameters constant for geologically simple scenarios (e.g. finite strain, steady-state flow) we examine the interdependence of the reciprocal parameters (kinematic vorticity and dilatancy number) and note model situations where degrees of freedom are limited. These interdependent parameters thereby provide a set of rules for integrating and modelling real field data. In particular we observe that for extrusion flow with a constant slab (or ‘channel’) width, degrees of freedom in kinematic vorticity and volume change at given finite strains are very restricted. We compare scenarios of low and high strain and low and high volume change on anatexis (related to partial melting of fertile sedimentary rocks and release of water upon crystallization) for different parts of the Himalaya.
Interpretation of deformation fabrics of infrastructure zone rocks in the context of channel flow and other tectonic models
Authors: Williams P. F. , Jiang D. , Lin S. 
Published: April 2008

Abstract
Infrastructure zones are essentially horizontal to shallowly dipping crustal-scale zones of non-coaxial flow, with two possible interpretations: (a) a crustal-scale shear zone, transporting upper crust over lower crust and/or mantle (transport flow); or (b) a zone of channel flow in which there is a flux of weak crust between relatively strong upper and lower crust and/or mantle, away from the centre of the orogen. Transport flow has a constant shear sense across the zone, whereas in channel flow the sense of shear reverses across the zone. Channel flow may be driven by extrusion (extrusive channel flow), due to the two channel walls approaching one another, or by a pressure gradient along the channel (normal channel flow), with no convergence of the walls necessary. Arguments based on strain compatibility and mechanics suggest that extrusive channel flow is unlikely. Kinematic vorticity numbers have been used to show that infrastructure and other shear zones have undergone flattening strains, but we show that the numbers are incompatible with extrusion. We also show that in addition to the problems inherent in determining kinematic vorticity numbers from fabric, the numbers cannot be related to bulk flow in mechanically heterogeneous zones, because of flow partitioning. Drag folds are a better indication, albeit qualitative, of whether a zone is thinning or not. They also give a conservative estimate of the minimum accumulated shear strains, and may inhibit strain localization. Like snowball garnets, they indicate shear strains that are so large that the pure-shear-thinning component for a steady-shear zone has to be small.
Structural evolution and vorticity of flow during extrusion and exhumation of the Greater Himalayan Slab, Mount Everest Massif, Tibet/Nepal: implications for orogen-scale flow partitioning
Authors: Jessup M. J. , Law R. D. , Searle M. P. , Hubbard M. S. 
Published: April 2008

Abstract

The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusion-related flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number (
Wm
) estimates of 0.74–0.91 (
c.
45–287fb3e69cure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield
Wm
estimates of 0.57–0.85 (
c.
62–357fb3e69cure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield
Wm
estimates of 0.63–0.77 (
c.
58–447fb3e69cure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.
Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome
Authors: Lee J. , McClelland W. , Wang Y. , Blythe A. , McWilliams M. 
Published: April 2008

Abstract

New U-Pb zircon, monazite,
40
Ar/
39
Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica
40
Ar/
39
Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar
40
Ar/
39
Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar
40
Ar/
39
Ar spectra yield ages of
c.
11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below
c.
115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from
c.
40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to
c.
350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica
40
Ar/
39
Ar chrontours are domed, whereas the low-temperature step potassium feldspar
40
Ar/
39
Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.
Heterogeneous ductile deformation along a mid-crustal extruding shear zone: an example from the External Hellenides (Greece)
Authors: Xypolias P. , Kokkalas S. 
Published: April 2008

Abstract

Petrofabric, finite strain and kinematic vorticity data were used to investigate the heterogeneous nature of ductile deformation along a 1.5–2 km thick extruding shear zone in the south Peloponnese, that formed under blueschist-facies conditions. Asymmetric quartz
c
-axis fabrics confirm westward thrust movements on an east-dipping shear zone and provide evidence for localized top-down-to-the-east shear sense at the front of the zone. Strain ratio (

R
xz

) is nearly constant (
c.
3.0–4.0) along the upper structural levels of the zone but increases systematically from the middle to the bottom, approaching a value of
c.
9.5 in the frontal parts, close to the basal thrust, and a value of
c.
7.0 in the inner parts. The distribution of kinematic vorticity number depicts a simple-shear-dominated domain in the lower half of the shear zone and shows that the pure shear component always increases upwards in the zone, becoming dominant at the top of the inner parts of the zone. Integration of strain and vorticity data yields a shear-parallel elongation of
c.
60–90% at the top and
c.
40–60% at the bottom of the zone, revealing that both upper and lower surfaces of the extruding slices were ‘stretching faults’. Minimum total displacements of 25 and 41 km and slip rates of 6.5 and 10 mm/year were estimated for the basal and roof faults, respectively.
An argument for channel flow in the southern Canadian Cordillera and comparison with Himalayan tectonics
Authors: Brown Richard L. , Gibson H. Daniel 
Published: April 2008

Abstract
Crustal thickening in excess of 55 km, and high heat flow, suggest that a high-standing plateau region in the Cordilleran hinterland was present in the Late Cretaceous. A low strength middle crust developed beneath the plateau, and parts of this layer were exhumed to upper crustal levels in Late Cretaceous to Eocene time. During Late Cretaceous time, structures in the hinterland were reactivated. Strata, buried to mid-crustal depths since the Jurassic, began to flow upward to higher levels; earlier structures were refolded and tightened, and a new transposition fabric developed. Some 10–20 km of the middle crust was involved in high temperature ductile flow. The lower boundary of the ductile zone lies with thrust sense on top of Precambrian rocks of Canadian Shield affinity, and splays upwards to the NE where it closely coincides with highly strained rocks in the hanging wall of the Purcell Thrust Fault. The upper boundary is marked by a normal-sense high strain zone, above which only minor Cretaceous deformation occurred. The boundaries were reactivated at upper crustal levels after cessation of flow in the mid-crustal channel. This reactivation resulted in formation of ductile to brittle extension faults such as the Okanagan Fault System. During final stages of flow, the Precambrian basement gneisses at the base of the channel became domed and exhumed to upper crustal levels. Comparisons with Himalayan tectonics are clearly drawn, but there are significant contrasts such as the long residence time of the proposed Cordilleran channel, and the nature of the channel boundaries.
Ductile thrusting versus channel flow in the southeastern Canadian Cordillera: evolution of a coherent crystalline thrust sheet
Authors: Carr Sharon D. , Simony Philip S. 
Published: April 2008

Abstract

The Late Cretaceous Gwillim Creek shear zone (GCSZ) exposed in the core of the Valhalla complex, and located in the hinterland of the southern Canadian Rocky Mountain thrust belt, is a 5–7 km thick, easterly verging, ductile thrust zone. It was active after
c.
90 Ma and during anatexis (800°C and 800 MPa), rose eastward in the direction of transport, and its base was refrigerated from below at
c.
60 Ma by thrust translation onto a cold footwall. Extensional shear zones are younger than the GCSZ, and there is no evidence of channel flow or ductile extrusion. Instead, a 30 km thick, coherent sheet was translated on the GCSZ, which at depth was linked to the Foreland thrust belt such as to form a composite crystalline thrust sheet. Doming of the Valhalla complex may be related to Eocene thrusting beneath the complex during the last stage of shortening. A channel flow, proposed by others for the region north of the Valhalla complex, could have evolved within the crystalline sheet by activation of lateral transition zones and an upper detachment, but is no wider than 250 km, does not represent the dominant orogenic process and may represent a nascent channel.
Possibility of channel flow in the southern Canadian Cordillera: a new approach to explain existing data
Authors: Kuiper Yvette D. , Williams Paul F. , Kruse Stefan 
Published: April 2008

Abstract
Existing structural, metamorphic and geochronological data in and close to the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model. Four general structural levels (domains) can be distinguished in the region, based on the orientation and vergence of folds. In the lowest three levels folds are mostly recumbent, whereas in the uppermost level they are upright. The lowest three levels are interpreted as a channel flow zone. NE-verging folds of the lowest level (Domain 1, e.g. the Monashee Complex) formed during top-to-the-NE detachment flow and/or in the lower part of a channel flow zone. When detachment flow changed to channel flow, the sense of shear changed in the upper part of the channel flow zone, resulting in overprinting of NE-verging folds by SW-verging folds (Domain 2, e.g. most parts of the Shuswap Metamorphic Complex to the west of the Monashee Complex). Temperature was probably increasing, weakening a progressively larger portion of the crust, and the crustal shear zone therefore widened. Thus, in the highest structural levels within the channel flow zone, SW-verging folds developed in areas where no NE-verging folds originally formed (Domain 3, e.g. the Cariboo Mountains). The channel flow model as presented here is compatible with many of the ductile structures and accommodates existing metamorphic and geochronological data in the part of the southern Canadian Cordillera described.
Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective
Authors: De Silva Shanaka , Zandt George , Trumbull Robert , Viramonte José G. , Salas Guido , Jiménez Néstor 
Published: April 2008

Abstract

The Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km
3
of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate.