bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Maastrichtian record at Blake Nose (western North Atlantic) and implications for global palaeoceanographic and biotic changes
Authors: MacLeod Kenneth G. , Huber Brian T. 
Published: April 2008

Abstract

Widespread biological, geochemical and sedimentological shifts within the Maastrichtian are well documented, but data are limited for the low-latitude Atlantic. New observations from Ocean Drilling Program (ODP) sites located on Blake Nose in the subtropical western North Atlantic increase information concerning the Maastrichtian history of this critical region. Planktonic δ
18
O results suggest up to 6 °C of local surface water warming (or 4‰ decrease in salinity) at the same time as most of the globe was cooling. Benthic δ
13
O and δ
13
C values of both planktonic and benthic taxa show little if any directional trend or excursions on long time scales; however, planktonic and benthic taxa exhibit strong δ
13
C and δ
18
O cycles (up to 0.8 and 0.6‰, respectively) across a short interval of high-resolution sampling. Other portions of the cores have not yet been studied at high resolution. The last occurrence of inoceramid shell fragments on Blake Nose matches previously documented global patterns, i.e. a mid-Maastrichtian extinction event that occurred later in low latitudes than in high southern latitudes. Models for Maastrichtian change seem to be converging on variation in intermediate to deep water ocean circulation as a unifying process. Blake Nose data are consistent with this conclusion, but demonstrate new regional patterns and emphasize the importance of precise and accurate chronostratigraphic correlation in understanding Maastrichtian change.
Geochemistry of the Cretaceous-Tertiary boundary at Blake Nose (ODP Leg 171B)
Authors: Martínez-Ruiz F. , Ortega-Huertas M. , Kroon D. , Smit J. , Palomo-Delgado I. , Rocchia R. 
Published: April 2008

Abstract
The Cretaceous-Tertiary (K-T) boundary at Blake Nose, in the NW Atlantic, is recorded by a coarse, poorly graded and poorly cemented layer mostly consisting of green spherules that are mainly composed of smectite. Geochemical patterns across the boundary are governed by the source material of the spherule bed and postdepositional processes. The chemical composition and the nature of this bed indicate that it derived from melted target rocks from the Chicxulub impact structure. Ir and other typical extraterrestrial elements do not present significant enrichments, which suggests that the spherule bed material derived from crustal rocks. Ir instead reaches its highest concentration in the burrow-mottled calcareous ooze above the spherule bed, suggesting that it is associated to the finest fraction deposited after the target-rock-derived material. Only the Ni and Co content show slight enrichments within the upper part of the spherule layer, although most of the trace element profiles resulted from diagenetic alteration. During the alteration of glass to smectite, the concentrations of certain trace elements, such as the rare earth elements, were severely changed. In addition, oxygen-poor conditions also led to the remobilization of redox-sensitive elements, which show enhanced concentration at the top or above the spherule bed. Diagenetic remobilization may have also affected the Ir concentration.
Astronomical calibration of the Danian time scale
Authors: Röhl Ursula , Ogg James G. , Geib Tricia L. , Wefer Gerold 
Published: April 2008

Abstract

Ocean Drilling Program Sites 1001A (Caribbean Sea) and 1050C (western North Atlantic) display obliquity and precession cycles throughout polarity zone C27 of the late Danian stage (earliest Cenozoic time). Sliding-window spectra analysis and direct cycle counting on downhole logs and high-resolution Fe variations at both sites yield the equivalent of 35–36 obliquity cycles. This cycle-tuned duration for polarity chron C27 of 1.45 Ma (applying a modern mean obliquity period of 40.4 ka) is consistent with trends from astronomical tuning of early Danian polarity chron C29 and
40
Ar/
39
Ar age calibration of the Campanian-Maastrichtian magnetic polarity time scale. The cycle-tuned Danian stage (
sensu
Berggren
et al.
1995, in SEPM Special Publications,
54
, 129–212) spans 3.65 Ma (65.5–61.85 Ma). Spreading rates on a reference South Atlantic synthetic profile display progressive slowing during the Maastrichtian to Danian stages, then remained relatively constant through late Palaeocene and early Eocene time.
Orbitally forced climate change in late mid-Eocene time at Blake Nose (Leg 171B): evidence from stable isotopes in foraminifera
Authors: Wade Bridget S. , Kroon Dick , Norris Richard D. 
Published: April 2008

Abstract

Previous stable oxygen isotopic data from surface-dwelling foraminifera indicate that Eocene tropical sea surface temperatures (SSTs) were significantly lower than at present. Here we show that stable isotopic analyses (δ
18
O, δ
13
C) of the late mid-Eocene mixed-layer dweller
Morozovella spinulosa
are consistent with mid-Eocene mid-latitude SSTs close to, or slightly lower than modern temperatures at Blake Nose, western North Atlantic. In contrast, isotopic analyses of the benthic foraminifer,
Nuttalides truempyi
reveal a gradual fall in mean bottom-water temperatures from 8 to 7 °C over
c
. 500 ka years. These deep intermediate-water temperatures are significantly higher than modern ones and are similar to intermediate- and bottom-water temperatures recorded from earlier in Palaeogene and late Cretaceous time.

Large shifts are seen in the δ
18
O and δ
13
C values of the planktonic foraminifers, of up to 1‰ and 2.6‰, respectively, that probably reflect temperature and nutrient fluctuations controlled by regional changes in upwelling intensity and runoff. The surface to benthos δ
18
O gradient decreases from 3‰ PDB to a minimum of
c.
0.5‰ PDB over 400 ka, which could relate to the intensity of upwelling. Spectral analysis reveals precessional forcing in the foraminiferal δ
18
O records, which shows the direct influence of low-latitude insolation on surface-water stratification. Monsoonal wind systems may have forced the upwelling cycles and/or freshwater input. The benthic foraminifer δ
18
O record also contains the obliquity cycle, in addition to the precessional cycles, indicating the inheritance of mid- and high-latitude forcing to subtropical deep waters.
Episodicity during orogenesis
Authors: Lister G. S. , Forster M. A. , Rawling T. J. 
Published: April 2008

Abstract

Deformation events and episodes of metamorphic mineral growth are usually regarded as relatively local phenomena. It is not expected that specific events and episodes within an orogenic sequence should exactly correlate over large distances. There is no obvious reason, for example, to assume that deformation and/or metamorphic events in the Western European Alps would directly correlate with events taking place in the Aegean continental crust,
c.
1000 km distant. Yet linked episodes of deformation and metamorphism appear to take place at the same time over large distances, even in these apparently unrelated segments of the same orogenic belt. This large-scale episodic behaviour appears to be associated with switches in tectonic mode, from compressional orogenesis to extensional tectonism, and may be the result of orgenic surges and/or periods of lithospheric extension following accretion events. The effect of these switches is greatest in back-arc environments, in the over-riding plate above major subduction zones. In these environments, roll-back of the subducting lithospheric slab after individual accretion events ensures that the amount of lithospheric extension after each accretion event is large. As a result this is where coherent high-pressure metamorphic terranes formed in the preceding accretion event are exhumed, and where remnants of newly emplaced ophiolite sheets are stranded by newly formed detachment faults.
The structure and rheological evolution of reactivated continental fault zones: a review and case study
Authors: Holdsworth R. E. , Stewart M. , Imber J. , Strachan R. A. 
Published: April 2008

Abstract
Repeated reactivation of structures and reworking of crustal volumes are characteristic, though not ubiquitous, features of continental deformation. Reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long-lived fault zones at different palaeo-depths. Ancient basement fault systems typically comprise heterogeneous, superimposed assemblages of fault rocks formed at different times and depths for which down-temperature thermal histories are most common. Several lithological and environmental factors influence the evolution of fault rock fabrics and rheology, but most fault/shear zone arrays appear to develop as self-organized deformation systems. Once mature, the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault/shear zone network.
A case study from the crustal-scale Great Glen Fault Zone (GGFZ), Scotland, reveals a complex evolution of mid- to upper-crustal deformation textures formed adjacent to the frictional-viscous transition. Fluid influx in the mid-crust has led to reaction softening of the rock aggregate as strong pre-existing phases such as feldspar are replaced by fine-grained, strongly aligned aggregates of weak phyllosilicates. In addition, a grainsize-controlled switch to fluid-assisted diffusional creep occurs in the highest strain regions of the fault zone. It is proposed that this led to a shallowing and narrowing of the frictional-viscous transition and to long-term overall weakening of the fault zone relative to the surrounding wall-rocks. Cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime, by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain-scale dilatancy. Equivalent processes are likely to occur along many other long-lived, crustal-scale fault zones.
Tectonic feedback, intraplate orogeny and the geochemical structure of the crust: a central Australian perspective
Authors: Sandiford M. , Hand M. , McLaren S. 
Published: April 2008

Abstract

The geological record of intraplate deformation in central Australia implies that past tectonic activity (basin formation, deformation and erosion) has modulated the response of the lithosphere during subsequent tectonic activity. In particular, there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin. This behaviour can be understood as a kind of ‘tectonic feedback’, effected by the long-term thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism. From a geochemical point of view, one of the most dramatic effects of intraplate orogeny in central Australia has been the exposure, in the cores of the orogens, of deep crustal rocks largely depleted in the heat producing elements. The geochemical structuring of the crust associated with the erosion of the heat-producing upper crust resulted in long-term cooling of the deep crust and upper mantle with associated lithospheric strengthening. This is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the
h-q
c
plane, where
h
is the characteristic length-scale for heat production distribution, and
q
c
is the total crustal heat production. Because rates of intraplate deformation in central Australia appear to be much slower than that typical of plate margin orogens, it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central Australia by providing a ‘thermal lock’. The diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central Australian intraplate orogens.
Tectonic evolution of the Reynolds-Anmatjira Ranges: a case study in terrain reworking from the Arunta Inlier, central Australia
Authors: Hand M. , Buick I. S. 
Published: April 2008

Abstract

The Reynolds-Anmatjira Range region forms part of the Arunta Inlier in central Australia and has undergone four tectonothermal cycles that span an interval of
c.
1450 Ma. The first two cycles were the Stafford Tectonic Event
c.
1820 Ma, and the Strangways Orogeny
c.
1770–1780 Ma, both of which were associated with regional low-pressure high-temperature metamorphism up to granulite grade that was coeval with the emplacement of voluminous sheet-like granites. The subsequent Chewings Orogeny occurred at around 1590–1570 Ma and was a long-lived event that produced regional low-pressure greenschist to granulite facies metamorphism without obvious associated magmatism. During the mid-Palaeozoic Alice Springs Orogeny (400–300 Ma), the terrain was dissected by a system of sub-greenschist to mid-amphibolite facies shear zones. In the Reynolds Range, the Proterozoic events produced a single regional foliation that is axial planar to simple large-scale folds. The composite regional Proterozoic foliation increases in grade smoothly from northwest to southeast, producing a pattern of isograds that is remarkably similar to those that formed during the mid-Palaeozoic Alice Springs Orogeny. Despite this simple pattern, the isograds reflect the superimposed metamorphic effects of four unrelated tectonothermal cycles. Without geochronological and stratigraphic information, the degree of terrain reworking in the Reynolds-Anmatjira Range region could have been largely obscured by the apparent simplicity of many of the structural and metamorphic relationships.
High-grade reworking of Proterozoic granulites during Ordovician intraplate transpression, eastern Arunta Inlier, central Australia
Authors: Scrimgeour Ian , Raith Johann G. 
Published: April 2008

Abstract

In the Huckitta region of the eastern Arunta Inlier, central Australia, two terrains with distinct metamorphic histories are separated by a zone of sinistral strike-slip mylonitic deformation and reworking, the Entire Point Shear Zone (EPSZ). To the south of the EPSZ, in the Harts Range Group, Ordovician (
c.
470 Ma) intraplate granulite facies metamorphism (
c.
800°C, 8–10 kbar) was followed by decompression to
c.
7 kbar. In contrast, the Kanandra Granulite, to the north of the EPSZ, is characterized by Palaeoproterozoic high-grade metamorphism at 770–850°C and 5–7 kbar, followed by inferred near-isobaric cooling. Juxta-position of these terrains along the EPSZ occurred at upper amphibolite facies conditions (700°C, 7 kbar), and resulted in extensive reworking of the Kanandra Granulite. Monazite growth within EPSZ mylonites is dated at 445 ± 5 Ma, whilst a garnet amphibolite gives a Sm-Nd isochron age of 434 ± 6. The timing of this deformation is broadly coincident with the inferred onset of south-vergent compressional deformation in the Harts Range region to the south. This suggests that juxtaposition of the Ordovician granulite terrain with the surrounding Proterozoic terrains occurred during intraplate sinistral transpression in the late Ordovician. Further reworking of the Kanandra Granulite occurred at mid-amphibolite to greenschist facies conditions, during north-vergent mylonitic deformation that exhumed the Ordovician high-grade terrain during the 400–300 Ma Alice Springs Orogeny. Although this zone of Palaeozoic reworking is <5 km wide, it forms the northern margin of Palaeozoic high-grade intraplate deformation and represents a major tectonic boundary in central Australia.
Fluid-rock interaction in the Reynolds Range, central Australia: superimposed, episodic, and channelled fluid flow systems
Authors: Cartwright Ian , Buick Ian S. , Vry Julie K. 
Published: April 2008

Abstract

The Reynolds Range, central Australia, is a polymetamorphic Proterozoic terrain within the Arunta Inlier. The terrain comprises a diverse sequence of metasedimentary rocks (including pelites, psammites, quartzites, marls and marbles) intruded by with two generations of granites. The Reynolds Range preserves evidence of undergoing several metamorphic events, including: phases of contact metamorphism at 1.82 Ga and 1.78 Ga; regional metamorphism at 1.6 Ga that varied in grade from greenschist facies (
c.
400°C) to granuite facies (750–800°C) at 400–500 GPa; and metamorphism at up to amphibolite facies (550–600°C at 500–600 MPa) in the Alice Springs Orogeny at
c.
334 Ma, the affects of which are recorded mainly within shear zones. Fluid flow occurred during both contact metamorphic events, cooling from the peak of regional metamorphism at 1.57–1.58 Ga, and additionally during Alice Springs shearing. By contrast, there is little fluid-rock interaction that can be attributed to the prograde stages of regional metamorphism, implying that fluids generated by metamorphic devolatilization at that time escaped relatively rapidly and did not interact with the rocks as a whole.

Fluid flow changed the mineralogy and stable isotope ratios of rocks, and locally caused extensive metasomatism. During cooling from the peak of regional metamorphism, the fluids were derived from crystallization of partial melts and reflect internal fluid recycling. However, at least some fluid flow during contact metamorphism and shearing involved external fluids. In both contact metamorphic episodes, igneous and locally surface-derived fluids interacted with the country rocks adjacent to the granite plutons. During Alice Springs shearing, large volumes of surface-derived fluids infiltrated the middle crust. Much of the fluid flow was channelled on scales ranging from hecto- to millimetres as a result of variations in intrinsic permeability caused by deformation or reaction enhancement.
An evolutionary model of glacial dispersal and till genesis in Maritime Canada
Authors: Stea Rudolph R. , Finck Phillip W. 
Published: April 2008

Abstract
Glacier process models of till genesis cannot fully explain the properties of tills in Maritime Canada. A succession of local ice caps, called the Appalachian Ice Complex, developed during the last glaciation and were drained by ice streams into the submarine channels bordering the region. The migration of these centres produced areas with widely differing flow patterns, landform assemblages and deposits. Early regional phases of ice flow were characterized by wide, rapidly-flowing ice-streams that formed thick exotic, silty tills. In later phases, ice divides developed over highland areas underlain by metamorphic and igneous rocks, forming stony local tills. Terrain zones characterized by distinct transport histories and depositional sequences were produced by the interplay and migration of regional ice sheets and local ice caps. The interaction of local glaciers and previously deposited tills formed hybrid tills through two reworking processes: inheritance and overprinting. Inheritance is incorporation of till components and/or fabric into a younger till by erosion and entrainment of material from an older till. Overprinting is the injection or imprint of matrix, clasts or fabric on older tills by overriding ice. Glacial dispersal of rocks, minerals and trace elements in this complex glaciated terrain is controlled by the location of former ice divides. Simple unidirectional trains are preserved in relict terrains under divides. In ice marginal areas, reworking processes result in complex dispersal fans produced by smearing and widening of previously formed trains and fans. These dispersal fans can be modelled by vector addition of discrete flow events within each dispersal zone. A simple empirical model of glacier dispersal is presented using exponential decay and uptake algorithms, and incorporating the reworking of older till material.
Permeability variation across an active low-angle detachment fault, western Woodlark Basin (ODP Leg 180), and its implication for fault activation
Authors: Kopf Achim 
Published: April 2008

AbstractIn the western Woodlark Basin, off Papua New Guinea, the variation from continental rifting to sea-floor spreading has profound effects on the mechanical response of the lithosphere. The extension is well expressed in a seismically active, shallow-dipping detachment fault. Recent Ocean Drilling Program drilling (ODP Leg 180) in the area obtained cores from the hanging wall, detachment fault gouge, and footwall, of which samples underwent permeability testing in the laboratory. Permeability variation was found to be critically dependent on (1) flow direction, i.e. fabric anisotropy of the rocks, and (2) deformational structures in the hanging wall to the fault. Regarding the first, a slight but distinct increase in permeability has been recorded parallel to the fabric (compared with flow normal to this, as indicated by anisotropy indices ofkhorizontal/kverticalof >1.7). This phenomenon appears most profound directly above fault zones in the hanging-wall block, which are interpreted to represent splays to the main detachment fault plane at depth. Here, shear-enhanced compaction seals fluid flow to the sea floor, so that conductive flow parallel to the fault plane is favoured (in general one order of magnitude higher). The fault gouge, mainly consisting of highly serpentinized basalt and chlorite, exhibits an increase in permeability relative to the clay- and siltstones of the hanging-wall block. In the metamorphic basalt from the tectonic footwall, permeability decreases again by three orders of magnitude (kisc.6e—17 to 5e—18 m2). Consequently, the detachment and synthetic splays related to it are zones of enhanced fluid migration in the fault plane direction. Fluid overpressure, and hence fault activity, is suggested to be trigered by seal of the top of the fault zone, owing to both shear fabrics and cementation processes.
Deformation microfabrics of clay gouge, Lewis Thrust, Canada: a case for fault weakening from clay transformation
Authors: Yan Yonghong , van der Pluijm Ben A. , Peacor Donald R. 
Published: April 2008

Abstract

A sequence of bentonite and shale samples in a gouge zone of the Lewis Thrust (Alberta, Canada) that display increasing degree of transformation of clay minerals toward the hanging wall of the thrust has been studied by X-ray diffraction (XRD), X-ray texture goniometry (XTG), scanning electron microscopy (SEM), and transmission and analytical electron microscopy (TEM-AEM), to examine the relations among mineral transformations, microfabrics and fault zone properties. TEM images of authigenic clays show abundant smectite in shale away from the hanging wall, characterized by anastomosing layers with an average orientation that parallels bedding, coexisting with uncommon
R
1 illite-smectite (I-S). In the sample nearest the hanging wall, by contrast, the dominant clay is mixed-layered, illite-rich ilite-smectite (
R
1 I-S), coexisting with discrete illite, occurring in individual packets of relatively straight layers with well-defined boundaries. Deformed clay packets are common. Pore space, where packets intesect at high angles to one another and to bedding, is abundant (
c.
25%). The microfabric and proportion of illite of intermediate samples are transitional to these end-members. Inter-layered bentonite samples show properties that are similar to those of shale. TEM observations are supported by quantification of the fabrics using XTG, which shows that the intensity of clay preferred orientation decreases significantly with increasing illitization. These relations imply that faulting was the cause of mineral transformations and formation of secondary pore space. The illitization reaction rate was enhanced both by stress-induced defects in clays, and by increased water/rock ratio resulting from deformation-related pore space, resulting in lowering of the effective stress. The deformation-enhanced reaction thus created a positive feedback for further faulting in clay gouge, leading to enhanced weakening of the fault zone.