bluuebunny/crossref_metadata_2025 huggingface dataset:
Helium isotope signature of lithospheric mantle xenoliths from the Permo-Carboniferous magmatic province in Scotland — no evidence for a lower-mantle plume
Authors: Kirstein L. A. , Dunai T. J. , Davies G. R. , Upton B. G. J. , Nikogosian I. K.
Published: April 2008

Noble gas studies of well-characterized spinel-peridotite-facies lithospheric mantle xenoliths and garnet megacrysts from Scottish Permo-Carboniferous dykes, sills and vents demonstrate that the mantle beneath Scotland during the late Palaeozoic was more radiogenic than the source of mid-ocean ridge basalts (MORB). The samples were collected from the Northern Highland Terrane and the Midland Valley Terrane, which vary from Archaean-Proterozoic to Proterozoic-Palaeozoic in age. Helium isotope ratios of between 3
R
a
and 6
R
a
(
R
a
= atmospheric ratio) indicate that there has been time-integrated U-Th enrichment of the subcontinental mantle. This enriched mantle was preferentially melted following the transition from early Palaeozoic compression to late Palaeozoic extensional tectonics. Helium isotope ratios provide no clear evidence for the presence of undegassed plume-type mantle beneath this part of Scotland during the Permo-Carboniferous. The measured helium ratios do not discount the presence of a low-helium plume similar to those of the European Cenozoic volcanic province. A passive origin, however, is preferred for the Permo-Carboniferous magmatism due to the protracted activity, relatively small-extruded volumes of mafic magma and the low-helium isotope ratios measured.
Permo-Carboniferous extension-related magmatism at the SW margin of the Fennoscandian Shield
Authors: Obst K. , Solyom Z. , Johansson L.
Published: April 2008

Permo-Carboniferous rifting in Europe was accompanied by the widespread emplacement of mantle-derived magmas forming regional dyke swarms and sills in northern England, Scotland, Norway and southern Sweden during the late Stephanian and early Autunian. The trends of the dyke swarms intersect at a focal point in the Kattegat south of the Oslo Graben, and are probably all related to a single magmatic centre that could be plume-related. The WNW- to NW-trending dyke swarm at the SW margin of the Fennoscandian Shield in southern Sweden is composed mainly of tholeiitic dolerites, with lesser amounts of alkaline mafic rocks (camptonites, alkali basalts and spessartites) and trachytes. The alkaline mafic rocks are enriched in Ba, Sr, Nb, P and CO
2
, implying a metasomatic enrichment of their upper-mantle source prior to melting. After generation of alkaline melts by relatively small degrees of partial melting, increased extension was accompanied by the formation of subalkaline tholeiitic magmas. Whole-rock compositions (Mg-numbers between 55 and 30) and mineral chemistry (olivine Fo
60
-Fo
40
; clinopyroxene approximately Wo
30
En
45
Fs
15
; plagioclase An
70
-An
50
) indicate relatively evolved melts that have undergone crystal fractionation of olivine and clinopyroxene. Two groups of dolerites can be distinguished on the basis of bivariate element plots, e.g. Zr-TiO
2
and La-Sm. Although both groups show enrichment in the whole range of incompatible trace elements, slight differences in mantle-normalized trace-element patterns and different Nb/La ratios suggest that they were generated from two different sources. Group I dolerites were formed from a (re-)enriched, but isotopically mildly depleted, sublithospheric garnet-bearing mantle source (Nb/La mostly >0.9, ɛNd
i
= +4 to +3, where ɛNd
i
is the initial Nd isotope ratio), whereas group II dolerites seem to indicate mixing of the asthenospheric-derived magmas with lithospheric mantle melts (Nb/La mostly <0.9, ɛNd
i
= 0 to −1). Increasing Th/Ta ratios together with decreasing U/Nb ratios from group I towards group II dolerites further reflect progressive crustal contamination.
Post-Variscan evolution of the lithosphere in the Rhine Graben area: constraints from subsidence modelling
Authors: Ziegler P. A. , Schumacher M. E. , Dèzes P. , Van Wees J.-D. , Cloetingh S.
Published: April 2008

In the area of the Cenozoic Rhine rift system, crustal and lithospheric thicknesses range between 24 and 35 km, and 60 and 120 km, respectively. This rift system transects the deeply truncated Variscan Orogen and superimposed Permo-Carboniferous wrench-induced troughs, and Late Permian and Mesozoic thermal sag basins. At the time of its Westphalian consolidation, the Variscan Orogen was probably characterized by 45–60 km deep-crustal roots that were associated with its Rheno-Hercynian-Saxo-Thuringian, Saxo-Thuringian-Bohemian and Bohemian-Moldanubian sutures, all of which are transected by the Cenozoic Rhine rift system. During the Stephanian-Early Permian wrench-induced disruption of the Variscan Orogen, subducted lithospheric slabs were detached causing upwelling of hot mantle material. During the resulting thermal surge, partial delamination and/or thermal thinning of the continental mantle-lithosphere induced regional uplift. At the same time the Variscan orogenic roots were destroyed and crustal thicknesses reduced to 28–35 km in response to the combined effects of mantle-derived melts interacting with the lower crust, regional erosional unroofing of the crust and, on a more local scale, by its mechanical stretching. Towards the end of the Early Permian, the potential temperature of the asthenosphere returned to ambient levels. With this, regional, long-term thermal subsidence of the lithosphere commenced, controlling the development of a new system of Late Permian and Mesozoic thermal sag basins. However, the evolution of these basins was repeatedly overprinted by minor short-term subsidence accelerations that reflect the build-up of far-field stresses related to rifting in the Tethyan and Atlantic domains. Comparison of observed and modelled subsidence curves suggests that in the area of the Rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the Cretaceous at depths of 100–120 km, before it became thermally destabilized again by Cenozoic rifting and plume-related magmatism. Modelled subsidence curves indicate that by the end of Early Permian times the thermal thickness of the remnant mantle-lithosphere ranged between 10 and 50 km in areas that were later incorporated into Mesozoic thermal sag basins; this corresponds to mid-Permian thermal lithosphere thicknesses of 40–80 km.
Timing of Upper Carboniferous-Permian horst-basin formation and magmatism in the NW Thuringian Forest, central Germany: a review
Authors: Zeh Armin , Brätz Helene
Published: April 2008

During Late Carboniferous-Early Permian times dextral transtensional movements along the NW-trending Franconian Fault System and parallel faults caused complex block faulting in the Thuringian Forest region, Germany, accompanied by intense magmatism. This is well constrained by geochronological data (
207
Pb/
206
Pb single zircon, SHRIMP,
40
Ar/
39
Ar mica, zircon fission-track ages), field relations, and the sedimentary record from the Ruhla Crystalline Complex (RCC) and surroundings. The combined dataset indicates that the Ruhla Crystalline Complex was faulted into three nearly N-S-trending segments, which underwent different exhumation histories during Late Carboniferous-Permian times. The central segment of the RCC was exhumed by several kilometres as a horst block, while the eastern and western segments subsided simultaneously, forming the basement to the Oberhof and Eisenach molasse basins, respectively. Late Carboniferous-Permian uplift of the central segment is constrained by
40
Ar/
39
Ar cooling ages of 311 ± 3 (muscovite) and 294–288 ± 3 Ma (biotite), a weighted zircon fission-track age of 272 ± 7 Ma and overlying Zechstein sediments. In contrast, the eastern segment shows much older
40
Ar/
39
Albiotite cooling ages between 336 ± 4 and 323 ± 3 Ma, was exposed at
c.
300 Ma, and subsequently covered by molasse sediments and volcanic rocks between 300 and
c.
275 Ma. A similar Late Carboniferous evolution is inferred for the western segment, as it is also overlain by Lower Permian volcanic rocks and has a 297 ± 29 Ma single zircon fission-track age. Simultaneous horst and basin formation is additionally constrained by granite pebbles in conglomerates of the Oberhof and Eisenach basins. These pebbles can partly be derived from granites in the central segment of the RCC.

Age data and the orientation of granitoid bodies and dykes in the Ruhla Crystalline Complex and its surroundings provide evidence for the opening of NE-trending structures between 300 and 294 Ma, and formation or reactivation of W- to NW-trending structures between 290 and 275 Ma. Magmatic activity in the Thuringian Forest region may have been caused by widespread mantle upwelling in central Europe during the Late Carboniferous-Early Permian.
Permian magmatism and basin dynamics in the southern Pyrenees: a record of the transition from late Variscan transtension to early Alpine extension
Authors: Lago M. , Arranz E. , Pocoví A. , Galé C. , Gil-Imaz A.
Published: April 2008

Permian magmatism in the Pyrenees is characterized by two compositionally different and temporally consecutive magmatic episodes: a calc-alkaline-transitional phase (andesites) and a midly alkaline phase (basalts and dolerites). These two magmatic episodes were related to the attenuation of late Variscan transtensional tectonics and the onset of extension related to regional rifting. The strike-slip fault systems that affected the Pyrenees in late Variscan times initially controlled the development and morphology of the sedimentary basins. These were periodically affected by phases of extension, which controlled the subsidence of the basins, and, in addition, the emplacement of magmas. The whole-rock trace-element and isotopic signature of the andesites suggests that they were derived from the upper mantle and variably hybridized with late orogenic crustal melts, whereas the alkali basalts could have been derived from a lithospheric mantle source, enriched as a consequence of Variscan subduction processes with the contribution, in some areas, of an enriched (asthenospheric) component.
Lower Permian magmatism of the Iberian Chain, Central Spain, and its relationship to extensional tectonics
Authors: Lago M. , Arranz E. , Pocoví A. , Galé C. , Gil-Imaz A.
Published: April 2008

Late Variscan magmatism in the Iberian Chain (Central Spain) is recorded by the presence of both pyroclastic units and high-level intrusions (sills and dykes). This magmatism encompasses a variety of subalkaline igneous rocks, from basalt to rhyolite; andesitic rocks are, however, dominant. The pyroclastic units contain plant fossils and pollen that suggest an Autunian age, which is consistent with the available K-Ar radiometric age data (283–292 Ma) obtained for the hypabyssal intrusions. Earlier amphibole-rich andesite, rich in crustal xenoliths (metapelite, granitoid and quartzite fragments) and xenocrysts (e.g. garnet) suggest the occurrence of significant crustal assimilation in the petrogenesis of the intermediate magmas, whereas later intrusions (pyroxene-rich andesites, basalts and some rhyolites) lack any crustal xenoliths or xenocrysts. Crustal assimilation is also suggested by the radiogenic
87
Sr/
86
Sr
290
and negative ɛNd
290
values for all the studied rocks. The cross-cutting relationships between the different rock types provide evidence for a multi-stage magmatic evolution, coeval with the post-orogenic transtensive evolution of the southern Variscan belt in Europe. A significant hiatus, covering the Middle Permian and most of the Upper Permian, separates this Lower Permian magmatism from the subsequent Triassic and Jurassic alkaline magmatism that represents different rifting events that affected the Iberian Chain, progressively thinning the Variscan crust as the Alpine cycle began.
Instability and localization of deformation in lower crust granulites, Minas fault zone, Nova Scotia, Canada
Authors: White Joseph C.
Published: April 2008

Blocks of granulite from within the megabreccia at Clarke Head, Nova Scotia, Canada contain extremely well preserved mylonitic and ultramylonitic textures developed in mineral assemblages for which thermobarometic calculations have indicated temperatures and pressures between 700–860 °C and 750–950 MPa. Deformation within these rocks is characterized by localization at several discrete length scales associated with the development of new microstructures comprising finer-grained material. Mylonitized granulite exhibits dislocation creep microstructures, with development of intense S-C fabrics and shear bands during the transition to ultramylonite. Dynamically recrystallization of plagioclase can be followed through progressive grain size reduction to about 5 μm, but there remain extensive zones with grains less than 1 μm in diameter. Localization of the these finest-grained ultramylonites occurs by transient frictional events associated with the introduction of partial igneous melts and formation of pseudotachylyte which produces abrupt decreases in grain size that cannot arise during dislocation mediated grain size reduction.

The heterogeneous response of these rocks demonstrates the importance of considering characteristic length scales when assigning evidence from the rock record (e.g. palaeopiezometry) to bulk behaviour of the lithosphere. Associated with the localization of strain and subsequent strain softening is the observation that microstructures formed during the event that initiated the instability can be an obliterated by ductile flow. In instances where critical components of the microstructural evolution are known to have been largely overprinted, it becomes possible to reconcile contradictions in the rock record, such as production of ultra-fine-grained superplastic aggregates in what otherwise appears to be a dominantly dislocation creep regime.
The application of GIS to unravel patterns of deformation in high grade terrains: a case study of indentor tectonics from west Greenland
Authors: Piazolo Sandra , Alsop G.I. , Nielsen B. Møller , Van Gool J.A.M.
Published: April 2008

The ability to compare, integrate and knit together multidisciplinary datasets in terms of subject, space and scale is critical to the recognition of geological patterns. In this contribution, we show that the use of Geographic Information Systems (GIS) is extremely valuable in detecting patterns associated with broad zones of deformation in high grade terrains. The GIS methodology facilitates the geological interpretation and development of models as it permits an easy and quick investigation of several geoscientific datasets by subject, space and scale. The GIS-based integration of structural, metamorphic, fabric type and aeromagnetic datasets collected in west Greenland shows that patterns seen within one dataset coincide with patterns observed in other datasets. Consequently, two major domains are recognized that are separated by a broad boundary zone. The southern block is characterized by a distinct, irregular magnetic signal coupled with granulite facies metamorphism and dominant S-type fabrics. The map scale geometry of this block controls the patterns observed within the amphibolite facies domain further north. Foliation and lineation patterns form an arcuate swing in strike about the southern block. Fabric types vary both around the strike swing and across strike. An indentor model that incorporates a rigid, cooled granulite block in the south bounded to the north by a rheologically weaker amphibolite facies domain can explain these patterns. The preserved metamorphic grade governs the rheology of the different, but essentially authochthonous blocks with the amphibolite facies domain being plastered and ‘moulded around’ the rigid granulite indentor. As patterns of remote geophysical and geological data closely correspond with one another, greater confidence may be placed in the application of remote geophysics in areas which lack abundant ground-based data.
Deformation in a complex crustal-scale shear zone: Errabiddy Shear Zone, Western Australia
Authors: Occhipinti S. A. , Reddy S. M.
Published: April 2008

Detailed mapping of four areas representing different geological units with varying formation histories within the crustal-scale Errabiddy Shear Zone shows an apparently simple temporal progression from foliation and mineral lineation development to folding and then to brittle deformation across the shear zone. However, in detail the structural evolution of the shear zone shows considerable complexity. The dominant foliation throughout the shear zone was formed in the upper greenschist to amphibolite facies during the 2000–1960 Ma Glenburgh Orogeny, which involved the accretion of the Archaean to Palaeoproterozoic Glenburgh Terrane onto the Archaean Yilgarn Craton and the subsequent formation of the Errabiddy Shear Zone. Orthorhombic kinematic indicators formed during the Glenburgh Orogeny as did the widespread mineral lineation. These fabrics were overprinted by a greenschist facies deformation and metamorphic event during the 1830–1780 Ma Capricorn Orogeny. During the Capricorn Orogeny mineral lineation development was rare, and mostly took place in high-Capricorn strain zones in areas where a pre-existing Glenburgh-aged mineral lineation was present. Such mineral lineations trend parallel to Capricorn-aged fold hinges. Regardless of the presence or absence of Capricornaged mineral lineations, dextral strike-slip kinematics and simple shear indicated by delta and sigma porphyroclasts, and displacement along detachment faults, are prevalent close to discrete shear zone boundaries, within the Errabiddy Shear Zone. However, between shear zone boundaries flattening and coaxial strain dominated during the Capricorn Orogeny. This difference in Capricorn Orogeny kinematics throughout the shear zone is caused by strain partitioning — although progressive deformation throughout the shear zone with dextral strike-slip faults overprinting older structures formed by pure shear also took place. These results suggest that analyses of small parts of shear zones may not give the complete history of an evolving transpressional shear zone because of the presence of strain partitioning and strain localization over time.
Strain and vorticity analysis of transpressional high-strain zones from the Virginia Piedmont, USA
Authors: Bailey Christopher M. , Francis Barbara E. , Fahrney Eleanor E.
Published: April 2008

Strain and vorticity analysis of two Late Palaeozoic high-strain zones from the southern Appalachian Piedmont indicates that these zones experienced general shear transpression with a monoclinic to triclinic symmetry. Granitic rocks in the Brookneal high-strain zone from the southwestern Virginia Piedmont were transformed into mylonites under greenschist facies conditions. Sectional strains, estimated from quartz grain shapes, in mylonites range from three to ten and three-dimensional fabrics record flattening strains. The mean vorticity number (
W
m
) estimated with the
R
s
/θ method ranges from 0.3 to 0.95. In the central Virginia Piedmont, lower amphibolite facies deformation in the Spotsylvania high-strain zone affected biotite gneisses, amphibolites, and granitic pegmatites. Minimum sectional strains, estimated from folded and boudinaged pegmatite dykes, of 8–20 are common and three-dimensional strains are dominantly constrictional. Porphyroclast hyperbolic distribution analysis of ultramylonites yields
W
n
values from 0.4 to 0.8. The kinematic significance of these transpressional high-strain zones is threefold: they record tens to hundreds of kilometres of strike-slip offset; 40 to 70% contraction normal to the zone; and significant orogen-parallel material elongation.
Dr John Snow and an early investigation of groundwater contamination
Authors: Price Michael
Published: April 2008

John Snow was a physician but his studies of the way in which cholera is spread have long attracted the interest of hydrogeologists. From his investigation into the epidemiology of the cholera outbreak around the well in Broad Street, London, in 1854, Snow gained valuable evidence that cholera is spread by contamination of drinking water. Subsequent research by others showed that the well was contaminated by sewage. The study therefore represents one of the first, if not the first, study of an incident of groundwater contamination in Britain. Although he had no formal geological training, it is clear that Snow had a much better understanding of groundwater than many modern medical practitioners. At the time of the outbreak Snow was continuing his practice as a physician and anaesthetist. His casebooks for 1854 do not even mention cholera. Yet, nearly 150 years later, he is as well known for his work on cholera as for his pioneering work on anaesthesia, and his discoveries are still the subject of controversy.
William Whitaker (1836–1925) — geologist, bibliographer and a pioneer of British hydrogeology
Authors: George William H.
Published: April 2008

William Whitaker was employed by the Geological Survey from 1857 until 1896 and subsequently worked as a consultant until his death in 1925. This paper examines the background to the era in which he worked and why he merits detailed consideration. Whitaker’s personal life, career in the Geological Survey, contribution to learned societies and field clubs, work in retirement and his death are detailed. His contradictory personality, contribution to hydrogeology and his claim to the title of ‘father of English hydrogeology’ are assessed.
19th century studies of the hydrogeology of the Permo-Triassic Sandstones of the northern Cheshire Basin, England
Authors: Tellam John H.
Published: April 2008

About a dozen workers were active in researching the hydrogeology of the Permo-Triassic sandstones of the northern Cheshire Basin, UK, in the 19th century. They were mostly amateur geologists, members of the geological societies of Liverpool and Manchester. Spurred by the water resource requirement of the two cities and by the formation of the societies, research burgeoned from the mid 1800s. Over the latter part of the century, a conceptual model of flow in the sandstones was developed which has most of the essential features of a present-day conceptual model, including intergranular and fracture flow, fault influence on flow, recharge reduction by drift and urban land cover, overspill recharge, influent river recharge (including estuarine intrusion), and cross-boundary flow. Water balances were undertaken to assess aquifer yield and attempt to understand well yields. Pollution from sewers, river water, estuary water, and graveyards was considered, as was water/rock interaction. Experimental work demonstrated: the proportionality between flow rate and pressure difference (1869); the importance of fractures to well yields (1850); the principle of specific yield (1869); the effects of lamination on unsaturated flow (1877); and the shape of breakthrough curves is sigmoidal (1878). Most of these findings were independent of previous work elsewhere. Whether this is because of the local, essentially amateur environment in which the researchers were operating, or whether this lack of communication was essentially a feature of research at this time, is uncertain. However, it does have implications for general theories of science.
Groundwater versus surface water in Scotland and Ireland — the formative years
Authors: Robins N. S. , Bennett J. R. P. , Cullen K. T.
Published: April 2008

Celtic interest in groundwater has continued to the modern era in much of Scotland and Ireland, despite abundant good quality surface waters. Groundwater investigation in the 19th and 20th centuries was prompted by the need to remove water from mine workings in Scotland and to provide water for industry in the Midland Valley of Scotland and the Lagan Valley in the north of Ireland. Little development took place in the south of Ireland until relatively recently. Champions of groundwater investigation include the venerable Scottish geologists Ben Peach and John Horne, as well as lesser known advocates of hydrogeology such as John Jerome Hartley in Ireland. These workers were supported by numerous people directly and indirectly involved with developing the understanding of the groundwater resources of Scotland and Ireland.
The first use of geophysics in borehole siting in hardrock areas of Africa
Authors: Barker Ron
Published: April 2008

The first resistivity soundings applied to borehole siting in Basement areas of Africa were probably measured by Dr. Sydney Shaw in 1933 in Southern Rhodesia, now Zimbabwe. Using a Megger Earth Tester and Wenner array, soundings were measured at a selection of both dry and successful boreholes, and compared. Re-examination of Shaw’s soundings using modern technology shows that the sounding curves are of surprisingly high quality even on today’s standards and indicate a strong relationship between interpreted depth to unfractured granite and drilling results. His survey helped the adoption in Africa of resistivity methods as standard practice in the location of drilling sites for water supply.
The development of groundwater in the UK between 1935 and 1965 — the role of the Geological Survey of Great Britain
Authors: Downing R. A.
Published: April 2008

After the drought of 1933–1934 the Geological Survey became responsible, under the Inland Water Survey, for collecting and collating data on groundwater. In 1935 a Water Unit was formed for this purpose. Following the Water Act of 1945, the Survey advised the Government on aspects of the Act relating to groundwater. The Act led to the introduction of quantitative hydrogeology in England and Wales. The groundwater resources of the main aquifers were assessed, well hydraulic theory was applied to British aquifers, and geophysical techniques and new instrumentation introduced.
Norman Savage Boulton (1899–1984): civil engineer and groundwater hydrologist
Authors: Downing R. A. , Eastwood W. , Rushton K. R.
Published: April 2008

Norman S. Boulton (Fig. 1) was a civil engineer who achieved international recognition for his work on groundwater hydraulics. He recognized that in unconfined aquifers water is released from storage by drainage under gravity from the pore-spaces in the cone of depression as it expands. This ‘delayed yield’ gave a characteristic S-shape to the log-log, time-drawdown graph of water levels in an observation well near a pumping well. Boulton developed a mathematical solution that reproduced the three segments of the curve. Most of his career was spent in academia mainly at the University of Sheffield where he was Professor of Civil Engineering between 1955 and 1964. His work embraced studies of structural engineering and soil mechanics as well as groundwater flow.
Recollections of a golden age: the groundwater schemes of Southern Water 1970–1990
Authors: Headworth H. G.
Published: April 2008

The creation of the river authorities and Water Resources Board in 1965 and the Water Authorities, Water Research Centre and Central Water Planning Unit in 1974 led to an explosion of groundwater investigation and development in England and Wales. In Southern Water’s region, from the Hastings Beds of the Wealden Series to the Recent beach gravels at Dungeness, a dozen or so schemes were carried out to investigate and develop aquifers and manage their groundwater resources. Six schemes are described here, including artificial recharge in Sussex, groundwater augmentation in Hampshire and the assessment of saline contamination from minewater disposal in East Kent.
Determining Precambrian crustal evolution in China: a case-study from Wutaishan, Shanxi Province, demonstrating the application of precise SHRIMP U-Pb geochronology
Authors: Wilde Simon A. , Cawood Peter A. , Wang Kaiyi , Nemchin Alexander , Zhao Guochun
Published: April 2008

SHRIMP U-Pb zircon analyses from eight samples of metamorphosed intermediate to felsic volcanic rocks from the lower, middle and upper ‘subgroups’ of the Wutai sequence in the North China Craton define a weighted mean
207
Pb/
206
Pb age of 2523 ± 3 Ma. Although individual rock ages range from 2533 ± 8 Ma to 2513 ± 8 Ma, all overlap within the error of the mean and do not support a stratigraphic interpretation for the sequence, since variations within individual previously assigned ‘formations’ in the sequence match the total age range. Contrary to previous interpretations, there is no correlation in age with metamorphic grade. These features highlight the need to reformulate stratigraphic schemes when defining the Precambrian geology of the North China Craton. The similarity in age between volcanic rocks of the Wutai Complex and higher-grade gneisses of the adjacent Fuping and Hengshan complexes supports the view that all three complexes represent portions of a Late Archaean arc complex that was tectonically dismembered and then re-assembled. There is no Fuping or Wutai orogeny in this, its type area: all three complexes were deformed and metamorphosed during collision of the eastern and western blocks of the North China Craton in the Lüliang orogeny
c.
1.8 Ga ago.
Precambrian tectonic evolution of the North China Craton
Authors: Zhai Mingguo
Published: April 2008

The North China Craton (NCC) is a major Archaean craton, covering an area of
c.
300 000 km
2
in north and northeast China. Almost all Archaean rocks on the craton experienced high-grade metamorphism and strong migmatization, so that the preserved greenstone belts underwent granulite-amphibolite-facies metamorphism, anatectic melting and strong deformation. This suggests that the NCC may have a more complicated early tectonic history than most other cratonic nuclei. The oldest NCC rocks are 3.8 Ga granitic gneisses in NE China and supracrustal rocks in eastern Hebei. Major continental growth occurred at 2.9–2.7 Ga. Two subsequent high-grade metamorphic events occurred at 2.6–2.45 Ga (‘2.5 Ga event’) and 1.9–1.75 Ga (‘1.8 Ga event’). The older episode is considered to mark an amalgamation event, whereas the 1.8 Ga event represents the final cratonization of the NCC. Some researchers have divided the 1.8 Ga event into a 1.9–1.8 Ga metamorphic event (interpreted as a continent-continent collision) followed by a 1.8–1.65 Ga rifting episode. Other workers have suggested that the metamorphism and rifting could be parts of a single tectonic event related to Palaeo-Mesoproterozoic mantle upwelling. The general consensus on the NCC for the period 2.5–1.8 Ga is that the craton was then in an inactive stage. However, in this paper it is proposed that several Palaeoproterozoic mobile belts existed (showing many of the characteristics of Phanerozoic orogens). During the Mesoproterozoic-Neoproterozoic, a set of sedimentary sequences (the Changcheng-Jixian-Qingbaikou systems) constituted a disconformable-pseudoconformable succession within an intra-cratonic aulacogen. The signature of a 1.4–0.9 Ga orogen and the Rodinia breakup is very weak, indicating that the NCC did not experience major deformation as it was amalgamated into the Rodinia supercontinent.
Subduction, collision and exhumation in the ultrahigh-pressure Qinling-Dabie orogen
Authors: Hacker Bradley R. , Ratschbacher Lothar , Liou J. G.
Published: April 2008

High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) attended suturing of the Yangtze craton to Rodinia during the
c.
1.0 Ga Grenvillian orogeny. The Qinling microcontinent then rifted from the Yangtze craton at
c.
750 Ma. The Erlangping intraoceanic arc formed in the Early Ordovician, was emplaced onto the Qinling microcontinent in the Ordovician-Silurian, and then both units were accreted to the Sino-Korea craton before being stitched together by the
c.
400 Ma Andean-style Qinling arc. Subsequent subduction beneath the Qinling-Sino-Korean plate created a Devonian-Triassic accretionary wedge that includes eclogites, and formed a coeval volcano-plutonic arc that stretches from the Longmen Shan to Korea. In the Late Permian-Early Triassic, the northern edge of the South China Block was subducted to >150 km depth, creating the diamond- and coesite-bearing eclogites of the Dabie and Sulu areas. Exhumation from the mantle by lithosphere-scale extension occurred between 245 and 195 Ma during clockwise rotation of the craton. The Yangtze-Sino-Korea suture locally lies tens of km north of the exhumed UHP-HP part of the South China Block, implying perhaps that the very tip of the South China Block was not subducted, or that the UHP-HP rocks rose as a wedge that peeled the upper crust of the unsubducted South China Block from the lower crust. The Tan-Lu fault is an Early Cretaceous to Cenozoic feature. The apparent offset of the Dabie and Sulu UHP terranes by the Tan-Lu fault is a result of this Cretaceous to Cenozoic faulting combined with post-collisional extension north of Dabie.
Jurassic intraplate magmatism in southern Hunan-eastern Guangxi:
40
Ar/
39
Ar dating, geochemistry, Sr-Nd isotopes and implications for the tectonic evolution of SE China
Authors: Li Xian-Hua , Chung Sun-Lin , Zhou Hanwen , Lo Ching-Hua , Liu Ying , Chen Chang-Hwa
Published: April 2008

The Mesozoic geology of SE China is characterized by intensive and widespread magmatism. However, the tectonic regime that accounted for the Mesozoic magmatism has been an issue with little consensus. A comprehensive study of
40
Ar-
39
Ar dating, geochemistry and Sr-Nd isotopes has been conducted on basalts from southern Hunan and syenite intrusions from eastern Guangxi. Three episodes of Jurassic magmatism, i.e. alkaline basalts of
c.
175 Ma in age, syenitic intrusions of
c.
160 Ma and high-Mg basalts of
c.
150 Ma, are identified. The older,
c.
175 Ma alkaline basalts are characterized by low Sr (
I
Sr
= 0.7035–0.7040) and high Nd (
ε
Nd
(
T
) = 5 to 6) isotopic compositions and OIB-like trace-element patterns (e.g. Nb/La > 1). In contrast, the younger,
c.
150 Ma high-Mg basalts have high Sr (
I
Sr
c.
0.7054) and low Nd (
ε
Nd
(
T
)
c.
-2) isotopic compositions and incompatible trace-element patterns of arc affinity. The
c.
160 Ma syenitic intrusions display a relatively large range of Sr and Nd isotopic compositions (
I
Sr
= 0.7032–0.7082,
ε
Nd
(
T
) = 5.5 to −4.1), with the Qinghu syenites having the lowest
I
Sr
, highest
ε
Nd
(
T
) and OIB-type incompatible trace-element patterns analogous to the
c.
175 Ma alkaline basalts. Such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources, which are inferred to have resulted from the post-Indosinian orogenic lithosphere extension and thinning. The
c.
175 Ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the asthenosphere or an enriched lithospheric mantle source accreted by asthenosphere-derived melts during the initial extension. The
c.
160 Ma syenitic and
c.
150 Ma high-Mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning. This interpretation is at odds with the active continental margin related to the subduction of palaeo-Pacific plate, but consistent with continental rifting and extension for the Mesozoic of SE China.
Evidence for the multiphase nature of the India-Asia collision from the Yarlung Tsangpo suture zone, Tibet
Authors: Aitchison Jonathan C. , Davis Aileen M.
Published: April 2008

Recent investigations in southern Tibet enable the testing and refinement of existing models for India-Asia collision. Presently available data indicate that marine deposition continued in the southern central portion of Tibet until at least the end of the Eocene. Sub-duction-related magmatism continued until the Mid-Oligocene, after which rapid uplift of the plateau was initiated. Mass-wasting of sediments into molasse basins did not commence until the latest Oligocene. The implications are that existing models, based on less-precise age constraints, invoking India-Asia collision at 55 Ma, are either flawed, or collision began at a different time. Recent work has produced sufficient data to allow the recognition of two different collisional events along the suture between India and Asia. Features related to each event require separate interpretation, and no collisional continuum should be assumed. In southern Tibet, a collision between the northern margin of India and a southfacing intra-oceanic island arc occurred at around 55 Ma, whereas continent-continent collision between India and Asia did not occur until at least 20 million years later.
Cenozoic tectonics of the China continental margin: insights from Taiwan
Authors: Teng Louis S. , Lin Andrew T.
Published: April 2008

The continental margin to the east and south of China comprises an active margin in the East China Sea, a collision mountain belt in Taiwan, and a passive margin in the South China Sea. These three segments were generally regarded as separate tectonic entities and their interrelations have long been the subject of debate. Here we synthesize available information to outline the tectonic and geological background of the China margin, examine the link between Taiwan and the neighbouring China margins, and thereby establish a Cenozoic evolutionary model.

The China margin is floored with a pre-Cenozoic continental basement covered with an up to 10-km-thick pile of Cenozoic sedimentary strata. The continental basement has been invariably stretched and moulded into a series of northeast-trending horsts and grabens. Except in the Okinawa Trough of the East China Sea, the Cenozoic sedimentary cover typically exhibits a two-tier tectonostratigraphic structure, with narrow Palaeogene rift basins draped by a blanket-like Neogene-Quaternary sequence. The two-tier structure prevails in the entire inner part of the China margin, including the Taiwan Strait off western Taiwan. In the outer China margin, however, the two-tier structure persists only in the South China Sea, and is in stark contrast with the collisional orogen of Taiwan and the Ryukyu arc of the East China Sea.

By untangling the contractional deformation of the northern Taiwan mountain belt, it has been possible to reconstruct a precollisional tectonostratigraphic section with a distinctive two-tier structure shown by a Palaeogene half-graben covered with a Miocene drape sequence. When put together with Palaeogene rift basins of the Taiwan Strait, it becomes clear that the precollisional continental margin of Taiwan resembles that of the South China Sea, characterized by two lines of Palaeogene rift basins. Hence before the collision started in Late Miocene times, Taiwan was part of the passive South China margin that extended northward to the southern Ryukyu area.

Ever since the end of the Cretaceous, the China continental margin has been dominated by extensional tectonics, regardless of the presence or absence of subduction zones. In the Early Cenozoic, extensive crustal attenuation resulted in region-wide subsidence and formation of rift basins. Extension in the South China Sea culminated in Late Oligocene times, when part of the outer margin was drifted away by the opening ocean basin. In the East China Sea, the margin remained intact and became separated from the South China Sea margin by a transform fault. From the Miocene onwards, the South China Sea margin has been passively subsiding, sporadically punctuated with basaltic volcanism. In the East China Sea margin, the Okinawa Trough has opened and the Ryukyu volcanic arc thrived. The NE edge of the South China Sea margin was deformed as the Taiwan orogen.
Precisely relocated hypocentres, focal mechanisms and active orogeny in Central Taiwan
Authors: Wu F. T. , Chang C. S. , Wu Y. M.
Published: April 2008

The 1999 Chi-Chi earthquake series occurred in Central Taiwan, where ongoing mountain building is most active. The pre- and post-Chi-Chi seismicity helps to clarify the internal orogenic activity. The 27 000 earthquakes from the 1993–2002 catalogues have been relocated with greater precision. By associating the seismicity with focal mechanisms, many structures inside the orogen have been mapped. Among them are a steeply dipping thrust fault in the deep crust; a 50-km-long left-lateral strike-slip fault in the south; and an Eastern Central Range NNE-striking normal fault. While the deep crustal thrust appears to contribute to the root-building, the southern strike-slip slip fault accommodates the mainshock fault motion, and the Eastern Central Range normal faulting probably occurs mainly after a major western Taiwan thrust type earthquake. Much of the Backbone Range and the Eastern Central Range were seismically quiescent before and after the Chi-Chi earthquake. The contrast in the seismicity of the Central Range and the surrounding regions implies different material behaviour in these different regimes of the orogen.
Strain modelling, seismic anisotropy and coupling at strike-slip boundaries: applications in New Zealand and the San Andreas fault
Authors: Savage M. K. , Fischer K. M. , Hall C. E.
Published: April 2008

Despite similar surface transform faulting behaviour, observed shear-wave splitting patterns in the California and New Zealand plate boundary regions are markedly different. To better understand the origin of the seismic anisotropy in these regions we model mantle flow and strain for a variety of strike-slip plate boundary scenarios. Simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths. Strain-controlled models fit the observations in New Zealand and California better than simplified flow-controlled models. Fast shear polarizations are progressively rotated toward the shear plane over time, and even a constant-viscosity model provides a good fit to the fast directions in New Zealand and southern California. The constant viscosity implies strong coupling between the surface and the deeper mantle. To fit the lack of decrease in delay times with distance from the fault, the relationship between delay time and strain must saturate at small strains. If this is the case, then strain in southern New Zealand and southern California may be small, equivalent to that achieved along an infinite fault by about 3–10 Ma of their present motion. Stratified viscosity allows more rapid rotation of fast directions toward fault-parallel than occurs in isoviscous models, and can explain the nearly fault-parallel fast directions in the central South Island. Different aspects of the northern California results are fitted with different models, but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy suggesting vertical decoupling.
Mantle-driven deformation of orogenic zones and clutch tectonics
Authors: Tikoff Basil , Russo Ray , Teyssier Christian , Tommasi Andréa
Published: April 2008

Compatible deformation between the upper crust and upper mantle is documented for a variety of ancient and neotectonic settings, suggesting that these lithospheric layers are coupled. Areas of neotectonic deformation are also characterized by high seismic attenuation, indicating that the uppermost mantle is rheologically weak and flowing in these regions. The flow of the mantle, both lithospheric and asthenospheric, potentially drives deformation in continental orogenic zones. Three-dimensional models, controlled by bottom-driven mantle flow, are proposed for obliquely convergent, transcurrent and obliquely divergent plate margins. Our analysis indicates that the absolute, and not just relative, plate motions play a critical role in the orogenic cycle.
Vertical-axis rotation of rigid crustal blocks driven by mantle flow
Authors: Giorgis Scott , Markley Michelle , Tikoff Basil
Published: April 2008

Vertical-axis rotation of rigid crustal blocks occurs in a variety of obliquely convergent and divergent plate boundaries. We quantify the rotation of these blocks using models of transpressional and transtensional kinematics, and corroborate our results using physical models where rigid blocks rotate in response to flow of a ductile substrate. Consequently, one can explicitly demonstrate a relationship between the amount of rotation of a rigid crustal block and strain recorded in ductile substrate. This strain should be reflected directly by the orientation of rock fabrics, such as those measured by shear-wave splitting in the
in situ
upper mantle.s

We apply this approach to southern California and New Zealand by using previously documented palaeomagnetic rotations and plate motion vectors, and calculate the strain recorded by the material below rigid blocks. These strain calculations are compared to shear-wave splitting data, which record upper mantle fabric, from the same region. Our model results suggest that similar deformation is recorded by the upper crust and lithospheric mantle. A bottom-driven flow, in which mantle deformation drives upper crustal rotations, is most consistent with these observations.
Growth and demise of continental arcs and orogenic plateaux in the North American Cordillera: from Baja to British Columbia
Authors: Whitney Donna L. , Paterson Scott R. , Schmidt Keegan L. , Glazner Allen F. , Kopf Christopher F.
Published: April 2008

In the North American Cordillera, crustal thickening, magmatism and flow of deep crust created an orogenic plateau, or series of related plateaux, in the Late Mesozoic-Early Cenozoic. From west to east, the plateaux extended from the continental arcs to the inboard crystalline belts of the Omineca-Sevier belt. From north to south, the plateaux ranged from British Columbia/SE Alaska to Baja California, Mexico. Although a vast region of western North America was characterized by thickened crust (60–70 km), unroofing of deep crust from >30 km was largely confined to the edges of the plateaux: the arcs and the eastern margins. Comparison of the unroofing histories of the Cordilleran arcs reveals that they differed dramatically from each other in the amount and style, but not timing, of exhumation. The northern Cordilleran arc and northern interior (Omineca) belt were exhumed from deep mid-crustal levels, with regional-scale Eocene extension accompanied by magmatism. In contrast, the central (Sierra Nevada) and southern (Peninsular Ranges) arcs were unroofed to much shallower levels (typically <15 km), primarily by erosion and local deformation. North to south differences in exhumation style and magnitude in the Cordilleran arcs may reflect differences in the degree of coupling between the subducting plate and the thickened continental lithosphere in the north v. south. In the northern Cordillera, relationships between Pacific-region plate activity and Tertiary continental extension/magmatism and deep exhumation suggest continued geodynamic coupling between subducting plates and orogenic crust following crustal thickening and plateau formation. In contrast, the central and southern Cordilleran arcs do not contain evidence for mechanical links with the subducting plate after the Late Cretaceous.