Journal of the Geological Society

Published by Geological Society
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Concentrations of organic carbon in samples from the Peterborough Member ranged from 0.5 to 16.6% and δ values of total organic carbon (TOC) ranged from -27.7 to -23.1‰ v. PDB. Isotopic composition of pristane and phytane in the Peterborough and Stewartby Members average -31.7‰, those in the Weymouth Member average -29.8‰. Values of δ for long-chain n-alkanes average -28‰. Together these indicate δ values for primary inputs as follows: terrestrial vascular plants, -23.5‰; Peterborough Member algae, -28.2; Stewartby Member algae, -29.1‰; Weymouth Member algae, -26.6‰. -from Authors
 
Here we argue that life emerged on Earth from a redox and pH front at c. 4.2 Ga. This front occurred where hot (c. 150 degrees C), extremely reduced, alkaline, bisulphide-bearing, submarine seepage waters interfaced with the acid, warm (c. 90 degrees C), iron-hearing Hadean ocean. The low pH of the ocean was imparted by the ten bars of CO2 considered to dominate the Hadean atmosphere/hydrosphere. Disequilibrium between the two solutions was maintained by the spontaneous precipitation of a colloidal FeS membrane. Iron monosulphide bubbles comprising this membrane were inflated by the hydrothermal solution upon sulphide mounds at the seepage sites. Our hypothesis is that the FeS membrane, laced with nickel, acted as a semipermeable catalytic boundary between the two fluids, encouraging synthesis of organic anions by hydrogenation and carboxylation of hydrothermal organic primers. The ocean provided carbonate, phosphate, iron, nickel and protons; the hydrothermal solution was the source of ammonia, acetate, HS-, H2 and tungsten, as well as minor concentrations of organic sulphides and perhaps cyanide and acetaldehyde. The mean redox potential (delta Eh) across the membrane, with the energy to drive synthesis, would have approximated to 300 millivolts. The generation of organic anions would have led to an increase in osmotic pressure within the FeS bubbles. Thus osmotic pressure could take over from hydraulic pressure as the driving force for distension, budding and reproduction of the bubbles. Condensation of the organic molecules to polymers, particularly organic sulphides, was driven by pyrophosphate hydrolysis. Regeneration of pyrophosphate from the monophosphate in the membrane was facilitated by protons contributed from the Hadean ocean. This was the first use by a metabolizing system of protonmotive force (driven by natural delta pH) which also would have amounted to c. 300 millivolts. Protonmotive force is the universal energy transduction mechanism of life. Taken together with the redox potential across the membrane, the total electrochemical and chemical energy available for protometabolism amounted to a continuous supply at more than half a volt. The role of the iron sulphide membrane in keeping the two solutions separated was appropriated by the newly synthesized organic sulphide polymers. This organic take-over of the membrane material led to the miniaturization of the metabolizing system. Information systems to govern replication could have developed penecontemporaneously in this same milieu. But iron, sulphur and phosphate, inorganic components of earliest life, continued to be involved in metabolism.
 
The hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera. In the interval from the Permian to Recent, these data encompass some 13,000 generic extinctions, providing a more sensitive indicator of species-level extinctions than previously used familial data. Extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 Ma intervals over the last 270 Ma. Most of these peaks correspond to extinction events recognized in more detailed, if limited, biostratigraphic studies. These new data weaken or negate most arguments against periodicity, which have involved criticisms of the taxonomic data base, sampling intervals, chronometric time scales, and statistical methods used in previous analyses. The criticisms are reviewed in some detail and various new calculations and simulations, including one assessing the effects of paraphyletic taxa, are presented. Although the new data strengthen the case for periodicity, they offer little new insight into the deriving mechanism behind the pattern. However, they do suggest that many of the periodic events may not have been catastrophic, occurring instead over several stratigraphic stages or substages.
 
This paper describes deformation fabrics developed in the northern part of the Hamisana zone in northeast Sudan. New structural data are presented which establish a structural chronology that characterizes distinct events of accretion, folding, and thrust faulting and reactivation of accretion-related faults. The structural data point to an intraplate compressional origin for the Hamisana zone. A review of available isotopic age data is carried out, and it is concluded that Pan-African accretionary processes may have been analogous to Phanerozoic ophiolite and island arc accretion in the western North American Cordillera, where penetrative deformation occurred in response to periodic intraplate shortening events, rather than an ultimate collision of unrelated crustal fragments.
 
Lunar geology evidence is examined for clues to the origin and evolution of the moon and earth. Seven evolutionary episodes, the last covering three billion years to the present day, are constructed for the moon. Parallel episodes in the earth's evolution are masked by the dynamic continuing evolution of the earth over a 4.5 billion year span, in contrast to the moon's quiescence and inability to retain fluids. Comparisons are drawn between the geochemistry and tectonics of the lunar basaltic maria and the earth's ocean basins. Lunar maria rocks differ strikingly in chemical composition from meteoritic matter and solar material. Inundation of frontside lunar maria basins by vast oceans of dark basalt mark the last of the major internally generated evolutionary episodes, and is attributed to consequences of meltdown of the lunar mantle and crust by radioisotope decay from below. Data are drawn primarily from Apollo missions 11-17, supplemented by other sources.
 
Two neighbouring areas of tholeiitic basalts, dolerites, gabbros and tonalites in E Jamaica are recognized as the upper section (approx 2.5 km thick) of an ophiolite. The basalts are overlain by a thick series of Maestrichtian volcaniclastic rocks and are faulted against a blueschist terrain. Representative rock analyses (12) and probe analyses of clinpyroxenes (5) are tabulated. Recognition of this subduction zone complex and regional considerations of island-arc polarity necessitates ocean crust being consumed by a southward-dipping subduction zone; the wider implications for Caribbean geology are discussed. -R.A.H.
 
When the San Andreas fault system of California and the Alpine fault system of New Zealand are compared within the reference frame of their regional slip vectors, their major tectonic elements are seen to occupy similar positions. This argues that a genetic relationship exists between the development of these complexities. Furthermore, three distinctive modes of slip, or 'seismic style', can be recognized for different segments of these major transform fault systems. These are: (I) slip during great (M ≥ 8) earthquakes separated by long periods of quiescence; (2) slip during more frequent large (6.5 ≤ M ≤ 7.5) earthquakes, and (3) aseismic slip (creep). These different seismic styles occur in distinctive tectonic settings and it is argued that they represent fundamental variations in the mechanical properties of the faults. I suggest that style 1 occurs along sections of the fault orientated such that a component of tectonic convergence occurs, leading to a high effective normal stress and hence high frictional strength. Style 2 occurs along sections of faults which strike close to the regional slip vector, and hence have a lower normal stress. Aseismic slip occurs only along sections of faults in central California where anomalously high pore pressures reduce the effective normal stress to a very low value and the fault is entirely within the stable sliding frictional field.
 
Three fault systems were responsible for Permian to Late Cretaceous deformation of the overriding plate of the Andean convergent margin in the Coastal Cordillera of northern Chile (25degrees30' to 27degrees00'S). Displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs. The Tigrillo Fault System, active from Triassic to Early Cretaceous time, was characterized by are-normal extension with increasing left-oblique extension (transtension) from Early Jurassic to Early Cretaceous time. Stretching of the crust created space for Triassic, Early Jurassic and Early Cretaceous arc basins where epiclastic, volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments. Tabular plutonic complexes were emplaced by roof uplift-floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension. The Atacama Fault System was initiated at c.132 Ma as a (mainly) left strike-slip fault during left-oblique extension of the margin. Elongate, tabular plutonic complexes were emplaced within the Atacama Fault System between c. 132 and c. 106 Ma, again by roof uplift-floor subsidence mechanisms. Ductile-brittle transitions in synplutonic mylonitic rocks of the Atacama Fault System provided the setting for Kiruna-type Fe-apatite, and Fe oxide (with Cu and/or An) ores. The Chivato Fault System was active as an extensional fault system at the eastern side of the Coastal Cordillera during displacement on the Tigrillo Fault System and later, between c.125 and c.93 Ma, as a partitioned left-oblique extensional fault system. In post-Early Cretaceous time the Chivato Fault System was inverted by left-oblique contraction (transpression) when NW-trending transfer faults, some probably reactivated lateral ramps in the Tigrillo Fault System, accommodated clockwise vertical-axis rotations of 35-45degrees. Contraction inverted the Atacama Fault System and Tigrillo Fault System and was responsible for west-vergent, thin-skinned, fold-thrust deformation in stratified rocks throughout the margin.
 
Compressive fabrics in the Late Palaeoproterozoic Mount Barren Group of the Albany-Fraser Orogen, southwestern Australia, record Mesoproterozoic collision between proto-Australia and proto-Antarctica. Petrographical evidence establishes that peak thermal metamorphism produced largely random growth of kyanite, staurolite, biotite, monazite and xenotime that overprinted those fabrics. SHRIMP U-Pb geochronology of xenotime and monazite yields an average age of 1205 10 Ma. Thermal metamorphism therefore occurred at least 45 Ma after fabric formation, and was unlikely to have been caused by collision. Rather, thermal metamorphism overlapped with the emplacement of 1215-1202 Ma dyke swarms into the Orogen and the adjacent Yilgarn Craton, and was followed by emplacement of 1200-1180 Ma granites. Regional heating associated with mafic magmatism was the probable cause of thermal metamorphism, but previous proposals that the dyke swarms were the consequence of collision or extensional orogenic collapse cannot be substantiated. A regional thermal anomaly, craton-scale extension and adiabatic decompression melting of the asthenosphere are implied, but causal mechanisms such as a mantle plume or intracontinental rifting require substantiation from other parts of East Gondwana. The significant time gap between orogenic deformation and thermal metamorphism implies that metamorphism in many other orogens may not necessarily be due to compressive tectonics.
 
Exhumation of rocks in extensional tectonic settings results from a combination of normal faulting and erosion but the relative contribution of these processes has rarely been quantified. Here we present new low-temperature thermochronological data and the first 10Be-based catchment-wide erosion rates from the Boz Daǧ region in the central Menderes Massif, which has experienced NNE-SSW extension since the Miocene. The slip rate of the shallow-dipping Gediz detachment fault, which defines the northern flank of the Boz Daǧ block, is 4.3 (+3.0/-1.2) mm/a, as constrained by zircon (U-Th)/He ages of ~4-2 Ma in the footwall (Buscher et al. in review with the Journal of the Geological Society, London). Apatite and zircon (U-Th)/He and fission track ages from the northern flank of the Boz Daǧ block yield exhumation rates of 0.6-2 km/Ma beneath the Gediz detachment, whereas those on the southern flank are only 0.2-0.6 km/Ma. Erosion of catchments on the northern and southern flanks proceeds at rates of 80-180 and 330-460 mm/ka, respectively. This marked contrast is a combined effect of the topographic asymmetry of the Boz Daǧ block and differences in rock erodibility. If these erosion rates persisted in the past, rock exhumation on the northern flank occurred predominantly by tectonic denudation, whereas rocks on the southern flank were mainly exhumed by erosion.
 
Geological map of the central Apennines. T., thrust sheet. Two grey stars on top of the Simbruini Mts show the location of exposed Triassic rocks, i.e. the most elevated rocks of that age in the central Apennines. At the northeastern front of the Simbruini thrust sheet, the lower Messinian ‘Brecce della Renga’ formation (i.e. pre-salinity crisis) constitutes the largest wedge of breccias in the central Apennines. Younger conglomerates (upper Messinian) occur on top of and at the front of the Simbruini thrust. On the right, synthetic stratigraphy of the central Apennines compiled according to the Puglia-1 (41.05 8 N, 16.20 8 E, depth 7070 m) and Trevi-1 (41.88 8 N, 13.20 8 E; depth 3549 m) wells and surface data (Accordi & Carbone 1986). 
( a ) Line drawing of the CROP-11 seismic profile. The relative location is shown in Figure 1. The 0 s datum corresponds to 500 m above sea level (a.s.l.). Depths in kilometres of the Moho in the Tyrrhenian (west) and Adriatic (east) domains are known from two seismic refraction profiles (Cassinis et al . 2003). A high-resolution image of the CROP-11 profile, the relative parameters and the station coordinates are available online at http:// www.geolsoc.org.uk/SUP18249. A hard copy can be obtained from the Society Library. ( b ) Regional gravity anomaly along the CROP-11 profile. In the central sector, the gap (shaded area) between the observed regional trend (continuous line) and the hypothesized (unaffected) regional trend (dashed line) indicates the effect of the mid-crustal antiform imaged on the CROP-11 profile. The gravity low is entirely compensated by assuming this structure is as thick as c . 10 km and as dense as c . 2570 kg m À 3 . 
( a ) Central segment of the CROP-11 profile (i.e. between the Tiber valley and the Fucino basin; see Fig. 2a). ( b ) Interpretation of first-order tectonic structures. The upper boundary of the hanging-wall area is drawn where a major change of seismic facies occurs. 
Schematic diagram showing the time–space migration of thrusting and the distribution of Messinian clastic deposits. Data are plotted along the profile D–E across the central Apennines (the location is shown in Fig. 1). Columns of conglomerates correspond, by number, to outcrops shown in Figure 1. ‘A’ is the approximate altitude for the outcrops of upper Messinian conglomerates. The location and linear extension of the antiform shown in Figure 3b is displayed in the top left of the diagram. Two grey stars show the location (corresponding to the crest of the mid-crustal antiform) along the profile D–E of exposed Triassic rocks on top of the Simbruini thrust sheet (see Fig. 1). This diagram shows that the central Apennines consist of an imbricate fan of shallow, carbonate, thrust sheets accreted in a piggyback sequence directed towards the foreland with episodic out-of-sequence thrusting. 
The CROP-11 deep seismic profile across the central Apennines, Italy, reveals a previously unknown, mid-crustal antiform here interpreted as a fault-bend fold-like structure. The seismic facies and gravity signature suggest that this structure consists of low-grade metamorphic rocks. Geomorphological, stratigraphic and tectonic evidence in the overlying shallow thrusts suggests that this structure developed in early to mid-Messinian time and grew out of sequence in late Messinian– Pliocene time. The out-of-sequence growth may reflect a taper subcriticality stage of the Apenninic thrust wedge, which induced renewed contraction in the rear. published 583–586
 
Samples from the Kola Superdeep Drillhole (12 262 m), a deep drillhole (1060 m), and from the surface, seaprated by only around 10 km, provided a unique opportunity for direct tracing of delta(13)C and delta(18)O changes through a low- to high-grade greenschist-facies transition within impure, C-13-rich Palaeoproterozoic dolostones. The least-altered dolostones have delta(13)C of +9 parts per thousand and delta(18)O of 22 parts per thousand. The metamorphic transition is expressed by dolomite + calcite, + quartz K-feldspar - tremolite + calcite(2) dolomite calcite, and defined by C-13 depletion of calcite(2) (c. 3.0 parts per thousand), calcite, (1.0-2.0 parts per thousand) and dolomite (<1 parts per thousand) which is associated with a Rayleigh distillation process. delta(18)O shows a considerable resetting in all carbonate components by around 6 parts per thousand caused by a Rayleigh distillation process coupled with isotopic exchange between the carbonates and fluids with an external source of oxygen. The retrograde alteration is expressed by the formation of quartz-chlorite veinlets within tectonically bound zones of brecciated and sheared dolostones. The maximum 180 depletion in dolomite (9 parts per thousand) and calcite(1) (c. 4 parts per thousand) were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration; delta(13)C remains largely unaffected.
 
Mass extinctions are recognized through the study of fossil groups across event horizons, and from analyses of long-term trends in taxonomic richness and diversity. Both approaches have inherent flaws, and data that once seemed reliable can be readily superseded by the discovery of new fossils and/or the application of new analytical techniques. Herein the current state of the Cretaceous-Tertiary (K-T) biostratigraphical record is reviewed for most major fossil clades, including: calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachio-pods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs). These reviews take account of possible biasing factors in the fossil record in order to extract the most comprehensive picture of the K-T biotic crisis available. Results suggest that many faunal and floral groups (ostracodes, bryozoa, ammonite cephalopods, bivalves, archosaurs) were in decline throughout the latest Maastrichtian while others (diatoms, radiolaria, benthic foraminifera, brachiopods, gastropods, fish, amphibians, lepidosaurs, terrestrial plants) passed through the K-T event horizon with only minor taxonomic richness and/or diversity changes. A few microfossil groups (calcareous nannoplankton, dinoflagellates, planktonic foraminifera) did experience a turnover of varying magnitudes in the latest Maastrichtian-earliest Danian. However, many of these turnovers, along with changes in ecological dominance patterns among benthic foraminifera, began in the latest Maastrichtian. Improved taxonomic estimates of the overall pattern and magnitude of the K-T extinction event must await the development of more reliable systematic and phylogenetic data for all Upper Cretaceous clades.
 
We describe the evolution of the volcanic activity and deformation patterns observed at Mount Etna during the July-August 2001 eruption. Seismicity started at 3000 in below sea level on 13 July, accompanied by moderate ground swelling. Ground deformation culminated on 16 July with the development of a NE-SW graben c. 500 in wide and c. I in deep in the Cisternazza area at 2600-2500 in above sea level on the southern slope of the volcano. On 17 July, the eruption started at the summit of Mount Etna from the SE Crater (central-lateral eruptive system), front which two radial, c. 30 in wide, c. 3000 in long fracture zones, associated with eruptive fissures, propagated both southward (17 July) and northeastward (20 July). On 18 July, a new vent formed at 2 100 in elevation, at the southern base of the Montagnola, followed on the next day by the opening of a vent further upslope, at 2550 in (eccentric eruptive system). The eruption lasted for 3 weeks. Approximately 80% of the total lava volume was erupted from the 2 100 in and the 2550 in vents. The collected structural data suggest that the Cisternazza graben developed as a passive local response of the volcanic edifice to the ascent of a north-south eccentric dyke, which eventually reached the ground surface in the Montagnola area (18-19 July). In contrast, the two narrow fracture zones radiating from the summit are interpreted as the lateral propagation, from the conduit of the SE Crater, of north-south- and NE-SW-oriented shallow dykes, 2-3 in wide. The evolution of the fracture pattern together with other volcanological data (magma ascent and effusion rate, eruptive style, petrochemical characteristics of the erupted products, and petrology of xenoliths within magma) suggest that the eccentric and central-lateral eruptions were fed by two distinct magmatic systems. Examples of eccentric activity accompanied by central-lateral events have never been described before at Etna.
 
The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo-Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above the Chari Tuff is particularly notable for its wealth of hominid fossils and archaeological sites. Silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation. Pumice clasts within the tuffs contain anorthoclase phenocrysts, ideal for 40Ar/39Ar single-crystal dating. Feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the KBS Tuff to the Silbo Tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the KBS Tuff (1.869 ± 0.021 Ma) and the Chari Tuff (1.383 ± 0.028 Ma) and to yet higher in the sequence to the Silbo Tuff (0.751 ± 0.022 Ma). These results provide a precise and accurate time scale for the upper part of the sequence in the whole of the Omo-Turkana Basin. A number of these tuffs are recognized elsewhere in East Africa; thus ages determined at Koobi Fora also apply to the wider region.
 
Detrital white micas from a restricted region of the Norwegian Sea have been analysed by the 40Ar/39Ar laserprobe. Examining the ages of individual white mica grains from offshore Triassic, Jurassic and Cretaceous sandstones reveals a narrow range of detrital white mica ages (369–424 Ma). Moreover, there is an inverse relationship between sediment age and detrital white mica age, with the Triassic sands containing a larger proportion of younger white micas, and the Cretaceous sands containing the oldest white micas. In addition, residence times of the detritus, which is the age of the sediment subtracted from the age of the detrital white mica, range from c. 150 to c. 300 Ma (for the Triassic and Cretaceous sands respectively). The inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion-deposition model involving at least two stages. Taking into account previous provenance studies in the Lofoten region of the Norwegian Sea, these new data suggest that detritus shed initially from the exhuming Caledonian nappes of NW Norway in late Silurian-early Devonian times was deposited in intramontane basins that were themselves, subsequently unroofed and re-eroded from Permo-Triassic times. This caused the recycling of the Caledonian detritus from the Devono-Carboniferous basins and led to the inverted detrital age distribution ovserved in the Triassic-Cretaceous sediments examined in this study. While sedimentary recycling is not a surprising feature for Mesozoic basins in the North and Norwegian Seas, this study documents a previously untapped method, 40Ar/39Ar laserprobe provenance studies, by which to identify its effect and extent.
 
Simplified geological map of central Chile (modified from SERNAGEOMIN 2003; Fock et al . 2006). Schematic geological profile ( c . 33 8 45 9 S) showing the Mesozoic–Cenozoic sequences both in the Coastal Range and in the Andean Cordillera (modified after Levi et al . 1989). Ages are from A  ̊ berg et al . (1984), Aguirre et al . (1999), SERNAGEOMIN (2003), Wilson et al . (2003), Fuentes et al . (2004) and Parada et al . (2005). 
SEM images of altered metabasites from the Andes of central Chile: (a) amygdale with celadonite, quartz and titanite; (b) titanite and celadonite infilling amygdale, sample GSII5A; (c) small anhedral crystals of titanite in the groundmass, sample STP-8; (d) fibrous actinolite and epidote infilling amygdale, sample STP-8. Scale bar represents 200 ìm. ttn, titanite; cel, celadonite; qtz, quartz; act, actinolite; ep, epidote.
Tera-Wasserburg diagrams (a, b) and weighted average 206 Pb/ 238 U (c, d), for laser ablation results obtained from titanite infilling amygdales and in the groundmass. Ages are given at 2ó confidence level. 40 Ar/ 39 Ar age, Ca/K, Cl/K and % 40 Ar* spectra (e) and inverse isochron diagram (f) were obtained for actinolite from a metamorphosed andesitic lava (sample STP-8). Plateau and integrated ages are given at the 2ó confidence level; apparent ages of individual step and ellipses are given at the 1ó level.
K-Ar, U-Pb and Ar-Ar ages for metamorphic minerals from metabasites of the Lo Valdés and Río Damas Fms, central Chile; datum SA56 K-Ar ages
Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic–Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K–Ar and U–Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes. This work was financially supported by the FONDECYT projects 1061266 (M.V.) and 3070028 (V.O.).
 
Infrared laserprobe 40Ar/39Ar dating has been used to date pseudotachylite and host-rock minerals from a crush belt in the Lewisian basement of Scotland. It has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host-rock clasts and mineral fragments in the pseudotachylite. In conjunction with the host-rock mineral laserprobe 40Ar/39Ar data it has been possible to simplify the pseudotachylite data for the samples, and the preferred ages for these are: 980 ± 39 Ma, 999 ± 31 Ma and 1024 ± 30 Ma (2). These ages are the first record of Grenville-aged brittle deformation in the Lewisian. Further, this study serves to illustrate the complexity of dating pseudotachylites, and the advantages and limitations of the IR laserprobe applied to such materials.
 
New calcite 87Sr/86Sr data for 47 limestones from the metamorphosed and deformed Neoproterozoic–Cambrian Dalradian Supergroup of Scotland and Ireland are used to identify secular trends in seawater 87Sr/86Sr through the Dalradian succession and to constrain its depositional age. Dalradian limestones commonly have Sr >1000 ppm, indicating primary aragonite and marine diagenesis. Low Mn, Mn/Sr <0.6, {delta}18O and trace element data indicate that many 87Sr/86Sr ratios are unaltered since diagenesis despite greenschist- to amphibolite-facies metamorphism, consistent with the documented behaviour of Sr and O during metamorphic fluid–rock interaction. Thus, the 87Sr/86Sr data are interpreted largely to reflect 87Sr/86Sr of coeval seawater. Currently available data show that Neoproterozoic seawater 87Sr/86Sr rose from c. 0.7052 at 850–900 Ma to c. 0.7085 or higher in the latest Neoproterozoic. Temporal changes at c. 800 Ma and c. 600 Ma bracket the range in 87Sr/86Sr values of calcite in Grampian, Appin and lowest Argyll Group (c. 0.7064–0.7072) and middle and uppermost Argyll Group (c. 0.7082–0.7095) limestones, consistent with a rise in seawater 87Sr/86Sr around 600 Ma. 87Sr/86Sr data are consistent with the sedimentary affinity of the Islay Subgroup with the underlying Appin Group, and with a possible time interval between deposition of Islay and Easdale Subgroup rocks. They indicate that the Dalradian, as a whole, is younger than c. 800 Ma.
 
(a) Map showing spatial relationship between the British Isles and the Shetland Islands. The box gives the location of (b). IS, Iapetus Suture; SUF, Southern Uplands Fault; HBF, Highland Boundary Fault; GGF, Great Glen Fault; MT, Moine Thrust; WBF, Walls Boundary Fault; WKSZ, Wester Keolka Shear Zone. The Moine Thrust denotes the NW limit of the Scottish Caledonides. The Iapetus Suture divides Laurentian and Iapetus-derived terranes to the north from Gondwana-derived terranes to the south. (b) Simplified map of the geology of the Shetland Islands. Sample locations for SH-10 and SH-11 are indicated.
Calculated P-T pseudosection for sample SH-11. The white ellipse shows the calculated average P-T conditions for sample SH-11. The black dashed ellipse indicates the position of the texturally early sillimanite-bearing assemblage, the black dotted ellipse indicates the position of the texturally late kyanite-bearing assemblage. The mineral abbreviations used are those of Kretz (1983).
(a-g) CL images of zircon grains from sample SH-11. (a)-(c) and one of the grains in (d) are interpreted to be detrital whereas the other grain in (d) and the grains in (e)-(g) are interpreted to be metamorphic in origin. Analyses depicted are 2o11c, 2o11a, 2o11n, 2o11j, 2o11i, 2111y2, 2o11c3 and 2o11g, respectively. (h) Concordia plot of all zircon analyses from sample SH-11. (i) Concordia plot of all interpreted metamorphic zircon analyses from sample SH-11. The regression age is also given.
Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U-Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 +/- 8 and 925 +/- 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migniatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c. 650-700 degrees C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c. 650 degrees C and 8-9 kbar. In situ electron probe microanalysis (EPMA) U-Th-Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician-Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia-Baltica provenance. Deposition of the Westing Group is constrained to between c. 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway.
 
Geological map of part of the NW Iberian Variscan belt showing the main units, tectonic features and granitoids. Location of samples collected for this study is also indicated.  
Concordia plots of U–Pb analytical data. Ellipses represent 2 uncertainties. Plot B is partially enlarged in plots C, D, and E.  
Plot of T DM v. age of granitoids and sedimentary rocks from Iberia and Cadomia (184 samples). Sources of data: A, Cadomian granitoids (D'Lemos & Brown 1993; Samson & D'Lemos 1998; Fernández-Suárez et al. 1998). B, Ollo de Sapo greywackes (Ortega et al. 1996). C, Variscan granitoids (white crosses, I-type granitoids; black crosses, leucogranites) (Beetsma 1995; Moreno Ventas et al. 1995; Galán et al. 1996; Villaseca et al. 1998; and own data). D, Sedimentary rocks (Beetsma 1995; Nägler et al. 1995).  
Detrital zircons from greywackes belonging to an Ordovician volcanosedimentary-igneous complex in NW Spain were dated using the laser ablation-inductively coupled plasma-mass spectrometry technique. U-Pb results indicate the existence of three Precambrian crustal sources in NW Iberia. The age grouping datetd at c. 590-620 Ma. confirms the presence of a Cadomian-Avalonian basement in this zone. The second age group (c. 1.1-1.2 Ga) is the first evidence for a Grenvillean age crustal component in NW Iberia. The age grouping dated at c. 1.9-2.0 Ga corresponds to the age of the Icartian igneous basement exposed in other areas of the European Variscides. mc
 
Recent data indicating that the Piaxtla Suite (Acatlan Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asis Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U-Pb (zircon) data indicate that the metasediments were deposited after c. 700 Ma and before intrusion of c. 470-420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (c. 10501250 Ma) and a few zircons in the range of c. 900-992 and c. 1330-1662 Ma. Their geochemical and Sm-Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a c. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a c. 0.8-1.1 Ga source (T-DM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.
 
Tectonic map of the southern Altaids that crosses the Chinese–Mongolian border showing main tectonostratigraphic units (modified after Badarch et al . 2002, Windley et al . 2002 and our own data). Inset is a map showing the tectonic position of the southern Altaids. 
Schematic sections demonstrating the tectonic evolution of the southern Altaids. ( a ) Cambrian to mid-Ordovician; ( b ) late Ordovician to Silurian; ( c ) early Devonian to mid-Carboniferous. The cross-sectional directions are present-day coordinates. 
The southern Altaids present a unidirectional section from Mongolia to China through an accretionary orogen that youngs progressively from Neoproterozoic in the north to Permian in the south. The orogen formed by forearc accretion of island arcs, accretionary wedges, ophiolites and Precambrian microcontinents. This regularity was upset by early growth within the ocean of arcs that later collided at the accreting continental margin, by imbrication of old ophiolites with young arcs, and by Himalayan-style thrust-nappe tectonics when an arc collided into a microcontinent. Lateral growth of the Southern Altaids represents a massive addition of juvenile material to the Palaeozoic crust.
 
The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.
 
Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S1. As the F1 folds became slightly overturned to the west, S1 was folded about west-vergent open F2 folds and an S2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by Rf /ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-tothe- east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c. 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c. 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.
 
The Late Triassic to Early Jurassic aged succession of SW Britain (the Penarth and lower Lias Groups) comprises mudstone, sandstone and limestone strata deposited in a variety of marine to non-marine environments. Faunal and floral characteristics of these successions have led to the proposal that one location in SW England, St Audrie's Bay, should serve as the Global Stratotype Section and Point (GSSP) for the base of the Hettangian Stage and, thus, for the Triassic–Jurassic (Tr–J) boundary. The sections of SW Britain have also been used previously to infer sea-level change history and relate this to potential kill mechanisms associated with the Tr–J boundary mass extinction. Chemostratigraphic, biofacies and lithofacies data are used here to suggest alternative models of sea-level change in relation to possible Tr–J boundary horizons in the sections of SW Britain. A sea-level lowstand surface of erosion is inferred to occur within the Cotham Member of the Lilstock Formation, a unit deposited in an environment that was often subaerially exposed. In contrast to previous interpretations, the top surface of the overlying Langport Member (here inferred to be deposited on a carbonate ramp of depositional or tectonic origin) represents a drowning event of at least regional extent. All horizons regarded as plausible levels at which to place the Tr–J boundary based on fossil distributions lie within strata deposited during relative sea-level rise. However, it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and, additionally, there is no positive evidence that sea-level fall had any relation to the genesis of proposed Tr–J marker horizons. It is unlikely that sea-level fall played a significant role in the Tr–J boundary extinctions in either a local or a global context.
 
The FAST deep seismic reflection profile traverses the whole width of the Faroe-Shetland Trough. The principal target of the profile was the structure of the crust beneath the Faroe basalts. In this region, bright reflections are seen from between 7 and 9 km depth beneath the basalts, dipping westwards in the opposite direction to the dip of the basalts and reflections within the basalts. These sub-basalt reflections are regarded as originating from near top basement. The Moho has not been imaged beneath the basalts, possibly because of the absence of any impedance contrast at the base of the crust. The profile shows that the basement of the Faroe-Shetland Trough thins to c. 10 km beneath the centre of the trough. Thinning of the crystalline basement is probably the result of more than one phase of extension, the most recent of which occurred in the mid- to late Cretaceous. Extension appears to have been concentrated on a series of east-dipping normal faults cutting through the basement. These faults may have originated during a Precambrian rifting event. It is suggested that opening of the NE Atlantic occurred to the west of the Faroe Islands, as Mesozoic rifting in the Faroe-Shetland Trough had strengthened the lithosphere in this region.
 
We have reinvestigated the marine mass extinction interval that occurred during the early Toarcian, which was a time of widespread marine anoxia. The ranges of marine benthic invertebrates are significantly altered using new observations from the Cleveland Basin, UK. Goniomya rhombifera is reported for the first time from the Whitby Mudstone Formation and together with an increased epifaunal bivalve diversity indicates a brief, relatively oxygenated period towards the end of the event. The new data, together with published results, suggest three apparent extinction horizons on a global scale; the first is just above the Pliensbachian–Toarcian boundary, and the following two are in the semicelatum ammonite Subzone. As a result of the Signor–Lipps effect there may be only one, or possibly two, true extinctions. The youngest extinction horizon coincides with the first of the abrupt carbon isotope shifts that characterize this interval, and with increases in sea surface temperature, continental weathering rates, and seawater anoxia. Pseudomytiloides dubius is the only abundant benthic macroinvertebrate during the most hostile environmental conditions but it and all other benthic species are almost entirely absent for many thousands of years immediately after each abrupt negative carbon isotope shift.
 
A high-resolution palynological study of the Triassic-Jurassic boundary in the St. Audrie's Bay section revealed a palynofloral transition interval with four pronounced spore peaks in the Lilstock Formation. Regular cyclic increases in palynomorph concentrations can be linked with periods of increased runoff, and correspond to the orbital eccentricity cycle. Spore peaks can be related to precession-induced variations in monsoon strength. An implication is that the initial carbon isotope excursion lasted for at least 20 ka. Emergence during deposition of the Cotham Member had an influence on one of the peaks, which is dominated by spore-producing pioneer plants (e.g. horsetails and liverworts). There is no compelling evidence of a global end-Triassic spore spike that, by analogy with the K-T boundary fern spike, could be related to a catastrophic mass extinction event. Climate change is a more plausible mechanism to explain the increased amount of spores.
 
Crystallochemical parameters of the Narcea Slates samples White Chlorite
X-ray diffraction analyses were performed on white-micas, formed under strain and low-grade metamorphic conditions, from sandstones and siltstones along a transect through the Narcea Antiform, a large structure that separates the external from the internal zones of the Variscan belt of NW Spain. The results indicate a close spatial relationship between the onset of metamorphism and the cleavage front. At the cleavage front, strain is characterized by oblate ellipsoids that have x = y:z ratios from 1 to 1.5. The x = y:z ratio increases westward (up to 3.0), as does the grade of metamorphism, and is accompanied by an increase in the 'crystallinity' index of the white-mica from very low to low metamorphic grade. This increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage. In the western part of the Narcea Antiform, the major deformation event was the development of kilometre-scale, reverse shear zones. Because of polyphase deformation, these rocks are not suitable for quantitative strain analysis, but qualitatively, finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector. The metamorphic grade is higher in the western part of the Narcea Antiform, with local maxima occurring next to the base of the shear zones. The b(0) cell parameter, a semi-quantitative geobarometer measured in the white-mica, is chaotic in the Eastern sector, showing a clear detrital inheritance. In the western sector it is rational due to complete re-equilibration during thermal resetting. The inverse correlation between finite strain and white-mica 'crystallinity' is close, showing that the anchizone-epizone limit can be related to the increase of finite strain above values of R(s) = 1.5. The close relation between white-mica 'crystallinity' and strain is suitable for tracking strain variations in extensive fine-grained siliciclastic rocks where no appropiate strain markers are found.
 
The Dariv Basin is an actively evolving intracontinental transpressional basin located on the eastern flank of the Mongolian Altai. The basin occupies a complex tectonic position between a restraining bend, a thrusted basement block, and two major conjugate strike-slip fault systems. Structures and sedimentary strata exposed within the Dariv Basin suggest a Mesozoic and Cenozoic two-stage evolution. Jurassic-Cretaceous strata fine upward and record alluvial fan, fluvial and lacustrine depositional environments. The distribution of Mesozoic sedimentary rocks and the presence of a suspected Jurassic normal fault array suggest that the Dariv Basin initially formed as an extensional basin. Following Palaeogene tectonic quiescence, Oligocene-Recent basin fill is dominated by alluvial fan sediments derived from basin-flanking ranges. The modern basin is deforming by thrusting, normal fault inversion and folding along discrete belts expressed as intrabasinal ridges and domes. These belts define a rhomboid of active deformation that compartmentalizes the basin. Sediments derived from these discrete deforming belts and from basin flanking ranges continue to accumulate in the basin centre. Thus, modern fans contain reworked older basin fill and competing processes of sedimentation, deformation, erosion and resedimentation can be observed. The Dariv Basin is an excellent example of a transpressional piggyback basin in the early stages of basin inversion and destruction.
 
The Great Glen Fault trends NNE-SSW across northern Scotland. According to previous studies, the Great Glen Fault developed as a left-lateral strike-slip fault during the Caledonian Orogeny (Ordovician to Early Devonian). However, it then reactivated right-laterally in the Tertiary. We discuss additional evidence for this later phase. At Eathie and Shandwick, minor folds and faults in fossiliferous Jurassic marine strata indicate post-depositional right-lateral slip. In Jurassic shale, we have found bedding-parallel calcite veins ('beef' and 'cone-in-cone') that may provide evidence for overpressure development and maturation of organic matter at significant depth. Thus, the Jurassic strata at Eathie and Shandwick accumulated deeper offshore in the Moray Firth and were subject to Cenozoic exhumation during right-lateral displacement along the Great Glen Fault, as suggested by previous researchers. Differential sea-floor spreading along the NE Atlantic ridge system generated left-lateral transpressional displacements along the Faroe Fracture Zone from the Early Eocene to the Late Oligocene (c. 47-26 Ma), a period of uplift and exhumation in Scotland. We suggest that such differential spreading was responsible for reactivation of the Great Glen Fault. Indeed, left-lateral slip along the Faroe Fracture Zone is compatible with right-lateral reactivation of the Great Glen Fault.
 
Published models for variations in 87 Sr/ 86 Sr i and Nd i in granitoids of the Antarctic Peninsula and Ellsworth-Whitmore Mountains crustal blocks. See text for description of fields. In each case, melting of high-87 Sr/ 86 Sr crust is invoked, although a variety of mafic end-members are suggested.
Variation in T DM through time. Granitoids and volcanic rocks of the Antarctic Peninsula, Ellsworth-Whitmore Mountains, and Thurston Island crustal blocks show similar overall trends, reflecting the changing geotectonic environment within West Antarctica through time. Additional data: Ellsworth-Whitmore Mountains granites, Pankhurst et al. (1991); Thurston Island, Pankhurst et al. (1991); Alexander Island, McCarron & Smellie (1998); silicic volcanic rocks, Pankhurst et al. (2000); NW Palmer Land dykes, Scarrow et al. (1998).
Magmatic rocks from the Antarctic Peninsula show marked variations in isotope composition, which reflect changes in the geodynamic evolution of the peninsula through time. Most Antarctic Peninsula granitoids formed as a result of subduction: they fall on well-defined trends on plots of Nd, Pb-207/Pb-204 and delta O-18 against Sr-87/Sr-86(i). between a component derived from subduction-modified mantle or juvenile basaltic underplate (epsilon Nd-i gt 6, Pb-207/Pb-204=15.61, delta O-18=5.5 parts per thousand, Sr-87/Sr-86 lt 0.704) and an end- member interpreted as a melt of Proterozoic lower crust (epsilon Nd=-7, Pb-207/Pb-204=15.67, delta O-18=10 parts per thousand, Sr-87/Sr-86=0.709). A small group of granitoids. emplaced before or during Gondwana break-up, plot on distinct trends towards high Sr-87/Sr-86(i) compositions. reflecting mixing between melts derived from Proterozoic lower crust and melts of middle-upper crustal rocks (epsilon Nd-i=-9, Pb- 207/Pb-204=15.64, delta O-18=10 parts per thousand, Sr-87/Sr- 86=0.726). with little or no input of new material derived from the mantle or from juvenile basaltic underplate. These granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of Gondwana break- up. Similar trends are shown by data from granitoids of the adjacent crustal blocks of West Antarctica, The isotope data suggest that an enriched Ferrar/Karoo-type lithosphere was not involved in the genesis of granitoids of the Antarctic Peninsula or of the Ellsworth-Whitmore Mountains crustal block.
 
The island of Rhodos represents an uplifted block in the largely submerged southeastern Aegean forearc. It has a complex history of subsidence, uplift and counterclockwise rotation during the Plio- Pleistocene, in response to the interplay between large-scale geodynamic processes. In this paper, we present a new chronostratigraphic framework for the continental Pliocene Apolakkia basin of southwestern Rhodos. We combine these time constraints with recently published chronostratigraphic data from the marine Plio- Pleistocene basins of northeastern Rhodos to reconstruct rotational and vertical motions. Our palaeomagnetic results identify two rotation phases for Rhodos: c .1 0 8 (9 � 68) counterclockwise (ccw) rotation between 3.8 and 3.6 Ma, and c .1 7� 68 ccw rotation since 0.8 Ma. Between these phases, Rhodos tilted to the SE, drowning the southeastern coast to a depth of 500-600 m between 2.5 and 1.8 Ma, then to the NW, which resulted in the re-emergence of the drowned relief between 1.5 and 1.1 Ma. We relate the rotations of Rhodos to incipient formation of the south Aegean sinistral strike-slip system and the foundering of the Rhodos basin. The previously shown absence of Messinian evaporites in the deep-marine Rhodos basin in combination with the 3.8 Ma onset of ccw rotation of Rhodos constrains the onset of the formation of the south Aegean strike- slip system between 5.3 and 3.8 Ma. The formation of this strike-slip system is probably related to the interplay of oblique collision between the southeastern Aegean region and the northward moving African plate, the westward motion of Anatolia, gravitational spreading of the overthickened Aegean lithosphere and the recently postulated southwestward retreat of the African subducted slab along a subduction-transform edge-propagator fault.
 
Structural and metamorphic data from the island of Amorgos (central Aegean Sea) show evidence for the existence of two distinct high-pressure units, the Metabasite Unit and the Basal Conglomerate Unit. These are exposed at the base of a thick marble sequence and overlying flysch deposits. The Metabasite Unit is characterized by a mineral assemblage of blue amphibole, garnet and clinopyroxene, indicating P-T conditions of 500-600 degrees C and > 13 kbar. It is juxtaposed below carpholite-bearing metaconglomerates and quartz-rich micaschists of the Basal Conglomerate Unit, for which metamorphic conditions of 300-450 degrees C and 10-14 kbar are estimated. The contact between the two units is interpreted as a low-angle detachment fault that accommodated top-to-the-NW sense of motion. The Amorgos succession above the Basal Conglomerate Unit collectively resembles the stratigraphy of external units in the Hellenides and could possibly be correlated with the so-called 'Basal Unit', which crops out in a number of tectonic windows throughout the Aegean Sea. This means that the Metabasite Unit in Amorgos could possibly represent the lowermost structural unit in the central Aegean Sea.
 
We constrain the slip and cooling history of the Mykonos detachment footwall using thermochronometry. A U–Pb zircon age of 13.5 ± 0.3 Ma dates intrusion of the Mykonos monzogranite. 40Ar/ 39Ar hornblende and biotite ages from the monzogranite are 12.7 ± 0.6 Ma and 10.9 ± 0.6 Ma, whereas zircon and apatite fission-track ages range from 13 ±0.8 Ma to 10.7 ± 0.8 Ma and 12.5 ± 2.2 Ma to 10.5 ± 1.8 Ma. (U–Th)/He ages range from 13.6 ± 0.6 Ma to 9.0 ± 0.7 Ma for zircon and 11.1 ±0.5 Ma to 8.9 ± 0.4 Ma for apatite. The ages in part overlap within 2s errors and together with the long apatite fission-track lengths (14µm) support rapid cooling at rates .100 8CMa. The low-temperature thermochronometric ages decrease east-northeastwards in the direction of hanging-wall transport on the Mykonos detachment. Age– distance relationships show that the Mykonos detachment slipped at an average rate of 6.0 +9.2/2.4 km Ma c. 30 km of offset and c. 12 km of exhumation. This result indicates that Miocene low-angle normal faulting was not important for the exhumation of the Cycladic blueschist unit. The opening of the Aegean Sea basin in the Miocene was controlled by a few large-magnitude low-angle normal faults.
 
The provenance and depositional setting of Late Palaeozoic and Early Mesozoic elastic sediments from the eastern Aegean archipelago are examined here for the first time using whole-rock geochemistry and composition of detrital chrome spinel. Major- and trace-element data for Late Palaeozoic and Permo-Triassic elastic sediments from the Lower and Upper Units of Chios are compatible with an acidic to intermediate source, minor input of (ultra)mafic detritus and recycling of older sedimentary components. Chondrite-normalized REE profiles are uniform with light REE enrichments (La-N/Yb-N c. 7.7), negative Eu anomalies (Eu/Eu* c. 0.67) and flat heavy REE patterns (Gd-N/Yb-N c. 1.5), indicating an upper continental crustal source and/or young differentiated arc material. Detrital chrome spinet from the elastic sediments of Chios has Cr-number (Cr/(Cr + Al)) values between 0.29 and 0.89 and Mg-number (Mg/(Mg + Fe2+)) values between 0.24 and 0.70, suggesting a probably mixed (ultra)mafic source involving ridge peridotites (mid-ocean ridge type), fore-are peridotites and island-arc basalts. The metasediments from the islands of Inousses and Psara have similar whole-rock chemical signatures to those of Chios, although no evidence was found for an (ultra)mafic source. We conclude that both the Late Palaeozoic sediments from the Lower and Upper Units of Chios and the metasediments from Inousses and Psara were deposited in a continental island-arc setting until at least Late Permian times, probably at a single Palaeotethyan margin. They are interpreted to be allochthonous, tectonically transported to their present position by Late Mesozoic to Cenozoic orogenic processes.
 
Sediments delivered to the South Atlantic Ocean by the Orange River are fractionated and dispersed northwards and westwards by a vigorous longshore drift system and a number of ocean currents. Gravels are accreted to the coastline for a distance > 300 km north from the Orange River mouth. Sands are transported alongshore for > 700 km but are, in places along this transport path, returned onshore by coastal winds to form the main Namib Sand Sea and other smaller dune fields. Mud is more widely dispersed westwards, northwards and southwards, probably by slow-moving, ocean-scale currents into basins on the shelf and onto the continental shelf edge. This dispersal system, operating since at least Eocene times, is believed to have originated during a time when there was a Late Cretaceous-Early Cenozoic uplift of southern Africa, which resulted in: (1) intensification of the existing southerly wind system; (2) incision of the Orange River, which, coupled with a shift in climate, resulted in a coarsening of its sediment load delivered to the coast; (3) a broad, weakly subsiding or mildly uplifting inner continental shelf with little accommodation space for the sediment load of the incising Orange River.
 
Trace-element, carbon, oxygen and lead-isotope analyses were carried out to determine the formation mode and crystallization age of magnesite from the Budd ultramafic complex of the Archaean Barberton Greenstone Belt, South Africa. Its significantly high Ti contents probably relate to a soluble Ti-rich accessory mineral, probably dissolved during magnesite precipitation. Primitive mantle-normalized REE patterns of the magnesite show negative Ce and Eu anomalies induced by two events: (1) the Eu anomaly indicates reducing conditions probably induced by the emplacement of the ultramafic source rock; (2) the Ce anomaly implies oxidizing conditions, probably during a hydrothermal event that favoured the precipitation of magnesite in veins of the rock. This Ce anomaly is among the oldest in Earth history. Negative {delta}13C (-2.9 to -3.3{per thousand} PDB) and high {delta}18O (30.5-31.2{per thousand} SMOW) suggest low-temperature precipitation in a near-surface Kraubath-type epithermal system. The magnesite contains considerable amounts of radiogenic lead (206Pb/204Pb is 125.5-153.7 and 207Pb/204Pb is 41.3-47.7). The resulting Pb-Pb age of 3043 ± 59 Ma is interpreted as dating a hydrothermal event related to the extensive plutonism episode that has been reported in the Barberton area at about 3.1 Ga. Previous models that proposed formation of the magnesite under recent climatic conditions can be discarded.
 
Maps showing the Molteno Formation of South Africa. (a) Distribution of the Molteno Formation. (b) The geological outcrop of the Molteno Formation of South Africa showing localities mentioned in the text. Lit 111, Little Switzerland; Umk 111, Umkomaas Valley; Wal 111, Waldeck; Pen 311, Peninsula; Gre 121, Greenvale; Bir 111, Birds River; Aas 411, Aasvoëlberg; Kap 111, Kapokkraal; Win 111, Winnaarspruit.  
Damage categories on leaves from the late Triassic Molteno Formation, South Africa 
The Molteno Formation of Late Triassic (Carnian) age in South Africa has yielded more than 200 plant species from 100 plant assemblages at 69 localities (30 000 catalogued slabs) as well as more than 300 species of insect. Damage to leaves caused by insects is widespread on many plant species at numerous localities. The damage includes feeding traces, predominantly continuous marginal feeding traces, leaf mines including linear and possible blotch varieties and probable leaf galls. Damaged taxa include a wide variety of gymnosperms including the conifer Heidiphyllum, the ginkgoaleans Ginkgo and Sphenobaiera, the peltasperms Dicroidium and Dejerseya, the pentoxyalean Taeniopteris and the gnetopsid Yabeiella. Quantitative data (>3000 specimens examined) on the insect damage were obtained for four sites: Aasvöelberg, Birds River, Kapokkraal and Waldeck. Quantitative data indicate that leaf damage between sites varies from 3 to 25% and within species from 1 to 50%. The variation in damage to the same taxon between sites, and even in the overall folivory seen at the four sites, makes the interpretation of the general levels of herbivory in the Molteno difficult to assess. Herbivory levels in Northern Hemisphere Triassic plant assemblages appear to be significantly lower than those in the Molteno, and succeeding Jurassic floras worldwide also show a low level of plant–insect interactions. It is not yet clear to what extent taphonomic bias may influence the calculation of overall herbivory levels for a given time period.
 
The relationships between meteorite impact craters, flood basalt volcanism and sudden environmental perturbation on Earth have been debated extensively. We present only the second example of contemporaneous meteorite impact and flood basalt volcanism: the 17 km diameter Logoisk impact structure (Belarus) and Afro-Arabian flood volcanism. The new precise 40Ar/39Ar age of 29.71 ± 0.48 Ma for Logoisk is coincident with the acme of flood volcanism. We argue that at least one crater the size of Logoisk is likely to form during the emplacement of a flood volcanic province, and that contemporaneity of impacts and flood volcanism was the norm. The absence of a biotic effect from the combined Logoisk–Afro-Arabian volcanism we attribute to the magnitude of these combined events, which are both significantly smaller than the other known example of the Chicxulub impact event and the Deccan Traps flood volcanism.
 
Map showing the distribution of the main geological units of the Scottish Highlands between the Great Glen and Highland Boundary faults. I, M and T represent sample locations for the Insch Gabbro, Morven Cabrach Gabbro and Tayvallich Tuff respectively. BV represents the Ben Vuirich Granite.
Back-scattered electron image of zircon from keratophyre dated by Halliday et al. (1989). Field of view 150 μm. 
Back-scattered electron image of zircon separated from the Morven Cabrach gabbro. Field of view 150 μm. 
Age constraints on Dalradian stratigraphy and tectonothermal events compared to models of Dalradian evolution. Major problems associated with models incorporating breaks in the succession (highlighted in dark orange) or no breaks in the succession (highlighted in pale orange) are emphasized. Uncertainties regarding the timing and presence of orogenic activity are illustrated in the stratigraphic column. Evidence for a break in the succession associated with the c.
The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ñ 4 Ma), and on the late Grampian 'Newer Gabbros' (470 ñ 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 ñ 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.
 
Geological map of the Caleu pluton and location of dated samples. Histograms show measured track length for apatite of samples from the Caleu pluton. 
Detailed 40 Ar/ 39 Ar analytical results obtained on biotites, amphiboles and plagioclases
(continued )
Cooling history of the Caleu pluton, as deduced from geochronological data. The U-Pb age range is based on the spread of data along the concordia curve. Error bars on 40 Ar/ 39 Ar plateau ages are given at the 2ó confidence level. For biotite, dashed lines represent older ages obtained on a sample (CA99-7) from the margin of the pluton (see text). The restricted indicated age range for the fission tracks is deduced from a thermal model (see text). The indicated closure temperature for each mineral (taken from Berger & York 1981; McDougall & Harrison 1999) and chronometer is approximate, and no error bar is given. The fast cooling of the pluton started coevally with its emplacement and ended when pluton and host rocks cooled together to c. 2508 C as a result of exhumation. The slower cooling to c. 1008 C could be continuous (curve A) or episodic (curve B) according to the rate of the concomitant exhumation.
Modelled thermal history of samples from the Caleu pluton based on 
The Caleu pluton, in the Coastal Range of central Chile, represents the last magmatic event related to the Early Cretaceous rifting along the western margin of South America. The pluton was emplaced into a c. 10 km thick pile of mainly basalts and basaltic andesites deposited in an Early Cretaceous subsiding basin, and affected by very low-grade metamorphism. The cooling history of the pluton is documented on the basis of U-Pb, Ar-40/Ar-39 step-heating and fission-track dating. The U-Pb date suggests an age of emplacement in the interval 94.2-97.3 Ma. Rapid subsolidus cooling between 550-500 degrees C and 250 degrees C is documented by Ar-40/Ar-39 plateau ages on amphibole, biotite and plagioclase between 94.9 +/- 1.8 and 93.2 +/- 1.1 Ma. Slower subsolidus cooling to c. 100 degrees C is identified at the 94-90 Ma interval by the fission-track thermal model. The geochronological data show that the emplacement of the pluton is coeval with the very low-grade metamorphism of the host rocks. Therefore, this metamorphism is probably not the result simply of burial, but also of a regional thermal gradient related to the plutonism. Exhumation of the pluton started coevally with its emplacement and continued to about 90 Ma, being associated with the closure of the Early Cretaceous rifting. The Caleu plutonism represents an asthenospheric-derived event during maximum extension, and marks a turning point between extensional- and compressional-related magmatism.
 
Basement gneiss inliers within the Scottish Caledonides have been conventionally correlated with the Archaean Lewisian Gneiss Complex of the Caledonian foreland. Alternatively, the inliers could represent allochthonous terranes accreted to Laurentia before or during the Caledonian orogeny. SIMS U-Pb zircon dating indicates that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are characterized by late Archaean protolith ages, and a period of isotopic disturbance in the late Palaeoproterozoic. The data are broadly consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. The c. 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, and a younger, c. 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane. None of the studied inliers shows a complete match of protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. Therefore, it remains probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny.
 
Map of Connemara, western Ireland, showing the simpli fi ed geology, after Leake et al . (1981) and Graham et al . (1989), the sample locations and K – Ar ages of the analysed hornblendes, separated from amphibolites and metagabbros. Group A samples have a circle with central spot, Group B is a local cluster of seven samples, Group C are three samples SW of Group A. Numbers indicate K – Ar ages in Ma. Sample numbers are in brackets. 
Scanning electron microscope backscattered electron images of A, B and C zoned hornblendes in sample TJ-7D; lighter rims (a), Al- and Fe-rich; darker cores (b), Si- and Mg-rich; D of uniform hornblende (TJ-44) with alteration to biotite and chlorite along cleavages. Qtz, quartz; plag, plagioclase; mt, magnetite. 
Comparisons of hornblende K-Ar ages with: ( a ) Flame emission spectrophotometer (FES) determined K wt.%; R for whole dataset= 0.576 (>99% signi fi cance); for Group B= 0.914 (>99% signi fi cance); ( b ) Electron microprobe analysed (EMPA) MgO/(MgO+FeO); R=0.525 (>99% signi fi cance); ( c ) Ionic Porosity, Z %; R= 0.487 (>99% signi fi cance); ( d ) H 2 O wt.%. Amphibolites; Group A, fi lled circles; Group B, asterisks; Group C, open diamonds; others, open squares. Metagabbros, fi lled stars; R, 0.354 (90 – 95% signi fi cance). Error bars are at 1 standard deviation. 
Comparisons of hornblende D values ( ‰ ) with: (a) H 2 O wt.%; R= 0.371 (>95% signi fi cance); (b) hornblende K-Ar ages: 
Calculated fl uid D (crosses) derived from hornblende D plotted against fl uid 18 O assuming hornblende 18 O equals fl uid 18 O. RGF is the measured retrograde fl uid (in fl uid inclusions) of Jenkin et al . (1997), and the open arrow indicates the deduced change in meteoric water composition as it moved through the Connemara rocks, taken from O ’ Reilly et al . (1997). 
Major element compositional analyses, K-Ar ages. δD ‰ and δ 18O ‰ values for 30 zoned and unzoned hornblendes from Dalradian amphibolites and metagabbros, mostly in north Connemara are reported. Although the cooling ages are expected from previous U-Pb zircon studies to be c. 475-450 Ma, the results obtained are from 556 ± 6 to 410 ± 9 Ma with an average of 470 Ma. Fluid movements, probably at 275 ± 15 °C, i.e. much below Ar closure temperature for hornblende, erratically reset the ages, as is shown by a negative correlation of hornblende δD and age and a wide scatter of ages even within 2 m. The changes were implemented by H-D exchange between fluid and hornblendes in which ionic porosity, Z, influenced the loss of Ar and possibly its gain from the fluid to give the excess Ar found in some samples. Z is controlled by hornblende chemical composition. High Mg, Si and Mg/Fe and low Fe, Al, Ti, Na and particularly low K, amphiboles giving low Z values retained Ar more firmly and gained Ar more readily than compositions which had higher Z values, which gave younger ages. These range down to c. 400 Ma, being the age of the intrusion of the Galway Granite suite that initiated the fluid circulation. The scatter of ages is a consequence of incomplete equilibration, mainly because of the slow H-D exchange rate below 350 °C and partly because the fluid permeated erratically in different areas and down cracks of all kinds, promoting irregular Ar movement. The meteoric fluid circulated through Connemara, the Galway Granite and at least some of the contiguous Silurian sediments of the South Mayo trough. These overlying sediments may have contributed to the water circulated in north Connemara which was slightly less negative δD than in central Connemara. For hornblende K-Ar ages to be a reliable measure of times of uplift and cooling, they need to be demonstrated to be free from the influence of hot fluids by showing no correlation of age with δD.
 
( a ) Simplified tectonic map of the Scandinavian Caledonides (modified from Gee & Sturt 1985) showing the location of the J  ̈mtland area. WGR, Western Gneiss Region. ( b ) Generalized map of the J  ̈mtland area (after Van Roermund 1985) showing the locations of the eclogites and and garnet pryoxenites analysed in this study. ( c ) Schematic cross-section illustrating the allochthons of the J  ̈mtland area. 
Sm-Nd mineral isochrons of eclogites and garnet pyroxenites from the Seve Nappe Complex in the Jämtland area of the Swedish Caledonides. Errors are smaller than the plotted points unless noted with an error bar. Ages were calculated using the Isoplot/Ex program of Ludwig (1998).
Weighted averages for eclogite and garnet pyroxenite ages from all samples analysed from the J  ̈mtland area ( a ), for two samples from the eastern belt ( b ) and from four samples from the central belt ( c ). Averages were calculated using the Isoplot/Ex program of Ludwig (1998). 
Eclogites and garnet pyroxenites from the Seve Nappe Complex in the Jamtland area of the Scandinavian Caledonides give Sm-Nd mineral ages (garnet-clinopyroxene-whole rock +/- orthopyroxene +/- amphibole) that are identical within error and give a weighted average age of 457.9 +/- 4.5 Ma (95% confidence). The age is 50 Ma younger than ages determined from eclogites from the apparently similar Norrbotten terrane, roughly 200 km to the north. Both terranes are correlated with the western margin of Baltica, suggesting that at least two eclogite-facies metamorphic events affected this margin prior to the final closure of Iapetus during the 430-400 Ma Scandian Orogeny. The c. 458 Ma ages are nearly identical to ages determined from the eclogite-bearing Tromso Nappe of the uppermost allochthon. The uppermost allochthon is generally considered part of the Laurentian margin, which, if true, requires that it evolved independently of Baltica until the Scandian Orogeny, when Laurentia and Baltica collided. Thus high-pressure eclogite-facies metamorphism and the introduction of mantle peridotite bodies into the crust appear to have occurred concurrently on opposite sides of Iapetus. We suggest that the LIP collision recorded in the Jamtland area be called the 'Jamtlandian Orogeny' if further studies confirm its Early Ordovician age.
 
In the Agly Massif (Pyrenees), two Variscan plutons, the Saint-Arnac pluton and the Ansignan charnockite, intrude different levels of a c. 10 km thick crustal section. The Saint-Arnac pluton intrudes through upper crustal rocks and the Ansignan charnockite cuts mid-crustal country-rocks. A structural study of these plutons based on the anisotropy of magnetic susceptibility technique, combined with a kinematic study of the country-rocks, shows that the structures and emplacement modes of the plutons are compatible with those of the other plutons of the Pyrenees emplaced during the D2 transpressive phase. U–Pb dating on zircons from the Saint-Arnac pluton yields a 308.3 1.2 Ma age for a diorite and a 303.6 4.7 Ma age for a granodiorite. The charnockite was previously dated at 315 Ma. The emplacement ages of these two intrusions are thus separated by at least 5 Ma. First, numerous sills and laccoliths, such as the Ansignan laccolith, were injected in the middle crust, and this induced heating and thickening. Subsequently, the large Saint-Arnac pluton was injected in the upper crust at the beginning of formation of a gneissic dome. This is a contribution of the LMTG–UMR 5563, Universite´ de Toulouse/CNRS/IRD/OMP.
 
Albitization is a common metasomatic process active in various geodynamic contexts. In the northern Pyrenees, there are several occurrences of albitites but, until now, only one occurrence has been dated (117 Ma, Ar-Ar dating). This paper presents new U-Th-Pb ages for several albitite occurrences throughout the Pyrenees to test whether they are contemporaneous and, if so, to specify the regional extent of the albitization event. Ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 ± 8 and 98 ± 2 Ma, respectively. The zircon U-Th-Pb isotopic system did not record this Cretaceous metasomatic event, even when grains were selected in metasomatically Zr-enriched rocks or in hydrothermal structures (millimetre-sized veins cross-cutting granitoids). We argue that the total time span of 20 Ma recorded by albitites corresponds to a long-lived hydrothermal system that was active during the rotation of Iberia around Europe, along the North Pyrenean Fault. Because albitization and talc mineralization have the same spatial and temporal distribution in the Pyrenees, we argue that these two metasomatic phenomena are two independent records of this single, regional-scale, long-lived hydrothermal system.
 
A thermo-chronological analysis of the Barlet basement unit (French Massif Central) reveals a four-stage history. Peak metamorphism (650 degrees C and 7 kbar) was followed by retrograde growth of albite blasts and development of the main foliation at 600 degrees C, and chloritization of the ferromagnesian minerals. The third stage is a marked reversal of cooling, with recrossing of the biotite isograd and local reappearance of garnet at 450 degrees C. This thermal event, inferred to result from hot fluid infiltration, is also recognized in the adjacent basin of Langeac, where it gives rise to anomalous coal grades (recording 200 degrees C at 1 km). A Stephanian age for this event correlates with a regional thermal event recognized throughout the Variscan, where it has been linked to delamination of the continental crust. This work represents the first instance in the Massif Central that recognizes this event in the shallow basement itself. Final cooling is accompanied by extensive fluid-induced sericitization, starting immediately after the peak of the thermal event and continuing to temperatures inferred for Sb-As ore deposition. This continuum leads us to conclude that reheating-related silicate reactions and ore deposition are caused by the same fluid and related to the wider regional Variscan thermal event.
 
Radiometric age determinations on syn-orogenic intrusive rocks coupled with evidence from field relationships, indicate that metamorphism and ductile deformation was taking place hi the Scottish metamorphic blocks coeval with extension on the Hebridean foreland. Consequently metamorphic rocks now found east and south of the Moine Thrust are regarded as substantially allochthonous, being located within the region where there was formerly a Cambrian-Early Ordovician passive margin sequence and its attendant oceanic crust. We regard these blocks as displaced terranes which acquired their metamorphic and structural signatures outside of the present Hebridean foreland regime. This may have occurred initially at the tune of megacontinent splitting and later on the destructive margin which was initiated hi Laurentia.
 
Top-cited authors
Brian Windley
  • University of Leicester
M. P. Searle
  • University of Oxford
Alfred Kröner
  • Johannes Gutenberg-Universität Mainz
Hugh C. Jenkyns
  • University of Oxford
Richard H. Sibson
  • University of Otago