Showing papers in "Journal of the Geological Society in 2011"

Crustal–lithospheric structure and continental extrusion of Tibet

Abstract: Crustal shortening and thickening to c . 70–85 km in the Tibetan Plateau occurred both before and mainly after the c . 50 Ma India–Asia collision. Potassic–ultrapotassic shoshonitic and adakitic lavas erupted across the Qiangtang ( c . 50–29 Ma) and Lhasa blocks ( c . 30–10 Ma) indicate a hot mantle, thick crust and eclogitic root during that period. The progressive northward underthrusting of cold, Indian mantle lithosphere since collision shut off the source in the Lhasa block at c . 10 Ma. Late Miocene–Pleistocene shoshonitic volcanic rocks in northern Tibet require hot mantle. We review the major tectonic processes proposed for Tibet including ‘rigid-block9, continuum and crustal flow as well as the geological history of the major strike-slip faults. We examine controversies concerning the cumulative geological offsets and the discrepancies between geological, Quaternary and geodetic slip rates. Low present-day slip rates measured from global positioning system and InSAR along the Karakoram and Altyn Tagh Faults in addition to slow long-term geological rates can only account for limited eastward extrusion of Tibet since Mid-Miocene time. We conclude that despite being prominent geomorphological features sometimes with wide mylonite zones, the faults cut earlier formed metamorphic and igneous rocks and show limited offsets. Concentrated strain at the surface is dissipated deeper into wide ductile shear zones.

Hafnium isotope record of the Ancient Gneiss Complex, Swaziland, southern Africa: evidence for Archaean crust―mantle formation and crust reworking between 3.66 and 2.73 Ga

Abstract: Combined U–Pb and Lu–Hf isotope analyses of zircons from 16 tonalite–trondjemite–granodiorite (TTG) gneiss and granite samples from Swaziland reveal that the oldest rocks of the Ancient Gneiss Complex in southern Africa formed by reworking of Early Archaean or perhaps Late Hadean crust at 3.66 Ga, and that new crust was extracted from a depleted mantle source during Palaeoarchaean events between 3.54 and 3.32 Ga. This interpretation is supported by eHf t of −1.6 ± 2.0 obtained from 3.66 Ga TTG gneisses, corresponding to hafnium model ages between 3.77 ± 0.18 Ga, for a presumed Hadean–Early Archaean chondritic mantle, and 4.08 ± 0.18 Ga, for a presumed Hadean depleted mantle reservoir, with the first model age being the most likely in the light of recent data from worldwide sources. Furthermore, it is reflected by superchondritic eHf t up to +2.2 ± 2.0 for TTGs formed at 3.54, 3.45 and 3.32 Ga. The new datasets additionally show that the Palaeoarchaean crust formed between 3.54 and 3.32 Ga was intensely reworked afterwards, without significant addition of depleted mantle derived material, during orogenic and intracratonic melting processes at 3.23, 3.1 and 2.7 Ga. This is well reflected by an array of decreasing eHf t from +2.2 to −7.2 between 3.3 and 2.7 Ga, which can be forced by 176 Lu/ 177 Hf of 0.0113, which is similar to that of present-day average continental crust, and might result from lower crust zircon fractionation during Archaean crust reworking. Supplementary material: Results of in situ U–Pb and Lu–Hf isotope zircon analyses and concordia diagrams are available at www.geolsoc.org.uk/SUP18465.

Early Permian high-K calc-alkaline volcanic rocks from NW Inner Mongolia, North China: geochemistry, origin and tectonic implications

Abstract: Knowledge of the characteristics of the extensive late Palaeozoic volcanic rocks across the northern China–Mongolia tract is essential for understanding the tectonic evolution and continental crustal growth in the Central Asian Orogenic Belt. This geochronological and geochemical study documents the Early Permian mafic and felsic volcanic rocks from northwestern Inner Mongolia. The mafic rocks form two magma series with distinctive geochemical characteristics; one showing large ion lithophile element (LILE) enrichment relative to high field strength elements (HFSE) and an asthenosphere-like Sr–Nd–Pb isotopic signature, and the other featuring an elevated Nb and lithospheric isotopic signature. This result indicates that two mantle source components are involved in the magma generation: the subduction-related metasomatized asthenosphere and lithospheric mantle. The felsic rocks show strong enrichment of LILE and light REE, depletion in HFSE, and indistinguishable isotopic compositions from mafic ones. Such features are consistent with partial melts of mixed sources composed of predominant juvenile basaltic underplates and minor ancient crustal materials. These mafic and felsic rocks constitute a post-subduction high-potassium calc-alkaline magmatic suite possibly under a geodynamic regime of Palaeo-Asian Ocean slab breakoff. This regime not only provides a feasible trigger for the flipping of subduction polarity in the Solonker suture zone, but also presents a favourable venue for vertical continental crustal growth.

Submarine slope degradation and aggradation and the stratigraphic evolution of channel–levee systems

Abstract: Two seismic-scale submarine channel–levee systems exposed in the Karoo Basin, South Africa provide insights into slope conduit evolution. Component channel fills in a levee-confined channel system (Unit C) and an entrenched channel system (Unit D) follow common stacking patterns; initial horizontal stacking (lateral migration) is followed by vertical stacking (aggradation). This architecture is a response to an equilibrium profile shift from low accommodation (slope degradation, composite erosion surface formation, external levee development, sediment bypass) through at-grade conditions (horizontal stacking and widening) to high accommodation (slope aggradation, vertical stacking, internal levee development). This architecture is likely common to other channel–levee systems. Supplementary material: A detailed correlation panel (presented schematically in Figure 2) is available at www.geolsoc.org.uk/SUP18456.

Tectonic escape of a crustal fragment during the closure of the Rheic Ocean: U–Pb detrital zircon data from the Late Palaeozoic Pulo do Lobo and South Portuguese zones, southern Iberia

Abstract: The Pulo do Lobo Zone, which crops out immediately north of the allochthonous South Portuguese Zone in southern Iberia, is classically interpreted as a polydeformed accretionary complex developed along the southern margin of the Gondwanan parautochthon (Ossa–Morena Zone), during the late Palaeozoic closure of the Rheic Ocean. This closure was a major event during the amalgamation of Pangaea. U–Pb laser ablation inductively coupled mass spectrometry dating of detrital zircons from late Palaeozoic Devono-Carboniferous clastic units in the South Portuguese Zone and Pulo do Lobo Zone yield contrasting age populations and attest to the exotic nature of both zones. Detrital zircons from the South Portuguese Zone display populations typical of detritus derived from either Gondwana (Ossa–Morena Zone), or peri-Gondwanan terranes. In contrast, rocks from the Pulo do Lobo Zone contain populations consistent with derivation from Baltica, Laurentia or recycled early Silurian deposits along the Laurentian margin. An example of one such deposit is the Southern Uplands terrane of the British Caledonides. Taken together, these data can be reconciled by a model involving tectonic transport of a crustal fragment that was laterally equivalent to the Southern Uplands terrane between the allochthonous South Portuguese Zone and Gondwana as a result of an early Devonian collision between an Iberian indenter with Laurussia. Supplementary material: U–Pb data tables, concordia diagrams, methods and representative back-scattered electron images are available at http://www.geolsoc.org.uk/SUP18441.

Pennsylvanian 'fossil forests' in growth position (T0 assemblages): origin, taphonomic bias and palaeoecological insights

Abstract: Fossil forests, buried in growth position in a geological instant (T 0 assemblages) are far more abundant in Pennsylvanian successions than in any other part of the geological record. In this review paper, we evaluate the fundamental controls on the origin of these phenomena, investigate the taphonomic biases that influence their composition, and summarize their palaeoecological significance. Following earlier workers, we highlight that high rates of burial and accommodation are essential for the formation and preservation of T 0 assemblages. Contexts especially favourable for their origin include ashfalls proximal to volcanic centres, coastal plains drowned by relative sea-level rise, and fluvial environments such as channel bars, crevasse splays, and distributary lobes. Long-term preservation requires high rates of subsidence. Consequently, the vast majority of Palaeozoic T 0 assemblages are confined to wetland settings at, or close to, sea level, whereas drylands are poorly represented and uplands rarely sampled, if ever. However, this is not the only major bias in the fossil record; taphonomic processes selectively preserve plants dependent on their anatomy and stature, and on groundwater chemistry. Thus, although T 0 assemblages offer unrivalled insights into the nature of ancient forests (whole-plant reconstructions, tree density, canopy height, productivity, plant hydraulics, cohort dynamics, spatial heterogeneity, ecological gradients, tree–sediment interactions, and animal–plant interactions, to name but a few), it is naive to believe they provide ‘photographic snapshots9 of palaeoecosystems. None the less, careful taphonomic analysis of T 0 assemblages offers the potential for a nuanced understanding of these evocative phenomena, and much remains to be learned from these important palaeoecological resources.

Basin architecture and growth folding of the NW Zagros early foreland basin during the Late Cretaceous and early Tertiary

Abstract: We present and use the chronostratigraphy of 13 field logs and detailed mapping to constrain the evolution of the early Zagros foreland basin, in NW Iran. Large foraminifera, calcareous nannofossil, palynological and 87 Sr/ 86 Sr analysis supplied ages indicating a Campanian–early Eocene age of the basin infill, which is characterizd by a diachronous, southwestward migrating, shallowing upwards, mixed clastic–carbonate succession. Growth synclines and local palaeoslope variations indicate syndepositional folding from Maastrichtian to Eocene time and suggest forelandward migration of the deformation front. We also illustrate the basin architecture with a synthetic stratigraphic transect. From internal to external areas, time lines cross the formation boundaries from continental Kashkan red beds to Taleh Zang mixed clastic–carbonate platforms, Amiran slope deposits and basinal Gurpi–Pabdeh shales and marls. The foreland basin depocentres show a progressive migration from the Campanian to Eocene ( c . 83–52.7 Ma), with rates of c . 2.4 mm a −1 during the early–middle Palaeocene ( c . 65.5–58.7 Ma) increasing to c . 6 mm a −1 during the late Palaeocene–earliest Eocene ( c . 58.7–52.8 Ma). Coeval subsidence remained at c . 0.27 mm a −1 during the first 12.7 Ma and decreased to c . 0.16 mm a −1 during the last 4.2 Ma of basin filling. Finally, we integrate our results with published large-scale maps and discuss their implications in the context of the Zagros orogeny. Supplementary material: Tables with dating results are available at http://www.geolsoc.org.uk/SUP18439.

Cambrian successions of the Meguma Terrane, Nova Scotia, and Harlech Dome, North Wales: dispersed fragments of a peri-Gondwanan basin?

Abstract: The Meguma Terrane of Nova Scotia and the Harlech Dome of North Wales preserve similar sedimentary successions of Cambrian age. Both successions comprise a thick succession of early Cambrian sandstone turbidites, overlain by early to middle Cambrian alternating mud-rich and sand-rich units in which manganese is concentrated in two stratigraphic intervals. Above these, both successions comprise anoxic, organic-rich turbidites, shallowing upward into paler, more bioturbated Tremadocian mudstone with Rhabdinopora . Within the limited constraints of the available biostratigraphic and geochronological data, major changes in environment occurred synchronously in the two successions. Both successions show much greater similarity to each other than to adjacent successions on ‘Avalonia9. A detrital zircon analysis from the Rhinog Formation, low in the Harlech Dome succession, reveals distinct clusters of ages around 537 Ma and 2.0–2.1 Ga. A close similarity to analyses from the Meguma Terrane suggests proximity between the two terranes on the margin of Gondwana during the Cambrian Period. We suggest the term Megumia for the palaeogeographical domain that included the two successions, which was dispersed during subsequent Appalachian and Caledonian movements. These observations suggest that Megumia may have separated terranes previously regarded as parts of Avalonia. Supplementary material: U/Pb detrital zircon analytical data are available at http://www.geolsoc.org.uk/SUP18431.

Insights of dyke emplacement mechanics from detailed 3D dyke thickness datasets

Abstract: Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from

A high-precision U-Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications

Abstract: An isotope dilution thermal ionization mass spectrometry U–Pb zircon age of 411.5 ± 1.3 Ma obtained from an andesitic lava occurring within the Lower Devonian Rhynie Outlier (Aberdeenshire, NE Scotland) effectively dates the Rhynie Chert Konservat-Lagerstatte. Biostratigraphical constraints on the Rhynie Chert-bearing succession indicate that this age lies within the interval early (but not earliest) Pragian–(?)earliest Emsian. Accordingly, the Pragian–Emsian boundary must post-date or closely approximate to 411.5 ± 1.3 Ma, while the Lochkovian–Pragian boundary must predate 411.5 ± 1.3 Ma. Integration of this new high-precision age with an improved temporal framework for late Caledonian intrusive activity in NE Scotland suggests that the Rhynie hot-spring system (the ‘parental9 hydrothermal system to the Rhynie cherts) was unrelated to any ‘Newer Granite9 intrusion. Rhynie was instead powered by a basaltic andesite magma whose generation and ascent were directly linked to the transcurrent fault movements responsible for the formation of the Rhynie basin. Supplementary material: Details of analytical techniques (ID-TIMS U–Pb geochronology) and photomicrographs of zircon and titanite grains recovered from the Milton of Noth Andesite are available at http://www.geolsoc.org.uk/SUP18463.

Sedimentation in deep-sea lobe-elements: implications for the origin of thickening-upward sequences

Abstract: The frequency and origin of thickening-upward packages in the sediments of deep-sea environments has been a topic of much recent debate. Excellent bed-scale exposures in the Carboniferous Ross Formation, western Ireland, allow single surfaces to be traced laterally and the detailed architecture of whole packages to be evaluated using multiple vertical logs. The deposits comprise architectural elements including bed-sets, lobe-elements and composite lobes, with the lobe-elements being arranged in thickening-upward depositional packages. Results show that these packages are the result of the progradation of each lobe-element, whereby each package records a vertical trend from distal to proximal deposits accompanied by an increasing frequency of megaflutes and ultimately the development of broad erosional surfaces. We propose a six-stage model for lobe-element evolution that documents successive phases of deposition, sediment bypass, erosion and lobe abandonment. This new model provides a mechanism for lobe-element switching and explains the development of thickening-upward successions. This allows the re-examination of existing process models for these sedimentary packages in deep-sea sediments.

Enhancing tectonic and provenance information from detrital zircon studies: assessing terrane-scale sampling and grain-scale characterization

Abstract: Determining detrital zircon U–Pb ages has become the method of choice for single- mineral-based provenance studies focused on the identification of potential source regions of siliciclastic sediments. Advances in microanalytical methods have significantly accelerated the acquisition rate of U–Pb ages, thus allowing for more statistically significant zircon age datasets to be acquired than previously. However, several studies have demonstrated limitations of relying solely on detrital zircon as a provenance proxy. To further assess the utility of this provenance indicator we measured U–Pb ages of detrital zircon derived from modern sediment collected from the French Broad River and its tributaries that drain portions of the Appalachian Orogen in southeastern USA. The results demonstrate that significant detrital zircon age variations occur along the length of the river. The age variations suggest that characterization of entire sedimentary formations by analysis of single samples may be misleading and that a multiple-sample approach is required. In addition, by incorporating high-magnification cathodoluminescence images with Th/U for each detrital grain, a more robust interpretation can be made regarding zircon source. Supplementary material: Global positioning system coordinates for each sampling location and complete zircon data are available at http:/www.geolsoc.org.uk/SUP18445.

Deformation partitioning during transpressional emplacement of a ‘mantle extrusion wedge': the Ronda peridotites, western Betic Cordillera, Spain

Abstract: Composite tectonic emplacement of the world9s largest outcropping body of subcontinental mantle rocks, the Ronda peridotites, occurred during Early Miocene continental subduction and as a result of oblique plate convergence. Partitioning of transpressional deformation between coeval orogen-parallel wrenching and orogen-perpendicular, pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks. Left-lateral shear, characterizing the deeper, high-pressure (eclogitic) portions of the continental subduction system, propagated through the mantle into the overlying continental crust of the overriding plate, whereas top-to-the-foreland frontal thrusting dominated at the leading edge of the hot peridotite body. Top-to-the-hinterland shear along the upper contact of the peridotites is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks. On the other hand, coeval strike-parallel extension and thinning of the crustal rocks overlying the peridotites confirms that, similarly to other Alpine–Mediterranean examples, partitioned transpression resulted in the development of a complex deformation pattern, with kinematically linked shear zones aiding exhumation. The proposed model provides a new, coherent scenario for syn-convergence peridotite emplacement and exhumation in the western Betic Cordillera.

Contrasting styles of mineralization in the Chinese Altai and East Junggar, NW China: implications for the accretionary history of the southern Altaids

Abstract: The Chinese Altai–East Junggar collage in the southern Altaids hosts three metallogenic belts, which are, from north to south: (1) a volcanogenic massive sulphide (VMS) Cu–Pb–Zn belt; (2) a belt of shear zone-related gold deposits; (3) a porphyry Cu–Au–Mo belt. The VMS deposits formed in two pulses ( c . 405 Ma and c . 375 Ma) in the Chinese Altai arc. The porphyry deposits developed in three pulses in the East Junggar arc, the first two synchronous with the VMS mineralization, and the third at c . 330 Ma. The shear zone-related gold deposits developed in the late Carboniferous to Permian at the contact between the Chinese Altai and East Junggar arcs. Time–space distributions of diverse ore deposits across the Altai–East Junggar collage indicate that the collage developed from two independent arcs, the Chinese Altai and the East Junggar. The VMS and porphyry deposits developed in the Chinese Altai and East Junggar arcs, respectively. The Chinese Altai arc is interpreted to be a Japanese-type arc, and the East Junggar arc a Mariana-type arc. During the latest Palaeozoic, the two arcs were juxtaposed by the Erqis Fault, when many shear zone-related lode gold deposits were emplaced. These metallogenic distributions were a likely response to spatially localized mechanisms of crust growth and to the tectonic evolution of the Altai–East Junggar collage, and they are consistent with interpretation of the Altaids as a multiple subduction–accretion collage.

Sediment transport processes in an ancient mud-dominated succession: a comparison of processes operating in marine offshore settings and anoxic basinal environments

Abstract: Few studies describe and compare the transport mechanisms operating to disperse mud in different parts of basins. Instead, the physical processes operating to disperse mud in offshore environments, where storm and tidal processes are interpreted to dominate, are generally considered in isolation from those occurring in basinal settings where changes in bottom-water anoxia and suspension settling from buoyant plumes are mostly interpreted to dominate. Using microtextural, mineralogical and geochemical data derived from the analyses of 151 thin sections obtained from the Lower Jurassic mudstone-dominated succession exposed on the coast of NE England, we investigate how varying sediment dispersal mechanisms, bioturbation and early diagenesis operated to produce the lithofacies variability observed. In particular, we consider the processes of sediment delivery while bottom waters were interpreted to be euxinic. Analyses of these samples reveal that the succession is highly variable at millimetre to centimetre scales. Six main lithofacies were observed: (1) sand- and clay-bearing, silt-rich mudstones; (2) silt-bearing, clay-rich mudstones; (3) clay-rich mudstones; (4) clay-, calcareous nannoplankton-, and organic carbon-bearing mudstones; (5) fine-grained muddy sandstones; (6) cement-rich mudstones. These units are organized typically into stacked successions of sharp-based, normally graded, thin (

Origin of the Aves Ridge and Dutch–Venezuelan Antilles: interaction of the Cretaceous ‘Great Arc' and Caribbean–Colombian Oceanic Plateau?

Abstract: In this paper we reassess the geochronology and geochemistry of three dredge hauls from the SE corner of the Aves Ridge (Caribbean Sea) originally sampled in 1968 by Duke University's R.V. Eastward. Two hauls consist of light rare earth element-enriched granitoids with a U–Pb zircon emplacement age of 75.9 ± 0.7 Ma. A further haul contains mostly calc-alkaline island arc basaltic andesites of uncertain age. Petrological, trace element and isotopic constraints indicate that the granitoids have an oceanic crustal source and were formed by melting of the lower arc, oceanic or oceanic plateau crust. The mafic rocks formed by partial melting of an incompatible trace element-enriched mantle wedge, which was probably composed of mantle plume material. Both the dredged rocks and data from the Dutch–Venezuelan Antilles indicate a period of west-dipping underthrusting and subduction beneath, or close to, the Caribbean–Colombian Oceanic Plateau between c. 88 and c. 59 Ma, concurrent with collision of part of the plateau with northwestern South America. Constraints from the geochemistry and geochronology of offshore southern Caribbean arc and plateau rocks suggest that in the southern Caribbean there was no pre-existing west-dipping subduction system during formation of the Caribbean–Colombian Oceanic Plateau, whereas long-lived SW-dipping subduction in the northern Greater Antilles is more probable.

Hydrothermal origin of elevated iron, manganese and redox-sensitive trace elements in the c. 635 Ma Doushantuo cap carbonate

Abstract: Major and trace element, including REE, concentrations of the Doushantuo cap carbonate ( c . 635 Ma) in South China show enrichment in Fe, Mn and redox-sensitive elements and slightly negative Ce anomalies, indicating anoxic environments during cap carbonate precipitation. High Fe T /Al ratios but very low concentration of extractable pyrites suggest ferruginous rather than euxinic conditions. The REE + Y patterns of samples show enrichment of heavy REE (HREE), positive Eu anomalies and positive Y anomalies, implying a hydrothermal origin for elevated concentration of Fe, Mn and redox-sensitive elements. The results suggest that ferruginous Ediacaran oceans may have rooted from hydrothermally induced iron accumulation in severely glaciated Cryogenian oceans.

Petrology and geochemistry of the Andaman ophiolite: melt-rock interaction in a suprasubduction-zone setting

Abstract: The Andaman ophiolite occurs as thrust slices in the outer arc of the Andaman–Java subduction zone This ophiolite preserves the mantle sequence, layered ultramafic–mafic rocks, intrusive and extrusive rocks The mantle sequence is represented by serpentinized lherzolite and harzburgite, hosting dunite and chromitite pods The low Cr-number (02–04), Cr-number–TiO 2 relation of the chromites, oxygen fugacity ( f O 2 ) values ( Δ log f O 2 (FMQ) = −190) and trace elements of mantle peridotites indicate a mid-ocean ridge basalt–suprasubduction-zone (MORB–SSZ) setting The MORB mantle underwent a low degree of melting ( c 10–15%) and interacted with the subduction-zone melts Melt–rock interaction of the peridotites in a suprasubduction zone is demonstrated by the replacement of pyroxene grains by olivine grains (Fo 90 ), composition of chromites and oxygen fugacity ( Δ log f O 2 (FMQ) = –190 to +216, where FMQ is the fayalite–magnetite–quartz buffer) The chromite composition of chromitite pods (Cr-number 072–075), f O 2 levels and trace elements for layered peridotites, and occurrence of the extrusive rocks as low-Ca boninite and island arc tholeiitic (IAT) basalt indicate interplay of both boninite and IAT melts for the Andaman ophiolite The MORB mantle of the subducting Indian plate accreted into the mantle wedge and then melting of the accreted mantle produced boninite melt at the first stage and tholeiitic melts at the second stage Supplementary material: Sample co-ordinates from the Andaman ophiolite, mineral analysis data for all the samples, modal compositions of the units of the Andaman ophiolite and details of the laboratory analyses including details of the standard are available at http://wwwgeolsocorguk/SUP18459

Age and composition of crystalline basement rocks on the Norwegian continental margin: offshore extension and continuity of the Caledonian–Appalachian orogenic belt

Abstract: Twenty-two wells on the Norwegian continental margin have penetrated underlying basement. We present U–Pb zircon, whole-rock geochemical, and Sm–Nd and Rb–Sr isotopic data from nine wells in the North Sea and Norwegian Sea with relevance to the offshore continuation of the Norwegian Caledonides, and their correlation throughout the Caledonian–Appalachian orogenic belt. Palaeozoic magmatism in the North Sea can be divided into two groups. The older group consists of 460 Ma calc-alkaline granites with evolved isotopic compositions, correlative with similar rocks in the Uppermost Allochthon. The younger group consists of a 430 Ma dacite and a 421 Ma leucogabbro, with less evolved isotopic compositions. In the Norwegian Sea, isotopically evolved granitic magmatism at 437 Ma and more juvenile dioritic magmatism at 447 Ma are correlative with magmatism in the Bindal and Smola–Hitra Batholiths in the Uppermost Allochthon. Metasedimentary basement rocks from the North Sea and Norwegian Sea, dominated by Late Palaeoproterozoic and Mesoproterozoic grains, resemble rocks found in the Caledonides of Scotland, Greenland and Svalbard. The new data, along with studies elsewhere along the belt, suggest that similar rocks may exist along much of the orogen. Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18471.

Timing of regional deformation and development of the Moine Thrust Zone in the Scottish Caledonides: constraints from the U–Pb geochronology of alkaline intrusions

Abstract: The Moine Thrust Zone in the Scottish Highlands developed during the Scandian Event of the Caledonian Orogeny, and now forms the boundary between the Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the formation of the Moine Thrust Zone, have previously been dated by a range of isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions localized along the thrust zone have provided the best age constraints for deformation. Recent British Geological Survey mapping has improved our understanding of the structural relationships of some of these intrusions, and this work is combined with new U–Pb dates in this paper to provide significantly improved ages for the Moine Thrust Zone. Our work shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within the orogenic belt to the east of the Moine Thrust Zone at 447.9 ± 2.9 Ma. A more significant pulse of magmatism centred in the Assynt area, which temporally overlapped movement in the thrust zone, occurred at 430.7 ± 0.5 Ma. Movement in the thrust zone had largely ceased by the time of emplacement of the youngest intrusions, the late suite of the Loch Borralan Pluton, at 429.2 ± 0.5 Ma, and the Loch Loyal Syenite Complex.

Normal faulting on Sifnos and the South Cycladic Detachment System, Aegean Sea, Greece

Abstract: We constrain the timing and kinematics of normal faulting on Sifnos, Cyclades, Greece. Penetrative top-to-the-NE kinematic indicators associated with a shallow dipping foliation occur in the deepest tectonic unit (Greenschist Unit). Combined P – T analysis and Rb–Sr dating indicates that this deformation was under way by 29.4 ± 3.4 Ma at P – T conditions of c . 16 kbar and c . 550 °C, and continued to operate at least until 22.8 ± 0.2 Ma at P – T conditions of c . 8 kbar and c . 400 °C. The later top-to-the-south directed Sifnos Detachment developed at the top of the Greenschist Unit and cuts the top-to-the-NE structures. Zircon fission-track ages suggest that deformation on the Sifnos Detachment was largely terminated by about 13–10 Ma. Subsequent extension was accommodated by high-angle normal faults. Regionally the top-to-the-south detachment phase on Sifnos correlates with top-to-the-south extensional faulting on nearby Serifos, which started there at c . 15–13 Ma. Top-to-the-south extensional shearing on Ios to the SE commenced earlier at c . 19–18 Ma, suggesting that top-to-the-south extension propagated westward from Ios to Sifnos and Serifos, or that extension was partitioned into different top-to-the-south detachment systems that operated at different times. Collectively these top-to-the-south detachments are named here the South Cycladic Detachment System, which played a key role in the opening of the Cretan Sea forearc basin.

Evolution of the Archaean Karelian Province in the Fennoscandian Shield in the light of U–Pb zircon ages and Sm–Nd and Lu–Hf isotope systematics

Abstract: In situ Lu–Hf (laser ablation microprobe–inductively coupled plasma mass spectrometry (LAM-ICPMS)) and U–Pb (LAM-ICPMS, secondary ionization mass spectrometry (SIMS)) analyses of zircon, and whole-rock Sm–Nd isotope analyses were performed on rocks formed during magmatic events in three Archaean complexes in the Karelian Province of Fennoscandia (Pudasjarvi, Koillismaa and Iisalmi). These complexes have U–Pb ages ranging from 3.5 to 2.6 Ga. In Pudasjarvi, sparse xenocrystic cores give ages of 3.6–3.7 Ga and initial 176 Hf/ 177 Hf suggesting influence of a crustal component T ≥ 4.0 Ga (assuming a CHUR-like mantle source). Ages and Nd and Hf isotope patterns indicate magmatic events at 3.6–3.7 Ga (Siurua, Pudasjarvi with ≥4.0 Ga precursor), 3.2 Ga (Iisalmi, Koillismaa), 2.8 Ga (Pudasjarvi) and 2.7 Ga (Pudasjarvi, Iisalmi). In the Meso- and Palaeoarchaean events, there is no evidence of sources equivalent to present-day depleted mantle; such sources were, however, involved in the 2.8–2.7 Ga events. e Hf and e Nd are strongly correlated. Contrasts between the Archaean complexes indicate that they evolved separately until c . 2.7 Ga. The age and e Hf pattern of ≤2.8 Ga rocks in the Karelian Province is compatible with a scenario in which the Karelia, Superior, Yilgarn and Slave cratons were part of a late Archaean supercontinent, but does not constitute proof of the existence of such a supercontinent. Supplementary material: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18430.

Pressure–temperature evolution and thermal regimes in the Barrovian zones, Scotland

Abstract: We constrain the P – T evolution of the Barrovian metamorphic zones from the southwestern to the northeastern coasts of Scotland using thermobarometry and pseudosection analysis based on mineral composition data, garnet zoning profiles and 2D garnet maps Twenty-five samples were investigated from the garnet to the sillimanite zones In the western half of the field area there was relatively high- P metamorphism (09–11 GPa) followed by near-isothermal decompression In and around the Barrovian type area of Glen Clova maximum pressures were also high ( c 08–09 GPa); however, peak- T conditions were driven by a brief (of the order of 1 Ma or less) thermal pulse or pulses during exhumation at c 06 GPa Pressures at peak- T conditions along the eastern coast were the lowest observed, c 04–05 GPa These rocks were probably affected by the same thermal pulse activity evident around Glen Clova All three regions initially developed during regional metamorphism associated with thermal relaxation of tectonically overthickened crust The eastern part of the sequence, including Glen Clova, is fundamentally different from the western part because it required additional advective heat input to achieve peak thermal conditions This heat was probably supplied by synorogenic magmas (eg Newer Gabbros) and the associated elevated crustal heat flow Supplementary material: Electron microprobe methods, mineral and rock analyses, mineral activity–composition relations, criteria for identifying prograde mineral compositions, pseudosection and diffusion modelling methods, and profiles of X Alm and X Prp from garnets in Figures 7,8,9,10 are available at http://wwwgeolsocorguk/SUP18491

Probabilistic longevity estimate for the LUSI mud volcano, East Java

Abstract: A new method for estimating the duration of a mud volcano eruption is applied to the LUSI mud volcano in East Java. The estimate is based upon carbonates at depths in the range 2500–3500 m being the water source, with an estimated area of 100–600 km 2 , thickness of 0.2–1.0 km, porosity of 0.15–0.25, an initial pressure between 13.9 and 17.6 MPa, and a separate, shallower source of mud ( c . 1200–1800 m depth). The resulting 50 percentile for the time it takes for flow to decline to −1 is 26 years. By analogy with natural mud volcanoes it can be expected to continue to flow at lower rates for thousands of years. Assuming subsidence rates of between 1 and 5 cm day −1 , land surface subsidence of between c . 95 and c . 475 m can be expected to develop within the 26 year time period.

Episodic fluvial incision of rivers and rock uplift in the Himalaya and Transhimalaya

Abstract: Seventeen strath terraces in northern India were dated using 10 Be surface exposure methods; ages range from c . 7 to c . 735 ka and provide fluvial incision rates of 0.02 ± 0.003 to 2.6 ± 1.9 mm a −1 . On the northern side of the Ladakh Range, incision rates are c . 1 mm a −1 ; in the northern Zanskar Range they are ≤0.06 ± 0.005 mm a −1 . New and published incision rates in southernmost Lahul range from 0.1 ± 0.02 to 13.2 ± 6.2 mm a −1 ; rates for ages >35 ka are ≤0.4 ± 0.2 mm a −1 . Across the Himalaya and Transhimalaya, Holocene fluvial incision rates range from c . 0.02 to c . 26.0 mm a −1 , whereas Pleistocene incision rates are ≤5 mm a −1 . Many of the Holocene incision rates exceed exhumation rates, whereas Pleistocene incision rates are comparable with exhumation rates. This suggests that long-term fluvial incision is in dynamic steady state with exhumation. The temporal pattern for rates of fluvial incision is probably controlled by episodic incision linked to significant precipitation changes throughout the Quaternary, suggesting that strath terraces with ages >35 ka can be used for assessing long-term rates of rock uplift. In contrast, rates of fluvial incision based on Late Glacial and Holocene strath terraces reflect changes in monsoon intensity and deglaciation events. By determining ages for multiple samples on flights of strath terraces, it is possible to document changes in incision rate, assess whether post-abandonment transient shielding has occurred, and help elucidate tectonic v. climatic controls on their formation. Supplementary material: Tables (DS1–4) for previously published data and recalculated ages for strath terraces, and figures (DS1 and DS2) showing plots of incision rates against time for the Himalaya and Transhimalaya are available at http://www.geolsoc.org.uk/SUP18454.

The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland

Abstract: The Mesoproterozoic Stoer Group of the ‘Torridonian9 has been dated at 1177 ± 5 Ma (2σ), based on 40 Ar/ 39 Ar dating of authigenic K-feldspars precipitated in degassing structures within the Stac Fada Member, interpreted previously as an impact ejecta deposit. Fluid inclusion analysis of the K-feldspar shows that it precipitated from hot fluids, and so must immediately post-date emplacement of the ejecta blanket. The oxygen isotopic composition of K-feldspar is remarkably constant at 9.7 ± 0.2‰ (V-SMOW) and is consistent with derivation from evaporated lake water. The new age supports previous determinations of 1199 ± 70 Ma from Pb–Pb dating of limestone and 1180 Ma from palaeomagnetic data, but provides much higher precision. Supplementary material: The raw 40 Ar/ 39 Ar data, J-parameters, correction factors, air calibrations and blank data are available at http://www.geolsoc.org.uk/SUP18453.

A refined chronology for the Cambrian succession of southern Britain

Abstract: Three dated (U–Pb, zircon) ash beds from biostratigraphically constrained Avalonian successions of Shropshire (England) and Pembrokeshire (Wales) delimit the traditional ‘Lower9–‘Middle9 Cambrian boundary and resolve a problematic regional correlation. In Shropshire, a date of 514.45 ± 0.36 [0.81 including tracer calibration and 238 U decay constant errors] Ma from near the top of the Lower Comley Sandstone Formation provides a maximum age for the boundary between Cambrian Stages 3 and 4, and a date of 509.10 ± 0.22 [0.77 including tracer calibration and 238 U decay constant errors] Ma from the basal Quarry Ridge Grits, Upper Comley Sandstone Formation, provides a minimum age for the boundary between Cambrian Stages 4 and 5 (and thus Series 2 and 3). These dates offer a calibration of early metazoan evolution by directly constraining the age of the intervening Comley Limestones, which contain diverse small shelly fossils in addition to trilobites, and also a key early occurrence of exceptional, three-dimensionally preserved arthropods. In Pembrokeshire, an ash bed from the Caerfai Bay Shales Formation dates to 519.30 ± 0.23 [0.77 including tracer calibration and 238 U decay constant errors] Ma, equivalent to a horizon low in the Lower Comley Sandstone Formation of Shropshire, possibly around the level at which trilobites make their first local appearance. Supplementary material: Appendix 1, a table of isotope data, is available at http://www.geolsoc.org.uk/SUP18444.

Late Miocene–Holocene canyon incision in the western Altiplano, northern Chile: tectonic or climatic forcing?

Abstract: Major fluvial incision (600–1000 m) affecting the Coastal Cordillera and Central Depression of northern Chile is analysed to evaluate supposed coeval uplift of the Altiplano and/or climatic changes in the Atacama Desert. The timing of the beginning of incision is constrained by the age of deposition of the Central Depression top. In the north (18–19°S), this top corresponds to fluvial gravels accumulated between 11.9 ± 0.6 Ma and 8.3 ± 0.5 Ma, which are genetically related to semiarid climate and to an eastward poorly dissected parallel drainage network that developed between 15.0 ± 0.6 and 11.2 ± 0.6 Ma; thus, gravel deposition ended at 11.9–11.2 Ma. To the south (19–20°S), the Central Depression top corresponds to c . 6 Ma alluvial deposits. Stratigraphically determined canyon ages and knickzone locations indicate that southward dissection began later and/or developed under a regime of lower erosion capacity owing to drier climate. Vertical incision rate evolution is compatible with eastward knickzone migration. Dissection required a considerable altitude difference between ancient and present-day river base levels, which was achieved predominantly by basin infill on an already partially elevated bedrock. Therefore subsequent incision would have been triggered by local semiarid climatic periods rather than by contemporaneous surface uplift. Exoreic canyons occur when climatic conditions in the catchments are arid–semiarid whereas endoreism is developed when these conditions in catchments are hyperarid.

Seismic interpretation of the Laga basin; constraints on the structural setting and kinematics of the Central Apennines

Abstract: The Messinian Laga basin is the largest foreland basin within the Central Apennines fold and thrust belt (Italy). This area, actively investigated in the 1980s and 1990s for hydrocarbon resources, is considered a valuable analogue for clastic reservoirs developed in confined structural settings. Furthermore, it represents a key area for understanding the evolution of the Apennines, as it links the internal, structurally uplifted Early Miocene fold and thrust belt of the western Central Apennines with the more external and recent belt to the east. Despite several papers published on this area, the only reconstruction of the substratum structure is an internal and classified industry report. During the present study, we had access to a seismic database comprising 200 km of seismic profiles that were collected between 1983 and 1990. These data allowed us to reconstruct the structural setting of the Laga basin substratum, define the lateral continuity of the main compressional structures within the basin, construct a balanced cross-section, and define the shortening values.

Radiation and extinction patterns in Permian floras from North China as indicators for environmental and climate change

Abstract: Permian continental sequences from North China contain previously unrecognized episodes of plant radiation and elevated extinction. The earliest extinction, in the Lower Shihhotse Formation (Roadian, Guadalupian), records a 45% floral species loss and is tentatively correlated with global extinctions amongst dinocephalian reptiles. Two younger extinctions are dated by correlating the Illawara Reversal and palaeomagnetic polarity sequences from Shanxi Province against global palaeomagnetic history. Missing data from the Shanxi sequence are evaluated using a novel approach estimating likely maximum and minimum sequence changes that provide age estimates for post-Illawara events in North China. The second extinction in the middle Upper Shihhotse Formation is more significant and is dated to the mid-Capitanian, with a loss of 56% of plant species coinciding with two phases of volcanism of the Emeishan Large Igneous Province in South China, previously linked to the mid-Capitanian marine mass extinction. The youngest extinction in the upper Upper Shihhotse Formation (late Capitanian to mid-Wuchapingian) is catastrophic and represents the end of range in the sequence. Changes in sedimentary facies suggest it to be related to global climatic warming and drying. Other viable causal mechanisms for the extinction episodes include plate motion and collision, global climate change, volcanism and biological competition.