Showing papers in "Journal of the Geological Society in 2013"

The full range of turbidite bed thickness patterns in submarine lobes: controls and implications

Abstract: A widely misused criterion to interpret lobe deposits in submarine fan systems at outcrop, and in core and well logs, is a thickening and/or coarsening upward profile. Lobe deposits from the Laingsburg depocentre, SW Karoo Basin, demonstrate that a full range of bed thickness patterns exists within lobes. When lobes are defined by their laterally extensive bounding surfaces that are marked by abrupt facies changes, five types of bed stacking patterns are identified: thickening upward, thinning upward, thickening then thinning upward, thinning then thickening upward, and constant. The abrupt bounding surfaces are interpreted to record avulsion of feeder distributive channels. The stratigraphic bed thickness pattern preserved in a lobe reflects the internal organization of smaller-scale lobe elements, rather than lobe-wide initiation and progradation as implied by a thickening-upward only pattern. The full range of bed thickness patterns in lobes can be used to understand the stacking of lobe elements, the evolution of sediment deposition through time and space, and the relative movement of depocentres.

Sicily’s fold–thrust belt and slab roll-back: the SI.RI.PRO. seismic crustal transect

Abstract: Sicily is a thick orogenic wedge formed by (1) the foreland (African) and its Sicilian orogen and (2) the thick-skinned, Calabrian–Peloritani wedge. The crust under central Sicily, from the Tyrrhenian margin to the coastline of the Sicily Channel, has been investigated by the multidisciplinary (SI.RI.PRO.) research project. The project dealt with the nature and thickness of the crust and depth and geometry of the Moho, which is essential in formulating subduction models and improving the knowledge of African and Tyrrhenian–European lithospheres. The results resolve features such as (1) the main orogenic wedge, (2) the very steep, NW–SE-trending regional monocline suggesting inflection of the foreland crust, (3) the deep Caltanissetta synform imaged, for the first time, to about 25 km, and (4) the top of the crystalline basement and the inferred crust–mantle boundary. The SI.RI.PRO. transect confirmed that the NNW-dipping, autochthonous Iblean platform of SE Sicily and its basement extends all the way into central Sicily. Further NW, towards the NNW end of the transect, a large uplift involves the Iblean platform and its underlying basement. The associated gravity anomaly is interpreted as the southern wedge edge of the Tyrrhenian mantle that splits the subducting Iblean–Pelagian (African) continental slab from an overlying synformal stack of allochthonous thrust sheets. Supplementary materials: Additional data are available at www.geolsoc.org.uk/SUP18594.

Origin and evolution of Avalonia: evidence from U–Pb and Lu–Hf isotopes in zircon from the Mira terrane, Canada, and the Stavelot–Venn Massif, Belgium

Abstract: Laser ablation inductively coupled plasma mass spectrometry U–Pb and Lu–Hf isotope analyses of detrital zircon from Neoproterozoic–Cambrian clastic sedimentary rocks in the Mira terrane (Cape Breton Island, Nova Scotia, Canada; West Avalonia) and the Stavelot–Venn Massif (East Belgium; East Avalonia) support deposition on an originally coherent microcontinent. Crustal evolution trends defined by ϵHf (t) values varying with age reflect juvenile magma production in the source continent at 1.2–2.2 and 2.4–3.1 Ga. Mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0.5–0.72, 1.4–1.7, 1.8–2.2 and 2.4–2.7 Ga. These results concur with the crustal evolution in Amazonia, the likely parent craton. Crustal evolution in Avalonia is recorded in detrital and magmatic zircon from Neoproterozoic arcs (680–550 Ma). Positive ϵHf (t) values suggest juvenile input and mixing with recycled crust. Most negative ϵHf (t) values represent recycling of predominantly Mesoproterozoic underlying crust. Avalonian arc magmatism was followed by late Neoproterozoic–early Cambrian sedimentation in various belts in West Avalonia. These belts were juxtaposed by strike-slip during late early Cambrian deposition in a rift basin. The youngest detrital zircon population ( c . 517 Ma) probably represents synrift magmatism before break-up of Avalonia. Migmatization at 406 ± 2 Ma in a xenolith from the East Avalonian crust reflects post-collisional heating. Supplementary materials: Details of sample locations and analytical results are available at www.geolsoc.org.uk/SUP18641.

New U–Pb SHRIMP ages from the Lubango region, SW Angola: insights into the Palaeoproterozoic evolution of the Angolan Shield, southern Congo Craton, Africa

Abstract: In an attempt to better understand the tectonic evolution of the continental crust forming SW Angola, zircon grains from the principal Precambrian rock types exposed in the Lubango area have been analysed using the sensitive high-resolution ion microprobe method U–Pb ages of 2038 ± 28 Ma and 1954 ± 6 Ma were obtained on weakly deformed granite samples from the basement below the Humpata Plateau The Chela Group on the Humpata Plateau is a relatively undeformed Palaeoproterozoic supracrustal sequence with an eruptive age of 1798 ± 11 Ma on ignimbrite of the Humpata Formation The age of the northern part of the Kunene Complex is constrained by zircon data from a xenolith of basement gneiss and a mangerite dyke cutting anorthosite, which give an emplacement age of 1385 ± 7 Ma The c 20 Ga granites below the Chela Group are part of a Palaeoproterozoic granitoid terrane that extends from north of Lubango in Angola into NW Namibia This terrane is referred to as the Angolan Shield Regionally, the Angolan Shield is interpreted to be part of a Palaeoproterozoic magmatic arc that extends NE from Angola and Namibia under Phanerozoic cover into NW Zambia The resultant crustal terrane defined the southern margin of the developing Congo Craton at c 20 ± 004 Ga Supplementary materials: Tables summarizing the zircon U–Pb data are available at wwwgeolsocorguk/SUP18577

Syncollisional tholeiitic magmatism induced by asthenosphere upwelling owing to slab detachment at the southern margin of the Central Asian Orogenic Belt

Abstract: The mafic–ultramafic complexes containing magmatic sulphides at the southern margins of the Central Asian Orogenic Belt have been recently proposed to result from an Early Permian mantle plume. However, in this study we show that the plume model cannot account for the observed geological characteristics of the Huangshan–Jingerquan mafic–ultramafic belt in the Northern Tianshan. Low K 2 O contents and positive correlation between TiO 2 and (Fe 2 O 3 ) T /MgO of the mafic–ultramafic complexes of this belt demonstrate a tholeiitic affinity. Enrichment of large ion lithophile elements and depletion of high field strength elements (in particular Nb and Ta) relative to mid-ocean ridge basalt indicate a subduction-modified mantle source. Lead isotope values and compositions of chromite indicate a significant contribution from the melting of asthenosphere. The absence of Late Carboniferous strata in the Huangshan–Jingerquan belt and Early Permian exhumation of blueschist and eclogite along the Aqikkuduk suture at the southern boundary of the belt indicate that an arc–continent collision occurred in the Late Carboniferous to Early Permian. We propose that the detachment of oceanic lithosphere from continental lithosphere during the collision induced asthenospheric upwelling, which resulted in melting of both the asthenosphere and the overlying metasomatized mantle wedge, and the formation of the mafic–ultramafic complexes with ages of 270–285 Ma along the Huangshan–Jingerquan belt. Supplementary materials: Zircon SHRIMP U–Pb ages of the Tudun and Huangshannan intrusions, major oxide and trace element compositions of the discussed intrusions, and clinopyroxene and chromite compositions of the selected intrusions are available at www.geolsoc.org.uk/SUP18656.

A palaeo Tibet–Myanmar connection? Reconstructing the Late Eocene drainage system of central Myanmar using a multi-proxy approach

Abstract: Strain resulting from the collision of India with Asia has caused fundamental changes to Asian drainage patterns, but the timing and nature of these changes are poorly understood. One frequently proposed hypothesis involves the connection of the palaeo Tsangpo drainage to a precursor to the Irrawaddy River of central Myanmar in the Palaeogene. To test this hypothesis, we studied the provenance of Palaeogene fluvio-clastic sedimentary rocks that crop out in central Myanmar, namely the Late Middle Eocene–Early Oligocene Pondaung and Yaw Formations. Isotopic analysis on bulk-rock and petrographic data indicate a primary magmatic arc source, and a secondary source composed of recycled, metamorphosed basement material. Although the exact location of both sources is hardly distinguishable because Burmese and Tibetan provinces share common lithological features, the presence of low-grade metamorphic fragments, the heterogeneity in Sr–Nd isotopic values of bulk sediments and westward-directed palaeoflow orientations indicate a proximal source area located on the eastern Asian margin. Central Myanmar was the locus of westward-prograding deltas opening into the Indian Ocean, supplied by the unroofing of an Andean-type cordillera that extended along the Burmese margin. We found no evidence to support a palaeo Tsangpo–Irrawaddy River, at least during the Late Eocene. Supplementary material: Data locations, and isotopic and petrographic results are available at www.geolsoc.org.uk/SUP18655.

Large sulphur isotopic perturbations and oceanic changes during the Frasnian–Famennian transition of the Late Devonian

Abstract: The Frasnian-Famennian transition of the Late Devonian was one of the most critical intervals in the Phanerozoic. Sulphur isotopic pairs of carbonate-associated sulphate and pyrite sulphide from coeval sections in South China and Poland reveal frequent perturbations of sulphur cycling during this time interval. These data suggest a sudden oceanic overturn during a rapid sea-level fall probably induced by jerky block tilting in the latest Frasnian. This event was followed by long-lasting photic-zone euxinia during a rapid sea-level rise in the earliest Famennian. Large increases in continental nutrient fluxes, and subsequent primary productivity and organic burial, could have greatly enhanced bacterial sulphate reduction, producing excessive sulphide through the water columns owing to iron depletion. Subsequently, rapid ventilation of oceanic basins occurred, dur- ing which direct aerobic oxidation of sulphide into sulphate predominated in bottom waters and even surface sediments with minimal fractionation. This oxygenation was probably induced by intensive climatic cooling and/or large-scale sea-level fall. The temporal coincidence of two extinction phases with the oceanic overturn and succeeding photic-zone euxinia suggests that these extreme oceanic events played an important role in the severe biotic crisis. Furthermore, photic-zone euxinia coupled with subsequent climatic cooling may have delayed post-extinction recovery of some taxa.

Baltican crustal provenance for Cambrian–Ordovician sandstones of the Alexander terrane, North American Cordillera: evidence from detrital zircon U–Pb geochronology and Hf isotope geochemistry

Abstract: Detrital zircon U–Pb geochronology and Hf isotope geochemistry allow us to decipher the crustal provenance of Cambrian–Ordovician backarc basin strata of the Alexander terrane, North American Cordillera, and evaluate models for its origin and displacement history relative to Baltica, Gondwana, Siberia, and Laurentia. Quartzose shallow-marine sandstones of the Alexander terrane contain a range of Neoproterozoic to Neoarchaean detrital zircons with the most dominant age groupings c . 565–760, 1000–1250, 1450, and 1650 Ma. Subordinate volcaniclastic sandstones yield Cambrian and Ordovician detrital zircons with a prominent age peak at 477 Ma. The detrital zircon age signatures resemble coeval strata in the Eurasian high Arctic, and in combination with faunal and palaeomagnetic constraints suggest provenance from local magmatic rocks and the Timanide orogenic belt and Fennoscandian Shield of NE Baltica. The Hf isotopic compositions of Palaeozoic to Neoarchaean detrital zircons strongly favour Baltican crustal sources instead of similar-aged domains of Gondwana. The Alexander terrane formed part of an arc system that fringed the Uralian passive margin, and its position in the Uralian Seaway allowed faunal exchange between the Siberian and Baltican platforms. The available evidence suggests that the Alexander terrane originated in the Northern Hemisphere and migrated to the palaeo-Pacific Ocean by travelling around northern Laurentia. Supplementary material: U–Pb and Lu–Hf data tables are available at www.geolsoc.org.uk/SUP18557.

Lithological controls on igneous intrusion-induced ground deformation

Abstract: Ground deformation commonly precedes volcanic eruptions, although its relationship to underlying intrusion networks is complex. We use 3D seismic reflection data to examine the link between a saucer-shaped sill and an overlying forced fold formed at the contemporaneous palaeosurface. Our results highlight a disparity in size between the sill and fold, which we attribute to accommodation of magma by overburden uplift and fluid expulsion from the host rock. Sill transgression occurred in response to plastic deformation of the host rock and did not produce seismically resolvable uplift. Inversion models of ground deformation should therefore acknowledge host rock behaviour during intrusion. Supplementary material: Uninterpreted seismic section and sill thickness/fold amplitude data are available at www.geolsoc.org.uk/SUP18663.

The sole of an ophiolite: the Ordovician Bay of Islands Complex, Newfoundland

Abstract: The early Ordovician ( c . 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician ( c . 470 Ma). The subduction zone was nucleated on an oceanic transform–fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a suprasubduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform–fracture zone–subduction zone system and NE of a trench–trench–ridge triple junction. A two-pyroxene garnet granulite–garnet amphibolite–epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of mid-ocean ridge basalt mafic rocks in the descending slab at about 10 kbar and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction, are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high- to lower-temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole amphibolites trend NNW (the subduction direction); those in the greenschist-facies, obducted, oceanic and continental margin rocks trend WSW (the obduction direction).

U–Pb and Hf isotope data of detrital zircons from the Barberton Greenstone Belt: constraints on provenance and Archaean crustal evolution

Abstract: Combined U–Pb and Hf isotope analyses of detrital zircons from the Fig Tree and Moodies Groups of the Barberton Greenstone Belt, South Africa, yield similar Hf isotope compositions and age populations, thus pointing to a similar provenance. Zircon populations of Fig Tree Group greywacke and Moodies Group quartzarenite are both dominated by age clusters at 3.53, 3.47, and 3.28 Ga, and a minor cluster at 3.36 Ga. The Moodies quartzarenite sample additionally contains a younger age population at 3.23–3.19 Ga. Hafnium isotope data indicate that the source area of both sediments was affected by new crust formation from depleted mantle sources at 3.53, 3.47, and perhaps at 3.36 Ga (ϵHf t between −1.7 and +4.5), accompanied by partial reworking of an Eoarchaean crustal component as old as 3.75–3.95 Ga. In contrast, crustal reworking was the predominant process between 3.28 and 3.22 Ga (ϵHf t between −6.0 and +0.9), probably related to subduction and collision of terranes along the Inyoka Fault system. The zircon U–Pb and Hf isotope datasets favour a southern provenance for the Fig Tree and Moodies sediments, comprising granitoids in the vicinity of the southern Barberton Greenstone Belt and in Swaziland. This finding is in contrast to the sedimentary record of the Moodies Group, which mostly suggests a northern and along-strike provenance. This discrepancy may be due to reworking of sediments during extensive syn- and postorogenic strike-slip faulting and high uplift or subsidence between 3.26 and 3.19 Ga. Supplementary material Results of in situ U–Pb and Lu–Hf isotope zircon analyses are available at www.geolsoc.org.uk/SUP18561.

Oligocene–Miocene burial and exhumation of the Southern Pyrenean foreland quantified by low-temperature thermochronology

Abstract: The central Pyrenees experienced an episode of rapid exhumation in Late Eocene–Early Oligocene times. Erosional products shed from the range during this time were deposited in large palaeovalleys of the southern flank of the Axial Zone, leading to significant sediment accumulation. A recent numerical modelling study of the post-orogenic exhumation and relief history of the central Axial Zone allowed us to constrain this valley-filling episode in terms of timing and thickness of conglomeratic deposits. This paper aims to test these results for the southern fold-and-thrust belt using apatite fission-track and (U–Th)/He analysis on detrital samples from the Tremp–Graus and Ager basins. Inverse thermal-history modelling of the low-temperature thermochronology data indicates that the fold-and-thrust belt was covered during the Late Eocene to Miocene by 0.7–1.6 km of sediments and confirms the timing of re-excavation of the valleys during the Miocene. A detailed analysis of the apatite (U–Th)/He results shows that the significant scatter in grain ages can be explained by the influence of alpha-recoil damage with varying effective uranium content together with distinct pre-depositional thermal histories; the age scatter is consistent with initial exhumation of the sediment sources during the Triassic and Early Cretaceous. Supplementary materials: Modelling results obtained using the Gautheron et al . (2009) diffusion algorithm are available at www.geolsoc.org.uk/SUP18560.

Precise timing of the Late Ordovician (Sandbian) super-eruptions and associated environmental, biological, and climatological events

Abstract: Two of the largest known eruptions in the Phanerozoic produced the Ordovician Millbrig K-bentonite of North America and the Kinnekulle K-bentonite of Scandinavia, which have been previously suggested to be coeval The Millbrig K-bentonite from Kentucky, USA and the Kinnekulle K-bentonite from Bornholm, Denmark yielded chemical abrasion thermal ionization mass spectrometry U–Pb zircon dates of 45286 ± 029 and 45441 ± 017 Ma (2σ analytical uncertainty), respectively, thus showing significant age differences contrary to what is generally held These data and four additional newly dated K-bentonites directly establish the first radioisotopic age constraints for the Ordovician Katian–Sandbian global stage boundary, refine global stratigraphic correlations, date associated chemostratigraphic events, and suggest an alternative volcanic–climate hypothesis for the Late Ordovician Supplementary material: U–Pb radioisotopic data table and analytical methods are available at wwwgeolsocorguk/SUP18636

Pre-existing basement structure and its influence on continental rifting and fracture zone development along Australia’s southern rifted margin

Abstract: Palaeogeographical reconstructions of the Australian and Antarctic margins based on matching basement structures are commonly difficult to reconcile with those derived from ocean-floor magnetic anomalies and plate vectors. Following identification of a previously unmapped crustal-scale structure in the southern part of the early Palaeozoic Delamerian Orogen (Coorong Shear Zone), a more tightly constrained plate reconstruction for these margins is proposed. This reconstruction places the Coorong Shear Zone opposite the Mertz Shear Zone in Antarctica and lends itself to a revised interpretation of continental rifting along Australia’s southern margin in which rift basin architecture, margin segmentation and the formation of ocean-floor fracture zones are all linked to pre-existing basement structure and the reactivation of a few deep-rooted crustal structures inherited from the Delamerian Orogeny in particular. Reactivation of the Coorong Shear Zone and other basement structures (Avoca–Sorell Fault Zone) during the earlier stages of rifting was accompanied by the partitioning of extensional strain and formation of late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and Otway basins with opposing NE–SW and NW–SE structural trends. Previously, the Mertz Shear Zone has been correlated with the Proterozoic Kalinjala Mylonite Zone in the Gawler Craton but this positions Australia 300–400 km too far east relative to Antarctica prior to breakup and fails to secure an equally satisfactory match in both basement geology and the superimposed extension-related structures.

U–Pb zircon ages of Late Cretaceous Nain–Dehshir ophiolites, central Iran

Abstract: Late Cretaceous Zagros ophiolites are part of the c . 3000 km long Late Cretaceous Ophiolite Belt of SW Asia including the Troodos (Cyprus), eastern Mediterranean (Turkey, Syria), Zagros (Iran) and Semail ophiolites (Oman). This ophiolite belt represents a magmatic forearc that formed when subduction of the Neotethys began along the SW margin of Eurasia. Geochronological data for Zagros ophiolites are limited to a few K–Ar and 40 Ar– 39 Ar ages. New thermal ionization mass spectrometry U–Pb zircon ages indicate that the Nain and Dehshir ophiolites of central Iran formed c . 101–103 Ma, with Nain (102.9 ± 0.3 Ma) being c . 1 Ma older than Dehshir (100.9 ± 0.2 Ma; 100.4 ± 0.1 Ma), and that these ophiolites were emplaced almost immediately after formation (Nain emplacement 101.2 ± 0.2 Ma; Dehshir emplacement 99.0 ± 1.1 Ma). These formation ages are significantly older than the 98–90 Ma U–Pb zircon ages of other Late Cretaceous ophiolites in this belt such as the Kizildag (Turkey), Semail (Oman) and Troodos ophiolites (Cyprus). If the subduction initiation model applies to this ophiolite belt, it suggests that subduction initiation began near the Zagros margin and propagated at c . 7 cm a −1 to the east (Semail) and c . 15 cm a −1 to the west (Troodos). Supplementary materials: Simplified geological maps of the Nain and Dehshir ophiolites, showing sample locations, are available at www.geolsoc.org.uk/SUP18554.

Non-dinosaurian dinosauromorpha

Abstract: Ichnological evidence suggests that dinosauromorphs originated by the Early Triassic, and skeletal remains of non-dinosaur representatives of the clade occur from the Anisian to the end of the Triassic. These taxa are small- to medium-sized, vary in feeding and locomotor features, and occurred over most of western Pangaea. They include the small lagerpetids from the Mid-Late Triassic of Argentina and the United States, and the larger, quadrupedal Silesauridae, with records in the Middle Triassic of Africa and Argentina, and in the Late Triassic of Europe, the Americas and northern Africa. The former group represents the earliest diverging dinosauromorphs, whereas silesaurids are more closely related to Dinosauria. Other dinosauromorphs include the archetypal early dinosauriform Marasuchus lilloensis (Middle Triassic of Argentina) and poorly known/controversial taxa such as Lewisuchus admixtus and Saltopus elginensis. The earliest diver- ging dinosauromorphs may have preyed on small animals (including insects), but cranio-dental remains are rare; by contrast, most silesaurids probably included plant material in their diet, as indi- cated by their modified jaw apparatus and teeth. Our knowledge of the anatomy and thus relation- ships of non-dinosaurian Dinosauromorpha is still deficient, and we suspect that future discoveries will continue to reveal novel patterns and hypotheses of palaeobiology and biogeography.

Neogene rock uplift and erosion in northern Borneo: evidence from the Kinabalu granite, Mount Kinabalu

Abstract: Thermochronological data from the Kinabalu granite, emplaced between c . 7.2 and 7.8 Ma, provide a unique record of northern Borneo’s exhumation during the Neogene. Biotite 40 Ar/ 39 Ar ages ( c . 7.32–7.63 Ma) record rapid cooling of the granite in the Late Miocene as it equilibrated with ambient crustal temperatures. Zircon fission-track ages ( c . 6.6–5.8 Ma) and apatite (U–Th–Sm)/He ages (central age c . 5.5 Ma) indicate rapid cooling during the Late Miocene–Early Pliocene. This cooling reflects exhumation of the granite, uplift and erosion bringing it closer to the Earth’s surface. Thermochronological age versus elevation relationships suggest exhumation rates of more than 7 mm a −1 during the latest Miocene and Early Pliocene. Neither the emplacement of the Kinabalu granite nor its exhumation is related to the Sabah orogeny, which terminated in the Early Miocene. Instead, granite magmatism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu granite was driven either by lithospheric delamination or break-off of a subducted slab beneath Sabah. Plio-Pleistocene tectonism offshore and onshore northern Borneo reflects continuing large-scale gravity-driven tectonics in the region. Supplementary material: Full 40 Ar/ 39 Ar, fission-track and (U–Th–Sm)/He analytical data and 40 Ar/ 39 Ar age spectra plots can be found at www.geolsoc.org.uk/SUP18613.

Palynostratigraphy and vegetation history of the Triassic–Jurassic transition in East Greenland

Abstract: We present a palynological study of a terrestrial Triassic–Jurassic (Tr–J; c . 200 Ma) boundary section at Astartekloft, East Greenland. We have generated a new palynostratigraphic scheme and vegetation history for this locality, and have integrated these with existing carbon isotope records. Samples for palynological analysis were collected from precisely the same stratigraphic horizons as plant macrofossils and samples used for geochemical analyses. Our results highlight four local sporomorph assemblage zones that are compositionally distinct from each other at Astartekloft. The extremely low abundance of Classopollis pollen in all samples, and the pronounced decline of Ricciisporites tuberculatus during the Late Rhaetian are notable features of the sporomorph record of Tr–J vegetation at Astartekloft. Correlation of Astartekloft and a marine Tr–J boundary section at St Audrie’s Bay, UK, provides no support for the idea that extinction and diversity loss in terrestrial ecosystems preceded biotic change in marine ecosystems at the Tr–J. Instead, the available data support suggestions that the onset of the Tr–J biotic crisis was synchronous in terrestrial and marine environments. Peak extinction among plants at Astartekloft occurred relatively late in the sequence of events across the Tr–J, and may represent a response to long-term cumulative effects of volcanism at this time. Supplementary material Plates of selected sporomorphs recovered from Astartekloft and a full pollen diagram are available at www.geolsoc.org.uk/SUP18553.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Abstract: Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Backarc basin inversion and subcontinental mantle emplacement in the crust: kilometre-scale folding and shearing at the base of the proto-Alborán lithospheric mantle (Betic Cordillera, southern Spain)

Abstract: To constrain the latest evolutionary stages and mechanisms of exhumation and emplacement of subcontinental peridotites in the westernmost Mediterranean, we present here a detailed structural study of the transition from granular spinel peridotite to plagioclase tectonite in the western Ronda Peridotite (Betic Cordillera, southern Spain). We show that the plagioclase tectonite foliation represents an axial surface particularly well developed in the reverse limb of a downward facing moderately plunging and moderately inclined synform at the base of the Ronda massif. The fold limbs are cut by several mylonitic and ultramylonitic shear zones with top-to-the-SW sense of shear. After restoring the middle to late Miocene vertical-axis palaeomagnetic rotation and the early Miocene tectonic tilting of the massif, these studied structures record southward-directed kinematics. We propose a geodynamic model in which folding and shearing of an attenuated mantle lithosphere occurred by backarc basin inversion during late Oligocene (23–25 Ma) southward collision of the Alboran Domain with the palaeo-Maghrebian passive margin, leading to the intracrustal emplacement of peridotites in the earliest Miocene (21–23 Ma).

Palaeoenvironmental and climatic changes during the Palaeocene–Eocene Thermal Maximum (PETM) at the Wadi Nukhul Section, Sinai, Egypt

Abstract: The Palaeocene–Eocene Thermal Maximum (PETM) interval at the Wadi Nukhul section (Sinai, Egypt) is represented by a 10 cm thick condensed clay-rich layer corresponding to the NP9a–NP9b nannofossil subzone boundary. The Wadi Nukhul Palaeocene–Eocene boundary (PEB) is characterized by (1) an abrupt negative excursion in carbonate and organic carbon isotope ratios (−6‰ in δ 13 C carb and −2‰ δ 13 C org ), (2) an abrupt persistent negative shift in organic nitrogen isotope values (δ 15 N org ), (3) a significant increase in phosphorus concentrations just above the carbon isotope excursion, (4) a decrease in carbonate content and significant increase in kaolinite and (5) high vanadium and low manganese contents coincident with the occurrence of framboidal pyrite. The abrupt correlative isotopic excursions of δ 13 C carb , δ 13 C org and δ 15 N suggest that the lowermost part of the PETM is missing. The decrease in carbonate content indicates dilution by high detrital input triggered by acid weathering and carbonate dissolution in response to increased atmospheric CO 2 resulting from the oxidation of methane. The sudden increase in kaolinite content reflects a short-lived change to humid conditions. The δ 15 N values close to 0‰ above the PEB suggest a bloom of N 2 -fixing cyanobacteria. Increased bacterial activity may be either the cause or the result of the anoxia locally associated with the PETM.

Lu–Hf and Sm–Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland

Abstract: Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc–continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). Lu–Hf and Sm–Nd garnet ages of c . 475–460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. Lu–Hf and Sm–Nd ages of c . 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event. The existing two-stage Grampian–Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution. The restriction of the Late Ordovician and Silurian events to the Northern Highland terrane reinforces the suggestion that it was far removed from the Grampian terrane until juxtaposition following major end-Caledonian (Devonian) sinistral displacement along the Great Glen Fault. A similar record of Mid- and Late Ordovician metamorphic events within the Laurentian-derived Uppermost Allochthons of Norway has been attributed to episodic accretion significantly prior to Silurian continent–continent collision and closure of the Iapetus Ocean. Supplementary materials: Results of trace element analysis of the garnets by laser ablation inductively coupled plasma mass spectrometry are available at www.geolsoc.org.uk/SUP18583.

Detrital zircon data reveal the origin of Australia’s largest delta system

Abstract: The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582.

Mercia Mudstone Formation caprock to carbon capture and storage sites: petrology and petrophysical characteristics

Abstract: Porous rocks in the subsurface are now used for carbon capture and storage (CCS), to help ameliorate the effects of greenhouse gas emissions. These porous reservoir rocks require a caprock to retain the CO 2 . It is important to characterize caprock quality and its stability in the presence of elevated partial pressures of CO 2 . Lower Triassic sandstones are common in the UK and NW Europe and are being considered for future CCS projects. The caprock to these sandstones is the Middle and Upper Triassic Mercia Mudstone Group. We have studied the Mercia Mudstone using mineralogy, petrology and mercury injection porosimetry to assess its caprock quality. Detrital minerals are dominated by quartz, K-feldspar, illite and chlorite; diagenetic pore-filling minerals are dominated by calcite, dolomite and gypsum. In samples with abundant clay minerals, there are only small quantities of pore-filling diagenetic cements. Porosity is broadly uniform for both clay-rich and clay-poor samples. The cleaner (clay-poor) samples had their initial pore spaces filled with early diagenetic pore-filling carbonates and gypsum. Despite the broadly uniform porosity, mean pore throat diameter displays a strong inverse correlation with clay content whereas threshold capillary entry pressure shows a strong positive correlation with clay content. The more clay-rich samples represent much better caprock than the coarser-grained samples that contain abundant pore-filling cement. The samples could support potential column heights of supercritical CO 2 between 70 and 540 m and have calculated permeabilities between 10 −20 and 10 −19 m 2 ; leakage would occur only on geological time scales, assuming that these samples are representative of the whole caprock. Because caprock quality correlates with illite content and illite will be relatively immune to elevated partial pressures of CO 2 , the Mercia Mudstone probably represents a durable caprock for future Lower Triassic sandstone CCS projects.

Investigation of the spatio-temporal relationship between normal faulting and arc volcanism on million-year time scales

Abstract: The spatio-temporal evolution of normal faulting and submarine volcanism during the Mid-Miocene to Recent ( c . 16 Ma has been attributed to progressive steepening and SE rollback of the subducting Pacific Plate. Similarly, the location and NE–SW strike of Late Miocene and younger normal faults mainly west of the North Island appear to have been controlled by the location and NE–SW strike of the underlying subducting plate. Stepwise changes in the locus of faulting at c . 8, 4 and 2 Ma could have been triggered by increases in the rates of vertical-axis rotation of the North Island associated with changes in plate convergence rates and southward migration of the rotation pole. The disparate spatio-temporal migration histories of subduction-related faulting and volcanism indicate that, over time scales of millions of years and distances of tens of kilometres, neither process controls the timing, location and rates of activity of the other.

Tectonic evolution of the European Variscan belt constrained by palaeomagnetic, structural and anisotropy of magnetic susceptibility data from the Northern Vosges magmatic arc (eastern France)

Abstract: Geochronology, structural and anisotropy of magnetic susceptibility data from the northern Vosges batholith, which belongs to the Northern Vosges–Mid-German Crystalline Rise arc, show contrasting emplacement modes of southern granodiorites and northern granites. The ENE–WSW-trending fabrics of granodiorites (346–334 Ma) are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression. The NNW–SSE-trending fabrics of younger granitoids ( c . 330 Ma) reveal an extensional emplacement mode, associated with a normal shear zone separating the two magmatic suites. Palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation. The 330–325 Ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith. Finally the system is rotated clockwise without any structural overprint. Data from the Early Carboniferous northern Vosges magmatic arc and remote Bohemian Massif suggest that this evolution is valid for the whole eastern branch of the European Variscan belt, for which the following model is proposed: (1) Late Devonian–Early Carboniferous east–west shortening of the Variscides above the Rhenohercynian subduction zone; (2) axial NNW–SSE shortening of the assembled Variscan orogen associated with (3) ‘internal’ sinistral rotation of inherited Rhenohercynian transform faults and shortening of intervening blocks; (4) east–west extension and dextral ‘external’ rotation of blocks between dextrally reactivated transforms. Supplementary materials: Analytical procedures for geochronology, magnetism, anisotropy of magnetic susceptibility and palaeomagnetism are available at www.geolsoc.org.uk/SUP18635.

Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’: Journal, Vol. 168, 863–872

Abstract: We welcome the contribution of Parry et al . (2011) concerning the age of the globally important Rhynie hot-spring system, Aberdeenshire, Scotland. However, although we accept that they have provided a valuable and robust U–Pb age constraint for the Milton of Noth Andesite and the Rhynie outlier stratigraphic succession, we do not accept that they have constrained the timing of hot-spring activity, as suggested. The geophysical trace of the unexposed Milton of Noth Andesite indicates that it lies immediately adjacent to the Rhynie Fault Zone and runs parallel to it for some 2 km. It may even lie within the Rhynie Fault Zone (see discussion by Rice & Ashcroft 2004). Its stratigraphic position is therefore uncertain. Parry et al . (2011) stated that the exact stratigraphic position is not important because their preferred stratigraphic locations lie within the Tillybrachty Sandstone and Dryden Flags Formations, both of which have consistently yielded spores belonging to the polygonalis–emsiensis Spore Assemblage Biozone. This assertion is not valid. The polygonalis–emsiensis Spore Assemblage Biozone may have lasted several million years and thus, potentially, there could be a significant hiatus between eruption of the andesite and the onset of hot-spring activity. They further assume that the andesite was linked to an andesitic magma chamber at depth, which powered the hydrothermal system. This remains speculation. Accepting the uncertainty in the stratigraphic position and by asserting a maximum thickness of sediment between the andesite and the Rhynie Cherts and a conservative deposition rate, Parry et al . (2011) attempted to temporally link the andesite to the cherts by calculating the time interval represented by the intervening sediment. The estimation of time depends on the absence of unconformities, disconformities and faulting, with such assumptions and uncertainties casting doubt on the thickness of the intervening sediments …

Sedimentary fill of the Chile Trench (32-46°S): volumetric distribution and causal factors

Abstract: The Chile Trench of the convergent continental margin of Central Chile is a sediment-filled basin that stretches over 1500 km in a north–south direction. The sediment fill reflects latitudinal variations in climate as well as in the morphology and geology of Chile, but also of sediment transport processes to the trench and within the trench. We try to untangle these signals by calculating the total volume and the latitudinal volume distribution of trench sediments and by relating this distribution to a number of factors that affect this pattern. The volume calculation is based on a model geometry of the top of the subducting oceanic plate that is buried beneath trench sediments and the sea floor as measured by swath bathymetry. We obtain the model geometry of the subducting plate by interpolating between depth-converted seismic reflection profiles that cross the trench. The total volume of the trench fill between 32 and 46°S is calculated to be 46000 ± 500 km 3 . The resulting latitudinal volume distribution is best explained by a sedimentation model that alternates between (1) glacial phases of high sediment flux from Southern Chile combined with active latitudinal sediment transport within the trench and (2) interglacial phases over which sediment input is dominated by local factors. Supplementary material: Top of the oceanic basement (TOB) grid is available as ascii raw data files (xyz) at www.geolsoc.org.uk/SUP18664.

The gigantic Seymareh (Saidmarreh) rock avalanche, Zagros Fold–Thrust Belt, Iran

Abstract: The Seymareh rock avalanche, first described by J. V. Harrison and N. L. Falcon in the 1930s, is the largest known subaerial non-volcanic landslide on Earth. The volume of its debris (44 Gm 3 ) is approximately equal to that of the largest known subaerial landslide of any kind, the 100 ka collapse of Mount Shasta volcano (45 Gm 3 ) and approaches those of gigantic landslides on Mars. Using satellite imagery, SRTM-3 data, and detailed field investigations, we gain a new understanding of the gigantic rock avalanche. The initial failure (38 Gm 3 ), involving a dip slope consisting of a relatively weak interbedded limestone–mudrock sequence capped by resistant Asmari Limestone, had an average thickness of 410 m along a 15.5 km width of the Kabir Kuh anticline. The rockslide transformed into a rock avalanche that travelled 19.0 km with a fahrboschung (the angle between the highest point of the pre-landslide source area topography and the distal limit of the debris) of only 3.6°. Initial failure involved complex planar sliding dictated by four structural elements: (1) bedding-parallel shears and (2) across-bedding break-throughs combined to produce a low-angle (11°) overall sliding surface that (3) broke out through the Asmari carapace at the toe of the failed slope while (4) tectonically weakened bedding-normal joints provided lateral release surfaces. Nine discrete bedding-parallel basal sliding surfaces divided the detached mass into nine stacked plates. The upper plate (23 Gm 3 ) included 20.7 Gm 3 of Asmari Limestone. Analysis of accelerator mass spectrometry radiocarbon dates from this study and by others indicates that the rock avalanche occurred between c . 8710 and 9800 14 C years BP. Supplementary material: A summary of the tectonics, stratigraphy and physiography of the Zagros Fold–Thrust belt together with a description of surficial gravity collapse structures in the region (Harrison & Falcon 1934, 1936) is available at www.geolsoc.org.uk/SUP18612.

Insights into the early Tibetan Plateau from (U–Th)/He thermochronology

Abstract: The central Songpan–Ganzi belt, located on the eastern margin of the Tibetan Plateau, has a similar high elevation and low relief to parts of central Tibet. Thermochronological studies from the central Tibetan Plateau reveal a history of slow exhumation (rates −1 ) since 45Ma; however, the exhumation history of the central Songpan–Ganzi belt is unknown. To address this, we conducted an apatite and zircon (U–Th)/He thermochronology study of bedrock samples collected across the central Songpan–Ganzi belt and into central parts of the Tibetan Plateau. Zircon (U–Th)/He ages range from 54.2±7.5 to 146.5±10.0Ma and the majority of apatite (U–Th)/He ages fall between 74.7±19.0 and 35.7±9.4Ma. Thermal history models of these data show rapid cooling in the late Mesozoic and much slower cooling diagnostic of low rates of erosion throughout most of the Cenozoic. The late Mesozoic rapid cooling is consistent with the existence of significant topography and relief at least in some parts of the Songpan–Ganzi belt at that time. We find no evidence for a regional Miocene acceleration in erosion, although three samples from the headwaters of the Salween and Mekong rivers gave younger AHe ages between 15 and 16Ma that reflect an acceleration in river incision. Supplementary material: Laser ablation–inductively coupled plasma mass spectrometry zircon U–Pb data from central and eastern Tibet are available at www.geolsoc.org.uk/SUP18647.