Showing papers in "Journal of the Geological Society in 2006"

Role of the Red River Shear zone, Yunnan and Vietnam, in the continental extrusion of SE Asia

Abstract: The 1000 km long NW–SE-striking, left-lateral Ailao Shan–Red River shear zone runs from the southeastern corner of Tibet to the Gulf of Tonkin and the South China Sea. It has been used as the prime example of a lithospheric-scale strike-slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of Indo-China away from the Indian plate indentor. Central to the model of continental extrusion is that such faults cut through the entire lithosphere, that shear heating resulted in high-grade metamorphism and local anatexis, and that the ages of sheared granites along the fault also date the timing of strike-slip shearing. However, structural data from the Red River shear zone clearly show that vertical strike-slip faulting post-dated metamorphism and granite emplacement. Most granites along the shear zone are mantle-related granodiorites or within-plate alkali granites formed prior to shearing along the Red River shear zone. Left-lateral kinematic indicators are ubiquitous within the Red River mylonites, but they are always lower-temperature fabrics, formed after peak sillimanite metamorphism and after granite crystallization. It is suggested that left-lateral strike-slip shearing along the Red River shear zone started after 21 Ma, not at 35 Ma as previously thought, and the fault was purely a crustal structure. None of the geological features used to propose the 500–1000 km offsets are robust, and the total finite offset remains unknown.

Active deformation within the Zagros Mountains deduced from GPS measurements

Abstract: We present and interpret the results of Global Positioning System (GPS) measurements at 35 stations in and beside the Zagros Mountain belt, SW Iran, for three campaigns ending March 1998, December 1999 and June 2001. Preliminary motion estimates show clearly the change in character along the strike of the belt. Stations to the SE move at 13–22 ± 3 mm a −1 towards N 7 ± 5°E with respect to Eurasia. Most of the shortening indicated by the GPS velocities seems to occur in the SE Zagros along two major seismic zones and along the Zagros front. To the NW, stations move oblique to the trend of the belt towards N 12 ± 8°W, at 14–19 ± 3 mm a −1 . Most of the shortening in the NW Zagros seems to occur along the Mountain Front Fault with its major earthquakes as well as along the Zagros front. The change in direction and magnitude of the velocity vectors across the north–south-trending Kazerun and Karebas faults involves extension of up to 4 mm a −1 along the strike of the Zagros belt.

Structure and evolution of hydrothermal vent complexes in the Karoo Basin, South Africa

Abstract: The Karoo large igneous province, formed at c . 183 Ma, is characterized by the presence of voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders. Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, slightly predating the onset of the main phase of flood volcanism.

Widespread arc-related melting in the mantle section of the northern Oman ophiolite as inferred from detrital chromian spinels

Abstract: We examined detrital chromian spinels, in recent river beds, derived from the mantle section to deterrmine whether the mantle peridotite of the northern Oman ophiolite is of oceanic or arc origin. The Cr-number (= Cr/(Cr + Al) atomic ratio) of the detrital chromian spinels mostly ranges from 0.4 to 0.8, and more than 60% of them have Cr-numbers higher than 0.6. This indicates a significant extent of island-arc nature because chromian spinels seldom have Cr-numbers higher than 0.6 in ocean-floor rocks. The high-Cr-number (>0.6) spinels, which are also low in TiO 2 (

Precise 40Ar/39Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya

Abstract: The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo–Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above the Chari Tuff is particularly notable for its wealth of hominid fossils and archaeological sites. Silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation. Pumice clasts within the tuffs contain anorthoclase phenocrysts, ideal for 40Ar/39Ar single-crystal dating. Feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the KBS Tuff to the Silbo Tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the KBS Tuff (1.869 ± 0.021 Ma) and the Chari Tuff (1.383 ± 0.028 Ma) and to yet higher in the sequence to the SilboTuff (0.751 ± 0.022 Ma). These results provide a precise and accurate time scale for the upper part of the sequence in the whole of the Omo–Turkana Basin. A number of these tuffs are recognized elsewhere in East Africa; thus ages determined at Koobi Fora also apply to the wider region.

Saucer-shaped sill with lobate morphology revealed by 3D seismic data: implications for resolving a shallow-level sill emplacement mechanism

Abstract: We use 3D seismic data to describe the 3D geometry of a large igneous intrusion, the Solsikke Compound Sill, and address a number of issues related to sill emplacement. The Solsikke Compound Sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer-shaped depressions and linear discontinuities. One of the saucer-shaped sub-elements of the compound sill, the Solsikke Lobate Sill, has a previously unrecognized morphology. It has a basal feeder and consists of a bifurcating network of interlinked lobe-shaped sill segments. We propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments. The lobate pattern indicates that the Solsikke Lobate Sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation. The feeder location is coincident with a fault intersection, suggesting that magma transport from the underlying source exploited the zone surrounding the intersection.

Two-phase orogenic convergence in the external and internal SW Alps

Abstract: The NW–SE-trending sector of the SW Alps includes the Dora Maira massif where Tertiary eclogites record ultrahigh pressures and rapid exhumation. Along a NE–SW crustal cross-section (Italy–France) compiled pressure–temperature–time data in internal zones are correlated with Tertiary stratigraphy in external zones to reconstruct orogen evolution, revealing a coherent two-phase convergence history. During the first, subduction–accretion phase (Eocene, 55–34 Ma) rapid north–south plate convergence caused the subduction and exhumation of high-pressure and ultrahigh-pressure (UHP) rocks in a steady-state subduction channel. This coincided with the north to NNW migration of an underfilled flexural basin across the European foreland. Nappe stacking within the subduction channel did not create significant relief, implying that primarily subduction forces generated this flexural basin. From 34 Ma onward, the second, collisional phase was characterized by slower NW–SE plate convergence. The internal units of the SW Alps underwent considerable anticlockwise rotation as they became involved in a NW–SE-oriented sinistral transpression zone between the European and Adriatic plates. To the north of the orogen the North Alpine Foreland Basin became overfilled as a result of high sediment supply from increasing orogen relief. In contrast, in SE France active flexure of the European plate appears to have ceased and sedimentation became limited to small thrust-sheet-top basins created by continuing gentle NE–SW shortening. Internal units were exhumed slowly from depths of c . 20 km, principally by erosion. In the SW Alps, the transition between these two phases was marked by the rapid subduction and exhumation of the Dora Maira UHP unit. Assuming lithostatic pressure, this unit would have been exhumed from 100 km depth, requiring a rate that exceeds that generated by plate convergence. Therefore, either exhumation was accelerated by additional stresses (locally generated by transpression, slab breakoff or high density contrasts) or, more controversially, the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure.

Convergent morphology in small spiral worm tubes (‘Spirorbis’) and its palaeoenvironmental implications

Abstract: Calcareous tube-worms generally identified as Spirorbis range from Ordovician to Recent, often profusely encrusting shells and other substrates. Whereas Recent Spirorbis is a polychaete annelid, details of tube structure in pre-Cretaceous ‘ Spirorbis ’ suggest affinities with the Microconchida, an extinct order of possible lophophorates. Although characteristically Palaeozoic, microconchid tube-worms survived the Permian mass extinction before being replaced in late Mesozoic ecosystems by true Spirorbis . Recent Spirorbis is stenohaline but spirorbiform microconchids also colonized freshwater, brackish and hypersaline environments during the Devonian–Triassic. Anomalies in the palaeoenvironmental distributions of fossil ‘ Spirorbis ’ are explained with the recognition of this striking convergence between microconchids and true Spirorbis .

Was Baltica right-way-up or upside-down in the Neoproterozoic?

Abstract: Baltica is a progeny of Rodinia, born from the breakup of the supercontinent in the Neoproterozoic. Within Rodinia, Baltica is generally placed adjacent to NE Laurentia but in a variety of configurations, which vary by up to 3000 km along the strike of the Laurentian margin and include both right-way-up and upside-down orientations (current coordinates). Geological and palaeomagnetic data show that the only viable reconstruction juxtaposes the western Scandinavian margin of Baltica, in its right-way-up orientation, against the Rockall–Scotland–SE Greenland segment of Laurentia.

The SNC meteorites: basaltic igneous processes on Mars

Abstract: A group of 32 meteorites, the SNC ( S hergotty, N akhla, C hassigny) group, was derived from Mars as a product of 4–7 ejection events, probably from Tharsis and Elysium–Amazonis. The SNCs either have basaltic mineralogy or some are ultramafic cumulates crystallized from basaltic melts. The SNCs can be classified both petrographically and geochemically. We classify the shergottite SNC meteorites on the basis of their light rare earth element (LREE) depletion into highly depleted, moderately depleted and slightly depleted. The slightly depleted samples (which are mainly but not exclusively aphyric basalts) show high log 10 f O 2 values (QFM −1.0, where QFM is quartz–fayalite–magnetite). Highly depleted samples, which are mainly olivine-phyric basalts, have low log 10 f O 2 values (QFM −3.5). On the basis of mixing calculations between La/Lu and 87 Sr/ 86 Sr we favour models linking the correlation between LREE abundances and log 10 f O 2 to mantle heterogeneity rather than contamination by oxidized, LREE-rich crustal fluids. SNC chemistry in general reflects the Fe-rich mantle of Mars (which contains twice as much FeO as the Earth9s mantle), the late accretion of chondritic material into the mantle, and possibly the presence of a plagioclase-rich magma ocean, which acted to variably deplete the mantle in Al. The high FeO contents of the SNC melts are associated with high melt densities (allowing the ponding of large magma bodies) and low viscosities, both of which are consistent with the large scale of many observed martian lava flows.

Severity and timing of Cenozoic exhumation in the southwestern Barents Sea

Abstract: The juxtaposition of the North Atlantic rift system with the borderlands of the Eurasian Arctic shelf has resulted in an unusual basin evolution in the southwestern Barents Sea. Exploration has concentrated on the Mesozoic sandstone plays of the Hammerfest Basin. These have yielded abundant gas and very little oil. Such findings are typical of peripheral North Atlantic margin basins that have undergone Cenozoic exhumation. This is generally thought to have occurred during the Palaeocene, Oligocene–Miocene or Pleistocene. Hammerfest Basin erosion is widely estimated to be in the range of 500–1500 m. However, the relative severity of individual episodes and impact on fluid dynamics within the basin are poorly constrained. We apply basin modelling tools to establish the sensitivity of the petroleum system to three cited scenarios for Cenozoic exhumation. Vitrinite reflectance and temperature data constrain burial and thermal histories. Calibration is not possible using heat flow variations alone; erosion timing and magnitude are required to match available data. The calibrated model indicates a significant thermal disequilibrium in the Hammerfest Basin at the present day resulting from Late Cenozoic exhumation. Forward modelling of Pleistocene ice sheets allows for an estimation of pressure and temperature fluctuations in response to glacial–interglacial cycles.

The Pennsylvanian tropical biome reconstructed from the Joggins Formation of Nova Scotia, Canada

Abstract: The Pennsylvanian (Langsettian) Joggins Formation contains a diverse fossil assemblage, first made famous by Lyell and Dawson in the mid-19th century. Collector curves based on c . 150 years of observation suggest that the Joggins fossil record is relatively complete. A key feature of the site is that fossils occur in (par)autochthonous assemblages within a narrow time interval (

K–Ar and 40Ar/39Ar geochronology of supergene processes in the Atacama Desert, Northern Chile: tectonic and climatic relations

Abstract: New K–Ar and 40 Ar/ 39 Ar data obtained for 29 samples of supergene alunite-, jarosite- and hollandite-group minerals from the Central Depression of Northern Chile document almost continuous supergene oxidation from 33 to 9 Ma. The downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16–35 m Ma −1 . These calculated rates are sensitive to pre-existing topography and show that the present relief observed in the Central Depression represents a modified Miocene landscape. The results presented here are in good agreement with previously published geochronological data for the Atacama Desert. The combination of all published data, including the results presented here, defines a continuous skewed distribution, with a maximum between 21 and 14 Ma. The relative paucity of ages greater than 30 Ma reflects the removal of supergene profiles by erosion during and immediately after the Incaic compressive deformation. From 29 to 9 Ma, supergene oxidation profiles were generated under semi-arid conditions throughout the Atacama region. Ages between 9 and 5 Ma on the northern and southern margins of the desert mark the end of supergene oxidation processes, consistent with a Pliocene onset of present-day hyperaridity.

Timing of early angiosperm radiation: recalibrating the classical succession

Abstract: New stratigraphically controlled pollen data from the late Early Cretaceous of Portugal allow a recalibration of the classical continental succession of the Potomac Group (USA), which has long been used to demonstrate the gradual morphological changes of angiosperm leaves and pollen. The Portuguese record represents the best dated succession of angiosperm pollen assemblages of the interval between the late Barremian and the mid-Albian. Comparison of the angiosperm pollen record of the Potomac Group with the pollen assemblages from Portugal and with other independently dated records indicates that the distinct differences in the angiosperm pollen assemblages between the three formations of this group (the Patuxent Formation, Arundel Clay Formation and Patapsco Formation) are related to discontinuities. Our revised age model for the Potomac Group implies a major discontinuity between the Arundel Clay (of early Albian age), and the Patapsco Formation (of mid-Albian–early Cenomanian age). The Portuguese record reveals a so far undocumented radiation phase of angiosperms within the early Aptian to mid-Albian in which monocolpate angiosperm pollen of monocot and/or magnoliid affinity appear as a highly diversified group. These new findings imply that the onset of the radiation of monocots–magnoliids preceded the radiation of eudicots by at least 10 Ma.

The morphology of intrusion-related vent structures and their implications for constraining the timing of intrusive events along the NE Atlantic margin

Abstract: Intrusion-related vent structures found within Palaeocene sediments and near the Palaeocene–Eocene transition in basins along the NE Atlantic margin have been studied in detail using 3D seismic data. The vents formed at the palaeo-seabed as a result of sill intrusion and the vent fill ranges in composition from entirely remobilized sedimentary to dominantly magmatic. The vents are commonly developed above sill edges and crests in sills to which they are linked by pipe- and diatreme-like structures. The vents also develop above the upper tips of deep-seated faults and at fault intersections. The vents can be used to constrain the approximate timing of igneous sill intrusion and duration of hydrothermal activity. It is estimated that using the seismically constrained stratigraphic context of these vents to establish the timing of sill emplacement is associated with an uncertainty of >100 ka and up to as much as 800 ka. Interpretation of the vents indicates that sill intrusion took place during a number of discrete phases within the Palaeocene and earliest Eocene. These structures and their timing of formation have important implications for climate change studies, basin-scale processes including fluid flow and diagenesis, as well as hydrocarbon exploration.

Provenance of late Palaeozoic metasediments of the SW South American Gondwana margin: a combined U–Pb and Hf-isotope study of single detrital zircons

Abstract: Combined U–Pb and Lu–Hf isotope measurements of single detrital zircon grains in Carboniferous metasediments from Patagonia delineate the source areas of the sediments. The detritus, represented by four metasandstone samples, was deposited prior to onset of subduction in Late Carboniferous time along the south Patagonian proto-Pacific Gondwana margin. A broad series of detrital zircon age peaks (0.35–0.7 Ga, 0.9–1.5 Ga) and a large spread (0.3–3.5 Ga) in the age spectra require numerous sources. A fifth metasediment was deposited after the onset of subduction. This syncollisional sample shows two distinct U–Pb age peaks at c . 290 Ma and 305 Ma. This points to a few sources only (Patagonia, West Antarctica). Initial Hf-isotope compositions of selected U–Pb dated zircons from the Carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources (eHf (T=0.4–3.5Ga) =−14 to +12). A comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of Gondwana (Extra-Andean Patagonia, the Argentine Sierra de la Ventana, southernmost Africa, East Antarctica), as well as northern Chile and northwestern Argentina. The sediment transportation paths are consistent with an autochthonous palaeogeographical position of Patagonia with respect to Gondwana in Carboniferous time.

Neoproterozoic to Early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina)

Abstract: The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c . 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c . 496 Ma El Penon pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208 Pb/ 232 Th monazite age of 478 Ma for a migmatite from the Nogoli Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c . 500 Ma and c . 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

Timing and kinematics of Eburnean tectonics in the central Reguibat Shield, Mauritania

Abstract: The Reguibat Shield of north Mauritania comprises a western Archaean terrane dominated by gneisses and granitic rocks and an eastern Eburnean terrane largely made up of Palaeoproterozoic granitic and metasedimentary rocks. These were juxtaposed during the c . 2.1–2.0 Ga Eburnean Orogeny, which formed an approximately north-trending front contiguous with the Eburnean belt of the Leo Shield in equatorial West Africa. Geological surveying in the Sfariat region of the Reguibat Shield has shown that a metamorphosed Palaeoproterozoic continental margin succession has been intruded by and intercalated with synorogenic granitoids and transported SW onto the Archaean foreland during sinistral oblique collision preserved as strongly partitioned D 1 shortening and D 2 strike-slip-dominated transpression within the Sfariat Belt. This was subsequently affected by retrogressive dextral transcurrent deformation and the propagation of the en echelon Imarkene and Tmeimichat fault zones. Laser ablation and conventional thermal ionization mass spectrometry U–Pb geochronology were carried out on five samples of granitic rock and migmatitic gneiss from the region, which reveal that anatexis and sinistral transpression took place between c . 2117 and 2064 Ma. Timing and kinematics of the Eburnean event in the Reguibat Shield are similar to those for the Leo Shield, which underwent SW-directed sinistral transpression at this time.

The late Neogene Sahabi rivers of the Sahara and their climatic and environmental implications for the Chad Basin

Abstract: This paper is based on the concept that the drainage system of c . 7.5–4.6 Ma is still reflected in the geomorphology of northeastern Chad and adjacent areas of Libya. During the Messinian and early Pliocene a large lake was present in the Chad Basin that is termed Neogene Lake Chad. It fluctuated in size in response to the precessional cycle and at times overflowed to the east, NE or north, giving rise to the Sahabi rivers. The Eosahabi flowed during the drawdown of the Mediterranean (late Messinian) and eroded the Erdi and part of the East Tibesti Valley. The post-rift Miocene deposits of the Chad Basin, some several hundred metres in thickness, record a transgression over an irregular and faulted terrain with the deposition of coarse and fine clastic material. Fluviatile and lacustrine environments are represented. At least part of the Miocene succession belongs to a phase of late Miocene lake development. Fluctuating climate during the Messinian and early Pliocene led to repeated changes in the environment of the Chad and Eosahabi Basins with over 100 climatic cycles developed. This would favour the development of animal species with high adaptability, for example to littoral, riparian, woodland and savanna habitats.

Upper Cretaceous carbon- and oxygen-isotope stratigraphy of hemipelagic carbonate facies from southern Tibet, China

Abstract: A high-resolution carbon-isotope curve derived from Upper Cretaceous hemipelagic sediments cropping out at Tingri, southern Tibet, shows similarities to patterns established on other continents, notably in the presence of a well-defined positive excursion across the Cenomanian–Turonian boundary where δ 13 C values exceed 3.5‰. From the upper Turonian to the lower Campanian, δ 13 C values generally decline, apart from a minor positive excursion in the middle Coniacian: a trend that departs from that recorded from Europe. Relatively low δ 13 C values ( c . 1‰) at the Santonian–Campanian and Campanian–Maastrichtian boundaries in Tibet define a prominent broad positive excursion centred in the middle Campanian and terminated by an abrupt fall towards the close of the stage. When compared with data from Europe and North Africa, the δ 13 C values of the Tibetan section are generally lower by c . 1.5‰, except for the middle Campanian positive excursion where values (δ 13 C c . 2‰) are comparable with those documented from Europe and North Africa. These differences are interpreted as reflecting variable mixing of water masses carrying different carbon-isotope signatures, such that areas close to the major sinks of marine organic carbon recorded higher δ 13 C values than those located in more distal regions. Oxygen-isotope ratios, albeit affected by diagenesis, may record a palaeotemperature signal.

Mare versus Lago-mare: marine fishes and the Mediterranean environment at the end of the Messinian Salinity Crisis

Abstract: During the late Messinian, c . 5.5 Ma ago, after evaporitic sedimentation and before the Pliocene flooding, the Mediterranean recorded a major environmental change testified by deposition of non-marine sediments of the so-called ‘Lago-mare’ facies. Such deposits are widespread in the Mediterranean basin, usually characterized by molluscs and ostracodes of brackish affinity. Here we present marine fish remains from ‘Lago-mare’ deposits of central Italy. The fossils, represented by both articulated skeletons and otoliths, were collected from nine stratigraphic horizons of the upper portion of the ‘Lago-mare’ sequence of Cava Serredi, Tuscany. Marine euryhaline fishes strongly dominate the assemblages but fully marine stenohaline fishes belonging to the families Myctophidae and Bythitidae are also present. The fossil fish remains are associated with classic ‘Lago-mare’ ostracodes and molluscs. These fossil fishes clearly demonstrate that normal marine conditions were present at least during the upper interval of the ‘Lago-mare’ event, implying that the marine refilling of the Mediterranean was intra-Messinian rather early Pliocene. We argue in favour of the necessity of a new palaeoenvironmental interpretation for the post-evaporitic Messinian of the Mediterranean.

Sequence of tuffs between the KBS tuff and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia

Abstract: This paper describes a sequence of tuffs between the KBS and the Chari Tuff of Omo Group formations in Kenya and Ethiopia. These tuffs have recently been shown to be 1.87 ± 0.02 Ma and 1.38 ± 0.03 Ma in age, respectively. The sequence of tuffs that is derived is consistent with 40Ar/39Ar ages reported separately, and provides the stratigraphic framework for interpreting those ages. Further, new correlations are established to the Konso Formation in southern Ethiopia. As drainage from the Ethiopian Rift to the Omo–Turkana Basin developed after deposition of the Konso Formation, pumice clasts in tuffs of the Omo–Turkana Basin probably were transported there by the Omo River. The tuffs are divided into five groups on the basis of their stratigraphic position in relation to extensive ash layers. The sequence of tuffs has import for the placement and age of archaeological sites in the Koobi Fora Formation, and for ages of mammalian faunas (including hominids). Many tuffs were deposited during a 90 ka interval during which Mediterranean sapropels are lacking, suggesting that Nile flow was reduced, and that the level of a lake that occupied the Omo–Turkana Basin at the time was low. Thus the record of climatic influence on deposition in the Omo–Turkana Basin, previously shown for the Kibish Formation (≤200 ka), extends at least to early Pleistocene time.

Flood lavas on Earth, Io and Mars

Abstract: Flood lavas are major geological features on all the major rocky planetary bodies. They provide important insight into the dynamics and chemistry of the interior of these bodies. On the Earth, they appear to be associated with major and mass extinction events. It is therefore not surprising that there has been significant research on flood lavas in recent years. Initial models suggested eruption durations of days and volumetric fluxes of order 10 7 m 3 s −1 with flows moving as turbulent floods. However, our understanding of how lava flows can be emplaced under an insulating crust was revolutionized by the observations of actively inflating pahoehoe flows in Hawaii. These new ideas led to the hypothesis that flood lavas were emplaced over many years with eruption rates of the order of 10 4 m 3 s −1 . The field evidence indicates that flood lava flows in the Columbia River Basalts, Deccan Traps, Etendeka lavas, and the Kerguelen Plateau were emplaced as inflated pahoehoe sheet flows. This was reinforced by the observation of active lava flows of ≥100 km length on Io being formed as tube-fed flows fed by moderate eruption rates (10 2 –10 3 m 3 s −1 ). More recently it has been found that some flood lavas are also emplaced in a more rapid manner. New high-resolution images from Mars revealed ‘platy–ridged’ flood lava flows, named after the large rafted plates and ridges formed by compression of the flow top. A search for appropriate terrestrial analogues found an excellent example in Iceland: the 1783–1784 Laki Flow Field. The brecciated Laki flow top consists of pieces of pahoehoe, not aa clinker, leading us to call this ‘rubbly pahoehoe’. Similar flows have been found in the Columbia River Basalts and the Kerguelen Plateau. We hypothesize that these flows form with a thick, insulating, but mobile crust, which is disrupted when surges in the erupted flux are too large to maintain the normal pahoehoe mode of emplacement. Flood lavas emplaced in this manner could have (intermittently) reached effusion rates of the order of 10 6 m 3 s −1 .

Sequence stratigraphy and architectural variability in growth fault-bounded basin fills: a review of Plio-Pleistocene stratal units of the Crotone Basin, southern Italy

Abstract: Field examples from the Pliocene to Pleistocene succession of the Crotone Basin document the variability of stratal architecture in syntectonic units deposited in normal fault-bounded basins. Relatively thick trangressive intervals are common within these successions, as shown in the Zinga 2, Zinga 3 and Strongoli stratal units. Transgressive packages are formed by lagoonal mudstones that are abruptly overlain by shoreface sandstones. Aggradational highstand shallow-marine packages are typical of the lower Pliocene succession in half-graben basins. These deposits resulted from balanced conditions between the rate of sediment supply and the rate of creation of accommodation. Relatively thick forced regressive deposits are common within basins during phases where the accommodation–supply ratio is low (e.g. the middle Pleistocene San Mauro Sandstone). These deposits, influenced by growth faulting and folding, recorded base-level lowering linked to glacio-eustasy. A complex stratal architecture may be the consequence of the alternation between tectonic subsidence and uplift (e.g. the Serra Piani stratal unit). Such variations are thought to be related to the onset of an earliest mid-Pliocene transpressional tectonic phase. The studied deposits, therefore, show a marked tectonic control that strongly influenced their stratal architecture. These examples represent interesting cases of sequence-stratigraphic analysis in growth-faulted contexts.

Shale smear along large faults: continuity of smear and the fault seal capacity

Abstract: Data from faults with core recovery offshore Norway and from outcrops in study areas onshore demonstrate the development and continuity of smear along large (seismic-scale) faults. Smear along these faults is typically associated with thick (tens of metres) shale source layers and fault segments that are slightly offset, where the overlap between the segments creates an extensional dip relay. It is demonstrated that rock types other than shale, such as coal, siltstones and carbonates, may smear and thereby contribute to the low- permeable fault gouge. A critical threshold for the shale smear factor (SSF), given by the fault throw divided by the thickness of the shale source layer, is established to separate continuous and discontinuous smears. An SSF ≤4 is likely to correspond to a continuous smear along large faults and thereby to a sealing membrane on the fault surface. For small (subseismic) faults, a continuous smear can be maintained for both shale and coal source layers for much higher smear factors compared with large faults. The continuity of smear associated with small faults also displays a greater variation (SSF in the range of 1–50), to become less predictable than smear along large faults.

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

Abstract: An unusual late Neoproterozoic ( c . 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Interpretation of palaeoweathering features and successive silicifications in the Tertiary regolith of inland Australia

Abstract: Detailed studies of morphological, micromorphological and geochemical characteristics of silcretes in the deep bleached and weathered regolith across a large area of inland Australia have provided a new interpretation of the history of the regolith and its climatic and morphological evolution during the Tertiary. Pedogenic silcretes have distinctive morphological and mineralogical features caused by a succession of phases of silica dissolution and recrystallization resulting from multiple episodes of water infiltration and percolation under alternately wet and dry climates. These are the oldest of the regolith features. Deep, bleached profiles formed over a wide area in a variety of substrates ranging from Precambrian granites to Palaeozoic sandstones, Cretaceous sediments and Tertiary deposits, and represent the second major stage in regolith development. These profiles, in which kaolinite coexists with gypsum, alunite and opal, formed by reaction of the substrates with saline groundwaters, the water-table levels of which progressively fell over the region. Extensive networks of termite burrows constructed to great depth in the bleached regolith followed the water tables down. The climate was warm and dry with a high water deficit. Groundwater silcretes formed near-horizontal lenses and pods of porcellanite and jasper in the bleached regolith. They preserve the primary fabric of the host rock. Groundwater silcretes post-date the construction of termite burrows and were formed during a rise in groundwater tables across the landscape, in places to near-surface environments in broad landscape depressions. The climate was more humid but the presence of gypsum during silicification demonstrates that the groundwaters were still saline. Red–brown hardpans are the youngest silicification features and represent periods of successive infiltration and percolation, and waterlogging, during high rainfall or flood events. They are confined to low regions in the landscape. Mineralogical and geochemical analysis of the bleached profiles, together with geochemical modelling, suggests that ferrolysis is the most likely cause of acidity in groundwater leading to the development of the bleach profiles and/or alunite. Present-day groundwater tables are both at low levels and sulphate-rich. It is possible that acidic alteration leading to bleaching is still active around the extensive playa landscapes in the region.

Early Silurian mafic–ultramafic and granitic plutonism in contemporaneous flysch, Magerøy, northern Norway: U–Pb ages and regional significance

Abstract: The Caledonian Mageroy sequence comprises Earliest Silurian clastic metasedimentary rocks, which underwent folding and regional metamorphism and were intruded by a mafic–ultramafic complex and various granitic plutons. U–Pb ages of 438.2 ± 0.7 Ma for gabbro in the Honningsvag Igneous Complex, 438.4 ± 0.9 Ma for an associated granite and 437.7 ± 1.6 Ma for the Finnvik granite coincide within error with the age of deposition inferred from fossils demonstrating a very rapid geological evolution. Plutons of peraluminous affinity are somewhat younger at 436.0 ± 1.0 Ma (Skarsvag granite) and 434.5 ± 1.5 Ma (a granitic dyke in the Skarsvag Nappe). The association of flysch deposition, folding and mafic–felsic magmatism suggests formation at a trench–ridge intersection. These Early Silurian events are the expression of a period of major magmatism in a variety of settings all along the Caledonides, probably reflecting rapid convergence and subduction during closure of the Iapetus Ocean. Re-evaluation of the palaeomagnetic signature of the Honningsvag Igneous Complex indicates that at the time of formation the suite was located in an equatorial position, probably close to the Laurentian margin, and was subsequently translated southward by some 1350 km prior to and during emplacement onto the Baltic margin.

Zircon thermometry and U–Pb ion-microprobe dating of the gabbros and associated migmatites of the Variscan Toledo Anatectic Complex, Central Iberia

Abstract: In the Central Iberian Zone there are several large thermal domes in which small bodies of ultramafic, mafic and intermediate rocks appear intimately associated with crustal granites and migmatites. The closest spatial association between the ultramafic, mafic and intermediate rocks and migmatites is in the Toledo Anatectic Complex, where field relationships suggest that these rocks are coeval and have an age close to 340 Ma. This, and the recent discovery in the neighbouring Ossa Morena Zone of a large mid-crustal seismic reflector interpreted as a 335–350 Ma mafic sill, reinforce the hypothesis that heat for crustal melting was supplied from early Variscan mantle magmas emplaced in the middle crust. However, precise ion-microprobe U–Pb zircon dating and Ti-in-zircon thermometry in Toledo do not support this idea. Whereas the mean age of four mafic bodies is 307 ± 2 Ma, the migmatites are c . 25 Ma older. The migmatites hosting ultramafic, mafic and intermediate bodies have the same age and Ti-in-zircon temperatures as migmatites far from any mafic intrusion. These data reveal that ultramafic, mafic and intermediate magmas are late Variscan; they were emplaced in already cooling anatectic zones once the extensional collapse was initiated, and their thermal impact on the mid-crustal Variscan anatexis of Central Iberia was negligible.

The mid-Norwegian margin: a discussion of crustal lineaments, mafic intrusions, and remnants of the Caledonian root by 3D density modelling and structural interpretation

Abstract: The high-density lower crustal body (LCB) on the mid-Norwegian margin is almost universally interpreted to represent magmatically underplated material, added to the crust during Early Tertiary opening of the NE Atlantic. The thickness of the LCB is uneven, and its distribution along the margin is sharply limited by margin-perpendicular lineaments. Three-dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments (Bivrost, Jan Mayen, Gleipne, etc.) in the crustal architecture, with and without the LCB. The Bivrost Lineament, separating the Voring and Lofoten margin segments, is clearly expressed and so is arguably a lineament in the SW Voring margin. Notably, most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from Late Caledonian orogenic collapse. An alternative interpretation of the LCB is thus that it represents high-grade metamorphic rocks, remnant from the Caledonian root. Determining the nature of the LCB has profound effects for the volume estimate of magmatic rocks in the North Atlantic Igneous Province, and consequently also for the degree of crustal thinning and heat flow.