| 1. |
- Aftabi, Pedram, et al.
(författare)
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InSAR mapping and modelling of an active Iranian salt extrusion
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:1, s. 155-170
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Tidskriftsartikel (refereegranskat)abstract
- Interferometric synthetic aperture radar (InSAR) imaging is a powerful technique that is increasingly used to extract detailed 3D information on Earth surface structures, including exposed diapiric surfaces. We have used InSAR to map, for the first time, the cumulative surface deformation in a 6 km × 3 6 km region surrounding an active salt diapir (Syahoo) and its associated surface salt flow (or namakier) exposed in the Zagros Mountains of southern Iran. Images provided by the European Space Agency were acquired in 12 increments (ranging in length between 35 and 1248 days) over a 14 year interval between July 1992 and May 2006. The deformation of the salt surface is non-steady, with (extrapolated) rates of displacement varying between surficial uplifts of +1.4 mm day-1 (+511 mm a-1) and subsidence of -2.2 mm day-1 (-803 mm a-1). Growth of a central topographic dome occurs following short wet intervals to create a salt fountain morphology, which then slowly decays during the intervening long dry periods. Salt associated with dynamic 'bulging' of the central dome during wet intervals may flow laterally via gravity spreading into the surrounding salt sheet, resulting in deflation and subsidence of the dome, which is counteracted by growth and inflation of the adjacent namakiers. Salt 'bulges' that migrate down the namakier, resulting in local inflationary and deflationary cycles of the surface, may be regarded as episodic pulses of gravity spreading. Areas of inflation and deflation are also observed to commonly reverse during dry to wet periods, as the overall salt system effectively self regulates as it continually strives for dynamic equilibrium. As long as the source of salt remains undepleted, gravity spreading of the dome ultimately results in more buoyancy-driven salt flowing up the diapiric neck to replenish and feed the extrusion and maintain the gross fountain morphology.
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| 2. |
- Cawood, P. A., et al.
(författare)
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Provenance of the Highland Border Complex : constraints on Laurentian margin accretion in the Scottish Caledonides
- 2012
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 169:5
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Tidskriftsartikel (refereegranskat)abstract
- Contrasting tectonic models for the Highland Border Complex in the Scottish Caledonides view it either as part of the rifted Laurentian margin of the Iapetus Ocean or as an oceanic terrane. Detrital zircon data from sandstones of the complex yield age peaks at 2.8–2.6, 1.3 and 1.1–1.0 Ga and minor peaks at c. 1.7–1.9 Ga. These characteristics compare closely with those of the upper Dalradian Supergroup of the adjacent Grampian terrane, and with the record of eastern Laurentia. The data are also consistent with the Laurentian provenance indicated by palaeontological evidence from the complex, and field evidence for continuity with the Dalradian Supergroup. Detrital ages for the Cambrian Salterella Grit of the Caledonian foreland compare with those for approximately age-equivalent sandstones from the Highland Border Complex. Both were contemporaneous with the regressive Hawke Bay event, accounting for similarities in provenance, and further linking the Highland Border Complex to Laurentia. The Grampian terrane was being uplifted and shedding detritus throughout the Ordovician and Silurian. The absence of this event from the detrital zircon records of either the Midland Valley or Southern Upland terranes suggests that these blocks cannot have been in their current location relative to the Grampian terrane before the end of the Silurian.Supplementary material: The complete analytical dataset and cathodoluminescence images are available at http://www.geolsoc.org.uk/SUP18531.
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| 3. |
- Emeleus, C. Henry, et al.
(författare)
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Lateral versus vertical emplacement in shallow-level intrusions? : The Slieve Gullion Ring-complex revisited
- 2012
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 169:2, s. 157-171
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies on shallow-level arcuate intrusions have identified numerous examples of horizontal mineral fabrics. These are commonly interpreted as reflecting considerable lateral flow during magma emplacement, thus querying established 'semi-vertical' ring-dyke models. We question the recent lateral emplacement model proposed for the Palaeocene Slieve Gullion Ring-complex, NE Ireland, where the absence of steep fabrics in parts of the ring-complex has been used to support a shallow, semi-horizontal sheet intrusion mechanism. We argue that such simple flow models cannot be applied to explosive ring-fissure eruptions and that fabric data alone do not warrant rejection of the ring-dyke model. Moreover, the apparent 'absence of steep intrusive contacts' along the intrusion's perimeter is readdressed and we present numerous examples of outcrops (27) with steep-sided geometries. The Camlough Breccias are reinterpreted as the product of gas-driven tuffisites injected along the active ring-fault (rather than of purely tectonic origin). Crucially, the porphyritic microgranite and porphyritic rhyolite ring-dyke rocks exhibit geochemical and petrographic signatures of contamination by the geographically restricted Palaeozoic Newry granodiorite and are best explained through crustal interaction vertically beneath the ring-complex. Subsequently, these silicic magmas rose into ignimbrite feeders along a caldera ring-fault system that was emplaced into near-surface vent-filling breccias.
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| 4. |
- Merle, Renaud E., 1976-, et al.
(författare)
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Occurrence of inherited supra-subduction zone mantle in the oceanic lithosphere as inferred from mantle xenoliths from Dragon Seamount (southern Tore–Madeira Rise)
- 2012
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 169:3
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Tidskriftsartikel (refereegranskat)abstract
- Spinel-bearing peridotite and pyroxenite xenoliths dredged from the Dragon Seamount (southern Tore–Madeira Rise, West Iberia and Morocco margin) give an insight into the composition of the underlying lithosphere. These xenoliths are devoid of evidence of strong host lava–peridotite interaction and re-equilibration or late impregnation in the plagioclase facies. The spinels and pyroxenes from the Dragon peridotites have compositions distinct from those of both lherzolites and harzburgites from the Iberia margin and the Mid-Atlantic Ridge. They display a highly depleted composition, in particular, high Cr-number, up to 0.63 in the spinels, consistent with a melting degree between 12 and 19%. Because of the strong chemical similarities between the Tore–Madeira Rise, Newfoundland peridotites, and peridotites from supra-subduction zones, we propose that the Dragon peridotites formed in a similar context. The pyroxenites display a cumulate texture and are probably a high-temperature–high-pressure cumulate formed by fractional crystallization from a melt. The Tore–Madeira Rise peridotites may represent a former mantle wedge in an oceanic arc, later included into the continental lithosphere and finally tectonically disseminated within the lithosphere during the rifting of the Newfoundland–Iberia continental lithosphere. As a consequence, rifting processes may produce heterogeneities in the oceanic lithosphere and influence isotopic compositions of ocean island basalt-type lavas during plume–lithosphere interactions, as inferred for the southern Tore–Madeira Rise.Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18512.
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| 5. |
- Pettersson, Carl Henrik, et al.
(författare)
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Detrital zircon U-Pb ages of Silurian-Devonian sediments from NW Svalbard: A fragment of Avalonia and Laurentia?
- 2010
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Ingår i: Journal of the Geological Society (London). - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:5, s. 1019-1032
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Tidskriftsartikel (refereegranskat)abstract
- Detrital zircon populations from Silurian-Devonian clastic rocks of NW Svalbard were analysed by U-Pb laser ablation inductively coupled plasma mass spectrometry to investigate the pre-Caledonian provenance of Svalbard's Northwestern Terrane. Changes in the resulting age spectra suggest a major shift in sources from the Laurentian-Avalonian suture in the latest Silurian to the local metasedimentary basement of the Northwestern Terrane in the Late Silurian-Early Devonian, and in the Lochkovian to Grenvillian-Sveconorwegian sources. These data, together with structural, additional geochronological and metamorphic data from Svalbard, East Greenland and Avalonia, support the amalgamation of Svalbard as the result of long-distance transport along sinistral strike-slip faults. A unifying model for the final amalgamation of Svalbard, consistent with the stratigraphical and tectonothermal history of Svalbard, involves fragments from the Grampian orogen and Avalonian crust originally accreted to the Laurentian margin being subsequently transported northward along sinistral strike-slip faults during Scandian deformation.
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| 6. |
- Pitcairn, Iain K., et al.
(författare)
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Structurally focused fluid flow during orogenesis : the Islay Anticline, SW Highlands, Scotland
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:4, s. 659-674
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Tidskriftsartikel (refereegranskat)abstract
- Displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny. This study investigates whether hydrothermal fluid flow was focused along the Islay Anticline, Islay, SW Highlands of Scotland, as shown in the axial zone of the neighbouring Ardrishaig Anticline. Four localities from the limb to the axial plane of the Islay Anticline were investigated for isotopic homogenization of metacarbonate units to silicate values. At Mull of Oa on the limb of the anticline, metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be <1 m(3) m(-2). Within the axial zone of the Islay Anticline, metacarbonate units from Port a' Chotain and Bagh an Da Dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes. Fluid flow was enhanced along localized parasitic folds such as at Port an t-Sruthain, where metacarbonates have been isotopically reset, and there are abundant quartz-carbonate veins that precipitated during D-1-D-2 deformation. Metamorphic fluid flow was higher in the axial zone of the Islay Anticline and in localized antiformal structures. Fluid fluxes are estimated to be considerably lower than at the neighbouring Ardrishaig Anticline.
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| 7. |
- Ring, Uwe, et al.
(författare)
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Faulting in Banks Peninsula : tectonic setting and structural controls for late Miocene intraplate volcanism, New Zealand
- 2012
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 169:6, s. 773-785
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of faulting in the late Miocene volcanic rocks of Banks Peninsula, South Island, New Zealand, shows that the formation of the volcanic edifice was largely controlled by NE-SW-striking dextral-oblique strike-slip faults. The data show a variable component of west-east-or NW-SE-oriented shortening and north-south or NE-SW extension. Synvolcanic faults reactivated Cretaceous normal faults and are interpreted to have formed a local pull-apart basin that controlled volcanism. Further east, the geometry of Akaroa Harbour is controlled by a north-south-striking oblique reverse fault. Limited fault-slip data collected from sub-recent loess deposits are not significantly different from the data collected in the volcanic rocks and appear to show that the kinematic field did not change significantly over the last c. 10 Ma. The overall kinematic field causing the recent series of earthquakes in the greater Christchurch region is also not fundamentally different from the one that controlled the eruption of the volcanic rocks. We conclude that the inherited Cretaceous faults controlled the development of the late Miocene volcanism on Banks Peninsula and largely provided a major anisotropy along which the recent faults ruptured.
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| 8. |
- Schmitz, Birger, et al.
(författare)
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The middle Darriwilian (Ordovician) delta C-13 excursion (MDICE) discovered in the Yangtze Platform succession in China: implications of its first recorded occurrences outside Baltoscandia
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 2041-479X .- 0016-7649. ; 167:2, s. 249-259
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Tidskriftsartikel (refereegranskat)abstract
- The middle Darriwilian delta C-13 excursion (MDICE), one of the least known of the Ordovician delta C-13 excursions, has previously been recorded only from the Middle Ordovician of Baltoscandia. Analysis of many recently collected limestone samples from the Guniutan Formation at two Yangtze Platform localities show elevated delta C-13 values in the same biostratigraphic interval (Microzarkodina ozarkodella Conodont Subzone) as the MDICE in Baltoscandia, which justifies identification of the Chinese delta C-13 excursion as the MDICE. These occurrences, which are in strata that show striking lithological and conodont faunal similarity to the Swedish Holen Limestone and some coeval units in Estonia, indicate that the MDICE, the stratigraphically oldest of the named Ordovician delta C-13 excursions, is likely to have a world-wide distribution and to have great potential for local and long-range chemostratigraphic correlations.
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| 9. |
- Skelton, Alasdair, et al.
(författare)
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Regional mapping of pre-metamorphic spilitization and associated chemical mobility in greenschist-facies metabasalts of the SW Scottish Highlands
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:5, s. 1049-1061
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Tidskriftsartikel (refereegranskat)abstract
- Both spilitic and non-spilitic metabasaltic sills are hosted by greenschist-facies metasediments in the SW Scottish Highlands. Spilitization is mainly characterized by enrichment in Na2O, elevated modal plagioclase and epidote segregations. Mapping of the spatial distribution of spilitic metabasalts reveals an ancient sub-sea-floor fluid cell centred on the extrusive Tayvallich Volcanics. Fluid circulation was most extensive at shallow levels where most sills were spilitized. We attribute this to pervasive flow of saline fluid, which was thermally driven by the cooling suite of lava flows and sills. Spilitization below this lithostratigraphic depth was restricted to only a few sills. Their spilitization is largely unrelated to specific properties of these sills (e. g. width, chemistry or host lithology). We conclude that fluid channelling was an intrinsic property of sub-sea-floor fluid flow either at deeper levels or earlier during fluid circulation. By profiling of the size distributions of relic phenocrysts in a partly spilitized sill, we conclude that spilitization proceeds with the symmetric propagation of a spilitization front from the sill margins towards the sill interior. Based on chemical profiling across the margin of an epidote segregation, we conclude that spilitization is associated with chemical transport on scales ranging from 0.1 to 10 m.
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| 10. |
- Virtasalo, Joonas J., et al.
(författare)
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Pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern Baltic Sea
- 2010
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:6, s. 1185-1198
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Tidskriftsartikel (refereegranskat)abstract
- Holocene postglacial lacustrine clays in the northern Baltic Sea were studied for ichnofossils,petrography, and microscale elemental and 34S/32S composition to understand the impact of benthos-producedorganic matrices on the early diagenesis of sediments. X-radiographs of these clays display intense burrowmottling with discrete Palaeophycus, ‘Mycellia’ and rare Arenicolites. Small burrow-like and irregularlyshaped concretions were separated from these clays. The burrow-like concretions are composed of framboidalpyrite and poorly crystalline FeS2 cement. They formed within worm-produced burrows by the decompositionof mucous coatings on the burrow walls by sulphate reduction. 34SCDT composition of the framboidsindicates saline inflows from the North Sea as the sulphate source. The irregular concretions are microcrystallinepyritic masses that formed in pore spaces with less reactive organic compounds. Their 34SCDT valuesreflect precipitation from the ambient porewater sulphate by sulphate reduction. The burrow-like and irregularconcretions are microcrystalline barite in the lower parts of the clays. ‘Mycellia’ are mineral-replacedfilaments produced by large sulphur-oxidizing bacteria or mycelial fungi. They concentrate in monosulphidebands in the lower substrate levels, but are associated with organic patches upward. This study demonstratesthat substantial amounts of pyrite formed within organic matrices close to the sediment–water interface. Thisdiffers from previous studies that proposed a deeper-burial origin and allochthonous sulphur source.
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