Preface
Preface
- James D. Floyd, Euan Clarkson, Phil Stone
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- 03 November 2011, p. v
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Preface
- W. E. Stephens, B. Barbarin
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- 03 November 2011, pp. v-vi
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Research Article
Southern Uplands geology: an historical perspective
- Gilbert Kelling
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- 03 November 2011, pp. 323-339
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ABSTRACT
For more than two centuries the rocks of the Southern Uplands have provided successive generations of geologists with both illumination and frustration. This area furnished vital evidence to support Hutton's ideas of petrogenesis and his cyclical view of earth history. However, for much of the succeeding century construction of a credible and consistent stratigraphy and structure eluded the best efforts of the many workers drawn to this region. It was an English-born schoolmaster, Charles Lapworth, inspired by the ideas of foreign practitioners and fascinated by the hitherto-despised graptolite, who applied a technique (zonal stratigraphy) and developed a structural paradigm (isoclinal folding) that provided an elegant and coherent solution to the myriad problems bequeathed by previous attempts.
So persuasive was Lapworth's model that the Geological Survey deployed its two finest mappers, Peach and Horne, to re-examine the entire region, using Lapworth's techniques. The outcome of that 10-year task is the monumental publication commemorated here. That work, and the ideas it espoused, remained virtually unchallenged for more than half a century and contributed to the application and elaboration of then-new geotectonic ideas, such as the geosynclinal concept.
However, in the 1950s the application of new or neglected techniques (way-up criteria, turbidite sedimentology, greywacke petrography, microstructural analysis, etc.) led to drastic reappraisal of prevailing structural and stratigraphic models and introduced a new paradigm (colloquially termed the ‘Southern Uplands paradox’) that envisaged a dominant role for strike-parallel major reverse faults. In contrast to Lapworth's shale-based approach this new model focussed on evidence derived predominantly from the thick intervening greywacke sequences. Investigations led by the Edinburgh and St Andrews schools extended and amplified this new interpretation of Southern Uplands geology, elucidated details of the palaeogeographic setting and the evolution of both the depositional basins and the source areas, and suggested comparisons with other parts of the Caledonian-Appalachian orogen.
The advent of plate tectonics revived interest in the Southern Uplands, first as a candidate subduction-related margin, then in the late 1970s an accretionary prism origin was proposed for the Southern Uplands imbricate thrust stack. The attractions of this hypothesis were manifest and it stimulated renewed activity by academics and the Geological Survey. Significant inconsistencies and perturbations in the simple accretionary prism concept have emerged from these more detailed studies and a range of convergence zone scenarios has been proposed. A Geological Society Meeting on the topic in 1986 furnished much new data and ideas but failed to yield conceptual consensus. Thus, as yet the latest Southern Uplands Controversy remains unresolved.
Changes in the level of geological research activity in the Southern Uplands can be assessed from an analysis of the numbers of relevant publications appearing over five-year intervals. This survey reveals a pattern that broadly accords with the narrative outlined above and supports the concept of two complete and one as-yet incomplete cycles of model development and adoption. Each of these developmental cycles appears to follow the progression of stages in scientific development identified by Thomas Kuhn and common to many scientific disciplines.
Fertility of metapelites and metagraywackes during leucogranite generation: an example from the Black Hills, U.S.A.
- Peter I. Nabelek, Cindy D. Bartlett
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- 03 November 2011, pp. 1-14
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In order to elucidate how mineralogy and composition of crustal sources influences production of leucogranite magmas, we modelled the potential fertility of a sequence of metapelites and metagraywackes from the Black Hills, South Dakota, U.S.A., using a least-squares mixing approach. Rocks analogous to the Black Hills schists were the sources of the Harney Peak leucogranite. Both muscovite and biotite fluid-absent melting reactions (MM and BM, respectively) were investigated. Using the Harney Peak Granite composition as the melt analogue and mineral compositions from the schists for mixing calculations, it is shown that MM of metapelites would lead to highly variable residue mineralogy in the investigated samples. The average residue includes 36 wt.% biotite, 32 wt.% quartz, 12 wt.% plagioclase, 8 wt.% K-feldspar, 9 wt.% sillimanite and 2 wt.% garnet. Melt production ranges from 5% to 23% with an average of 14%. It is limited by the amount of H2O that must be in the melt at the conditions of melting, relative to the amount that is in muscovite in the source rocks. Plagioclase-rich metagraywackes contain little to no muscovite, thus MM cannot occur in them.
Although BM is continuous over a wide temperature range, for the purposes of modelling melting at 975°C and 10kbar was chosen. The temperature is near the terminal stability of biotite, thus the calculations give near-maximum melt production. At this temperature, the mineralogy of the model residues from both metapelites and metagraywackes is dominated by garnet. The potential melt production in the metapelites ranges from 0% to 58% with an average of 32%. It is limited by the availability of plagioclase in the source rocks. Potential melt production in the metagraywackes ranges from 9% to 37% with an average of 23%. At the chosen conditions of melting, melt production is limited by the available K in biotite, although at lower temperatures, the available H2O limits melt production. The total potential melt production (MM + BM) in the metapelites is higher because they have on average a low normative An/Ab ratio (0·14) that approaches the ratio in the leucogranites (0·04). The paragonite component in muscovite significantly contributes to the low ratio in the metapelites. The higher ration (0·27) in the metagraywackes is denned by the feldspar composition.
Using the calculated melt fractions and residue mineralogies, we modelled the concentrations of Rb, Sr and Ba in the melts, as these elements are important indicators of melt-generating processes. The results indicate that both Sr and Ba are likely to be heterogeneous in extracted melt batches and will be depleted in partial melts relative to their pelitic sources, irrespective of whether the melting is fluid-absent or fluid-present.
Micro structural and mineralogical evidence for limited involvement of magma mixing in the petrogenesis of a Hercynian high-K calc-alkaline intrusion: the Kozárovice granodiorite, Central Bohemian Pluton, Czech Republic
- Vojtěch Janoušek, D. R. Bowes, Colin J. R. Braithwaite, Graeme Rogers
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- 03 November 2011, pp. 15-26
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Textural and mineralogical features in the high-K calc-alkaline Kozárovice granodiorite (Hercynian Central Bohemian Pluton, Bohemian Massif) and associated small quartz monzonite masses imply that mixing between acid (granodioritic) and basic (monzonitic/monzogabbroic) magmas was locally petrogenetically significant.
Net veining, with acicular apatite and numerous lath-shaped plagioclase crystals present in the quartz monzonite, and abundant mafic microgranular enclaves (MME) in the granodiorite, indicate that as the monzonitic magma was injected into the granodioritic magma chamber, it rapidly cooled and was partly disintegrated by the melt already present. Evidence from cathodoluminescence suggests that the two magmas exchanged early-formed plagioclase crystals. In the quartz monzonite, granodiorite-derived crystals were overgrown by narrow calcic zones, followed by broad, normally zoned sodic rims. In the granodiorite, plagioclase crystals with calcic cores overgrown by normally zoned sodic rims are interpreted as xenocrysts from the monzonite. After thermal adjustment, crystallisation of the monzonitic magma ceased relatively slowly, forming quartz and K-feldspar oikocrysts.
Although the whole-rock geochemistry of the quartz monzonite and the MME support magma mixing, major- and trace-element based modelling of the host granodiorite has previously indicated an origin dominated by assimilation and fractional crystallisation. Magma mixing therefore seems to represent a local modifying influence rather than the primary petrogenetic process.
The use of graptolites in the stratigraphy of the Southern Uplands: Peach's legacy
- A. W. A. Rushton
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- 03 November 2011, pp. 341-347
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ABSTRACT
Lapworth, at the time of his 1878 paper on the Moffat Series, was the world's foremost expert on graptolites, and in that paper he gave the first great demonstration of the biostratigraphical value of graptolites. Peach & Home's resurvey of the Southern Uplands of Scotland extended Lapworth's ideas and his use of graptolites across the entire region. Peach's graptolitic work for the Survey is discussed: even though he identified a smaller repertoire of graptolites than Lapworth had, and often identified their general horizons rather than exact zone, his results are considered broadly correct. His faunal lists often emphasise the oldest faunas from the Moffat Shale inliers, presumably in order to stress their supposedly anticlinal structure. Subsequent work has seen a great extension of graptolite taxonomy and provided more detailed biostratigraphical subdivision, especially in the Silurian. The model of the Southern Uplands as an imbricate thrust stack is constrained by identifying the youngest (rather than oldest) fauna from the Moffat Shale inliers or, where possible, graptolites from the overlying greywacke formations. Such work has enabled the identification of about 25 thrust tracts in SW Scotland and of out-of-sequence thrusting in the Moniaive and Peebles areas to the NE.
The Southern Uplands Terrane: a stratigraphical review
- James D. Floyd
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- 03 November 2011, pp. 349-362
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ABSTRACT
The Southern Uplands is a major Ordovician-Silurian accretionary terrane which developed as a marine basin over a period of c.75 Ma (495-420 Ma). The terrane extends to c. 10,000 km2 in Scotland alone and correlates with the similar-scale Longford–Down terrane in Ireland. Despite tectonic complexity, a detailed lithostratigraphy has been erected. The oldest strata are mudstones, cherts and lavas of mid-Arenig age known only in the Leadhills Imbricate Zone. The next youngest rocks are of similar lithology but of late Llanvirn-early Caradoc age. These oceanic sediments are succeeded by black shales of the Moffat Shale Group which are, in turn, diachronously overlain by huge volumes of turbiditic sandstones, siltstones, mudstones and minor conglomerates (greywackes) of Caradoc to Wenlock age. Overall, the terrane is sandstone-dominated, with other components such as lavas, volcaniclastics and cherts representing only a tiny proportion of the total volume. The conglomerates have a broadly northerly provenance, whereas the sandstones exhibit both marginal (NW and SE), and axial (NE and SW) derivation. During the Ordovician, strongly contrasting sources alternated through time. The youngest sandstones (Hawick and Riccarton groups), are notably rich in detrital biogenic carbonate, a rare component in the Leadhills Supergroup and Gala Group.
Origin of heterogeneous mafic enclaves by two-stage hybridisation in magma conduits (dykes) below and in granitic magma chambers
- W. J. Collins, S. R. Richards, B. E. Healy, P. I. Ellison
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- 03 November 2011, pp. 27-45
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Field, petrographic and geochemical evidence from the K-feldspar megacrystic Kameruka pluton, Lachlan Fold Belt, southeastern Australia, suggests that complex, multicomponent, mafic microgranular enclaves (MME) are produced by two-stage hybridisation processes. Stage 1 mixing occurs in composite dykes below the pluton, as mafic and silicic melts ascend through shared conduits. Pillows formed in these conduits are homogeneous, fine-to medium-grained stage 1 MME, which typically range from basaltic to granitic compositions that plot as a sublinear array on Harker diagrams. Stage 2 hybridisation occurs in the magma chamber when the composite dykes mix with the resident magma as synplutonic dykes. The stage 2 hybrids also form linear chemical arrays and range from basaltic to granodioritic compositions, the latter resembling the more mafic phases of the pluton. Stage 2 MME are distinguished from stage 1 types by the presence of K-feldspar xenocrysts and a more heterogeneous nature: they commonly contain stage 1 enclaves. Subsequent disaggregation and dispersal of stage 2 hybrid synplutonic dykes within the magma chamber produces a diverse array of multi-component MME.
Field evidence for conduit mixing is consistent with published analogue experimental studies, which show that hybrid thermo-mechanical boundary layers (TMBL) develop between mafic and silicic liquids in conduits. A mechanical mixing model is developed, suggesting that the TMBL expands and interacts with the adjacent contrasting melts during flow, producing an increasing compositional range of hybrids with time that are mafic in the axial zone, grading to felsic in the peripheral zones in the conduit. Declining flow rates in the dyke and cooling of the TMBL zones produce a pillowing sequence progressing from mafic to felsic, which explains the general observation of more MME in more silicic hosts.
The property of granitic magmas to undergo transient brittle failure in seismic regimes allows analogies with fractured solids to be drawn. The fracture network in granitic magmas consists of through-going ‘backbone’ mafic and silicic ± composite dykes, and smaller ‘dangling’ granitic dykes locally generated in the magma chamber. Stage 1 hybrids form in composite backbone dykes and stage 2 hybrids form where they intersect dangling dykes in the magma chamber. With subsequent shear stress recovery, the host magma chamber reverts to a visco-plastic material capable of flow, resulting in disaggregation and dispersal of these complex, hybrid synplutonic dykes, and a vast array of double and multicomponent enclaves potentially develop in the pluton.
Structural interpretations of the Southern Uplands Terrane
- T. Bernard Anderson
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- 03 November 2011, pp. 363-373
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ABSTRACT
Bounded by sutures and demonstrating a unique geological history and structure, the Lower Palaeozoic rocks of the Southern Uplands–Down–Longford form a definitive Caledonian suspect terrane. The geological history of the final closure of Iapetus is encrypted in their structural fabric.
Across the terrane, NW-younging turbidites predominate but graptolites invariably indicate the presence of younger sediments to the SSE. This fundamental Southern Upland paradox is soluble only by recognizing many strike-parallel faults, dividing the terrane into more than thirty tracts, each with its own variant of the stratigraphy and structure, and each having a lateral extent far in excess of what might be expected from the probable mechanical strength of the composing sediments. Structural interpretations of the terrane's unique tectonostratigraphic pattern are critically reviewed and the accretionary prism model, modified by strong sinistral transpression from the late Llandovery onward, is preferred. Transpression was apparently triggered when the converging continents of Laurentia and Avalonia made solid contact, so establishing a mechanically effective coupling of sialic crustal elements beneath and across the closing Iapetus ocean basin.
The geometry of the terrane's internal structural fabric is analysed. Tentative area-balancing calculations indicate a crustal shortening from a basin width of at least 1,000 km to the current terrane width of 75 km. Continuing sinistral transpression was expressed in fault reactivation and the development of a major shear zone. Late Palaeozoic strike-parallel extension produced W-facing half-grabens and the associated rotation may account for the easterly plunge of most fold axes.
Thermal evolution of silicic magma chambers after basalt replenishments
- Takehiro Koyaguchi, Katsuya Kaneko
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- 03 November 2011, pp. 47-60
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In order to understand the governing factors of petrological features of erupted magmas of island-arc or continental volcanoes, thermal fluctuations of subvolcanic silicic magma chambers caused by intermittent basalt replenishments are investigated from the theoretical viewpoint. When basaltic magmas are repeatedly emplaced into continental crust, a long-lived silicic magma chamber may form. A silicic magma chamber within surrounding crust is composed of crystal-melt mixtures with variable melt fractions. We define the region which behaves as a liquid in a mechanical sense (‘liquid part’) and the region which is in the critical state between liquid and solid states (‘mush’) collectively as a magma chamber in this study. Such a magma chamber is surrounded by partially molten solid with lower melt fractions. Erupted magmas are considered to be derived from the liquid part. The size of a silicic magma chamber is determined by the long-term balance between heat supply from basalt and heat loss by conduction, while the temperature and the volume of the liquid part fluctuate in response to individual basalt inputs. Thermal evolution of a silicic magma chamber after each basalt input is divided into two stages. In the first stage, the liquid part rapidly propagates within the magma chamber by melting the silicic mush, and its temperature rises above and decays back to the effective fusion temperature of the crystal-melt mixture on a short timescale. In some cases the liquid part no longer exists. In the second stage, the liquid part ceases to propagate and cools slowly by heat conduction on a much longer timescale. The petrological features of the liquid part, such as the amount of unmelted preexisting crystals, depend on the intensity of individual pulses of the basalt heat source and the degree of fractionation during the first stage, as well as the bulk composition of the silicic magma.
Phase equilibrium constraints on the viscosity of silicic magmas II: implications for mafic–silicic mixing processes
- Bruno Scaillet, Alan Whittington, Caroline Martel, Michel Pichavant, François Holtz
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- 03 November 2011, pp. 61-72
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Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic, dacitic and more mafic compositions have their crystallinities linearly correlated with temperatures. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modelling of felsic magma chambers. Given the typical dyke widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallising intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenisation can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.
Caledonian and related events in Scotland
- B. J. Bluck
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- 03 November 2011, pp. 375-404
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ABSTRACT
The Scottish Caledonides, sited near the triple-junction between Laurentia, Amazonia and Baltica, is divided into at least five discrete blocks, each with a history incompatible with that of the block now lying adjacent to it. With the exception of the Hebridian margin, with its extensional Torridonian basins and Cambrian passive margin sequence, all blocks have undergone terrane-scale movements. The Moine, Central Highland Division and the bulk (if not all) of the Grampian Group have shared a common regional metamorphism, involving thickening and uplift, at c. 800 Ma and c. 450–480 Ma. This is incompatible with their being in the extensional regime that appears to characterise much of Neoproterozoic Laurentia. They, along with the polymetamorphic Dalradian block, now replace a region of passive margin and an unknown width of attendant Iapetus oceanic crust. These metamorphic blocks are grossly out-of-place.
The Midland Valley is a severely contracted block of ancient crust, once fringed by extensive oceanic basins to the N and S. An Ordovician–Devonian arc was founded on this older craton, and supplied sediment to basins on either side of it. This arc, during its Lower Palaeozoic life, matured, finally to yield relatively quartz-rich sediment, but was re-activated during the Devonian. An arc, similar to that of the Midland Valley, also supplied sediment to the Southern Uplands. Metamorphic debris in the Southern Uplands had a provenance in either this arc-basement or in a basement somewhere along the orogen. No metamorphic detritus in the Highland Border Complex has yet been demonstrated to have a Dalradian source.
Much of Scotland was assembled in a strike-slip regime. Evidence for strike-slip tectonics can be seen from the Late Proterozoic through to the Devonian. In periods of transtension, basins opened to accumulate sediment; in periods of transpression, those sediments were compressed and uplifted to yield sediments to successor basins. In the Neoproterozoic, during the phase of transpression, the basins were buried and metamorphosed, but during the Palaeozoic the basins were at a much higher level and escaped metamorphism.
A substantial volume of the Neoproterozoic–Palaeozoic sediment that accumulated in Scotland was derived from two orogens, both of which were sited some distance away. During the Neoproterozoic, the Grenville orogen was the main source, and in later (Devonian) time sediment accumulated in Scotland from the major, Late Palaeozoic continent–continent collision of Greenland–Scandinavia. These two external sources were augmented by a substantial contribution of sediment supplied from the Midland Valley arc or its lateral equivalent and by mild uplifts within the Scottish basements.
Grain-scale and outcrop-scale distribution and movement of melt in a crystallising granite
- E. W. Sawyer
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- 03 November 2011, pp. 73-85
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The distribution of melt has been mapped in a granite pluton deformed whilst it contained melt. At the outcrop scale, leucomonzogranite melt was segregated from hornblende monzogranite when the rigid crystal framework was tectonically compacted. The melt collected in well-defined, structurally controlled sites that formed during dextral, non-coaxial, strike-slip shearing. The segregations are generally isolated, but locally they link to form extensive branched arrays which drained larger volumes of granite in a two-step process. First, melt drained from the compacting matrix through the array and pooled along dilatant foliation planes; later, the melt moved farther away when a single planar melt-transfer channel formed.
Thin section maps show that most melt was distributed in the foliation plane and along the lineation in the crystallising matrix. The location of melt at the grain scale is primarily controlled by the feldspar-dominated shape fabric of the crystal framework, and not by tectonic stresses as at the outcrop scale. Tectonic stresses account for the relatively small proportion of melt films located in grain boundaries normal to the lineation. The distribution of melt-bearing grain boundaries outlines larger domains in the thin sections that form a linked three-dimensional network through which melt moved within the crystallising framework.
A global perspective on the Scottish Caledonides
- Ian W. D. Dalziel
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- 03 November 2011, pp. 405-420
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ABSTRACT
The Scottish Caledonides constitute less than 10% of the length of the Caledonian-Appalachian orogen, the rocks of which define one major margin of the Laurentian craton in Neoproterozoic-Palaeozoic times. Scotland was located, however, in a critical position at the tip of a major cratonic promontory bounded by the Caledonian and Appalachian segments of that margin. Isotopic dates from minerals and rocks collected in the Scottish Highlands have been regarded for 40 years as indicating a Neoproterozoic history of compressive orogenesis that is absent in N America and Greenland. They have therefore been taken by some authors to indicate an origin exotic to Laurentia for rocks of the Northern and Grampian Highlands S and E of the Moine thrust belt. An alternative explanation is that the Neoproterozoic rocks in the Scottish Highlands are all related to the two-stage ‘breakout’ of a discrete rift-bounded Laurentian continent from the core of the Rodinian supercontinent, believed to have assembled at the end of the Mesoproterozoic.
Traditional reconstructions of the late Neoproterozoic–Early Palaeozoic Earth oppose the proto-Caledonian/Appalachian margin of Laurentia and the W African craton of the newly assembled Gondwanaland. However, consideration of the global inventory of late Precambrian rifted margins, their relation to Grenvillian orogenic belts and of scale, leads to the hypothesis that the conjugate was the proto-Andean margin of S America. Recent recognition that the Cambrian and Lower Ordovician strata of the northwestern Argentine Precordillera and their underlying Grenvillian basement are unquestionably of Laurentian derivation, while not definitive, does point in this direction. If correct, this means that even the presence of Neoproterozoic orogenesis need not imply an exotic origin, as Neoproterozoic orogens are widespread in S America.
Traditional models show an Early Ordovician lapetus ocean basin approximately 4500 km wide, but the remarkably synchronous Ordovician collision of arcs and other terranes with the Laurentian and Gondwanan cratons from Argentina to the British Isles, suggests that this premise may be incorrect. The Appalachian–Caledonian orogen may rather have resulted from close and complex tectonic interaction between Laurentia and Gondwana, involving intervening volcanic arcs and other terranes. The interaction may have taken place during a clockwise transit of Laurentia around the proto-Andean margin to its late Caledonian–Scandian collision with Baltica, and the final suturing of Pangaea at the close of the Palaeozoic era. A modern analogue may be the interaction between Australia and Asia, involving intervening volcanic arcs and other terranes of the western Pacific Ocean basin, from ~ 50 Ma through the Present, and into the future.
Dyke widths and ascent rates of silicic magmas on Venus
- Nick Petford
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- 03 November 2011, pp. 87-95
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The ascent of silicic magmas in dykes and diapirs on Venus is investigated using magma transport models for granitic melts on Earth. For fixed planetary thermal and melt properties, differences in critical minimum dyke widths, and hence magma ascent rates, are controlled by gravitational strength alone. For density contrasts of 200–600 kg/m3 and a solidus temperature of 1023 K, minimum critical dyke widths (wc) on Venus range from c. < 1–1200 m for a transport distance of 20 km. Dyke widths are especially sensitive to small changes in the far-field lithospheric temperature at values close to a critical Stefan number (S∝crit) of 0·83 where dyke magma temperatures are equal to the mean surface temperature. Typical magma ascent rates range from 0·02m/s (ηm = 105 Pa s) to 10−9 m/s (ηm = 1017 Pa s) giving transport times of between 12 days and c. 105 years. Dyke ascent velocities for highly viscous melts are compared with diapiric rise of a hot Stokes body of radius comparable with the pancake dome average (c. 12 km), and require dyke widths of the order of 100 times the average width of low viscosity flows to prevent freezing. In both cases, magma flow is characterised by Péclet numbers between 1 and 4, although even at high viscosities (> 1014 Pa s), dyke ascent is still 100 to 1000 times faster than diapiric rise. At a melt viscosity of 1017 Pa s, critical dyke widths are between c. 1% and 5% the diameter of an average width pancake dome on Venus, indicating that even for extreme melt viscosities, domes can easily be fed by dykes. Given the abundance of dome structures and associated surface features related to hyperbasal magmatism, batholithic volumes of silicic rocks may be present on Venus. Intermediate to high silica melts formed by partial melting of the Venusian crust should be compositionally more akin to Na-rich terrestrial adakites and trondhjemites than calc-alkaline dacites or rhyolites.
The Northern Belt 100 years on: a revised model of the Ordovician tracts near Leadhills, Scotland
- R. A. Smith, E. R. Phillips, J. D. Floyd, H. F. Barron, E. A. Pickett
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- 03 November 2011, pp. 421-434
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ABSTRACT
A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone-dominated sequence was deposited in a sand-rich submarine fan environment, overlying sparse hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose-dominated succession was punctuated by the input of fresh volcanic detritus shed from an oceanic/continental island-arc situated to the W/NW of the Northern Belt basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Northern Belt basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands terrane broadly corresponds to uplift and erosion of earlier obducted ophiolite in both Scotland and Ireland, possibly in response to collision of Cambrian–early Ordovician island-arc systems with the Laurentian continental margin. If this interpretation is correct, then the possibility arises that the Southern Uplands–Midland Valley terranes record the dismembering of this oceanic/continental island-arc complex within an overall transpressional regime.
Analogue and numerical modelling of shape fabrics: application to strain and flow determination in magmas
- Laurent Arbaret, Angel Fernandez, Josef Ježek, Benoît Ildefonse, Patrick Launeau, Hervé Diot
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- 07 October 2019, pp. 97-109
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ABSTRACT:
We summarise numerical and analogue models of shape fabrics, and discuss their applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplacement of partially crystallised magmas often employ the analytical and numerical solutions of the Jeffery's model, which describe the movement of noninteracting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas, however, are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mechanical interactions between crystals whose shape distribution is not necessarily homogeneous and constant during emplacement deformation. Experiments carried out in both monoparticle and multiparticle systems show that shape fabrics begin to develop early in the deformation history and evolve according to the theoretical models for low-strain regimes. At large strains and increasing crystal content, the heterogeneous size distribution of natural crystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This effect has been observed in all threedimensional experiments with particles of similar size and for strain regimes of high vorticity. On the other hand, studies of feldspar megacryst sub-fabrics in porphyritic granites suggest that these record a significant part of the strain history. Thus, the fabric ellipsoid for megacrysts evolves closer to the strain ellipsoid than for smaller markers. This behaviour results from the fact that the matrix forms of the melt and smaller crystals behave like a continuous medium relative to the megacrysts. Consequently, in the absence of these markers, and because the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminate. In that case, strain quantification and geometry of the flow requires the addition of external constraints based on other structural approaches.
Silurian subduction-related assembly of fault-defined tracts at the Laurieston Fault, Southern Uplands accretionary terrane, Scotland, U.K.
- M. C. Akhurst, A. A. McMillan, G. S. Kimbell, P. Stone, R. J. Merriman
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- 03 November 2011, pp. 435-446
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ABSTRACT
Subduction-related accretion of fault-defined tracts built up the Southern Uplands terrane during the final stages of closure of the Iapetus Ocean (Llandovery to Wenlock). Contrasts in depositional environment and pronounced differences in geochemical composition, provenance studies and metamorphic grade across the Laurieston Fault between the Gala and Hawick groups, suggests that it has a greater regional significance than most other tract-bounding structures. Initiated by underthrusting, and acting as a locus for subsequent sinistral strike-slip, the fault overlies a regional gravity anomaly gradient that is interpreted to be due, in part, to a concealed NW-ward dipping shallow basement surface. This is modelled as an open ramp in the NE that steepens to a near-vertical step along-strike to the SW. A change in structural geometry noted at the Laurieston Fault, with excision of accretionary tracts, is related to a period of oblique closure of the Iapetus Ocean. The youngest Gala Group tracts were accreted during a period of intense transpression to form a regional strike-slip duplex over the shallow basement ramp with termination of the tracts at the Laurieston Fault, its surface expression. The ramp acted as an obstacle to forward-breaking thrust progress, forcing the out-of-sequence thrusting and repetitive thrust imbrication noted in the eastern Southern Uplands. Upper Palaeozoic reactivation of this basement structure may have transferred strain between extensional Permian basins.
Silurian provenance variation in the Southern Uplands terrane, Scotland, assessed using neodymium isotopes and linked with regional tectonic evolution
- P. Stone, J. A. Evans
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- 03 November 2011, pp. 447-455
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ABSTRACT
The progressive changes in the provenance of Silurian greywacke turbidites in the Southern Uplands terrane reflect geotectonic events at the Laurentian continental margin during closure of the Iapetus Ocean. In the northern Gala Group, juvenile andesitic detritus in some beds gives εNd values no lower than −4·2; more commonly, quartzo-feldspathic greywackes have εNd values in the −5·5 to −6·7 range, produced by the mixing of juvenile plutonic and Proterozoic basement detritus during arc unroofing. In the southern (younger) Gala Group, Proterozoic εNd values range down from −7·7 to −11·2 with only sporadic evidence for a juvenile component. An abrupt change is seen between the Gala Group and its tectonostratigraphical successor, the Hawick Group. In the latter, εNd values have a compact range between −4·7 and −6·6, indicating the renewed dominance of a more juvenile, plutonic provenance. Regional variations in the Sr/Rb ratio suggest that this was more evolved than the source of the Gala Group plutonic material. The Wenlock greywackes of the Riccarton Group have εNd values in the range −5·1 to −7·8, overlapping with the Hawick Group and with coeval greywackes from both the Midland Valley and Lakesman terranes. Overall, the data support proposals that the Iapetus Ocean had effectively closed by mid-Silurian times. Conversely, data from greywacke boulders in the basal Old Red Sandstone conglomerate of the Midland Valley terrane militate against its Wenlock juxtaposition with the Southern Uplands.
Analogue scale models of pluton emplacement during transpression in brittle and ductile crust
- Keith Benn, Francis Odonne, Sharon K. Y. Lee, Ken Darcovich
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- 07 October 2019, pp. 111-121
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ABSTRACT:
Analogue experiments were used to investigate pluton emplacement during transpression in a layered crust. Models consisted of (1) a silicone gum-PbO suspension as analogue magma, (2) a silicone gum-Pb suspension as a basal ductile layer, and (3) an overlying sand pack representing brittle crust. The models were transpressed at 3 mm/hr causing the extrusion of the analogue magma from a progressively closing slot, and its emplacement into the ductile layer. The thicknesses of the layers were critical in controlling the shapes of intrusions and the structures that developed in the brittle overburden. Thicker sand packs led to flattened, symmetrical laccolith-shaped intrusions and the nucleation of one oblique thrust in the sand pack above the extremity of the intrusion. Thinner sand packs led to thicker, asymmetrical laccolith-like intrusions with uplift of the overburden on an oblique thrust, and the formation of a shallow graben in the extrados of a bending fold. Reducing the thickness of the basal ductile layer resulted in a larger number of shear zones in the sand pack, and structural geometries approaching those produced in experiments involving only a brittle analogue crust and no ductile layer. Shear zones in the sand pack were localised by intrusions, and also played a key role in displacing analogue brittle crust to make space for intrusions. The results suggest that tectonic forces may play an important role in displacing blocks of crust during pluton emplacement in transpressional belts. They also suggest that pluton shapes, and the geometries and kinematics of emplacement-related shear zones and faults, may depend on the depth of emplacement. In nature, depending on the structural level exposed in the map plane, faults and shear zones that helped make space for emplacement may not appear to be spatially associated with the pluton.