Regional Dykes Research Articles

Evidence of gas formation and venting in organic soils: experimental evidence and modelling approach

Peatlands have been recognised to provide a natural carbon sink thanks to waterlogged conditions, which keep summertime temperatures relatively low, increase their water holding capacity, decrease the organic soil decomposition rate by creating anoxic conditions and eventually keeping high water table. However, unfavourable environmental conditions due to increasing temperatures and more frequent droughts will reduce water retention of peats and the summertime insulation, in turn increasing their temperature sensitivity and their decomposition rate [1]. As a result, peatlands may start inverting their positive cycle and emitting greenhouse gases, including CO2 and CH4 [2], which suggests better investigating how increasing climate stresses will affect the efficiency of peats in the greenhouse gases cycle and CO2 sequestration.
 Some evidence of gas production from increasing decomposition rate in the Netherlands is coming from continuous pore pressure measurements in saturated layers below the water table, which are monitored to assess the safety of the water defence and the transportation infrastructures. Increasing water pressure in closed piezometers compared to vented ones seem to suggest that gas is produced and capped in the ground, until the breakthrough pressure is reached and the gas vents from cracks opened in the soil matrix. Besides the environmental issues, increasing gas production from decomposition is becoming of concern for the stability of embankments made of organic soils, where the effective stress may be lowered to such an extent to endanger their stability. As a matter of fact, in the last ten years, gas overpressure has been claimed to be a triggering or a contributing factor in few small failures experienced by regional dykes in the Netherlands. In spite of the evidence [e.g. 3] and the risk increasing with heat waves and drought events, the role of gas on the coupled hydromechanical response of organic soils has been seldom investigated nor properly understood yet.
 In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains [4, 5]. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results [6].
 A similar model was used to numerically investigate the relevance of gas production and venting on the response of a regional dyke in the Netherlands, where gas bubbles from venting were observed after excavating - unloading - the toe of the dyke during a stress test. Fully coupled three-phases hydromechanical numerical analyses were performed with CODE_Bright [7] to include gas overpressure. A gas content of 6% in volume was artificially generated in the peat layer, capped by a clay layer on top, and let reaching an equilibrium distribution, which depends on the stress-strain response of the different layers and their volumetric compressibility (Figure 1(a)). Gas venting is triggered by simulating excavation at the toe of the dyke, which allows gas escaping after the capping clay is removed.
 The variation in the operative stress, on which stiffness and strength are assumed to depend [6], is shown in Figure 1(b) over gas generation and venting. In spite of the small amount of gas generated, the predicted overpressure is enough to bring the operative stress to zero in the upper meter of soil at the toe of the embankment due to the light weight of peat and cover soil, which temporarily reduces the factor of safety of the water defence against global stability. As soon as the gas overpressure is released, the operative stress increases above the effective stress which would characterise saturated conditions, bringing the system back to safer conditions.
 These preliminary analyses are supporting an undergoing experimental and numerical thorough effort to better quantify the dynamics of gas generation and venting in organic soils to reduce the hazard associated with increasing climatic stresses.

Effect of uncertainties in geometry, inter-layer boundary and shear strength properties on the probabilistic stability of a 3D embankment slope

ABSTRACT This paper investigates the influence of three forms of uncertainty on the probabilistic stability of an idealised 3D embankment slope. These are: 1D spatial variability in the external geometry of the slope along its length, 2D spatial variability in the depth of the boundary between the embankment material and the foundation layer, and 3D spatial variability in the shear strength properties of the slope and foundation materials. The relative influence of each uncertainty has been investigated using the random finite element method, based on statistics consistent with a Dutch regional dyke. The results indicate that, for such a structure, the soil spatial variability has a much greater influence than uncertainties relating to embankment geometry and inter-layer boundary. In particular, it is demonstrated that the spatial correlation of material properties along the length of the embankment has a greater influence on the probabilistic characteristics of the embankment slope stability and failure consequence than the spatial correlation of properties perpendicular to it. A worst case scale of fluctuation for the material properties is identified.

Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes

Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions.Here, we present new S concentration and isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and δ34S of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province have δ34S above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated δ34S values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones.Comparing the various generations of Gardar magmas, we find that δ34S values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in 34S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.

Mesozoic mafic dykes in the North China Craton: magmatic evolution and implications for gold mineralization

ABSTRACT The North China Craton (NCC), which underwent multiple subduction and lithospheric destruction during the Mesozoic, is well known for hosting some of the world-class gold deposits. In many cases, Mesozoic mafic dykes are associated with the gold deposits, although there has been no systematic studies aimed to integrate the characteristics of these dykes in different regions of the NCC. Here we investigate diabase and lamprophyre dyke suites in the Xiaoqinling region along the southern margin of the NCC, and compile the published geochronological, geochemical and isotopic data for regional mafic dykes and gold deposits from the major gold belts including Jiaodong, Xiaoqinling and central NCC. Zircon U-Pb data show the mafic dykes in Xiaoqinling were emplaced in two episodes at 219–227 Ma and 121–137 Ma, and their εHf(t) values (ranging from −2.1 to 3.8 and −13.6 to 1.6 respectively) suggest increasing input of lithospheric materials in the Early Cretaceous dykes. The compiled geochemical data on the mafic dykes from different regions in the NCC show that the magmatic evolution involved olivine and clinopyroxene fractional crystallization with limited crustal contamination and subduction-related fluid metasomatism. The geochemical and isotopic signature suggests multiple sources for the parent magma of the mafic dykes ranging from convective asthenospheric mantle to ancient lithospheric mantle, with the magma source characterized by enriched mantle (EM2 or EM2+ EM1), and that signatures might be potential for gold mineralization. Further, most dykes are coeval and cospatial with gold deposits in the NCC. The geochronological data show that small amount of both Mesozoic dykes and gold deposits formed in Triassic whereas the peak formed in Cretaceous following slab roll back of the Pacific plate subducting from the east during 145–110 Ma. We envisage that the tectonic extension associated with Pacific slab subduction contributed to the magmatism and gold mineralization in the NCC.

Late Paleoproterozoic mafic-intermediate dykes from the southern margin of the North China Craton: Implication for magma source and Columbia reconstruction

Dyke swarms formed in extensional settings and are important proxies to track mantle source characteristics as well as continent and supercontinent break-up. The late Paleoproterozoic mafic-intermediate dykes in the Xiaoqinling region, southern margin of the North China Craton (NCC), provide a window to investigate the late Paleoproterozoic magma sources and tectonic evolution. Although many studies have been conducted on this region, the timing of late Paleoproterozoic extension and tectonic setting remain debated. Here we present results from LA-ICP-MS zircon U-Pb geochronology, Lu-Hf isotopic analyses and geochemistry of these dykes, together with an evaluation of the published geochronological and geochemical data for the regional dyke activity. The zircon U-Pb data show emplacement ages of 1751–1841 Ma for the dykes and compiled age data indicate widespread late Paleoproterozoic dyke emplacement during 1.85–1.75 Ga, with the majority of formed at 1.84–1.80 Ga. The geochemical characteristics, along with zircon εHf(t) values (showing two distinct ranges: 0.7–7.4 and −5.1–0), indicate multiple magma sources for the 1.84–1.80 Ga dykes from asthenosphere and depleted subcontinental lithosphere sources followed by interaction and mixing with continental crust. Geochemically, the dykes are enriched in Ba, K and P and depleted in Nb, Ta and Sr. The geological and geochemical features of these dykes are consistent with their formation within a post-collisional extension setting. We also evaluate the significance of these dykes in terms of paleogeographic correlations within the Columbia supercontinent through correlations with the North China Craton, Indian Shield and West Australia Craton.

Calibration of a simple 1D model for the hydraulic response of regional dykes in the Netherlands

A simple numerical model was set up to investigate the hydraulic behaviour of a regional dyke to improve understanding of the response under variable atmospheric conditions. The unsaturated hydraulic properties of the dyke body and the relevant foundation layers were calibrated either on the results of laboratory tests or on a national database, namely the Staringreeks, compiled for typical Dutch soils. The boundary conditions were imposed according to the weather history at the top, and to the pore pressures measured in the field at the bottom of the representative soil column. The results indicate that a simple 1D model is able to accurately reproduce the suction time history in the dyke core, provided the hydraulic conductivity and soil water retention properties are properly calibrated. The optimised hydraulic conductivities are typically two orders of magnitude higher than the saturated hydraulic conductivity from the laboratory tests, but comparable to the ones suggested in the database developed on field data. The work highlights that cautious evaluation of laboratory data is needed for field applications, and that direct information from the field should be used to validate numerical models in the presence of organic soils.

Thermal mapping of the 2009 dyke emplacement at Harrat Lunayyir, Saudi Arabia

The 2009 seismic episode at Harrat Lunayyir signalled a renewed geohazard and resulted in a regional dyke that propagated to a very shallow depth (a few hundred metres) below the surface. Since then, there has been an extensive research debate over the potential links between the volcanic/intrusive activity and tectonic processes, particularly because the earthquake swarm and dyke propagation did not eventually result in an eruption. The current study seeks to estimate the relative changes in surface temperatures as a means for detecting an impending dyke-fed eruption or, alternatively, dyke arrest at a shallow depth. An analysis of thermal (infrared) data with a focus on the spatial distribution of land surface temperatures over a longer period of observation may help reveal the link between volcanic activity and dyke propagation. Here, the land surface temperature changes in the centre of Harrat Lunayyir were recorded when the 2009 dyke was propagating toward the surface. The spatial distribution of the land surface temperatures in the area indicated the segmentation of the dyke and suggested the segments were arrested at somewhat different depths below the surface.

Modelling of surface stresses and fracturing during dyke emplacement: Application to the 2009 episode at Harrat Lunayyir, Saudi Arabia

Correct interpretation of surface stresses and deformation or displacement during volcanotectonic episodes is of fundamental importance for hazard assessment and dyke-path forecasting. Here we present new general numerical models on the local stresses induced by arrested dykes. In the models, the crustal segments hosting the dyke vary greatly in mechanical properties, from uniform or non-layered (elastic half-spaces) to highly anisotropic (layers with strong contrast in Young's modulus). The shallow parts of active volcanoes and volcanic zones are normally highly anisotropic and some with open contacts. The numerical results show that, for a given surface deformation, non-layered (half-space) models underestimate the dyke overpressure/thickness needed and overestimate the likely depth to the tip of the dyke. Also, as the mechanical contrast between the layers increases, so does the stress dissipation and associated reduction in surface stresses (and associated fracturing). In the absence of open contacts, the distance between the two dyke-induced tensile and shear stress peaks (and fractures, if any) at the surface is roughly twice the depth to the tip of the dyke. The width of a graben, if it forms, should therefore be roughly twice the depth to the tip of the associated arrested dyke. When applied to the 2009 episode at Harrat Lunayyir, the main results are as follows. The entire 3–7km wide fracture zone/graben formed during the episode is far too wide to have been generated by induced stresses of a single, arrested dyke. The eastern part of the zone/graben may have been generated by the inferred, arrested dyke, but the western zone primarily by regional extensional loading. The dyke tip was arrested at only a few hundred metres below the surface, the estimated thickness of the uppermost part of the dyke being between about 6 and 12m. For the inferred dyke length (strike dimension) of about 14km, this yields a dyke length/thickness ratio between 2400 and 1200, similar to commonly measured ratios of regional dykes in the field.

Restoration of geometry and emplacement mode of the Permian mafic dyke swarms in Keping and its adjacent areas of the Tarim Block, NW China

Abstract The Permian mafic dyke swarms are widely distributed in the Tarim Block, East Tianshan and the western Junggar Block in northwestern China. These mafic dyke swarms, together with coeval basalts, mafic–ultramafic intrusions and A-type granitic plutons, constitute the Permian Tarim large igneous province (LIP), which is considered to be mantle plume-derived. In this study, multi-source high-resolution remote sensing images are used to identify the geometry and emplacement mode of the mafic dykes, sills and flood basalts in Keping and its adjacent areas including Bachu (northwestern Tarim Block), Beishan and Kuluketag (northeastern Tarim Block) and Kelamyi (western Junggar Block). Structural analysis and paleomagnetic results are used to restore the primary geometry of the mafic dykes. The results indicate that the lengths and thicknesses among a total of 117 extracted mafic dykes in Keping follow negative exponent size distributions. The mafic dykes have a mean thickness of 3.8 m with a maximum of 21.4 m. The length of unconnected mafic dyke segments ranges from 0.127 km to 17.1 km, with an arithmetic length of 1.8 km. The restoration of geometry of the mafic dykes indicates that the Permian mafic dyke swarms in Keping may have extended as far as 61–69 km along a primary orientation of about N320°W, and the flood basalts of the Tarim LIP may have extended to Keping so that the areal extent of the Tarim LIP could be enlarged for about 12,000 km 2 compared with that originally reported (about 250,000 km 2 ). The geometric features of the mafic dyke swarms in Keping and its adjacent areas are different from those of the giant radiating dyke swarms due to radial fractures associated with domal uplift. The mafic dyke swarms and sills in the Tarim LIP make up the plumbing system of the mantle plume. Accompanied by the upwelling of mantle plume, many eruptive centers and regional dykes are generated in Beishan, East Tianshan and Tarim basin. The flood basalts in Keping are fissure-type, which may have been fed by magma conduits from the plume center; whereas in Bachu, the central volcanic eruption is dominant.

The Christmas Cove Dyke of coastal Maine, USA, and regional sources for Early Mesozoic flood basalts in northeastern North America

&lt;p&gt; &lt;/p&gt;&lt;p style="margin: 1em 0px; line-height: 200%; text-indent: 0.5in;"&gt;&lt;span style="line-height: 200%; font-family: 'Minion Pro','serif'; font-size: 12pt;"&gt;A large Early Mesozoic quartz tholeiite dyke has been mapped discontinuously for 190 km in southern coastal Maine, USA. Including its type locality at Christmas Cove (South Bristol, Maine, USA), the dyke has features of a generally ENE strike; dip usually steep to the SSE but abruptly turning very shallow in short sections; and widths of 9 to 35 m. &lt;/span&gt;&lt;span style="line-height: 200%; font-family: 'Minion Pro','serif'; font-size: 12pt; mso-ansi-language: EN-US;" lang="EN-US"&gt;The dyke rock has a distinctive cross-columnar field appearance, and a subophitic to micro-porphyritic&lt;/span&gt;&lt;span style="line-height: 200%; font-family: 'Minion Pro','serif'; font-size: 12pt;"&gt; texture with abundant Ti-bearing augite, calcic plagioclase, scattered euhedral orthopyroxene phenocrysts, and coarse-grained glomerophyric clumps of augite with plagioclase. Several &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar whole-rock dates for this and associated regional dykes are close to 201 Ma, in agreement with ages of other Early Mesozoic dykes and basalts of the Central Atlantic Magmatic Province (CAMP). The geographic position, age, whole-rock chemistry, and petrography indicate that the Christmas Cove Dyke is co-magmatic or contiguous with the Higganum-Holden Dyke of southern New England, which was a source for the Talcott Basalt of the Early Mesozoic Hartford rift basin. The dyke system is a 700-km long fissure source for the earliest rift basin basalts preserved in northeastern North America, and it virtually connects the Hartford Basin and the Fundy Basin in Atlantic Canada. The Caraquet Dyke of New Brunswick and central Maine may be co-magmatic with the Buttress Dyke and Holyoke Basalt of southern New England, but lava from it is not preserved in Atlantic Canada or Maine.&lt;/span&gt;&lt;/p&gt;&lt;p&gt; &lt;/p&gt;

Structure of regional dykes and local cone sheets in the Midhyrna-Lysuskard area, Snaefellsnes Peninsula (NW Iceland)

This paper provides the first detailed structural description of 48 vertical dykes, 384 inclined sheets and two large intrusions and the geometry (strike, dip direction and dip) of 1116 fractures in the central area of the Snaefellsnes peninsula, NW Iceland. Our data show a more complex setting than that depicted by the WNW-ESE en-echelon trend of the volcanic structures at the surface. In the Miocene basement lavas, dykes dominantly strike N50–100°E whereas other directions are also present with a higher dispersion. Two main swarms of centrally dipping sheets have also been recognized, focussing towards two areas. Sheet dips range from 2 to 75° with the higher frequency between 10 and 45°. In section view, there is no systematic variation of sheet dip with distance from the focus area. Gabbro and granophyre laccoliths are present in the studied area but cross-cutting relations indicate that most of the inclined sheets are younger. Comparison with regional tectonics suggests that the N50–80°E-striking dykes are coherent with emplacement under the stress field of the pre-6 Ma Snaefellsnes Rift dominated by a NNW-SSE-directed least principal stress (σ3). The N80–100°E dykes and the late Quaternary WNW-trending sub-aerial volcanic features are instead consistent with the development of a more recent E-W, right-lateral shear zone affecting the Snaefellsnes peninsula. Coherent sets of fractures have also been found. Within the inclined sheet swarms, the stress tensor rotated in response to an excess magma pressure linked to two underlying magma chambers of lobate shape, located at an estimated depth of about 400 and 500 m below sea level. This local magmatic stress also produced the centrally inclined fracture swarms that have been found in this area.

An alternative emplacement model for the classic Ardnamurchan cone sheet swarm, NW Scotland, involving lateral magma supply via regional dykes

Sub-volcanic intrusive networks, of which cone sheets are recognised as a major constituent, partially control volcano growth and eruption style. The accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip-parallel magma flow. However, the emplacement mechanism of cone sheets remains poorly understood. The ∼58 Ma cone sheet swarm of Ardnamurchan, NW Scotland, offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets, through studying magma flow, and their importance in volcanic edifice construction. Structural measurements and anisotropy of magnetic susceptibility (AMS) analyses have constrained a lateral magma flow pattern, consistently oriented NW–SE, in the majority of the Ardnamurchan cone sheets. Field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry. Our observations suggest cone sheet formation may be linked to laterally propagating NW–SE trending regional dykes, sourced from laterally adjacent magmatic systems (likely the Palaeogene Mull Central Complex), which are deflected around the central complex by stress field interference. Implicitly, edifice construction and potential eruption precursors observed at a volcano may instigate, or result from, magmatic activity in laterally adjacent volcanic systems.

Explanatory notes to the Geological map of Greenland, 1:500 000, Thule, Sheet 5

These explanatory notes cover part of North-West Greenland defined by latitudes 75°15'N and 78°N and longitudes 57°W and 73°W, a region with appreciable ice cover. The bedrock is dominated by two Precambrian provinces that extend across Baffin Bay into Canada: the highgrade Archaean-Palaeoproterozoic shield overlain by the intracratonic Mesoproterozoic-?Neoproterozoic Thule Basin. Map units are systematically described and introductory sections cover the physical environment, logistics, data sources and geoscientific research.&#x0D; The crystalline shield embraces seven complexes: three of Archaean age, two of Archaean-?Palaeoproterozoic age and two of Palaeoproterozoic age. Isotopic ages of c. 2900 Ma indicate that Neoarchaean orthogneisses are present in part of the region while the existence of Mesoarchaean crust is indicated by c. 3200 Ma detrital zircons. The high-grade orthogneisses and paragneisses of the Thule mixed-gneiss complex were intruded by two plutonic suites, the Kap York meta-igneous complex at c. 2700 Ma and the Smithson Bjerge magmatic association that includes a major anorthosite body. Subsequent deformation, metamorphism and migmatisation led to the formation of gneisses recognised within the Melville Bugt orthogneiss complex . Palaeoproterozoic sedimentation and volcanism represented by the Prudhoe Land supracrustal complex took place after c. 2250 Ma but had ceased by c. 1985 Ma when the Prudhoe Land granulite complex was emplaced. Rocks within the Lauge Koch Kyst supracrustal complex may correlate with the Palaeoproterozoic Karrat Group of West Greenland. Polyphase deformation with isoclinal folding, and regional metamorphism up to granulite-facies grade, affected the region c. 1900 Ma ago, with cooling until c. 1650 Ma.&#x0D; Extensional faulting, intracratonic basin formation and periods of basaltic magmatism occurred during the last 1000 million years of Proterozoic time. After regional dyking at c. 1630 Ma (Melville Bugt dyke swarm) followed by mature peneplanation, the Thule Basin developed as an interior fracture and sag depocentre across the area that is now northernmost Baffin Bay. Defined by the unmetmorphosed Thule Supergroup at least 6 km thick, the basin records fluvial to shallow-marine sedimentation and tholeiitic volcanism at least 1270 million years old. The basin is dissected by the Thule half-graben system dominated by WNW-ESE-trending faults thought to have developed during the final tectono-magmatic period dated at c. 700 650 Ma. Conspicuous products of this are a major sill complex (Steensby Land sill complex) and a regional dyke swarm that parallels the half-grabens (Thule dyke swarm). Fault reactivation is probably related to the late Phanerozoic tectonic evolution of Baffin Bay.&#x0D; In addition to the four metallic commodities included on the map - magnetite, copper, iron suphides, ilmenite - there is potential for gold and other mineralisations. The Neoarchaean magnetite province, traceable for over 400 km through the map region, is spatially the largest in Greenland and it is a correlative of the Mary River iron deposits of Baffin Island, Canada. Several raw materials have potential for local handicraft industries, including soapstone and agate.&#x0D; The region hosts a multi-event glacial and marine Middle-Late Quaternary stratigraphy dating back to at least the Saalian/Illinoian (pre-130 ka B.P.). The entire region was probably overriden by the Inland Ice during the Weichselian/Wisconsinian glacial maximum and deglaciated in the early Holocene, 11 000 to 9000 years ago.

Explanatory notes to the Geological map of Greenland, 1:500 000, Thule, Sheet 5

Abstract Dawes, P.R. 2006: Explanatory notes to the Geological map of Greenland, 1:500 000, Thule, Sheet 5. Geological Survey of Denmark and Greenland Map Series 2, 97 pp. + map&#x0D; These explanatory notes cover part of North-West Greenland defined by latitudes 75°15'N and 78°N and longitudes 57°W and 73°W, a region with appreciable ice cover. The bedrock is dominated by two Precambrian provinces that extend across Baffin Bay into Canada: the highgrade Archaean-Palaeoproterozoic shield overlain by the intracratonic Mesoproterozoic-?Neoproterozoic Thule Basin. Map units are systematically described and introductory sections cover the physical environment, logistics, data sources and geoscientific research.&#x0D; The crystalline shield embraces seven complexes: three of Archaean age, two of Archaean-?Palaeoproterozoic age and two of Palaeoproterozoic age. Isotopic ages of c. 2900 Ma indicate that Neoarchaean orthogneisses are present in part of the region while the existence of Mesoarchaean crust is indicated by c. 3200 Ma detrital zircons. The high-grade orthogneisses and paragneisses of the Thule mixed-gneiss complex were intruded by two plutonic suites, the Kap York meta-igneous complex at c. 2700 Ma and the Smithson Bjerge magmatic association that includes a major anorthosite body. Subsequent deformation, metamorphism and migmatisation led to the formation of gneisses recognised within the Melville Bugt orthogneiss complex . Palaeoproterozoic sedimentation and volcanism represented by the Prudhoe Land supracrustal complex took place after c . 2250 Ma but had ceased by c. 1985 Ma when the Prudhoe Land granulite complex was emplaced. Rocks within the Lauge Koch Kyst supracrustal complex may correlate with the Palaeoproterozoic Karrat Group of West Greenland. Polyphase deformation with isoclinal folding, and regional metamorphism up to granulite-facies grade, affected the region c. 1900 Ma ago, with cooling until c . 1650 Ma.&#x0D; Extensional faulting, intracratonic basin formation and periods of basaltic magmatism occurred during the last 1000 million years of Proterozoic time. After regional dyking at c . 1630 Ma ( Melville Bugt dyke swarm ) followed by mature peneplanation, the Thule Basin developed as an interior fracture and sag depocentre across the area that is now northernmost Baffin Bay. Defined by the unmetmorphosed Thule Supergroup at least 6 km thick, the basin records fluvial to shallow-marine sedimentation and tholeiitic volcanism at least 1270 million years old. The basin is dissected by the Thule half-graben system dominated by WNW-ESE-trending faults thought to have developed during the final tectono-magmatic period dated at c. 700650 Ma. Conspicuous products of this are a major sill complex ( Steensby Land sill complex ) and a regional dyke swarm that parallels the half-grabens ( Thule dyke swarm ). Fault reactivation is probably related to the late Phanerozoic tectonic evolution of Baffin Bay.&#x0D; In addition to the four metallic commodities included on the map - magnetite, copper, iron suphides, ilmenite - there is potential for gold and other mineralisations. The Neoarchaean magnetite province, traceable for over 400 km through the map region, is spatially the largest in Greenland and it is a correlative of the Mary River iron deposits of Baffin Island, Canada. Several raw materials have potential for local handicraft industries, including soapstone and agate.&#x0D; The region hosts a multi-event glacial and marine Middle-Late Quaternary stratigraphy dating back to at least the Saalian/Illinoian (pre-130 ka B.P.). The entire region was probably overriden by the Inland Ice during the Weichselian/Wisconsinian glacial maximum and deglaciated in the early Holocene, 11 000 to 9000 years ago.

Geochemistry of the Early Jurassic Messejana-Plasencia dyke (Portugal-Spain); Implications on the Origin of the Central Atlantic Magmatic Province

The Messejana–Plasencia dyke (MPD) is one of the largest doleritic dykes of the so-called Central Atlantic Magmatic Province (CAMP). Emplaced at ∼200 Ma in the northernmost sector of this province, it crops out for some 530 km from South Portugal to Central Spain with variable width ranging from 5 to 200 m. The bulk-rock composition is that of a typical continental tholeiitic basalt and there is a remarkable geochemical homogeneity along the dyke, with only minor petrographic variations from the margin (microdolerite) to the centre (gabbro). Major and trace element and Sr–Nd–Pb isotopic compositions of the samples from the MPD suggest that they were the result of combined assimilation plus fractional crystallization (AFC). Quantitative modelling shows that the parental magma originated from a source that was 87Sr enriched and 143Nd depleted (87Sr/86Sr ∼0·7050 and 143Nd/144Nd ∼0·51244) relative to Primitive Mantle, which may represent an enriched portion of the mantle–lithosphere. The Pb isotope composition of the MPD is comparable with that of other CAMP dolerites and suggests that the enrichment of the mantle lithosphere occurred during a metasomatic event at ∼1 Ga. Before their emplacement at upper-crustal levels, the primitive magmas crystallized up to ∼30% and assimilated <10% of lower-crustal felsic granulitic rocks. Comparison of the MPD data with those from other parts of the CAMP suggests that their petrogenesis can be linked to the arrival of a Central Atlantic mantle Plume (CAP) at the base of the lithosphere before the onset of continental break-up. The presence of the CAP provides an explanation for several other geological and geochemical features of this province (e.g. the large-scale regional dyke orientations and the presence of magmatic rocks with plume-like signatures), and could have provided the heat source required for inducing melting of enriched, easily melted domains within the overlying refractory mantle lithosphere. However, the scarcity of rocks with plume-like signatures and the results of our petrogenetic modelling suggest that the CAP was only rarely the direct source of the magmas. The explanation for this is that the plume head was probably detached, cooling quickly after impingement and spreading or channelling in the form of a wide, hot sheet following favourable paths of thinned lithosphere. This scenario explains why the CAMP is mainly constituted by dykes whereas lava flows are scarce.

Emplacement and arrest of sheets and dykes in central volcanoes

Sheet intrusions are of two main types: local inclined (cone) sheets and regional dykes. In Iceland, the inclined sheets form dense swarms of (mostly) basaltic, 0.5–1 m thick sheets, dipping either at 20–50° or at 75–90° towards the central volcano to which they belong. The regional dykes are (mostly) basaltic, 4–6 m thick, subvertical, subparallel and form swarms, less dense than those of the sheets but tens of kilometres long, in the parts of the volcanic systems that are outside the central volcanoes. In both types of swarms, the intrusion intensity decreases with altitude in the lava pile. Theoretical models generally indicate very high crack-tip stresses for propagating dykes and sheets. Nevertheless, most of these intrusions become arrested at various crustal depths and never reach the surface to supply magma to volcanic eruptions. Two principal mechanisms are proposed to explain arrest of dykes and sheets. One is the generation of stress barriers, that is, layers with local stresses unfavourable for the intrusion propagation. The other is mechanical anisotropy whereby sheet intrusions become arrested at discontinuities. Stress barriers may develop in several ways. First, analytical solutions for a homogeneous and isotropic crust show that the intensity of the tensile stress associated with a pressured magma chamber falls off rapidly with distance from the chamber. Thus, while dyke and sheet injection in the vicinity of a chamber may be favoured, dyke and sheet arrest is encouraged in layers (stress barriers) at a certain distance from the chamber. Second, boundary-element models for magma chambers in a mechanically layered crust indicate abrupt changes in tensile stresses between layers of contrasting Young’s moduli (stiffnesses). Thus, where soft pyroclastic layers alternate with stiff lava flows, as in many volcanoes, sheet and dyke arrest is encouraged. Abrupt changes in stiffness between layers are commonly associated with weak and partly open contacts and other discontinuities. It follows that stress barriers and discontinuities commonly operate together as mechanisms of dyke and sheet arrest in central volcanoes.

40Ar– 39Ar age of the St. Mary’s Islands volcanics, southern India: record of India–Madagascar break-up on the Indian subcontinent

The felsic volcanics (rhyolites and rhyodacites) of the St. Mary’s Islands (SMI), southern India (∼13°N), were originally interpreted as a distant outlier of the ∼65 Ma Deccan volcanic province of west–central India, comprising dominantly flood basalts. Later the SMI volcanics were dated at ∼93 Ma by the K–Ar technique. However, this K–Ar ‘age’ was dubious, being merely an average of five out of six widely varying dates and arbitrary data selectivity being involved in this averaging. Our first 40Ar– 39Ar dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6±0.9 Ma (2 σ). Interestingly, the southern Indian Precambrian terrain is intruded by numerous mafic–doleritic dyke swarms ranging in age from Proterozoic to the latest Cretaceous (69–65 Ma, Deccan-related), and indeed, two regional dykes (a leucograbbro and a felsite) from the Kerala region of southwestern India remain previously dated at ∼85 Ma, but again with the K–Ar technique. However, this age for the SMI volcanics also corresponds excellently with 40Ar– 39Ar ages of ∼89–85 Ma (weighted mean isochron age 87.6±1.2 Ma, 2 σ: equivalent to 88.1±1.2 Ma corresponding to MMhb-1 age of 523.1±2.6 Ma) for the Madagascar flood basalt province. Together, therefore, the Madagascar flood basalt province, the SMI volcanics, and possibly the Kerala dykes could represent volcanic activity associated with the break-up of Greater India (India plus Seychelles) and Madagascar, thought to have occurred in the Upper Cretaceous at ∼88 Ma.

Infrastructure and mechanics of volcanic systems in Iceland

The divergent plate boundary in Iceland consists of more than two dozen systems where most of the volcano-tectonic activity takes place. At the surface the volcanic systems are characterised by 5–20-km-wide and 40–100 km-long swarms of tension fractures (~10 2 m long), normal faults (~10 3 m long) and volcanic fissures. The Holocene fissure swarms are confined to less than 10,000-year-old basaltic lava flows, mostly pahoehoe, occurring near the centre of the active rift zone. In any particular swarm, the number of tension fractures exceeds that of normal faults. All tension fractures and normal faults are vertical at the surface, indicating that the surface parts were generated by an absolute tension. In addition to a fissure swarm, many volcanic systems have a central volcano some of which have developed collapse calderas. In the late Tertiary and Pleistocene lava pile of Iceland, extinct volcanic systems are represented by local sheet swarms and regional dyke swarms. The sheet swarms are normally circular or slightly elliptical, several kilometres in radius, and are confined to the extinct central volcanoes. Many swarms are associated with large plutons, exposed at 1–2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers, and in short traverses up to 90% of the rock may consist of sheets. The sheets have a very variable strike, dip on average 45–65 °, mostly towards the centre of the associated volcano, and have an average thickness of about 1 m. The regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5–10 km wide. In several-kilometre-long traverses, commonly 1–5% of the rock consist of dykes but occasionally as much as 15–28%. Most regional dykes are subparallel and subvertical. The average dyke thickness in the Pleistocene swarms is less than 2 m but 4–6 m in the Tertiary swarms. While active, the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8–12 km at the boundary between the crust and upper mantle. The reservoirs are partially molten, with totally molten top regions, and of cross-sectional areas similar to those of the volcanic systems that they feed. Some active volcanic systems, especially those that develop central volcanoes, high-temperature areas and calderas, have, in addition to the deep-seated magma reservoirs, shallow crustal magma chambers, located at 1–3 km depth, which, in turn, are fed by the deeper magma reservoirs. It is proposed that the regional dyke swarms are supplied with magma largely from the deep-seated reservoirs, whereas the local sheet swarms are mainly fed from the associated crustal magma chambers. Because the volume of a crustal chamber is less than that of its deeper source reservoir, a single magma flow (dyke intrusion) from the reservoir may trigger tens of magma flows (sheet intrusions) from the chamber, which is one explanation for the enormous number of sheets associated with many central volcanoes. The sheets follow the stress trajectories of the local stress fields around the source chambers, whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes. It is suggested that many dykes develop as self-affine (some as self-similar) structures. When applied to the Krafla volcanic system in northern Iceland, model calculations suggest that a magma flow from the Krafla reservoir, with regional dyke formation, should occur, on average, once every several hundred years, and that tens of sheets might be injected from the shallow Krafla chamber and triggered by a single magma flow from the Krafla reservoir. These results are in broad agreement with the available data.

Brittle deformation and fracture patterns on oceanic rift shoulders: the esja peninsula, SW Iceland

This paper provides detailed structural data on the crustal deformation of the Esja peninsula, at the southwestern margin of the rift zone in SW Iceland. Forty percent of the faults are strike-slip with steep dips, 35% are normal dip-slip with dips ranging from 50° to 90°. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indicate three main states of stress. Two extensional regimes ( σ 3 N180°E ±10° and N110°E ±10°) are related to normal slip faults. The third is a compressive stress regime with σ 1 trending N10°E-N30°E. Associations between strike-slip and dip-slip faults are often observed, resulting from both the geometrical requirements of interactions between faulted blocks and from local-regional permutations between σ 1 and σ 2. We note a constant obliquity of 10–20° between the trend perpendicular to the regional structures and the computed direction of extension. This conclusion is also supported by the major trends of dykes and tensional fractures. There are two mechanisms of volcanic injection (local sheets and regional dykes) resulting in power-law cumulative thickness distributions with exponents of 1.5 and 3.0, respectively.

Effect of local and regional stress fields on sheet emplacement in West Iceland

At the Tertiary Reykjadalur central volcano in West Iceland, there is a remarkable change in strike, dip, thickness and percentage of sheets at a distance of about 9 km from the centre of the volcano. At distances of less than 9 km, the sheets strike in several main directions, their average (arithmetic mean) dip is 45° and the average thickness 1 m. Locally, the sheets make up nearly 90% of the rock, but in 1–1.5-km-long profiles they are 5.8–7.9%. One-third of the sheets are porphyritic, containing as much as 50% of plagioclase phenocrysts. Within, and in the vicinity of, the central volcano there is thus a clear swarm of inclined (cone) sheets. At distances greater than 9 km the sheets suddenly dip much more steeply (average 81°), become thicker (average 3.15 m), strike parallel to the nearby volcanic zones, and have all the characteristics of regional dyke swarms. In 2.5–10.5-km-long profiles these dykes are only 0.83–1.84% of the rock. One-fourth of the dykes are porphyritic, but the percentage of plagioclase phenocrysts is normally less than 10%. We interpret this abrupt change from a local swarm of inclined sheets to swarms of regional dykes as reflecting changes in the stress field. We propose that a local stress field controlled the strike, dip and, partly, thickness of the sheets to a distance of 9 km from the centre of the volcano. This local stress field, which is also thought to have given rise to the collapse caldera associated with the volcano, is attributed to a crustal magma chamber at a shallow depth below the volcano. This crustal magma chamber is regarded as the source of most of the inclined sheets. The regional stress fields associated with the volcanic zones is thought to have controlled dyke emplacement at greater distances from the volcano. We suggest that many of the regional dykes were injected from a deep-seated magma reservoir, located at boundary between the crust and upper mantle.