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The Zaozigou gold deposit lies south of the Xiahe–Hezuo regional fault zone in the western Qinling orogenic belt and contains many intermediate to felsic dikes. Diorite porphyry, quartz diorite porphyry and biotite diorite porphyry are mineral‐bearing dikes, whereas granite porphyry is unrelated to gold mineralization. To compare the relationship between different dikes and mineralization, this study analyzed the cathodoluminescence (CL) of zircon crystals in all four types of dikes exposed in the Zaozigou deposit, using a zircon typological classification. The formation temperature of the granite porphyry (734°C) was higher than the average temperature of the other three types of dikes (704°C), whereas the former's alkalinity index (395) was lower than the average alkalinity index of the other three dikes (425). The ΣREE amount of granite porphyry (147.18 ppm) was smaller than the average ΣREE amount of the other three dikes (246.80 ppm) and its δEu value (0.33) was larger than the average δEu values of the other three dikes (0.30). The U‐Pb ages of zircon in the four types of dikes were relatively consistent with the crystallization ages of approximately 240 Ma, which indicates that all four types of dikes intruded in the Middle Triassic. Compared to the other three ore‐bearing dikes, the morphology of the zircon crystals in the barren dikes evolved from S13 to S5 and then returned to S13. These results indicate that the barren dike underwent an increase in temperature and a decrease in alkalinity, which may be an important reason for the absence of mineralization. [ABSTRACT FROM AUTHOR]

The paper presents detailed geological, petrographic, geochemical, and isotope data on the dolerites from dykes of the Baikal dike subswarm in the central part of the Baikal basement inlier of the Siberian craton. The main geochemical and isotope criteria were identified for classifying the dolerites that are similar in geological–structural position and mineral composition into three geochemical groups. Provisional age estimates of the dolerites are presented. The dolerites of the first and, perhaps, second groups compose Neoproterozoic (715 Ma) dikes. The first group includes medium- to coarse-grained dolerites, which form thick (more than 5–10 m) dikes. The dolerites of this group are characterized by low concentrations of Th (0.6–2.1 ppm) and Nb (3.3–9.2 ppm) and by ɛNd(T) = ‒0.5 to ‒3.9. Considered together, the geochemical and isotope data show that these dolerites could be produced as a result of the melting of a mantle source produced by mixing mantle components close in composition to oceanic plateau basalts and subduction-related subcontinental lithospheric mantle. The second group includes fine-grained dolerites from thin (1–5 m) dikes, including dikes located in contact with the dikes of the first group. The dolerites of the second group are characterized by higher concentrations of Th (3.0–5.3 ppm) and Nb (9.8–21.1 ppm) and by ɛNd(T) = –5.3 to ‒6.0. Geochemical and isotopic data on the dolerites of the second group indicate that continental crustal material was added to the mantle source unified for the dolerites of first and second groups. The third group includes medium-grained dolerites, which make up individual dikes that are contrastingly different in geochemical and isotope characteristics from the Neoproterozoic dolerites of the first and second groups. Dolerites of this group show low concentrations of Th (0.6–1.6 ppm) and Nb (2.7–5.1 ppm) at low values of (Th/La)pm (0.29–0.71) and 143Nd/144Nd (0.511223‒0.511544), which indicates that these dolerites may have been generated as a result of the melting of the subcontinental lithospheric mantle enriched in subduction components. Dolerites of the third group exhibit geochemical characteristics close to those of Paleoproterozoic (1.84 Ga) dikes of the South Siberian post-collisional magmatic belt. [ABSTRACT FROM AUTHOR]

Due to dissolution, weathering and erosion, limestone and marble form karstic landscape with precipitous hills, rugged valleys, caves and sinkholes. In the event of an intrusion, the magma, which resides below the limestone bedrock will ascend toward the earth's surface and fill up the fractures within the pre-existing rocks, forming dikes and sills. Dikes and veins can also be formed when minerals precipitate from hydrothermal fluids within a fracture. The objective of this study was to determine the texture and mineralogy of the dikes cutting the marble in the study area. The findings suggest that the mineral composition and physical structure of the dikes differ from the marble host rock based on its mineral, chemical and physical properties. Three samples were collected and analyzed using Scanning Electron Microscopy (SEM), X-ray diffraction analysis (XRD) and petrographic microscope. The results show that there are at least two types of dike, which are quartz and a coarse-grained granite dike at the study area. The presence of these dikes may affect the chemical and mechanical properties of aggregates produced from the quarry, and in turn affect the excavated rocks of its usage. [ABSTRACT FROM AUTHOR]

The flood protection embankments of Hungary and Europe face numerous challenges. Some dike bases were constructed more than 200 years ago; since then, they have been elevated and extended. Because of these iterative adaptations, the dikes bear many construction errors, which can trigger failures and slides. Due to climate change, droughts and low-water periods of the rivers in central Europe are becoming more frequent. As a result of these effects, the water balance of the dikes can alter and desiccate in the long term. The most staggering fissures appeared on dikes built from clays susceptible to volume change. The General Directorate of Water Management ordered a comprehensive survey of dike pavement cracks in Hungary. This was one of the most extensive surveys of such kind. Hungary has some 4400 km of primary flood protection embankments, out of which 1250 km is paved. There are multiple reasons why the pavement of an embankment can crack. The main features of crack patterns related to clays with shrink-swell potential are identified. The results of international studies and the present survey are synthesised. The main objective of this paper is to draw a correlation between drought (aridity) zones, plasticity index of the soil samples, and crack thickness. [ABSTRACT FROM AUTHOR]

Xiangshan volcanic-related uranium ore field, as the historic largest uranium producer in China, is characterized by a large volcanic-intrusive complex and a number of uranium deposits. In most of the deposits, various types of dikes are spatially associated with the uranium ore bodies, although there have been no systematic attempts to integrate the geochronology and petrogenesis of these dykes and their link with uranium mineralization. In this study, whole-rock major and trace elements, zircon U–Pb dating and zircon Hf–O isotopic analysis were carried out on the Xiagnshan felsic-intermediate dikes, including granitic porphyry, quartz monzonitic porphyry and dioritic porphyry. These felsic-intermediate dikes have the similar geochemical characteristics with the major ore-hosting volcanic rocks, such as enrichment in large lithophile elements (i.e., Rb and Th) and some high-field strength elements (i.e., Zr, Y), but relative depletion in Sr, Ba, P and Ti. These dikes also exhibit zircon εHf(t) values concentrating from -9 to -4 and zircon δ18O values ranging from 7.6 to 10.0 ‰. Geochemical and isotopic data suggest that these dikes were predominantly derived from partial melting of crustal materials and possibly mingled with different proportions of mantle-derived materials during the formation of melts. SIMS and SHRIMP U–Pb dating of zircon grains from these felsic-intermediate dikes yield high-precision ages ranging from 136 to 131 Ma. Combined with previously reported age data indicates a rapid and concentrated dike emplacement event, although a few mafic-felsic dikes were sporadically emplaced at 126 -122 Ma. Such a relatively concentrated dike emplacement in the Early Cretaceous were most likely caused by the back-arc extension or intra-arc rift due to progressive rollback of the Paleo-Pacific Plate. Previous metallogenetic studies supported that the late-stage dikes at Xiangshan were temporally and spatially closely related to uranium mineralization. However, our new geochronological data and field observations indicate that the two episodes of uranium mineralization were not synchronous with the majority of dikes intrusions, indicating that they probably have no direct relationship in origin and that spatial coexistence were more likely due to multistage activities of faults. Rather, these dikes might act as a relatively reduced environment, and thus could facilitate the precipitation of uranium. In addition, all of these dikes can be considered favorable prospecting indicators in the Xiangshan ore field. [ABSTRACT FROM AUTHOR]

The results of petromagnetic and paleomagnetic studies of rocks from mafic dykes of the Sergeevka Terrane that intruded the basement rocks and are widespread in coastal outcrops of the eastern part of Peter the Great Bay of the Sea of Japan between the Anna and Priboinaya bays in the southern part of Primorskii krai are reported. The structural information allows us to conclude that dykes of both the eastern (Srednyaya Bay–Priboinaya Bay) and western (Anna Bay–Cape De Livron) clusters intruded from the igneous feeding channels of the same orientation. Rocks of the dykes were subjected to significant secondary alterations. Inverse magnetic fabric in the dykes is due to high-temperature decomposition and granulation of primary titanomagnetites upon postmagmatic deformations and metamorphism, traces of which are clearly visible in the microscopic and microprobe study of secondary alteration products. This provides evidence for a significant secondary change in rocks of the dykes after their intrusion. This is the secondary heating of the dykes above the temperature of 600°C, i.e., above the Curie point of magnetite (578°C). The nature of the isolated high-temperature NRM component of the dykes is not primary, but secondary, that is, metachronous. The paleomagnetic data constrained the time of the metamorphic event leading to the remagnetization of mafic dykes of the Sergeevka Terrane to about 250 Ma. The calculated paleolatitude of the Sergeevka Terrane at the time of acquisition of the metachronous NRM component by mafic dykes (21.8° ± 4.2° N) is consistent with the paleolatitudes of the northeastern edge of the North China Craton at the Late Permian/Early Triassic boundary. [ABSTRACT FROM AUTHOR]

The paper presents data on dolerite dikes in the Murmansk block in the northeastern Kola Peninsula. The dikes belong to a Middle Paleozoic flood-basalt association, which is a principal element of the large-scale plume–lithospheric process in the area of the Barents Sea. According to their geological, petrological, and geochemical features, the dolerite dikes are classified into two groups: older and younger, which pertain to the initial and final evolution of the large igneous province produced under the effect of the Barents Sea plume at 450–340 Ma. One of the results of the plume–lithosphere interaction was the origin of the Eastern Barents Basin, which is filled with large volumes of basalts, and this predetermined the further subsidence of the area in Meso-Cenozoic time and the accumulation of thick sedimentary sequences with significant hydrocarbon resources. The peripheries of the area with products of basaltic magmatism are marked by alkaline rock complexes. The largest of the latter are the intrusions of the Kola Province and kimberlites in Archangelsk oblast. The dolerites of the dikes abound in xenogenic grains of zircon, which were dated at 2.7, 1.8, 1.3, 0.7 Ga. These age values suggest that the parental melts may have been derived in the lower crust and that the magma-generating processes may have evolved in the root zone of an ancient rift system. [ABSTRACT FROM AUTHOR]

Abstract—Granitoids and dikes of Svyatoy Nos pluton of Cape Svyatoy Nos in the northwestern part of Verkhoyansk-Kolyma folded area, cutting terrigenous and volcanogenic-sedimentary rocks of Upper Jurassic–Lower Cretaceous, are considered. Obtained U‒Pb SIMS geochronological data on zircons from granitoids and dike rocks indicate, that their crystallization ages correspond to Aptian‒Albian (119–111 Ma). Petrography and geochemical affinitites of rocks allow to refer granitoids and felsic dikes to I-type granites. Similarity in chemical composition with coeval granitoids of Great Lyakhovsky Island, the most northern member of Chokhchuro‒Chokurdakhsky submeridional belt of granitoids and Aptian‒Albian granitoids of sublatitudinal belt of Anyui‒Chukotka fold belt is shown. Earlier it was established by many authors, that intrusion of Aptian-Albian granitoids and dikes (117–105 Ma) on the Arctic margin of Chukotka and in the northwestern part of Verkhoyansk‒Kolyma folded area occurred during extension. Aptian‒Albian age, post-shortening character of intrusion and similarity of Svyatoy Nos granitoids in chemical composition with mentioned above Aptian-Albian granitoids allow to refer them to a single stage of postcollisional extension. [ABSTRACT FROM AUTHOR]

The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak–Pechenga–Imandra–Varzuga belt and dikes and sills of the Murmansk and Kola–Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06–1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra–Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra–Varzuga zones intruded at 2010–1970 and 1970–1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra–Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts. [ABSTRACT FROM AUTHOR]

Mixed‐mode fluid‐filled cracks represent a common means of fluid transport within the Earth's crust. They often show complex propagation paths which may be due to interaction with crustal heterogeneities or heterogeneous crustal stress. Previous experimental and numerical studies focus on the interplay between fluid overpressure and external stress but neglect the effect of other crack parameters. In this study, we address the role of crack length on the propagation paths in the presence of an external heterogeneous stress field. We make use of numerical simulations of magmatic dike and hydrofracture propagation, carried out using a two‐dimensional boundary element model, and analogue experiments of air‐filled crack propagation into a transparent gelatin block. We use a 3‐D finite element model to compute the stress field acting within the gelatin block and perform a quantitative comparison between 2‐D numerical simulations and experiments. We show that, given the same ratio between external stress and fluid pressure, longer fluid‐filled cracks are less sensitive to the background stress, and we quantify this effect on fluid‐filled crack paths. Combining the magnitude of the external stress, the fluid pressure, and the crack length, we define a new parameter, which characterizes two end member scenarios for the propagation path of a fluid‐filled fracture. Our results have important implications for volcanological studies which aim to address the problem of complex trajectories of magmatic dikes (i.e., to forecast scenarios of new vents opening at volcanoes) but also have implications for studies that address the growth and propagation of natural and induced hydrofractures. Plain Language Summary: Fluids move within the Earth by means of different mechanisms. One of the most relevant mechanisms, particularly for magma transport within the lithosphere, is the propagation through fluid‐filled fractures: the fluid (or magma) can create its own path through the crustal rocks by fracturing them. If the density of the fluid is lower than the density of the rocks, the fluid would be pushed upward by buoyancy (similarly to a gas bubble in water). However, the propagation path followed by these fluid‐filled fractures may be complex. This may be due to several factors, including the forces (stresses) acting within the crust because of plate tectonic or because of remarkable topographic features. Here we make use of computer simulations and laboratory experiments to test how fluid‐filled fractures interact with such crustal stresses. We quantify how the competition between (i) crustal stresses, (ii) fluid (or magma) pressure, and (iii) the length of a fluid‐filled fracture may affect its direction of propagation. We define a critical range of values for a parameter which may help identifying the path of a fluid‐filled fracture propagating through the Earth crust. Our results may have important implications for volcanological studies which aim to forecast scenarios of new eruption locations. Key Points: We make use of analogue experiments and numerical simulations to study the propagation path of fluid‐filled cracks in interaction with crustal stressesWe show and quantify how the competition between crustal stresses, fluid pressure, and crack length affect the path of fluid‐filled cracksWe provide a critical range of values for a parameter which may help predicting the propagation path of a fluid‐filled crack [ABSTRACT FROM AUTHOR]

Based on geological classification of Iran, Oligocene Lakhshak pluton is situated in the Sistan suture zone that composed of siliciclastic and ophiolitic melanges. The Lakhshak pluton is a host of numerous northeast dikes intruded the siliciclastic sediments. The pluton is granodiorite, granite, diorite, quartz diorite in composition. The dikes occur along shear zones and show moderate to high deformations. Dikes are mainly diorite and granodiorite in composition and consist of plagioclase, biotite, hornblende and quartz. The deformation evidences are quartz microboudines, mica fish (especially in biotite), and fractures in minerals, inflation in quartz, asymmetric porphyroclasts, preferred orientation and rotation of minerals. These evidences show that the dikes suffered from brittle to ductile deformation. Dynamic recrystallization of quartz and feldspar grains is mainly similar as bulge type. Mylonitic structures in the dikes probably formed between 300 and 500°C. Oriented thin sections, three-dimensional block diagram and field studies show dextral shears in dikes. [ABSTRACT FROM AUTHOR]

Numerous inselbergs rise above the regional erosional surface in semiarid northeastern Brazil. The inselbergs are underlain by the Quixadá Pluton and cut by cm-thick granite dike swarms. Five inselbergs with abundant dikes from two different parts of the pluton were selected to investigate the role of dike patterns in the evolution of residual granite topography, including morphological features of hillslopes. The methods included field mapping of landforms, structural measurements of accessible dike arrays, and rock strength determinations using a Schmidt hammer. Unmanned Aerial Vehicle (UAV) photogrammetry was used to build detailed digital surface models of inselbergs and to recognize geometric patterns of dike arrays. In addition, 34 rock samples were collected to determine rock composition and evaluate their resistance to weathering. The results show that >60 % of dikes display a sub-horizontal array composed of low- and medium-angle dikes (up to 45° dip), which tend to increase rock-mass resistance, providing effective barriers against weathering and vertical fracturing, and are associated with the most prominent, steep-sided residual hills. Besides, dike swarm geometry controls the slope outlines of some inselbergs, which commonly conform to dike attitudes. By contrast, steeply-dipping dikes (>60° dip) promote weathering along the dike-host rock interface and are hypothesized to underlie the plains between the inselbergs. At a small scale, dikes control the occurrence and evolution of minor rock landforms such as weathering pans, solution runnels, overhangs, and split boulders. Our findings show that the resistance of inselbergs to erosion is not homogeneous in the granitic pluton but is controlled by local factors such as dike occurrence and dip. [Display omitted] • Inselbergs are primarily cut by low-angle felsic dike swarms. • Structural control exerted by dike swarm geometry influences erosion pathways. • Sub-horizontal dikes create structural anisotropy halting erosion. • Vertical dikes favor the propagation of fractures and break-up of rock mass. • Low outcrops and solution runnels are often associated with high-angle dikes. [ABSTRACT FROM AUTHOR]

This report is brief in context and rich in unexpected discovery. With > 2 km of erosion, kimberlite models predict the near-complete removal of pipes with exposures to the pipe-root-zones of dikes. Exploration in NW Liberia has, indeed, uncovered eight kimberlite dikes (~ 10 m wide) but also an en echelon pipe, comparable in size to the Kimberley pipe and De Beers' pipe in South Africa. Discoveries are in a narrow 200–300 m wide valley of extraordinary thick bush, undergrowth, and organic overburden. Ilmenite and co-existing leucoxene were used as diagnostic tracers for detecting hard rock kimberlite in this tropical terrane. Micro-diamonds show that the redox state of ilmenite is a potentially useful proxy as an index for macro-diamond preservation. The tectonic control of kimberlites is complex, with diverse lithologies. Discoveries include a well-defined regional trend for kimberlite dikes along paleo-fracture zones, Precambrian in age (Liberia Trend), coupled with kimberlite dikes on the craton that are traced to Mesozoic oceanic transform faults (the Sierra Leone Trend). Although long predicted, this is the first report of kimberlite dike-trends in Liberia that are similar in orientation to those in Sierra Leone. An explosive blow on a Liberia-Trend dike demonstrates a similarity to the dynamics attendant in rich (50–500 cpht) diamond-bearing dikes in Sierra Leone, and in South Africa of comparable age. The potentially high grade dikes, along with the pipe (~ 500 × 50 m), now more reasonably accounts for the enormous number of alluvial diamonds (blood and non-conflict), recovered over more than seven decades, downstream from the discovery cluster. A neglected region since the classic work by Bardet (1974), and with few contributions on Liberia since then, an update is considered timely, particularly in the context of discoveries of diamond-bearing kimberlite. [ABSTRACT FROM AUTHOR]

We have studied the processes of dike-induced graben nucleation through the analytical modeling of the failure sequence, i.e., the timing, location, and mode, of the discontinuities formed between the dike's upper tip and the surface. To this end we use as input parameters dike geometry and the elastic and frictional properties of a homogeneous medium. We applied this analytical model to three dike-induced graben in the Elysium region of Mars; Galaxias, Elysium, and Cerberus Fossae, some of which have been found to show active extensional tectonics. Firstly, we calculated dike aperture and depth from graben topography using an area balance technique. At each site we modeled with different combinations of input parameters which resulted in a total 27 models, of which 14 were found to be compatible with the inferred present-day graben-bounding faults. Based on these results we propose two conceptual models of dike-induced graben nucleation: 1) in a shallow dike intruding a low-compliance host rock, nucleation occurs through the linkage of near-surface mode I cracks and mode II discontinuities propagated from the dike tip, and 2) in a deep dike intruding a stiff host rock, shallow mode I cracks propagate to depth and collapse into normal faults in a dominantly tensile regime. A comparison of equivalent models under Martian versus Terrestrial conditions shows that in the former case failure is more likely to occur in a tension-dominated regime due to the greater weight of the dike induced stresses in a medium with a reduced lithostatic load. On Mars, dense networks of open fractures may have facilitated large fissure eruptions. On Earth, tensile failure occurs at very shallow depths whilst faults accommodate most of the deformation at depth. Our analytical modeling methodology explains intrusion experiments and observed dike-induced deformation. Therefore, the models provide plausible mechanisms of graben nucleation above intruding dikes. These need to be complemented by models which simulate long-term graben subsidence above dikes, which may be key to understand seismicity in tectonomagmatically active regions of Mars. • Dike aperture, depth, and host-rock properties control the shape of dike-induced faults. • Shallow, narrow dikes in a weak host nucleate graben through both joints and faults. • Deep, wide dikes in a strong host nucleate graben mostly through tensile cracks. • Martian conditions favor a tensile regime and the formation of highly dipping faults. • Terrestrial conditions favor a compressive regime and the formation of faults with smaller dips. [ABSTRACT FROM AUTHOR]

Hydrothermal activity along the Mid-Atlantic Ridge is predominantly high-temperature venting controlled by volcano-tectonic processes confined to the ridge axis and neotectonic zone, which extends ~ 20 km on each side of the axis (e.g. TAG or Logatchev 1). These vents cannot, however, account for all the heat which needs to be removed to cool the plate and a significant amount of heat is probably removed in the off-axis regions as well. These regions have previously not been systematically surveyed for hydrothermal activity due to a lack of predictive models for its nature, location or controlling structures. Here we use hot springs in the Tertiary Westfjords of Iceland as onshore analogs for hydrothermal activity along the off-axis Mid-Atlantic Ridge to better understand tectonic and volcanological controls on their occurrence, as well as the processes which support hydrothermal circulation. Our results show that even crust ≥ 10 Ma has abundant low-temperature hydrothermal activity. We show that 66% of hot springs investigated, and 100% of those for which a detailed geological setting could be determined, are associated with basaltic dikes cross-cutting the sub-horizontal lava sequence. This is in strong contrast to on-axis springs, which are known (both from underwater and on land) to be predominantly associated with faults. Absence of earthquakes in Westfjords suggests that the faults there are no longer active and possibly sealed by secondary minerals, suppressing fluid circulation. In such a situation, the jointed and fractures dike margins may provide the major pathways for fluid circulation. Extrapolating this idea to the off–axis regions of the Reykjanes Ridge, we suggest, based on bathymetric maps, potential sites for future exploration for off-axis hydrothermal systems. [ABSTRACT FROM AUTHOR]

Tharsis is the largest volcanic province on Mars and in the solar system. This region includes major volcanoes (Olympus Mons, Alba Mons, Arsia Mons, Pavonis Mons, and Ascraeus Montes) and hundreds of small volcanic cones and vents, whose origin is not yet fully understood. Although the main Tharsis' edifices plumbing system has been extensively studied, smaller volcanoes' origin remains unknown. The formation of those minor volcanic landforms may be related to the large volcanic edifices' evolution, and/or controlled by the fault systems through which magma migrates towards the Martian surface. In this study, we analyzed the central part of the Tharsis volcanic province using satellite images with a resolution of ~6 m/px from the Context Camera onboard the Mars Reconnaissance Orbiter (CTX/MRO). We identified and mapped 659 volcanic edifices >1 km in diameter. We analyzed (1) their spatial distribution, (2) alignment of summit craters, and (3) surface model ages derived from crater counting. We found that volcanic edifices are unevenly distributed across the Tharsis province and proposed that their formation is controlled by at least six individual magma-plumbing systems associated with major edifices: Olympus Mons, Alba Mons, Arsia Mons, Pavonis Mons, Ascraeus Mons, and Uranius Mons. Their summit alignment orientations indicate they were controlled by radial and circumferential dikes originated from magma sources beneath the six central volcanoes, either magma chambers in the crust or other magmatic underplates at the base of the crust. Volcano flanks of distributed volcanoes are of similar age or younger than the summit calderas of the associated central volcano indicating a common magmatic system. Magma migration and eruptions from distributed volcanoes may thus extend beyond the magma waxing periods when magma supply was high enough to sustain summit eruptions. The relatively young age of documented volcanic activity within the Tharsis province may imply recently active hydrothermal systems triggered by magma-water interaction. • Tharsis on Mars hosts at least 659 distributed volcanoes. • Our crater count dating shows most volcanoes are < 200 Ma. • The 659 volcanic edifices were classified into 6 subprovinces of central volcanoes. • They are supplied by systems of radial and circumferential dikes. • The dikes are controlled by central volcanoes' magmatic systems. [ABSTRACT FROM AUTHOR]

For the first time for the region, the composition of the composite dikes is studied on the example of the Takalkan ore-magmatic cluster in the Polousny Range. Mineralogical, petrochemical, and geochemical study shows that the dikes were formed by the mechanical and chemical interaction of subsimultaneous mantle trachybasaltic and crustal granitic melts during their emplacement, with the formation of a transitional monzonite (trachyandesite) zone. [ABSTRACT FROM AUTHOR]

Abstract: Petrologic and geochemical features enabled us to constrain the petrogenesis of dikes spatially associated with the Golden Revenue (GR) Au–Cu and the Stoddart Mo–Cu±W mineralization, in the Mountain Freegold, Yukon Territory, Canada. Both dike sets are aplitic and porphyritic and characterized by broken and embayed feldspar and quartz phenocrysts/phenoclasts set in a fine-grained groundmass. In both GR and Stoddart mineralized zones, aplitic dikes are typically related to the lower T magma relative to the porphyritic dikes, as indicated by zircon and apatite saturation temperatures. GR dikes are essentially dacitic to rhyolitic in compositions, whereas dikes from Stoddart display a wide compositional range spanning from basaltic–andesitic to rhyolitic compositions. GR and Stoddart dike suites are classified as high-K calc–alkaline units and exhibit adakitic affinities with their depletion in HREE+Y relative to LREE and subsequently high Sr/Y and La/Yb ratios. Dikes associated with the Stoddart Mo–Cu±W mineral occurrence are oxidized (as suggested by the elevated XMg of biotite and hornblende, and magnetic susceptibility data) relative to dikes associated with Au–Cu mineralization, and the former were fed by a high-level magma chamber, whereas the latter are associated with a deep-seated magma chamber. 87Sr/86Sri values from GR and Stoddart dikes are near equal (0.7052–0.7083 and 0.7054–0.7076, respectively), and the εNdi of both dike suites is equal as well (−0.1). While Pb isotope compositions of GR dikes are heterogeneous and span a wide composition range (206Pb/204Pbi: 18.823–19.577; 207Pb/204Pbi: 15.593–15.681, and 208Pb/204Pbi: 38.539–39.017), by contrast dikes from Stoddart yielded relatively homogeneous Pb isotope compositions (206Pb/204Pbi: 19.103–19.389; 207Pb/204Pbi: 15.606–15.667, and 208Pb/204Pbi: 38.672 to 38.99). The difference in fO2 and in isotope compositions, as well as in the depth of magma emplacement between the two dike sets indicates magma derived from different sources, and it is suggested that those differences further controlled the variation in styles of mineralization. The preservation of petrographic features, such as miarolitic cavities, quartz phenocrysts and phenoclasts, and the presence of intermineral porphyry dikes suggest a close temporal relationship between dike emplacement and the hydrothermal activity, thus consistent with the magmatic–hydrothermal transition. [Copyright &y& Elsevier]

Abstract: The island of São Vicente has undergone continuous volcanic activity from Pliocene to Pleistocene times. The earliest evidence of activity corresponds to some 9million years ago when a submarine edifice of alkaline basaltic affinity had already started to develop. The island resulted from the growth of a single major edifice (the São Vicente Edifice) built up in several distinguishable growth stages. The early main stage occurred at 6.5 to 4.5Ma giving rise to an edifice of about 10–12km in radius and more than 2500m in height whose centre was located south of the present city of Mindelo. Although the edifice in its earlier phase showed typical characteristics of a shield volcano (the Mindelo Formation) it adopted later on those of an ordinary composite volcano (the Madeiral–Monte Cara Formation). A giant landslide event, the Praia Grande landslide, destroyed the NE sector of the edifice and left a 10×12km large depression that was successively refilled by nephelinitic lava flows and some associated carbonatites (the Monte Verde Formation, 4.5–3.1Ma). The volcanic activity ceased about 3–2Ma ago when the edifice started to be deeply eroded until disappearing about 90% of its total volume. Only in recent times (0.3Ma) very scarce and local strombolian activity has been developed aside the São Vicente Edifice, in the eastern sector of the island. [ABSTRACT FROM AUTHOR]

Abstract: The Midcontinent Rift (MCR) of North America comprises a series of basaltic sheets, flows and intrusive rocks emplaced in the Lake Superior region during the Mesoproterozoic. The mafic rocks preserved on the northern flank of Lake Superior represent the older portions of the rift sequence and offer insights into the early development of the rift. New geochronological, geochemical and paleomagnetic data are presented for the dikes and sills located in and south of Thunder Bay, Ontario. Three sill suites are recognized within the study area; an earlier, spatially restricted ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the north of the study area. In addition three dike sets are recognized, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt. Mollie dike. The geochemical data demonstrate that the majority of sills south of Thunder Bay are of Logan affinity and distinct from those of broadly similar age in the Nipigon Embayment to the north. The Pigeon River dikes that intrude the sills are geochemically coherent but distinct from the Logan sills and could not be feeders to the sills. The new age of 1109.2±4.2Ma for the Cloud River dike and its R polarity are consistent with published magnetostratigraphy. The Mt. Mollie dike age (1109.3±6.3Ma) indicates that it is not coeval with the spatially associated Crystal Lake gabbro as previously thought. The complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south. The geochemistry and geochronology of the intrusions suggest a long-lived and complex magmatic history for the Midcontinent Rift. [ABSTRACT FROM AUTHOR]

Abstract: East–west extension has been a prominent feature of Cenozoic tectonics in central-southern Tibet, and the onset of this extension has been interpreted as indicating the surface uplift of the Tibetan Plateau to a critical level. Previous studies suggested that extension started in the Neogene between 18–13Ma and 8Ma, based on dating results from north–south trending normal faults, grabens or rifts, and dikes. We report here the discovery of widespread north–south trending diabase and andesitic porphyry dikes in and around the Shuang Hu graben, central Tibet, where crustal extension has previously been argued to start from 13.5Ma based on mineral ages from graben-bounding faults. Our results show that dikes in this area were mainly formed in the Eocene (47–38Ma). Geochemical and Nd–Sr isotopic data suggest that the diabase dikes originated from partial melting of an enriched lithospheric mantle source metasomatized by melts from subducted continental crust, and the andesitic porphyry dikes were probably derived from interactions between the subducted continental crust-derived melts and the mantle. Given that north–south trending Eocene dikes are found widely dispersed in central Tibet, we argue that the onset of east–west extension, and likely regional uplift of the Tibetan Plateau, began much earlier than previously thought. Our study implies that a large part of the Tibetan Plateau had already attained near-maximum elevation in the Eocene and is consistent with recent investigations regarding the Middle–Late Eocene (47–34Ma) initiation of aridification in Asia, evidence from contemporary global oceanic O–Sr isotope curves and temperatures, and the carbon cycle. [ABSTRACT FROM AUTHOR]

Abstract: The anisotropy of magnetic susceptibility (AMS), coupled with mineral magnetic identification, has been investigated in about 640 samples from 27 basaltic dikes of Tertiary age in Eastern Iceland. The dikes are a few meters thick and they have been sampled all across. Three types of magnetic fabrics with well-defined mean principal axes of anisotropy are recognized. In the first one the maximum axis is vertical and the minimum axis coincides with the pole of the dike plan (“geometrically normal” fabric). The second fabric shows opposite orientation (“geometrically inverse” fabric). The third type is when these two fabrics coexist with the “normal” one at the margins and the “inverse” one in the center of the dike (“mixed” fabric). No relationship is observed between the type of fabric and the width of the dikes. Each fabric is carried by clear magnetic assemblages made of low (x ∼0.15) Ti-content fine-grained magnetites for the “normal” one and high Ti-content (around TM60) coarse magnetite grains for the “inverse” fabric. Upon heating, the magnetic properties and the orientation of the anisotropy ellipsoid characterizing the “inverse” fabric in the mixed dikes change and evolve toward the “normal” type. These two geometrically “normal” and “inverse” fabrics are interpreted as primary ones, representing different physico-chemical conditions during the emplacement of the magma. The AMS results indicate that, during the volcanic events occurring about 10Myr ago, the extrusion of the magma was vertical at the depth of the studied sites in eastern Iceland. [Copyright &y& Elsevier]

Abstract: From geophysics, two thin and narrow sill-like melt lenses occur directly beneath the ridge axis on the East Pacific Rise (EPR) near 9°N. One is at the base of sheeted dikes, and the other at the base of the crust in the mantle transition with largely crystalline material in between. Along-axis continuity indicates that the upper one is a steady-state feature along much of the EPR. What causes melt to pool at two levels in the crust, with little melt in between? Normative densities of gabbro sampled from Hess Deep, eastern Pacific, at solidus temperatures are greater than melt densities calculated from compositions of basalt glass from the EPR. Therefore, abyssal-tholeiite liquids of all compositions are buoyant in a hot gabbroic crystalline matrix with no more than a few percent melt. There is therefore no level in the gabbroic layer at which pure basaltic melt attains neutral buoyancy, and by this means produces sill-like rather than dike-like intrusives. Instead, the upper melt lens appears to collect at a combined compositional and permeability barrier, defined by the coarse-grained base of the dike sequence sampled on the northern rift margin at Hess Deep. About 200m of massive dike rocks with ophitic to subophitic coarse-grained texture occur just above gabbronorite and oxide–gabbro cumulates which are the crystallized relics of highly differentiated ferrobasaltic and ferroandesitic liquids in the melt lens. At temperatures of crystallization of cumulates at the level of the melt lens (1100–1000°C), the basal dike rocks of less differentiated basaltic composition just above this were 90–98% crystalline, and they annealed to the point where dike contacts cannot be seen on outcrop surfaces. These slightly molten rocks need to be fractured by rift dilatancy or overpressure from beneath for magmas to ascend beyond the melt lens. Neutral buoyancy occurs in fine-grained dikes above where densities of escaping magma finally match those of the fractured and porous formation and this is the level that can be laterally intruded. The base of the crust, on the other hand, presents an abrupt reduction in the density contrast between ascending melt and surrounding rock, and a higher concentration of melt pockets and channels. Neutral buoyancy can be attained where primitive gabbros are first encountered if the ascending magmas are picritic, laden with olivine phenocrysts. High olivine concentrations in the narrow bodies of moving magma can be attained by a combination of sinking, relative plagioclase flotation, and flow differentiation. Sill-like intrusions that crystallize to dunite and plagioclase–peridotite cumulates will result. Such rocks are found but their structural orientation is unknown at the base of the intra-rift ridge at Hess Deep; sill-like analogs are found in some ophiolites. Lateral leakage of deep melt lenses into transform faults might account for both primitive picritic tholeiites and strongly plagioclase-phyric basalts found, for example, at Siqueiros Fracture Zone at 8°30′N along the EPR. [Copyright &y& Elsevier]

Abstract: Images from MESSENGER''s first flyby of Mercury have shown convincing evidence for surface volcanism. Here we report on evidence in the new data for several features that are characterized by fractures and graben — rare features on a planet dominated by contractional deformation — that may be linked to intrusive activity. These features include: (1) A floor-fractured crater, interpreted to have been the site of laccolith-like sill intrusions; the feature is similar to some floor-fractured craters on the Moon and shows evidence for individual fractured dome-like uplifts on the floor. (2) A concentric complex of graben, observed inside the peak ring on the floor of the ~250-km-diameter Raditladi basin and associated with dark plains and possibly embayed by them; the feature may represent an unusual type of floor-fracturing associated with deeper intrusions and related ring dikes or cone sheets, or the graben may instead be the product of non-magmatic uplift of the basin floor. (3) A large radial graben swarm, Pantheon Fossae, located near the center of the Caloris basin, thus far unique on Mercury, and characterized by hundreds of individual graben segments ranging from ~5 km to ~110 km in length. In the nexus, graben crosscut one another and produce a local polygonal pattern; others curve away from the center as the nexus is approached. Two scales of graben length are observed; the radius of the dense radially symmetric plexus of graben is ~175 km, and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor. Two width scales of graben are observed; a large graben about 8 km wide emerges from the nexus and extends for ~100 km; most graben are less than half this width. Some graben walls appear cuspate, with convex-outward wall segments that resemble crater chain segments. One crater chain with distinctive raised rims parallels nearby graben. Locally, some graben appear in en echelon patterns, and smaller graben sometimes show cross-cutting (superposition) relationships. Abundant impact craters, the most prominent being Apollodorus, and secondary crater clusters and chains are superposed on the graben system; there is little evidence that craters greater than 5 km in diameter have been cut by a graben. This relation implies that the graben swarm formed soon after the emplacement of the Caloris floor plains. These graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir. Similar features, in which the dikes contribute to a near-surface stress field that favors radial graben, are known on the Earth, Venus, and Mars. The location of Pantheon Fossae in the center of the Caloris basin suggests that formation of the radial graben structure is linked to basin evolution. [Copyright &y& Elsevier]

Abstract: Eruptions are fed by dikes; therefore, better knowledge of dike propagation is necessary to improve our understanding of how magma is transferred and extruded at volcanoes. This study presents an overview of dike patterns and the factors controlling dike propagation within volcanic edifices. Largely based on published data, three main types of dikes (regional, circumferential and radial) are illustrated and discussed. Dike pattern data from 25 volcanic edifices in different settings are compared to derive semi-quantitative relationships between the topography (relief, shape, height, and presence of sector collapses) of the volcano, tectonic setting (presence of a regional stress field), and mean composition (SiO2 content). The overview demonstrates how dike propagation in a volcano is not a random process; rather, it depends from the following factors (listed in order of importance): the presence of relief, the shape of the edifice and regional tectonic control. We find that taller volcanoes develop longer radial dikes, whose (mainly lateral) propagation is independent of the composition of magma or the aspect ratio of the edifice. Future research, starting from these preliminary evaluations, should be devoted to identifying dike propagation paths and likely locations of vent formation at specific volcanoes, to better aid hazards assessment. [Copyright &y& Elsevier]

Abstract: The emplacement conditions for 39 igneous dikes cutting basalts in northwestern Ethiopia are evaluated by analyzing their displacement–length scaling relations. Maximum opening displacements and lengths of the dikes demonstrate displacement–length scaling of the form D max =0.088L 0.48, consistent with other populations of dikes and veins and different than the power-law scaling relation typically found for faults. The dikes propagated through the thin Trap basalt sequence under conditions of constant fracture toughness, with values corrected for three-dimensional dike geometry of ~77–273 MPa m1/2. The large values of fracture toughness are likely associated with (1) the toughening effects of near-tip damage, (2) mixed-mode dike propagation, as shown by magma flow fabric analysis through anisotropy of magnetic susceptibility (AMS) and image analysis of thin sections, and (3) elevated temperature within the blocky and ductile basaltic host rock, evidence of which has been found in the field. [Copyright &y& Elsevier]

Abstract: The capture of vertically rising dikes by faults is examined with a combination of an analytic solution, numerical simulations and field observations. One of the numerical solutions and the field observations also provide insight into the intrusion of sills into the hanging wall of faults that are reactivated when lubricated by magma. The approximate analytic solution indicates that dike capture by a fault will be limited primarily to steeply-dipping faults. Capture will become easier, meaning that it will be possible for faults with shallower dips, as the intersection of the dike with the fault occurs at smaller depths. Capture will also be easier as it is harder for crack to grow in the hanging wall; this may be interpreted in terms of a higher tensile strength or in terms of shorter pre-existing cracks or higher fracture toughness. Numerical analysis using a discrete element code confirms this conclusion but also shows that the analytic solutions overestimate the difficulty of intruding a pre-existing fault, the maximum deviation occurring for faults dipping at 45°. This finding that dike capture by faults should be restricted to high-angle faults and shallow depths is supported by field observations of exhumed Miocene basaltic volcanic terrain in central Nevada, USA. Differences in style seen in other regions are attributed to both the rock properties and the regional stress field. The numerical solutions also indicate that synintrusive fault slip is likely to lead to formation of sills in the hanging wall at depths of a few hundred meters or less: a finding that is also consistent with the field observations in Nevada. [Copyright &y& Elsevier]

Volcano-tectonic (VT) seismicity is commonly recorded prior to and during eruptions. VT seismicity may reflect stresses induced by dike propagation, as indicated by propagating hypocenters and fault-plane solutions (FPS) reflecting regional stresses, or stresses induced by dike inflation, indicated by randomly distributed hypocenters and FPS with pressure axes rotated ~90° to regional maximum compression. Although numerical models of dike-induced stresses indicate that both regimes should occur during dike emplacement, this has not yet been observed, according to published seismic data. Instead, ~90° rotated FPS are observed in some cases, while propagating hypocenters mark dike formation in other cases. We suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma-wall-rock interactions. Ultimately, it may be possible to use in- formation contained in VT seismicity to forecast changes in a volcano's behavior by establishing the characteristic stress field response for a given volcano, or through a deeper understanding of the complex relationships between VT seismicity, local crustal stresses, and the physical mechanisms of magma migration. [ABSTRACT FROM AUTHOR]

The Heart Mountain block slide of northwestern Wyoming and southwestern Montana is one of the largest slides known to have occurred in Earth's history. This early Eocene block slide covered an area of over 3400 km² and moved a minimum of 45 km across open terrain. The initial 2- to 4-km-thick Heart Mountain block slide moved on a slope of about 2°, detaching for half its length on a nondescript bedding plane in the Ordovician Big Horn Dolomite (BHD). Given our current understanding of fundamental mechanics, such a great mass of rock should not have begun sliding on such a gentle slope without some special condition. Here we suggest that a special condition existed during the interval between extensive upper-plate dike injections and the initial movement phase. In our model, the dike injections increased horizontal stresses and heated the surrounding layers. Both the increased stresses and the heat input elevated fluid pressure of water trapped within the BHD. In addition, vertical hydrofracturing was retarded as horizontal stress approached vertical, thus allowing a critical buildup of fluid pressure. Fluid overpressuring is a mechanism that can overcome the mechanical problem of initiating movement on a low-angle surface. Moreover, this mechanism explains the observed fluidized features found along the basal contact of the slide block as well as the observed lack of deformation in the lower plate. [ABSTRACT FROM AUTHOR]

Finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the Mount Stuart Batholith, Washington and the Tuolumne Intrusive Suite, California, yield strain rates in the range of 10-2 to 10-13 s-1. Compared to published regional strain rates (10-13 to 10-15 s-1), wallrock strain rates associated with dike-fed expansion (10-7 s-1), and rates resulting from numerical modeling of crystal-plastic creep in aureoles (10-11 s-1), our fast rates are several orders of magnitude higher. Field and microstructural observations suggest that multiple material transfer processes, including rigid rotation, ductile flow, cracking, and potentially melt-assisted granular flow operated in the aureoles to accommodate emplacement of these two plutons. Calculated durations of pluton construction and bulk shortening in the aureoles indicate that aureole deformation requires only slow, long-term strain rates of 10-14 s-1. Thus our local fast strain rates indicate that aureoles may be characterized by pulsating high strain rate surges. We suggest that magmatically folded dikes in these and other pluton aureoles around the world may be used as evidence for fast host rock strain rates during pluton emplacement. [ABSTRACT FROM AUTHOR]

The La Peña Complex (LPC) is a silica-undersaturated alkaline potassic intrusive system, with a subduction-related signature, linked to the early Miocene retroarc magmatism of the Southern Central Andes, in the flat slab segment. The LPC is composed of several intrusions, predominantly plutonic (clinopyroxenite, malignite and syenite), cross cut by a voluminous swarm of radial and annular dikes with mostly volcanic-subvolcanic textures and variable compositions (foid-bearing alkali feldspar trachyte, trachyte, benmoreite, ledmorite, syenite, tephrite, tephriphonolite and alkaline lamprophyre). In the TAS classification these rocks plot in the alkaline series covering a wide spectrum of compositions following two different trends: 1) alkaline (potassic) strongly silica-undersaturated series, from tephrite, phonotephrite to tephra-phonolite, and 2) mid-alkaline, less silica-undersaturated series, ranging from basaltic trachyandesite to trachyandesite (benmoreite), and trachyte. Dikes from the alkaline series show higher K 2 O/Na 2 O ratios and Sr, La, Ce, contents compared to those from the mid-alkaline series. Rocks of the alkaline series are richer in K-feldspar, sodalite, leucite (pseudoleucite), biotite, potassic-ferro-pargasite and garnet than the less silica-undersaturated (trachytic) rocks, reflecting a stronger alkaline potassic affinity. A review of geochemical, isotopic and mineralogical data, and a new geochemical modeling performed on the LPC dikes, suggests that both trends represent separated magmatic series that evolved from two different parental magmas lodged ∼30 km deep in the crust. Our results suggest that the compositional variations observed in LPC dikes, cannot be explained by a simple magmatic evolution via fractional crystallization from a unique parental magma, and that an assimilation and fractional crystallization (AFC) process is required to explain some compositional differences. Our results suggest an upper crustal contaminant (evolved rocks) with a Grenvillian isotope signature. On the other hand, analyses of feldspar crystals from the tephriphonolitic dikes indicate local mixing effects, between an evolved tephriphonolitic melt and a less evolved and hotter mafic magma. The origin of both parental magmas could be explained by different melting degrees of the same mantle source, a phlogopite-bearing spinel lherzolite metasomatized by subduction derived fluids. We consider as a possible explanation that alkaline and coeval calc-alkaline magmatism in this part of the Andes, is due to local heterogeneities in the mantle source, and different degrees of partial melting Similar isotopic compositions of the LPC dikes, with those from other Miocene magmatic occurrences with arc-signature and similar age (e.g., Paramillos de Uspallata, Las Máquinas basalt, Abanico Fm and Farellones Fm) suggest an analogous mantle source for these rocks, from arc and retroarc in the Pampean flat slab regions. However, our results suggest that the isotopic trend contamination of LCP is different from that of Paramillos de Uspallata and other arc rocks of the Southern Volcanic Zone. The crustal contaminant of LPC possibly has another composition that those of Precordillera and Principal Cordillera Miocene rocks. The age of LPC rocks (∼19 Ma) and their arc-related signature agree with the eastward broadening of the arc magmatism between 17 and 19 Ma in this part of the flat slab. According to our interpretations, the LPC is a singular occurrence of two alkaline magmatic series on destructive plate margins, associated with calc-alkaline magmatism, occurring closely in time and space. • The LPC dikes belong to alkaline and mid-alkaline series following two trends. • Modeling suggests separated series evolving from two different parental magmas. • The parental magmas result by different melting degrees of the same mantle source. • Compositional variations can be explained via AFC and local mixing process. • Isotope suggest similar source for magmatic manifestations of the flat slab region. [ABSTRACT FROM AUTHOR]

Abstract: It is well established that shear cracks in rock (faults) obey linear displacement–length scaling and thus have scale invariant driving stresses. Several recent papers have claimed that for opening mode cracks in rock (joints, veins, and dikes) displacement obeys square root scaling with fracture length. This is a fundamentally different mode of behavior, because, unlike shear cracks, opening mode cracks would then be unstable under constant stress boundary conditions. Here the same data are reexamined and it is found, to the contrary, that for opening mode cracks in rock, fracture toughness K c scales with √L and hence displacement scales linearly with L. The conflicting view resulted from data misinterpretation. This resolves the discrepancy between the behavior of shear and opening mode cracks in rock. [ABSTRACT FROM AUTHOR]

• Late-orogenic dikes in southern Jordan were emplaced in the period ~607 to ~590 Ma. • Post-orogenic dikes represent the last Ediacaran magmatism are dated at ~580 Ma. • The change in NFS tectonics from compressional to extensional is between 607 and 580 Ma. • The magmatic activity ended 50 Ma before the regional unconformity dated at ~530 Ma. A spectacular feature of the Arabian-Nubian Shield (ANS) is the abundance of well-exposed and extensive Neoproterozoic dike swarms of variable compositions. Most of these dikes are late to post-orogenic with respect to the East African Orogen (EAO) and are unmetamorphosed. We dated a composite dike with latite margins and a rhyolite core (607 ± 6 Ma, U-Pb), a biotite rhyolite dike (600 ± 4 Ma, 40Ar/39Ar age of biotite), an andesite dike (594 ± 3, 40Ar/39Ar age of amphibole) and a dolerite dike (~579 Ma, 40Ar/39Ar whole rock total gas age). We propose that the first three dikes represent one generation that was emplaced at different episodes extending between 607 and 590 Ma. Time and composition equivalent dikes are common in the northern ANS. The dikes crosscut late collisional granitoids and geochemically display a subduction-related character as evidenced by a negative Nb-Ta anomaly. These dikes are absent in the alkali feldspar A-type Humrat Syenogranite dated at 586 ± 5 Ma in Jordan and equivalent rocks in the northern ANS, which are crosscut only by the (~579 Ma) dolerite dikes. The within-plate character of the dolerite dikes is supported by the absence of the Nb-Ta anomaly and high field strength element geochemistry. We propose that the dolerite dikes are a generation, distinct from the ~607–590 Ma dikes, that reflects a change in tectonic regime and represents the last magmatic activity of the Neoproterozoic in the northern ANS. The ages of the dikes dated in this study agree with the published age range of the transitional stage from late orogenic calc-alkaline to extensional alkaline tectono-magmatic setting for the ANS. We propose that the magmatic activity was terminated ~50 m.y. before the age of the Cambrian unconformity at ~530 Ma. Correlation with ages of dikes and magmatic rocks in the northern ANS favors this supposition. The dike geochemistry and geochronology are compatible with a tectonic model that involves mantle lithosphere delamination from below the northern ANS after a significant crust-mantle thickening caused by the EAO, followed by thermal relaxation, subsidence and gradual denudation until the age of the unconformity at ~530 Ma. [ABSTRACT FROM AUTHOR]

The stability of a dike is influenced strongly by its water content, via both changing its weight and strength. While safety calculations using both analytical and numerical methods are well studied, the impact of surface boundaries exposed to natural conditions is rarely considered, nor is the fact that this surface is covered in vegetation and is susceptible to cracking. This paper presents a numerical study of stability of dikes covered with grass, subject to meteorological forcing and crack formation due to drying conditions. Building on a previous study and adding the impact of cracking, a crop model and a Finite Element Method (FEM) model are integrated together using an optimisation method to ensure mass balance and consistency. The crop model, used to simulate vegetation growth and infiltration/evaporation in response to meteorological forcing, is modified to consider preferential flow due to cracking. The FEM model, used to simulate the dike stability and hydro-mechanical behaviour, has the material properties modified to simulate the impact of cracks. Results simulating a ten-year period indicate a strong impact of cracking on the factor of safety. The vegetation was found to be responsive to both crack presence and an increase in the amount of cracks, which suggests that monitoring vegetation could be a useful tool to identify cracked (vulnerable to cracking) locations along dikes. [ABSTRACT FROM AUTHOR]

We use a 3D seismic dataset and wireline logs obtained during hydrocarbon exploration to describe the three-dimensional geometry and characteristics of mafic igneous rocks in the Cerro Dragón oilfield, Golfo San Jorge Basin, southern Argentina. Igneous intrusions occur within Upper Cretaceous and Paleocene sedimentary rocks at depths ranging from 2600 m to a ~200 m below of contemporaneous surface. Three distinctive types of shallow intrusions were recognized (layer-parallel, saucer-shaped, and transgressive sills). The largest sills are layer-parallel, some with estimated volumes up to 9,5 km3, whereas saucer-shaped sills and transgressive sills show widely variable intrusion geometries due to interaction with previous extensional tectonic structures and intrusion-related dikes. Sill intrusions were sourced either through vertical mafic dikes of NNW to NNE orientation, some up to 19 km in length and by sill-sill vertical connections. The presence of three imaged lava flows documents subaerial volcanic events coincident with the deposition of the Sarmiento Formation (late Eocene to early Miocene), whose trajectories were traced for 25–46 km of length. Cenozoic igneous intrusions produce both changes in the temperature history of the basin, and the remobilization of hosted hydrocarbon through the creation of supra-intrusion forced folds and new pathways generated during the igneous emplacement. Besides, sills and dikes act as barriers to both lateral and vertical fluid migration, and thus they need to be considered in petroleum prospect analysis. • Intrusive and extrusive igneous rocks of Cenozoic age were imaged in a subsurface dataset. • Vertical dikes and several sill varieties (layer-parallel, saucer-shaped, and transgressive) occur. • Lava flows within the Paleogene succession can be traced laterally for up to 46 km long. • The mechanism of emplacement and lateral propagation of igneous sills are discussed. • The importance of igneous rocks within hydrocarbon-bearing sedimentary basins are highlighted. [ABSTRACT FROM AUTHOR]