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  • 1.
    Berg, S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Troll, V. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Riishuus, M. S.
    Burchardt, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Krumbholz, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Eroded Neogene Silicic Central Volcanoes in Northeast Iceland Revisited2012Conference paper (Refereed)
  • 2.
    Berg, S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Troll, V. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Riishuus, M. S.
    Burchardt, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Krumbholz, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Silicic Magma Genesis in Neogene Central Volcanoes in Northeast Iceland2012Conference paper (Refereed)
    Abstract [en]

    We report on a geological expedition to NE Iceland in August 2011. A comprehensive sample suite of intrusive and extrusive rocks, ranging from basaltic to silicic compositions, was collected from the Neogene silicic central volcanic complexes in the region between Borgarfjörður eystri and Loðmundarfjörður. The area contains the second-most voluminous occurrence of silicic rocks in Iceland, including caldera structures, inclined sheet swarms, extensive ignimbrite sheets, sub-volcanic rhyolites and silicic lava flows. Yet it is one of Iceland's geologically least known areas (c.f. Gústafsson, 1992; Martin & Sigmarsson, 2010; Burchardt et al., 2011). The voluminous occurrence of evolved rocks in Iceland (10-12 %) is very unusual for an ocean island or a mid-oceanic ridge, with a typical signal of magmatic bimodality, often called "Bunsen-Daly" compositional gap (e.g. Bunsen, 1851; Daly, 1925; Barth et al., 1939). The Bunsen-Daly Gap is a long-standing fundamental issue in petrology and difficult to reconcile with continuous fractional crystallization as a dominant process in magmatic differentiation (Bowen, 1928), implying that hydrothermal alteration and crustal melting may play a significant role. Our aim is to contribute to a solution of this issue by unravelling the occurrence of voluminous evolved rhyolites in NE Iceland. We will use a combined petrological, textural, experimental and in-situ isotope approach. We plan to perform major, trace element and Sr-Nd-Hf-Pb-He-O isotope geochemistry, as well as U/Pb and Ar/Ar geochronology on rocks and mineral separates. In addition, high pressure-temperature partial melting experiments aim to reproduce and further constrain natural processes. Using the combined data set we intend to produce a comprehensive and quantitative analysis of rhyolite petrogenesis, and of the temporal, structural and geochemical evolution of the silicic volcanism in NE Iceland. The chosen field area serves as a good analogue for active central volcanoes in Iceland, such as Askja and Krafla, where a close interaction of basaltic and more evolved magma has led to explosive eruptions.

  • 3.
    Berg, Sylvia E.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Troll, Valentin R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Univ Las Palmas Gran Canaria, GEOVOL, Las Palmas Gran Canaria, Spain.
    Deegan, Frances M.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Georg August Univ Gottingen, Geosci Ctr, Goldschmidtstr 1-3, D-37077 Gottingen, Germany.
    Mancini, Lucia
    SCpA, Elettra Sincrotrone Trieste, SS 14 Km 163,5 AREA Sci Pk, I-34149 Trieste, Italy.
    Polacci, Margherita
    Univ Manchester, Sch Earth & Environm Sci, Williamson Bldg,Oxford Rd, Manchester M13 9PL, Lancs, England.
    Carracedo, Juan Carlos
    Univ Las Palmas Gran Canaria, GEOVOL, Las Palmas Gran Canaria, Spain.
    Soler, Vicente
    CSIC, Estn Vulcanol Canarias, Avda Astr Fco Sanchez 3, Tenerife 38206, Spain.
    Arzilli, Fabio
    SCpA, Elettra Sincrotrone Trieste, SS 14 Km 163,5 AREA Sci Pk, I-34149 Trieste, Italy.; Univ Manchester, Sch Earth & Environm Sci, Williamson Bldg,Oxford Rd, Manchester M13 9PL, Lancs, England.
    Brun, Francesco
    SCpA, Elettra Sincrotrone Trieste, SS 14 Km 163,5 AREA Sci Pk, I-34149 Trieste, Italy.; Univ Trieste, Dept Engn & Architecture, Via A Valerio 10, I-34127 Trieste, Italy.
    Heterogeneous vesiculation of 2011 El Hierro xeno-pumice revealed by X-ray computed microtomography2016In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 12, article id 85Article in journal (Refereed)
    Abstract [en]

    During the first week of the 2011 El Hierro submarine eruption, abundant light-coloured pumiceous, high-silica volcanic bombs coated in dark basanite were found floating on the sea. The composition of the light-coloured frothy material ('xeno-pumice') is akin to that of sedimentary rocks from the region, but the textures resemble felsic magmatic pumice, leaving their exact mode of formation unclear. To help decipher their origin, we investigated representative El Hierro xeno-pumice samples using X-ray computed microtomography for their internal vesicle shapes, volumes, and bulk porosity, as well as for the spatial arrangement and size distributions of vesicles in three dimensions (3D). We find a wide range of vesicle morphologies, which are especially variable around small fragments of rock contained in the xeno-pumice samples. Notably, these rock fragments are almost exclusively of sedimentary origin, and we therefore interpret them as relicts an the original sedimentary ocean crust protolith(s). The irregular vesiculation textures observed probably resulted from pulsatory release of volatiles from multiple sources during xeno-pumice formation, most likely by successive release of pore water and mineral water during incremental heating and decompression of the sedimentary protoliths.

  • 4.
    Berg, Sylvia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Troll, Valentin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Deegan, Frances
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Riishuus, Morten S.
    Nordic Volcanological Center. Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik.
    Whitehouse, Martin J.
    Dept. of Geosciences, Swedish Museum of Natural History, SE-104 05, Stockholm, Sweden.
    Harris, Chris
    Dept. of Geological Sciences, University of Cape Town, Rondebosch, South Africa,.
    Freda, Carmela
    Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy.
    Ellis, Ben S.
    Inst. f. Geochemie und Petrologie, ETH, Clausiusstrasse 25, 8092, Zurich, Switzerland.
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Gústafsson, Ludvik E.
    Samband Islenskra Sveitarfélag, Borgartúni 30, pósthólf 8100, 128 Reykjavik, Iceland.
    Rapid high-silica magma generation in basalt-dominated rift settings2015Conference paper (Other academic)
  • 5.
    Berg, Sylvia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Troll, Valentin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Riishuus, M.
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Gústafsson, L.E.
    Iceland's best kept secret2014In: Geology Today, ISSN 0266-6979, E-ISSN 1365-2451, Vol. 30, no 2, p. 54-60Article in journal (Refereed)
    Abstract [en]

    The ‘forgotten fjords’ and ‘deserted inlets’ of NE-Iceland, in the region between Borgarfjörður Eystri and Loðmundarfjörður, are not only prominent because of their pristine landscape, their alleged elfin settlements, and the puffins that breed in the harbour, but also for their magnificent geology. From a geological point of view, the area may hold Iceland's best kept geological secret. The greater Borgarfjörður Eystri area hosts mountain chains that consist of voluminous and colourful silicic rocks that are concentrated within a surprisingly small area (Fig. 1), and that represent the second-most voluminous occurrence of silicic rocks in the whole of Iceland. In particular, the presence of unusually large volumes of ignimbrite sheets documents extremely violent eruptions during the Neogene, which is atypical for this geotectonic setting. As a group of geoscientists from Uppsala University (Sweden) and the Nordic Volcanological Center (NordVulk, Iceland) we set out to explore this remote place, with the aim of collecting material that may allow us to unravel the petrogenesis of these large volumes of silicic rocks. This effort could provide an answer to a long-standing petrological dilemma; the question of how silicic continental crust is initially created. Here we document on our geological journey, our field strategy, and describe our field work in the remote valleys of NE-Iceland.

  • 6.
    Berg, Sylvia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Univ Iceland, Nord Volcanol Ctr, Inst Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland.
    Troll, Valentin R.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Harris, Chris
    Univ Cape Town, Dept Geol Sci, ZA-7701 Rondebosch, South Africa.
    Deegan, Frances
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Riishuus, Morten S.
    Univ Iceland, Nord Volcanol Ctr, Inst Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Exceptionally high whole-rock delta O-18 values in intra-caldera rhyolites from Northeast Iceland2018In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1147-1168Article in journal (Refereed)
    Abstract [en]

    The Icelandic crust is characterized by low delta O-18 values that originate from pervasive high-temperature hydrothermal alteration by O-18-depleted meteoric waters. Igneous rocks in Iceland with delta O-18 values significantly higher than unaltered oceanic crust (similar to 5.7 parts per thousand) are therefore rare. Here we report on rhyolitic intra-caldera samples from a cluster of Neogene central volcanoes in Borgarfjorour Eystri, Northeast Iceland, that show whole-rock delta O-18 values between +2.9 and +17.6 parts per thousand (n = 6), placing them among the highest delta O-18 values thus far recorded for Iceland. Extra-caldera rhyolite samples from the region, in turn, show delta O-18 whole-rock values between +3.7 and +7.8 parts per thousand (n = 6), consistent with the range of previously reported Icelandic rhyolites. Feldspar in the intra-caldera samples (n = 4) show delta O-18 values between +4.9 and +18.7 parts per thousand, whereas pyroxene (n = 4) shows overall low delta O-18 values of +4.0 to +4.2 parts per thousand, consistent with regional rhyolite values. In combination with the evidence from mineralogy and rock H2O contents, the high whole-rock delta O-18 values of the intra-caldera rhyolites appear to be the result of pervasive isotopic exchange during subsolidus hydrothermal alteration with O-18-enriched water. This alteration conceivably occurred in a near-surface hot spring environment at the distal end of an intra-caldera hydrothermal system. and was probably fed by waters that had already undergone significant isotope exchange with the country rock. Alternatively, O-18-enriched alteration fluids may have been produced during evaporation and boiling of standing water in former caldera lakes, which then interacted with the intra-caldera rock suites. Irrespective of the exact exchange processes involved, a previously unrecognized and highly localized delta O-18-enriched rock composition exists on Iceland and thus probably within the Icelandic crust too.

  • 7.
    Burchardt, Steffi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology. Univ Gottingen, Geosci Ctr, D-37077 Gottingen, Germany.
    Tanner, D.C.
    Univ Gottingen, Geosci Ctr, D-37077 Gottingen, Germany.
    Krumbholz, Michael
    Univ Gottingen, Geosci Ctr, D-37077 Gottingen, Germany; Leibniz Inst Appl Geophys, D-30655 Hannover, Germany.
    The Slaufrudar pluton, southeast Iceland: An example of shallow magma emplacement by coupled cauldron subsidence and magmatic stoping2011In: Geological Society of America Bulletin, ISSN 0016-7606, E-ISSN 1943-2674, Vol. 124, no 1-2, p. 213-227Article in journal (Refereed)
    Abstract [en]

    The Tertiary Slaufrudalur pluton is the largest granitic intrusion exposed in Iceland. Five glacial valleys cut through the uppermost 900 m of the pluton, exposing spectacular sections through its roof, walls, and interior. The wall contacts are subvertical and sharp. Only in the northeast and southwest is the wall contact characterized by brittle faulting. The pluton roof is smooth at map scale, so that the overall cross-sectional shape of the pluton and its internal layering indicate emplacement by incremental floor sinking through cauldron subsidence. A pronounced elongation of the pluton, parallel to the trend of regional fissure swarms, and its angular shape in map view indicate strong tectonic control on horizontal ring-fault propagation, whereas faulted wall contacts represent step-over structures between the earlier-formed ring faults. On outcrop scale, the roof contact exhibits numerous steps, faults, and apophyses associated with conjugate fracture sets that are parallel and perpendicular to the strike of the length of the pluton. These structures were presumably formed by sequential inflation and deflation of the pluton during episodic magma intrusion and therefore are closely coupled to cauldron subsidence. As a result of roof fracturing and magma injection along the fractures, roof material is found partly or completely detached within the granite. The Slaufrudalur pluton therefore provides new insight into the coupling of the emplacement mechanisms of cauldron subsidence and magmatic stoping in the upper crust.

  • 8. Kosminska, Karolina
    et al.
    Majka, Jaroslaw
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Mazur, Stanislaw
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Klonowska, Iwona
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Manecki, Maciej
    Czerny, Jerzy
    Dwornik, Maciej
    Blueschist facies metamorphism in Nordenskiold Land of west-central Svalbard2014In: Terra Nova, ISSN 0954-4879, E-ISSN 1365-3121, Vol. 26, no 5, p. 377-386Article in journal (Refereed)
    Abstract [en]

    Recent fieldwork in Nordenskiold Land, Svalbard's Southwestern Basement Province, has established the presence of high-pressure (HP) lithologies. They are strongly retrogressed blueschists consisting mainly of garnet and Ca-amphibole with remnants of ferroglaucophane and phengite. The pressure-temperature (P-T) conditions were estimated using phase equilibrium modelling in the NCKFMMnASHTO system. P-T estimates based on the garnet, phengite and ferroglaucophane compositional isopleths and modelled paragenetic assemblage indicate peak metamorphism at 470-490 degrees C and 14-18 kbar. These data fall close to the 7-8 C km(-1) geo-therm, which is similar to that from Motalafjella, the only previously known occurrence of blueschists in Svalbard's Caledonides. The newly discovered blueschists could have formed during the early stage of the Caledonian Orogeny and may represent a vestige of missing marginal basins of the western Iapetus developed at the onset of subduction. The likely counterpart to Svalbard's blueschists is the ophiolitic sequence in the Pearya Terrane of northern Ellesmere Island.

  • 9.
    Krumbholz, Michael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Bock, M
    Institute of Geosciences, Johannes-Gutenberg-University of Mainz, Mainz, Germany.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Kelka, U.
    Institute of Geosciences, Johannes-Gutenberg-University of Mainz, Mainz, Germany.
    Vollbrecht, A.
    Geoscience Center, Georg-August-University of Göttingen, Göttingen, Germany.
    A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust2012In: Solid Earth, ISSN 1869-9510, E-ISSN 1869-9529, Vol. 3, no 2, p. 401-414Article in journal (Refereed)
    Abstract [en]

    In recent years, the ElectroMagnetic Radiation (EMR) method has been used to detect faults and to determine main horizontal stress directions from variations in intensities and directional properties of electromagnetic emissions, which are assumed to be generated during micro-cracking. Based on a large data set taken from an area of about 250 000 km2 in Northern Germany, Denmark, and Southern Sweden with repeated measurements at one location during a time span of about 1.5 yr, the method was systematically tested. Reproducible observations of temporary changes in the signal patterns, as well as a strongly concentric spatial pattern of the main directions of the magnetic component of the EMR point to VLF transmitters as the main source and hence raise serious concerns about the applicability of the method to determine recent crustal stresses. We conclude that the EMR method, at its current stage of development, does not allow determination of the main horizontal stress directions.

  • 10.
    Krumbholz, Michael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Qualitative and quantitative analyses of magmatic stoping in the roof of the Proterozoic Åva ring complex2013Conference paper (Refereed)
    Abstract [en]

    Daly (1903) defined magmatic stoping as magma emplacement due to the detachment of blocks of magma-chamberroof- and wall rocks and their incorporation into the magma chamber. Stoping itself involves a number of interrelated processes, e.g. hydraulic fracturing, partial melting, and explosive exfoliation, that are a product of the complex thermal, mechanical, and chemical interaction of magma and the country rocks. However, the individual processes, as well as the influence of the main controlling parameters, are poorly understood. This makes it difficult to quantify the contribution ofmagmatic stoping as a magma-emplacement process, which has resulted in vigorous debates about its efficiency and overall significance. To resolve this controversy, detailed, qualitative and quantitative studies to better understand the involved processes and the interaction of forces are essential. We studied strongly foliated amphibolite-facies volcaniclastic metasedimentary rocks that were intruded by granitic magmas of the Åva ring complex (Finland), a 1.76 Ga intrusion which formed at 5 to 6 km depth (Eklund and Shebanov, 2005). In the roof region of the main intrusion, the country rock is strongly fragmented and incorporated into the granite as xenoliths ranging in size (area) from tens of m2 to mm2. We systematically recorded subhorizontal, glacially polished coastal outcrops that contain thousands of xenoliths. The xenoliths show signs of brittle deformation resulting in intense fragmentation caused by the intrusion of granitic veins and dyklets, i.e. the fragments are angular. Bigger blocks are often split along the foliation and are surrounded by a cloud of smaller blocks. In many places, the blocks still fit to each other like a jig saw puzzle, while in other domains, they appear to have tumbled around. In contrast, some outcrops contain rounded xenolithic blocks that show signs of ductile deformation. From the outcrop maps, we carefully recorded all xenoliths to determine their size, orientation, and shape. In addition, we measured the strike of the internal foliation in relation to the undisturbed country rock for each individual xenolith. The spatial xenolith distribution pattern and the close assemblage of fragments of a wide range of sizes indicate that stoping is a rapid and efficient process. The size distribution closely resembles a power-law distribution over several orders of magnitude, even if modified by stereographic effects. The results of the shape analysis indicate that the fragmentation process is strongly controlled by the host-rock foliation, expressed in alternating aspect ratios with respect to the xenolith size. First fragmentation occurs parallel to the foliation, resulting in high aspect ratios of large xenoliths. Further fragmentation reduces block aspect ratios cracking the blocks perpendicular to their long axis, before fragmentation parallel to the foliation becomes dominant again, producing small blocks with high aspect ratios. References Daly, R. A., 1903. The mechanics of igneous intrusion. American Journal of Science 15, 269- 298. Eklund, O. and Shebanov, A. D., 2005. Prolonged postcollisional shoshonitic magmatism in the southern Svecofennian domain - a case study of the Åva granite-lamprophyre ring complex

  • 11.
    Krumbholz, Michael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Hieronymus, Christoph
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Troll, Valentin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Tanner, David
    Leibniz Institute of Applied Geophysics.
    Friese, Nadine
    Wintershall Norge AS.
    Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength2014In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 3272-Article in journal (Refereed)
    Abstract [en]

    Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

  • 12.
    Krumbholz, Michael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Vollbrecht, Axel
    Aschoff, Marius
    Recent horizontal stress directions in basement rocks of southern Sweden deduced from open microcracks2014In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 65, p. 33-43Article in journal (Refereed)
    Abstract [en]

    The strike direction of open intragranular microcracks in quartz and feldspar host grains was determined using optical transmission and reflection microscopy on eight oriented samples taken in two study areas in Precambrian basement rocks of southern and south-central Sweden. For an area of about 160 km(2) (SW of Vastervik) and two sample locations (W of Uppsala), the vast majority of open microcracks displays a strong preferred NW-SE strike direction. According to the common assumptions that natural cracks in crystalline rocks are predominantly extensional (mode I), and that open cracks belong to the latest microcrack generation, these strike directions should reflect the (sub-) recent main horizontal stress direction (sigma H) of the recent tectonic stress field. This conclusion is supported by corresponding directions known from in situ stress measurements and focal plane solutions in the vicinity of the study areas. It is remarkable that even in samples taken close (i.e. a few hundred metres) to recently active large scale faults the orientation of microcracks does not deviate from this common direction. This may point to slip on already softened faults, very local stress reorientations (e.g. m-scale) or that local stress relief was accomplished by other processes at microscale, e.g. mechanical twinning in favourably oriented feldspar crystals, or slip on grain boundaries.

  • 13.
    Mathieu, Lucie
    et al.
    Univ Quebec Chicoutimi, CONSOREM, Chicoutimi, PQ G7H 2B1, Canada.;Uppsala Univ, Dept Earth Sci, CEMPEG, S-75236 Uppsala, Sweden..
    Burchardt, Steffi
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Troll, Valentin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Krumbholz, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
    Delcamp, Audray
    Vrije Univ Brussel, Fac Sci, Dept Geog, B-1050 Brussels, Belgium..
    Geological constraints on the dynamic emplacement of cone-sheets - The Ardnamurchan cone-sheet swarm, NW Scotland2015In: Journal of Structural Geology, ISSN 0191-8141, E-ISSN 1873-1201, Vol. 80, p. 133-141Article in journal (Refereed)
    Abstract [en]

    Cone-sheets are a significant constituent of many central volcanoes, where they contribute to volcano growth by intrusion and through flank eruptions, although the exact emplacement mechanisms are still controversially discussed. In particular, it is not yet fully resolved whether cone-sheets propagate as magma-driven, opening-mode fractures or as shear fractures, and to what extent pre-existing host-rock structures and different stress fields influence cone-sheet emplacement. To shed further light on the role of these parameters in cone-sheet emplacement, we use detailed field and remote sensing data of the classic Ardnamurchan cone-sheet swarm in NW-Scotland, and we show that the cone-sheets primarily propagated as opening-mode fractures in the sigma(1)-sigma(2) plane of the volcanic stress field. In addition, more than one third of the Ardnamurchan cone-sheet segments are parallel to lineaments that form a conjugate set of NNW and WNW striking fractures and probably reflect the regional NW SE orientation of sigma(1) during emplacement in the Palaeogene. Cone-sheets exploit these lineaments within the NE and SW sectors of the Ardnamurchan central complex, which indicates that the local volcanic stress field dominated during sheet propagation and only allowed exploitation of host-rock discontinuities that were approximately parallel to the sheet propagation path. In addition, outcrop-scale deflections of cone-sheets into sills and back into cone-sheets (also referred to as "staircase" geometry) are explained by the interaction of stresses at the propagating sheet tip with variations in host-rock strength, as well as the influence of sheet-induced strain. As a consequence, cone-sheets associated with sill-like segments propagate as mixed-mode I/II fractures. Hence, cone-sheet emplacement requires a dynamic model that takes into account stress fields at various scales and the way propagating magma interacts with the host rock and its inherent variations in rock strength.

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