Progress in seismic methodology and ambitious large‐scale seismic projects are enabling high‐resolution imaging of the continental crust. The ability to constrain interpretations of crustal seismic data is based on laboratory measurements on rock samples and calculations of seismic properties. Seismic velocity calculations and their directional dependence are based on the rock microfabric, which consists of mineral aggregate properties including crystallographic preferred orientation (CPO), grain shape and distribution, grain boundary distribution, and misorientation within grains. Single‐mineral elastic constants and density are crucial for predicting seismic velocities, preferably at conditions that span the crust. However, high‐temperature and high‐pressure elastic constant data are not as common as those determined at standard temperature and pressure (STP; atmospheric conditions). Continental crust has a very diverse mineral composition; however, a select number of minerals appear to dominate seismic properties because of their high‐volume fraction contribution. Calculations of microfabric‐based seismic properties and anisotropy are performed with averaging methods that in their simplest form takes into account the CPO and modal mineral composition, and corresponding single crystal elastic constants. More complex methods can take into account other microstructural characteristics, including the grain shape and distribution of mineral grains and cracks and pores. Dynamic or active wave propagation schemes have recently been developed, which offer a complementary method to existing static averaging methods generally based on the use of the Christoffel equation. A challenge for the geophysics and rock physics communities is the separation of intrinsic factors affecting seismic anisotropy, due to properties of crystals within a rock and apparent sources due to extrinsic factors like cracks, fractures, and alteration. This is of particular importance when trying to deduce crustal composition and the state of deformation from seismic parameters.

SUMMARY Digital rock physics combines modern microscopic imaging with advanced numerical simulations to analyse the physical properties of rocks. Elastic‐wave propagation modelling based on the microstructure images is used to estimate the effective elastic properties of the rock. The goal of this paper is to describe and understand how laboratory experiments compare with digital rock physics results using Berea sandstone. We experimentally measure pressure‐dependent ultrasonic velocities and the pore size distribution. The effective elastic properties resulting from numerical simulations are based on microcomputed tomography (micro‐CT) images, which are systematically stiffer than the laboratory measures. Because the tomographic images do not resolve the small‐scale pore and crack network of the sample, we hypothesize that the numerical overprediction is attributable to the smallest pores and grain‐to‐grain contacts that are missing in the images. To reconcile the difference between numerical and experimental data, we suggest to use a grain boundary reconstruction algorithm. This allows to implement and approximate so far unresolved features in the virtual rock model. As a result, we can predict pressure‐dependent effective velocity using micro‐CT images.

No abstract

Abstract. The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project focuses on mountain building processes in a major mid-Palaeozoic orogen in western Scandinavia and its comparison with modern analogues. The project investigates the subduction-generated Seve Nape Complex. These in part under ultra-high-pressure conditions metamorphosed outer continental margin and continent-ocean transition zone assemblages were emplaced onto the Baltoscandian platform and there influenced the underlying allochthons and the basement. COSC-1 is the first of two ca. 2.5 km deep, fully cored drill holes located in the vicinity of the abandoned Fröå mine, close to the town of Åre in Jämtland, central Sweden. It sampled a thick section of the lower part of the Seve Complex and was planned to penetrate its basal thrust zone into the underlying lowergrade metamorphosed allochthon. The drill hole reached a depth of 2495.8 m and nearly 100 % core recovery was achieved. Although planning was based on existing geological mapping and new high-resolution seismic surveys, the drilling resulted in some surprises: the Lower Seve Nappe proved to be composed of rather homogenous gneisses, with only subordinate mafic bodies, and its basal thrust zone was unexpectedly thick (> 800 m). The drill hole did not penetrate the bottom of the thrust zone. However, lower-grade metasedimentary rocks were encountered in the lowermost part of the drill hole together with garnetiferous mylonites tens of metres thick. The tectonostratigraphic position is still unclear, and geological and geophysical interpretations are under revision. The compact gneisses host only eight fluid conducting zones of limited transmissivity between 300 m and total depth. Downhole measurements suggest an uncorrected average geothermal gradient of ∼ 20 • C km −1 . This paper summarizes the operations and preliminary results from COSC-1 (ICDP 5054-1-A), drilled from early May to late August 2014, and is complemented by a detailed operational report and the data repository.Published by Copernicus Publications on behalf of the IODP and the ICDP.

No abstract

Synchrotron radiation X-ray tomographic microscopy is a nondestructive method providing ultra-high-resolution 3D digital images of rock microstructures. We describe this method and, to demonstrate its wide applicability, we present 3D images of very different rock types: Berea sandstone, Fontainebleau sandstone, dolomite, calcitic dolomite, and three-phase magmatic glasses. For some samples, full and partial saturation scenarios are considered using oil, water, and air. The rock images precisely reveal the 3D rock microstructure, the pore space morphology, and the interfaces between fluids saturating the same pore. We provide the raw image data sets as online supplementary material, along with laboratory data describing the rock properties. By making these data sets available to other research groups, we aim to stimulate work based on digital rock images of high quality and high resolution. We also discuss and suggest possible applications and research directions that can be pursued on the basis of our data.

Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. Deformation during laccolith emplacement is primarily considered to occur in the host rock. However, shallowly emplaced laccoliths (cryptodomes) show extensive internal deformation. While deformation of magma in volcanic conduits is an important process for regulating eruptive behavior, the effects of magma deformation on intrusion emplacement remain largely unexplored. In this study, we investigate the emplacement of the 0.57 km 3 rhyolitic Sandfell laccolith, Iceland, which formed at a depth of 500 m in a single intrusive event. By combining field measurements, 3D modeling, anisotropy of magnetic susceptibility (AMS), microstructural analysis, and FEM modeling we examine deformation in the magma to constrain its influence on intrusion emplacement. Concentric flow bands and S-C fabrics reveal contact-parallel magma flow during the initial stages of laccolith inflation. The magma flow fabric is overprinted by strain-localization bands (SLBs) and more than one third of the volume of the Sandfell laccolith displays concentric intensely fractured layers. A dominantly oblate magmatic fabric in the fractured areas and conjugate geometry of SLBs, and fractures in the fracture layers demonstrate that the magma was deformed by intrusive stresses. This implies that a large volume of magma became viscously stalled and was unable to flow during intrusion. Fine-grained groundmass and vesicle-poor rock adjacent to the fracture layers point to that the interaction between the SLBs and the flow bands at sub-solidus state caused the brittle-failure and triggered decompression degassing and crystallization, which led to rapid viscosity increase in the magma. The extent of syn-emplacement fracturing in the Sandfell laccolith further shows that strain-induced degassing limited the amount of eruptible magma by essentially solidifying the rim of the magma body. Our observations indicate that syn-emplacement changes in rheology, and the associated fracturing of intruding magma not only occur in volcanic conduits, but also play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.

[1] A differential effective medium (DEM) model is used to predict elastic properties for a set of porous and anisotropic aggregates, comprised of mixtures of calcite and muscovite. The DEM takes into consideration an anisotropic background medium with triclinic or higher symmetry, in which inclusions of idealized ellipsoidal shape are added incrementally. In general, the calculated elastic properties of a solid that contains inclusions representing "dry" pores/cracks are strongly dependent on the orientation and aspect ratio of the inclusions. Aspect ratios of inclusions in the synthetic aggregates, which consist of air-filled pores, are estimated from anisotropy of magnetic susceptibility (AMS) of samples whose pore space has been impregnated with a colloidal ferrofluid. The AMS derived pore shape geometry is used as an input value for inclusions in the DEM. Modeling results are compared with laboratory determined elastic properties, measured with ultrasonic waves. Calculated shear wave velocities agree in general well with laboratory measured S wave velocities, whereas calculated P wave velocities are typically 0.5-1.1 km/s higher than measured values. Differences between calculated and measured P wave velocities are attributed mainly to incomplete and biased ferrofluid saturation of pores. Spherical pores are preferably filled during imbibition, in comparison to thin cracks, which leads to overprediction of the calculated P wave velocities. The amount of ferrofluid that fills the pore space is dependent on the ratio of calcite to muscovite and the load used for compaction during sample synthesis. The permeability decreases with increasing muscovite content and increasing compaction load. Incomplete saturation of samples with high-muscovite content is confirmed by X-ray microtomography density contrast imaging of dry and ferrofluid saturated specimens.Citation: Almqvist, B. S. G., D. Mainprice, C. Madonna, L. Burlini, and A. M. Hirt (2011), Application of differential effective medium, magnetic pore fabric analysis, and X-ray microtomography to calculate elastic properties of porous and anisotropic rock aggregates,