Richard J. Walker

Fault zone permeability structure evolution in basalts

A combination of field, microstructural and experimental static permeability characterization is used to determine fault permeability structure evolution in upper crustal basalt-hosted fault zones in the Faroe Islands. The faults comprise lower strain fracture networks, to higher strain breccias that form tabular volumes around a principal slip zone hosting gouge or cataclasite. Samples representative of these fault zone components are used for static experimental permeability measurement. Results indicate that within the appropriate effective pressure (depth) range (10–90 MPa: ~0.3 to ~3.0 km), basalt-hosted faults evolve from low strain ( 10–17 m2) structures. Sample analyses reveal that static permeability is controlled by the development of: a) fault-parallel clay alteration (decreasing permeability); and b) porous zeolite vein connectivity due to hydrofracture (increasing permeability). Fault-parallel permeability is increased relative to the host rock, while fault-normal permeability is low throughout fault rock evolution. This configuration will tend to promote across-fault compartmentalization and along-fault fluid flow, facilitating migration between relatively high-permeability horizons (e.g. vesicular flow unit tops and siliciclastic horizons), bypassing the bulk of the stratigraphy.

U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin

Constraining the timing of brittle faulting is critical in understanding crustal deformation and fluid flow, but many regional-scale fault systems lack readily available techniques to provide absolute chronological information. Calcite mineralization occurs in crustal faults in many geological settings and can be suitable for U-Pb geochronology. This application has remained underutilized because traditional bulk dissolution techniques require uncommonly high U concentration. Because U and Pb are distributed heterogeneously throughout calcite crystals, high-spatial-resolution sampling techniques can target domains with high U and variable U/Pb ratios. Here we present a novel application of in-situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) to basaltic fault rock geochronology in the Faroe Islands, northeast Atlantic margin. Faults that are kinematically linked to deformation associated with continental break-up were targeted. Acquired ages for fault events range from mid-Eocene to mid-Miocene and are therefore consistently younger than the regional early Eocene onset of ocean spreading, highlighting protracted brittle deformation within the newly developed continental margin. Calcite geochronology from LA-ICP-MS U-Pb analysis represents an important and novel method to constrain the absolute timing of fault and fluid-flow events.

Onshore evidence for progressive changes in rifting directions during continental break-up in the NE Atlantic

Abstract: Current models for the Palaeogene-aged continental break-up in the North Atlantic invoke a NW–SE extension vector throughout the rift-to-drift process, accommodated by margin-parallel normal faults, and margin-oblique (strike-slip) transfer zones. However, Palaeogene and younger faults in the Faroe Islands provide evidence for a progressive anticlockwise rotation in extension vectors immediately prior to and following North Atlantic break-up. Six deformation stages are recognized: (1) east–west to NE–SW extension, accommodated by dip-slip north–south- and NW–SE-trending faults; (2) continued NE–SW extension accommodated by NW–SE- and NNE–SSW-oriented dykes; (3) north–south extension accommodated by ENE–WSW and ESE–WNW conjugate dykes; (4) crustal extrusion involving both east–west shortening and further north–south extension along ENE–WSW (dextral) and ESE–WNW (sinistral) conjugate strike-slip faults; (5) during the final stages of rifting, the regional extension vector became NW–SE and was accommodated predominantly by slip along NE–SW dextral oblique-slip faults; (6) pre-existing structures were locally reactivated as tensional and extensional–shear features, characterized by the entrainment of clastic material along fault planes. The present study reveals a distinct period of margin-parallel extension immediately prior to break-up that is not accounted for by existing models, illustrating the importance of conducting field-based studies to validate otherwise widely accepted margin-scale models worldwide.

Fault-zone evolution in layered basalt sequences: A case study from the Faroe Islands, NE Atlantic margin

Few studies have focused on the geological characterization of exhumed subsurface faults and fractures within continental flood basalt provinces. We present field and microstructural observations of basalt-hosted fractures and faults from the Faroe Islands, NE Atlantic margin. For a given displacement, the thickness of these highly mineralized faults varies by over three orders of magnitude. Fault-zone thickness and displacement data from the Faroe Islands span nearly four orders of magnitude in displacement, but there is no strong positive correlation between fault-zone thickness and displacement. Fault-rock characterization reveals important breccia distinctions, including collapse/infill, crush/wear/abrasion, and implosion breccias, each with a respective increase in sealing potential. Collapse/infill breccias indicate sustained fluid-migration pathways, as they require open, subterranean cavities that are formed faster than mineral precipitation can seal them. Crush/wear/abrasion and implosion breccias record crack-seal behavior during successive slip events. Despite having distinctly different fault-rock assemblages, fault-zone thickness and displacement data from basalt-hosted faults are indistinguishable from comparable data obtained from sediment-hosted faults. This observation suggests that the first-order controls on fault development are the same in layered basalts and sediments, namely, fault surface bifurcation and linkage, asperity removal, and the accommodation of geometrically necessary strains in the wall rocks.

Controls on transgressive sill growth

Igneous sills represent an important contribution to upper crustal magma transport and storage. This study focuses on an exemplary 20–50-m-thick transgressive sill in the Faroe Islands on the European Atlantic passive margin, which is hosted in layered lavas (1–20 m thick) and basaltic volcaniclastic units (1–30 m thick). Preserved steps in the sill, and offset intrusive segments, are consistent with initial propagation via segmented fractures followed by inflation to create a through-going sheet. Although steps correspond to the position of some host rock interfaces and volcaniclastic horizons, most interfaces are bypassed. Transgressive sill contacts are subparallel to thrust faults that record ENE-WSW shortening, which are observed within the surrounding country rock and within the sill. Remnant sill segments are elongate along a NNW-SSE axis, parallel to the derived intermediate stress axis for thrust faults. The overall transgressive geometry is consistent with regional horizontal shortening, with steps indicating transitions between transgressive and lateral sill propagation are controlled locally by layering. This work emphasizes the importance of scale of observation in considering the controls on sill emplacement, and in particular, that layering is not the primary control on geometry.

Igneous sills as a record of horizontal shortening: The San Rafael subvolcanic field, Utah

Igneous sills can facilitate significant lateral magma transport in the crust; therefore, it is important to constrain controls on their formation and propagation. Close spatial association between sills and dikes in layered (sedimentary) host rocks has led to a number of sill emplacement mechanisms that involve stress rotation related to layering; from horizontal extension and dike emplacement, to horizontal compression and sill emplacement. Here, we used field observations in the San Rafael subvolcanic field (Utah, USA), on the Colorado Plateau, supported by mechanical modeling, to show that layering is not the dominant control in all cases of sill formation. We found no compelling evidence of large sills fed by dikes; all observed cases showed that either dikes cut sills, or vice versa. Local sill contacts activate and follow host layer interfaces, but regionally, sills cut the stratigraphy at a low angle. The sills cut and are cut by reverse faults (1−3 m displacement) and related fractures that accommodate horizontal shortening. Minor sill networks resemble extension vein meshes and indicate that horizontal and inclined geometries were formed during coaxial horizontal shortening and vertical thickening. Although sills elsewhere may be related to mechanical layering during tectonic quiescence, our mechanical models show that the observed San Rafael subvolcanic field geometries are favored in the upper crust during mild horizontal shortening. We propose that sill geometry provides an indication of regional stress states during emplacement, and not all sill geometry is a response to bedding. Constraining sill geometry may therefore present a useful tool in plate-tectonic studies.

First ground-based 200-μm observing with THUMPER on JCMT — sky characterization and planet maps

We present observations that were carried out with the Two HUndred Micron PhotometER (THUMPER) mounted on the James Clerk Maxwell Telescope (JCMT) in Hawaii, at a wavelength of 200 ?m (frequency 1.5 THz). The observations utilize a small atmospheric window that opens up at this wavelength under very dry conditions at high-altitude observing sites. The atmosphere was calibrated using the sky-dipping method and a relation was established between the optical depth, ?, at 1.5 THz and that at 225 GHz: ?1.5 THz= (95 ± 10) ×?225 GHz. Mars and Jupiter were mapped from the ground at this wavelength for the first time, and the system characteristics measured. A noise-equivalent flux density (NEFD) of ? 65 ± 10 Jy (1? 1s) was measured for the THUMPER–JCMT combination, consistent with predictions based upon our laboratory measurements. The main beam resolution of 14 arcsec was confirmed and an extended error beam detected at roughly two-thirds of the magnitude of the main beam. Measurements of the Sun allow us to estimate that the fraction of the power in the main beam is ?15 per cent, consistent with predictions based on modelling the dish surface accuracy. It is therefore shown that the sky over Mauna Kea is suitable for astronomy at this wavelength under the best conditions. However, higher or drier sites should have a larger number of useable nights per year.

THUMPER: a 200-μm photometer for ground-based astronomy

Atmospheric modelling predicts that a window at 200-μm occurs under very dry conditions at high altitude sites. The transmission can reach up to 30 % in the driest conditions, but also exists for as many as 80 nights per year at Mauna Kea. A 200-μm photometer, THUMPER, is currently under construction at Cardiff University for use at the JCMT to exploit this atmospheric window. THUMPER consists of a seven-element hexagonal array of stressed Ge:Ga photoconductors cooled to liquid helium temperature. Initial laboratory testing suggest an NEFD of (formula available in paper)should be possible, under conditions of 0.5-mm pwv. A dichroic splits the beam between SCUBA and THUMPER, allowing simultaneous observations with THUMPER effectively acting as a third SCUBA array. Photometric measurements at 200-μm, in conjunction with SCUBA, will provide valuable information on cold dust sources in the temperature range 10 to 50 K. Since SCUBA fails to sample the peak of the Planck function at these temperatures, it is not possible to differentiate between temperature and density variations across a source using SCUBA data alone. THUMPER will provide these additional data at the same spatial resolution as SCUBA. This will provide an unprecedented combination of wavelength coverage and resolution when imaging sources such as protostars and pre-stellar cores.

Dike propagation and magma flow in a glassy rhyolite dike: A structural and kinematic analysis

Exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems. We use a 5−14-m-thick flow-banded rhyolite dike in Arran (Scotland) to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow, based on mesoscale kinematic indicators. The dike has concave-inward dike-margin segments with plumose-like structures that record vertical and horizontal propagation of lobes, which inflated and linked to form a through-going sheet. Devitrified rhyolite zones at the dike margins show gentle to open folds. In contrast, glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal, curvilinear folds and sheath folds that record sustained progressive deformation. The inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions. In the dike center, fold axes plunge 27°NE along the dike, and parallel to elongation lineations. Combined with shear sense indicators (σ- and δ-objects, sheared vesicles, and asymmetric folds), these features indicate that magma flow was obliquely upward, to the southwest, and locally ≤60° to the propagation direction of the dike. The distribution of structures within the rhyolite indicates local accretion of the (now) devitrified material to the margins, with localization of flow into the center of the dike. We find that the initial magma flow direction was controlled by fracture propagation and interaction, with the subsequent flow record controlled by accretion and flow localization in the conduit. This study demonstrates that analysis of mesoscopic structural and kinematic features (several of which have not previously been reported from dikes) is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes.

Excess 180 W in IIAB iron meteorites: Identification of cosmogenic, radiogenic, and nucleosynthetic components

The origin of 180W excesses in iron meteorites has been a recently debated topic. Here, a suite of IIAB iron meteorites was studied in order to accurately determine the contribution from galactic cosmic rays (GCR) and from potential decay of 184Os to measured excesses in the minor isotope 180W. In addition to W isotopes, trace element concentrations (Re, Os, Ir, Pt, W) were determined on the same samples, as well as their cosmic ray exposure ages, using 36Cl-36Ar systematics. These data were used in combination with an improved model of GCR effects on W isotopes to correct effects resulting from neutron capture and spallation reactions. After these corrections, the residual 180W excesses correlate with Os/W ratios and indicate a clear contribution from 184Os decay. A newly derived decay constant is equivalent to a half-life for 184Os of (3.38 ± 2.13) × 1013 a. Furthermore, when the data are plotted on an Os-W isochron diagram, the intercept (ε 180Wi = 0.63 ± 0.35) reveals that the IIAB parent body was characterized by a small initial nucleosynthetic excess in 180W upon which radiogenic and GCR effects were superimposed. This is the first cogent evidence for p-process variability in W isotopes in early Solar System material.