Katherine A. Daniels

Dyke propagation and sill formation in a compressive tectonic environment

Sills could potentially form as a result of dykes modifying their trajectory in response to remote tectonic compression. Here, we use analogue experiments to investigate how a buoyant vertical dyke adjusts its trajectory to a compressive remote stress to form a sill, and over which vertical distance this sill formation does occur. Our investigation is restricted to an intrusion propagating through a homogeneous solid, which enables us to identify the characteristic length-scale over which a dyke responds to remote stress compression, independently of the presence of crustal layers. The experiments involve the injection of air in a gelatine solid that experiences lateral deviatoric compression. The response of the buoyant air crack to the compressive stress in not instantaneous but operates over some distance. An important observation is that some cracks reach the surface despite the compressive environment. Dyke-to-sill rotation occurs only for large compressive stress or small effective buoyancy. Dimensional analysis shows that the length-scale over which this rotation takes place increases exponentially with the ratio of crack effective buoyancy to horizontal compressive stress. Up-scaled to geological conditions, our analysis indicates that a dyke-to-sill transition in response to tectonic compression in homogeneous rocks cannot occur over less than two hundred meters and would need several kilometers in most cases. This is typically greater than the average thickness of lithological units, which supports the idea that crustal heterogeneities play an important role in determining the fate of dykes and in controlling where sills could form.

The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

We document the shape of dikes from well exposed field locations in the Isle of Rum, Scotland, 14 and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the 15 Helam kimberlite dikes and have a smaller length to thickness ratio (~100:1 Isle of Rum, 16 ~1000:1 Helam Mine). We compare dike thickness field measurements with the geometry 17 predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic 19 theory, being too thick at the dike ends and too narrow in the middle. Our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength. Dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike’s tapering edges preventing its closure as magma pressure declines 23 during emplacement. The permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement

An experimental investigation of dyke injection under regional extensional stress

Dyke injection is a fundamental process of magma transport in the crust, occurring in all tectonic settings. The effect of extensional stress regimes on dyke injections is particularly important to understanding a wide spectrum of processes including continental rifting and volcanic activity. Yet dyke injection in extensional regimes has been relatively understudied. In addition, the effect of dyke-dyke interaction modifying the surrounding stress field and leading to dyke rotation about the vertical axis has not been addressed. We present the results from 23 laboratory analogue experiments investigating lateral dyke injections in a remote extensional stress field. This study is unique in that it addresses the effect of both extension and dyke-dyke interaction on the lateral propagation and rotation of dykes. The experiments study the interrelationship between successive lateral dyke injections by examining dyke injection thickness, injection spacing, injection orientation, extension, and structural relationship. A relationship between the rotation angle between two successive intrusions and the distance separating them under given extensional stress conditions is established. The rotation angle depends on two dimensionless numbers: the ratio of fluid overpressure of the first injection and remote tensile stress, and the ratio of the spacing between injections and the height of the first intrusion. The experiments show how the stress field is perturbed by an intrusion and how the remote stress field is locally relieved by this intrusion. The results show furthermore that measuring or estimating the rotation angles between successive intrusions within rift zones allows the spatial distribution of these intrusions to be estimated. In the case of the actively spreading Red Sea rift in Afar, Ethiopia, we find that the vast majority of the dykes are predicted to intrude within 10 km of each other and most frequently between 4 and 5 km, in good agreement with independent geophysical observations.

The formation of columnar joints produced by cooling in basalt at Staffa, Scotland

Columnar jointing in basaltic lava flows on the island of Staffa, NW Scotland, was studied using a combination of field mapping and measurement of column dimensions, sample petrology and measurements of plagioclase crystal size distributions (CSDs) interpreted using theoretical models of cooling. Four different lava flow units were measured, and column ordering was assessed using the hexagonality index and relative standard deviations of column side length, top area and internal angle. Upper and lower colonnades consist of dominantly 5-, 6- and 7-sided columns, with a hexagonality index value very similar to that of Giant’s Causeway and other basaltic columnar jointed localities. CSDs from samples at different heights within one colonnade were used to infer the propagation of the solidus isotherm, which was consistent with a convective cooling mechanism within the colonnade interior. Sample petrology and CSD measurements suggest that entablature can form both by the interaction of propagating joint sets and flooding of the flow surface by water, and the most widely exposed unit on Staffa shows evidence of both mechanisms operating on the same flow. Crystal size distribution measurements can provide a useful tool for field interpretation of lava flow cooling mechanisms.

The current state of CCS: Ongoing research at the University of Cambridge with application to the UK policy framework

The Earths climate is changing and the release of carbon dioxide (CO2) is recognised as the principal cause. To meet legally binding targets, UK GHG emissions need to be cut by at least 80% of the 1990 levels by 2050. With an increase in future fossil fuel use, Carbon Capture and Storage (CCS) is the only method of meeting these targets. Some key challenges face the deployment of CCS including cost, uncertainty of CCS deployment, the risks of long-term CO2 storage, public communication and scale. Research at the University of Cambridge is resolving these issues and assisting the deployment of CCS technology. The right regulatory framework also needs to be set so that the technology is commercially deployed. The current UK policy framework for CCS is outlined in this document and the immediate barriers to deployment are highlighted. The ongoing CCS research taking place primarily at the University of Cambridge is described. There are many steps that need to be taken if CCS deployment is to ultimately succeed; this document attempts to highlight these steps and address them.

A simple model predicts energetically optimised jumping in dogs

ABSTRACT It is generally accepted that animals move in a way that minimises energy use during regular gait and there is evidence that the principle might extend more generally to locomotor behaviour and manoeuvres. Jumping during locomotion is a useful manoeuvre that contributes to the versatility of legged locomotion and is within the repertoire of many terrestrial animals. We describe a simple ballistic model that can be used to identify a single unique trajectory of the bodys centre of mass that minimises the mechanical work to initiate a jump, regardless of the approach velocity or take-off position. The model was used to show that domestic dogs (Canis lupus familiaris) demonstrate complex anticipatory control of locomotor behaviour by systematically using jump trajectories close to those that minimised the mechanical energy of jumps over raised obstacles. It is unclear how the dogs acquired the complex perception and control necessary to exhibit the observed behaviour. The model may be used to investigate whether animals adopt energetically optimised behaviour in any similarly constrained ballistic task. Summary: A simple model of jumping mechanics is used to show that domestic dogs use complex anticipatory control to systematically choose jump trajectories close to those that minimise mechanical energy.

Whole-body biomechanical differences between limbs exist 9 months after ACL reconstruction across jump/landing tasks

Previous studies examining jump tasks after anterior cruciate ligament reconstruction (ACLR) have focused on performance measures without examining joint kinematic and kinetic variables. The aim of this study was to identify differences in biomechanical and performance measures between limbs across tests 9 months after surgery.

Biomechanical but not timed performance asymmetries persist between limbs 9 months after ACL reconstruction during planned and unplanned change of direction

Whilst anterior cruciate ligament injury commonly occurs during change of direction (CoD) tasks, there is little research on how athletes execute CoD after anterior cruciate ligament reconstruction (ACLR). The aims of this study were to determine between-limb and between-test differences in performance (time) and joint kinematics and kinetics during planned and unplanned CoD. One hundred and fifty-six male subjects carried out 90° maximal effort, planned and unplanned CoD tests in a 3D motion capture laboratory 9 months after ACLR. Statistical parametric mapping (2 × 2 ANOVA; limb × test) was used to identify differences in CoD time and biomechanical measures between limbs and between tests. There was no interaction effect but a main effect for limb and task. There was no between-limb difference in the time to complete both CoD tests. Between-limb differences were found for internal knee valgus moment, knee internal rotation and flexion angle, knee extension and external rotation moment and ankle external rotation moment with lower values on the ACLR side (effect size 0.72-0.5). Between test differences were found with less contralateral pelvis rotation, distance from centre of mass to the ankle in frontal plane, posterior ground reaction force and greater hip abduction during the unplanned CoD (effect size 0.75-0.5). Findings demonstrated that kinematic and kinetic differences between limbs are evident during both CoD tests 9 months after surgery, despite no statistical differences in performance time. Biomechanical differences between tests were found in variables, which have previously been associated with ACL injury mechanism during unplanned CoD.

Fixation Prediction and Visual Priority Maps for Biped Locomotion

This paper presents an analysis of the low-level features and key spatial points used by humans during locomotion over diverse types of terrain. Although, a number of methods for creating saliency maps and task-dependent approaches have been proposed to estimate the areas of an image that attract human attention, none of these can straightforwardly be applied to sequences captured during locomotion, which contain dynamic content derived from a moving viewpoint. We used a novel learning-based method for creating a visual priority map informed by human eye tracking data. Our proposed priority map is created based on two fixation types: first exploiting the observation that humans search for safe foot placement and second that they observe the edges of a path as a guide to safe traversal of the terrain. Texture features and the difference between them, observed at the region around an eye position, are employed within a support vector machine to create a visual priority map for biped locomotion. The results show that our proposed method outperforms the state-of-the-art, particularly for more complex terrains, where achieving smooth locomotion needs more attention on the traversing path.