Structural development and fluid flow along strike-slip faults in the Variscan belt of SW England

Applications are invited for a four-year PhD studentship. The studentship will start on 1 October 2022.

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Director of Studies Professor Mark Anderson, University of Plymouth.

2nd Supervisor Dr Michelle Harris, University of Plymouth, UK.

3rd Supervisor Dr Catherine Mottram, University of Portsmouth, UK.

4th Supervisor Dr David Peacock, University of Göttingen, Germany.

Applications are invited for a four-year PhD studentship as part of the Centre for Doctoral Training (CDT) in Geosciences and the Low Carbon Energy Transition. The studentship will start on 1 October 2022.

Project description

Global ambitions for net zero emissions by the second half of the 21st century require development and investment in ”green energy” such as geothermal. This has led to a resurgence of interest in the bedrock geology of SW England for a number of reasons, each of which requires an improved understanding of fluid flow in the sub-surface: (a) geothermal heat production, such as the United Downs Deep Geothermal Project (UDDGP) in Cornwall and other local low enthalpy geothermal projects; (b) increased global demand for critical metals, such as lithium for use in the production of rechargeable batteries; and (c) a UK-wide search for safe underground storage for waste products, be that carbon capture and sequestration (CCS) or nuclear waste (a Geological Disposal Facility, GDF). This project will focus on exploration for UK geothermal potential and has two principal aims, each of which will be cross-referenced with existing and potential geothermal projects across SW England: (1) constrain structural controls over fluid flow in the upper levels of the continental crust; (2) method development/enhancement for models of strike-slip deformation associated with the forelands of orogenic belts.

Faults and associated damage zones (Figure 1) are of specific interest because they are considered to exert significant control on sub-surface fluid flow. Two trends of strike-slip faults are present in both the Palaeozoic “Variscan basement” and in the Mesozoic “cover” rocks of SW England; a prominent NW-SE set and a less-prominent NE-SW set[1]. Some of these faults in the basement rocks show extensive mineralisation (the Cornubian orefield) that is thought to be related to infiltration of mineralising fluids along these structures, predominantly during the late Palaeozoic or early Mesozoic[2]. Faults with similar trends in the Mesozoic cover rocks[3], along with Cenozoic deposits in pull-apart basins, suggest further development during phases of post-Variscan deformation[4]. These strike-slip faults also represent potential conduits for fluid flow in the sub-surface at the present day. For example, the NW-SE trending Porthtowan Fault Zone is the target (at 4.5 km depth) for the UDDGP, where the associated natural fracture network (possibly with additional artificial stimulation) is being used to connect fluid flow between injector and production wells[5].

Figure 1: Example of small sinistral strike-slip fault with damage zone in Carboniferous low grade metasediments, Westward Ho!

It is presently unclear how much of this strike-slip faulting and associated fracturing occurred during the Variscan Orogeny and how much occurred during later deformation events. The Variscan mountain building event affected the southern parts of the UK through the late Devonian and Carboniferous periods[6], with the latest stages represented by emplacement of significant volumes of late- or post-orogenic granites in the earliest Permian[7]. E-W or ENE-WSW trending structures and associated mineralisation characterise deformation and fluid flow associated with the Variscan Orogeny. Although many of these features are cross-cut by the two sets of strike-slip fault, this relationship alone does not preclude significant Variscan strike-slip movement. Larger faults (hundreds of metres of displacement) tend to show evidence of Variscan and later deformation, whilst smaller ones are less well understood. Any particular strike-slip fault in the Palaeozoic rocks may therefore have: (a) only been active during the Variscan; (b) formed during the Variscan but reactivated later; or (c) formed entirely post-Variscan. There are currently no criteria for establishing which of these options is correct for any particular structure.

It is now generally acknowledged that careful, multi-scale structural analysis can produce reliable geometric and kinematic/dynamic models of brittle deformation histories spanning several hundreds of millions years, even in geologically complex and multiply-deformed terrains[8]. For example, this broad approach has been applied successfully by Prando et al. (2020)[9] to study cyclical deformation and associated fluid flow along strike-slip faults at Onkalo, the site in Finland chosen for the permanent disposal of high-grade radioactive waste.

The aim of this project is to date the movement history of the two dominant strike-slip fault sets, to see if different generations of faults have different associated damage zones and mineralisation, and to predict whether different ages of faults and damage zones have significance for fluid flow in the sub-surface. This information is an important aid for geothermal prospect evaluation, and for geological and reservoir modelling. Two research objectives are therefore particularly timely: (1) to establish more rigorous geometrical characteristics for these faults and associated damage zones; and (2) to better constrain the timing of movement(s) and structural development along these structures. Results will be considered within this regional perspective but also in the context of other multiply deformed terranes globally.

Figure 2: Simplified bedrock geology map of SW England, showing selected NW-SE trending faults and a selection of potential field sites for the proposed study.

Project work packages

1. Fieldwork in SW England on target faults and three-dimensional modelling of fault core and damage zones (Figure 2). 

This work will be used to understand the geometries of fracture networks in the fault zones and how these have evolved over time. Fieldwork will integrate, high resolution drone imagery of structures in both metasedimentary and granitic host rocks. The Sticklepath- Lustleigh Fault Zone, a structure known to have experienced multiple displacements[4], will be used as a control. The student will build and interpret 3D structural models of the fault zones. Dr Peacock is an expert in the safe and effective use of drones for geological studies[10] and will supervise these aspects of research. This project will develop virtual outcrop models of these structures for use by education and industry.

2. SEM-based microstructural analysis of samples from the fault zones to detect the grain-scale fluid pathways (e.g. micro-fractures, dilational grain boundaries) and quantify their density and geometry. 

This study will make use of the recently installed Focussed Ion Beam SEM (FIB-SEM) at Plymouth, which will enable a quantitative analysis of the three-dimensional geometry of grain boundaries. FIB- SEM studies have rapidly become a state-of-the-art technique for the visualization and quantification of the micro- and nano-scale porosity associated with fluid flow in crystalline rocks[11].

3. Multi-system radiometric dating of fault rocks. 

The relatively fast, low temperature upper crustal fluid flow and faulting processes are notoriously difficult to date as the minerals usually used to record geological time (such as zircon) typically do not crystallise or record any deformation under these conditions. Techniques have recently been developed for directly dating brittle structures[12-15], opening up a whole new realm of tectonic investigation in the upper crust. This project will use U-Pb and K-Ar dating techniques to determine absolute timings of faulting and fracturing processes for these structures for the first time.

(A) U-Pb dating will be used to analysis calcite slickenfibres and veins, ubiquitous in many brittlely deformed rocks, and most notably in the middle to upper Devonian of SW England (Figure 1). Recent cutting-edge developments in laser ablation geochronology techniques mean that it is possible to date calcite in-situ using the U-Pb method. High-precision Laser ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) techniques yield U-Pb ratios that allow calcite to be dated within ~± 3% uncertainty. Calcite commonly yields variable amounts of U and (common) Pb, meaning that careful characterisation of the geochemistry of crystals is vital for success in dating. Dr Harris has pioneered a method for multi-element ICP-MS analysis of carbonates developed during hydrothermal alteration and will supervise this element of the research. Calcite with as little as 100 ppb U has been successfully dated[12-15] and Dr Mottram is an expert in this area of research. U-Pb dating will be undertaken at the University of Portsmouth. The feasibility of this method has been successfully demonstrated in a sister PhD project quantifying deformation and fluid flow in the Bristol Channel Basin.

(B) K-Ar dating of authigenic clay formed in fault rocks will be employed using the workflow developed by Kemp et al. (2020)[16], whereby most recent fault activity is revealed by mineralogical and isotopic investigations of the finest particle size fractions of fault gouge from the fault core. Movement histories on reactivated faults will be evaluated by analysis of other phyllosilicate size fractions and from sampling multiple failure surfaces in the damage zones of individual faults. Novel, capillary-encapsulated X-ray diffraction analysis will be employed to facilitate the identification and quantification of the clay minerals in the various clay fractions. Sample preparation and XRD discrimination of clay fractions will be conducted with Simon Kemp at the British Geological Survey (BGS). K-Ar age determinations on the various clay fractions will be undertaken at the radiometric isotope facility of the Norwegian Geological Survey in Trondheim (NGU).


This project will, for the first time, allow the construction of time-constrained brittle evolutionary models for the strike-slip faults of SW England. The student will refine the criteria for understanding the ages and associated damage zones of brittle strike-slip fault zones in multiply-deformed terranes more generally. These results will have important implications for the development of UK- geothermal capabilities, critical for helping the nation achieve net-zero carbon emissions. The successful candidate will develop sought after skill sets in geochemistry and structural geology with the ability to integrate these various datasets to address societally and globally relevant scientific questions on fluid flow in the crust.

Eligibility

Applicants should have (at least) a first or upper second class honours degree in an appropriate subject and preferably a relevant MSc or MRes qualification.

The studentship is supported for four years and includes full home tuition fees plus a stipend (currently £15,609 per annum for 2021/22. Prevailing rates will apply for 2022/23 onwards). The studentship will only fully fund those applicants who are eligible for home fees with relevant qualifications. Applicants normally required to cover international fees will have to cover the difference between the home and the international tuition fee rates (approximately £12,670 per annum).

If you wish to discuss this project further informally, please contact Professor Mark Anderson.

Please see a list of supporting documents to upload with your application.

For more information on the admissions process contact Dr Michelle Harris.

The closing date for applications is 12 noon on Friday 28 January 2022.

Shortlisted candidates will be invited for interview week commencing 28 February/7 March 2022. We regret that we may not be able to respond to all applications. Applicants who have not received an offer of a place by 1 May 2022 should consider their application has been unsuccessful on this occasion.

 

References

1. DEARMAN, W.R. 1963. Wrench faulting in Cornwall and South Devon. Proceedings of the Geologists’ Association, 74, 265–287. https://doi.org/10.1016/S0016-7878(63)80023-1

2. SCRIVENER, R.C., DARBYSHIRE, D.P.F. & SHEPHERD, T.J. 1994. Timing and significance of cross- course mineralisation in SW England. Journal of the Geological Society, London, 151, 587–590. http://dx.doi.org/10.1144/gsjgs.151.4.0587

3. ALEXANDER, A.C., SHAIL, R.K. & LEVERIDGE, B.E. 2019. Late Paleozoic extensional reactivation of the Rheic–Rhenohercynian suture zone in SW England, the English Channel and Western Approaches. From: WILSON, R. W., HOUSEMAN, G. A.,MCCAFFREY, K. J. W., DORÉ, A. G. & BUITER, S. J. H. (eds) 2019. Fifty Years of the Wilson Cycle Concept in Plate Tectonics. Geological Society, London, Special Publications, 470, 353–373. https://doi.org/10.1144/SP470.19

4. HOLLOWAY, S. & CHADWICK, R.A. 1986. The Sticklepath–Lustleigh fault zone: tertiary sinistral reactivation of a Variscan dextral strike-slip fault. Journal of the Geological Society, London, 143, 447–452. http://dx.doi.org/10.1144/gsjgs.143.3.0447

5. REINECKER, J. GUTMANIS, J. FOXFORD, A. COTTON, L. DALBY, C. & LAW, R. 2021. Geothermal exploration and reservoir modelling of the United Downs deep geothermal project, Cornwall (UK). Geothermics, 97, 102226. https://doi.org/10.1016/j.geothermics.2021.102226

6. SHAIL, R.K. & LEVERIDGE, B.E. 2009. The Rhenohercynian passive margin of SW England: development, inversion and extensional reactivation. Comptes Rendus Geoscience, 341, 140– 155. https://doi.org/10.1016/j.crte.2008.11.002

7. CHESLEY, J.T., HALLADAY, A.N., SNEE, L.W., MEZGER, K., SHEPHERD, T.J. & SCRIVENER, R.C. 1993. Thermochronology of the Cornubian batholith in southwest England: implications for pluton emplacement and protracted hydrothermal mineralization. Geochimica et Cosmochimica Acta, 57, 1817–1835. https://doi.org/10.1016/0016-7037(93)90115-D

8. VIOLA, G., SCHEIBER, T., FREDIN, O., ZWINGMANN, H., MARGRETH, A. KNIES, J. 2016. Deconvoluting complex structural histories archived in brittle fault zones. Nature Communications, 7:13448. https://doi.org/10.1038/ncomms13448

9. PRANDO, F., MENEGON, L., ANDERSON, M.W., MARCHESINI, B., MATTILA, J. & VIOLA, G. 2020. Fluid-mediated, brittle-ductile deformation at seismogenic depth: Part II –stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland). Solid Earth, 11(2), 489-511. https://doi.org/10.5194/se-2019-142

10. PEACOCK, D.C.P. & CORKE, E. 2020. How to use a drone safely and effectively for geological studies. Geology Today, 6(4), 146-155. https://doi.org/10.1111/gto.12320

11. PLÜMPER, O., BOTAN, A., LOS, C., LIU, Y., MALTHE-SORENSSEN, A. & JAMTVEIT, B. 2017. Fluid- driven metamorphism of the continental crust governed by nanoscale fluid flow. Nature Geoscience, 10, 685-690. https://doi.org/10.1038/ngeo3009

12. ROBERTS, N.M.W. & WALKER, R.J. 2016. U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin. Geology, 44, 531- 534. https://doi.org/10.1130/G37868.1

13. NURIEL, P., WEINBERGER, R., KYLANDER-CLARK, A.R.C., HACKER, B.R., & CRADDOCK, J.P. 2017. The onset of the Dead Sea transform based on calcite age-strain analyses. Geology, 45, 587-590

14. GOODFELLOW, B.W., VIOLA, G., BINGEN, B., NURIEL, P., AND KYLANDER-CLARK, A.R.C. 2017. Palaeocene faulting in SE Sweden from U-Pb dating of slickenfibre calcite. Terra Nova, 29, 321– 328. https://doi.org/10.1111/ter.12280

15. PARRISH R.R., PARRISH C.M. AND LASALLE S. 2018. Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England. Journal of the Geological Society. 175, 425–42.

16. KEMP, S.J., GILLESPIE, M.R., LESLIE, G.A., ZWINGMANN, H., DIARMAD, S. & CAMPBELL, G. 2019. Clay mineral dating of displacement on the Sronlairig Fault: implications for Mesozoic and Cenozoic tectonic evolution in northern Scotland. Clay Minerals, 54 (2), 181–196. https://doi.org/10.1180/clm.2019.25