Tectonic Studies Group Shetland trip, June 2013
The Tectonic Studies Group Shetland field trip was held from the 24th to the 28th of June 2013. The field trip was organised by Natalie Farrell, currently a postgraduate at the University of Aberdeen. It was attended by 20 geologists (16 postgraduate students and 4 staff members) from 11 different Universities including Universities of Aberdeen, Bristol, Durham, Keele, Leeds, Liverpool, Oxford, Royal Holloway, Southampton, University College Dublin and University College London.
The field trip ran over 5 days investigating the tectonic history of Shetland by visiting localities throughout the Mainland, and the North Isles, focusing on a number of structural features including the Walls Boundary Fault – thought to be an extension of the Great Glen Fault, Shetland’s oldest rocks – the sheared basement gneisses in North Roe, the Shetland ophiolite and Funzie conglomerate on the islands of Unst and Fetlar and copper vein deposits in the south Mainland.
Details of 16 localities visited and photos of over 30 rock types observed are documented below with comments and thoughts from some of the field trip attendees.
Day 1 Brittle and Ductile Faulting
The first day of the TSG Shetland fieldtrip focussed on the Walls Boundary Fault Zone, a continental strike-slip system which is of particular interest to me as the subject of my PhD research is the Dead Sea Fault system, a fault of similar nature and scale. Our first two stops at Ollaberry Ness and Ness of Haggrister displayed excellent exposures of the Walls Boundary Fault Zone (Figure 1). Superb examples of both brittle and ductile deformation are visible at both localities, including fault gouges, breccias, folding and evidence of mylonitisation (Figure 2). At Lunnister, these two faulting styles are combined in a unit of brecciated mylonite, indicating polyphase deformation with brittle reactivation of earlier ductile faulting. Despite the weather’s best efforts at the start of the day, the rain passed, giving us chance to see evidence of low-temperature hydrothermal alteration of granites (Figure 3) and interesting structures formed by magma mixing (Figure 4) in two disused quarries west of the Walls Boundary Fault. The first day ended with sunshine and fish & chips, having been a fantastic start to a wonderful fieldtrip – it was great to have the opportunity to see some fantastic geology as part of a ‘group-led’ trip, the format of which allowed for some very interesting discussions with fellow geologists at many different stages of their research careers. Thanks to TSG and Natalie Farrell for organising an informative and inspiring fieldtrip!
Siân Evans is a second year PhD student at Durham University researching deformation processes along continental transform faults
Day 2 Basement Gneisses and Caledonian Orogeny
The opportunity to experience the outstanding geology and complex deformation expressed on the Shetland Islands was something that, as a structural geologist, could not be missed. I particularly enjoyed investigating the structures displayed at North Roe, where we walked through three shear zones seeing; Moine type ribboned gneiss, amphibolite facies gneiss and Lewisian gneiss. We were lucky to get good weather for this day and the walk took in some spectacular cliffs and views (Figure 5). Some other interesting features seen on Day 2 included large felsic dykes with a silica-rich spheroid texture (Figure 6 and 7). The quality of the outcrop allowed observation of the structures at a level of detail usually only confined to textbooks, which, as a soft rock geologist, has furthered my appreciation of how strain is expressed at deeper levels within the crust. The trip not only broadened my geological horizons, but also introduced me to both the Shetland Islands and fellow geologists with common interests (Figure 8). Thanks to Natalie Farrell for organising, and TSG for funding the trip.
Andrew Bladon is a third year PhD student at Keele University researching the structural evolution of a Paleogene continental rift-basin in northwest India
Day 3 Unst-Fetlar Ophiolite
The Shetland Islands are a geological theme park as they host a multitude of world class geological localities, with enough variety to interest all geological types. As a hard rock geologist, my favourite locality during the TSG field trip was viewing the ophiolite complex, which outcrops on the islands of Unst and Fetlar. The group examined outcrops from the structurally lowest rocks of the ophiolite to the uppermost kilometre of the mantle and overlying metadunite layer in very sunny weather on Day 3 (Figure 9). One cannot fail to be inspired by walking a section through the mantle and into the oceanic crust, and being able to place a finger on the petrological MOHO (Figure 10). The locality was also an important reminder of the importance of fluids in metamorphism, as large tracts of the peridotite were pervasively altered to various polymorphs of serpentine, fundamentally altering the rheology of the unit. In particular, the quarry outcrop by the Unst ferry terminal provided excellent exposure of the base of the ophiolite, and is a highly recommended field stop if you ever find yourself waiting for a ferry there (Figure 11)! Many thanks to Natalie Farrell and the TSG for organising and funding the trip.
Owen Weller is a third year PhD student at the University of Oxford researching the structural and metamorphic evolution of the Tibetan plateau.
Day 4 Strain analysis and the Flinn plot
During our trip to the Shetland Islands we visited many renowned localities where Derek Flinn worked. As a structural geologist, for me one of the most profound was the exposure of deformed Funzie conglomerate on Fetlar where Flinn developed his seminal concepts in strain analysis (Figure 12). In the course of my research I develop and apply advanced methods of strain analysis to crustal shear zones, so it was fascinating to examine the outcrops where the fundamental methods were developed. The large cobbles of the conglomerate and the similar clast-matrix rheologies make this an exceptional lithology for the study of strain and one where it is easy to visualise (Figure 13). We were also able to observe the variations in strain intensity and geometry approaching the base of the over-riding Shetland ophiolite – a world class outcrop. At the base of the ophiolite we also had the opportunity to observe deformation fabrics and shear sense indicators which, along with the Funzie conglomerate, provided a remarkably complete description of the kinematics of ophiolite emplacement. A cross section of these relationships has been constructed by Geopark Shetland as a ‘Geowall’ at the car park for Funzie Bay (Figure 14).
David Wallis is a third year PhD student at the University of Leeds researching methods for quantifying strain distribution across the Karakoram Fault Zone in the NW Himalaya
Day 5 Copper deposits
I always associated Shetland with ponies and puffins and never knew these islands were a true paradise if it comes to geology. With two of my fellow PhD-colleagues we decided to join the TSG trip to surprisingly Sunny Shetland. My PhD is on volcanic/magmatic/tectonic implications of porphyry copper deposits and Shetland. The first day a beautiful example of magma mixing was studied in the Northmavin plutonic complex. Another of my favorite outcrops was the Skaw granite with large feldspar phenocrysts showing intense shear structures formed during its transportation along a fault (Figure 15). We saved the last for best when we saw good outcrops of copper and pyrite deposits (Figure 16) and off course a lot of Puffins (Figure 17)! The group existed of students and staff with different backgrounds coming from all over the UK, which lead to interesting and educational discussions at each stop. An extra bonus was that the best geology required beautiful hikes along white beaches and through green fields, which were an absolute treat with the sunny weather. I would like to thank Natalie Farrell and TSG for organizing this amazing trip.
Marit van Zalinge, 2nd year PhD student at the University of Bristol researching the chronostratigraphic evolution of the central Andes and porphyry copper-related processes in Northern Chile
Shetland is a small archipelago on the edge of the north-west European continental shelf about 100 km north of Scotland and 300 km west of Bergen. Shetland consists partly of ancient sedimentary rocks which were metamorphosed and intruded by igneous rocks during the Caledonian orogeny and partly of sedimentary and volcanic rocks of the Old Red Sandstone (Devonian) age, which were laid down and folded during the final phases of that orogeny (Figure 18). It forms the a link between the Norwegian, Scottish and East Greenland portions of the Caledonian Orogenic Belt, as they were before continental drift.
Professor Derek Flinn and Shetland
The idea for this fieldtrip came about after years of hearing brilliant geo- adventure stories about the University of Liverpool geologist Derek Flinn. For me, Flinn encompassed my reasons for choosing to study geology; spending lots of time outdoors having rough and ready adventures and the opportunity for genuine creative scientific thinking. Only later during my masters degree that I came to appreciate the importance of Flinn’s research on assigning mathematical values to geometry and strain analysis.
Derek Flinn was born 1922. He left school in 1939 age 17 as he hadn’t got the grades to stay on for his higher school certificate. After school Flinn first worked in a bank and later joined the Marines. He joined the SBS unit as a sergeant (refusing a commission) in 1941 and was posted to Ceylon. After WWII, Flinn did a pre degree year at Chelsea then moved to Imperial College for his BSc. In the final year of his undergraduate degree, Flinn went to Shetland to do an independent study. This influenced him to go on and do a PhD on the geology of Shetland under the supervision of H.H. Read. From his PhD Flinn realised that Unst was too complicated to work on out of context of the surrounding geology and after his PhD Flinn, “started [mapping] at Fitful Head in 1953 and began to work north systematically and reached Out Stack north of Unst about 43 years later”. In 1953, Flinn was appointed lecturer at the University of Liverpool where he remained as an active researcher until shortly before his death in 2012.
During his career, Flinn published 58 papers specifically on the geology of Shetland as well as maps and memoirs together with many other papers arising from his Shetland work including his influential work on the Flinn Plot.
TSG Public outreach event at Lerwick Museum and Archives
On the evening of Day 4, TSG teamed up with Geopark Shetland to hold a public outreach lecture and poster evening ‘The Geological Fabric of Shetland’ (Figure 19) at the impressive Shetland Museum and Archives building in Lerwick. The event was well attended by around fifty local Shetlanders and tourists of a range of ages who enjoyed talks on; the Life and Research of geologist Derek Flinn, Fault Finding: Flinn’s contribution to Earthquake Geology and Unst and Fetlar: Showing the Strain as well as geological posters on current geology research by TSG PhD students. After the talks, children and adults formed a long queue to receive ‘top trumps’ style, Top Rocks and Minerals of Shetland geo-packs (designed by Natalie Farrell with funding from the University of Aberdeen) intended to encourage amateur geologists to get out in the field and investigate Shetland’s diverse geology for themselves (Figure 20 and 21).
The outreach event was positively received with requests for more geology lecture events including from William Moore at Scalloway Museum on Shetland who emailed to say, “Thanks for the splendid evening of ‘geology for all’ in Lerwick – a really interesting, well presented programme perfectly tailored for the heterogeneous audience that we were. Carry on the good work – and roll on your next visit!”
Figure 1. Walls boundary fault, Ollaberry, Mainland.
Figure 2. Folding around WBF, Ollaberry, Mainland.
Figure 3. Hydrothermally altered granite and serpentinised ultramafic rock and scapolite veining, Loch of Haggrister, Mainland.
Figure 4. Magma mixing structures at Mavis Grind, Mainland.
Figure 5. Shear zone cliffs, North Roe, North Mainland.
Figure 6. Felsic dykes, Wilgi Geos, North Mainland.
Figure 7. Interesting textures in silca-rich dykes, Wilgi Geos, North Mainland.
Figure 8. Group photo, North Roe, North Mainland
Figure 9. Lunch stop at Swarta Skerry, Unst.
Figure 10. Petrological Moho, Hagdale, Unst.
Figure 11. Sheared serpentinite and fibrous serpentinite, Belmont ferry terminal, South Unst.
Figure 12. Funzie Conglomerate, Funzie Bay, Fetlar.
Figure 13. Deformed elongate pebbles of quartzite, Funzie Bay, Fetlar.
Figure 14. Group photo on Unst-Fetlar ophiolite Geo-wall, Fetlar.
Figure 15. Feldspar phenocrysts in Skaw granite, North Unst.
Figure 16. Massive ore deposit, Garths Ness, South Mainland
Figure 17. Puffins, Sumburgh Head, South Mainland.
Figure 18. Shetland Geological map, compiled by Derek Flinn.
Figure 19. TSG lecture evening poster.
Figure 20. Top Rocks and Minerals of Shetland geo-packs, Lerwick.
Figure 21. Surprisingly lots of adults wanted a geo-pack as well as kids, Lerwick