CRACKING CANISP: DEEP VOID EVOLUTION DURING ANCIENT EARTHQUAKES

Image: Yerko Espinoza/shutterstock.com

he —a suite of a range of depths throughout the upper Kit Hardman Precambrian metamorphic rocks crust. The Canisp shear zone (CSZ), which unravels void outcropping in the NW is part of the Lewisian Complex, highlands and has long-held the represents rocks that were deformed at formation and T interest of many in the geological depths above and below the brittle-ductile community. So much so that you could be transition zone. So, I ventured to the collapse during forgiven for thinking there is little more to Scottish Highlands to investigate the CSZ, learn from this region of the North West which is exposed in Bay and seismogenic slip Highlands. But the truth is, as our Clachtoll. These rocks represent the oldest understanding of sub-surface processes and deepest of all my study areas. in the Canisp shear develops and our analytical tools and Specifically, I wanted to assess how fault methods improve, we continue to uncover voids develop, fill and collapse in zone, North West a wealth of knowledge hidden in those crystalline basement that deformed at well-weathered outcrops. depth. Rocks in the CSZ have undergone a The processes that occur during long and often intense deformation earthquake slip at different depths are still history that includes a phase of coeval poorly known. So, in the hope of gaining seismogenic-fluid circulation, frictional new insights, I am investigating a variety melting, hydrothermal mineralisation and of different rock outcrops to assess how complex brecciation. Using detailed fracture-hosted cavities, or fault voids, mapping, microstructural characterisation, develop and evolve during deformation at kinematic analyses and fracture-attribute

10 | FEBRUARY 2019 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | FEBRUARY 2019 | 11 Annotated aerial photo of the north side of Achmelvich Bay. Taken with a DJI Mavic pro UAV. Annotations show the trace of the largest of the mapped fracture corridors in the Laxfordian brittle shear zone

analyses, I wanted to develop a better fracture attribute and topological data, Lewisian Complex in the North West understanding of the processes controlling and give an excellent view of the Highlands has been debated in one form fault-void evolution. contemporaneous interlinked systems of or another for over 100 years. There is no I worked together with Bob faults, fractures and fills of the CSZ. consensus on the absolute timings of the Holdsworth, Eddie Dempsey, Ken various deformational and metamorphic McCaffrey and Tom Utley to use the latest GEOLOGICAL HISTORY episodes recorded in these ancient rocks photogrammetry techniques to develop Wikipedia summarises the Lewisian (Butler, Geol. Soc. London SP 2010), but terrain and perspective-corrected fracture Complex as “…a suite of Precambrian here I present my interpretation of the and outcrop maps from aerial imagery. We metamorphic rocks that outcrop in the geological and structural history of collected these data using a newly- northwestern part of Scotland, forming Achmelvich, based on my field acquired unmanned aerial vehicle (UAV) part of the Hebridean Terrane and the observations and the published literature. quad-copter. Despite the predictable North Atlantic Craton. These rocks are of The earliest discernible geological problems flying a brand-new drone for the Archaean and Paleoproterozoic age, events in the CSZ of the Lewisian first time on the coast in ranging from 3.0–1.7 Ga and form the Complex are shallowly-dipping foliations, September, the resulting imagery and basement on which the Torridonian and small intrafolial folds and streaky textures, models are exceptional. The Moine Supergroup sediments were or schlieren, around ultramafic pods or photogrammetric techniques enable us to deposited”. Whilst undoubtedly an boudins. These are best seen on the develop ultra-high-resolution maps and over-simplification, this is an excellent northern limb of the monocline 3D virtual outcrop models, which, when starting point to summarise 3 billion years (see illustration Annotated aerial photo of combined with detailed field observations of Earth history in a few hundred words. the north side of Achmelvich Bay’) in the and field maps, allow easy collection of The Precambrian geology of the beautiful water-washed exposures either

10 | FEBRUARY 2019 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | FEBRUARY 2019 | 11 side of Achmelvich beach. These dykes 2.4 billion years ago. Thus, the features of the Inverian fabric are the granulite metamorphic textures are orogeny occurred somewhere in the development of tight minor folds and products of the Badcallian orogeny, 100-million-year window between 2.5 a regional NW-SE trending foliation. formed about 2.5 billion years ago and 2.4 billion years ago (Park, The Inverian orogeny is also (Park, Scottish J. Geol. 1970), at great Scottish J. Geol. 1970). The Inverian responsible for the Lochinver depths under extremely high here is expressed by the development monocline and regional amphibolite temperatures and pressures. of a strong sub-vertical foliation and facies metamorphism (Attfield,Geol. The next major event was the steeply-plunging mineral lineation Soc. London SP 1987). This is a Inverian orogeny, which is post-dated forming the 1.5 to 2-km-wide ductile lower-temperature and pressure by the emplacement of the many CSZ immediately north of the metamorphism compared to the NW-SE trending Scourie metadolerite Lochinver monocline. Characteristic Badcallian event, suggesting a progressive regional uplift toward the Plan view of the brittle-ductile transition at about 15 Lewisian Gneiss. Well-developed km depth. schlieren Scourie dykes are spectacularly textures were exposed in the southern and northern formed during the Badcallian headlands of Achmelvich Bay. These orogeny. Lens cap dykes cross-cut the Badcallian for scale gneissic foliation fabric, but are reactivated along the margins in brittle-ductile centimetre-scale shear zones related to the next orogenic phase, the Laxfordian. The 1.4 to 1.7-billion-year-old Laxfordian event has at least two distinct phases of deformation that track events across the brittle-ductile transition and into the earthquake realm. An earlier greenschist- amphibolite facies ductile fabric Annotated map forms the centre of the CSZ, where it of Achmelvich intensifies the pre-existing Inverian Bay. Map shows the collected fabric in a zone 0.5 km wide. This is structural data, associated with a shallowly plunging field photographs mineral lineation, swarms of tight to and mapped boundaries isoclinal sheath folds and associated dextral shear criteria. A ‘late- Laxfordian’ event involving widespread brittle reactivation of the shear zone foliation sees a reversal to sinistral shearing. In the CSZ, a series of interlinked high-intensity fracture corridors were formed. 2 main fracture sets are recognized: sinistral foliation-parallel faults and dextral- extensional foliation-perpendicular faults. These coeval fractures are locally associated with significant Fault breccias volumes of frictional melt— and cataclasites. pseudotachylite—which is injected (right page) into neighbouring dilational voids A-D, collection of epidote and that follow the foliation- quartz mineralised perpendicular fault set, often fault breccias intermixed with well-developed fault and cataclasites from foliation breccias in zones up to 1 m wide and perpendicular several metres long. The fractures are faults, formed easily picked-out as they are stained a during the late-Laxfordian deep red colour by the percolation of tectonic episode iron-rich fluids. This Laxfordian

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transition from ductile-to-brittle deformation shows a continued shallowing/uplift history as the rocks cross the brittle-ductile transition at around 15 km. The final phase of deformation comprises a series of tensile/dilational faults and fractures that opened close to the surface and were invaded by Stoer Group unlithified sediment. The best locality to observe this is in a small bay between A’Chlach Thuill and Rubha Leumair headlands. These fractures record the final arrival of the presently exposed Lewisian complex at the surface about 1.2 billion years ago, the age of the Stoer Group, where the gneiss has lain ever since (except for a few shallow burials and exhumation events during the last billion years). FIELD DAYS Like many undergraduate experiences of the NW Highlands, my field excursion was met with biblical winds and sideways rain, which started and ended on my first and last days in the field. Intermittent breaks in wind and rain allowed for brief Clachnessie and along the Loch dextral-extensional foliation- flights of the UAV to collect photos for the fault. These outcrops featured very similar perpendicular faults are thought to have detailed digital maps, but most work epidote/quartz breccias, kinematics, and formed simultaneously during repeated, continued regardless of weather, deformational styles. I’m still investigating and likely seismogenic, shear events alternating between notebooks to allow the hypothesised “Late-Laxfordian” event under a regional ENE-WSW compressive time for the other to fully dry. During the and, over the next few years, these stress. Despite being coeval, detailed heaviest downpour, fieldwork was observations will be compared and observations show that the fault-sets are “briefly” suspended to seek shelter and contrasted to observations and data remarkably different. The foliation- prevent the loss of collected work. This collected from equivalent studies of parallel faults show significant offsets of method of stop-start fieldwork was almost fracture networks with fault-void fills several metres, with locally large volumes successful, apart from the third day, when formed across a broad range of of generated frictional melt and the I watched the central half of my palaeodepths; ranging from the near development of small drag folds in the meticulously drawn, detailed field map surface (Brixham SW England, Rona wall rocks. The foliation-perpendicular rip from my mapping board and Ridge West of Shetland UKCS, Calabria, faults show less slip, but have a much gracefully fly away across the North Italy), to other deep crustal settings larger damage zone and complex shapes. Minch, evidently heading back to the (Adamello massif in Italy) as part of my Conceivably this makes sense: if I were to home of the Lewisian Gneiss on the Isle of larger PhD research goals. shear a block of wood it would be far Lewis. The fieldwork was mostly solitary easier to fault it “with the grain” as except for a sea-otter, which followed me VOIDS, FILLS AND FLUIDS opposed to “against the grain”, and the at a distance between coastal outcrops, My core aim is to assess the influence of resultant damage to the wood would be and would observe with apparent interest reactivation of the ductile CSZ fabrics on different in either case. In addition, the my attempts at recording structural faulting and fault void development. smaller foliation-perpendicular faults measurements at some of the less- I want to test a hypothesis that the fault commonly connect the foliation-parallel accessible outcrops. voids and fills formed due to the rapid faults like the rungs on a wonky ladder. Given some of the meteorological- opening and partial collapse of large As they grow, the ladder fill collapses into challenges, a return trip to the NW dilatant voids during ancient earthquakes. a dilatant cavity to form a heavily Highlands took place in the spring of What mechanisms could open and fill iron-stained, clast-supported breccia. The 2018, where I verified and tidied-up some fault voids at large depths and great chaotic assemblage of wall-rock clasts of the field observations and collected pressure, and what are the impacts for shows no uniform rotation direction and further data. I was also able to tie in some short- and long-term fluid flow through no evidence of attrition. This implies that “Late-Laxfordian” field observations the crystalline basement-rock? the cavities opened enough to allow wall made in Achmelvich, with outcrops in The sinistral foliation-parallel and rock collapse, with free rotation and

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displacement of the clasts, before the gaps between them. During opening, the observations I made at the CSZ cannot cavity partially collapsed, trapping the cavity would be under a negative settle the debate, but they can tell us that breccia clasts in place and propping open hydrostatic pressure and would draw in at this specific location we have coseismic the fault cavity. any fluid, bits of wall rock or frictional percolation of iron-rich fluids through a We initially thought that such a melt. When a cavity collapsed, larger clasts deep-seated fault network in a crystalline sequence of events might be caused by would be held in place, propping open the basement rock, driven by the energetic high fluid pressure widening the fracture cavity, but anything mobile, such as fluid formation and collapse of fracture cavities aperture, before fluid pressure dropped or any suspensions, would be driven out, at significant depth (about 10 km or more). and the cavity closed. But we have since circulating fluid (and perhaps melt) The fracture cavities here act as fluid discarded this idea. Instead, our work throughout the adjacent fracture network. pathways not just in the short term, but in shows that the opening and closing of The movement of these blocks is driven, the long term, providing a conduit for fracture cavities was controlled by we suggest, by seismic activity, and what fluid to circulate through the deep seismogenic movement along the larger we observe is a greater proportion of basement over long geological timescales. foliation-parallel faults that host friction frictional melt along the foliation-parallel This is counterintuitive because it is often melts with complex flow-lineation faults that surround these tensile fractures, thought that any faults or fractures in the patterns. It appears that as movement compared to those that are not associated deep crust will immediately close-up, along the foliation-parallel faults occurred, with foliation-normal ladder fractures. offering only transient short-lived fluid blocks of the adjacent wall rocks moved in One possible reason for this is that during flux. Yet, in Achmelvich, although the the same direction, but at different times/ a seismic event, frictional melt is generated seismic activity is transient, we see speeds, causing them to move apart from along the weak foliation-parallel faults, evidence that fracture cavities never and collide with one another, rapidly which lubricates the faults and facilitates completely close-up, instead being held opening and closing the foliation-normal the block shunting process. open by the infilling breccia material. fracture cavities forming blocks. This The idea of seismically-driven fluids is This allows for the distribution of fluids model is conceptually similar to shunting not new. But, whilst we see many across a large volume of rock deep in the carriages of a train, with each of the examples of fluids being expelled at the Earth’s crust. carriages represented by adjacent fault surface during earthquakes, the idea of The mineralogy we observe in thin blocks, bound by the different types of seismic “pumping” at depth remains section is surprisingly diverse, with fractures. Thus, if one carriage were contentious (Sibson et al., J. Geol. Soc. complex secondary mineral interactions. shunted, it would move away from one London 1975; Muir-Wood & King, J. These secondary minerals include: adjacent carriage, and collide with the next Geophys. Res. 1993; Matthäi, J. prehnite, chlorite, epidote, clinozoisite, down the line, closing and opening the Geochemical Explor. 2003). The pyrite, quartz and chalcedony. In addition

Major fractures. A, Orthorectified aerial image of CSZ outcrop [58°10’33.3”N 5°18’17.1”W], Psuedotachylite. In thin section, cross section (complete with major fractures traced and manual structural measurements. B, Structural with “wing-crack injections”), and plane view (showing flow measurements isolated, with an equal area rose plot of fracture angles (equal area, lineation patterns of injected melt); fingers for scale n=1293), and ITX fracture connectivity plot (calculated using Dave Healey’s FracPaQ module for MATLAB) 14 | FEBRUARY 2019 | WWW.GEOLSOC.ORG.UK/GEOSCIENTIST WWW.GEOLSOC.ORG.UK/GEOSCIENTIST | FEBRUARY 2019 | 15 GEOSCIENTIST FEATURE

to veins, these secondary minerals compressive stress regime, with a major ACKNOWLEDGEMENTS frequently form both matrix and clast ENE-WSW compression and minor My thanks to the Geological Society for their support material in fault breccias, cataclasites and SSE-NNW extension. The ratio of via the Annie Greenly Fund. Thanks, too, to my gouge (often with injection textures). The principle stress differences was consistent supervisory team, Bob Holdsworth, Eddie Dempsey, sections also show locally significant across all Late-Laxfordian structures at Ken McCaffrey and Tom Utley. volumes of pseudotachylite, with lateral around 0.30. The fracture topology and injections. Many are bubbly—consistent attribute analyses show that these fault FURTHER READING with rapid changes in pressure, perhaps networks are well-connected and ◆ Park, R.G. (1970) Observations on Lewisian associated with the seismic cycle and the orientated, with distinct fault sets Chronology. Scottish J. Geol. 6, 379-399. proposed ‘block shunting’ process. The separated by length. ◆ Butler , R.W.H. (2010) The Geological Structure lack of universal cross-cutting of the North-West Highlands of Scotland relationships between the different THE UPSHOT – revisited: Peach et al. 100 years on. In secondary minerals and/or the From this work, we can draw some Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. Law, R.D., et al. psuedotachylite demonstrates the iterant, significant conclusions that have real- (eds.) Geol. Soc. London, Spec. Publ. 335, 7-27. cyclical and broadly synchronous nature world applications. We describe a ◆ Attfield, P. (1987) The structural history of the of these mineralisation episodes. mechanism by which cm-scale cavities Canisp Shear Zone. Geol. Soc. London, Spec. Evidently during the Late-Laxfordian have been generated and held open in an Publ. 27, 165-173. deformation there is a whole host of otherwise impermeable crystalline rock at ◆ Sibson, R.H., et al. (1975) Seismic pumping--a different fluid chemistries, local stress extreme depths in the upper crust. hydrothermal fluid transport mechanism.J. conditions and brecciated material Elsewhere similar cavities may be filled Geol. Soc. London 131, 653–659 migrating through these faults and with economic minerals or hydrocarbons ◆ Muir -Wood, R. & King, G.C.P. (1993) fractures. that lie unconsidered as an exploration Hydrological signatures of earthquake strain. J. Geophys. Res. Solid Earth 98, 22,035–22,068 Statistical work was carried out in two opportunity due to their high formation ◆ Matthäi, S.K. (2003) Fluid flow and (reactive) fronts: analysis of the structural slip-data depths. Going forward, a better transport in fractured and faulted rock. J. to generate a palaeostress field for the understanding of the internal-architecture Geochemical Explor. 7879, 179–182. Late-Laxfordian-aged deformation, and of fault and fracture cavities throughout ◆ Healy , D. et al. (2017) FracPaQ: A MATLABTM topological and fracture attribute analysis the upper-crust can guide us to better toolbox for the quantification of fracture of the ortho-rectified drone images of key utilise these resources. ◆ patterns. doi:10.1016/j.jsg.2016.12.003 outcrops. The palaeostress inversion was ◆ WinT ensor: http://damiendelvaux.be/Tensor/ conducted using WinTensor (by Kit Hardman is a PhD student at Durham University; WinTensor/win-tensor.html

D. Delvaux) and yielded a strike-slip e-mail: [email protected]

Conceptual sketch of the “train-carriage” model. A, Demonstrates opening and partial collapse of fault voids in N-S tensile fractures. Voids rapidly open and dilate, leading to fluid, melt and fault rock infill followed by collapse and partial void closure. Wall rock clasts and fills act as props leading to the development of potentially long-lived fluid migration pathways. B, C, Also shown are field

photographs of tensile fractures filled with erratically rotated breccia clasts (phone and lens cap for scale) ▼

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