Evidence for Inherited Basement Faults in the London Basin

Accepted Manuscript Quarterly Journal of Engineering Geology and Hydrogeology Major faulting in London: Evidence for inherited basement faults in the London Basin Tom Morgan, Richard Ghail & James Lawrence DOI: https://doi.org/10.1144/qjegh2018-193 This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin Received 28 November 2018 Revised 28 February 2020 Accepted 29 April 2020 © 2020 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics When citing this article please include the DOI provided above. Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. The manuscript will undergo copyediting, typesetting and correction before it is published in its final form. 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Major faulting in London: Evidence for inherited basement faults in the London Basin Inherited faults in London Tom Morgan1*, Richard Ghail2, James Lawrence1 1Department of Civil and Environmental Engineering, Imperial College London, SW7 2AZ *Corresponding author (email: [email protected]) 2Department of Earth Sciences, Royal Holloway, University of London, TW20 0EX Abstract The near-surface of London is faulted; however, their locations, architecture and tectonic origins are broadly unknown. This presents serious issues for geotechnical engineering in London and has implications for our structural understanding of the London Basin. The region is a product of Alpine compression, yet it is unclear if these major faults were new Alpine shears or reactivated basement faults. Here the plausibility of Alpine reactivation and inheritance of basement faults in London is assessed through three investigations: analysing structures in the near- surface; mechanically assessing the feasibility of basement fault reactivation under Alpine stress conditions; and comparing inheritance mechanisms with observations in London and the Thames Estuary. Three major en échelon fault sets have been identified. These appear to have compartmentalised London’s near-surface geology and are interpreted to all be products of Alpine reactivation of underlying basement faults. Fault interaction and linkage is evidenced by complex zones of intense faulting identified by tunnelling projects. The role of new structure development in accommodating Alpine compression is considered to have been comparatively minor. The lack of major faulting in the Basin’s interior may reflect the competence of the underlying Anglo- Brabant Massif in restricting Alpine strains to its margins. Key Words: London; Faulting; Variscan; Alpine orogeny; Fault Reactivation; Inheritance; London Basin ACCEPTED MANUSCRIPT The London Basin (fig. 1) has historically been interpreted as a broad, shallow asymmetrical syncline formed by the flexural shortening of Late Cretaceous–Palaeogene sequences in response to Alpine compression. Despite this tectonic origin, the internal geology of the region is assumed to have undergone minimal deformation (Sherlock et al. 1962), with few major structures (of unclear origin) being recognised (fig. 2) (Ellison et al. 2004). Consequently, the region has traditionally been viewed as geologically simplistic, attracting limited attention. Contrary to this, anomalous structural and geological features were historically recognised or suspected within the Basin’s southern region (fig. 2) (Whitaker et al. 1872; Woodward 1909; Wooldridge 1923), with underlying Palaeozoic faults hypothesised as their origin (Boswell 1915; Wooldridge 1923; Wooldridge 1926). De Freitas (2009) and Royse et al. (2012) have recently revived the conversation about the London Basin’s unappreciated structural complexity by illustrating further geological structures indicative of structural inheritance, identified primarily through geotechnical engineering projects (fig. 2). The role of recurring basement fault activity in the development of the region is now strongly- suspected, as recognised elsewhere in southern Britain (e.g. Nemčok et al. 1995; Mansy et al. 2003; Westhead et al. 2018), raising new questions about how the London Basin formed in response to Alpine compression. This paper assesses the validity of whether major faults in the London region were inherited from the underlying basement by collating new and existing near-surface structural observations, analysing the mechanical feasibility of basement fault reactivation under Alpine stress conditions, and identifying potential inheritance mechanisms. The paper focuses primarily on the southern portion of the Basin, in particular London, due to the availability of data. Tectonic Framework The London Basin (fig. 1) comprises approximately 300 m of Cretaceous–Palaeogene sequences that unconformably rest on the Anglo–Brabant Massif, a shallow Palaeozoic basement comprised of Upper Devonian and Lower Palaeozoic sequences sited around an Ediacaran igneous core (Pharaoh 2018). The Variscan Front underlies the southern margin of the Basin (fig. 1), representing a major tectonic interface between the shallow basement of the Massif and the deeper Variscan basement beneath the inverted Weald Basin (to the south). A detailed overview of the tectonic history of southern Britain and its implications for the basement underlying the London Basin has been provided by Cosgrove et al. (2020). The region has experienced episodic tectonism throughout the Phanerozoic. The northern portion of the Massif was affected by the Caledonian orogeny (Pharaoh et al. 1987). Variscan fault sets (thrust and strike-slip pairs) are expected along the southern region of the Massif (fig. 1) due to its proximity to the Variscan Front; these are comparable to exposed Variscan faults in Pembrokeshire that are situated immediately north of the Front (Royse et al. 2012). Post-Variscan gravitational collapse and subsequent Mesozoic extension generated normal faults that can also be expected in the basement, having exploited favourably aligned pre-existing Variscan thrusts (fig. 3), as occurred elsewhere in southern Britain (Brooks et al. 1988). Throughout the Mesozoic the Massif remained a structural high (the London Platform) with only isolated evidence of submergence (Prestwich 1878; Owen 1971)ACCEPTED until the Aptian. Mesozoic extension ceased MANUSCRIPT by the Aptian, however, there is evidence for syn- depositional faulting affecting Early Cretaceous sediments on the southern margin of the Platform (Owen 1971). A transition to compression during the Late Cretaceous caused renewed syn- depositional faulting (Mortimore & Pomerol 1997; Kley & Voigt 2008; Mortimore et al. 2011) in response to the initiation of the Alpine Orogeny; with eventual uplift and partial-denudation of the White Chalk Group on the Platform by the end-Cretaceous. Deposition episodically resumed throughout the Late Palaeocene and Eocene, with further syn-depositional tectonism suspected (Ellison et al. 1996; Royse et al. 2012). Alpine compression culminated during the Late Eocene-Early Oligocene in response to the Pyrenean Orogeny (Parrish et al. 2018), deforming and partially- denuding the Platform’s Late Cretaceous–Palaeogene cover (Ellison et al. 2004; Hillis et al. 2008), producing the London Basin syncline. As Chadwick (1993) identified in the Wessex Basin, the deformation behaviour of a sedimentary mass above a faulted basement is controlled by an interplay between the mechanical properties of the cover and the feasibility of reshearing major underlying faults. The tectonic history demonstrates that, prior to the generation of the London Basin, the region was a thin sedimentary mass of weak, sub-horizontal Late Cretaceous-Palaeogene sediments covering a shallow Palaeozoic basement that has been faulted by both Variscan compression and post-Variscan extension. This has been schematically summarised in figure 3 to illustrate how Variscan compression and post-Variscan extension may have controlled the region’s deformation under Alpine compression. The major faults observed in London could thus be a product of either new Alpine shearing in the cover, or, reactivation and inheritance of pre-existing basement faults. Major faulting in London and the Thames Estuary The majority of known faults in the London Basin are restricted to its southern region (fig. 1), with the majority of published faults following three distinct trends: NE–NNE, NNW, and E–ENE (fig. 4). New observations of faulting indicate greater prevalence than previously recognised and have additional structural complexities associated with them. An example of this is in the NNW-trending Lea Valley, where a long-suspected major fault (Wood 1882) has been confirmed by recent and historic observations (Wooldridge 1923; de Freitas 2009; Ghail et al. 2015; Linde-Arias et al. 2018). Examination

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