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

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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 of known faulting Faults in London are under-represented despite historic and recent observations of them, reflecting both anthropogenic and natural constraints that inhibit their recording (Aldiss 2013). A number of known faults mapped in the London Basin have been re-examined in the context of the local geology, with additional observations offering new insights into their origin and architecture. The en échelon Wimbledon–Streatham–Greenwich fault series intersect south and east London along a NE-NNE trend (fig. 2) (Ellison et al. 2004). They are described as en échelon because of their parallel strikes and overlapping nature, with their relative position in the fault system implying sinistral (leftward) slip (Mandl 2000) and a right-stepping arrangement. Elevated stratigraphic inliers, such as the Greenwich ‘Anticline’ (fig. 2), occur between these en échelon faults. Such raised inliers are indicative of the localised development and existence of push-up structures (fig. 5.a), occurring where overlapping en échelon faults (fig. 2) have linked, causing uplift internally. The Cliffe Graben (fig. 6.a) in the basement underlying the Thames Estuary (section A-A’, fig. 6.b) was first proposed by Owen (1971) who identified a band of Oxford Clay preserved in a suspected graben between northern Silurian and southern Devonian basement floor. Graben inversion and propagation into the overlying London Basin sediments is inferred as evidenced by reversal of postulated normal faults, with variable stratigraphic elevations internally (fig. 6.a) implying lateral shortening also. The graben is interpreted to continue westwards into east London (fig. 42 in Ellison et al. 2004). Identification of Oxford Clay in boreholes west of the Cliffe Graben [TQ67SW561, TQ67SW18, TQ67SW12, TQ67SW11] appears to validate this, indicating that it is part of a larger graben structure beneath the Thames Estuary that strikes approximately E–W (fig. 6.b). The offsets between the inliers (fig. 6.b) indicate that the graben is not a continuous structure, but insteadACCEPTED composed of several en échelon or potentially MANUSCRIPT laterally displaced grabens, with the Cliffe Graben being the easternmost member: the Thames Estuary Grabens.

A series of en échelon E–W trending faults are observed within the North Downs, which separate the London and Weald Basins (Sherlock et al. 1962). Seismic reflection line ‘UKOGL‐RG‐001’ (Butler & Jamieson 2013), summarised in figure 7, intersects the North Downs (fig. 1) and indicates that these represent major Mesozoic E-W striking normal faults that have subsequently reversed (Chadwick 1993) along the northern margin of the Weald Basin during late Palaeogene inversion. This provides a clear example of post-Variscan normal faults linking with and exploiting the presence of the underlying pre-existing thrust fault (fig. 3), and the reactivation and inheritance of the former into the overlying sedimentary cover. Hidden fault network beneath London Away from known structures, engineering projects have encountered major faults (fig. 2), implying that additional faults are present beneath London that are not observed at the surface. Multiple lines of evidence indicate the presence of an underlying fault network in London, however, in isolation these inferences are insufficient to confirm their presence. As noted by de Freitas (2009) both the Thames and its tributaries follow consistent, sub-linear trends (NW-NNW, NE-NNE, & E); he proposed that they reflect an underlying fault fabric, with the river system exploiting these lines of weakness. Aldiss (2013), however, highlighted the potential unreliability of utilising solely drainage patterns to map underlying structures. The geometric arrangement of a hidden fault network has been further indicated by InSAR investigations. Mason et al. (2015) demonstrated that the surface of London is moving as a series of discrete blocks, with vertical and E–W lateral displacements of ≤ 2 mm.a-1 recorded internally within them. They interpreted the boundaries of these blocks to represent major faults due to their consistent, sub-linear trends with opposing senses of motion measured across them. However, these are inferred, as neither the postulated fault positions suggested from river morphologies or InSAR data have been corroborated. Major elevation changes have been identified in the upper surface of the White Chalk Group with contours steepening along consistent sub-linear trends separating areas of comparatively uniform elevation. This has led to inferences of both folding and faulting in the near-surface under London (Buchan 1938; Ellison et al. 2004; Ford et al. 2008; Ford et al. 2010; Aldiss 2013). To assess whether these independent lines of evidence reflect an underlying fault network, the positioning of abrupt changes in vertical displacement InSAR data (Mason et al. 2015) was compared with major elevational changes identified in the upper surface of the White Chalk reinterpreted from Ellison et al. (2004). Vertical offsets from both datasets, when spatially coincident, provide indirect evidence of faulting in London rather than folding, despite differences in data resolution. For further comparisons, the positions of known faults and the fault zones observed by geotechnical projects (fig. 2) have also been plotted. As seen in figure 8, there is a good correlation between these three independent datasets, indicating the presence of multiple major faults beneath London. Three fault sets are interpreted from these findings, striking approximately: 020°, 080° & 335° (fig. 8, inset) that are consistent with measured fault trends observed across the southern region of the London Basin (fig. 4) and the orientations of the Thames’ river system (de Freitas 2009). There are discrepancies between these interpretations and observations, such as the NW-trending Lea Valley Fault appearing on InSAR (fig. 8) but not on the chalk surface, potentially reflecting issues associated with data coverage and the interpretation of structural contours. It may indicate that the Lea Valley Fault is primarily lateral slip and thus has minimal vertical offset. Local divergence in trends may reflect errors in the assumed linearity of elevational boundaries in the chalk surface, with resultant fault positions also being linearised. The upper surface of the chalk is erosional, thus any episodes of fault activity and offset prior to its masking by later Palaeogene deposition may be partially-to-completely removed by denudation. Further investigations are requiredACCEPTED to confirm the geometries of individual MANUSCRIPT faults.

Block Compartmentalisation De Freitas (2009) argued that major faults have discontinuously partitioned the geology and hydrogeology of the underlying Late Cretaceous–Palaeogene cover. The confirmed presence of a fault network beneath London and the observations of grid-like movement (fig. 8) of its surface indicates that the area has been compartmentalised into a suite of discrete fault-bounded blocks. Dislocation between blocks is evidenced by elevation differences in Cretaceous–Palaeogene sequences across London (Ellison et al. 2004; Paul 2016) following consistent trends that are comparable within both inferred and known fault strikes (fig. 8, inset; 4), and by the localised preservation of Jurassic sediments in inferred grabens beneath the London Basin (fig. 6) (Owen 1971). These faults appear to be actively compartmentalising London at present, with InSAR observations of opposing senses of vertical (fig. 8) and lateral E–W movement (Mason et al. 2015) recorded between fault-bounded blocks. Hydrogeological behaviour is demonstrated to vary across fault-bounded blocks, with variation in standing water levels in the Lower Aquifer (Buchan 1938) mirroring the position of inferred faults in figure 8. This hydrogeological discontinuum was verified by Bischoff et al. (2017; 2020) who identified an irregular cone of depression formed during the dewatering and recharge of the Lower Aquifer at the Limmo shaft site for Crossrail. The cone’s irregular shape follows the strike of the NW- trending Lea Valley Fault (Linde-Arias et al. 2018; Bischoff et al. 2020), indicating that this major fault impeded inter-block flow. This indicates that within the chalk aquifer this fault’s core has a lower permeability than the surrounding chalk, thereby hindering fluid migration. Additionally, highly- variable permeability observed in the overlying Thanet Sands (Linney & Withers 1998) proximal to the Lea Valley Fault may indicate the dilational distortion of the sand immediately above the fault. Such evidence provides a case study for the impact of faulting on hydrogeological compartmentalisation of the Lower Aquifer; however, it should be noted that other major faults facilitate inter-block flow, rather than impede it (Environment Environment Agency 2018). The compartmentalisation of the Lower Aquifer by low-permeability faults is suspected of causing the differential displacement observed by Mason et al. (2015) between fault-bounded blocks. Impedance of flow between blocks will inhibit hydrogeological interactions between blocks and cause variations in aquifer behaviour. Consequently high-permeability faults that enable hydrogeological continuum between blocks are unlikely to be identified by InSAR. This indicates that additional major faults remain unrecorded in London.

Fault zones: Complex features of block-bounding faults Fault zones – kilometre-wide bands of extensive faulting that separate sub-horizontal relatively undeformed strata – exist throughout central London. The number of such fault zones in London, and their geometries, is unknown. Some are spatially coincident with recognised (fig. 2) and inferred (fig. 8) major faults, indicating that fault zones are causally linked with them. For example, an inferred major fault that intersects Southwark and London Bridge (fig. 8, dashed lines) is coincident with a fault zone (fig. 9.a; marked on figs. 2 & 8) that was intercepted by both the Thames Tideway Tunnel (Newman 2017) and the Jubilee Line Extension (Entwisle et al. 2013). Fault zones have been recorded up to 3 km wide in London (fig. 9.b), and are characterised by extensive brittle deformation, an increase in joint prevalence, and increased displacement towards their centres (Newman 2009; Linde-Arias et al. 2018). Two-dimensional sections (fig. 9) initially indicated that they were solely dip-slip structures, e.g. Plaistow Graben (fig. 2) (Newman 2008), however, limited three-dimensional analysis (Carter & Hart 1977; Linde-Arias et al. 2018) indicates that the architecture of these fault zones is more complex than this. Observations at the Thames Barrier fault zone (fig. 2) indicate that fault zones are internally characterised by lateral-slip dominant en échelon faults. Through a detailed site investigation, Carter & HartACCEPTED (1977) identified that these NNE-trending MANUSCRIPT en échelon faults displayed dextral slip, with minor folding between the interior faults (fig. 10). The structural arrangement of this particular fault zone is comparable to a shear zone characterised by transpressional stress conditions internally, with the en échelon shears interpreted to be Riedel shears (R or R’) with the minor folds illustrating shortening between them (Mandl 2000). Fault zones are products of transpressional and transtensional stress, as indicated by the dominance of lateral-slip faulting (fig. 10) and both positive relief (fig. 9.b) and negative relief (fig. 9.a) of strata internally. Their limited known coverage (fig. 2) indicates that these stress conditions are localised phenomena on major faults (fig. 8), rather than a continuous feature along them. The Greenwich Connection Tunnel Fault Zone (GCTFZ) is an example of a positive relief fault zone (fig. 9.b) coincident with elevated structural contours of the chalk surface (Ellison et al. 2004) between two overlapping right-stepping en échelon faults (fig. 2), and is interpreted to represent the internal architecture of a transpressional push-up structure (fig. 5.a). This indicates that fault zones observed in London (i.e. localised occurrences of extensive faulting) are products of linkage between en échelon faults (fig. 5). Isolated folds of limited lateral extent (e.g. Greenwich Anticline, fig 2) have been inferred in the southern part of the London Basin based on local variations in structural contour elevations (Bromehead et al. 1925; Sherlock et al. 1962; Howland 1991). The axial traces of these folds (Wooldridge 1923; Ellison et al. 2004) are consistent with known fault strikes from the area (fig. 4.a). Some of these folds are oblique to the anticipated Alpine stress field (Cosgrove et al. 2020), bringing into question whether they are indeed products of buckling. It is suspected that many of these inferred folds are instead misidentified fault zones at points of linkage between en échelon faults, with poor borehole data coverage and limited exposure giving the impression of a fold-like geometry. To illustrate this, the Greenwich Connection Tunnel Fault Zone (fig. 9.b) has been re-reinterpreted from five hypothetical boreholes (fig. 11). The wide borehole spacing suggests an anticlinal geometry, leading to a low-resolution misinterpretation of the fault zone. Some folds oblique to the Alpine stress field may also represent fault-capping folds where underlying faults at depth have reactivated and propagated upwards to bend and deform the overlying layers, as suggested by Blundell (2002) for the Windsor Anticline. Block interiors Away from these major faults, the interiors of compartmentalised blocks appear to be comparatively undeformed, so preserving the horizontal ‘layer-cake’ character historically attributed to the London Basin. However, it should be recognised that discrete minor faults, folds and joints are prevalent throughout the London Basin as summarised in table 1.

Kinematic feasibility of basement fault reactivation beneath the London Basin As summarised in figure 3, the Palaeozoic basement beneath the southern region of the London Basin is interpreted to contain Variscan thrusts and conjugate strike-slip pairs, and a post-Variscan normal fault (Cosgrove et al. 2020). The reshearing of a pre-existing fault is primarily dependent upon the applied stress magnitudes (σ’) and orientation of the fault plane relative to the tectonic stress field (θ) (Sibson 1974; Sibson 1985), the frictional properties of the plane itself (μ) (Byerlee 1978; Copley & Woodcock 2016; Copley 2017) and the fluid pressure acting on the fault plane (Sibson 1994). Therefore, the tectonic stress history of the region from the late Palaeozoic onwards will affect whether the reactivation was indeed feasible during the Alpine orogeny. The tectonic history of southern Britain is reviewed by Cosgrove et al. (2020), with figure 12 providing a simplified overview in this paper. It should be recognised that spatio-temporal variations in the magnitude and orientation of the stress field are anticipated over the time periods of each tectonic regime, as illustrated by Cosgrove et al. (2020), who subdivided the Alpine Orogeny into three phases to reflect its evolution. In this paper, the Alpine orogeny (fig 12.d) has beenACCEPTED represented by its main phase in southern MANUSCRIPT Britain only: the Pyrenean Phase (Parrish et al. 2018). The orientation of the stress axes have remained approximately coaxial since the late Palaeozoic (fig. 12), despite major changes in magnitude and principal stress alignments. Pre-existing basement structures (fig. 3) that formed under compressive (fig. 12.a,b) and extensional (fig. 12.c) stress regimes may have been favourably aligned to reshear in either their original or reverse sense of slip during Alpine compressive phases (fig. 12.d). It is considered that any reshear of Variscan sets would introduce a minor oblique-slip component due to the stress fields not being purely coaxial (fig. 12.a,b, vs. d).

Feasibility of reshear under Alpine compression The geometric arrangement of the strike-slip sets, and dip-slip sets under Alpine stress conditions are summarised in figure 13. The orientations of the Variscan strike-slip conjugate pair (fig. 13.a) are based on their comparative orientations in south Pembrokeshire. The dip-slip sets shown are idealised orientations for both thrusts and normal faults (fig. 13.b). Reshear feasibility for all four fault sets was mechanically assessed (fig. 14) based on Sibson (1985) under the Alpine stress field (fig. 12.d), assuming frictional sliding conditions of μ = 0.3 (Copley 2017). For these conditions, optimal alignment for reactivation (θ*) was determined to be θ = 36°. The Variscan conjugate pair does not form an acute bisector with the Alpine stress field (fig. 13.a; θ = 30°, 20°), reflecting minor asymmetry with the late stage Variscan stress field (fig. 12.b,d). Consequently, the two sets will not share the same value of θ, therefore requiring different stress magnitudes to initiate reactivation (fig. 14.a). The dextral NW-set is near-optimally aligned for reactivation, with sinistral NE-set less ideally oriented but still considered feasible for reshearing. The comparative optimality of θ for the dextral NW-sets could explain the prevalence of NW-trending structures in post-Variscan sequences across southern England (Lake & Karner 1987). It should be noted that under the applied (Pyrenean phase) stress regime, both the dextral and sinsitral sets would reactivate with their original sense of shear (fig. 13.a). In order to calculate reactivation feasibility of the dip-slip faults their strikes have been idealised to perpendicular to the Alpine stress field, as coaxiality with the applied stress axis is required for Sibson’s (1985) mechanism. The Variscan thrust set (θ = 30°) is near-optimally aligned for reactivation, whilst the post-Variscan normal fault set (θ = 60°) is approaching stress conditions that inhibit reshear (fig. 14.b). Despite the apparent kinematic preference for Variscan thrust set reactivation, normal faults are recognised to undergo considerable reversal under compressive stresses, as evidenced by their inversion across southern Britain (Lake & Karner 1987; Chadwick 1993; Turner & Williams 2004; Westhead et al. 2018), and in analogue modelling (Del Ventisette et al. 2006; Bonini et al. 2012). Furthermore, across Europe, thrusts are noted for being less susceptible to compressional reshearing than normal faults (Ziegler 1987). The unexpected dominance of post- Variscan normal fault reversal likely reflects two conditions. Firstly, these faults are listric, curving at depth to form shallow-angles (fig. 13.b) that have attached onto and exploited the pre-existing Variscan thrust (fig. 7); this low-angled portion of the fault would have been favourably aligned for reactivation and reversal (fig. 6 in Sibson 1995), even more so than thrusts (fig. 14.b) . Secondly, elevated fluid pressures during inversion facilitate the reactivation of unfavourably-oriented normal faults by reducing the effective stresses acting on the fault plane (Sibson 1995; Turner & Williams 2004); thereby further lowering the reactivation envelope to enable reshear of steeper portions of the normal fault at lower differential stress conditions (σ1’/σ3’).

Structural inheritance in the London Basin: Evidence and mechanisms Alpine reactivation of the anticipated basement fault sets beneath the southern region of the London Basin (fig. 3) was mechanically feasible (fig. 14). However, unlike adjacent geological regions where penetrative geophysical exploration programmes have provided confirmation of basement- derivedACCEPTED fault inheritance, such investigations areMANUSCRIPT absent in the London Basin. Consequently, fault inheritance has only been observed twice in the Basin: the inverted grabens in the Thames Estuary (fig. 6), and along seismic line RG-001 (fig. 7,1). To assess whether fault sets inferred in London (fig. 8, inset) and observed in the Basin’s southern region (fig. 4) are indeed reactivated basement faults that have been inherited or represent new Late Cretaceous-Palaeogene compressive shears, their strikes were compared with the orientations of basement fault sets from across southern Britain (fig. 13). The fault traces published by British Geological Survey (BGS) (fig. 15.a; 4) vary in length (hectometres-kilometres), with the degree of observed scatter implies that both inherited faults and new Alpine shears may be present. Comparatively, the inferred km-scale faults in London (fig. 8) display minimal scatter; however, this may reflect the sample size. If these major fault sets in London represent new shear under Alpine stress, it is anticipated that they would be aligned with this stress axis (fig. 12.d). The major NE- and NW-trending fault sets form an acute bisector of ~355-360° (fig. 15.b), potentially making them compatible with formation under both the Late Variscan and Alpine stress axes (fig. 12.b,d). Consequently, their trends alone (fig. 15.b) are considered insufficient to deduce their origins. The misalignment of the ENE–WSW set (fig. 15.a,b) with both basement fault trends (fig. 15.c,d) and the Alpine stress field (fig. 12.d) by approximately 10-20° is unexpected. The inverted Thames Estuary graben (fig. 6) indicates that the km-scale faults in this trend (fig. 15.b) may be inherited normal faults that propagated into the overlying cover during their reversal. The ESE-WNW set may potentially imply spatial differences in orientation of the Early Variscan stress field (and consequently Mesozoic extensional axis) between basement underlying the Wessex Basin and along the Anglo-Brabant Massif’s southern margin. The smaller shears in this trend (fig. 15.a) may represent the development of new thrusts and/or partially exposed inherited faults. Inheritance mechanisms The respective geometries of each basement fault set (fig. 3; 15.c,d) relative to the Alpine stress field (fig. 12.d) will affect their respective propagation behaviour into the Cretaceous–Palaeogene cover upon reactivation. These mechanisms have been determined from published analogue modelling and analogous geological observations of structural inheritance, being validated where possible with structures in London. The anticipated Variscan strike-slip sets in the underlying Palaeozoic basement can be idealised to sub-vertical faults (fig. 3). Analogue modelling of the strike-slip reactivation of a sub-vertical fault beneath an unsheared sedimentary mass demonstrates that it does not lead to the propagation of that fault as a single shear into the cover (Naylor et al. 1986). Instead reactivation causes basal shearing at the interface with the cover, locally disrupting the applied stress field (fig. 16). This localised distortion generates sufficient shear stresses for the fault to propagate by splaying into multiple en échelon Riedel shears that are oblique to the basement fault trend (Mandl 2000). As the Riedel shears propagate upwards, they become less influenced by the stress distortion at the basement-cover interface (Mandl 1988). This causes progressive rotation of the shear planes into alignment with the applied stress field and the development of helicoidal geometries (fig. 16) (Dooley & Schreurs 2012). Continued shearing of the basement fault will cause eventual overlap and linkage of en échelon Riedel shears, locally developing transpressional push-up structures (fig. 17, 5.a). This proposed mechanism is evidenced in London by NE-trending en échelon observations (fig. 2) withACCEPTED inliers situated between them, the presence MANUSCRIPT of NW- and NE-trending fault sets concordant with Variscan strike-slip trends (fig. 8; 15), the spatial coincidence between fault zones and major faults (fig. 8), and the identification of internal positive relief (fig. 9.a) and oblique-slip features (fig. 10) in fault zones. It is interpreted that the major NW- and NE-trending fault sets identified in London (fig. 8) are inherited Riedel shears from underlying reactivated dextral and sinistral Variscan strike-slip faults respectively located in the Palaeozoic basement. They should therefore be treated as en échelon rather than continuous structures. Seismic scan RG-001 (fig. 7) indicates that Variscan thrusts were initially exploited during Mesozoic extension as a décollement surface for listric post-Variscan normal faults (fig. 13.b), as observed elsewhere in southern Britain (Chadwick 1986; Brooks et al. 1988; Mansy et al. 2003) and by analogue modelling (Ivins et al. 1990; Faccenna et al. 1995). The Variscan thrust in RG-001 (fig. 7) is not observed to have propagated into and displaced the Cretaceous–Palaeogene cover, implying no inheritance from Variscan thrusting. Post-Variscan normal faults appear to have ceased extension by the Early Cretaceous (Owen 1971), and likely underwent minor reverse reactivation during the Late Cretaceous (Mortimore & Pomerol 1997). The main inversion phase likely occurred in the late Palaeogene, as demonstrated by the offset of Palaeogene sequences (fig. 6.a), agreeing with the observations of Parrish et al. (2018) in southern England. Analogue modelling (Del Ventisette et al. 2006; Bonini et al. 2012) and geological observations (Turner & Williams 2004; Westhead et al. 2018) indicate that approximately coaxial inversion (fig. 12.c,d; 13.b) would be characterised by normal fault reversal and propagation, with localised generation of secondary thrusts and capping anticlines. Observations of propagated graben-bounding fault reversal and the internal bulk shortening of Thames Estuary Graben’s fill (fig. 6.a) indicate that the structure contains at least two inverted and consequently inherited post- Variscan normal faults. The apparent offsets (fig. 6.b) may imply an en échelon arrangement or laterally offsetting by inherited strike-slip faults.

Discussion A fault network comprised of at least three en échelon sets has been identified in the Cretaceous– Palaeogene sequences under London (fig. 8) that are interpreted to be inherited from the underlying Palaeozoic basement (fig. 3) during Alpine compression (fig. 13, 14).

New shear or reshear: How was Alpine strain accommodated in the London Basin? Alpine compressive strain in the London Basin was accommodated primarily by brittle deformation, as demonstrated by the widespread presence of faulting at all scales and the limited coverage and magnitude of folding (fig. 2; table 1). As schematically illustrated in figure 3, this would have been controlled by a combination of new shear development in the cover, and the inheritance of resheared pre-existing basement faults. The distribution of strain accommodated by these two mechanisms, and consequently the comparative magnitudes of their structures, would have been controlled by which was mechanically more favourable to undertake. Evidence provided in this paper indicates that the major fault sets in London are a product of inherited basement faulting under Alpine stresses (fig. 13; 14), with the more prevalent but minor brittle structures attributed to new shear development (table 1). The comparative difference in scale in their products and the presence of highly deformed fault linkage zones indicates that basement fault reshear accommodated the majority of Alpine compressive strain in the region. The propagationACCEPTED of inherited basement faults mayMANUSCRIPT also indirectly contribute to the development of new shears in the cover by locally perturbing the stress field to initiate faults under alignments that contradict the Alpine stress axes, e.g. within fault zone interiors, or, by generating short-cut structures (Turner & Williams 2004). Spatial extent of structural inheritance in the London Basin The investigation has primarily focussed on the southern region of the London Basin, specifically London (fig. 2). The spatial extent of structural inheritance across the Basin remains unknown due to comparative data sparsity and issues associated with fault identification in highly homogeneous lithologies (Aldiss 2013). Structural inheritance from the Palaeozoic basement appears to cease north of Chelmsford (fig. 1) towards the centre of the Basin, as indicated by an assessment of published structural contour and faulting data northward into the interior of the London Basin (Bristow et al. 1985; Ellison et al. 1986; Lake et al. 1986; Pattison et al. 1993). Contrary to this however, the Lilley Bottom structure near Hitchen (Hopson et al. 1996; Mortimore et al. 2001) and the Glinton Thrust along the northern margin of the Basin with East Anglia (Woods & Chacksfield 2012) provide evidence of structural inheritance further into the Basin’s interior. The strike of the latter is oblique to the Variscan thrust regime (fig. 12.a), potentially implying a transition to a Caledonian fabric as anticipated by Pharaoh et al. (1987) and Pharaoh (2018). It is suspected that the comparatively undeformed interior of the London Basin reflects the behaviour of the underlying Massif with Alpine strain primarily distributed along its margins, spatially restricting the extent of structural inheritance to its southern, and probably northern, boundaries. Paradoxical observations There is limited evidence of major NW-trending faults beneath London (fig. 8; 15.b), such as the Lea Valley Fault, despite kinematic analysis (fig. 14.a) demonstrating that the underlying Variscan NW- trending dextral strike-slip set (fig. 13.c; 3) was near-optimally aligned for reshear and subsequent inheritance. Furthermore, inherited NW-trending Variscan dextral faults are recognised as a dominant feature across southern England (Holloway & Chadwick 1986; Lake & Karner 1987; Miliorizos & Ruffell 1998; Kelly et al. 1999). Paradoxically, the observations indicate that less favourably aligned NE-trending sinistral set are prevalent beneath London (fig. 8; 15.b). Rather than indicating that reshear of Variscan dextral strike-slip faults was minimal, this is interpreted to reflect their respective reactivation behaviours in response to relative obliquities to the Alpine stress field (fig. 12.d; 13.a). The near-optimal alignment of the dextral set (fig. 14.a) would cause reactivation and inheritance into the cover to be principally strike-slip, hence resultant elevational variations would be restricted to linkage zones between individual en échelon dextral Riedel shears (fig. 5.a). The less favourably aligned Variscan NE-trending sinistral set (fig. 14.a) would reactivate more obliquely and contain a dip-slip component that would be exhibited in inherited Riedel shears. As the identified fault network (fig. 8) was interpreted primarily from observations of vertical changes (Ellison et al. 2004; Mason et al. 2015), the minimal dip-slip component of the Variscan dextral set beneath London would cause its inherited products to be less apparent in the near- surface (fig. 8). It is anticipated that utilising E–W displacement InSAR data from Mason et al. (2015) may have offered a potential solution. Transtensional structures in the London Basin LocalisedACCEPTED transtensional conditions (fig. 5.b) are MANUSCRIPT indicated by the presence of negative fault zones, such as the London Bridge Fault Zone (fig. 9.a) and the Plaistow Graben (fig. 2) (Newman 2008; Newman et al. 2016), and depressed synclinal-like structure contours observed adjacent to major faults, such as the Greenwich Syncline proposed by Howland (1991). This is unexpected, as under Pyrenean compression (fig. 13.a), the Variscan strike-slip pair are anticipated to have resheared under their original sense of slip to produce transpressional push-up structures between en échelon Riedel shears of the same fault set (fig. 17; 5.a), and the post-Variscan normal faults to have reversed and inverted. The presence of transtensional and/or extensional structures in the London Basin demonstrate that there are other mechanisms that could locally perturb the stress axis from the compressive Alpine regime, in addition to the ones previously presented. Cosgrove et al. (2020) identified sinistral reversal of the initially dextral NW-trending Variscan strike- slip set under the later Helvetic phase of the Alpine orogeny. If their inherited en échelon Riedel shears developed dextral left-stepping faults as expected (Mandl 2000) during the earlier Pyrenean Phase (fig. 12.d), then sinistral reversal would induce transtensional stress conditions in linkage zones (fig. 5.b). The geometry of the inverted Thames Estuary Graben indicates that it may be comprised of en échelon normal faults (fig. 6.b). If so, their linkage may develop relay ramps that are internally tilted and deflected. The presence of E-W trending faults, anticipated to also be normal faults, elsewhere in London (fig. 8) indicate that relay ramps may provide local transtensional conditions also. Future investigations are necessary to validate the presence of these structures. The Plaistow Graben (fig. 2) and Greenwich Syncline are situated proximal to the anticipated intercept between sinistral Greenwich fault (fig. 2) and the NW-trending Lea Valley Fault, both interpreted to be inherited en échelon Riedel shears (fig. 17). These two structures may represent interaction between different fault sets, either in the basement or the cover, that disrupted the stress field near points of interception to enable oblique stress conditions locally. Finally, the underlying basement faults (fig. 3) are unlikely to be purely linear, and instead contain bends. The reactivation of faults along both releasing and restraining bends will locally cause oblique stress conditions (Fossen 2016) in both the basement and the overlying cover. Interaction within and between fault sets, changes in global stress field and the geometry of underlying basement faults, can all locally disrupt the applied stress conditions, producing both localised transpressional and transtensional structures. Consequently, this indicates that fault zones in London (fig. 9; 10) are a product of several different mechanisms caused by the complicated behaviour of inherited basement faults.

Conclusion The southern portion of the London Basin is structurally complex. A network of three major fault sets has been identified under London (fig. 8) and the Thames Estuary (fig. 6) that have compartmentalised the area into a series of blocks. Interactions between and within these fault sets has caused development of localised transpressional (fig. 9.b; 10) and transtensional (fig. 9.a) fault zones. These major faults are interpreted to have been inherited from underlying basement faults that reactivated during the Alpine Orogeny. Kinematic assessments (fig. 13; 14) demonstrate that their reactivation was feasible under Alpine stresses, with observations in the near-surface (fig. 2) being comparable to known inheritance mechanisms from geological investigations and analogue modelling (fig. 16; 17), and known basement fault trends (fig. 15). The lack of observed major structures in the Basin’s interior is inferred to reflect the role of the underlying, rigid Anglo-Brabant Massif in distributing strain along its margins, spatially restricting the regional extent of structural inheritancACCEPTEDe. Alternatively, this potential absence MANUSCRIPT may instead reflect comparative data sparsity, and issues associated with fault identification in the London Basin (Aldiss 2013). The findings indicate that Alpine compressive strain in the London Basin was primarily accommodated by basement fault reactivation, with new structure development providing a comparatively minor contribution. The near-surface geology in London is faulted, however, the styles, spatial extent and engineering impact of this brittle tectonism is broadly unknown. These structures have major implications for civil engineering projects undertaken in London and further research is needed to address this lack of understanding. Acknowledgements The authors would like to thank both Dr Michael de Freitas and an anonymous second reviewer for their insights and constructive comments throughout. The authors wish to thank Tim Newman and Thames Tideway for allowing adaptation of their cross- sections (fig. 9), and Jackie Skipper & John Davis of GCG who have provided valuable insights and observations from civil engineering projects in London. Stereonets were produced using the Stereonet software produced Richard Almendinger and his team. Fault data presented in figures 4.a & 15.a is from Geological Map Data BGS©UKRI 2018. Funding Funding for research provided by the Engineering and Physical Sciences Research Council (EPSRC) Award EP/L016826/1 and the Skempton Scholarship of Imperial College London.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 – The London Basin, a region of Cretaceous-Palaeogene sequences folded into a gentle, open syncline during the Alpine Orogeny. The approximate position of the Variscan Front (from Pharaoh et al. 1996) is highlighted, in addition to the positions of figures 7 and 8.

Fig. 2 – The positions of observed geological structures from historical studies. Also shown is faulting encountered by recent engineering projects in London. Labelled structures are referred to in the paper. Intense zones of faulting, called here “fault zones”, on map represent locations where they have been intersected by engineering projects. Aside from the Plaistow Graben, the geometry of these has yet to be defined. Fault zone locations sourced from: Carter & Hart (1977), Newman et al. (2016), Newman (2017), Black (2017). Fig. 3 – Mechanical schematic summarising the key structural components that are interpreted to have influenced and controlled deformation along the southern margin of the London Basin (fig. 1) during Alpine compression. A Palaeozoic basement containing four fault sets that acts as planes of weakness, overlain by a cover of weak Late Cretaceous-Palaeogene sediments. Fig. 4 – 77 strikes from across the southern region of the London Basin were measured digitally from published 1:50,000 BGS data. Linear approximations were made for faults with mild curvature, and two major faults were segmented into five consistent strikes. © Geological Map Data BGS © UKRI 2018. Stereonet constructed using Stereonet 10 (Allmendinger et al. 2011; Cardozo & Allmendinger 2013). (a) Rose plot of measured fault strikes. (b) Area of London Basin from which fault strikes were measured.

Fig. 5 – Linkage of overlapping en échelon shears through continued shearing causes localised stress concentration and the development of deformed zones. The style of linkage is dependent on the geometry of the overlap and the direction of shearing; in both scenarios presented the en échelon shears are right-stepping but with opposing senses of slip. (a) Push-up structures, where converging en échelon shears generate transpressive conditions internally, causing localised shortening and uplift. An uplifted chalk inlier, the ‘Greenwich Anticline’, situated between two en échelon sinistral faults (fig. 2) is suspected of being a push-up structure. (b) Pull-apart basins, where diverging en échelon shears generate transtensive conditions internally, causing localised extension and depression.

Fig. 6 – Evidence of an inverted graben structures in the Thames Estuary. (a) Cross-section of the Cliffe Graben, first identified by Owen (1971), demonstrating evidence of inversionACCEPTED through normal fault reversal and internal MANUSCRIPT bulk shortening. (b) Proposed westward extension of the inverted Thames Estuary Grabens as previously documented in Ellison et al. (2004), of which the Cliffe Graben is the easternmost member. This is evidenced by the identification of Oxford Clay intercepted by additional boreholes in Gravesend, and the westward continuation of the E-W trending chalk outlier, indicating fault-bounded uplift. It is suspected that the graben-bounding normal faults are either en echelon or have been laterally displaced, with further investigations required to define this structure.

Fig. 7 – Geological cross-section illustrating the major basement interface along the Weald Basin- London Basin boundary reflecting the role of Variscan thrust and later, exploitative post-Variscan normal faults developed during Mesozoic extension. Reversal of the latter during Alpine inversion caused upward warping of the North Downs and inheritance of propagating E-W trending en échelon faults. The section is adapted from Butler & Jamieson’s (2013) interpretation of seismic line RG-001 (position marked on figure 1). Vertical axis is presented in terms of two-way time (TWT, secs) and approximate elevation (m AOD). Fig. 8 – Fault map of London developed through coupling indirect evidence of faulting. Boundaries of vertical displacement identified by InSAR (adapted from Mason et al. 2015) spatially-correlate with both known faults (solid black lines) and approximated boundaries between major elevation changes in the chalk surface at depth indicative of faulting (dashed black lines) (adapted from Ellison et al. 2004). Fault zones (black dots) identified by engineering projects (fig. 2) have also been plotted. The inset compares strikes of both datasets: Red – chalk | Black – InSAR. Fault positionings on the map are linear approximations, and it is considered likely that will curve. Fig. 9 – Cross-sections of fault zones identified by Thames Tideway Tunnel ground investigations, adapted from Newman (2017). Locations of these fault zones provided on figures 2 & 8, as identified by ground investigations along tunnel alignments. (a) London Bridge Fault Zone, a negative relief fault zone. (b) Greenwich Connection Tunnel Fault Zone, a positive relief fault zone. Note how with limited borehole coverage, such structures could easily be interpreted as folds (fig. 11). Fig. 10 – Plan view of Thames Barrier fault zone, adapted from Carter & Hart (1977), who identified a series of dextral slip NNE-trending en échelon faults and two minor folding sets onsite, through a combination of contour mapping, micropalaeontology and geophysics. The geometric arrangement is comparable to a shear zone, with evidence of lateral shortening between the dextral slip en échelon fault trends implying transpressive stress conditions internally. Location provided on figure 2. Fig. 11 – Illustration of how the Greenwich Connection Tunnel Fault Zone misidentified as an anticline, from five widely spaced hypothetical boreholes using stratigraphic positions from figure 9.b. This demonstrate how poor data coverage may cause misinterpretations of fault zones as folds, due to their fold-like geometries at lower resolutions. Fig. 12 – Inferred tectonic palaeostress field orientation (± 10°) from late Palaeozoic to Cenozoic for the London Basin relative to present day North, as determined from published and unpublished structural data across southern Britain. (a) Thrust regime during early-stage Variscan orogeny. (Dixon & Strahan 1977; Wright et al. 2009; Woodcock et al. 2014; Morgan 2015) (b) Late-stage Variscan strike-slip regime. (Roberts 1966, 1975; Dixon & Strahan 1977; Roberts 1979; Wright et al. 2009; Woodcock et al. 2014; Morgan 2015) (c) PostACCEPTED-Variscan to Mesozoic extensional regime. MANUSCRIPT (Roberts 1966, 1975, 1979; Nemčok et al. 1995; Glen et al. 2005; Morgan 2015) (d) Alpine compressive regime, specifically the Pyrenean Phase (Parrish et al. 2018). Additional sources include: (Skempton et al. 1969; Ameen & Cosgrove 1990; Nemčok et al. 1995; Belayneh & Cosgrove 2004; Glen et al. 2005; Doherty 2012)

Fig. 13 – Mechanical schematics of basement faults (fig. 3) and their anticipated reactivation style behaviour under the Alpine stress field (fig. 12.d). θ represents the angle of the shear plane relative to the σ1. (a) Mechanical schematic of Variscan strike-slip conjugate pair in plan view. Orientations are inferred from comparative strikes of structures exposed in south Pembrokeshire. (b) Mechanical schematic of dip-slip fault sets in section view ad anticipated slip directions. Orientations for each fault type (thrust & normal) have been idealised, with the listric component of the post-Variscan normal fault inferred to range between both as it links onto the underlying Variscan thrust. For simplicity, both sets are assumed to strike perpendicular to the Alpine stress axis (fig. 12.d), in order to satisfy the coaxial conditions required for reactivation feasibility assessment (fig. 14.b).

Fig. 14 - Kinematic assessment of reshear feasibility for anticipated basement fault sets (fig. 3) under Alpine stress conditions (fig. 12.d), calculated using Sibson (1985) and assuming a friction sliding coefficient (μ) of 0.3 (Copley 2017). The closer the fault plane is oriented to the optimal reactivation angle (θ*), the lower the stress conditions required to induce reshearing. (a) Reactivation feasibility assessment of the Variscan strike-slip conjugate pair (fig. 13.a). (b) Reactivation feasibility assessment of the idealised dip-slip faults (fig. 13.b), and the listric component (fig. 7) of the post-Variscan normal fault set.

Fig. 15 – Comparison of published (fig. 4) and inferred (fig. 8, inset) fault strikes identified in the Late Cretaceous-Palaeogene sequences of the London Basin (a,b) with the strikes of Variscan compressional (c) and post-Variscan extensional (d) fault sets in southern Britain. Locations of the London Basin fault strike datasets is provided by figures 4.b & 8. Stereonet constructed using Stereonet 10 (Allmendinger et al. 2011; Cardozo & Allmendinger 2013). Fig. 16 – Inheritance mechanism for a reactivated vertical strike-slip basement fault into the overlying cover by the localised disruption of the applied stress field and the development of en échelon Riedel shears (adapted from Dooley & Schreurs (2012), based on Mandl (1988)). The maxima stress (σ1) axis is represented by the solid bold lines, and the helicoidal geometry of the Riedel shear by the orange planes. Fig. 17 – 3D interpretation of the development and linkage of two en échelon Riedel shears from a single underlying basement fault. Eventual linkage of the overlapping shears will cause localised shortening and uplift, generating a transpressive push-up structure. Table 1. Observations of minor structures in the London Basin Source Observations Locations Whitaker et al. (1872) Faulting; Folding London & Thames Valley Fissuring; Jointing; Faulting Ward et al. (1959) London & Thames Valley (London Clay-specific) Fissures; Jointing; Shear zones Fookes & Parrish (1969) Wraysbury ACCEPTED(London ClayMANUSCRIPT-specific) Skempton et al. (1969) Fissures; Joints; Faulting Wraysbury & Edgware Low-angled shear zones; Folding; Chandler et al. (1998) West London Flexural slip Mortimore et al. (2001) Faulting London Basin Ellison et al. (2004) Jointing; Faulting; Folding London Mortimore et al. (2011) Jointing; Faulting East London Doherty (2012) Fissures; Jointing Isle of Sheppey Royse et al. (2012) Jointing; Faulting London Entwisle et al. (2013) Jointing; Faulting London Basin (Lambeth Group-specific) Black (2017) Faulting London

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