The geological context and evidence for incipient inversion of the London Basin Contexte géologique et arguments en faveur de l’inversion naissante du bassin londonien R.C. Ghail*1, P.J. Mason2 , J.A. Skipper3 1 Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK 2 Department of Earth Sciences & Engineering, Imperial College London, London SW7 2AZ, UK 3 Geotechnical Consulting Group, 52A Cromwell Road, London SW7 5BE, UK ABSTRACT A reappraisal of ground investigation data across London reveal that a range of unexpected ground conditions, encountered in engineering works since Victorian times, may result from the effects of ongoing inversion of the London Basin. Site investigation bore- hole data and the distribution of river terrace deposits of the Thames and its tributaries reveal a complex pattern of block movements, tilting and dextral transcurrent displacement. Significant displacements (~10 m) observed in Thames terrace gravels in borehole TQ38SE1565 at the Lower Lea Crossing, showing that movement has occurred within the last ~100 ka. Restraining bends on reactivated transcurrent faults may explain the occurrence of drift filled hollows, previously identified as fluvially scoured pingos, by faulting and upward migration of water on a flower structure under periglacial conditions. Mapping the location of these features constrains the location of active transcur- rent faults and so helps predict the likelihood of encountering hazardous ground conditions during tunnelling and ground engineering. RÉSUMÉ Une réévaluation des résultats provenant d’études de terrain à travers Londres a révélé une diversité de phénomènes géologiques inattendus. Rencontrés lors de travaux d’ingénierie depuis l’époque victorienne ils pourraient être causés par les suites de l’inversion du basin londonien en cours. Les données provenant de forages et la répartition des sédiments de la terrasse fluviale de la Tamise et ses affluents révèlent un schéma complexe de mouvements de blocs et de failles de cisaillement inclinées ou dextres. Les mouvements significatifs (~10m) observés dans le forage TQ38SE1565 des graviers de la terrasse de la Tamise au niveau du Lower Lea Crossing indiquent un âge inférieur à ~100ka. Des courbes de retenue sur des failles de cisaillement réactivées pourraient expliquer l’existence de creux remplis de sédiments, précédemment catégorisés en tant que pingos érodés par des processus fluviaux, qui seraient plutôt créé par des failles et la migration ascendante d’eau sur des structures en fleur en conditions périglaciaires. En cartographiant ces caractéristiques, la localisation de failles de cisaillement pourra être contrainte et ainsi aider à prédire la probabilité de rencontrer des terrains hasardeux lors de percement de tunnels ou de travaux d’ingénierie. 1 INTRODUCTION collision and North Atlantic ridge push (Musson, 2007), but these forces appeared not to have affected The London Basin, a wedge-shaped region ex- the London Basin. However, a reappraisal of the data tending from Newbury to Rochester and Great Yar- show that the geology of the London Basin, its pat- mouth in southern England, is one of a number of tern of drainage, and the nature of its periglacial fea- post-Variscan sedimentary basins (Busby and Smith, tures, all strongly indicate recent and ongoing inver- 2001) NW-SE trending basement fractures that were sion of the basin. reactivated as dextral transcurrent faults at various points in the Palaeogene and Neogene, causing inver- sion of the Hampshire and Weald basins (Blundell, 2 GEOLOGY AND TECTONICS 2002; Chadwick, 1993) the Weald, for example, has been uplifted by more than 1500 m (Jones, 1999a) The London Basin (Figure 1) is one of a number since the Cretaceous. The inversion is likely a result of Permo-Triassic, post-Variscan, sedimentary basins of the combined compressive effects of the Alpine (Busby and Smith, 2001) many of which were reac- tivated during Neogene inversion (Chadwick, 1993). were tentatively added (Sumbler, 1996). The many They opened above basement Variscan transcurrent instances of unexpected ground conditions encoun- and thrust faults (Chadwick, 1993) and while each tered in London since Victorian times (Chandler et opened at a slightly different time, all were generally al., 1998; Lenham et al., 2006; Mortimore et al., depositional during the Mesozoic. However, the 2011; Newman, 2009) point to greater structural London Basin lies immediately north of the Variscan complexity than has ever been represented on geo- Front, on the margin of the Midlands microcraton, logical maps. and is bounded by the chalk hills of the Chilterns and Countless site investigations have revealed brittle the North Downs. It formed a subaerial high failures in Palaeogene sediments across London throughout much of the Mesozoic but underwent (Figure 2). A Neolithic trackway, dating from be- complex and rapid deformation during the late Palae- tween 1520 and 1100 BC, unearthed in 1993 in ogene (Knox, 1996), and since then it has remained Beckton [TQ 427 820] was found to be cut by a fault subaerial and quiescent (Gibbard and Lewin, 2003). which is infilled with clay (Greenwood and Maloney, 1994), implying a significant London earthquake within the last 3000 years. The Colchester Earth- quake of 1884, which occurred at the northern edge of the London Basin, was probably the most damag- ing in Britain in the last 400 years (Musson and Winter, 1996). Figure 2. a) Low angle reverse faults (thrusts) in the Lambeth Group sediments beneath the Canary Wharf Crossrail station, 27 m below ground level. White box indicates the position of the detail shown in b) faults associated with periglacial features; c) Uplift of Thames terrace deposits by more than 20 m at Sheppey; and d) persistent tectonic shears in London Clay (exposed in a landslip). Scale bar is 0.5 m in each case. Figure 1. The Cretaceous/Tertiary transtensional basins and Var- Although Pleistocene (Berry, 1979; Hutchinson, iscan basement fractures of southern England. The region is histori- 1980) and Holocene deposits (Akeroyd, 1972) mask cally aseismic but nonetheless influenced by NW-directed Alpine the deeper tectonic features, a regional uplift of at stresses. (Developed from Musson 2007, with Geological Map Data -1 BGS © NERC 2013 and Ordnance Survey Data © Crown copy- least 0.07 mm a across the Thames valley in the last right/database right 2012. An Ordnance Survey/EDINA supplied 900 ka has been detected (Maddy et al., 2000). A service) higher rate in the last 400 ka has also been suggested, despite recent deglaciation (Nunn, 1983). Blundell Although its structural complexity was recognised (2002) estimates that only 50-70% of the uplift of the nearly a century ago (Wooldridge, 1923). Faults have British Isles, Norway and France in the last 2.5 Ma rarely been recognised in London (Aldiss, 2013) and years can be explained by glacier retreat and denuda- evidence for recent fault displacement is rarer still. A tional offloading. very simple unfaulted synclinal model (Sherlock, 1947) persisted into the 1990s, when some faults Reactivation of basement transcurrent faults dur- 3 DRAINAGE ing the Palaeogene and Neogene has caused inver- sion of both Hampshire and Weald basins (Blundell, The drainage network and chalk aquifer in London 2002; Chadwick, 1993). In the case of the Weald reflect a complex pattern of faulting (de Freitas, more than 1500 m of uplift has occurred since the 2009) that appears to be controlled by the Variscan Cretaceous (Jones, 1999a). Apart from a few scat- basement fractures noted above. The Thames is tered outcrops of Oligocene and Pliocene sediments, commonly considered to be a meandering river but the end of the Palaeocene in the London area is closer inspection reveals a number of sharp bends marked by a gap in the stratigraphy of some 40 Ma; a and straight sections indicative of an active tectonic significant period of subaerial conditions under a environment (Figure 4) with fault-controlled mean- compressive regime between the Alpine orogeny and ders. North Atlantic spreading (Musson, 2007), although it is not generally accepted that these forces affected the London Basin. However, a reappraisal of new and existing data in light of these observations shows that inversion is now affecting the London Basin (Royse et al., 2012); an interpretation in agreement with earlier inferences (Blundell, 2002; Wood and Johnson, 1978). Evidence for fault reactivation and inversion can be found in the borehole records held at the British Geological Survey and available publically online. Approximately 2000 borehole records of more than 20 m depth were analysed for the London Basin Fo- rum Atlas Project through a number of student pro- jects. Borehole TQ38SE1565 in the Lower Lea Crossing [TQ 394 809] has a 10.3 m repeated se- quence of Kempton Park Gravel (described as Thames Ballast) and London Clay (Figure 3). Figure 4. Drainage patterns across London; note the sharp bends and straight sections on the Thames and Lea rivers. 1. Pre-Anglian drainage consists of NE-directed tributaries to a proto-Thames across the north of this map. 2. Modern drainage is almost perpen- dicular and ignores the path of the modern Thames. Places referred to in the text are: L=London, R=Richmond, H=Hampton, and W=Walton. Furthermore, many tributaries to the Thames join at a high angle, implying a regional slope independ- ent of the river floodplain. This relationship is a par- ticularly evident east of Richmond, including in the so-called lost rivers of London (Barton, 1992; de Figure 3. Cross-section in the Lea Valley/Thames confluence showing the repetition of Kempton Park Gravels and London Clay Freitas, 2009). Upstream of Richmond, the Thames encountered in two boreholes, which has been interpreted to have takes a long detour south through Hampton, only been caused by thrusting. turning north again west of Walton. These large-scale deviations have previously been interpreted to result Devensian, 30 to 140 ka ago (Maddy et al., 2001).