Under-representation of faults on geological maps of the London region: reasons, consequences and solutions

Abstract

London lies mainly within an area of long-term tectonic stability known as the London Platform. This is characterised by relatively thin Cretaceous and Palaeogene sequences overlying Palaeozoic basement at shallow depths, less seismic activity than surrounding areas and, according to published geological maps, little faulting.However, observations of temporary exposures and borehole records, and other studies, show that in reality faults are numerous and widespread in the London region. Their relative absence on the geological maps is a consequence of past mapping methods, coupled with the relative uniformity of extensive bedrock units such as the London Clay Formation and the Chalk Group, and the widespread presence of Quaternary and anthropogenic deposits, and of urban development. However, complementary approaches to geological surveying, including the use of geophysical data and satellite-based radar interferometry, together with geological modelling in three dimensions using subsurface information, provide the means to accurately survey fault systems even in the most densely urbanised areas.Such work shows that earth movements in the London area, apparently including nearsurface fault displacements, have taken place during the late Quaternary and continue at the present. These findings are important to civil engineering projects and hydrogeological studies in the London area and to understanding local tectonic development.2

Anomalous buried hollows in London: development of a hazard susceptibility map

“…The review also suggests that there are isolated hollow occurrences that do not occur near a fault or within modern river valleys, indicating that they may result from a different mode of formation. The possibility that these features are associated with unmapped faults (Aldiss 2013), such as strike-slip faults (dextral shear associated with the Variscan orogeny; de Freitas 2009) that have not been identified in the 3D modelling of the geology, should also be considered.…”

“…Of the 31 buried hollows originally identified, nine lie within 500 m of a mapped (major) fault and a further six lie within 1000 m of one. However, to date BGS mapping has focused on the faults that can be traced through unexposed ground because they juxtapose different rock formations along much of their outcrop (Aldiss 2013;Royse 2010;Fig. 4) and it is suspected that faults are under-represented by current mapping (Aldiss 2013).…”

Response to Owen, H.G. (2014), Discussion on “Aldiss, D.T., Under-representation of faults on geological maps of the London region: reasons, consequences and solutions” [Proc. Geol. Assoc. 124 (2013) 929–945]

“…Although this interpretation is attributed by Owen (2014) to Lake (1975), Lake's figure 1 shows a questioned north-south line of faulting -presumably placed in a more-or-less arbitrary position for lack of evidence -on the west side of the coalfield, following ShephardThorn et al (1972). More recent geological mapping in east Kent inferred the existence of some NNW-SSE faults in that area (Aldiss, 2013, fig. 4b), substantiating the interpretation given by Owen, but also found extensive NE-SW faulting, suggesting that faulting on that trend would also be expected to be present in the Palaeozoic basement in that area, and might play some part in determining the extent of the coalfield.…”

“…1) suggests that the Variscan Front (as then understood) could be as far south as a line between Dorking and Biddenden. Lake did not, however, have the benefit of modern regional gravity anomaly maps, which show a similar pattern of Bouguer anomalies throughout the Weald, at least as far north as the Lower Greensand escarpment on the northern fringe of the Weald (Aldiss, 2013, fig. 11).…”

“…I thank Dr Owen for his comments on my paper (Aldiss, 2013) and for his useful exploration of several aspects of the tectonic development of the London region. I am especially grateful for him drawing attention to his evidence for Quaternary faulting of the Gault in the south London area, and to several other pertinent and useful papers, notably Lake's (1975) discussion of the tectonics of the Weald and those by Owen (1971Owen ( , 2012.…”