Structures in the basement beneath the London Basin affect the geology of relevance to geotechnical engineering within London. Unfortunately, the basement beneath London is covered by Cretaceous and Tertiary sediments. It is cut by major faults linked to the compressive phases of the Hercynian and Alpine Orogenies and to the regional extension that occurred during the Mesozoic between these compressive events. Evidence is presented that movement on basement fractures beneath London played a major role in the distribution and deformation of sediments within the Basin, causing local folding and faulting significant to engineering works. Basement rocks are exposed in SW England where the type and orientation of these fractures (faults and joints) can be examined in outcrop. This study, complemented by seismic sections in the southern UK, enable the architecture of this fault network within the basement to be determined. Understanding the fracture system in the basement provides a basis for (i), interpreting the lateral facies variations of sediments in the Basin and hence provides a means for predicting from a ground investigation the likely presence, activity or influence on site of such structures at depth and (ii), understanding the extent of local, steeply inclined and sub-horizontal planar zones of shearing when encountered on site.

Thematic collection: 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

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