TheLondon–Brabant Massif orLondon–Brabant Platform is, in thetectonic structure of Europe, astructural high ormassif that stretches from theRhineland in western Germany across northernBelgium (in theprovince of Brabant) and theNorth Sea to the sites ofEast Anglia and the middleThames in southern England.
The massif also occurs in the Belgian subsurface, where it is bounded to the northeast by theRoer ValleyGraben that runs diagonally throughDutch Limburg.
TheMidlands Microcraton (southeasternWales and part of western England) is often considered part of the massif and to reflect this the namesWales–Brabant Massif,Wales–London–Brabant Massif andWales–Brabant High are sometimes used. This massif was also formerly referred to, at least in part, asSt George's Land.[1]
The London–Brabant Massif is part of the formermicrocontinentAvalonia. To the south it borders theRhenohercynian Zone of theHercynian orogeny. To the northeast it is flanked by theAnglo-Dutch Basin in the subsurface of the North Sea.
At times in geologic history the London–Brabant Massif formed an island, which is called theLondon–Brabant Island.
The massif is composed ofcrystallinebasement (metamorphic andigneous rocks) withProterozoic to earlyPaleozoic ages. It wasdeformed andmetamorphosed during theCadomian orogeny (Ediacaran, about600 million years ago) andCaledonian orogeny (Silurian, about420 million years ago). This basement is almost everywhere overlain by youngersedimentary rocks, except for some places in the southwest of England and in Wales.
The continentAvalonia was until theOrdovician (465 million years ago) part of the large southern continentGondwana, but then begandrifting independently to lowerlatitudes. As it passed through the dry latitudes represented today by theNamib Desert1, it waseroded and thesoils becamelaterite. Thestrata, particularly of thePrecambrian are complex. Their continuity is also poorly understood because they are beyond the reach of mostboreholes.
The period from which the island has exercised most economic influence on modern Europe was theCarboniferous. As the continent was drifting past theEquator, on the island's shores, there grew a richtropical forestswamp. On the island's southern shore, it left theDinantian,Namurian andWestphaliancoal fields of France, Belgium and westernGermany. SeeAachener Revier (in German).
To its northwest, the thinner crust between it and theMarket Weighton Axis was crumpled between the blocks leaving low ridges of wet land between strips of water such as theWidmerpool Gulf. On the wet land, the coal fields ofLeicestershire, Nottinghamshire andDerbyshire were deposited. These extend further east but are now at ever greater depth. At the modern eastYorkshire and northLincolnshire coast for example, their upper surface is at about 2 km depth. These Carboniferous beds are part of a system linking with those of Westphalia, around the north side of the island. On the northNorfolk coast, the line of the Carboniferous shore roughly coincides with the modern one.2
As the continent drifted northwards, away from the Equator, through the latitudes represented today by theSahara desert, the erosion was renewed. This time, the lateritic soils are represented by theNew Red Sandstone and the red soils of Leicestershire andRutland.
Theearly Permian was the time of the height of theVariscan earth movements as the crust to the south was crushed against the island. The great disturbances seen at the surface inBrittany, theArdennes and theRhineland also lie below the Paris Basin. They fade out in the gentleranticline of the downs and Weald of southern England which overlies the edge of the island. The axis of this anticline is normally called the northern Variscan front. However, the chalk of the downs is Upper Cretaceous, so the process continued well after the Permian. The point in the present context is that the stability of the island contrasts with the relatively unstable crust to its south, which was forced into a long mountain ridge.
To the north, economically important things were happening. Western Britain was pushed up as part of the Variscan Orogeny while the east of Britain, including the island began to subside leaving a broad basin, north of the island and south of Scandinavia. This formed a shallow sea in a very dry climate. Desert sands and salt basins were a result but there are also mudstones. This provided the alternating porous and impervious rocks which have trapped the gas escaping when the coal measures, below were subjected togeothermal heat. This has left a group of gas fields off the Norfolk coast. That is to say, off the coast of the island.
In the earlyJurassic, theRhaetic sea flooded much of thePermian plain. On the margin of the London–Brabant Island, theestuarine conditions which left theLower Estuarine Series prevailed for a while before the sea rose so as to deposit theLincolnshire limestones before falling again so that theUpper Estuarine Series was left. Again the sea rose to deposit theBlisworth Limestone, theBlisworth Clay and theUpper Jurassic clays.
The same general pattern occurred in France leaving theParis Basin flooded from Anjou to Luxembourg.
By theCretaceous the island had sunk much further in relation to the sea level. Before the end of the period, the British end was buried inUpper Cretaceous chalk. This happened because the Pacific Ocean bed swelled up causing the world's seas to rise but also, the process released much carbon dioxide.
It is now best viewed as a block of dense crust floating deeply sunk into the mantle and overlain with less dense superficial rocks. It depresses the boundary of the crust and the mantle (Mohorovičić discontinuity, commonly Moho) to depths greater than 40 kilometres as against a figure at the top of thecontinental shelf of about thirty and less than fifteen belowoceanic depths.3
The map shows that there is some tendency for such seismic activity as there is in the region to occur around the margin of the massif. It was into this pattern that theDover Straits earthquake of 1580 and the2008 Lincolnshire earthquake, the latter marked by an orange star, fell.