Evidence on the deep structure of the Anglo-Brabant Massif from gravity and magnetic data

Long term planning for flood risk management in coastal areas requires timely and reliable information on changes in land and sea levels. A high resolution map of current changes in land levels in the London and Thames estuary area has been generated by satellite-based persistent scatterer interferometry (PSI), aligned to absolute gravity (AG) and global positioning system (GPS) measurements. This map has been qualitatively validated by geological interpretation, which demonstrates a variety of controlling influences on the rates of land level change, ranging from near-surface to deep-seated mechanisms and from less than a decade to more than 100,000 years' duration.During the period 1997 to 2005, most of the region around the Thames estuary subsided between 0.9 and 1.5 mm a -1 on average, with subsidence of thick Holocene deposits being as fast as 2.1 mm a -1 . By contrast, parts of west and north London on the Midlands Microcraton subsided by less than 0.7 mm a -1 , and in places appear to have risen by about 0.3 mm a -1 . These rates of subsidence are close to values determined previously by studies of Quaternary sequences, but the combined GPS, AG and PSI land level change data demonstrate a new level of local geological control that was not previously resolvable.

Section: Geological Setting Of London and The Thames Estuary Areamentioning

confidence: 99%

Geological interpretation of current subsidence and uplift in the London area, UK, as shown by high precision satellite-based surveying

Aldiss

Burke

Chacksfield

et al. 2014

“…Cornwell & Walker (1989) suggested a Precambrian basement source. ESH East Shetland High Donato & Tully (1982) suggested an association with a granite pluton but Holloway et al (1991) (Pharaoh et al , 1995; early Palaeozoic magnetic metasediments (Lee et al, 1993); Precambrian basement (Wills 1978); Devonian metamorphism (Allsop 1987) FVP Forties Volcanic Province Jurassic volcanic province (Ritchie et al 1988;Smith & Ritchie 1993). …”

Section: Discussionmentioning

confidence: 99%

The deep crustal magnetic structure of Britain

Beamish

Kimbell

Pharaoh

2016

The deep crustal magnetic structure of Britain has not previously been described in a uniform manner. We provide a new assessment of the deep crustal magnetic bodies responsible for the long wavelength magnetic features. The study area contains deep crustal relics of the destruction of early Palaeozoic oceanic lithosphere along the Thor-Tornquist Suture and primarily the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Spectral decomposition is applied to a merged onshore and offshore magnetic anomaly data set. Thirty idealised basement bodies are compared with a representation of the subsurface obtained by a coarse 3D inversion of the data. The central area separating Laurentia and Avalonia, is largely characterised by an absence of high susceptibilities throughout the whole crustal volume. We find that the idealised basement bodies are largely consistent with relatively high susceptibility zones at depths in excess of 10 km. The zones of higher relative susceptibility are referenced to the tectonic-terrane framework of the area and possible geological explanations for the contrasts are reviewed. In the north, the Laurentian terranes are diverse, comprising crust first created in the Archaean (Hebridean Terrane), Palaeoproterozoic (Rhinns Terrane), Mesoproterozoic? (Midland Valley Terrane), Neoproterozoic (sub-Southern Upland rocks) and Ordovician. Magnetic anomalies further record the assembly of the Gondwanan (Eastern Avalonian) part of the country through Neoproterozoic and Ordovician (Tornquist) arc magmatism and accretion. The convergence zones between Laurentia, Avalonia and Baltica have all left a magnetic imprint, as has Variscan convergence to the south.

“…These data have been used to compile the 1:625 000 scale gravity and magnetic maps of the UK (British Geological Survey, 2007a, b). Although regional gravity and magnetic data have been previously discussed and interpreted in the context of deep basement structure (Lee et al, 1990(Lee et al, , 1993Busby and Smith, 2001;Busby et al, 1993Busby et al, , 2006, they have not hitherto been used to understand patterns of Mesozoic sedimentation in East Anglia. Figure 7 shows a colour shaded relief residual Bouguer gravity anomaly map with the key borehole locations plotted; Figure 8 shows the same boreholes plotted on a greyscale shaded relief version of the residual Bouguer gravity anomaly map, overlain by a flat colour reduced-to-pole magnetic anomaly map.…”

Section: Regional Gravity and Magnetic Datamentioning

confidence: 99%

Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations

Woods

Chacksfield

2012

New borehole geophysical log interpretations between Wiltshire and north Norfolk show detailed lateral changes in the spatial relationships of Chalk Group marker beds. They show how marker beds in the Turonian and Coniacian Chalk Group in East Anglia pass laterally into their correlatives further west, and reveal unusual lateral thickness changes affecting stratigraphical intervals in the East Anglian succession. Newly enhanced regional gravity and magnetic data indicate that these thickness changes are probably related to WNW to ESE trending structural lineaments in the Palaeozoic basement rocks of the buried Anglo-Brabant Massif.The later part of the Mid Turonian and early part of the Late Turonian succession across East Anglia is greatly thickened, and shows almost no lateral variability. These relatively soft, smooth-textured chalks equate with thinner, hard, nodular beds formed in both shallow marine and deeper basinal settings elsewhere in southern England. Since it seems unlikely that there was greater sediment accommodation space across East Anglia at this time compared to basinal areas, this thickening may reflect a localised coccoliths productivity pulse, or perhaps a sheltered palaeogeographical position that protected the area from sediment-winnowing marine currents.A residual gravity low across north Norfolk, previously interpreted as a granite pluton, may instead represent two elongated (?fault-bounded) sedimentary basins.