Tertiary Rivers: Tectonic and structural background
Fig.1 Tectonic map of the British region in the early Palaeogene.
Two major tectonic influences have affected the NW European continent during the Cenozoic: fragmentation of the Eurasian-North American plate, and Alpine orogenesis. The interplay of these two processes has resulted in great complexity of both structural and depositional patterns. In essence these two processes work in opposing ways, i.e. the break-up of the northern hemisphere plate produces tensional or extensional features whereas Alpine mountain building, arising from continent-to-continent collision between the Eurasian and African plates, produces compressional features. Active tectonics led to widespread reactivation of older structures, particularly those of Variscan origin (Fig. 1).
Particularly important for British landscape development was rifting of the Greenland-European plate in the early Palaeocene. This caused thermal uplift of Scotland and the East Shetland platform and volcanic activity. This activity and uplift was attributed to magmatic underplating resulting from a mantle plume that occurred beneath northern Britain during the early Tertiary. After about 54 Ma opening of the North Atlantic by seafloor spreading resulted in the westwards movement of the magmatic plume from beneath the continent, leaving the northern Britain uplifted to the present. However, in southern Britain extensional development involving Variscan structures initially led to the development of depobasins that were later deformed by northwards transmission of lithospheric stresses generated during Alpine orogenesis.
After early Tertiary fragmentation of the Eurasian-North American crustal plate, the rift system of the North Sea Basin became inactive. After the continental separation the crustal relaxation resulted in continued uniform basinal subsidence, which followed from a pre-existing pattern of differential subsidence initiated in the Jurassic .
The Channel may have had a similar origin, in that ocean crust may possibly have been formed here, but that events affecting the northwest European continent may have deflected this zone south-eastwards beneath the land mass. Inversion events can be equated to four progressive deformation stages recognised in the North Alpine Foreland Basin. The first is Laramide tectonism in the early -middle Palaeocene caused mild inversion movements in the Celtic Sea, Western Approaches and the Channel region. This was followed in the middle Eocene by uplift of the Weald and Celtic Sea basins, and subsidence of the London and Hampshire-Dieppe basins. Uplift and folding of Eocene strata also occurred on the Isle of Wight. Later uplift in post-Eocene times related to Alpine compression reached a peak in the middle-late Miocene, with tectonic activity continuing intermittently into the Pliocene and even the Pleistocene resulting in inversion of major Mesozoic basins, including the Hampshire-Dieppe and Channel Basins and the Bristol Channel and Western Approaches Trough, and updoming of the Pays de Bray Anticline in the Paris Basin and the Weald-Artois Anticline. Uplift of basement block massifs, particularly aligned along the western and northern part of the region, also occurred during these phases. This was accompanied by rapid subsidence of local pull-apart basins during the Oligocene, such as the Lough Neagh basin in Northern Ireland, the Cardigan Bay basins and in Cornwall, which occurred as a consequence of short-lived wrench faulting along the NW-SE-aligned Sticklepath-Lustleigh system. The post-Miocene uplift of southern Britain cannot be attributed to basin inversion but more probably results from compensatory isostatic rebound following denudational offloading, possibly accompanied by strain release. However, uplift during the Neogene fits a pattern of late Cenozoic intraplate uplift that is known from around the North Atlantic.
The loss of Mesozoic rocks that once covered large areas of western and northern Britain has been the subject of extended discussion. The pulsed uplift resulting from periodic magma injection, noted above, resulted in surface rejuvenation, producing substantial siliclastic deposition in the North Sea between 54-62 Ma. Fission-track and vitrinite reflectance data have demonstrated that denudation of the eastern Irish Sea Basin has undergone between 1.5 to 3 km of exhumation with attendant removal of surface rock cover in the early Tertiary. Moreover, fission-track studies have shown evidence for a mid-Cenozoic cooling event that began c. 30 Ma that may have caused a further 1 km of denudation along the NW European continental margin resulting from changes in mantle heat-flow. This relief creation and erosional activity was much more pronounced in the north than further south and later in the Tertiary, which reflects the position of the magmatic plume and its movement west as the North Atlantic opened. This process has been invoked to explain the early Tertiary history of Britain, for example. It has been suggested that uplift centred on the north-west of Britain may explain the south-eastwards tilt of the English-Welsh block, attributed, to a Cretaceous-aged hotspot centred on the Irish Sea. Such a hotspot would have led to the injection of igneous material below the surface causing dome-like uplift of an area 500 km in diameter. The resulting updomed area has been particularly deeply eroded but its effect is thought to explain the alignment of the modern lowland British drainage pattern, albeit modified by Pleistocene glaciation, although this is not universally accepted.
By far the largest Cenozoic basin is the Northwest European Basin, which extends from Poland to the northern North Sea. This basin had become stabilised by the Miocene and subsequently subsided in an irregular manner . This, together with the North Sea Basin, received huge volumes of sediment during the Cenozoic, particularly during the Palaeogene. The latter basin is estimated to contain up to 3500 m of sediment for the whole period compared with a thickness of over 1000 m from the Quaternary alone.