North West European Rivers
1. TECTONIC BACKGROUND
Throughout the Cainozoic era, north west Europe has been affected by two
major tectonic influences: fragmentation of the Eurasian-North American
plate, and Alpine orogenesis. The interplay of these two factors has resulted
in a 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. The forces
have led to activity along pre-existing tectonic lines that have been prevalent
for long periods and in many instances have been inherited from previous
phases (map 1).
MAP 1. NW Europe: Tectonic map of the sub-Tertiary surface.
After the early Tertiary fragmentation of the Eurasian-North American crustal plate, the rift system of the North Sea Basin became inactive. This rift system was formed in the Mesozoic as a secondary rifting line in response to extensional stresses.
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. Later widespread uplift in post-Eocene times reached a peak in the Miocene resulting in inversion of the Hampshire and Channel Basins and the Bristol Channel and Western Approaches Trough, and updoming of the Pays de Bray Anticline in the Paris Basin. This uplift was related to late phases of the Alpine orogeny. Local minor activity has continued into the Pleistocene.
By far the largest Cainozoic basin is the Northwest European Basin, extending from Poland to the northern North Sea. This basin had become stabilized by the Miocene and subsequently subsided in an irregular manner. This, together with the North Sea Basin, received huge volumes of sediment during the Cainozoic. 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. This implies a ten-fold increase in sedimentation during the Quaternary. Its existence and infill would have been encouraged by the uplift of the Fennoscandian Shield that took place during the Pleistocene.
In contrast to this basin, the foredeep basins of the Alps were much smaller and they subsided strongly during the Tertiary. Their infill was greatly deformed by late Alpine orogenic phases. Further, by late Tertiary times the basins had become uplifted and their sediments had been eroded as a result of post-orogenic isostasy of the mountain belt.
According to the same author, the Rhine and Rhne Grabens form part of a late Cainozoic collapse system that began to subside during the Eocene and has continued intermittently since then. This subsidence is contemporaneous with the orogenesis such that, during formation of the Jura Mountains in the Pliocene, thrust sheets overrode the southern end of the Rhine Graben and the eastern margin of the Rhone Graben. The Rhine Graben extends north-northwest into the Lower Rhine Embayment as the graben and horst structure of the southern North Sea Basin. There is, however, no evidence of reactivation of the North Sea rift system during the Late Tertiary. The Rhine Graben also continues northward into the North German Plain. Moreover, rifting of the Rhine Graben is also accompanied by alkaline volcanism that continued until very recently.
Uplift of the Massif Central and the Bohemian Massif also occurred during the Neogene. In both cases this was accompanied by alkaline volcanism that began in the Oligocene and has continued up to the present. Dissection of the Alpine foreland rocks resulted from this uplift, together with that of the Vosges-Black Forest rift dome. This uplift also isolated the Paris Basin.
These important events have had profound effects on the fluvial system, which has undergone considerable adjustment and modification to adapt to the changes. As can be seen from a comparison of the tectonic and late Pliocene palaeogeography maps (maps 1 and 2) there is a close correspondence of major drainage lines and distribution of structural elements. For example, the Baltic River is aligned along the long axis of the North west European Basin, the central German rivers drain northwards towards this basin, the rivers Thames, Meuse and Rhine drain towards the North Sea Basin and the Seine drains towards the Channel and follows a major fault zone. In addition, the Rhine occupied the Central Graben region of the southern North Sea, Lower Rhine Embayment and the Upper Rhine Graben. Subsequent disruption of this pattern by later glaciation, sea-level change and river migration and capture has occurred, but nevertheless the tectonic effects remain an important controlling element.