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Models of Extensional Basins
Intraplate Stresses and Apparent Changes in Sea Level: The Basins of Northwestern Europe
Apparent sea level fluctuations deduced from seismic stratigraphy and facies analyses have been attributed to either global changes in sea level or to subsidence and uplift of shorelines by tectonic processes. A specific tectonic model is examined in which changes in sea level relative to land are produced by an interaction between fluctuating horizontal stress fields in the lithosphere and basin subsidence resulting from lithospheric cooling and sediment loading. Relative changes of a few tens of metres can be produced by changes in the stress regime of about 100 MPa. If this hypothesis is valid it suggests that regional relative sea level curves can be used as paleostress indicators, and that these observations contain information on basin formation and basin subsidence mechanisms. The hypothesis has been tested for the region of northwestern Europe where sea level changes can be associated with major rifting and compressional episodes during Mesozoic and Tertiary times. The major Tertiary lowerings in relative sea level at the basin margins can be interpreted in terms of increasing horizontal compression; the major drops in sea level do coincide with the compressive folding phases of the Alpine Orogeny.
The model can be generalized to include basin subsidence produced by lithospheric stretching: an increase in the tensional stress producing onlap or an apparent rise in sea level at the basin margin, and a decrease in tension producing an offlap or apparent fall in sea level. The gradual rise in sea level recorded in the northwestern European basins throughout the Jurassic and Cretaceous can then be associated with the extensional tectonics, and the periods of offlap can be associated with relaxation of the regional stresses as would result locally when active rifting occurs. Most of the Jurassic and Cretaceous North Sea offlaps do in fact coincide with active rifting episodes.
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