Origin of anomalous Tertiary subsidence adjacent to North Atlantic continental margins

Abstract

At the beginning of the Tertiary, the northern North Atlantic continental margins were characterized by voluminous igneous activity caused by continental rifting in the presence of the Iceland hot-spot. Many of the Mesozoic sedimentary basins close to these margins have been permanently uplifted by igneous underplating and remain close to or above sea level at the present day. However, a significant number of other basins underwent rapid subsidence at the same time, sometimes by as much as 3 km of sediment-loaded subsidence in 15–20 Ma. Several hypotheses attempting to explain the cause of this Tertiary subsidence have been proposed. The observational data are summarized here and then each hypothesis is considered in turn. It is argued that many of these are implausible. One possible alternative explanation links Early Tertiary subsidence/uplift and magmatism. If basaltic melts are trapped within the lithosphere, it is possible to produce either uplift or subsidence depending on the density of the trapped melt. At depths greater than 50 km, melts will solidify rapidly in the garnet stability field to form primary eclogite, which is dense enough to cause subsidence, while satisfying the constraints of long-wavelength Airy isostasy. Unfortunately, such an hypothesis is difficult to test. The simplest explanation for Tertiary subsidence is still a minor episode of lithospheric stretching, although to date there is only limited evidence for sufficient Tertiary normal faulting. Further work must be carried out before this simple and testable hypothesis can be confidently rejected.