ORIGIN OF ANOMALOUS TERTIARY SUBSIDENCE ADJACENT TO NORTH-ATLANTIC CONTINENTAL MARGINS

At the beginning of the Tertiary, the northern North Atlantic continen tal margins were characterized by voluminous igneous activity caused b y continental rifting in the presence of the Iceland hot-spot. Many of the Mesozoic sedimentary basins close to these margins have been perm anently uplifted by igneous underplating and remain close to or above sea level at the present day. However, a significant number of other b asins underwent rapid subsidence at the same time, sometimes by as muc h as 3 km of sediment-loaded subsidence in 15-20 Ma. Several hypothese s attempting to explain the cause of this Tertiary subsidence have bee n proposed. The observational data are summarized here and then each h ypothesis is considered in turn. It is argued that many of these are i mplausible. 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 fo rm primary eclogite, which is dense enough to cause subsidence, while satisfying the constraints of long-wavelength Airy isostasy. Unfortuna tely, such an hypothesis is difficult to test. The simplest explanatio n for Tertiary subsidence is still a minor episode of lithospheric str etching, although to date there is only limited evidence for sufficien t Tertiary normal faulting. Further work must be carried out before th is simple and testable hypothesis can be confidently rejected.