Deep-Water Margins, Dynamic Topography and Sequence Stratigraphy

An evolving pattern of convective circulation within the Earth's mantle generates and maintains dynamic topography, which is some fraction of observed topography. Spatial variations of dynamic topography are easy to measure within the oceanic realm since the subsidence history ofoceanic lithosphere as a function of age is well understood. A group of us have used substantial inventories of seismic reflection and wide-angle profiles to determine dynamic topography of the oldest oceanic lithosphere which abuts passive rifted continental margins. Our results show that this old oceanic lithosphere has dynamic topographic anomalies of +/- 1 km with wavelengths of 500-1000 km. These significant anomalies often intersect coastal shelves and so we expect that the development of these anomalies has affected the evolution of deep-water rifted margins and their sequence stratigraphic architecture in important ways. A series of examples will be used to illustrate how the configuration of rifted margins can be profoundly influenced by changing patterns of dynamic topography. First, along the West African and Brazilian margins sets of dynamic topographic domes intersect the adjacent shelves. Onshore, Neogene growth o f these domes is recorded by emergent marine terraces and by radial drainage patterns. Offshore, switches from aggradation to progradation together with a series of younger disconformities have modified stratigraphic architectures of these shelves. Secondly, along the Northwest Shelf of Australia there is excellent evidence for about 700 metres of dynamic drawdown of the oldest oceanic floor which abuts this shelf. Regional mapping and backstripping of clinoformal geometries within a Miocene carbonate reef complex shows that there is a dramatic switch from progradation to aggradation which cannot be attributed to glacio-eustatic sea-level variations. Instead, this switch appears to reflect growth of dynamic drawdown within the mantle. Finally, the Icelandic plume is a large convective upwelling which has controlled vertical motions along fringing North Atlantic continental margins over the last 60 million years. There is independent evidence that the temperature of this plume fluctuates as a function of time over the last 60 million years. These fluctuations are indirectly recorded within the sequence stratigraphic architecture of fringing margins where a series of ephemeral terrestrial landscapes have been mapped on three-dimensional seismic volumes. In this way, transient activity of the Earth's convecting mantle is stratigraphically recorded. Thus the sequence stratigraphic architecture of many, if not all, rifted continental margins appears to be an important repository of details about convective circulation which are otherwise difficult to obtain.