The rift to drift transition and sequence stratigraphy at passive continental margins.
Most passive margins display a prominent breakup unconformity coinciding
with the rift to drift transition. The unconformity, as defined by Falvey, (1974) is
of broad regional extent affecting both basins and highs and is easily recognised on
seismic sections. Criteria for the recognition of the breakup unconformity include an
inflection in the subsidence curve, fault terminations and volcanic strata (and/or
evaporites) at the level of the unconformity. Falvey considered that it was caused by
"erosion during the final uplift pulse associated with pre-breakup upwelling in the
mantle". It is more likely that the uplift is caused by magmatic underplating in
response to the passive upwelling of the mantle and the flexural isostatic effects of
erosion throughout the syn-rift phase.
The primary objective has been to quantify the amount of uplift and erosion
associated with the breakup unconformity / breakup megasequence boundary. This
is of particular importance in hydrocarbon exploration as it quantifies the potential
loss of old reservoirs and predicts the provenance of new reservoir clastics.
Two data sets, from the Grand Banks and the Northwest Shelf of Australia,
have been studied. In both cases there are multiple breakup events and breakup
megasequence boundaries form part of a complex tectono-stratigraphy. Regional
seismic lines have been interpreted, depth converted and modelled using a new
technique of combined reverse post-rift and forward syn-rift modelling. The results
of this process, together with seismic megasequence analysis, show that the
morphology of the breakup megasequence boundary varies systematically across a
passive margin. It is strongly erosional at about 70 km landward of the continentocean
boundary, where regional "breakup" uplift outweighs extensionally controlled
subsidence, but may be depositional on either side of this zone.
A coupled, quantitative magmatic-tectonic model has been constructed by
combining the Bickle-McKenzie melt generation model with the flexural cantilever
model for continental extension. The magnitude of underplating can be estimated
using the Bickle-McKenzie model, in which the amount of melt produced is controlled
by the extension factor, ß, and the proximity of a mantle plume convection cell.