Modelling overpressure development and the mechanical behaviour of sediments in a complex, tectonically active setting: East Coast Basin, New Zealand [dataset]

The East Coast Basin (ECB) is a complex, active convergent margin within the North Island of New Zealand, where variable but commonly very high overpressures occur in Cretaceous to Pleistocene stratigraphy, including near-lithostatic pressures at shallow burial depths (<300 m). This study investigates the controls on overpressure generation through time and the changes in mechanical properties of sedimentary rocks as a result of rapid burial, uplift, active tectonic compression, and changes in effective stress. A regional investigation of the Cretaceous to Pleistocene tectono-stratigraphy was combined with seismic and well-log interpretation to understand the structural, stratigraphic, and sedimentation history of the ECB. 1D hydro-mechanical models, which simulate the geological histories of five key wells across the ECB, were built to investigate the overpressure contribution generated by burial disequilibrium compaction and tectonic compression. Present-day overpressure is mainly the result of recent (<6 Ma) tectonic compression because of the subduction of the Pacific Plate beneath the Australian Plate, generating up to ~18 MPa overpressure at 2,000 m TVDSS depth. Burial disequilibrium compaction played a secondary role in overpressure generation, up to 4 MPa at 2,000 m TVDSS depth. Whilst erosive events result in overpressure dissipation, the effects of all but the most recent/ongoing erosion event are masked and overprinted by subsequent sedimentation. Critical state soil mechanics models were used to track the rheological behaviour of low permeability intervals through tectonic and erosive events, with mechanical changes for mudstones and fine-grained carbonates predicted to be through compaction rather than dilation (shear).