APPLYING AN EPEIROGENIC TYPE UPLIFT TO SUPPOSED PASSIVE MARGINS: GEOMETRY AND TECTONIC HISTORY OF THE CANTERBURY BASIN, SOUTH ISLAND, NEW ZEALAND

Epeirogenic is a term used to describe the long wavelength to amplitude uplift or subsidence effecting a region of a tectonic plate not attributed to compression or extension. This type of tectonism is due to density changes in the mantle causing changes in the regional stress field. Density changes caused by underplating and upper mantle removal can cause uplift or subsidence. Recent data interpretation from IODP Expedition 317 suggests a regional epeirogenic uplift event has affected the Canterbury Basin while preserving the overall passive margin geometry.

The Canterbury Basin is classified as a passive margin that formed after rifting of the Zealandia from Gondwana at ~83 Ma. Cores from IODP Expedition 317 shelf Sites U1351, U1353, and slope U1352 have been resampled with emphasis on benthic foraminifera from the Late Eocene to the Late Pliocene. Using these data together with water-depth data from the nearby Clipper1 industry well, a new basin subsidence model has been generated for the Canterbury Basin. Corresponding tectonic uplift at both Site U1352 and Clipper1 suggest uplift at ~4.3 Ma, causing the basin to reach its current elevation by ~3.7 Ma. Inconsistent tectonic subsidence results observed between the wells can be resolved by making the assumption that contamination has occurred due to shallow water benthic foraminifera transported down-dip. All observations of Miocene benthic foraminifera at shelf sites U1351 and U1353 are neritic, requiring down-dip transport of shelf sediments if the deep-water Miocene depths observed at Site U1352 and the Clipper1 well are accepted as correct.

This uplift event was epeirogenic in nature and can be explained by migration of the South Island over a slab graveyard. In this model, water released from these subducted slabs causes metamorphism of lithospheric mantle to a low viscosity, eclogite facies. This negatively buoyant material then separates from the overlying crust along or below the Moho and sinks into the mantle in a process called delamination. In response, asthenosphere upwells to replace this delaminated upper mantle material. The resulting decrease in lithospheric thickness and, thus, density produces epeirogenic uplift. Based on this hypothesis, both the rapid uplift and the preservation of the passive margin geometry of the Canterbury Basin shelf basin is explained.