Dr. Garry Karner
Earth Institute Contact: Dr. Garry Karner
The project will undertake an integrated geodynamic/seismic stratigraphy analysis of the subsidence history of the extended margins off northwest Australia. Many passive continental margins are characterized by a regional distribution and thickness of syn-rift and post-rift sediment packages that are not consistent with the minor amounts of brittle deformation observed in either seismic sections across the margin (e.g., Exmouth Plateau, northwest Australia; Marion Plateau, northeast Australia; Grand Banks; Brazilian and West African margins; and West of Shetlands basins) or from field mapping of syn-rift systems (e.g., Brazilian and West African margins). While the geological details and sedimentary facies differ between the various margins, the style of deformation and the regional distribution of accommodation are remarkably similar. The development of significant post-rift accommodation in the same region characterized earlier by minor syn-rift faulting and shallow depositional environments has been explained by Driscoll & Karner (1998) in terms of depth-dependent extension that is partitioned vertically across a zone of decoupling that results in the development of a relatively non-deforming upper crust (i.e., the upper plate) from a ductile-deforming lower crust and lithospheric mantle (i.e., the lower plate), the boundary between them having a ramp-flat-ramp geometry. A major problem with this hypothesis is that extension within the lower plate needs to be balanced by an equal amount of extension within the upper plate, although the lateral distributions need not be the same. The exact form and location of the counterbalancing upper crustal extension presumably exists in the vicinity of the ocean/continent boundary where the extensional balance through the crust probably occurs by a combination of thinned and “rafted” crustal blocks that expose the continental mantle. Nevertheless, it remains to be shown that this strain balance actually exists for the Exmouth Plateau. Analysis completed thus far of R/V Ewing cruise EW0113, which collected over 2000 km of multi-channel seismic reflection and more than 800 km of OBS wide-angle refraction data across the continent-ocean transition zone of the Exmouth Plateau and Cuvier passive margin segments in 2001, has allowed: 1) Mapping of the ocean-continent transition, which has shown the existence of a broad transition zone ~200 km wide containing landward and seaward dipping reflectors and upper continental blocks, 2) Re-examination of the sequence strati-graphic framework for the western edges of the Exmouth Plateau and the Cuvier margin, requiring refinements to the early Cretaceous rifting stratigraphy and the effect of early late Cretaceous inversion, and 3) testing the extensional strain balance across the margin and ocean-continent transition. Kinematic and flexural modeling of the stratigraphic and structural development of the Western Ex-mouth Plateau indicates an approximate balance between a highly extended lower crust, which is the foundation of much of the ocean-continent transition, and extended, broken and translated upper crustal blocks. Underplating has occurred, but is relatively minor and has helped source the basalts that comprise the seaward- and landward-dipping reflectors.
Cross Cutting Themes:
Hazards and Risk
National Science Foundation