SEMINAR: Geohazards and Seafloor Stability of Australia�s North West Shelf

The North West Shelf of Australia is characterized by two types of plate boundaries: a passive Atlantic-type margin on the west and an active collisional boundary on the north. Our research is developing information which demonstrates that the stresses imposed on the plate due to the collision in the north are causing regional lithospheric scale warping, surface faulting, and reactivation of former rift-related structures along the continental margin. The broad scale regional tilting along the North West Shelf has resulted in the subsidence of former coastal depositional environments below the recorded late Quaternary sea-level minima. The presence of these depositional environments can have a significant impact on the stability of soils within the foundation zones of offshore infrastructure and due to the down-warping these deposits can occur at greater depths than otherwise would be expected.

Although deformation is occurring at low rates, some structures along the continental margin of Western Australia (WA) are capable of producing large magnitude earthquakes. For example, the Mt. Narryer fault zone in west central Australia may have produced the 1885 ML 6.6 Mt. Narryer and the 1941 ML 7.1 to 7.3 Meeberrie earthquakes. The fault zone is a 120-km long north-trending structure that has captured and diverted active stream flow,formed sag ponds, and impounded Lake Wooleen. Scarp heights of 3 to 8 meters suggest either single large magnitude displacements, or repeated surface deformation events (i.e. repeated large magnitude earthquakes). Additional structures trend offshore and these have caused local inversion of older rift basins and in places have offset the seafloor indicating recent movement. Structures with evidence of Neogene to historical deformation are present over a length of more than 2,000 km along the continental margin of west-central and northwestern WA.

Large scale slope failures are present along the continental slope and relict landslide deposits are present along the continental rise. Some slides have widths of several 10’s of kilometres with run out lengths of up to 200 km. Smaller slide deposits are concentrated at the base of the slope with run out lengths commonly of <10 km. Expulsion features are common on the Gorgon slide and other slide complexes indicating that dewatering may have occurred during mobilization of the slide masses, or that gas expulsion may have played a role is landslide triggering. The presence of a zone of seismogenic structures with the potential to produce earthquakes in the magnitude Mw 7.0 to 7.5 range along the continental margin suggests that earthquake loading also may be a trigger for submarine slope failures on the North West Shelf.