Motion Of Australia Research Articles

Reversible subsidence on the North West Shelf of Australia

The Northwest Shelf (NWS) of Australia is characterized by offshore basins associated with Permian and Jurassic rifting and was only slowly subsiding by the Neogene. International Ocean Discovery Program (IODP) Expedition 356 targeted this region by coring four sites in the Northern Carnarvon and Roebuck Basins and two sites in the Perth Basin to the south on the Australian western margin. We use detailed lithological, physical property and age data with paleobathymetric interpretations, to infer tectonic subsidence apparently confined to the NWS that reverses (uplifts) with about the same amplitude and rate as an earlier subsidence event. About 300 m of tectonic subsidence occurred over one million years from 6 to 5 million years ago and then reverses when 300 m of tectonic uplift occurred from 2 to 1 Ma. The along strike extent of this subsidence pattern is ∼ 400 km. The similarity of magnitude and duration of the subsidence and uplift phases suggest that the subsidence is reversible. The results cannot be explained by glacial eustatic variability nor can the uplift event be attributed to sediments filling the accommodation space generated earlier. Reversible subsidence is a key fingerprint of dynamic topography. Although the rates of subsidence and uplift are roughly ∼ 300 m/Myr, a substantial portion of the changes occur over less than 1 Myr and the rates inferred from a detailed least squares analysis can reach up to about 500 m/Myr. These rates are incompatible with dynamic topography associated with motion of Australia over large-scale convection (10 to 40 m/Myr) or that associated with instability of the base of the lithosphere (<15 m/Myr). The vertical motions are too large to be associated with simple flexure of a plate and plate buckling in that the required amplitudes would lead to permanent deformation of the plate. A new geodynamic mechanism is required to fit the observations.

Slip Rate Estimation of the Lembang Fault West Java from Geodetic Observation

The Sunda arc forms the southern border of the Indonesia Archipelago, where the Indo-Australian plate is subducted beneath Eurasia. The age of subducting plate increases from Sumatra in the west to Flores in the east. The increase in age is consistent with an increase in plate dip along the arc and an increasing depth of seismic activity. The motion of Australia with respect to West Java is 68 mm/yr in a direction N11E orthogonal to the trench. A number of active faults characterizing this area include Cimandiri fault, Lembang fault and Baribis fault. This research uses campaign and continues GPS data to make a preliminary estimation of the slip rate of Lembang fault. Our GPS measurements suggest that Lembang fault has shallow creeping and deeper locking portion. The estimated slip rate is 6 mm/yr with fault locking at 3-15 km and shallow creeping with the same rate. While the results are preliminary and we need more data for reliable estimations, we point out that these data can contribute to earthquake risk assessment by constraining earthquake recurrence relationships.

Outline of Phanerozoic History of Australia and Surrounding Oceanic Region: ABSTRACT

The Tasman orogenic zone has been accreted onto the eastern side of a Precambrian shield. A series of transitional to postorogenic basins has been superimposed. The Tasman orogenic zone, which was mobile from the Cambrian to Early Cretaceous, underwent progressive but stepwise cratonization with culminations or orogenesis in the Early Ordovician, Middle Devonian, Early Permian, and Early Cretaceous. Relicts of this orogenic zone crop out in New Zealand and New Caledonia, and are inferred in the foundered continental blocks of the Queensland Plateau, Lord Howe Rise, Norfolk Ridge, and the Campbell Plateau. The Tasman orogenic zone, which differs considerably from the classic model, can be explained as an active plate boundary between a Paleopacific-oceanic plate and a continental plate. Mountain building has been of a modest scale and no great crustal thicknesses have been developed. The sediment filling the various basins is mainly clastic and, in the early and middle Paleozoic at least, had a dominant southerly source. Antarctica must have been a significant contributor, and probably remained so even in the Permian and early Mesozoic when parts of the Eastern Highlands were elevated. In the early Paleozoic, carbonate rocks are more abundant in the north of the continent. Fragmentation of this part of Gondwanaland commenced in the west in the Late Jurassic to Early Cretaceous with the formation of the Wharton basin. In the east, the Tasman Sea started to form in the Late Cretaceous. Although an incipient rift formed across southern Australia in the middle Mesozoic, new seafloor formation and northward motion of Australia away from Antarctica did not commence until the Paleocene. Eastward accretion of oceanic crust took place in the Eocene and Oligocene (the Coral Sea and the South Fiji basin). Some phases of plate convergence occurred in this time interval resulting in overthrusts of oceanic crust. Relocation of the Pacific plate boundary inside the Melanesian arcs resulted in some of the sea floor, formed during extension, being consumed from the Mioc ne onward. Folding and uplift in the island of New Guinea also took place in the later Tertiary and Quaternary. The continental shelf and thick sediment accumulations in marginal basins have formed as a response to rifting. End_of_Article - Last_Page 1453------------