Structure and formation of the Argo Abyssal Plain and the adjacent shelf off the North West Australia

The Argo Abyssal Plain northwest of Australia is one of the few preserved patches of Jurassic ocean crust that has not been subject to reheating by mantle plumes. This study examines the crustal configuration and subsidence history of the south-east Argo Abyssal Plain and adjacent continental margin. Wide-angle seismic data from ocean bottom seismographs (OBS), together with gravity and multichannel reflection data were used to define the crustal-scale features across the transition of the Canning offshore basin from continental to oceanic crust of the Argo Basin. Twenty three OBS instruments were deployed across the transition of the Canning offshore basin from continental to oceanic crust of the southeast Argo Abyssal Plain. OBS data were corrected for clock drift and the position of each OBS instrument. Finally the data were organised in SEG-Y format files . Additional processing was carried out on the OBS data to enhance the seismic images. The additional processing consisted of two-dimensional filtering in the wave number and frequency domains. The travel times of the most prominent compressional and shear wave arrivals were digitised and modeled using dynamic forward ray tracing and the Zelt and Smith's [1993] inversion scheme. The main findings from the seismic model of the oceanic and continental portions of the crust can be summarised as follows: • The oceanic crust is 7-7.1 km thick, which is typical for normal oceanic crust adjacent to the non-volcanic passive margins. Thus, the anomalous depth of the Argo Abyssal Plain cannot be explained by anomalous thin crust. • Compressional velocities ('P' ) in the lower part of the lower oceanic layer (Oceanic Layer 3) are significantly different for two seismic transects. The average 'P ' velocity at the base of the Layer 3 in the strike line (7.4 km/s) is --0.15 km/s higher than the average 'P' velocity found for the oceanic lower crust in the dip line, which is 7.25 km/s. It is proposed that high magnitude of horizontal compressional stress at present day oriented NE-SW and a horizontal minimum in stress parallel to the spreading direction [Cloeti11gh and Wortel, 1985; Bull and Scrutton, 1992] may account for the differences in 'P ' velocity in the lower part of the oceanic layer 3 observed on two seismic lines: 7.25-7.3 km/s for the dip line and 7.4 km/s for the strike line. Results of modeling of OBS data from the Canning offshore basin show that the preexisting continental crust is thinned to a thickness of~ 14.5 km. In the continentocean transition the ~6.7 km thick lower crust and 7.7 km thick upper crust extend for ~45 km and exhibit 'P' velocities of 6. 7-7.1 +/O. lkm/s typical for the continental lower crust, and 5.9 6.1 +/-0.1 km/s for the continental upper crust. The results of two dimensional velocity models of twenty three OBS instruments were constrained along two seismic refraction transects and integrated with regional well data. The models and associated studies on the formation of the North West Shelf off Australia provide an outline of crustal and upper mantle architecture of the area and support a new interpretation of the geological history of the Argo Abyssal Plain and adjacent segment of the northwest continental margin Canning offshore transect Oceanic basement in the Argo Abyssal Plain is anomomalously deep by about 300 m, assuming that it formed at an average ridge elevation of 2500 m and has subsi~ed according to the plate model. However, geochemical studies by Keen et al. [1990] indicate that the normalised oceanic crustal Na20 content as well as other major element ox.ides of the Argo basalts at ODP site 765d in the Argo Abyssal Plain, aged about 140 Ma, indicate that it formed at much shallower depths of 1.8±0.5 km, at least 200 m shallower than average ocean crust. It follows that the Argo Abyssal Plain is at least 500 m deeper than expected, assuming that its original ridge depth was at least as shallow as 2.3 km. This work shows that the anomalous depth cannot be explained by anomalous thin crust, as the average measured crustal thickness is about 7-7. l km, based on two-dimensional compressional and shear velocity models. These observations may indicate that the Argo Abyssal Plain has subsided according to the thermal boundary layer model, as ocean crust 140 m.y. old would show an excess depth of -600 m if it subsided as a thermal boundary layer compared with the plate model. However, associated studies of the North West Shelf show that the observed excess subsidence in the Argo Abyssal Plain is of the same order as the anomalous tectonic subsidence recorded on the outer northwest shelf of Australia in the last 10-20 million years. This may indicate that both subsidence anomalies are due to the same cause. In this case the cause would be unrelated to thermal boundary layer cooling, as all excess subsidence would have occurred recently, since about 20 Ma. The large area affected by Miocene-Pliocene accelerated subsidence on the North West Shelf, together with contemporaneous accelerated subsidence off western India, suggests that both are related to the complex breakup of the IndoAustralian Plate into the Indian, Australian and Capricorn plates. This interpretation is consistent with the status of compressional deformations in the areas adjacent to the Argo Abyssal Plain [Bui! and Scrutton, 1992; Royer and Gordon, 1997]. Additional evidence for the recent tectonic processes being responsible for anomalous subsidence is found in reverse faults observed in the Argo oceanic crust, and in elevated compressional wave velocities in the lower oceanic crust in the direction of present maximum horizontal stress (NE). This suggests that these reverse faults are associated with the reactivation of basement blocks near the fracture zones due to intraplate compressive stresses. The seismic model constraints along the Canning transect have been integrated with the regional well data to explain the timing and subsequent structural events. Estimations based on stretching and strain-rate information and the prerift configuration suggest that the structural evolution of the Canning transect is related to multiphase rift formation where a progressive shifting of extension occurred. Combined seismic modeling and well data provide evidence for four stages of the rift centre migration in space and time. The analysis of the regional crustal configuration based on the results of extensional modeling [Dunbar and Sawyer, 1989] suggests that the nature of preexisting weaknesses responsible for the concentration of strain in the four documented rifting stages is more likely associated with initial weaknesses localised in the crustal strong zone. However, a full characterisation of the rifting mechanism must await comparison with a detailed kinematic models for the crustal response to combinations of driving extensional forces. Chapter J Introduction