No abstract

Crustal thicknesses have been determined by receiver function analysis of broadband teleseismic waveforms recorded during the Broadband Experiment Across the Alaska Range (BEAAR). Typical crust beneath the northern lowlands is 26 km thick, while beneath the mountains it is 35-45 km thick. The transition from thick to thin crust coincides with the location of the Hines Creek fault, a major tectonostratigraphic boundary. Crustal thicknesses determined by receiver functions agree with those predicted from topography assuming Airy type isostasy, suggesting that the Alaska Range is compensated by its crustal root. North of the range, however, the crust is systematically thinner than predicted by simple Airy isostasy. A crustal density contrast of 4.6% across the Hines Creek fault, 2700 kg m ؊3 to the north and 2830 kg m ؊3 to the south, explains the observed difference between the crustal thicknesses predicted by simple Airy isostasy, and the crustal thicknesses determined by receiver function analysis.

Quality improvements in marine reflection seismic data over recent years have lead to a better understanding of the relationships between seismo-stratigraphical sequences present in the Dampier Sub-basin and those in adjacent areas. The "drift-onset" unconformity, which separates the syntectonic rift sequence from the post-tectonic drift sequence, can now be seismically recognised as a single unfaulted surface. Previously this unconformity was interpreted to be faulted. In places this surface had some 2400 m of palaeotopography in the form of an escarpment. This escarpment was formed by tectonic movements and subsequent erosion some time in the Callovian, probably as a consequence of the opening of the Indian Ocean.The presence of Callovian and Upper Jurassic marine sands on the Rankin Platform shows that the Rankin Platform was in places submerged during Callovian and Upper Jurassic times. Furthermore, the Dampier Sub- basin must have been more than 2000 m deep immediately following the causal tectonic event. The escarpment was rapidly buried, with 1200 m of sediments locally deposited by the end of the Callovian and was finally buried by Neocomian times. Thereafter the Rankin Platform and Dampier Sub-basin have subsided.