Structural Evolution of the Onshore Northern Perth Basin, Western Australia. A. J. Mory and R. P. IaskyI Abstract Analysis of fault trends from seismic, and aeromagnetic and gravity images, shows that the onshore northern Perth Basin has a complex tectonic history. Two major phases of tectonism are recognised: 1) extension in a northerly direction in the Permian resulted in east-west normal faults, and implies sinistral strike-slip along the Darling Fault. This phase of extension possibly continued into the Triassic; 2) the breakup of Greater India from Australia in the Early Cretaceous, as a result of dextral transtension with extension to approximately 255 degrees, produced normal faults striking 345 degrees. Sinistral movement on transfer faults oriented at 300 degrees in the onshore northern Perth Basin implies that the region to the north, underlain by the West Australian Craton, has acted as a • stationary fault block during breakup. An analysis of the structural trends and tectonic history of the onshore northern Perth Basin between latitudes 29° S to 31 o S (Fig. 1) was can'ied out for this study. This analysis is based largely on subsurface structure maps showing the Middle Jurassic (top of Cattamarra Coal Measures), top Permian and basement levels, derived from a regional seismic grid of 11 800 line kilometres, and 127 petroleum exploration and development wells. Aeromagnetic and gravity images were also used to establish the orientation of faults. \ 11 115° CARNARVON BASIN The Perth Basin is a north-south elongate trough that contains sediments of Silurian to Pleistocene age (Fig. 1). The basin lies in the southwest of Western Australia, with the Darling Fault as its eastern boundary; about one half is located onshore. The discovery of several oil and gas fields onshore, and numerous hydrocarbon shows both onshore and offshore, confirms the petroleum potential of the basin. The early 1990s has been a period of exploration revival, with nearly all the offshore areas of the basin being under permit to companies with active exploration programs. Continuing exploration onshore and improvements in seismic imaging led to the discovery of the Beharra Springs gas field in 1990 (Hall & Kneale, 1992). The understanding of the basin's structure and its evolution has increased greatly over the last five years. Interpretations by Stein et al. (1989), Marshall et al. (1989), Byrne and Harris (1992), and Marshall et al. (1993) now recognise a transtensional faulting regime as a prime factor in the development of the basin. 1WA Geological Survey, Mineral House, 100 Plain Street, Perth, WA 6004. Acknowledgments: We thank Woodside, Enterprise Oil and WAPET for the loan of aeromagnetic images which greatly assisted in the regional structural interpretation of the northern Perth Basin. This paper is published with the permission of the Director of the Geological Survey of Western Australia. Introduction -- 30° --32° Dongara Saddle Tÿrtle D( Ridge AREA OF INVESTIGATION CONTINENTALÿ OCEAN ÿ "b Beagle BOUNDARY ÿ Ridge Cadda Terrace PERTH "ÿ BASIN 113° 115° Figure 1: Tectonic elements of the northern Perth Basin. Basin Subdivisions The onshore northern Perth Basin includes the Dandaragan Trough, Dongara Saddle, Beagle Ridge, and the Cadda and Irwin terraces (Fig. 1). The southern part of