NEOGENE TECTONICS, GREATER TIMOR SEA, OFFSHORE AUSTRALIA: IMPLICATIONS FOR TRAP RISK

Abstract

The present day tectonic configuration of the Timor Sea area is the product of multiple phases of deformation occurring from the Proterozoic to the Holocene. Each major phase of deformation has produced structural hydrocarbon traps, which, in many cases, have subsequently undergone considerable reactivation, and in some cases, breaching. The last phase of tectonism affecting the region is the Neogene collision of the Australian and SE Asian Plates in the Banda Arc region. Although its importance in trap modification and integrity has been acknowledged, a cohesive tectonic model integrating observations that cross the international border has been lacking. The observed Neogene structural deformation and subsidence/uplift trends of the Australian shelf in the Greater Timor Sea can be explained by a combination of strike-slip faulting and flexure as the result of oblique convergence and partial subduction of the Indo-Australian Plate under the Southeast Asian plate. Neogene subsidence and deformation of the Cartier Trough and Malita Graben are interpreted to be the result of rhomb graben (i.e. pull-apart) basin development associated with sinistral shear and reactivation of relict rift structures. These basins developed along 'releasing bends' on the west-northwestward dipping flanks of rigid basement highs. Conversely, in areas where existing basement structure trends define 'confining bends' (e.g. southeast-eastward dipping flanks of basement high features), older rift structures have been reactivated transpressionally, in some cases resulting in large-scale inversion. The present day bathymetry suggests that these rhomb-grabens are currently active. In addition to the wrench-related deformation associated with oblique plate convergence and partial subduction, the Australian shelf has undergone flexure resulting in extension. It is proposed that the Neogene palaeo-and the present day stress fields of the Australian shelf reflect the combination of shear stresses related to wrenching and the tensional stresses related to flexure as the Australian Plate is obliquely subducted. Understanding of this modus operandi has direct bearing to hydrocarbon exploration as areas of structural risk (i.e. structures prone to trap breaching) can be delineated. In general trap breaching risk is lower in areas lying away from the shelf margin and away from 'releasing bends' along shear zones.