Structural framework of the eastern Otway Basin: inversion and interaction between two major structural provinces

Abstract

Integrating reflection seismic interpretation of the onshore eastern Otway Basin and offshore Torquay Embayment with published and in progress thermochronology has had an impact upon some of the recent models proposed for the Mesozoic to Recent history of the area. Analysis of Early Cretaceous-age faulting with significant heaves in the Eastern Otway-Torquay offshore regions suggest that much of it strikes E-W, implying a general N-S extension at that time. To the east of an area centred around the Stoneyford Gravity High the strike of Early Cretaceous faults gradually changes from E-W to NNE-SSW over several tens of km. There is no evidence for an abrupt discrete transfer fault, controlling the change in fault orientation. The sedimentary section around 143°E in the Port Campbell Embayment records a near orthogonal change in fault strike from WNW-ENE to SW-NE to the east. This change has been recognised for some time from surface data and published tectonic maps and has been used as corroborative evidence for locating a major structural boundary. The WNW-ESE fault trends seen on regional maps in the Port Campbell Embayment were initiated in the Cenomanian-Santonian; thus the orthogonal relationship of the fault trends seen around 143°E is a consequence of juxtaposing faults of different ages. A large scale boundary in this general area is still supported by the significant change in Late Cretaceous subsidence patterns evident in the sedimentary record inferred from the seismic and local thermochronological interpretation. However that boundary, at least at shallow basement levels, is not manifested as a transfer zone and may lie further east, around the Stoneyford Gravity High, rather than on trend with Woorndoo Fault zone.