Kinematic Analysis of Normal Faults from 3D Seismics within the Otway Basin, Australia: Evidence for Oblique Movement in a Passive Margin

Abstract:

The onshore Otway Basin is dominated by normal faults and only minimal strike-slip faulting has been reported. The basin is part of the NW-striking, passive margin that records the break-up of Gondwana and the Antarctic-Australian separation between Late Jurassic and Miocene times. From a 3D seismic cube (ca. 8 km x 7 km x 4 km depth) around the CO2CRC Otway site in SE Australia, we interpreted eight major, Cretaceous and Tertiary stratigraphic horizons (all syn-rift) and over 24 major normal faults.

Detailed analysis of the fault system in our 3D geological model shows that the area is characterised by southwest-dipping, normal faults that have a listric character in the SW and planar character in the NE. North-dipping, antithetic faults developed secondarily due to movement on the major faults. Syn-sedimentary movement caused fourfold strata thicknesses in the hanging-wall of the faults. Fault offset increases towards the south; up to 800 m in the lower stratigraphy. Most faults, however, die out before the Eocene.

Using different techniques we determined that 60% the major faults didn't move in a pure dip-slip fashion, rather the movement contained a component of dextral strike-slip. In particular, juxtaposition maps of the stratigraphic horizons on the fault surfaces shows that in the obliquely-displaced faults the tip-lines of the faults are extremely asymmetric, with one flatly-dipping tip-line and one steep, overturned tip-line. Dip-slip faults demonstrate symmetrical convergence of the tip-lines. However faults with different kinematics are not grouped, but heterogeneously distributed over the model, suggesting strain-partitioning took place. This evidences that faults react differently when the stress field in the passive margin changes direction, but in a way that suggests that they are interacting.