The Australian stress map

doi:10.1071/EG998420

The Australian stress map

R.R. Hillis, J.J. Meyer and S.D. Reynolds

Exploration Geophysics 29(4) 420 - 427
Published: 1998

Abstract

The Australian stress map project has compiled 357 quality ranked stress orientation analyses for the Australian Continent (including New Guinea). Of these, 206 provide reliable stress orientations, approximately doubling the number from the 1992 world stress map compilation. Most new data are from borehole breakouts. Regionally, maximum horizontal stress (sH) is oriented northeast-southwest from New Guinea along most of the North West Shelf, rotating to 100°N in the Carnarvon Basin. An 010°N?020°N sH orientation is observed in the Amadeus and Bowen Basins. However, in the southern half of the continent a broad east-west sH trend is observed in the Yilgarn Block, Cooper-Eromanga Basins, and from a number of isolated indicators. In the Otway and Gippsland Basins sH is oriented 130°N. The rotation of stresses along the North West Shelf and from east-west in the Yilgarn Block to north-south in the Amadeus Basin can be explained in the context of the heterogeneous plate boundary forces acting along the convergent plate boundary to the north of Australia. However, plate boundary forces can not explain the rotation of sH from east-west in the cooper-Eromanga Basins to approximately north-south in the Amadeus Basin, which may be linked to second order influences on the stress field. The vertical stress (sV) gradient in the Bonaparte and Cooper-Eromanga Basins increases with depth, and is around 20 MPa/km at 1000 m, attaining 23 MPa/km around 3 000 m. The Amadeus Basin displays an overburden gradient of 25 MPa/km that is little affected by depth. In situ measurements in hard rock terranes suggest a higher average overburden stress gradient of 27 MPa/km. Leak-off pressures suggest that the minimum horizontal stress (sH) is the least principal stress (60?70% of sV) in the Bonaparte and Cooper-Eromanga Basins. Hence in'neither basin is the stress regime one associated with reverse faulting (where sH > sh > sV). Consideration of the frictional limits to faulting suggests that, if in a state of incipient faulting, the stress regime is approximately on the boundary between normal (sV >sh>sh) and strike-slip (sh >sV > sh) faulting environments in the Bonaparte Basin and strike-slip in the Cooper-Eromanga Basins. Applications of the stress data include assessing both natural and induced fluid flow directions in the subsurface. For example, hydraulic fractures induced for geothermal exploitation of hot-dry-rock in the Cooper-Eromanga Basins would tend to be vertical, and not, as previously suggested horizontal.