ROTATION OF HORIZONTAL STRESSES IN THE AUSTRALIAN NORTH-WEST CONTINENTAL-SHELF DUE TO THE COLLISION OF THE INDO-AUSTRALIAN AND EURASIAN PLATES

There is a 40 degrees rotation of regional maximum horizontal stress ( sigma(hmax)) orientation between the western end of the Australian Nor th West Continental Shelf (Carnarvon Basin) and its eastern end (Bonap arte Basin). A total of 625 borehole breakouts covering a cumulative l ength of 7.7 km in 42 wells in the Carnarvon Basin indicates a sigma(h max) orientation of 090 degrees-100 degrees N. A total of 616 borehole breakouts over 6.8 km in 46 wells in the Bonaparte Basin indicates a sigma(hmax) orientation of 055 degrees N-060 degrees N. Together with extant data from the World Stress Map, these results indicate that reg ional sigma(hmax) orientation is 050 degrees-060 degrees N from New Gu inea westward through the Bonaparte Basin to the Canning Basin (centra l North West Shelf). Between the Canning Basin and the Carnarvon Basin , sigma(hmax) rotates to 090 degrees-100 degrees N. The parallelism of sigma(hmax) orientation in the Bonaparte Basin to the Australia/Banda Are collisional zone indicates that this collision is not generating significant net push. Rather, the 050 degrees-060 degrees N sigma(hmax ) orientation of much of the northern Australian margin is probably co ntrolled by the more mature New Guinea orogen to which it is approxima tely orthogonal. The observed rotation of sigma(hmax) can be explained solely by the focusing of the forces balancing ridge push along colli sional segments of the northeastern boundary of the Indo-Australian Pl ate (such as the New Guinea orogen). Although not required to account for the observed stress rotation, a slab pull force from oceanic Indo- Australian Plate being subducted beneath the Sunda Are cannot be dismi ssed.