The response to extensional stresses that deformed and finally ruptured the northwest Australian passive margin was achieved by an interplay of simple and pure shear mechanisms. The results of a two-ship seismic reflection and refraction experiment show that the outer Exmouth Plateau is dominated by large rotated blocks bounded by deeply penetrating normal faults, some of which are heavily intruded. Magmatic underplating attended this extension but is absent in the central region where a set of prominent, subhorizontal, mid-crustal arrivals are interpreted to be detachment surfaces. Extensive exploration seismic coverage, drilling, and sampling suggest that deformation on the outer plateau postdates that in the central plateau. We propose that initial deformation of the region was dominated by a simple shear detachment system along low-angle fault planes. This passed laterally and temporally into a region of pure shear deformation characterized by high-angle normal faults and magmatism, and this became the locus of final rupture.

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