Recently acquired deep crustal refraction measurements made aboard R/V Vema in January and February, 1976, are combined with new and existing seismic reflection, shallow-refraction, gravity, magnetic and other data, to present a picture of the structure of the Magnetic Quiet Zone south of Australia from its surface morphology down to the mantle.

We deduce that the quiet zone is bounded by primary discontinuities in the earth's crust that extend to great depths beneath the surface. The inner (landward) boundary is marked by a prominent magnetic trough, a steplike change in crustal thickness and velocity structure, and commonly an isostatic gravity high. Its outer (seaward) boundary is marked by anomalously shallow basement topography, and represents the sharp boundary with normal oceanic crust produced at the Southeast Indian Ridge.

The crust within the quiet zone lies at a relatively uniform depth, but has a variable velocity structure. Computed seismic results indicate "continental" and "oceanic" sections and still others that are neither "oceanic" nor "continental." No systematic gradation from one kind to the others was observed. Rather, the quiet zone appears to be an inhomogeneous amalgamation of different crustal types. We propose that this type of crust may be unique, being neither continental nor oceanic.

We suggest that the evolution of this margin took place in two stages. In the first stage, a continental rift valley about 150 to 200 km wide was formed. Its width stayed nearly constant while predominantly vertical motions took place within it and the unique rift crust evolved during a period of several tens of millions of years. During the second stage, on the other hand, horizontal motions dominated. The rift valley broke apart at its axis and seafloor spreading started. Thus, in contrast to a continuous evolution of the margin as many previous authors have suggested, we propose a two-stage evolution: the first in which the width of the rift valley remained constant but the crustal composition evolved, and the second in which the geometry changed (that is, drift took place) but the composition of the new oceanic crust remained relatively constant.