PLANAR DEFORMATION FEATURES IN THE WOODLEIGH IMPACT STRUCTURE, WESTERN AUSTRALIA, AND THEIR BEARING ON THE DEGREE OF STRUCTURAL UPLIFT IN THE STRUCTURE

We have studied four samples from the central uplift of the buried Woodleigh impact structure, western Australia (26°03'19"S, 114°39'56"E) (between 198 m and 253 m in GSWA borehole 'Woodleigh 1'). Intense pressures present during the impact have induced shock deformation features in quartz, known as planar deformation features (PDFs). The crystallographic orientations of the PDFs are sensitive to the shock pressure experienced by the grain, and were recorded for the Woodleigh samples using a universal stage.

The moderately foliated granitoid samples comprise 0.5 to 3 mm sized quartz exhibiting minor to moderate undulose extinction, microcline, plagioclase, and locally kinked biotite ± minor muscovite. Patchy clay veining locally corrodes surrounding phases.

Of the 133 PDFs measured in 50 grains, 94% correspond to known PDF orientations. They are only moderately decorated and are spaced between 2 to 10 mm. 13% of grains contain PDFs in one orientation, 51% contain two, 18% contain three, 16% contain four, and 2% contain five. The most common PDF orientations are parallel to {10-13} (58%) and {10-12} (14%). Average shock pressures inferred for the four samples are 16.6 to 20.2 GPa. There is no clear decrease in shock pressure with depth in the drill core.

Pressure attenuation rates in the target were estimated with the following assumptions: the top of the uneroded central peak was at ~30 GPa, and there has been ~1 km of erosion of the peak. An average measured pressure of 18.2 GPa thus yields a pressure (P) attenuation rate of (P~Rx), where R is distance from the impact centre, and the exponent x is -3.6.

The diameter of Woodleigh has been interpreted as 40 km to 120 km, depending on how the geophysical data for the area are interpreted (Mory et al., 2000; Reimold and Koeberl, 2000). Crater scaling relations indicate that at the interpreted diameters of 40 km or 120 km, there should be a corresponding 4 km or 12 km of structural uplift, respectively. Applying our shock attenuation rate indicates that the sampled rocks were shocked to an average 18.2 GPa, implying they were originally 6 km below the point of impact, and were subsequently uplifted during the crater modification phase by this amount. An inferred pressure at the top of the uneroded central peak of 25 GPa yields a shock attenuation rate exponent, x of -2.2 and an uplift amount of ~6.3 km.