Magnetic signature and morphology of the Acraman impact structure, South Australia

Acraman is a major complex impact structure located in the Mesoproterozoic Gawler Range Volcanics of the Gawler Craton, South Australia, and has parts of its distal ejecta preserved in late Neoproterozoic (~590 Ma) strata of adjacent basins. Geomorphology and satellite images reveal a severely degraded structure comprising a near-circular central topographic depression 30 km across and arcuate features at 85-90 and ~150 km diameter. Disrupted bedrock of Yardea Dacite displaying shatter cones, shatter cleavage, and multiple sets of planar shock lamellae in quartz grains, and a dyke of melt rock, crop out at the centre of the structure. High-resolution digital aeromagnetic data (400-m line-spacing, 80-m ground clearance) for Acraman reveal a conspicuous circular magnetic low, 20 km in diameter, exhibiting subdued magnetic relief and a central high-amplitude dipolar anomaly. A subdued magnetic signature in places reaches ~30 km diameter, which is at the limit of the topographic depression and disrupted dacite bedrock. Several discontinuous magnetic lineaments that roughly parallel the boundary of the circular magnetic low occur at 85-90 km diameter. The 20-km-diameter magnetic low is unrelated to topography, and its subdued character evidently reflects a measured decrease in susceptibility of the disrupted dacite relative to undisturbed dacite. The central dipolar anomaly, whose axis is deflected by remanent magnetisation with a direction similar to that of the melt rock, may indicate a concentration of impact melt material at shallow depth that was magnetised in the ambient geomagnetic field at the time of the impact. Acraman originally was a complex crater with central peak. Suggested original diameters of major structural features are ~20 km (peak ring and central uplift marked by the magnetic low), ~40 km (estimated extent of transient cavity/excavated area), 85- 90 km (possible final structural rim of the collapse crater), and ~150 km (possible outer limit of structural disturbance). The ratio of estimated original diameters of adjacent structural features is about 2, which is near the upper limit suggested by theoretical models and the dimensions of other terrestrial complex impact structures.

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1996-01-01T00:00:00
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Geoscience Australia Persistent Identifier/https://pid.geoscience.gov.au/dataset/ga/81456

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Australian Geological Survey Organisation

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Williams, G.E.

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Schmidt, P.W.

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Boyd, D.M.

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AGSO Journal of Australian Geology and Geophysics

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16:4:431-442

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