DISTRIBUTION AND SHOCK PRESSURE FORMATION FOR SHATTER CONES: INSIGHTS FROM THE MANICOUAGAN IMPACT STRUCTURE, CANADA

Shatter cones are the only macroscopic evidence of shock metamorphism observed as a result of the meteorite impact process. However, their exact formation mechanism and the range of shock pressures at which they form remain poorly constrained. Recent field work and drilling at the ~80-90 km diameter Manicouagan impact crater, Canada provide new insights into the distribution of shatter cones both radially from the centre of, and vertically within the crater as well as their formation pressures.

A WNW-ESE exposed transect from ~10 km out to 23 km from the geometric centre of the structure reveals a radial decrease in the observed occurrence of shatter cones in the fractured floor of the crater immediately underlying the impact melt sheet and basal, melt-bearing breccias. Shatter cones and curvilinear fracture surfaces are ubiquitous in the exposures closest to the point of impact and are observed in both finer and coarser grained gneisses, manifest as smaller and larger scale cones respectively. There is a systematic decrease in the number of shatter cones observed in exposures towards the edge of the crater. This suggests a zone of enhanced shatter cone formation at a radius of ~10-12 km from the centre of the structure and their formation over a range of shock pressures.

Shatter cones also occur within the centrally uplifted, anorthositic gneisses of the structure, in close proximity to shock veins with associated, relatively high pressure phases stishovite and maskelynite. There are two interpretations for the shatter cone associations within the central uplift at Manicouagan. The first is that they may have formed at higher shock pressures than those typically attributed to their formation. The second is that the centrally uplifted lithologies were exposed to lower, bulk shock pressures, which resulted in the generation of the shatter cones and that the stishovite and maskelynite represent the manifestation of localized shock excursions.

Petrographic examination of the shatter coned lithologies from the centrally uplifted material, from within drill core and the radial transect at Manicouagan will identify shock effects preserved in the associated minerals and elucidate their formation pressures. This will aid in better understanding and constraining the conditions for shatter cone formation.