SHOCK-INDUCED CRUSTAL OSCILLATION RECORDED IN ROCKS AND MINERALS OF THE MANIITSOQ IMPACT STRUCTURE, WEST GREENLAND

The giant, 3001 Ma Maniitsoq impact structure is deeply eroded and presently exposed 25 km below the impacted surface. Its 35 x 50 km large core, the Finnefjeld domain, was not only crushed but also mechanically homogenised to a degree where almost all previous rock units and geochemical variations have been obliterated. In the collar zone around the core, up to metre-scale veins and dykes of crushed and fluidised rock preserve a record of multiple injection and folding simultaneously with the presence of direct K-feldspar melts. During the subsequent K-feldspar crystallisation in dykes and wall rocks, several phases of plagioclase with declining Ca content were stepwise exsolved from ductile K-feldspar grains during simple and pure shear in local, crystal-scale stress fields of variable orientation. Comparable features also occur in wall rock biotite, where repeated fracturing and concomitant intracrystalline shear folding of these fractures are recorded. These kilometre- to micron-scale features show that the shock-heated lower crust was reverberating immediately after the passage of the initial shock wave. Such radial oscillations in the deep crust would dissipate outward but create a space problem inward and thus drive the intense crushing and mixing observed in the Finnefjeld core domain at Maniitsoq. The observed oscillations may also for the first time substantiate models of upper-crustal crater reorganisation that imply acoustic fluidisation.