Distinguishing friction- from shock-generated melt products in hypervelocity impact structures

Abstract

Field, microtextural, and geochemical evidence from impact-related melt rocks at the Manicouagan structure, Québec, Canada, allows the distinction to be made between friction-generated (pseudotachylite) and shock-generated melts. Making this distinction is aided by the observation that a significant portion of the impact structure’s central peak is composed of anorthosite that was not substantially involved in the production of impact melt. The anorthosite contrasts with the ultrabasic, basic, intermediate, and acidic gneisses that were consumed by decompression melting of the >60 GPa portion of the target volume to form the main impact melt body. The anorthosite was located below this melted volume at the time of shock loading and decompression, and it was subsequently brought to the surface from 7–10 km depth during the modification stage. Slip systems (faults) within the anorthosite that facilitated its elevation and collapse are occupied by pseudotachylites possessing anorthositic compositions. The Manicouagan pseudotachylites were not shock generated; however, precursor fracture-fault systems may have been initiated or reactivated by shock wave passage, with subsequent tectonic displacement and associated frictional melting occurring after shock loading and rarefaction. Pseudotachylites may inject off their generation planes to form complex intrusive systems that are connected to, but are spatially separated from, their source horizons. Comparisons are made between friction and shock melts from Manicouagan with those developed in the Vredefort and Sudbury impact structures, both of which show similar characteristics. Overall, pseudotachylite has compositions that are more locally derived. Impact melts have compositions reflective of a much larger source volume (and typically more varied source lithology inputs). For the Manicouagan, Vredefort, and Sudbury impact structures, multiple target lithologies were involved in generating their respective main impact melt bodies. Consequently, impact melt and pseudotachylite can be discriminated on compositional grounds, with assistance from field and textural observations. Pseudotachylite and shock-generated impact melt are not the same products, and it is important not to conflate them; each provides valuable insight into different stages of the hypervelocity impact process.