Large Meteorite Impacts and Planetary Evolution VI
CONTAINS OPEN ACCESS
This volume represents the proceedings of the homonymous international conference on all aspects of impact cratering and planetary science, which was held in October 2019 in Brasília, Brazil. The volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures—on Earth and Mars. Many contributions report results from state-of-the-art investigations, for example, several that are based on electron backscatter diffraction studies, and deal with new potential chronometers and shock barometers (e.g., apatite). Established impact cratering workers and newcomers to the field will appreciate this multifaceted, multidisciplinary collection of impact cratering studies.
Distinguishing friction- from shock-generated melt products in hypervelocity impact structures
Published:August 02, 2021
John G. Spray, Marc B. Biren, 2021. "Distinguishing friction- from shock-generated melt products in hypervelocity impact structures", Large Meteorite Impacts and Planetary Evolution VI, Wolf Uwe Reimold, Christian Koeberl
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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.