Rock magnetic and paleomagnetic study of the Keurusselkä impact structure, central Finland

Abstract

Abstract-There are 31 proven impact structures in Fennoscandia-one of the most densely crater-populated areas of the Earth. The recently discovered Keurusselka¨impact structure (62°08¢ N, 24°37¢ E) is located within the Central Finland Granitoid Complex, which formed 1890-1860 Ma ago during the Svecofennian orogeny. It is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism, e.g., planar deformation features in quartz. New petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented. The suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility. The presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization. Four different remanent magnetization directions were isolated: (1)

Preservation and detectability of shock‐induced magnetization

“…While many attempts have been made to identify SRM in natural samples [ Robertson , ; Halls , ; Jackson and Van der Voo , ; Fuller and Cisowski , ; Iseri et al ., ; Schmidt and Williams , ; Pesonen et al ., ; Carporzen and Gilder , ; Elbra et al ., ; Kontny et al ., ; Louzada et al ., ; Raiskila et al ., ; Carporzen et al ., ], reports of confirmed SRM in studied impact craters and extraterrestrial samples are rare to nonexistent. A key question is why SRM has not yet been conclusively identified.…”

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure, Finland

“…; Raiskila et al. ). Hietala and Moilanen (, ) proposed a minimum age of ~600 Ma based on the observable erosion level of the impact structure.…”

“…, ; Raiskila et al. ). However, the genetic link between the geophysical anomalies and the impact remains speculative.…”

“…Impact‐related lithologies comprise the Paleoproterozoic shatter cone‐bearing crystalline rocks (e.g., Hietala and Moilanen , ; Raiskila et al. ; Fig. B) and allochthonous boulders and pebbles of lithic impact breccias that exhibit planar fractures (PF) and PDF in quartz (Hietala and Moilanen ).…”

“…More recently, Raiskila et al. () suggested an age close to ~1120 Ma according to new paleomagnetic studies; however, it has been shown before that the paleomagnetic dating of impact structures is rather imprecise and not always successful (e.g., Jourdan et al. ).…”