The Keurusselkä impact structure, Finland—Impact origin confirmed by characterization of planar deformation features in quartz grains

Although the meteorite impact origin of the Keurusselkaimpact structure (central Finland) has been established on the basis of the occurrence of shatter cones, no detailed microscopic examination of the impactites from this structure has so far been made. Previous microscope investigations of in situ rocks did not yield any firm evidence of shock features (Raiskila et al. 2008; Kinnunen and Hietala 2009). We have carried out microscopic observations on petrographic thin sections from seven in situ shatter cone samples and report here the discovery of planar fractures (PFs) and planar deformation features (PDFs) in quartz and feldspar grains. The detection and characterization of microscopic shock metamorphic features in the investigated samples substantiates a meteorite impact origin for the Keurusselkastructure. The crystallographic orientations of 372 PDF sets in 276 quartz grains were measured, using a universal stage (U-stage) microscope, for five of the seven distinct shatter cone samples. Based on our U-stage results, we estimate that investigated shatter cone samples from the Keurusselkastructure have experienced peak shock pressures from approximately 2 GPa to slightly less than 20 GPa for the more heavily shocked samples. The decoration of most of the PDFs with fluid inclusions also indicates that these originally amorphous shock features were altered by postimpact processes. Finally, our field observations indicate that the exposed surface corresponds to the crater floor; it is, however, difficult to estimate the exact diameter of the structure and the precise amount of material that has been eroded since its formation.

[1]  R. Dietz.  Meteorite Impact Suggested by Shatter Cones in Rock. , 1960, Science.

[2]  D. Stoeffler.  Deformation and transformation of rock-forming minerals by natural and experimental shock processes. I - Behavior of minerals under shock compression. , 1972 .

[3]  L. Nicolaysen,et al.  Vredefort shatter cones revisited , 1999 .

[4]  C. Koeberl,et al.  Shatter cone and microscopic shock-alteration evidence for a post-Paleoproterozoic terrestrial impact structure near Santa Fe, New Mexico, USA , 2008 .

[5]  M. Lehtinen.  Lake Lappajärvi, a meteorite impact site in western Finland , 1976 .

[6]  W. Engelhardt,et al.  Shock induced planar deformation structures in quartz from the Ries crater, Germany , 1969 .

[7]  M. Schmieder,et al.  A High-Precision Late Mesoproterozoic 40Ar/39Ar Age for the Keurusselk Impact Structure (Finland) , 2009 .

[8]  L. Pesonen,et al.  Rock magnetic and paleomagnetic study of the Keurusselkä impact structure, central Finland , 2011 .

[9]  H. Leroux,et al.  A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa , 1994 .

[10]  B. Schmitz,et al.  Impact structures and events - a Nordic perspective , 2008 .

[11]  Christian Koeberl,et al.  Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation , 2008, Science.

[12]  F. Wieland,et al.  New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation , 2006 .

[13]  John G. Spray,et al.  Shock-induced crystal-plastic deformation and post-shock annealing of quartz microstructural evidence from crystalline target rocks of the Charlevoix impact structure, Canada , 2006 .

[14]  L. Hecht,et al.  THE ORIGIN OF “ TOASTED ” QUARTZ IN IMPACTITES REVISI , 2009 .

[15]  Markku Poutanen,et al.  The Keurusselkä meteorite impact structure, Central Finland: Geophysical data , 2005 .

[16]  L. Pesonen,et al.  Lake Karikkoselkä impact structure, central Finland: New geophysical and petrographic results , 1999 .

[17]  J. White,et al.  Micro-Shock Deformation adjacent to the Surface of Shatter Cones from the Beaverhead Impact Structure, Montana , 1996, The Journal of Geology.

[18]  Bevan M. French,et al.  Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures , 1998 .

[19]  R. C. Emmons.  The universal stage : with five axes of rotation , 1943 .

[20]  W. Reimold,et al.  Experimental constraints on shock-induced microstructures in naturally deformed silicates , 1996 .

[21]  Christian Koeberl,et al.  Systematic study of universal‐stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results , 2009 .

[22]  J. Spray,et al.  Origin of “toasted” quartz in terrestrial impact structures , 2002 .

[23]  W. Reimold.  The Lappajärvi meteorite crater, Finland: petrography, Rb-Sr, major and trace element geochemistry of the impact melt and basement rocks , 1982 .

[24]  F. Hoerz,et al.  Statistical measurements of deformation structures and refractive indices in experimentally shock loaded quartz , 1968 .

[25]  Falko Langenhorst,et al.  Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory* , 1994 .

[26]  H. Leroux,et al.  Formation mechanisms of planar deformation features in naturally shocked quartz , 1992 .

[27]  H. Leroux.  Weathering features in shocked quartz from the Ries impact crater, Germany , 2005 .

[28]  Susan Werner Kieffer,et al.  Shock processes in porous quartzite: Transmission electron microscope observations and theory , 1976 .

[29]  Falko Langenhorst,et al.  Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience* , 1996 .

[30]  W. Manton.  THE ORIENTATION AND ORIGIN OF SHATTER CONES IN THE VREDEFORT RING , 1965 .