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Marius Ramirez-Cardona, State University of Hidalgo (Mexico)
Hernando Nuņez del Prado, INGEMMET (Peru)
Jose Machare, INGEMMET (Peru)
Luisa Macedo, INGEMMET (Peru)
Humberto Chirif, INGEMMET (Peru)
Walter Pari, INGEMMET (Peru)
Dafne Ramirez-Mendoza, State University of Hidalgo (Mexico)
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This study attempts to estimate the shock pressure (level of shock metamorphism) causing the crater of Carancas, in Peru. In this aspect, we identified some microscopic and megascopic shock-deformation features, which occurred in the sedimentary rocks (crater rim). This methodology is used assuming that certain features are related with different pressure ranges and are distinctive for determining a stage of shock-metamorphism. The geoform of Carancas offers a good opportunity to perform a complementary study by a combination among impact dynamic processes, fresh shock metamorphosed materials and meteoritic remnants.
The target material (sedimentary rock samples) used in this study has been classified as graywacke (about 60% matrix), and predominantly monocrystalline and polycrystalline quartz grains (poorly-sorted, and sub-rounded). A small amount of feldspar grains is also present. The matrix contains abundant opaque iron oxide and a small fraction of clay (<5%) material. According to these latter observations, samples are minerallogically matured and texturally submatured. Porosity is measured about 10%.
Several 20 to 30-cm-long shatter cones of the shock-metamorphosed graywacke have been collected to perform a megascopic description. Only individual shatter cones (partial and complete) have been identified and collected. Radiating striations (horsetailing), which are very common in non-porous rocks, are not detected in this study. Only some subparallel striations occur on transversal and longitudinal surfaces, and they are interpreted as a result of a frictional motion during the impact event. They form subparallel sets without any indication of fun-like disposition as expected from those originated by shock wave.
Arrays of small fluid inclusions are identified along PDFs (decorated Planar Deformation Features). Fluid inclusions recently formed lead us to consider that they are the result of trapping fluids during the shock event without a devitrification transition. This idea is also supported by the fact that target material is porous with a high saturation grade. The presence of some deformation microstructures within shatter cones constrains the shock pressure into a range between 8 and 12 GPa.
The availability of a very young and unaltered structure provides several advantages concerning the estimation of the kinetic energy of the bolide. 1) It represents a unique opportunity to determine the amount of displaced material volume as well as some geometrical magnitudes of original features. 2) The deformation microstructures in fresh materials are not altered and allow characterizing the dynamic behaviour in the early stages after the shock.
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