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Enigmatic Meteorite Impact Signature: Field Evidence and Ground Penetrating Radar (GPR) Measurements Suggest Megascopic Impact Spallation Features.

Authors:

Abstract

Spallation is a well-known process in technical fracture mechanics. It describes strong tensile pulses, reflected from incident compression pulses at free surfaces, which can lead to decisive material damage due to the usually significantly lower tensile strength. In meteoric impacts with shock propagations, they play an important role in all dimensions, from mega to micro (Ernstson 2014). Early considerations led to the model of so-called spall plates (Melosh 1989), which describes near-surface platy rock bodies ejected at high speed during the formation of impact structures, a consequence of the spallation superposition of the propagating shock front with the rarefaction wave reflected at the free Earth surface. We describe known terrain observations (Ries impact event) of structurally very conspicuous impact layer deformations in Upper Jurassic limestones (Chao et al. 1978, Rutte 2003) , which so far cannot be explained geologically. GPR measurements show an extrapolation into the depth (8 m) and suggest that, based on the model of spall plates, shock spallation during the propagation of impact shock waves is the only reasonable explanation so far.-Very unusual GPR reflections are associated with a suggested impact airburst in Lower Bavaria. A 500 m x 50 m area of surficially melted granites shows narrow, circular curved, isolated reflection bands of high energy at apex depths down to a few meters. Conventional GPR interpretations and geologic considerations do not explain the observation. It is suggested that the GPR reflection traces an arcuate thin weak zone in the hard granite, caused by a rarefaction front of strong tensile stress propagating into the subsurface in response to an incident shock front. A specification of such a process in more detail is still pending, but shows how impressively GPR measurements can contribute to the clarification of near-surface impact processes (Poßekel & Ernstson 2019, Ernstson & Poßekel 2017). Plain Language Summary Spallation is a well-known process in technical fracture mechanics that describes the particularly destructive effect of dynamic tensile stresses as a result of reflected dynamic compressive stresses. In meteorite impacts, it is primarily reflected shock waves (rarefaction waves) that leave their mark in dimensions from mega to micro. We describe the investigation of quite unusual and so far geologically not understood layer deformations with geophysical ground penetrating radar (GPR) measurements and consider impact shock spallation as causal force.
EP53F-2239 - Enigmatic Meteorite Impact
Signature: Field Evidence and Ground
Penetrating Radar (GPR) Measurements
Suggest Megascopic Impact Spallation
Features.
Abstract
Spallation is a well-known process in technical fracture mechanics. It describes strong tensile pulses,
reflected from incident compression pulses at free surfaces, which can lead to decisive material damage
due to the usually significantly lower tensile strength. In meteoric impacts with shock propagations, they
play an important role in all dimensions, from mega to micro (Ernstson 2014). Early considerations led to
the model of so-called spall plates (Melosh 1989), which describes near-surface platy rock bodies ejected
at high speed during the formation of impact structures, a consequence of the spallation superposition of
the propagating shock front with the rarefaction wave reflected at the free Earth surface. We describe
known terrain observations (Ries impact event) of structurally very conspicuous impact layer deformations
in Upper Jurassic limestones (Chao et al. 1978, Rutte 2003) , which so far cannot be explained
geologically. GPR measurements show an extrapolation into the depth (8 m) and suggest that, based on
the model of spall plates, shock spallation during the propagation of impact shock waves is the only
reasonable explanation so far. - Very unusual GPR reflections are associated with a suggested impact
airburst in Lower Bavaria. A 500 m x 50 m area of surficially melted granites shows narrow, circular
curved, isolated reflection bands of high energy at apex depths down to a few meters. Conventional GPR
interpretations and geologic considerations do not explain the observation. It is suggested that the GPR
reflection traces an arcuate thin weak zone in the hard granite, caused by a rarefaction front of strong
tensile stress propagating into the subsurface in response to an incident shock front. A specification of
such a process in more detail is still pending, but shows how impressively GPR measurements can
contribute to the clarification of near-surface impact processes (Poßekel & Ernstson 2019, Ernstson &
Poßekel 2017).
Plain Language Summary
Spallation is a well-known process in technical fracture mechanics that describes the particularly
destructive effect of dynamic tensile stresses as a result of reflected dynamic compressive stresses. In
meteorite impacts, it is primarily reflected shock waves (rarefaction waves) that leave their mark in
dimensions from mega to micro. We describe the investigation of quite unusual and so far geologically not
understood layer deformations with geophysical ground penetrating radar (GPR) measurements and
consider impact shock spallation as causal force.
Authors
Kord Ernstson
Friday, 13 December 2019
13:40 - 18:00
Moscone South - Poster Hall
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