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8/2/2019 Shock spallation - a typical impact process in the Chiemgau meteorite crater strewn field
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8/2/2019 Shock spallation - a typical impact process in the Chiemgau meteorite crater strewn field
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For a better understanding we add that fractures always begin at a definite point
within the material propagating from there with a certain fracture velocity which may
change during propagation and may even become zero. Then the fracture stops
unless it is again fed with energy and continues running.
As for the Chiemgau impact and shock spallation quite peculiar conditions are met
namely particularly in the form of very solid cobbles of Alpine lithology. Apart from the
occurrence as components of the strongly cemented Nagelfluh plates, the cobbles are
in general found in loose bulk and, hence, predestined for a reaction to the passage of
shock waves with resulting spallation. Its not just the extreme contrast of impedance at
the cobbles surface, also their frequently nodular shape may boost the effect by
internal focusing of the shock and rarefaction waves in part yielding enormous energy
densities.
As early as in the beginning of our impact research in the Chiemgau crater strewn fieldwe have reported on these deformations and have shown typical photos of spallation
fractures down to microscopic scales. Here, we present new examples from recent
investigations near the small town of Obing north of Lake Chiemsee. The reader may
forgive our keeping secret about the precise coordinates of the impact sites. Our bad
experiences with ransacked smaller craters and with the Tttensee crater where
practically all rocks with impact-typical deformations have been removed by rock
hunters or people disliking our impact research are forcing us in order to preserve these
peculiar impact features for science and interested scientists.
Fig. 2. A quartzite cobble exhibiting a prominent spallation fracture which like the rock in Fig. 1
has not completely split the cobble. In Fig. 3 we point to features very characteristic of
spallation fracturing.
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Fig. 3. A close-up of the spallation fracture in Fig. 2 shows some typical behavior: Frequently,
the pathway of the spallation fracture proves to be a mirror image of the cobbles surface
curvature, and in the case under discussion we have marked the axis of mirror symmetry by a
blue dashed line. This may be understood as a consequence of the reflection of the shock
(compressive) wave at the free surface for geometrical reasons leading to a mirrored front ofthe reflected rarefaction wave.
Fig. 4. Spallation fractures in a gneiss cobble. Here again the open fractures do not split the
cobble completely, and here again the geometry of the roughly perpendicularly oriented
ruptures mirrors the shape of the angular cobble.
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Fig. 5. Open spallation fractures in a garnet amphibolite.
The examples of spallation fractures in quartzite, limestone, gneiss and amphibolite
cobbles demonstrate that the process is independent of rock lithology und produces
recurrent features.
To obviate objections these deformations have already originated from tectonics in the
Alps (regularly claimed by local geologists) and have been transported in the form of
cobbles in rapid glacial and post-glacial streams and in the end to have been deposited
near Obing north of Lake Chiemsee, we point to the frequently very fragile character of
the cobbles. Moreover, any strong pressure having acted on the cobbles can basically
be excluded because they would inevitably have been broken and sheared.
For comparison spallation like in cobbles from the Chiemgau crater strewn field is
shown in the next figures with examples from the Ries impact crater und the Spanish
8/2/2019 Shock spallation - a typical impact process in the Chiemgau meteorite crater strewn field
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Azuara http://pubs.giss.nasa.gov/abs/er01000b.html Rubielos de la Crida impact
structures. The latter occurrences have been investigated more intensively including
spallation experiments. A related article has been published in the prestigious
GEOLOGY journal (see http://pubs.giss.nasa.gov/abs/er01000b.html) where the full
article can be downloaded), and an extended report may be read here:
http://www.impact-structures.com/impact-spain/shock-deformation-in-triassic-buntsandstein-conglomerates-spain/.
Fig. 6. Limestone cobble from the ejecta (Bunte Breccia) of the Ries impact structure(Nrdlinger Ries crater) showing spallation fractures that have not split the cobble. After the
shock spallation the deformation of the cobble continued probably in the course of
excavation without dissecting it.
Fig. 7. Quartzite cobbles from the Spanish large Azuara and Rubielos de la Crida impact
structures showing very typical shock-induced open spallation fractures. For some of thefissures the rough mirror symmetry of surface and fracture geometry becomes again evident.