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SHATTER CONES AND ASSOCIATED SHOCK-INDUCED MICRODEFORMATIONS IN MINERALS –
NEW INVESTIGATIONS AND IMPLICATIONS FOR THEIR FORMATION. L. Ferrière and G. R. Osin-
ski, Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7,
Canada (ludovic.ferriere@uwo.ca).
Introduction: Shatter cones are the only shock-
deformation feature (i.e., diagnostic evidence of hy-
pervelocity impact) that develop on a macro- to
megascopic (i.e., hand specimen to outcrop) scale
[e.g., 1]. However, despite being one of the most dis-
tinctive products of hypervelocity impact events, the
shatter cone formation mechanism remains unclear.
Different models for their formation exist in the litera-
ture [2-7], but none of them account for all of the cur-
rent field observations of shatter cones [7,8]. In addi-
tion, it is notable that very few studies, combining ob-
servations of shatter cones from the outcrop to the mi-
croscopic scale, have been conducted [e.g., 9].
We report here on preliminary results of an ongo-
ing study of shatter cones from several terrestrial im-
pact structures, including the Charlevoix (“CS”),
Haughton (“HS”), Keurusselkä (“KS”), Rochechouart
(“RS”), Siljan (“SiS”), Sierra Madera (“SmS”), Stein-
heim (“StS”), and Sudbury (“SuS”) structures.
The challenge of this study is to combine observa-
tions on the occurrence, distribution, and characteris-
tics of shatter cones at the scale of the impact structure
with macroscopic observations (e.g., shatter cone mor-
phology, etc.) and microscopic properties of (shocked)
minerals (mainly quartz), to infer the course of events
that result in the formation of shatter cones. The abun-
dance and crystallographic orientations of planar de-
formation features (PDFs) in quartz grains, based on
universal stage (U-stage) microscope examination
[10], was studied to estimate the peak shock pressure
recorded by the samples.
Results and discussion: Based on the compilation
of an extensive literature database, we estimate that
shatter cones have been reported for more than half of
the currently 177 proven impact structures on Earth, in
extremely different lithologies (from fine- to coarse-
grained), with large variations of cone size (cm to m),
but generally occurring in-situ only in the central part
of the impact structure. In a few cases, such as at
“HS”, “RS”, and “SmS”, shatter cones occur within
clasts in crater-fill impact breccias and melt rocks, and
also, at “HS”, within megablocks of the ballistic ejecta
blanket. The shatter cones studied here are in various
types of lithologies; in limestone, gneiss, and in meta-
greywacke at “CS”; principally in limestone and in
sandstone at “HS”, in granodiorite and in orthogneiss
at “KS”, in microgranite at “RS”, in granite at “SiS”,
in limestone and in sandstone at “SmS”, in limestone
at “StS”, and in quartzite, sandstone, metagreywacke,
and in gabbro at “SuS”. Note that shatter cones are for
the first time reported here in metagreywacke at “CS”
and in gabbro at “SuS”.
In all the studied structures, shatter cones display
reciprocal positive and negative curved, oblate, in
some cases nearly flat, or conical surfaces, with striae
that radiate outward from the cone apex and with sub-
sidiary divergent striations ("horsetailing"; Fig. 1a).
Shatter cones are generally found as composite groups,
rarely as single specimens, of commonly partial to
complete cones, mainly at “HS”, but also at “CS” and
“StS”, and rarely at “SuS”, with very frequently oppo-
site orientations at the centimeter to decimeter scale.
Samples with apices pointing in opposite directions
were mainly observed at “HS” (Fig. 1b).
Fig. 1: Macrophotographs of shatter cones in lime-
stone. a) Horsetailing shatter cone surfaces (“StS”). b)
Two complete cones pointing in opposite directions.
Clast from crater-fill impact breccia (“HS”).
1392.pdf41st Lunar and Planetary Science Conference (2010)
Based on these observations it is obvious that the
use of shatter cone apex orientation to determine the
centre of an impact structure is likely to yield incorrect
results. However, the study of the distribution of in-
situ shatter cones can be used for the estimation of a
minimum crater diameter.
It is also important to combine field observations
with laboratory investigations. Our petrographic study
confirm that a large number of micro-deformation fea-
tures occur in shatter cones, including random penetra-
tive fractures (in all samples), kink bands (mainly in
micas at “CS”, “RS”, and “SuS”, but also rarely in
quartz and feldspar grains at “CS”), planar fractures
(PFs) and PDFs in quartz grains in samples from “CS”
(Fig. 2), “HS”, “KS”, “SiS”, and rarely “RS”. So-
called “feather textures” were also noted in quartz
grains from “CS” and “HS”. Detailed U-stage investi-
gations are in progress for quartz-bearing shatter cones
from “CS”, “HS”, “RS”, and “SiS”, and are only fully
completed for samples from “KS”. In this case, we
estimate that the investigated shatter cones have ex-
perienced peak shock pressures comprised between ~2
GPa to slightly less than 20 GPa for the more heavily
shocked samples (results are reported in [11]).
Fig. 2: Photomicrograph (in cross-polarized light) of a
quartz grain, in a shatter cone from “CS”, containing
three decorated PDF sets; one set (*), with ω’{3101 }-
equivalent orientation, is hardly visible on this photo-
micrograph, but is evident under the U-stage.
No glassy patches or venners, as reported e.g., for
shatter cones from Beaverhead [12], “SuS” [13], and
Vredefort [7], were observed in any of the investigated
samples. In addition, we were not able to confirm that,
as currently recognized in the literature [e.g., 9,12],
and based on qualitative investigations, PFs and/or
PDFs occur in minerals only within 1–2 mm of the
cone surfaces.
Furthermore, our petrographic and U-stage study
shows that even from the same outcrop, shatter cone
samples recorded significantly different peak shock
pressures. This implys that at a certain distance from
the crater center, where shatter cones form, at shock-
pressures of ~2–20 GPa, the shock wave that propa-
gates through the target rocks is highly scattered, re-
fracted, and/or reflected.
Conclusions: Our study provides some important
insights into the mechanism of shatter cone formation:
(1) as previously suggested by some [7,8], our obser-
vations of shatter cones within crater-fill breccias, at
“HS”, “RS”, and “SmS”, indicate that they must form
very early in the cratering process (i.e., prior to crater
excavation); (2) the occurrence of shatter cones with
complete cones and apices pointing in opposite direc-
tions reject the models by Sagy et al. [5,6]; (3) The
record of shock pressures up to ~20 GPa also poses
problems for the model of Baratoux and Melosh [4], as
their proposed mechanism only operates “at pressures
of 3–6 GPa”. It is also, so far, not clear if shatter cone
formation occurs during the post-shock phase of the
compression stage of cratering and/or immediately
thereafter during shock unloading, by decompression,
as suggested by [7]. Thus, while we have begun to
answer some questions, more arise and the mechanism
for shatter cone formation remains elusive. Currently,
none of the various proposed models can account for
all of the observations of natural shatter cones.
Acknowledgments: Thanks to W.R. Church for
shearing his field experience on “SuS”. This work was
supported by the Department of Foreign Affairs and
International Trade (DFAIT) (L.F.) and the Natural
Sciences and Engineering Council of Canada (G.R.O.),
Government of Canada, and the Ontario Ministry of
Innovation Early Researcher Award fund to G.R.O.
References: [1] Dietz R. S. (1968) In Shock meta-
morphism of natural materials, French B. M. & Short
N. M., Eds, Baltimore: Mono Book Corporation. pp.
267–285. [2] Johnson G. P. and Talbot R. J. (1964)
Air Force Institute of Technology, Dayton, Ohio,
USA, 92 pp. [3] Gash P. J. S. (1971) Nature Phys. Sci.,
230, 32–35. [4] Baratoux D. and Melosh H. J. (2003)
Earth Planet. Sci. Lett., 216, 43–54. [5] Sagy A. et al.
(2002) Nature, 418, 310–313. [6] Sagy A. et al. (2004)
JGR, 109, B10209, 20 pp. [7] Wieland F. et al. (2006)
Meteoritics & Planet. Sci., 41, 1737–1759. [8] Osinski
G. R. and Spray J. G. (2006) Proc. 1st Int. Conf. on
Impact Cratering in the Solar System, ESA SP-612. [9]
Fackelman S. P. et al. (2008) Earth Planet. Sci. Lett.,
270, 290–299. [10] Ferrière L. et al. (2009) Meteorit-
ics & Planet. Sci., 44, 925–940. [11] Ferrière et al.
(2010) LPS XXXXI, Abstract #1072. [12] Hargraves R.
B. and White J. C. (1996) J. Geol., 104, 233–238. [13]
Gibson H. M. and Spray J. G. (1998) Meteoritics &
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