Impact Cratering and the Formation of Shatter cones
The enigma remains
David Baratoux
Observatory Midi-Pyrénées, University of Toulouse
Jay Melosh, Purdue University, Indiana, USA.
Uwe Reimold, Museum für Naturkunde, Berlin, Germany

Shatter cones in shale, Vredefort impact crater. See satellite image of the Vredefort dome (80 km in diameter).

Outline
What is a shatter cone ?
Definition and criteria for identification

Where can you find shatter cones ?
Occurrence and distribution of shatter cones

Shape, orientation, size – what do we know ?

Model of shatter cone formation – The enigma
remains…but what do we know ?

Map of the impact sites

What is a shatter cone ?
It looks like a cone (but actually the shape israther curved or planar rather than conical).

Shatter cones in Shales, Vredefort
impact crater.

Shatter cones in limestone, Ries
(Germany)What is a shatter cone ?
Shape ? Conical, curvi-planar, planar ?

Haughton impact site, Canada, fine-grained
limestones (Osinski, 2006)

Vredefort impact site, quartzite, South Africa

Vredefort impact site, quartzite, South Africa.
(Sagy et al. 2004)

What is a shatter cone ?
Shape

Unfortunately, measurement of the shapes of shatter cones are rare…

1-D topographic profile of shatter cones (Vredefort impact crater, Sagy et al. ,2002)

What is a shatter cone ?
It has characteristic striations on the surface
30 cm

Shatter cones in quartzite, Vredefort impact crater.

What is a shatter cone ?
It has characteristic striations on the surface

Shatter cones in shale, Vredefort impact crater.

What is a shatter cone ?
It is a pervasive fracture

Shatter cones in shale, Vredefort impact crater.

Where are the valid shatter cones ?

Is it possible to make a mistake ?
Example of possible confusions
Striations on surface faults - Slickenslides

Salvador, Limestone (Photo L. Baratoux)

Is it possible to make a mistake ?
Example of possible confusions

Quartzite at Vredefort impact crater

Slickensides and shatter cone striations canoccur at the same outcrop

Is it possible to make a mistake ?
Example of possible confusions
Cone-in-cone structures (mostly in calcareous rocks)

photo courtesy of Dr. Diane M. Burns

Growth of calcareous fibers – exact mechanism (also) debatedIs it possible to make a mistake ?
Example of possible confusions
Eolian features, ventifacts

Gilf Kebir plateau, Paillou et al., 2006

Is it possible to make a mistake ?
Example of possible confusions
Eolian features, ventifacts

Gilf Kebir plateau, Paillou et al., 2006

Is it possible to make a mistake ?
Example of possible confusions
Eolian features, ventifacts

Gilf Kebir plateau, Paillou et al., 2006

Is it possible to make a mistake ?
Example of possible confusions

Eolian features, ventifacts

Gilf Kebir plateau, Paillou et al., 2006

Is it possible to make a mistake ?
Example of possible confusions

Eolian features, ventifacts

Gilf Kebir plateau, Paillou et al., 2006

Even if the sites look like impact craters…

Gilg Kebir plateau, Egypt, Paillou et al. 2006.

Even if the sites look like impact craters…

Gilg Kebir plateau, Egypt, Paillou et al. 2006.

To tell all the story…
Landsat image (1978)

From Desert Landforms ofsouthwest Egypt: A basis
from comparison with Mars.
F. El Baz, T.A. Mawxell(editors)

To tell all the story…

From Desert Landforms of southwest Egypt: A basis from comparison withMars. F. El Baz, T.A. Mawxell (editors), 1979.

To tell all the story…

From Desert Landforms of southwest Egypt: A basis from comparison with Mars.
F. El Baz, T.A. Mawxell (editors), 1979.

Shatter cones - Evidence for an impact crater

World map distribution of shatter cones, , Dietz, 1984, Meteoritics
Dietz, 1947 – Association of shatter cones with meteoritic impacts.
Since the 80’s, shatter cones have been clearly seen as one of the evidence
on the field for an impact crater.

Shatter cones - Evidence for an impact crater

World map distribution impact crater
Since the 80’s, shatter cones have been clearly seen as one of the evidence
on the field for an impact crater.

Outline
What is a shatter cone ?
Definition and criteria for identification

Where can you find shatter cones ?
Occurrence and distribution of shatter cones

Shape, orientation, size – what do we know ?

Model of shatter cone formation – The enigma
remains…but what do we know ?

Impact metamorphism

Post – shock
temperature

Impact metamorphism

Where can you find shatter cones ?

Shatter cones at Vrederfort arefound at distances ranging from 10– 60 km from the center of thecrater.

Shatter cones are generally foundwith a precise range from thecenter of the crater.

Where can you find shatter cones ?
Distribution of shatter cones inthe Haughton impact structure,
Canada.

Shatter cones are found-In place in the central uplift-Within ejecta blocks

Osinski and Spray, 2006 (ESA Conference)
Compilation of pressure ranges at various sites indicate values
between 1 GPa (10kbars) and 20 GPa (200 kbars).

Orientation of shatter cones apices

Manton, 1977
Orientation of shatter cone apices point toward the center of the crater (afterrotation of the bedding).

Shatter cones – how big ?

Sudbury, Ontario, Canada
12 meters high shatter conesShatter cones – how big ?
As small as few centimeter or even millimeters

Samples from Steinheim impact crater (Germany)

Outline
What is a shatter cone ?
Definition and criteria for identification

Where can you find shatter cones ?
Occurrence and distribution of shatter cones

Shape, orientation, size – what do we know ?

Model of shatter cone formation – The enigma
remains…but what do we know ?

Models for the formation of shatter cones

1) Previous ideas- Johnson and Talbot (1964)- Gash (1971)

2) Front waves (Sagy and Reeches, 2002)

3) Baratoux and Melosh (2003)

Models for the formation of shatter cones
Some background about shock wave propagation

Melosh, 1989

Models for the formation of shatter cones
Some background about shock wave propagation
Threeregimes:Low
pressure(<Elastic waves (speed: CL)

HEL)- Intermediatepressure<Gpa
Plastic wave with propagation speed lower
than the elastic wave. Elastic precursor +plastic wave.
Highpressure>>
GPa
Shock wave propagating at a velocity
greater than the elastic wave.

K0: Bulk modulus
: Shear modulus

Melosh, 1989

Models for the formation of shatter cones
Some background about shock wave propagation

Elastic precursor in nuclear tests
Melosh, JAP, 2003.

Elastic precursor in numericalsimulations
Baratoux and Melosh, EPSL, 2003.

Models for the formation of shatter cones
Stress distribution during the spherical propagation of a shock wave

Baratoux and Melosh, 2003

Models for the formation of shatter cones
Stress distribution during the spherical propagation of a shock wave

Compressive stress

Extensive stressModels for the formation of shatter cones
Stress distribution during the spherical propagation of a shock wave

Elastic precursor

Compressive stress
Extensive stress

Baratoux and Melosh, 2003

Models for the formation of shatter cones
Stress distribution during the spherical propagation of a shock wave

Shock front

Models for the formation of shatter cones
Stress distribution during the spherical propagation of a shock wave

Shock front

Johnson and Talbot (1964)
The elastic precursor
Starting point: Elastic precursor is scattered by a rock inclusion

Elastic stresses in thevicinity of an rock inclusion.

Johnson and Talbot (1964)
The elastic precursor
Consequence: Plastic yielding in a conical zone

Assumption : stress removal before the arrival of the main plastic wave
Elastic zone returns back to its initial state, but not the plastic zone
Separation or fracture occurs at the interface between the pastic zone and the elastic zone

Johnson and Talbot (1964)
The elastic precursor
Main problem: There is no stress removal before the arrival of the plastic wave. The
whole rock will be exposed immediately after elastic precursor to higher stresses, anddeformed plastically.

Elastic precursor in nuclear tests
Melosh, JAP, 2003.

Elastic precursor in numericalsimulations
Baratoux and Melosh, EPSL, 2003.

Gash - 1971
Interferences with reflected waves at the surface

Ray paths (left) and wave front (right) for a spherical wave stress emanating from a point
source adjacent to a free surface. Incident stress (plain) and reflected stress (dashed)Gash - 1971
Interferences with reflected waves at the surface“zero stresscorridor”

Possible planes of fractures forming a (noncomplete) conical surface

σ1 trajectories (fracture plane are // to σ1
for fracture resulting from tensile stress)
Contour for σ3 values.

Gash - 1971
Interferences with reflected waves at the surface
This model has actually received little attention by other authors…
No real discussion of this model can be found in the literature.
I see two main problems :Complete cones are impossible to form with this model, as there is no stress when
the conical surface intercepts, as mentioned by the authors. Complete cones arerare indeed but they do exist!

Orientations of shatter cones usually indicate that apices point toward the centerof the crater, (ie, the source of the main shock wave) which is in contradiction
with the prediction of this model.

Front wave
Sagy and Reeches, 2002, 2004
Theoretical studies [Ramanathan and Fisher, 1997;
Morrissey and Rice, 1998, 2000], and experimental
work [Sharon et al., 2001, 2002] have revealed a
new type of localized wave, termed a ‘‘frontwave’’ (FW), which is excited when a rapidly
moving fracture front encounters an inhomogeneity
in the material.
Inhomogeneity will induce a pair of propagating
front waves that create a pair of tracks on thefracture surface emanating from inhomogeneity[Sharon et al., 2001, 2002].
V : Velocity of the fracture front
VFW : Velocity of the front wave
= 0.96 – 1.0 Rayleigh wave velocity.

Rayleigh wave velocity : maximum velocity at which acrack can propagate, independent of shock pressure).

An important prediction of this model:
The fracture velocity will decrease withthe decreasing stress away from thecenter should increase with the distance to thecenter of the crater

Front wave
Sagy and Reeches, 2002, 2004

Observations: increase of the striation angle with the distance to the center of thecrater (Sagy et al., 2002, 2004).

Front wave
Sagy and Reeches, 2002, 2004

Internal consistency: Relation between crack velocity and compressive stress(estimated from numerical modeling of Vredefort impact crater). Sagy et al., 2004.

Front wave – Difficulties..
Sagy and Reeches, 2002, 2004
Careful analysis of the distribution ofstriation angles for each outcrop.

Wieland et al., 2006 (MAPS)

Front wave – Difficulties..
Sagy and Reeches, 2002, 2004
There are more variations at a given outcrop than between one outcrop and another!

Wieland et al., 2006 (MAPS)

Front wave – Difficulties..
Sagy and Reeches, 2002, 2004

Comparison between Sagy et al.
2002, 2004 and Wieland et al.,2006 observations

Dependence of striation angles
with the distance to the center ofthe crater is seriously questionable.

Wieland et al., 2006 (MAPS)

Interferences at rock heterogeneities
Baratoux and Melosh, 2003

Interferences at rock heterogeneities
Numerical simulation of a shatter cone

S.A.L.E. (2D) Simplified Arbitrary Lagragian Eulerian- Hydrocode: Navier Stokes equations- Mass conservation- Energy conservation- Equation of state (P = f(,E)) – Murnhagan EOS P = K0/n[(/0)n – 1]

+ Artificial viscosity (to spread the shock wave over several cells of the grid, with Hugoniot equationssatisfied)

Baratoux and Melosh, 2003

Interferences at rock heterogeneities
Numerical simulation of a shatter cone
Goemetric parameters of the spherical shell
Internal radius : 7 m
Angle of the spherical shell : 30°
Heterogeneity = 4 cells : 4 x 7.2 cm, at 7.60 m from the source

Baratoux and Melosh, 2003

Interferences at rock heterogeneities
Numerical simulation of a shatter cone

Size of the heterogeneity (1 cell) : 4 x 7.2 cm

For more information about Damage modeling see:
H.J. Melosh, E.V. Ryan, Dynamic fragmentation in impacts: Hydrocode simulation of laboratory impact, J.
Geophys. Res. 97 (1992) 14735-14759.
Baratoux and Melosh, 2003

Interferences at rock heterogeneities
Numerical simulation of a shatter cone

Damage: Graddy-Kipp-Meloshfragmentation model
Fracturation is represented by a scalarparameter (D) for each cell decreasing
the value of elastic modulii whenstresses are tensional.
D = 0 : no damage
D =1 : Everything is damaged andtensile stresses are not transmitted
The parameter is responsible for aprogressive (linear) decraese of elastic
modulii of the media.

For more information about Damage modeling see:
H.J. Melosh, E.V. Ryan, Dynamic fragmentation in impacts: Hydrocode simulation of laboratory impact, J.
Geophys. Res. 97 (1992) 14735-14759.
Baratoux and Melosh, 2003

Interferences at rock heterogeneities
Numerical simulation of a shatter cone

Shatter cone simulation for ice
inclusion in a basaltic rock.

The fracture develop along a surface defining a pseudo-conical region. As the extensive
stress is not transmitted once the fracture is formed the area within the cone is protected.
Cells inside can be eventually partially fractured.
Baratoux and Melosh, 2003

The shape of shatter cones: determination of 3D required

Sagy and Reeches, 2004

In Baratoux and Melosh, 2003, we predict that theshatter cone shape will result from the intersection of
two expanding spheres. The exact shape will depend on
the structure of the shock front (rise time) and of theparameters of the heterogeneities.

Baratoux and Melosh, 2003

It would be possible to test
the model with a 3D shattercone shape measurement.

Implications
Range of pressures
Shatter cones are observed in numerical simulations for a range of pressure between 1 and 10 GPa, arange similar to the observed range of pressure in terrestrial impact craters. (~ structures d’impactterrestre).
The range of pressure for which shatter cones can occur is controlled by the resistance of the material
in tension (and not by the Hugoniot Elastic limit, like in the Johson and Talbot model).
Duration of the rise time of the shock wave

The rise times is should be short

What is the rise time of a shock wave in geological media ?
H.J. Melosh, 2003Implications
The rise time of a shock wave in geological media: nuclear tests

Rise time as a function of particle velocity inhalite (NaCl).
Rise time mostly depends on shock
wave intensity.

Rise time as a function of particle velocity in
granite.
The geometrical properties of shatter cones (size,apical angles) should depend on material affected by
the shock waves.
H.J. Melosh, 2003

Implications
The rise time of a shock wave in geological media

Hugoniot equations

Extrapolation at 3 Gpa => 1 ms (still one or two orders of magnitude too long…).

What is the real rise time of strong shock waves in geological media ?

Conclusions
Johnson andTalbot (1964)
Heterogeneities
required✔

Elastic wave
interferences✔

Sagy at al.,2002, 2004

✔
✔

✔
✔

Front waves
Important flaws

Baratoux andMelosh (2003)

✔
✔

✔
✔

Tensile fracture
Fractured linked withplastic yielding

Gash (1971)✔

Conclusions- Shatter cones are formed by extensive stresses- Shatter cones are formed during the passage of the shock wave

- Striations are explained by Sagy et al., 2002, 2004, but relationships between
striations angle and distance to the center of the crater is not established.

- Shapes and sizes are well explained by Baratoux and Melosh, 2003 (but thestriations are not explained with this model).

Combination of the two models as
proposed in Wieland et al., 2006 ?

Shatter cones in basalts – Craters in Brazil
Few words about Vargeão and Vista Allegre

R. Trinidade, E.
Yokoyama, University ofSaõ Paolo

Shatter cones in basalts – Craters in Brazil
Few words about Vargeão and Vista Allegre

Parana trapps – overview of the basaltic pile

Shatter cones in basalts – Craters in Brazil
Few words about Vargeão and Vista Allegre

Parana trapps – contact with the Botucatu sandstoneShatter cones in basalts – Craters in Brazil
First observations for a summer field campaign

Shatter cones in basalts – Craters in Brazil
First observations for a summer field campaign

Timing of the formation of shatter cones is clear!

Session proposal

Impact Cratering on Solid Planets - Shocks on Basalt
Baratoux, David1, Crosta, Alvaro2, Arakawa, Masahiko3
1Observatoire Midi-Pyrénées, Toulouse, France. 2University of Campinas, Campinas, Brazil.
3University of Nagoya, Nagoya, Japan.

Impact craters on solid planets are often formed on magmatic rocks. For the Moon andMars, new magnetic, mineralogic and chemical data motivates a better quantification ofshock effects on basalt. Recent analyses of craters on terrestrial igneous provinces offer theperspective of comparative studies. Contributions providing insights into the formation ofimpact craters on magmatic bodies, experimental or field studies (e.g., Vargeão and Cerro
do Jarau, or Lonar), and analyses of extra-terrestrial impact structures will be appreciated.
A 2-day field trip to Vargeão and Vista Alegre craters is being proposed in association
with this session.

Can we produce shatter cones artificially ?

D.J. Roddy and Davis, 1977, Shatter cones in large scale experimental explosion craters.

Relationships between shatter cones and other fractures
Few words about the MSJS
Shatter cones are generally defined as curved fracture surfaces with striations. Entire
or more often partial cones are found on the field.
MSJS (Multipled-Striated-Joint-Set) are also fractured surfaces with striations
MSJS and shatter cones may be the expression of the same phenomenon,
occurring during the passage of the shock wave.

MSJS at Vergeão impact
crater in Brazil.

I. Impact dans le système solaire

Impacts Météoritiques et Ondes de Chocs dans les roches

Limite élastique de Hugoniot
Les roches ne peuvent pas supporter des différences de contraintes au-delà d’une certaine valeur seuil

Fracture (critère de Coulomb)

Déformation plastique

Différence de contrainte maximale :Limite élastique de Hugoniot :Melosh, 1989
MECADYMAT 08 - LorientImpacts Météoritiques et Ondes de Chocs dans les roches

I. Impact dans le système solaireTraitement de l’onde de choc: Equation de Hugoniot- Conservation de la masse- Conservation de la quantité demouvement- Conservation de l’énergie

MECADYMAT 08 - LorientImpacts Météoritiques et Ondes de Chocs dans les roches

I. Impact dans le système solaire

Une première approximation des valeurs de pression: l’impact plan(Equation d’état)Représentation du contact entre un projectile et la surface d’uneplanète dans l’approximation de l’impact plan. Le projectile sedéplace à une vitesse Vp/c par rapport `a la cible.Up/p et up/preprésentent respectivement la vitesse du choc et d’un petit volume dematière relativement au projectile en mouvement. Uc/c et uc/c
représentent respectivement la vitesse du choc et d’un petit volume dematière relativement à la cible.
MECADYMAT 08 - Lorient

Anorthosite (roche lunaire)Impacts Météoritiques et Ondes de Chocs dans les roches

MECADYMAT 08 - Lorient

I. Impact dans le système solaireImpacts Météoritiques et Ondes de Chocs dans les roches

I. Impact dans le système solaire

Limites de Hugoniot pour les roches terrestres (ou planétaires)

MECADYMAT 08 - LorientImpacts Météoritiques et Ondes de Chocs dans les roches

II. Métamorphisme d’impact

Après le passage de l’onde de choc:- Décompression adiabatique- Augmentation de température, changement de densité, transition de phase

MECADYMAT 08 - LorientImpacts Météoritiques et Ondes de Chocs dans les roches

III. Cônes de percussion

Cratère d’impact de Vredefort, 2.023 Gy, 280 km de diamètre

MECADYMAT 08 - LorientFront wave – Difficulties..
Sagy and Reeches, 2002, 2004

Assuming compressive stressequal tensile stresses.