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Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin
~ecron~p~y~jcs, 171 (1990) 185-200 Plsevier Science ~blish~s B.V., Amsterdam - Printed in The Neth~lands
185
Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin * R.A.F. GRIEVE, J.M. CODERRE, P.B. ROBERTSON and J. ALEXOPOULOS Geophysics Division, Geological Survey of Canada, Oztawa, Ont. KlA 0Y3 (Canada) (Revised version accepted August 11, 1988)
Abstract Grieve, R&F., Coderre, J.M., Robertson, P.B. and Alexopoulos, J., 1990. Microscopic planar deformation features in quartz of the Vredefort structnre: Anomalous but still suggestive of an impact origin. In: L.O. Nicolaysen and W.U. Reimold (Pditors), Cryptoexplosions and Catastrophes in the Geological Record, with a Special Focus on the Vredefort Structure. Tectonophykcs, 171: 185-200. The orientation and distribution of diagnostic shock-produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the Vredefort structure in South Africa. These features are widespread, occurring in all but four samples. The majority of the planar features have a basal (CKKtl)orientation, with a few higher pressure so-called w features and three n features occurring near the center of the structure. Their relative distribution and preservation are, however, anomalous compared to those at other large terrestrial impact structures and recorded shock pressures, calculated on the basis of planar feature orientation, do not show a regular decrease radially outward as observed at other known impact structures. The apparently anomalous development of planar features is interpreted as a result of post-shock recrystallization. If the effect of recrystallization of quartz, which generally increases inward toward the core of the structure, is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure, then higher pressures are recorded in the core compared to the collar rocks. No evidence of a previously reported, additional post-r~~st~~tion shock event is found in this study, although previous researchers report that p~udotachy~te, which may or may not be associated with the shock event, cuts recrystallized quart&es. In is concluded that, although anomalous, the evidence from planar features in quartz is still consistent with the Vredefort structure being an erosional remnant of a large, complex, impact structure.
Introduction The circular Vredefort structure in South Africa consists of a central core of predominantly Archean granites - 44 km in diameter, a collar of steeply dipping to overturned Proterozoic sedimentary and volcanic rocks - 18 km wide, and an outer broad synclinorium of predominantly gently dipping Proterozoic sedimentary and volcanic
* Contribution of the Geological Survey of Canada 32887. oo40-1951/90/$03.50
0 1990 Elsevier Science Publishers B.V.
rocks - 28 km wide. The southeastern half of the structure is covered by younger sandstones and shales. The general circular form of the structure with an uplifted central core, the occurrence of the shatter cones, stishovite, coesite and diagnostic shock-produced microscopic planar features in quartz have been cited as evidence that the Vredefort structure is a very large, complex impact structure (e.g. Dietz, 1961; Carter, 1965, 1968; Manton, 1965; Martini, 1978). This interpretation has not gone unchallenged and alternative endogenic origins have been pro-
RAF. GRIEVE ET AL.
186
posed (Hart et al., 1981; Schreyer, 1983; Winter, 1986). Indeed, the evidence generally considered diagnostic of an impact origin has itself been challenged. For example, Lilly (1981) and Simpson (1981) have presented arguments that the temporal aspects of the formation of microscopic planar features in quartz and shatter cones can be correlated with other thermal and structural events and are inconsistent with an impact origin. Reimold and H&z (1986) have stated that there are no diagnostic, shock-produced planar features in quartz at Vredefort, only planar fractures. Planar fractures are produced by dynamic deformation and may occur in shocked lithologies, but are not unique to an impact origin. Clearly, there is some controversy regarding microscopic deformation features in quartz at Vredefort and their exact significance for the origin of the structure. In an effort to clarify these issues, we have examined a large number of samples from Vredefort and analyzed the distribution and orientation of planar features. We have also evaluated the extent of recrystallization in quartz. Before presenting our observations, we briefly summarize
previous systematic analyses of microscopic planar features in quartz at Vredefort to set in context the argument that planar features do not indicate an impact origin for the Vredefort structure. Previous analyses
The presence of planar elements considered diagnostic of a meteorite impact origin in quartz has been reported in the Vredefort structure in several studies. For example, Carter (1965, 1968) described “deformation lamellae” in virtually all of the quartz grains in the samples that he examined. He concluded, from extrapolation of static deformation tests, that stress differences in the range 35-60 kbar were required for their formation and that, in nature, pressures of this magnitude must have been derived from shock due to impact of an extraterrestrial body. Lilly (1981) has examined the occurrence of microscopic planar features in quartz in samples along three radial traverses through part of the collar rocks at Vredefort. He noted the abundance and orientation of “planar features” and used the
PALEOZOIC I_ ~~~~
(Shale,
PROTEROZOIC m zFt$gAAL
fEj
m
*SAMPLE
Fig. 1. Geology
of the Vredefort
structure
showing
locations
of samples
examined
(Andesite
WITWATERSRAND
ARCHEAN m @#[~
in this study. Geology
f
(%&e,~uuprttile,
~~~~&SDoRP SYSTEM
sandstone
)
(S)huaSr~~itc,
(Granite)
LOCATIONS
taken from Nel(l925).
QUARTZ
DEFORMATION
FEATURES,
VREDEFORT
pressure calibration scheme of Grieve and Robertson (1976), which is based on planar feature orientation, to argue for a relatively regular decrease in recorded shock pressure declining outward from 15 GPa at the core-collar boundary. He also noted that the degree of recrystallization of quartz increases progressively inward from the outer collar rocks toward the core. In examining some of the more thoroughly recrystallized samples close to the core-collar contact, Lilly noted that planar features in quartz were confined to the basal orientation, parallel to (0001). In the scheme of Grieve and Robertson (1976) basal features are indicative of the lowest grade of shock necessary to produce planar features. Thus, the calculated recorded shock pressures in these core-collar boundary samples were lower than those in collar samples at greater radial distances from the core. He interpreted this observation as indicating that a second, lower pressure shock event (which produced only basal features), was separated temporally from an earlier, higher pressure shock event (which produced basal and other planar feature orientations) by a period of thermal metamorphism (recrystallization). From this, Lilly argued that the spatial overlap of two shock events is highly unlikely, if the sources of the shock waves were related to impact events, which are spatially random. He concluded that this not only argues against an impact origin for the Vredefort structure but also calls into question the argument that, in nature, microscopic planar features in quartz are unique indicators of impact. This latter view has been quoted by a number of authors who oppose an impact origin for specific terrestrial impact structures (Sage, 1978; Muir, 1984) as well as by those who oppose an impact origin for planar features in quartz from Cretaceous-Tertiary boundary sites (Officer and Drake, 1985; Officer et al., 1987). The studies by Reimold and Horz (1986) and Reimold (1987) even question the occurrence of diagnostic planar features at Vredefort. They note the occurrence of only so-called “planar fractures” and, in some cases, spatially relate their relative abundances to the local development of pseudotachylite. If one accepts the argument that diagnostic planar features of the type observed at
187
other impact structures are not present, then Vredefort may not be an impact structure. It is difficult, however, to understand the denial of the occurrence of planar features at Vredefort by these authors, in light of the previous observations of Lilly (1981) and earlier workers. There is possibly some confusion regarding the definitions of “ planar fractures” and “planar features”. In our experience in examining samples from a number of impact structures, planar fractures are microscopic, open, parallel to subparallel fissures. These extension fractures may be up to 10 pm wide and may be relatively widely separated by distances of up to several tens of micrometers. Planar features in shocked quartz on the other hand, occur as multiple, parallel elements, l-2 pm wide, narrowly spaced (2-5 pm), generally a cross entire grains, and have specific crsytallo~ap~c orientations. Stoffler (1972) notes that planar fractures in quartz have low Miller indices, including the basal orientation (0001). While this may be the case, planar features produced at relatively low shock pressures (7.5-10 GPa) correspond to the (0001) orientation (French and Short, 1968; Walzebuck and Engelhardt, 1979). Although this may account for some of the apparent confusion, Reimold and H&z (1986) and Reimold (1987) also note what they refer to as planar fractures in quartz oriented at high angles to the e-axis. We can only assume that these studies at Vredefort have incorporate both planar features, mostly with {OOOl} orientations, together with planar fractures, with various orientations, and classed them all as planar fractures (Reimold and H&z, 1986). On the basis of different optical properties Carter (1965, 1968) distinguishes “deformation lamellae” that form parallel to the basal plane from “planar features” that lie parallel to other specific c~stallograp~c o~entations. However, Robertson et al. (1968) were unable to maintain a clear distinction, on the basis of optical appearance, between coexisting features oriented parallel to the basal plane and to other orientations. They have, thus, continued to classify all these elements with their specific orientations as “planar features”. It is clear, regardless of the nomenclature, that Carter and Robertson et al. are describing the same suite of deformations and
R.A.F. GRIEVE
188 TABLE 1 Planar feature statistics for the Vredefort structure Sample
Recrystal-
Distance
Grains
Type A
Type B
Type C
Type D
lization
from center
with planar
(I%)
(5%)
(W)
(9)
level
(km)
features (W)
4
0.00
0
AVG-32
3
4.40
44
AVG-36
3
5.40
0
5.63
I
GV-35
VNT-17 AVG-34
4
WIT-16
_
_
_ _
24 _
8
12 _
_
_
_
_
6.00
0
7 _
7.13
40
40
_
_
_
_
8.20
47
47
_
_
GV-40
3
8.50
32
8
24
_
AVG-38
2
9.10
56
56
WIT-15
_ 4
_
AVG-37
2
9.40
16
AVG-17
10.00
8
8
VNT-14
3 _
10.00
53
53
_
_
GV-16
2
10.80
28
28
_
_
11.25
72
_
_
60
60
11.80
40
40
VNT-12
2 _
12.50
60
60
_
GV-33
3
12.90
32
32
_ _
VNT-13 AVG-39
_ _ _
_ _ _
13.50
72
72
VNT-11
2 _
_
14.00
74
67
3
14.10
12
12
7 _
_
AVG-43 GV-13
2
14.10
72
68
4
_
GV-15
2
14.10
36
32
_
_
AVG-53
2
15.00
20
20
4 _
_
_
AVG-44
4 _
15.30
0
_
_
_
VNT-10
15.63
47
47
_
_
_
AVG-14
2
15.80
45
45
_
_
AVG-30
3
15.80
20
20
_
_
GV-18
16.30
24
24
_
_
VNT-9
3 _
17.25
66
66
AVG-59
2
17.80
48
48
AVG-41
_
_ _ _
_
AVG-1
3
17.80
12
12
_
_
AVG-54
2
17.90
70
70
_
_
GV-4
2
17.90
8
8
AVG-52
3 _
18.00
16
16
18.00
33
33
VNT-8 GV-9 VNT-7
2 _
_
_ _ _
18.40
32
32
_
18.75
20
20
_
GV-50
3
18.80
16
16
_
AVG-50
2
18.90
52
52
_
GV-7
2
19.00
36
36
_
AVG-55
2
19.00
44
44
AVG-61
2
19.10
12
12
AVG-20
19.80
96
92
VNT-6
2 _
19.88
27
27
AVG-58
2
20.00
48
GV-45
2
20.00
AVG-8
2
AVG-47
2
_
_ _ _
_
_
_
_
_ _
4
_
48
_
_
_
16
16
_
_
_
20.30
52
52
_
_
_
20.40
36
36
_
_
_
_
ET AL.
QUARTZ
DEFORMATION
FEATURES,
189
VREDEFORT
TABLE 1 (continued) Sample
GV-24
Rectystal-
Distance
Grains
Type A
TypeB
Type C
Type D
hzation
from center
with planar
(%)
(%)
(%)
(%)
level
(km)
features (W)
2
20.40
44
44
_
_
20.50
66
66 52
VNT-5 AVG-31
2
20.60
52
AVG-46
2
20.90
52
52
AVG-12
2
20.90
60
60
GV-22
2
VNT-4
20.90
72
68
21.25
60
60
AVG-21
2
21.30
64
64
GV-29
2
21.30
76
76
21.75
33
33
VNT-3 AVG-9
2
21.80
50
50
AVG-27
2
21.90
60
60
AVG-28
2
22.00
80
80
AVG-56
2
22.50
32
32
22.50
40
40
22.80
44
44
22.88
80
80
23.50
75
15
VNT-2 2
AVG-10 VNT-1 VT-94
2
GV-26
2
23.80
68
68
MV-0.06
2
24.00
92
48
MV-0.08
2
24.00
28
28
MV-0.09
1
25.60
52
52
25.60
0
26.40
80
80
26.80
72
12
AVG-73 2
VT-88 MV-0.15
26.90
21
21
VT-90
1
27.00
64
64
MV-0.12
1
27.90
84
84
28.00
80
80
AVG-76
MV-0.14 MV-0.19
1
28.10
64
64
VT-81
1
28.80
40
40
VT-83
2
29.60
67
61
h4V-0.31
2
31.50
72
72
VT-86
1
32.50
80
80
_
Recrystalhr.ation level: 1 = recoverey; 2 = primary; 3 = moderate; 4 = extensive. Distance from center: Most samples beyond radial range of AVG-10 are from collar rocks, given that the core is not a perfect circle. Percentage grains with planar features: In most samples, 25 grains/thin section were examined for planar features. Type-A features are produced by the lowest level of shock and Type-D
by the highest level of shock. Type A= basal {OOOl); Type B= w{lOi3);
Type C=r
& I {loil},
(Olil};
Type
d = s(loi2).
attribute the entire suite to high strain rate deformation due to meteorite impact. Where fresh, planar features are filled by diaplectic glass, but, more commonly, they are partially annealed and contain minute inclusions or decorations. In the case of Vredefort the obscuring effect of annealing on planar features is due to - 2.0 b.y. of geologic
history since formation and also to grain recrystallization, for which there is clear evidence. Planar features
We have examined the occurrence and orientation of planar features in quartz in 83 samples
R.A.F.
190
from both the core and collar rocks at Vredefort
progressively
(Fig. 1). Our samples
rocks
of Lilly (1981), however,
do not correspond
except
cover the same radial
collar rocks. The seventeen traverse
of Reimold
In general,
were examined
in each sample
were measured
stage techniques.
Qualitative
ing the optical character, ity, and clarity and the general
25 quartz
by standard
the
such as spacing, features
feature universal
observations
regardcontinu-
were made
of recrystallization
structures this may
of the
samples was also noted. We found that planar features in quartz are widespread at Vredefort but their relative distri-
features
1977).
at Vredefort,
the unusual
of the original
may
1988)
represent
planar
trails
and
are
typically
recognized
grains.
Bohor
(pers.
these
trails
that
remnants
their
in
of decorated
recognition,
the anomalous
the Vredefort structure. defined by the alignment that
are
suggested
annealed
features
for
of the higher shock (Type-
quartz
has
would help explain
for
it does not fully account
association
commun.,
(Robertson account
of lower shock Type-A
D) features. Bubble or inclusion many
ET AL.
in the uplifted
While
some of the predominance
grains
Grieve,
material
impact
and
are included
and planar
of the planar degree
within
of complex
do,
from the VNT
(1987), however,
suite.
They
range
samples
in our sample orientations
to those
by coincidence.
less shocked
GRIEVE
as such,
observations
at
These bubble trails are of inclusions with traces
slightly
curved.
quartz
grains contain
both decorated
bubble
There
proximately 90% of the planar have a basal (0001) orientation
tween the straight, well defined decorated planar features and the bubble trails within these same
features observed and are so-called
Type-A features (Robertson et al., 1968) (Table 1 and Fig. 2a). In all but nine samples, basal features alone are observed in quartz grains. This typifies Type A, or the lowest level (5 10 GPa) of quartz deformation in the terminology of Robertson et al. (1968). In most cases the Type-A features observed at Vredefort are equivalent to those
trails.
and
difference
be-
The average recorded shock pressure for each sample was determined from the planar feature orientations by the method of Grieve and Robertson (1976). This method to a selected grains/sample)
highest level of quartz deformation, Type D (15-25 GPa), characterized by the presence of
covery
samples displaying Type-D features in other structures (Figs. 2c and d). The heavy bias towards the occurrence of basal (Type-A) features relative to other orientations at Vredefort compared to other known impact structures is illustrated in Fig. 3. This may be, in part, an erosional effect. Increasing erosion exposes
is a distinct
features
grains. The bubble trails display irregular development and an apparent lack of crystallographic control.
observed at known impact structures (Figs. 2a and b). Planar features parallel to w (1013) (Type B, Table 1) (Robertson et al., 1968) were identified in nine samples and correspond to an intermediate level of shock deformation (- lo-15 GPa). The
planar features parallel to 7~ (1072) (Robertson et al., 1968), was observed in only one sample (Table 1 and Fig. 2~). Although the orientations of the Type-D features are equivalent to those at known impact structures, the appearance compared to the features at Vredefort differs. The overall development and frequency of occurrence of planar features in this sample is less than that observed in
planar
Some
bution and preservation are anomalous compared to those at other known impact structures. Ap-
assigns
a pressure
value
number of quartz grains (usually 25 in the sample based on the planar
feature development in that grain. Estimates of the required pressures for development of each of the planar feature types are based on shock reexperiments
(H&z,
1968;
Mtiller
and
Defourneaux, 1968) and are as follows: Type A = 8.8 GPa, B = 12 GPa, C= 15 GPa, and D= 23 GPa, and grains without planar features = 5.5 GPa if they lie within the outer limit of observed shatter cone formation. An average shock pressure for the entire sample is calculated from the arithmetic mean of the individual grain pressures. Using this method, no well-defined relationship between pressure and radial distance from the center of the structure could be discerned (Fig. 4). This does not compare with the relatively regular outward decrease in recorded shock pressure observed at other impact structures (Fig. 5) (Dence, 1968; Robertson, Marakushev, We concur
1972; Grieve 1981).
and
Robertson,
with Lilly (1981) in finding
1976; that the
QUARTZ
Fig.
basal
DEFORMATION
2.
Planar
features
FEATURES,
in quartz
{OOOl} form (crossed
(crossed
polars).
confinement
Compare
191
VREDEFORT
grains. a. Primary
polars).
b. Quartz
quartz grain from Vredefort
grain
with decorated
with (a). c. Development
to the core of the quartz
the inner core rocks and is moderately
of Type-D
basal
planar
recrystallized
(crossed
impact
polars).
structure
highest average shock pressures have been recorded near the core-collar contact (Fig. 4). Unlike Lilly, however, we fail to observe a regular decrease in recorded shock pressure outward in the collar rocks, using the scheme of Robertson and Grieve (1976) (Fig. 4). Nowhere do we observe recorded pressures greater than 10 GPa, in contrast to the maximum values of 15 GPa reported by Lilly (Fig. 4). This is difficult to explain except by hypothesizing that Lilly measured not only the orientation of bona fide planar features but also that of planar features. For example, the relatively widely spaced features in Lilly (1981, Fig. 3), presented as an illustration of planar features in the most highly shocked sample of collar rock, would be classed by us as annealed planar fractures. Consistent with the possible con-
features
oriented
from the Manicouagan
features
parallel
to the n (1012)
This Vredefort
d. Development
(crossed
planar
features
grain and the lack of other orientations.
quartz gram from the Mistastin
with decorated
planar
polars).
sample,
of Type-D
Note difference
planar
orientation.
AVG-32, features
compared
parallel
impact
Note
was collected (oriented
to the
structure their from
N-S)
in
to (c).
fusion between different types of planar elements is the observation that Lilly (1981) records planar features in 100% of the quartz grains in the more highly shocked samples near the core-collar contact. He also notes a large range of so-called planar feature orientations in samples from the collar rocks. These observations are at variance with ours (Table 1 and Fig. 6). In polycrystalline targets such as rocks interaction of the shock wave with different phases, phase boundaries, irregular grain margins and discontinuities produce localized stress variations resulting in a range of planar feature development within a sample. Often, individual grains in a low-shocked sample are limited to Type-A deformation but, at higher levels of shock, A-C-Type or B-D-Type deformations occur typically within
R.A.F.
192
50
10 rWW..E
2&WEE,““,-AX~3*k0
POLES
TO
F%RES
70 80 (DEGREES)
10
90
20
ANGLE
BETWEEN
30 C-AXIS
40 SD 60 AND POLES TO FEATURES
GRIEVE
70 (DEGREES)
GO
ET AL.
90
Fig. 3. Comparison of the relative abundance of various planar feature orientations in quartz at (A) Vredefort (n = 737) and (B) the combination of data from ten known impact structures in Canada (n = 1202). n is the number of measured planar feature sets with specific orientations.
a single sample, representing grain to grain variations in recorded shock pressure of - 25% (Robertson and Grieve, 1977). The occurrence of
15’0/
. Core
14.0-p
;i 13.0-&
A
0
planar features between grains within a single sample at Vredefort, however, is even more variable than that at other known impact sites. This
Collar
.
Lilly (1991) This study
);: 12.0--
. 8 5
ll.O-A
e
8 lO.O--
.
l
8 t 4 3 3
I
9.0--
l
%.O-7.0-6.0--
$
0.0
I
I
16.0
0.0 DISTANCE
FROM CENTER
24.0
32.0
(km)
Fig. 4. Variation of calculated, average recorded shock pressures with increasing radial distance from the center of the Vredefort structure. See text for method of calculation. Triangles are the data of Lilly (1981) from the collar rocks.
QUARTZ
DEFORMATION
FEATURES,
193
VREDEFORT
l
‘:: e
e
-00 l o l 4.0
I 2.0
0.0
t 6.0
l
o l I 10.0 DISTANCE
l
*
l l 1 ‘ 14.0 18.O FROM CENTER (km)
I 22.0
i
‘.O
Fig. 5. Variation of calculated, average recorded shock pressures with increasing radial distance from the center of the Charlevoix structure, based on the occurrence of planar features and, at the lowest pressures, the spatial limit of shatter cone development. Compare with Fig. 4.
observation is emphasized by sample AVG-32. The Type-D features observed (Fig. 2~) occur in the cores of three grains, whereas other grains in the same sample show no planar features (Table
A,NGCT BETWEEN
C-AXIS
AND POLESTO FEATURES
1). This represents an apparent grain to grain variability in recorded shock pressure of - 200%. At other impact structures where Type-D features are observed, all grains in a sample show planar
IDEQREESI
Fig. 6. Comparison of planar feature orientations in quartz from the collar rocks at Vredefort. A. This study (n = 252). B. Lilly (1981) (n = 861). n is explained in Fig. 3.
R.A.F.
194
features
at this elevated
(Robertson the
and Grieve,
high-level
consistent
However,
for estimating
pressure
shock
formation
estimated
20 km uplift
is
of uplift
may
location
using
within
pressures
features of
4.5 km
the standard
the average
Type-D
(Robertson
to be 15 km and,
of
at Vredefort
recorded
grams is averaged (Grieve and Robertson, this sample has a recorded shock pressure require
Vredefort
features
(when the result from a number
9.9 GPa, although
shock
1977). The occurrence
with the sample
of the center. nique
Type-D
level of recorded
of quartz 1976)) of only
are believed
- 20 GPa
and Grieve,
techshock
to
for their
1977).
normal
thermal
- 525 o C. Post-shock could
add
Ahrens, strain
Assuming
gradient
another
energy
population
density)
could
defect healing,
a
core
rocks
were
at
for tectosili-
GPa
high
highly
are not C
(Raikes
shocked
state
of the minerals
of planar
sheet would
the uplifted
150°-250’
The
that
temperatures
to lo-25
1979).
the
of 15 o C/km
of the pre-erosional
shocked
with
of the collar rocks, 35 km
at the time of impact,
at the center cates
combined
occurred.
continental
prevailed
order Recrystallization
have
ET AL.
GRIEVE
features
contributes
with their
to recrystallization
serve to reduce
and (high
and increased
while any overlying
but
disand
breccia
or melt
the rate of heat loss.
The recrystallization of quartz at Vredefort has been subdivided qualitatively into four levels, ranging from low-level recovery in the outer collar
We believe recrystallization
samples to extensive recrystallization in the central core samples (Fig. 7). These four levels have been described in detail by Hobbs et al. (1976) and Nicolas and Poirier (1976). We suggest that re-
The quartzites in the outer collar were at a shallower depth than rocks of the inner collar and core before uplift and, as a result, they were at
crystallization is directly related to the amount of uplift of shocked rocks at Vredefort. Hart et al. (1981) reported the amount of uplift in the core at
resulted
as the shocked
and uplifted core and collar rocks reacted changed conditions.
to these
relatively lower temperatures. Their temperature and shock-induced disorder were only sufficient to allow the process of recouely to take place. This process attempts to repair the internal fragmenta-
15’
PALEOZOIC I-] $$;$j$
(Shale.sondstone)
PROTEROZOIC /// TRANSVAAL
’ h
SYSTEM
(Shale, dolomite)
-1
k$~~~~sDoRP
m
WITWATERSRAND($uyo~tt)zite, SYSTEM
ARCHEAN 1 $$$@~ I
(Andes(k)
( Granite)
LEVELS OF RECRYSTALLIZATION
q q
Fig. 7. Spatial relationship
of the levels of recrystallization
in the Vredefort
structure.
quortzlte.
FfWWy Primary
I3
Moderote
ccl
Extensive
Geology
taken from Nel(l925).
Fig. 8. Recrystallization textures in quartz grains at Vredefort. See text for details. a. Recovery displayed in the central quartz grain of this outer collar quartzite; level 1 (crossed polars). b. Primary recrystallization of quartzite of the inner collar; level 2 (crossed polars). c. Primary recrystallization between primary quartz grains in granite of the outer core; level 2 (crossed polars). d. Moderate recrystallization and normal grain growth in granite of the inner core; level 3. A single remnant, primary quartz grain remains in the right of the photograph (crossed polars). e. Extensive recrystallization, as shown by the recrystallization of all primary quartz grains in the core granite; level 4 (crossed polars).
of the crystal structure, which is seen as mosaicism, and attempts to return deformed, original grams to lower, more stable, energy states with little development of new quartz by recrystallization. Groups of slip planes or dislocation arrays within deformed grains may diffuse by rearrangement of atoms or may glide in order to tion
achieve a lower energy level (Hobbs et al., 1976). At this level (1 in Fig. 7), subgrains are developed with poorly defined boundaries within the original strained quartz grams. These subgrains are slightly misoriented optically within the parent grain (Fig. 8a). Planar features, were developed within the original grains, are well-preserved and continuous.
196
Rocks in the inner collar and outer core were subjected to greater sh~k-indu~d disorder and were originally at higher temperatures than rocks in the outer collar. These conditions prompted recovery and primary recrystallization. Primary recrystallization removes most of the stored strain energy that remains after recovery. At this level (2 in Fig. 7), numerous small (0.05-0.1 mm) new quartz grains surround relic original grains in the quartzite samples of the inner collar (Fig. 8b), In quartzite, virtually all the grains have the potential to form subgrains, resulting in a high density of nucleation sites and in the formation of a large number of small, recrystallized grains with many developing polyhedral, straight grain boundaries. In the granite samples from the outer core, new grains are developed along quartz grain boundaries and within defo~ation bands (Fig. 8~). The relatively smaller number of quartz grains in the granitic rocks provides fewer nucleation sites and thus new quartz grains are relatively scarce. Most original quartz grains remain, with many displaying well-defined planar features. As noted earlier, decorated basal planar features in these samples are equivalent in appearance to those in known impact structures (cf. Figs. 2a and b). At this level, feldspar grains display little or not alteration. According to the hypothesis that Vredefort is a complex impact structure, samples from the center of the core were subjected to the greatest shock-induced disorder and deformation and had the highest original temperatures, as they were uplifted from the greatest depth. These conditions allowed not only recovery and primary recrystallization but also two levels of normal grain growth. Normal grain growth reduces the grain boundary tension of the polycrystalline aggregate and brings the grains to an even lower energy state by reducing surface energy. At the rn~e~a~e recrystallization level (3 in Fig. 7), most original quartz grains are recrystallized to small (0.05-0.1 mm) grains which display both polyhedral and irregular boundaries. Some quartz, however, has grown to larger (0.3-1.0 mm), new grains with stable, lowenergy polyhedral boundaries (Fig. 8d). A few remnant quartz cores are preserved, with faint and discontinuous planar features. The sample containing the Type-D feature is recrystallized to this
R.A.F.
GRIEVE
ET AL.
level and, as noted earlier, the appearance of these faint Type-D features is anomalous compared to the Type-D features at known impact structures (cf. Figs. 2c and d). Alteration of coexisting feldspar grains to sericite and clay minerals is minor. At the extensive recrystallization level (4 in Fig. 7), essentially all quartz grains are recrystallized with many reaching size of between 0.5 and 1.O mm. Many have developed low-energy, straight grain boundaries and no planar features are apparent (Fig. 8e). Coexisting feldspar grains are moderately altered. Contrary to observations by Lilly (1981), we have not observed planar features in completely recrystallized quartz grains. Lilly (1981) however, restricted his observations within the recrystallized sector of the collar rocks to quartz grains within and adjoining pseudotachylite veins. We do, however, observe planar features in the cores of some relict primary grains surrounded by secondary recrystallized quartz. Figure 8 in Lilly (1981), which purports to show planar features in recrystallized quartz, is, based on our observations, a moderately recrystallized sample with planar features in a remnant core enclosed by polyhedral, recrystallized quartz. The effect of r~~~lIizatio~ estimates
on shock pressure
The progressive increase in recrystallization toward the center and the variability of recrystallization from grain to grain in individual samples, in our opinion, can account for the apparently anomalous distribution of recorded shock pressures in the quartz at Vredefort. If shock pressures are calculated by the method of Grieve and Robertson (1976), the central core rocks have low recorded shock pressures and there is no apparent decrease in recorded shock pressure with radial distance from the core (Fig. 4). This distribution is consistent with increasing inward recrystallization (Fig. 9) if planar features have been progressively annealed by r~~stall~tion. It is apparent that the lowest recorded shock pressures occur in the relatively more recrystallized samples (Fig. 10). Also, on average, the more recrystallized a sample, the smaller the percentage of grains with planar
QUARTZ
DEFORMATION
FEATURES,
197
VREDEFORT
4.0* Cwe
I
I
I
I
8:0
o%.O
DISTANCE
6.0 FROM
CENTER
,. . \K~J
-I
32.0
24.0
Fig. 9. Variation in recrystallization level with radial distance from the center of the Vredefort structure.
method of estimating shock pressure may be invalid at Vredefort. This is most apparent in the case of the sample containing Type-C and -D features.
features (Table 1). As the calculation of recorded shock pressure is made from an average of observations on 25 grains and grain to grain recrystallization is variable, we would argue that this
4.0
3.0 g i= ifI
i a E-J & 2.0 ! b Gi 5
1.0,
0.0;
I
8.0
1
7.0 CALCULATED
I
S~!=RESSURE
I
&ii,
I
10.0
I
Fig. 10. Variation in calculated, recorded shock pressures with respect to levels of recrystallization.
D
R.A.F.
198
GRIEVE
ET AL.
Collar
2 A4 3
0~~00
0* 0
I
I
6 DISTANCE
Vredefort
structure.
00
I I I I I I 4 I I I I I
I
Fig. 11. Variation
00
00
00
in the highest
pressure
Note the tendency
planar
features
for a progressive
16 FROM CENTER
in quartz
ous types of planar features does show some relation to radial distance from the center of the Vredefort structure. Progressively higher pressure type planar features are confined to the central area of the structure (Fig. 11). The figure does indicate, however, a number of anomalous samples with no shock-induced planar features at
32
(km)
in each sample
increase in planar the center.
If, instead, the occurrence of planar features indicating the highest recorded shock pressure observed in each sample is plotted against radial distance, the maximum radial extent of the vari-
24
feature
with radial
type observed
distance (increased
from
the center
shock pressure)
of the toward
Here, as at Vredefort, anomalously low shock pressures are recorded. For example, the rocks that contain high-level Type-C features in quartz have planar features in only lo-308 of the grains, as opposed to the expected close to 100% (Robertson and recorded
Grieve, 1977), and have low calculated shock pressures of 6-7 GPa (Dressler,
1984). Conclusions
various radial distances. Type-C features are absent in this figure because they occur in a sample with higher pressure Type-D features. We have no definitive explanation for the heavy bias toward the preservation of Type-A basal features (Fig.
The conclusions of this study can be summarized as follows. (1) Diagnostic, shock-produced planar features in quartz occur at Vredefort. (2) Their distribution and the calculated auer-
ll), except to note that shock represents increasing disorder, and thus entropy, and that the lower pressure basal features correspond to a smaller increase in disorder and may be relatively more resistant to recrystallization. The annealing of planar features by a thermal event has been previously observed at the Sudbury impact structure.
age recorded shock pressures are, however, anomalous with respect to those observed at other large terrestrial impact structures. (3) The degree of recrystallization in quartz increases inward toward the core. (4) If the degree of recrystallization and its grain to grain variability are taken into account
QUARTZ
DEFORMATION
FEATURES.
VREDEFORT
and only the occurrence of planar features indicative of the highest shock pressures in a sample is used as an estimate of the shock pressure, there does appear to be a tendency for an increase in recorded shock pressure toward the core. (5) No evidence of a post-recrystallization, second shock event is observed. Lilly’s (1981) conclusion is based on the interpretation that weak planar features in the Vredefort collar rocks are superimposed on quartz after total recrystallization by an earlier thermal event. Our observations suggest that these features are faint, discontinuous, relict planar deformation features in the remnant, unrecrystallized cores of original quartz grains. (6) Much of what is observed with respect to planar features at Vredefort is at first glance anomalous relative to established impact sites. This perhaps befits the highly eroded Vredefort structure as the largest and oldest known impact structure on the Earth. While aspects of this structure will obviously remain controversial, we believe the evidence is sufficient to class Vredefort as an impact structure. Acknowledgements
The authors would like to express appreciation to B.M. French, D.P. Gold, R.B. Hargraves and U.W. Reimold for providing samples from then collections. Reviews by B. Bohor, N. Carter, F. H&z and N. Gay are appreciated. References Carter, N.L., 1965. Basal quartz deformation Iamellae-a criterion for recognition of impactites. Am. J. Sci., 263: 786-806. Carter, N.L., 1968. Dynamic deformation of quartz. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 453-474. Dence, M.R., 1968. Shock zoning at Canadian craters: Petrography and structural implications. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 164-184. Die& R.S., 1961. Vredefort ring structure: Meteorite impact scar. J. Geol., 69: 499-516. Dressfer, B.O., 1984. The effects of the Sudbury event and the intrusion of the Sudbmy Igneous Complex on the footwall rocks of the Sudbury Structure. In: E.G. Pye, A.J. Naldrett and P.e. Giblin (Editors), The Geology and Ore Deposits of the Sudbury Structure. Ont. Geol. Surv., Spec. Vol., 1, pp. 97-138.
199 French, B.M. and Short, N.M, (Editors), 1968. Shock Metamorphism of Natural Materials. Mono Book, Baltimore, 644 PP. Grieve, R.A.F. and Robertson, P.B., 1976. Variations in shock metamorphism at the Slate Islands impact structure, Lake Superior. Contrib. Mineral. Petrol., 58: 37-49. Hart, R.J., Welke, H.J. and Nicolaysen, L.O., 1981. Geochronology of the deep profile through Archaean basement of Vredefort, with implications of early crustal evolution. J. Geophys. Res., 86: 10663-10680. Hobbs, B.E., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, Toronto, pp. 73-118. H&Z, F., 1968. Statistical measurements of deformation structures and refractive indices in experimentally shock loaded quartz. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 243-254. Lilly, P.A., 1981. Shock metamorphism in the Vredefort collar: Evidence for internal shock sources. J. Geophys. Res., 86: 10689-10700. Manton, W.I., 1965. The o~entation and origin of shatter cones in the Vredefort Ring. N.Y. Acad. Sci. Ann., 123: 1017-1049. Marakushev, A.A., 1981. Impactites. Moscow State Univ. Press., 240 pp. Martini, J.E.J., 1978. Coesite and stishovite in the Vredefort Dome, South Africa. Nature, 272: 715-717. Muir, T.L., 1984. The Sudbury structure: Considerations and models for an endogenic origin. In: E.G. Pye, A.J. Naldrett and P.E. Giblin (Editors), The Geology and Ore Deposits of the Sudbury Structure. Ont. Geol. Surv., Spec. Vol., 1, pp. 449-490. Miiller, W.F. and Defoumeaux, M., 1968. Deformationsstrukturen in Quart ah Indikator fti Stosswellen: Eine experimentelle Untersuchung an Quarz-Einkristallen. Z. Geophys., 34: 483-505. Nel, L.T., 1925. Geological map of the country around Vredefort. Scale, 1: 63, 360. Geol. Surv. S. Afr., Pretoria. Nicolas, A. and Poirier, J.P. (Editors}, 1976. Crystalline Plasticity and Solid State Flow in Methodic Rocks. Wiley, Toronto, pp. 49-376. Officer, C.B. and Drake, C.L., 1985. Terminal Cretaceous environmental events. Science, 227: 1161-1167. Officer, C.B., Hallan, A., Drake, C.L. and Devine, J.D., 1987. Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions. Nature, 326: 143-149. Raikes, S.A. and Ahrens, T.S., 1979. Post-shock temperatures in minerals. Geophys. J. R. Astron. Sot., 58: 717-747. Reimold, W.U., 1987. Fracture density statistics along radial traverses through the crystalline basement at the Vredefort Dome, South Africa. Lunar Planet. Sci., 8: 826-827. Reimold, W.U. and H&z, F., 1986. Textures of experimentally shocked (5.1-35.5 GPa) Witwatersrand quart&e. Lunar Planet. Sci., 17: 703-704. Robertson, P.B., 1972. Zones of shock metamorphism at the Charlevoix impact structure. Bull. Geol. Sot. Am., 86: 1630-1638.
R.A.F.
Robertson,
P.B. and Grieve,
at terrestrial
impact
and R.B. Merrill ing. Pergamon, Robertson,
(Editors),
tion in rock-forming B.M. French
1977. Shock attenuation
In: D.J. Roddy,
Impact
R.O. Pepin
and Explosion
Crater-
M.R. and Vos, M.A., 1968. Deformaminerals
and N.M.
of Natural
from
Short
Materials.
Canadian
(Editors), Mono
craters.
Shock
Book,
In:
Metamor-
Baltimore,
pp.
Sage, R.P., 1978. Diatremes of the Slate
and shock features
Islands,
Northeastern
W., 1983. Metamorphism
basement
of the Vredefort
to the origin of the structure. Simpson,
C., 1981. Occurrence
Dome,
J. Petrol.,
guidelines
24: 26-47.
and orientation
of shatter
Winter,
of the Vredefort
J. Geophys.
Res.,
and transformation
natural
and
of minerals
86:
Mineral.,
of quartz
shock
shock compres-
49: 50-113.
influenced
Observations
of rock-
experimental under
J.P. and Von Engelhardt,
W., 1979. Shock defor-
by grain
size and shock direc-
on quartz-plagioclase
of the Ries crater,
Germany.
rocks
from
Contrib.
the
Mineral.
70: 267-271.
H. de la R.,
Witwatersrand arc plate-tectonic
cones
by
I. Behavior
sion. Forts&r. Walzebuck,
Petrol.,
South Africa:
minerals
processes.
basement
in the
in the collar
precluded,
D., 1972. Deformation
forming
Lake
and fluid inclusions
quartzites origin
ET AL.
10701-10706.
tion:
Superior.
group
Impact
in Precambrian
Bull, Geol. Sot. Am., 89: 1529-1540. Schreyer,
Dome:
mation
433-452. rocks
in Pretoria
Stoffler,
New York, pp. 687-702.
P.B., Dence,
phism
R.A.F.,
structures.
GRIEVE
Witwatersrand
1986.
A cratonic-foreland
Basin-development setting.
Econ.
model
in a continental, Geol.
Inf. Circ., 193: 36 p.
Res.
Unit.
for back-
Univ.
185
Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin * R.A.F. GRIEVE, J.M. CODERRE, P.B. ROBERTSON and J. ALEXOPOULOS Geophysics Division, Geological Survey of Canada, Oztawa, Ont. KlA 0Y3 (Canada) (Revised version accepted August 11, 1988)
Abstract Grieve, R&F., Coderre, J.M., Robertson, P.B. and Alexopoulos, J., 1990. Microscopic planar deformation features in quartz of the Vredefort structnre: Anomalous but still suggestive of an impact origin. In: L.O. Nicolaysen and W.U. Reimold (Pditors), Cryptoexplosions and Catastrophes in the Geological Record, with a Special Focus on the Vredefort Structure. Tectonophykcs, 171: 185-200. The orientation and distribution of diagnostic shock-produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the Vredefort structure in South Africa. These features are widespread, occurring in all but four samples. The majority of the planar features have a basal (CKKtl)orientation, with a few higher pressure so-called w features and three n features occurring near the center of the structure. Their relative distribution and preservation are, however, anomalous compared to those at other large terrestrial impact structures and recorded shock pressures, calculated on the basis of planar feature orientation, do not show a regular decrease radially outward as observed at other known impact structures. The apparently anomalous development of planar features is interpreted as a result of post-shock recrystallization. If the effect of recrystallization of quartz, which generally increases inward toward the core of the structure, is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure, then higher pressures are recorded in the core compared to the collar rocks. No evidence of a previously reported, additional post-r~~st~~tion shock event is found in this study, although previous researchers report that p~udotachy~te, which may or may not be associated with the shock event, cuts recrystallized quart&es. In is concluded that, although anomalous, the evidence from planar features in quartz is still consistent with the Vredefort structure being an erosional remnant of a large, complex, impact structure.
Introduction The circular Vredefort structure in South Africa consists of a central core of predominantly Archean granites - 44 km in diameter, a collar of steeply dipping to overturned Proterozoic sedimentary and volcanic rocks - 18 km wide, and an outer broad synclinorium of predominantly gently dipping Proterozoic sedimentary and volcanic
* Contribution of the Geological Survey of Canada 32887. oo40-1951/90/$03.50
0 1990 Elsevier Science Publishers B.V.
rocks - 28 km wide. The southeastern half of the structure is covered by younger sandstones and shales. The general circular form of the structure with an uplifted central core, the occurrence of the shatter cones, stishovite, coesite and diagnostic shock-produced microscopic planar features in quartz have been cited as evidence that the Vredefort structure is a very large, complex impact structure (e.g. Dietz, 1961; Carter, 1965, 1968; Manton, 1965; Martini, 1978). This interpretation has not gone unchallenged and alternative endogenic origins have been pro-
RAF. GRIEVE ET AL.
186
posed (Hart et al., 1981; Schreyer, 1983; Winter, 1986). Indeed, the evidence generally considered diagnostic of an impact origin has itself been challenged. For example, Lilly (1981) and Simpson (1981) have presented arguments that the temporal aspects of the formation of microscopic planar features in quartz and shatter cones can be correlated with other thermal and structural events and are inconsistent with an impact origin. Reimold and H&z (1986) have stated that there are no diagnostic, shock-produced planar features in quartz at Vredefort, only planar fractures. Planar fractures are produced by dynamic deformation and may occur in shocked lithologies, but are not unique to an impact origin. Clearly, there is some controversy regarding microscopic deformation features in quartz at Vredefort and their exact significance for the origin of the structure. In an effort to clarify these issues, we have examined a large number of samples from Vredefort and analyzed the distribution and orientation of planar features. We have also evaluated the extent of recrystallization in quartz. Before presenting our observations, we briefly summarize
previous systematic analyses of microscopic planar features in quartz at Vredefort to set in context the argument that planar features do not indicate an impact origin for the Vredefort structure. Previous analyses
The presence of planar elements considered diagnostic of a meteorite impact origin in quartz has been reported in the Vredefort structure in several studies. For example, Carter (1965, 1968) described “deformation lamellae” in virtually all of the quartz grains in the samples that he examined. He concluded, from extrapolation of static deformation tests, that stress differences in the range 35-60 kbar were required for their formation and that, in nature, pressures of this magnitude must have been derived from shock due to impact of an extraterrestrial body. Lilly (1981) has examined the occurrence of microscopic planar features in quartz in samples along three radial traverses through part of the collar rocks at Vredefort. He noted the abundance and orientation of “planar features” and used the
PALEOZOIC I_ ~~~~
(Shale,
PROTEROZOIC m zFt$gAAL
fEj
m
*SAMPLE
Fig. 1. Geology
of the Vredefort
structure
showing
locations
of samples
examined
(Andesite
WITWATERSRAND
ARCHEAN m @#[~
in this study. Geology
f
(%&e,~uuprttile,
~~~~&SDoRP SYSTEM
sandstone
)
(S)huaSr~~itc,
(Granite)
LOCATIONS
taken from Nel(l925).
QUARTZ
DEFORMATION
FEATURES,
VREDEFORT
pressure calibration scheme of Grieve and Robertson (1976), which is based on planar feature orientation, to argue for a relatively regular decrease in recorded shock pressure declining outward from 15 GPa at the core-collar boundary. He also noted that the degree of recrystallization of quartz increases progressively inward from the outer collar rocks toward the core. In examining some of the more thoroughly recrystallized samples close to the core-collar contact, Lilly noted that planar features in quartz were confined to the basal orientation, parallel to (0001). In the scheme of Grieve and Robertson (1976) basal features are indicative of the lowest grade of shock necessary to produce planar features. Thus, the calculated recorded shock pressures in these core-collar boundary samples were lower than those in collar samples at greater radial distances from the core. He interpreted this observation as indicating that a second, lower pressure shock event (which produced only basal features), was separated temporally from an earlier, higher pressure shock event (which produced basal and other planar feature orientations) by a period of thermal metamorphism (recrystallization). From this, Lilly argued that the spatial overlap of two shock events is highly unlikely, if the sources of the shock waves were related to impact events, which are spatially random. He concluded that this not only argues against an impact origin for the Vredefort structure but also calls into question the argument that, in nature, microscopic planar features in quartz are unique indicators of impact. This latter view has been quoted by a number of authors who oppose an impact origin for specific terrestrial impact structures (Sage, 1978; Muir, 1984) as well as by those who oppose an impact origin for planar features in quartz from Cretaceous-Tertiary boundary sites (Officer and Drake, 1985; Officer et al., 1987). The studies by Reimold and Horz (1986) and Reimold (1987) even question the occurrence of diagnostic planar features at Vredefort. They note the occurrence of only so-called “planar fractures” and, in some cases, spatially relate their relative abundances to the local development of pseudotachylite. If one accepts the argument that diagnostic planar features of the type observed at
187
other impact structures are not present, then Vredefort may not be an impact structure. It is difficult, however, to understand the denial of the occurrence of planar features at Vredefort by these authors, in light of the previous observations of Lilly (1981) and earlier workers. There is possibly some confusion regarding the definitions of “ planar fractures” and “planar features”. In our experience in examining samples from a number of impact structures, planar fractures are microscopic, open, parallel to subparallel fissures. These extension fractures may be up to 10 pm wide and may be relatively widely separated by distances of up to several tens of micrometers. Planar features in shocked quartz on the other hand, occur as multiple, parallel elements, l-2 pm wide, narrowly spaced (2-5 pm), generally a cross entire grains, and have specific crsytallo~ap~c orientations. Stoffler (1972) notes that planar fractures in quartz have low Miller indices, including the basal orientation (0001). While this may be the case, planar features produced at relatively low shock pressures (7.5-10 GPa) correspond to the (0001) orientation (French and Short, 1968; Walzebuck and Engelhardt, 1979). Although this may account for some of the apparent confusion, Reimold and H&z (1986) and Reimold (1987) also note what they refer to as planar fractures in quartz oriented at high angles to the e-axis. We can only assume that these studies at Vredefort have incorporate both planar features, mostly with {OOOl} orientations, together with planar fractures, with various orientations, and classed them all as planar fractures (Reimold and H&z, 1986). On the basis of different optical properties Carter (1965, 1968) distinguishes “deformation lamellae” that form parallel to the basal plane from “planar features” that lie parallel to other specific c~stallograp~c o~entations. However, Robertson et al. (1968) were unable to maintain a clear distinction, on the basis of optical appearance, between coexisting features oriented parallel to the basal plane and to other orientations. They have, thus, continued to classify all these elements with their specific orientations as “planar features”. It is clear, regardless of the nomenclature, that Carter and Robertson et al. are describing the same suite of deformations and
R.A.F. GRIEVE
188 TABLE 1 Planar feature statistics for the Vredefort structure Sample
Recrystal-
Distance
Grains
Type A
Type B
Type C
Type D
lization
from center
with planar
(I%)
(5%)
(W)
(9)
level
(km)
features (W)
4
0.00
0
AVG-32
3
4.40
44
AVG-36
3
5.40
0
5.63
I
GV-35
VNT-17 AVG-34
4
WIT-16
_
_
_ _
24 _
8
12 _
_
_
_
_
6.00
0
7 _
7.13
40
40
_
_
_
_
8.20
47
47
_
_
GV-40
3
8.50
32
8
24
_
AVG-38
2
9.10
56
56
WIT-15
_ 4
_
AVG-37
2
9.40
16
AVG-17
10.00
8
8
VNT-14
3 _
10.00
53
53
_
_
GV-16
2
10.80
28
28
_
_
11.25
72
_
_
60
60
11.80
40
40
VNT-12
2 _
12.50
60
60
_
GV-33
3
12.90
32
32
_ _
VNT-13 AVG-39
_ _ _
_ _ _
13.50
72
72
VNT-11
2 _
_
14.00
74
67
3
14.10
12
12
7 _
_
AVG-43 GV-13
2
14.10
72
68
4
_
GV-15
2
14.10
36
32
_
_
AVG-53
2
15.00
20
20
4 _
_
_
AVG-44
4 _
15.30
0
_
_
_
VNT-10
15.63
47
47
_
_
_
AVG-14
2
15.80
45
45
_
_
AVG-30
3
15.80
20
20
_
_
GV-18
16.30
24
24
_
_
VNT-9
3 _
17.25
66
66
AVG-59
2
17.80
48
48
AVG-41
_
_ _ _
_
AVG-1
3
17.80
12
12
_
_
AVG-54
2
17.90
70
70
_
_
GV-4
2
17.90
8
8
AVG-52
3 _
18.00
16
16
18.00
33
33
VNT-8 GV-9 VNT-7
2 _
_
_ _ _
18.40
32
32
_
18.75
20
20
_
GV-50
3
18.80
16
16
_
AVG-50
2
18.90
52
52
_
GV-7
2
19.00
36
36
_
AVG-55
2
19.00
44
44
AVG-61
2
19.10
12
12
AVG-20
19.80
96
92
VNT-6
2 _
19.88
27
27
AVG-58
2
20.00
48
GV-45
2
20.00
AVG-8
2
AVG-47
2
_
_ _ _
_
_
_
_
_ _
4
_
48
_
_
_
16
16
_
_
_
20.30
52
52
_
_
_
20.40
36
36
_
_
_
_
ET AL.
QUARTZ
DEFORMATION
FEATURES,
189
VREDEFORT
TABLE 1 (continued) Sample
GV-24
Rectystal-
Distance
Grains
Type A
TypeB
Type C
Type D
hzation
from center
with planar
(%)
(%)
(%)
(%)
level
(km)
features (W)
2
20.40
44
44
_
_
20.50
66
66 52
VNT-5 AVG-31
2
20.60
52
AVG-46
2
20.90
52
52
AVG-12
2
20.90
60
60
GV-22
2
VNT-4
20.90
72
68
21.25
60
60
AVG-21
2
21.30
64
64
GV-29
2
21.30
76
76
21.75
33
33
VNT-3 AVG-9
2
21.80
50
50
AVG-27
2
21.90
60
60
AVG-28
2
22.00
80
80
AVG-56
2
22.50
32
32
22.50
40
40
22.80
44
44
22.88
80
80
23.50
75
15
VNT-2 2
AVG-10 VNT-1 VT-94
2
GV-26
2
23.80
68
68
MV-0.06
2
24.00
92
48
MV-0.08
2
24.00
28
28
MV-0.09
1
25.60
52
52
25.60
0
26.40
80
80
26.80
72
12
AVG-73 2
VT-88 MV-0.15
26.90
21
21
VT-90
1
27.00
64
64
MV-0.12
1
27.90
84
84
28.00
80
80
AVG-76
MV-0.14 MV-0.19
1
28.10
64
64
VT-81
1
28.80
40
40
VT-83
2
29.60
67
61
h4V-0.31
2
31.50
72
72
VT-86
1
32.50
80
80
_
Recrystalhr.ation level: 1 = recoverey; 2 = primary; 3 = moderate; 4 = extensive. Distance from center: Most samples beyond radial range of AVG-10 are from collar rocks, given that the core is not a perfect circle. Percentage grains with planar features: In most samples, 25 grains/thin section were examined for planar features. Type-A features are produced by the lowest level of shock and Type-D
by the highest level of shock. Type A= basal {OOOl); Type B= w{lOi3);
Type C=r
& I {loil},
(Olil};
Type
d = s(loi2).
attribute the entire suite to high strain rate deformation due to meteorite impact. Where fresh, planar features are filled by diaplectic glass, but, more commonly, they are partially annealed and contain minute inclusions or decorations. In the case of Vredefort the obscuring effect of annealing on planar features is due to - 2.0 b.y. of geologic
history since formation and also to grain recrystallization, for which there is clear evidence. Planar features
We have examined the occurrence and orientation of planar features in quartz in 83 samples
R.A.F.
190
from both the core and collar rocks at Vredefort
progressively
(Fig. 1). Our samples
rocks
of Lilly (1981), however,
do not correspond
except
cover the same radial
collar rocks. The seventeen traverse
of Reimold
In general,
were examined
in each sample
were measured
stage techniques.
Qualitative
ing the optical character, ity, and clarity and the general
25 quartz
by standard
the
such as spacing, features
feature universal
observations
regardcontinu-
were made
of recrystallization
structures this may
of the
samples was also noted. We found that planar features in quartz are widespread at Vredefort but their relative distri-
features
1977).
at Vredefort,
the unusual
of the original
may
1988)
represent
planar
trails
and
are
typically
recognized
grains.
Bohor
(pers.
these
trails
that
remnants
their
in
of decorated
recognition,
the anomalous
the Vredefort structure. defined by the alignment that
are
suggested
annealed
features
for
of the higher shock (Type-
quartz
has
would help explain
for
it does not fully account
association
commun.,
(Robertson account
of lower shock Type-A
D) features. Bubble or inclusion many
ET AL.
in the uplifted
While
some of the predominance
grains
Grieve,
material
impact
and
are included
and planar
of the planar degree
within
of complex
do,
from the VNT
(1987), however,
suite.
They
range
samples
in our sample orientations
to those
by coincidence.
less shocked
GRIEVE
as such,
observations
at
These bubble trails are of inclusions with traces
slightly
curved.
quartz
grains contain
both decorated
bubble
There
proximately 90% of the planar have a basal (0001) orientation
tween the straight, well defined decorated planar features and the bubble trails within these same
features observed and are so-called
Type-A features (Robertson et al., 1968) (Table 1 and Fig. 2a). In all but nine samples, basal features alone are observed in quartz grains. This typifies Type A, or the lowest level (5 10 GPa) of quartz deformation in the terminology of Robertson et al. (1968). In most cases the Type-A features observed at Vredefort are equivalent to those
trails.
and
difference
be-
The average recorded shock pressure for each sample was determined from the planar feature orientations by the method of Grieve and Robertson (1976). This method to a selected grains/sample)
highest level of quartz deformation, Type D (15-25 GPa), characterized by the presence of
covery
samples displaying Type-D features in other structures (Figs. 2c and d). The heavy bias towards the occurrence of basal (Type-A) features relative to other orientations at Vredefort compared to other known impact structures is illustrated in Fig. 3. This may be, in part, an erosional effect. Increasing erosion exposes
is a distinct
features
grains. The bubble trails display irregular development and an apparent lack of crystallographic control.
observed at known impact structures (Figs. 2a and b). Planar features parallel to w (1013) (Type B, Table 1) (Robertson et al., 1968) were identified in nine samples and correspond to an intermediate level of shock deformation (- lo-15 GPa). The
planar features parallel to 7~ (1072) (Robertson et al., 1968), was observed in only one sample (Table 1 and Fig. 2~). Although the orientations of the Type-D features are equivalent to those at known impact structures, the appearance compared to the features at Vredefort differs. The overall development and frequency of occurrence of planar features in this sample is less than that observed in
planar
Some
bution and preservation are anomalous compared to those at other known impact structures. Ap-
assigns
a pressure
value
number of quartz grains (usually 25 in the sample based on the planar
feature development in that grain. Estimates of the required pressures for development of each of the planar feature types are based on shock reexperiments
(H&z,
1968;
Mtiller
and
Defourneaux, 1968) and are as follows: Type A = 8.8 GPa, B = 12 GPa, C= 15 GPa, and D= 23 GPa, and grains without planar features = 5.5 GPa if they lie within the outer limit of observed shatter cone formation. An average shock pressure for the entire sample is calculated from the arithmetic mean of the individual grain pressures. Using this method, no well-defined relationship between pressure and radial distance from the center of the structure could be discerned (Fig. 4). This does not compare with the relatively regular outward decrease in recorded shock pressure observed at other impact structures (Fig. 5) (Dence, 1968; Robertson, Marakushev, We concur
1972; Grieve 1981).
and
Robertson,
with Lilly (1981) in finding
1976; that the
QUARTZ
Fig.
basal
DEFORMATION
2.
Planar
features
FEATURES,
in quartz
{OOOl} form (crossed
(crossed
polars).
confinement
Compare
191
VREDEFORT
grains. a. Primary
polars).
b. Quartz
quartz grain from Vredefort
grain
with decorated
with (a). c. Development
to the core of the quartz
the inner core rocks and is moderately
of Type-D
basal
planar
recrystallized
(crossed
impact
polars).
structure
highest average shock pressures have been recorded near the core-collar contact (Fig. 4). Unlike Lilly, however, we fail to observe a regular decrease in recorded shock pressure outward in the collar rocks, using the scheme of Robertson and Grieve (1976) (Fig. 4). Nowhere do we observe recorded pressures greater than 10 GPa, in contrast to the maximum values of 15 GPa reported by Lilly (Fig. 4). This is difficult to explain except by hypothesizing that Lilly measured not only the orientation of bona fide planar features but also that of planar features. For example, the relatively widely spaced features in Lilly (1981, Fig. 3), presented as an illustration of planar features in the most highly shocked sample of collar rock, would be classed by us as annealed planar fractures. Consistent with the possible con-
features
oriented
from the Manicouagan
features
parallel
to the n (1012)
This Vredefort
d. Development
(crossed
planar
features
grain and the lack of other orientations.
quartz gram from the Mistastin
with decorated
planar
polars).
sample,
of Type-D
Note difference
planar
orientation.
AVG-32, features
compared
parallel
impact
Note
was collected (oriented
to the
structure their from
N-S)
in
to (c).
fusion between different types of planar elements is the observation that Lilly (1981) records planar features in 100% of the quartz grains in the more highly shocked samples near the core-collar contact. He also notes a large range of so-called planar feature orientations in samples from the collar rocks. These observations are at variance with ours (Table 1 and Fig. 6). In polycrystalline targets such as rocks interaction of the shock wave with different phases, phase boundaries, irregular grain margins and discontinuities produce localized stress variations resulting in a range of planar feature development within a sample. Often, individual grains in a low-shocked sample are limited to Type-A deformation but, at higher levels of shock, A-C-Type or B-D-Type deformations occur typically within
R.A.F.
192
50
10 rWW..E
2&WEE,““,-AX~3*k0
POLES
TO
F%RES
70 80 (DEGREES)
10
90
20
ANGLE
BETWEEN
30 C-AXIS
40 SD 60 AND POLES TO FEATURES
GRIEVE
70 (DEGREES)
GO
ET AL.
90
Fig. 3. Comparison of the relative abundance of various planar feature orientations in quartz at (A) Vredefort (n = 737) and (B) the combination of data from ten known impact structures in Canada (n = 1202). n is the number of measured planar feature sets with specific orientations.
a single sample, representing grain to grain variations in recorded shock pressure of - 25% (Robertson and Grieve, 1977). The occurrence of
15’0/
. Core
14.0-p
;i 13.0-&
A
0
planar features between grains within a single sample at Vredefort, however, is even more variable than that at other known impact sites. This
Collar
.
Lilly (1991) This study
);: 12.0--
. 8 5
ll.O-A
e
8 lO.O--
.
l
8 t 4 3 3
I
9.0--
l
%.O-7.0-6.0--
$
0.0
I
I
16.0
0.0 DISTANCE
FROM CENTER
24.0
32.0
(km)
Fig. 4. Variation of calculated, average recorded shock pressures with increasing radial distance from the center of the Vredefort structure. See text for method of calculation. Triangles are the data of Lilly (1981) from the collar rocks.
QUARTZ
DEFORMATION
FEATURES,
193
VREDEFORT
l
‘:: e
e
-00 l o l 4.0
I 2.0
0.0
t 6.0
l
o l I 10.0 DISTANCE
l
*
l l 1 ‘ 14.0 18.O FROM CENTER (km)
I 22.0
i
‘.O
Fig. 5. Variation of calculated, average recorded shock pressures with increasing radial distance from the center of the Charlevoix structure, based on the occurrence of planar features and, at the lowest pressures, the spatial limit of shatter cone development. Compare with Fig. 4.
observation is emphasized by sample AVG-32. The Type-D features observed (Fig. 2~) occur in the cores of three grains, whereas other grains in the same sample show no planar features (Table
A,NGCT BETWEEN
C-AXIS
AND POLESTO FEATURES
1). This represents an apparent grain to grain variability in recorded shock pressure of - 200%. At other impact structures where Type-D features are observed, all grains in a sample show planar
IDEQREESI
Fig. 6. Comparison of planar feature orientations in quartz from the collar rocks at Vredefort. A. This study (n = 252). B. Lilly (1981) (n = 861). n is explained in Fig. 3.
R.A.F.
194
features
at this elevated
(Robertson the
and Grieve,
high-level
consistent
However,
for estimating
pressure
shock
formation
estimated
20 km uplift
is
of uplift
may
location
using
within
pressures
features of
4.5 km
the standard
the average
Type-D
(Robertson
to be 15 km and,
of
at Vredefort
recorded
grams is averaged (Grieve and Robertson, this sample has a recorded shock pressure require
Vredefort
features
(when the result from a number
9.9 GPa, although
shock
1977). The occurrence
with the sample
of the center. nique
Type-D
level of recorded
of quartz 1976)) of only
are believed
- 20 GPa
and Grieve,
techshock
to
for their
1977).
normal
thermal
- 525 o C. Post-shock could
add
Ahrens, strain
Assuming
gradient
another
energy
population
density)
could
defect healing,
a
core
rocks
were
at
for tectosili-
GPa
high
highly
are not C
(Raikes
shocked
state
of the minerals
of planar
sheet would
the uplifted
150°-250’
The
that
temperatures
to lo-25
1979).
the
of 15 o C/km
of the pre-erosional
shocked
with
of the collar rocks, 35 km
at the time of impact,
at the center cates
combined
occurred.
continental
prevailed
order Recrystallization
have
ET AL.
GRIEVE
features
contributes
with their
to recrystallization
serve to reduce
and (high
and increased
while any overlying
but
disand
breccia
or melt
the rate of heat loss.
The recrystallization of quartz at Vredefort has been subdivided qualitatively into four levels, ranging from low-level recovery in the outer collar
We believe recrystallization
samples to extensive recrystallization in the central core samples (Fig. 7). These four levels have been described in detail by Hobbs et al. (1976) and Nicolas and Poirier (1976). We suggest that re-
The quartzites in the outer collar were at a shallower depth than rocks of the inner collar and core before uplift and, as a result, they were at
crystallization is directly related to the amount of uplift of shocked rocks at Vredefort. Hart et al. (1981) reported the amount of uplift in the core at
resulted
as the shocked
and uplifted core and collar rocks reacted changed conditions.
to these
relatively lower temperatures. Their temperature and shock-induced disorder were only sufficient to allow the process of recouely to take place. This process attempts to repair the internal fragmenta-
15’
PALEOZOIC I-] $$;$j$
(Shale.sondstone)
PROTEROZOIC /// TRANSVAAL
’ h
SYSTEM
(Shale, dolomite)
-1
k$~~~~sDoRP
m
WITWATERSRAND($uyo~tt)zite, SYSTEM
ARCHEAN 1 $$$@~ I
(Andes(k)
( Granite)
LEVELS OF RECRYSTALLIZATION
q q
Fig. 7. Spatial relationship
of the levels of recrystallization
in the Vredefort
structure.
quortzlte.
FfWWy Primary
I3
Moderote
ccl
Extensive
Geology
taken from Nel(l925).
Fig. 8. Recrystallization textures in quartz grains at Vredefort. See text for details. a. Recovery displayed in the central quartz grain of this outer collar quartzite; level 1 (crossed polars). b. Primary recrystallization of quartzite of the inner collar; level 2 (crossed polars). c. Primary recrystallization between primary quartz grains in granite of the outer core; level 2 (crossed polars). d. Moderate recrystallization and normal grain growth in granite of the inner core; level 3. A single remnant, primary quartz grain remains in the right of the photograph (crossed polars). e. Extensive recrystallization, as shown by the recrystallization of all primary quartz grains in the core granite; level 4 (crossed polars).
of the crystal structure, which is seen as mosaicism, and attempts to return deformed, original grams to lower, more stable, energy states with little development of new quartz by recrystallization. Groups of slip planes or dislocation arrays within deformed grains may diffuse by rearrangement of atoms or may glide in order to tion
achieve a lower energy level (Hobbs et al., 1976). At this level (1 in Fig. 7), subgrains are developed with poorly defined boundaries within the original strained quartz grams. These subgrains are slightly misoriented optically within the parent grain (Fig. 8a). Planar features, were developed within the original grains, are well-preserved and continuous.
196
Rocks in the inner collar and outer core were subjected to greater sh~k-indu~d disorder and were originally at higher temperatures than rocks in the outer collar. These conditions prompted recovery and primary recrystallization. Primary recrystallization removes most of the stored strain energy that remains after recovery. At this level (2 in Fig. 7), numerous small (0.05-0.1 mm) new quartz grains surround relic original grains in the quartzite samples of the inner collar (Fig. 8b), In quartzite, virtually all the grains have the potential to form subgrains, resulting in a high density of nucleation sites and in the formation of a large number of small, recrystallized grains with many developing polyhedral, straight grain boundaries. In the granite samples from the outer core, new grains are developed along quartz grain boundaries and within defo~ation bands (Fig. 8~). The relatively smaller number of quartz grains in the granitic rocks provides fewer nucleation sites and thus new quartz grains are relatively scarce. Most original quartz grains remain, with many displaying well-defined planar features. As noted earlier, decorated basal planar features in these samples are equivalent in appearance to those in known impact structures (cf. Figs. 2a and b). At this level, feldspar grains display little or not alteration. According to the hypothesis that Vredefort is a complex impact structure, samples from the center of the core were subjected to the greatest shock-induced disorder and deformation and had the highest original temperatures, as they were uplifted from the greatest depth. These conditions allowed not only recovery and primary recrystallization but also two levels of normal grain growth. Normal grain growth reduces the grain boundary tension of the polycrystalline aggregate and brings the grains to an even lower energy state by reducing surface energy. At the rn~e~a~e recrystallization level (3 in Fig. 7), most original quartz grains are recrystallized to small (0.05-0.1 mm) grains which display both polyhedral and irregular boundaries. Some quartz, however, has grown to larger (0.3-1.0 mm), new grains with stable, lowenergy polyhedral boundaries (Fig. 8d). A few remnant quartz cores are preserved, with faint and discontinuous planar features. The sample containing the Type-D feature is recrystallized to this
R.A.F.
GRIEVE
ET AL.
level and, as noted earlier, the appearance of these faint Type-D features is anomalous compared to the Type-D features at known impact structures (cf. Figs. 2c and d). Alteration of coexisting feldspar grains to sericite and clay minerals is minor. At the extensive recrystallization level (4 in Fig. 7), essentially all quartz grains are recrystallized with many reaching size of between 0.5 and 1.O mm. Many have developed low-energy, straight grain boundaries and no planar features are apparent (Fig. 8e). Coexisting feldspar grains are moderately altered. Contrary to observations by Lilly (1981), we have not observed planar features in completely recrystallized quartz grains. Lilly (1981) however, restricted his observations within the recrystallized sector of the collar rocks to quartz grains within and adjoining pseudotachylite veins. We do, however, observe planar features in the cores of some relict primary grains surrounded by secondary recrystallized quartz. Figure 8 in Lilly (1981), which purports to show planar features in recrystallized quartz, is, based on our observations, a moderately recrystallized sample with planar features in a remnant core enclosed by polyhedral, recrystallized quartz. The effect of r~~~lIizatio~ estimates
on shock pressure
The progressive increase in recrystallization toward the center and the variability of recrystallization from grain to grain in individual samples, in our opinion, can account for the apparently anomalous distribution of recorded shock pressures in the quartz at Vredefort. If shock pressures are calculated by the method of Grieve and Robertson (1976), the central core rocks have low recorded shock pressures and there is no apparent decrease in recorded shock pressure with radial distance from the core (Fig. 4). This distribution is consistent with increasing inward recrystallization (Fig. 9) if planar features have been progressively annealed by r~~stall~tion. It is apparent that the lowest recorded shock pressures occur in the relatively more recrystallized samples (Fig. 10). Also, on average, the more recrystallized a sample, the smaller the percentage of grains with planar
QUARTZ
DEFORMATION
FEATURES,
197
VREDEFORT
4.0* Cwe
I
I
I
I
8:0
o%.O
DISTANCE
6.0 FROM
CENTER
,. . \K~J
-I
32.0
24.0
Fig. 9. Variation in recrystallization level with radial distance from the center of the Vredefort structure.
method of estimating shock pressure may be invalid at Vredefort. This is most apparent in the case of the sample containing Type-C and -D features.
features (Table 1). As the calculation of recorded shock pressure is made from an average of observations on 25 grains and grain to grain recrystallization is variable, we would argue that this
4.0
3.0 g i= ifI
i a E-J & 2.0 ! b Gi 5
1.0,
0.0;
I
8.0
1
7.0 CALCULATED
I
S~!=RESSURE
I
&ii,
I
10.0
I
Fig. 10. Variation in calculated, recorded shock pressures with respect to levels of recrystallization.
D
R.A.F.
198
GRIEVE
ET AL.
Collar
2 A4 3
0~~00
0* 0
I
I
6 DISTANCE
Vredefort
structure.
00
I I I I I I 4 I I I I I
I
Fig. 11. Variation
00
00
00
in the highest
pressure
Note the tendency
planar
features
for a progressive
16 FROM CENTER
in quartz
ous types of planar features does show some relation to radial distance from the center of the Vredefort structure. Progressively higher pressure type planar features are confined to the central area of the structure (Fig. 11). The figure does indicate, however, a number of anomalous samples with no shock-induced planar features at
32
(km)
in each sample
increase in planar the center.
If, instead, the occurrence of planar features indicating the highest recorded shock pressure observed in each sample is plotted against radial distance, the maximum radial extent of the vari-
24
feature
with radial
type observed
distance (increased
from
the center
shock pressure)
of the toward
Here, as at Vredefort, anomalously low shock pressures are recorded. For example, the rocks that contain high-level Type-C features in quartz have planar features in only lo-308 of the grains, as opposed to the expected close to 100% (Robertson and recorded
Grieve, 1977), and have low calculated shock pressures of 6-7 GPa (Dressler,
1984). Conclusions
various radial distances. Type-C features are absent in this figure because they occur in a sample with higher pressure Type-D features. We have no definitive explanation for the heavy bias toward the preservation of Type-A basal features (Fig.
The conclusions of this study can be summarized as follows. (1) Diagnostic, shock-produced planar features in quartz occur at Vredefort. (2) Their distribution and the calculated auer-
ll), except to note that shock represents increasing disorder, and thus entropy, and that the lower pressure basal features correspond to a smaller increase in disorder and may be relatively more resistant to recrystallization. The annealing of planar features by a thermal event has been previously observed at the Sudbury impact structure.
age recorded shock pressures are, however, anomalous with respect to those observed at other large terrestrial impact structures. (3) The degree of recrystallization in quartz increases inward toward the core. (4) If the degree of recrystallization and its grain to grain variability are taken into account
QUARTZ
DEFORMATION
FEATURES.
VREDEFORT
and only the occurrence of planar features indicative of the highest shock pressures in a sample is used as an estimate of the shock pressure, there does appear to be a tendency for an increase in recorded shock pressure toward the core. (5) No evidence of a post-recrystallization, second shock event is observed. Lilly’s (1981) conclusion is based on the interpretation that weak planar features in the Vredefort collar rocks are superimposed on quartz after total recrystallization by an earlier thermal event. Our observations suggest that these features are faint, discontinuous, relict planar deformation features in the remnant, unrecrystallized cores of original quartz grains. (6) Much of what is observed with respect to planar features at Vredefort is at first glance anomalous relative to established impact sites. This perhaps befits the highly eroded Vredefort structure as the largest and oldest known impact structure on the Earth. While aspects of this structure will obviously remain controversial, we believe the evidence is sufficient to class Vredefort as an impact structure. Acknowledgements
The authors would like to express appreciation to B.M. French, D.P. Gold, R.B. Hargraves and U.W. Reimold for providing samples from then collections. Reviews by B. Bohor, N. Carter, F. H&z and N. Gay are appreciated. References Carter, N.L., 1965. Basal quartz deformation Iamellae-a criterion for recognition of impactites. Am. J. Sci., 263: 786-806. Carter, N.L., 1968. Dynamic deformation of quartz. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 453-474. Dence, M.R., 1968. Shock zoning at Canadian craters: Petrography and structural implications. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 164-184. Die& R.S., 1961. Vredefort ring structure: Meteorite impact scar. J. Geol., 69: 499-516. Dressfer, B.O., 1984. The effects of the Sudbury event and the intrusion of the Sudbmy Igneous Complex on the footwall rocks of the Sudbury Structure. In: E.G. Pye, A.J. Naldrett and P.e. Giblin (Editors), The Geology and Ore Deposits of the Sudbury Structure. Ont. Geol. Surv., Spec. Vol., 1, pp. 97-138.
199 French, B.M. and Short, N.M, (Editors), 1968. Shock Metamorphism of Natural Materials. Mono Book, Baltimore, 644 PP. Grieve, R.A.F. and Robertson, P.B., 1976. Variations in shock metamorphism at the Slate Islands impact structure, Lake Superior. Contrib. Mineral. Petrol., 58: 37-49. Hart, R.J., Welke, H.J. and Nicolaysen, L.O., 1981. Geochronology of the deep profile through Archaean basement of Vredefort, with implications of early crustal evolution. J. Geophys. Res., 86: 10663-10680. Hobbs, B.E., Means, W.D. and Williams, P.F., 1976. An Outline of Structural Geology. Wiley, Toronto, pp. 73-118. H&Z, F., 1968. Statistical measurements of deformation structures and refractive indices in experimentally shock loaded quartz. In: B.M. French and N.M. Short (Editors), Shock Metamorphism of Natural Materials. Mono Book, Baltimore, pp. 243-254. Lilly, P.A., 1981. Shock metamorphism in the Vredefort collar: Evidence for internal shock sources. J. Geophys. Res., 86: 10689-10700. Manton, W.I., 1965. The o~entation and origin of shatter cones in the Vredefort Ring. N.Y. Acad. Sci. Ann., 123: 1017-1049. Marakushev, A.A., 1981. Impactites. Moscow State Univ. Press., 240 pp. Martini, J.E.J., 1978. Coesite and stishovite in the Vredefort Dome, South Africa. Nature, 272: 715-717. Muir, T.L., 1984. The Sudbury structure: Considerations and models for an endogenic origin. In: E.G. Pye, A.J. Naldrett and P.E. Giblin (Editors), The Geology and Ore Deposits of the Sudbury Structure. Ont. Geol. Surv., Spec. Vol., 1, pp. 449-490. Miiller, W.F. and Defoumeaux, M., 1968. Deformationsstrukturen in Quart ah Indikator fti Stosswellen: Eine experimentelle Untersuchung an Quarz-Einkristallen. Z. Geophys., 34: 483-505. Nel, L.T., 1925. Geological map of the country around Vredefort. Scale, 1: 63, 360. Geol. Surv. S. Afr., Pretoria. Nicolas, A. and Poirier, J.P. (Editors}, 1976. Crystalline Plasticity and Solid State Flow in Methodic Rocks. Wiley, Toronto, pp. 49-376. Officer, C.B. and Drake, C.L., 1985. Terminal Cretaceous environmental events. Science, 227: 1161-1167. Officer, C.B., Hallan, A., Drake, C.L. and Devine, J.D., 1987. Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions. Nature, 326: 143-149. Raikes, S.A. and Ahrens, T.S., 1979. Post-shock temperatures in minerals. Geophys. J. R. Astron. Sot., 58: 717-747. Reimold, W.U., 1987. Fracture density statistics along radial traverses through the crystalline basement at the Vredefort Dome, South Africa. Lunar Planet. Sci., 8: 826-827. Reimold, W.U. and H&z, F., 1986. Textures of experimentally shocked (5.1-35.5 GPa) Witwatersrand quart&e. Lunar Planet. Sci., 17: 703-704. Robertson, P.B., 1972. Zones of shock metamorphism at the Charlevoix impact structure. Bull. Geol. Sot. Am., 86: 1630-1638.
R.A.F.
Robertson,
P.B. and Grieve,
at terrestrial
impact
and R.B. Merrill ing. Pergamon, Robertson,
(Editors),
tion in rock-forming B.M. French
1977. Shock attenuation
In: D.J. Roddy,
Impact
R.O. Pepin
and Explosion
Crater-
M.R. and Vos, M.A., 1968. Deformaminerals
and N.M.
of Natural
from
Short
Materials.
Canadian
(Editors), Mono
craters.
Shock
Book,
In:
Metamor-
Baltimore,
pp.
Sage, R.P., 1978. Diatremes of the Slate
and shock features
Islands,
Northeastern
W., 1983. Metamorphism
basement
of the Vredefort
to the origin of the structure. Simpson,
C., 1981. Occurrence
Dome,
J. Petrol.,
guidelines
24: 26-47.
and orientation
of shatter
Winter,
of the Vredefort
J. Geophys.
Res.,
and transformation
natural
and
of minerals
86:
Mineral.,
of quartz
shock
shock compres-
49: 50-113.
influenced
Observations
of rock-
experimental under
J.P. and Von Engelhardt,
W., 1979. Shock defor-
by grain
size and shock direc-
on quartz-plagioclase
of the Ries crater,
Germany.
rocks
from
Contrib.
the
Mineral.
70: 267-271.
H. de la R.,
Witwatersrand arc plate-tectonic
cones
by
I. Behavior
sion. Forts&r. Walzebuck,
Petrol.,
South Africa:
minerals
processes.
basement
in the
in the collar
precluded,
D., 1972. Deformation
forming
Lake
and fluid inclusions
quartzites origin
ET AL.
10701-10706.
tion:
Superior.
group
Impact
in Precambrian
Bull, Geol. Sot. Am., 89: 1529-1540. Schreyer,
Dome:
mation
433-452. rocks
in Pretoria
Stoffler,
New York, pp. 687-702.
P.B., Dence,
phism
R.A.F.,
structures.
GRIEVE
Witwatersrand
1986.
A cratonic-foreland
Basin-development setting.
Econ.
model
in a continental, Geol.
Inf. Circ., 193: 36 p.
Res.
Unit.
for back-
Univ.