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Australian crustal structure
Tectonophysics. 20 (1973) 241-248 0 Elsevier Scientific Publishing Company,
Amsterdam
- Printed
in The Netherlands
AUSTRALIANCRUSTALSTRUCTURE JOHN CLEARY
Research School of Earth Sciences, Australian National University, Canberra, A.C. T. (Australia) (Accepted
for publication
November
22, 1971)
ABSTRACT Cleary,
.I., 1973. Australian
crustal
structure.
In: S. Mueller
Crust, based on Seismic Data. Tectonophysics, 20 (l-4): There velocities in eastern from east Australia, an average topography
(Editor), 241-248.
The Structure of the Earth’s
have been eight large-scale refraction experiments in Australia during the last fifteen years. PI derived from these experiments are significantly higher in the Precambrian shield region than Australia. Pn-velocities are also higher beneath the shield, and appear to increase systematicall:/ to west across the continent. There is good evidence for an intermediate layer in all parts of with an average depth of about 20 km to the Conrad discontinuity. The crustal thickness has value of about 40 km, and the observed variations in thickness are apparently unrelated to in most cases.
INTRODUCTION
The following analysis is based on the results of large-scale crustal refraction experiments performed in Australia during the last fifteen years. No attempt has been made to incorporate results from surface wave studies and seismic reflection surveys. A comprehensive sum. mary which includes this information
has been presented
recently by Dooley (1972).
The positions of shot points for the various experiments are indicated in Fig. I. Relevant details will be provided later in the text. Also shown in the figure is a line indicating the eastern limit of exposed Precambrian venient division of the continent
REGIONAL
rocks (Howard and Sass, 1964), which serves as a con-
into shield and non-shield
for the purposes of this analysis.
DATA
Southeast region Fig. 2 shows shot points, traverse lines and formal velocity determinations for three experiments in the area. The first, in 1956-57, was based on large explosions at Eaglehawk quarry in the Snowy Mountains (Doyle et al., 1959). This was complemented in 1965 by a series of timed explosions off the central coast of New South Wales, which gave a reversal of the northeast traverse of the previous experiment (Doyle et al., 1966). Finally, the 1966
J. CLEAKk
242
ORD 500
WRAMP
RIVER
TON 1970.71
I
SHOTS
EXPT.
TON SHOTS. 1966 _*.
m;;
%~~;T~:p:HOTS
QUARRY 50-100
TON
SHOTS
1956.57 ATOMIC
TESTS _. BUMP EXPT I TON SHCiTS 1966
W. A. GEOTRAVERSE l-41/2 TON SHOTS I969
Fig. 1. Seismic refraction experiments in Australia. The line indicates the eastern limit of exposed Precambrian rocks.
b-
7.96
BASS STRAIT
N.S.W. -r
,68,4
E
VIC.
SYDNEY
N.S.W. ._
BASIN 4,9
0
6.0 --\_
-
---_
--
_____------C
Fig. 2. PI-, Pz- and Pn-velocities, and a crustal section derived from experiments in southeast Australia. E indicates the position of Eaglehawk quarry.
243
AUSTRALIANCRUSTALSTRUCTURE series of explosions
in Bass Strait (Project BUMP) was widely recorded along traverses in
southeast Australia and Tasmania (Underwood,
1969, 1970).
Analysis of data from the first two experiments
by Doyle et al. (1966) resulted in a two-
layer model of the crust with P, -, P2 - and P,-velocities of 6.04,6.15 and 7.86 km/set, the crustal thickness increasing from 25 km at the eastern continental margin to 42 km beneath the Snowy Mountains.
Data from the western line of BUMP gave an approximate
reversal
of the readings of Doyle et al. (1959) southwest from Eaglehawk, and Underwood (1969) derived a single-layer model with a P,-velocity of 7.86 and crustal thickness decreasing from 37 km beneath the Snowy Mountains
to 25 km beneath Bass Strait. The presence of an in-
termediate layer would increase the calculated thicknesses by 3 or 4 km. A combination of the above results provides a consistent model of the crust across southeast Australia, as shown in Fig. 2. The data from the unreversed eastern line of BUMP is reasonably
consistent
with this
model, confirming the presence of an intermediate layer and a P,-velocity less than 8 km/set.. A somewhat different interpretation based on delay time analysis has been reported by Underwood (1969) but the interpretation appears to be invalidated fication of some near-station arrivals as P,.
by an incorrect
identi-
Information from the region shown in Fig. 3 has been derived from the offshore experiment CRUMP (Finlayson, 1968) the associated WRAMP experiment (Underwood, 1967; Underwood et al., 1968). and the eastern traverse recording a large quarry blast at the Ord River damsite (Denham et al., 1972). Underwood’s results from WRAMP have been modified slightly by the exclusion of doubtful observations. The CRUMP data give a structure similar to that found for southeast Australia: a twolayer crust thickening from about 25 km at the coast to about 45 km in the interior, with the Conrad discontinuity at a depth of about 20 km away from the coast. The Ord River data indicate a crustal thickness of about 37 km, based on a two-layer model. This profile is unreversed, as are others to be discussed in the next section, but the profiles are up to 1000 km long in the P,-range and the effects of dip should be minimal. A notable feature of the northern Australian data is a systematic increase in P,-velocity from 7.84 kmlsec at the eastern margin to 8.17 km/set in central Australia.
Western region The results of experiments in the western part of Australia are shown in Fig. 4. These include: (I) a line south of the Ord River blast (Denham et al., 1972); (2) traverses west and southeast of the Maralinga atomic explosions (Doyle, 1957; Bolt et al., 1958; Doyle and Everingham, 1964); and (3) the “Geotraverse” within the Archaean shield conducted by the Bureau of Mineral Resources in 1969 (Gregson and Paull, 1971; Mathur et al., 1973)
A -
-
_
2
TO
fJ
8’
Fig. 3. PI-, Pz- and P,-velocities from experiments in northern Australia. The crustal sections are derived from contours given by Finlayson (1958).
Fig. 4. PI - and P,-velocities
from experiments in the western part of Australia
AUSTRALIAN
CRUSTAL
r
245
STRUCTURE
100
.
MARALINGA
P,:,
20
4’ ~
v
/’
0
SE. W.
/j
I
,,”
100
I
I
200
300 A
Fig. 5. Pz-observations
from traverses
I
,
400
500
L 600
km.
west and southeast
Although PZ was not positively identified
of Maralinga.
from recordings along either of the Marahnga
traverses, the published
times of some unidentified
to a line corresponding
to 6.5 km/set arrival from a discontinuity
Analysis of the Geotraverse layers (~athur
data also indicates
or tentatively
identified
phases he close
at 20 km depth (Fig. 5).
the presence of an intermediate
layer or
et al., 1973).
The P,-velocities
from the southern and eastern lines of the Ord River experiment
are
almost the same, but the times for the southern line are systematically earlier by about 1 sec. indicating a crustal thickness of about 42 km in this direction. The western and southeastern traverses from Maralinga give thicknesses of 37 and 42 km respectively. The east-west
gradation in P,-velocity
found for northern
this region, especially when taken in conjunction for southeast Australia. A P,-velocity
Australia is also observed in
with the velocity of 7.86 km!sec found
of 8.4 km/set along the east-west
traverse supports this trend, but the situation
is complicated
line of the Geo-
by a derivation
of 8.1 1 km/set
for the second line near the western edge of the Archaean shield. The discrepancy
between
the two results is puzzling, and may require further irlvestigatioii.
DISCUSSION
pi The individual determinations of P, -velocity throughout Australia may be misleading, as some are based only on a few points. If all P, -times are plotted against distance on a reduced
P, --
times
100
300
200
A, Fig. 6. Summary graph of P1-times throughout
km.
Australia.
time scale (Fig. 6), the times are seen to be earlier to the west of the shield boundary,
corre-
sponding to higher PI -velocities within the shield. The scatter in velocities may be slightly exaggerated by this representation,
because intercept
Nevertheless, there appears to be a clear demarcation the shield boundary at about 6 km/set.
times are not taken into account. between velocities on either side of
There is good evidence for the presence of an intermediate layer in all parts of Australia. occurs at an average depth of about 20 km, with a Pz -velocity
The Conrad discontinuity of 6.5-6.7 km/set.
The tendency for a systematic increase in P,-velocity across Australia from east to west follows the pattern of other geophysical observations. The isotopic ages of crustal rocks vary from about 3. lo9 years within the Archaean shield on the western side of the conti-
AUSTRALIAN
CRUSTAL
241
STRUCTURE
nent to about 0.2. lo9 years on the eastern side (Evernden and Arriens,
and Richards,
1962; Compston
1968). Heat-flow values are lowest in the Archaean shield, and highest in south-
east Australia (Jaeger, 1970). A similar variation
is observed in travel time anomalies at
seismic stations, with times recorded earliest in the Archaean shield and latest in southeast Australia (Cleary, 1967). The nature of the relationship
between
these parameters
remains
undetermined.
At sufficient distances from the continental margin, the thickness of the crust does not vary by more than 5 km from an average value of about 40 km. The variation in thickness does not appear to be related to present topography, with the possibly fortuitous exception of southeast Australia.
ACKNOWLEDGEMENTS
I wish to thank Dr. D. Denham and Mr. D. Simpson, and officers of the Bureau of Miner:~l Resources, for information concerning the Ord River and Geotraverse experiments, respectively. I am grateful also to the International Upper Mantle Committee for providing me with financial support to attend the Symposium.
REFERENCES
Bolt, B.A., Doyle, H.A. and Sutton, D.J., 1958. Seismic observations from the 1956 atomic explosions in Australia. Geophys. J., 1: 135.-145. Cleary, J., 1967. P-times to Australian stations from nuclear explosions. Bull. S&mot. Sot. Am., 57: 7733781. Compston, W. and Arriens, P.A., 1968. The Precambrian geochronology of Australia. Can. J. Eu‘arth Sci, 5: 561-583. Denham, D., Simpson, D.W., Gregson, P.J. and Sutton, D.J., 1972. Travel times and amplitudes from explosions in northern Australia. Geophys. J., 28: 225-235. Dooley, J.C., 1971. Seismological studies of the upper mantle in the Australian region. In: Puoc. lnd. Symp. Upper~a~tle Project, Znd, ~yderabffd, 1970. Geoph. Res. Board Nat. Geophys. Res. Inst.. Hyderabad, pp. 113-146. Doyle, H.A., 1957. Seismic recordings of atomic explosions in Australia. Narure, 180: 132-134. Doyle, H.A. and Everingham, I.B., 1964. Seismic velocities and crustal structure in southern Australia. J. Geol. Sot. /lust., 11: 141-1.50. Doyle, H.A., Everingham, LB. and Hogan, T.K., 1959. Seismic recordings of large explosions in southeastern Australia. Ausr. J. Phys., 12: 222-230. Doyle, H.A., Underwood, R. andPolak, E.J., 1966. Seismic velocities from explosions off the central coast of New South Wales. J. Geol. Sot. Amt., 13: 355-372. Everndcn, J.F. and Richards, J.R., 1962. Potassium-argon ages in eastern Australia. J. Geol. Sot. Amt.. 9: I-50.
J. CLEAKL Finlayson, D.M., 1968. First arrival data from the Carpentaria Region Upper Mantle Project ((‘RUMP). J. Geol. Sot. Aust., 15: 33-50. Gregson, P.J. and Paull, E.P., 1971. Geotraverse refraction data 1969. Bur. Min. Resour. Aust. Rec., 1971(75). Howard, LE. and Sass, J.H., 1964. Terrestrial heat flow in Australia. J. Geophys. Res.. 69: 1617 -1616. Jaeger, J.C., 1970. Heat flow and radioactivity in Australia. Earth Planet. Sci. Lett., 8: 285 -292. Mathur, S.P., Bramson, J.C. and Moss, F.J., 1973. Geotraverse seismic survey W.A. 1969. Bur. Min. Resour. Aust. Rec. (in preparation). Underwood, R., 1967. The Seismic Network and its Applications. Thesis, Australian National Universtty . Canberra, A.C.T., 298 pp. (unpublished). Underwood, R., 1969. A seismic refraction study of the crust and upper mantle in the vicinity of Bass Strait. Aust. J. Phys., 22: 513. 587. Underwood, R., 1970. A large cooperative seismic experiment: Project BUMP. Aust. Phys.. 7: 21-22. Underwood, R., Elliston, J. and Mathews, K.E., 1968. Shooting for deep refraction experiments. Geephysics, 33: 135-136.
Amsterdam
- Printed
in The Netherlands
AUSTRALIANCRUSTALSTRUCTURE JOHN CLEARY
Research School of Earth Sciences, Australian National University, Canberra, A.C. T. (Australia) (Accepted
for publication
November
22, 1971)
ABSTRACT Cleary,
.I., 1973. Australian
crustal
structure.
In: S. Mueller
Crust, based on Seismic Data. Tectonophysics, 20 (l-4): There velocities in eastern from east Australia, an average topography
(Editor), 241-248.
The Structure of the Earth’s
have been eight large-scale refraction experiments in Australia during the last fifteen years. PI derived from these experiments are significantly higher in the Precambrian shield region than Australia. Pn-velocities are also higher beneath the shield, and appear to increase systematicall:/ to west across the continent. There is good evidence for an intermediate layer in all parts of with an average depth of about 20 km to the Conrad discontinuity. The crustal thickness has value of about 40 km, and the observed variations in thickness are apparently unrelated to in most cases.
INTRODUCTION
The following analysis is based on the results of large-scale crustal refraction experiments performed in Australia during the last fifteen years. No attempt has been made to incorporate results from surface wave studies and seismic reflection surveys. A comprehensive sum. mary which includes this information
has been presented
recently by Dooley (1972).
The positions of shot points for the various experiments are indicated in Fig. I. Relevant details will be provided later in the text. Also shown in the figure is a line indicating the eastern limit of exposed Precambrian venient division of the continent
REGIONAL
rocks (Howard and Sass, 1964), which serves as a con-
into shield and non-shield
for the purposes of this analysis.
DATA
Southeast region Fig. 2 shows shot points, traverse lines and formal velocity determinations for three experiments in the area. The first, in 1956-57, was based on large explosions at Eaglehawk quarry in the Snowy Mountains (Doyle et al., 1959). This was complemented in 1965 by a series of timed explosions off the central coast of New South Wales, which gave a reversal of the northeast traverse of the previous experiment (Doyle et al., 1966). Finally, the 1966
J. CLEAKk
242
ORD 500
WRAMP
RIVER
TON 1970.71
I
SHOTS
EXPT.
TON SHOTS. 1966 _*.
m;;
%~~;T~:p:HOTS
QUARRY 50-100
TON
SHOTS
1956.57 ATOMIC
TESTS _. BUMP EXPT I TON SHCiTS 1966
W. A. GEOTRAVERSE l-41/2 TON SHOTS I969
Fig. 1. Seismic refraction experiments in Australia. The line indicates the eastern limit of exposed Precambrian rocks.
b-
7.96
BASS STRAIT
N.S.W. -r
,68,4
E
VIC.
SYDNEY
N.S.W. ._
BASIN 4,9
0
6.0 --\_
-
---_
--
_____------C
Fig. 2. PI-, Pz- and Pn-velocities, and a crustal section derived from experiments in southeast Australia. E indicates the position of Eaglehawk quarry.
243
AUSTRALIANCRUSTALSTRUCTURE series of explosions
in Bass Strait (Project BUMP) was widely recorded along traverses in
southeast Australia and Tasmania (Underwood,
1969, 1970).
Analysis of data from the first two experiments
by Doyle et al. (1966) resulted in a two-
layer model of the crust with P, -, P2 - and P,-velocities of 6.04,6.15 and 7.86 km/set, the crustal thickness increasing from 25 km at the eastern continental margin to 42 km beneath the Snowy Mountains.
Data from the western line of BUMP gave an approximate
reversal
of the readings of Doyle et al. (1959) southwest from Eaglehawk, and Underwood (1969) derived a single-layer model with a P,-velocity of 7.86 and crustal thickness decreasing from 37 km beneath the Snowy Mountains
to 25 km beneath Bass Strait. The presence of an in-
termediate layer would increase the calculated thicknesses by 3 or 4 km. A combination of the above results provides a consistent model of the crust across southeast Australia, as shown in Fig. 2. The data from the unreversed eastern line of BUMP is reasonably
consistent
with this
model, confirming the presence of an intermediate layer and a P,-velocity less than 8 km/set.. A somewhat different interpretation based on delay time analysis has been reported by Underwood (1969) but the interpretation appears to be invalidated fication of some near-station arrivals as P,.
by an incorrect
identi-
Information from the region shown in Fig. 3 has been derived from the offshore experiment CRUMP (Finlayson, 1968) the associated WRAMP experiment (Underwood, 1967; Underwood et al., 1968). and the eastern traverse recording a large quarry blast at the Ord River damsite (Denham et al., 1972). Underwood’s results from WRAMP have been modified slightly by the exclusion of doubtful observations. The CRUMP data give a structure similar to that found for southeast Australia: a twolayer crust thickening from about 25 km at the coast to about 45 km in the interior, with the Conrad discontinuity at a depth of about 20 km away from the coast. The Ord River data indicate a crustal thickness of about 37 km, based on a two-layer model. This profile is unreversed, as are others to be discussed in the next section, but the profiles are up to 1000 km long in the P,-range and the effects of dip should be minimal. A notable feature of the northern Australian data is a systematic increase in P,-velocity from 7.84 kmlsec at the eastern margin to 8.17 km/set in central Australia.
Western region The results of experiments in the western part of Australia are shown in Fig. 4. These include: (I) a line south of the Ord River blast (Denham et al., 1972); (2) traverses west and southeast of the Maralinga atomic explosions (Doyle, 1957; Bolt et al., 1958; Doyle and Everingham, 1964); and (3) the “Geotraverse” within the Archaean shield conducted by the Bureau of Mineral Resources in 1969 (Gregson and Paull, 1971; Mathur et al., 1973)
A -
-
_
2
TO
fJ
8’
Fig. 3. PI-, Pz- and P,-velocities from experiments in northern Australia. The crustal sections are derived from contours given by Finlayson (1958).
Fig. 4. PI - and P,-velocities
from experiments in the western part of Australia
AUSTRALIAN
CRUSTAL
r
245
STRUCTURE
100
.
MARALINGA
P,:,
20
4’ ~
v
/’
0
SE. W.
/j
I
,,”
100
I
I
200
300 A
Fig. 5. Pz-observations
from traverses
I
,
400
500
L 600
km.
west and southeast
Although PZ was not positively identified
of Maralinga.
from recordings along either of the Marahnga
traverses, the published
times of some unidentified
to a line corresponding
to 6.5 km/set arrival from a discontinuity
Analysis of the Geotraverse layers (~athur
data also indicates
or tentatively
identified
phases he close
at 20 km depth (Fig. 5).
the presence of an intermediate
layer or
et al., 1973).
The P,-velocities
from the southern and eastern lines of the Ord River experiment
are
almost the same, but the times for the southern line are systematically earlier by about 1 sec. indicating a crustal thickness of about 42 km in this direction. The western and southeastern traverses from Maralinga give thicknesses of 37 and 42 km respectively. The east-west
gradation in P,-velocity
found for northern
this region, especially when taken in conjunction for southeast Australia. A P,-velocity
Australia is also observed in
with the velocity of 7.86 km!sec found
of 8.4 km/set along the east-west
traverse supports this trend, but the situation
is complicated
line of the Geo-
by a derivation
of 8.1 1 km/set
for the second line near the western edge of the Archaean shield. The discrepancy
between
the two results is puzzling, and may require further irlvestigatioii.
DISCUSSION
pi The individual determinations of P, -velocity throughout Australia may be misleading, as some are based only on a few points. If all P, -times are plotted against distance on a reduced
P, --
times
100
300
200
A, Fig. 6. Summary graph of P1-times throughout
km.
Australia.
time scale (Fig. 6), the times are seen to be earlier to the west of the shield boundary,
corre-
sponding to higher PI -velocities within the shield. The scatter in velocities may be slightly exaggerated by this representation,
because intercept
Nevertheless, there appears to be a clear demarcation the shield boundary at about 6 km/set.
times are not taken into account. between velocities on either side of
There is good evidence for the presence of an intermediate layer in all parts of Australia. occurs at an average depth of about 20 km, with a Pz -velocity
The Conrad discontinuity of 6.5-6.7 km/set.
The tendency for a systematic increase in P,-velocity across Australia from east to west follows the pattern of other geophysical observations. The isotopic ages of crustal rocks vary from about 3. lo9 years within the Archaean shield on the western side of the conti-
AUSTRALIAN
CRUSTAL
241
STRUCTURE
nent to about 0.2. lo9 years on the eastern side (Evernden and Arriens,
and Richards,
1962; Compston
1968). Heat-flow values are lowest in the Archaean shield, and highest in south-
east Australia (Jaeger, 1970). A similar variation
is observed in travel time anomalies at
seismic stations, with times recorded earliest in the Archaean shield and latest in southeast Australia (Cleary, 1967). The nature of the relationship
between
these parameters
remains
undetermined.
At sufficient distances from the continental margin, the thickness of the crust does not vary by more than 5 km from an average value of about 40 km. The variation in thickness does not appear to be related to present topography, with the possibly fortuitous exception of southeast Australia.
ACKNOWLEDGEMENTS
I wish to thank Dr. D. Denham and Mr. D. Simpson, and officers of the Bureau of Miner:~l Resources, for information concerning the Ord River and Geotraverse experiments, respectively. I am grateful also to the International Upper Mantle Committee for providing me with financial support to attend the Symposium.
REFERENCES
Bolt, B.A., Doyle, H.A. and Sutton, D.J., 1958. Seismic observations from the 1956 atomic explosions in Australia. Geophys. J., 1: 135.-145. Cleary, J., 1967. P-times to Australian stations from nuclear explosions. Bull. S&mot. Sot. Am., 57: 7733781. Compston, W. and Arriens, P.A., 1968. The Precambrian geochronology of Australia. Can. J. Eu‘arth Sci, 5: 561-583. Denham, D., Simpson, D.W., Gregson, P.J. and Sutton, D.J., 1972. Travel times and amplitudes from explosions in northern Australia. Geophys. J., 28: 225-235. Dooley, J.C., 1971. Seismological studies of the upper mantle in the Australian region. In: Puoc. lnd. Symp. Upper~a~tle Project, Znd, ~yderabffd, 1970. Geoph. Res. Board Nat. Geophys. Res. Inst.. Hyderabad, pp. 113-146. Doyle, H.A., 1957. Seismic recordings of atomic explosions in Australia. Narure, 180: 132-134. Doyle, H.A. and Everingham, I.B., 1964. Seismic velocities and crustal structure in southern Australia. J. Geol. Sot. /lust., 11: 141-1.50. Doyle, H.A., Everingham, LB. and Hogan, T.K., 1959. Seismic recordings of large explosions in southeastern Australia. Ausr. J. Phys., 12: 222-230. Doyle, H.A., Underwood, R. andPolak, E.J., 1966. Seismic velocities from explosions off the central coast of New South Wales. J. Geol. Sot. Amt., 13: 355-372. Everndcn, J.F. and Richards, J.R., 1962. Potassium-argon ages in eastern Australia. J. Geol. Sot. Amt.. 9: I-50.
J. CLEAKL Finlayson, D.M., 1968. First arrival data from the Carpentaria Region Upper Mantle Project ((‘RUMP). J. Geol. Sot. Aust., 15: 33-50. Gregson, P.J. and Paull, E.P., 1971. Geotraverse refraction data 1969. Bur. Min. Resour. Aust. Rec., 1971(75). Howard, LE. and Sass, J.H., 1964. Terrestrial heat flow in Australia. J. Geophys. Res.. 69: 1617 -1616. Jaeger, J.C., 1970. Heat flow and radioactivity in Australia. Earth Planet. Sci. Lett., 8: 285 -292. Mathur, S.P., Bramson, J.C. and Moss, F.J., 1973. Geotraverse seismic survey W.A. 1969. Bur. Min. Resour. Aust. Rec. (in preparation). Underwood, R., 1967. The Seismic Network and its Applications. Thesis, Australian National Universtty . Canberra, A.C.T., 298 pp. (unpublished). Underwood, R., 1969. A seismic refraction study of the crust and upper mantle in the vicinity of Bass Strait. Aust. J. Phys., 22: 513. 587. Underwood, R., 1970. A large cooperative seismic experiment: Project BUMP. Aust. Phys.. 7: 21-22. Underwood, R., Elliston, J. and Mathews, K.E., 1968. Shooting for deep refraction experiments. Geephysics, 33: 135-136.