Explosion seismic determination of crustal structure beneath the Adelaide Geosyncline, South Australia

Physics of the Earth and Planetary Interiors, 58 (1989) 323—343

Elsevier Science Publishers BY., Amsterdam

—

323

Printed in The Netherlands

Explosion seismic determination of crustal structure beneath the Adelaide Geosyncline, South Australia S.A. Greenhaigh and C.C. von der Borch School of Earth Sciences, Flinders Unipersity of South A ustralia, Bedford Park, S.A., 5042 (Australia)

D. Tapley Esso Australia Ltd., Kent St. Sydney, N. S. W, 2000 (Australia)

(Received February 9. 1989; accepted May 3. 1989)

Greenhalgh, S.A., von der Borch, C.C. and Tapley, D.. 1989. Explosion seismic determination of crustal structure beneath the Adelaide Geosyncline. South Australia. Phys. Earth Planet. Inter., 58: 323—343. The structure of the Earth’s crust beneath the Adelaide Geosyncline, South Australia, has been investigated over the last few decades using both earthquake and explosion seismic techniques. These studies have led to the simple average model of a single, homogeneous layer having a P-wave velocity of 6.32 km s~ and thickness of 38 km overlying a homogeneous mantle with a velocity of 8.05 km s~. The only compelling evidence for crustal layering comes from two recent long-range refraction profiles, recorded along (N—S) and perpendicular (E—W) to the axis of the geosyncline, using quarry blast sources. Head waves and wide-angle reflections have been identified as originating from a mid-crustal boundary separating an upper crust of velocity 5.94 km s~. from a lower crust of velocity 6.42 km s ~, at a depth of 10 to 18km. The depth variation to this boundary, and the corresponding Moho topography, were interpreted using an iterative ray tracing technique. Both discontinuities shallow towards the Murray Basin which flanks the Geosyncline to the south-east. The crustal model obtained was further investigated by means of a synthetic aperture seismic array experiment, carried Out on the N—S refraction profile, at a source offset of 120 km. Recordings were made on a 21 element unreversed linear profile, using digital seismographs spaced at 0.5 km intervals. An accurate P 1 velocity of 5.88 km s~ was obtained. Secondary P arrivals were studied with the aid of beam formers and tau-p processors. Tentative reflections from within the crust and upper mantle were obtained. However, the variation in seismic waveform generated by different blasts was a definite drawback of the experiment. Prominent dipping reflectors and faults at depths of several kilometres have been observed in near-normal incidence reflection surveys shot in Porterozoic rocks just west of the Adelaide Geosyncline. The reflectors may represent layering within the crystalline basement, or they may relate to the rifting stage of basin development.

I. Introduction Controlled-source seismic refraction investigations of crustal and upper mantle structure in Australia have been carried out since the early 1950s by the Bureau of Mineral Resources, and various University groups. Results are reviewed by Cleary (1973) and Finlayson (1982). The upper crustal P1 velocity lies in the range 5.7 to 6.3 km s~ with the Moho P~velocityranging from 7.8 to

t. The average crustal thickness is 40 8.4 km s km. A regional trend in the P 1 and P~seismic velocities has been established, with velocities increasing from east to west across the continent. This regional seismic trend is in accord with other geophysical observations such as electrical conductivity and heat flow, and is related to the relative ages of the upper crustal rocks which are Precambrian in the west and Phanerozoic in the east.

324

The Adelaide Geosyncline in South Australia forms the transitional boundary between these two contrasting tectonic terrains, In recent years deep reflection profiling experiments, akin to the U.S. Consortium for Continental Reflection Profiling (COCORP) programme, have been carried out in Eastern and Central Australia to investigate crustal layering (Mathur, 1983; Finlayson et al., 1984; Moss and Mathur, 1984: Wright et al., 1987) These studies have elucidated more detailed structure of the crust, and located a mid-crustal boundary at a depth of around 20 km. which had been detected in some earlier refraction and wide-angle reflection profiles. The lower crustal velocity ranges from 6.5 to 6.7 km s~ The structure of the Earth’s crust in South Australia has been investigated by a number of workers in the past, but to date no deep crustal reflection experiments have been conducted. These studies have led to the simple model of a single, homogeneous layer having a P-wave velocity of 6.32 km s~,overlying a homogeneous mantle at a depth of 37 km, with a velocity of 8.05 km s~. This is the model which has been routinely used by the Adelaide University seismology group for determination of focal co-ordinates of South Australian earthquakes (Parham, 1981). The only evidence for a mid-crustal discontinuity in South Australia is provided by Shackleford and Sutton (1981), who carried out the first detailed long-range refraction profiles in the Adelaide Geosyncline, using quarry blast sources. Refinement of the velocity model in South Australia would lead to improvements in the accuracy of hypocentral solutions, which in turn would enhance the effectiveness of seismic risk evaluation and focal mechanism studies. Knowledge of crustal and upper mantle velocity distributions are also of fundamental importance in studies of Earth composition and evolution, In this paper we review the crustal seismic information available for South Australia and give a re-interpretation of the data of Shackleford and Sutton (1981) using an iterative ray tracing technique. We present the results of a recent digital seismic array experiment conducted in the Flinders Ranges, along one of the refraction profiles. Fi-

S.A. OREFNHAFGH. FT AL

nally, we present evidence of intra-basement layering obtained from two exploration seismic reflection surveys.

2. Tectonic setting Two contrasting regions of outcropping late Proterozoic (Adelaidean) sediments are recognized in south-central South Australia (Fig. 1). These are; (1) the Stuart Shelf, a broad region of undeformed strata several hundred metres in thickness, overlying the Gawler Domain and (2) the Dclamerian Fold Belt, an approximately meridional zone of strata which is considerably thicker than that on the Stuart Shelf and which was deformed by the Cambro-Ordovician Delamerian Orogeny. These regions are collectively termed the Adelaide Geosyncline, in which earliest sedimentation may have begun around 1076 Ma (see discussion by Webb et al., 1983) and extended into the Cambrian. The Delamerian Foldbelt in turn has been further subdivided by Rutland et al. (1981) into several tectonically distinct regions which indude a series of arcuate fold belts and the Central and North Flinders Zones. The Stuart Shelf and Delamarian Fold Belt are separated by an approximately north—south transitional zone known as the Torrens Lineament or Hinge Zone, which is of uncertain origin (Rutland et al., 1981; Murrell, 1977). Boundaries of the Adelaide Geosyncline are defined to the west by a greater than 1400 Ma basement block known as the Gawler Domain. Here, Adelaidean sediments on the Stuart Shelf exhibit a distinct onlapping geometry towards the west (Preiss, 1983). In the northeast the greater than 1400 Ma Curnamona Nucleus which includes the Willyama Domain and Mt. Painter Inlier, forms a basement boundary, although details of the structural relationships between the Nucleus and the fold belt are poorly understood. Elsewhere bounding relationships are obscured by undeformed Mesozoic and younger basin sediments. North of the North Flinders Zone, Adelaidean sediments outcrop in the Peake nd Denison Ranges, and subcrop in a north—west trend across the state of South Australia into Western Australia.

325

EXPLOSION SEISMIC DETERMINATION, ADELAIDE 138’E

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Basement

THZ .1’s• tIw. Adelaide U ~

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MURRAY BASIN 0

100

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SCALE

—

Fig. 1. Generalised geologic map of south-central South Australia. showing the major tectonic units. The Adelaide Geosyncline comprises the three units, Delamerian Fold Belt (zone of arcuate fold belts), and the North and Central Flinders Zones.

Von der Borch (1980) and Rutland et al. (1981) cite evidence that a major late Proterozoic rifting event triggered the formation of the geosyncline. Von der Borch (1980) and Preiss (1983) have suggested alternative stratigraphic levels for a “breakup” unconformity which separates syndine-rift from passive subsidence strata. Flat-lying Adelaidean strata on the Stuart Shelf are considered to represent deposition during the post-rift thermal equilibration phase (Preiss, 1983; von der Borch and Grady, 1986).This suggests that thinned

continental crust must occur beneath the Stuart Shelf. In terms of current understanding of major rift structures, thinned crust is also inferred to exist beneath the Delamerian Fold Belt. Compressive deformation within the Dclamerian Fold Belt, associated with local intrusions of granites, produced the typically open style of folding observed in the Adelaide Geosyncline, with distinct similarity being noted by Rutland et al. (1981) between the arcuate fold belts of the geosyncline and the Jura Mountains of Europe.

326

S.A. GREENHALGH. ET AL.

Evidence for thrusting with westward vergence occurs in regions immediately east of the Torrens Hinge Zone. Sukanta (1986) and Jenkins (1986)

tamed from MA1 was substantially higher than that deduced from MA2 for Western Australia. Similarly, differences in P1 velocity found by Bolt

have suggested that major thrust faulting occurred in southern regions near Adelaide with all of the Proterozoic units there being allochthonous. Considering available evidence, there is little doubt that the Adelaide Geosyncline began its history as an intracratonic rift. What remains to he determined is whether or not oceantc crust accretion occurred following the rifting. If this did occur, then the phase of obvious compressive deformation in the Cambra-Ordovician could have been related to oceanic crust subduction with corresponding accretion of possible allochthonous terrains which would include at least some of the major basement blocks. An alternative explanation, in which the Adelaide Geosyncline is entirely intracratonic in origin, is discussed in Rutland et al. (1981). In this model, the geosyncline is cornpared with a multiple rifted arch system described by Veevers and Cotterill (1978). The actual passive margin in this modelaswould further to theclearly east. The above, possibly well aslieother models, require testing. and deep crustal studies using seismic profiling can provide critical data.

et al., (1958) to the west of Maralinga were later confirmed by Mathur (1974). The depth to the Moho in South Australia was slightly greater than that for Western Australia. Neither study of the Maralinga nuclear explosions identified any arrival from a lower crustal layer, but both reported some unidentified phases which may have originated from such a layer. Hawkins et al., (1965) reported results from several marine refraction traverses obtained off the coast of South Australia. The average crustal velocity was 6.26 km s~. The P1 velocity of 8.2 km was substantially higher than that obtained onshore by Doyle and Everingham (1964). The Moho depth beneath the ocean basin was 12 km. In 1971 an unreversed refraction profile (OD in Fig. 2) was recorded across South Australia and the Northern Territory from explosions at the Ord River Dam1(Denham et al., 1972). A P1 velocity of was obtained, At distances greater 8.27 km than 1400s km, the apparent velocity exhibited a sudden jump to 8.82 km s~.The Moho depth was calculated to be 40 km. assuming a somewhat low average crustal velocity of 6 km s~. Finlayson et al., (1974) recorded several refraction profiles across South Australia using explosions at Mt. Fitton and Kunanalling. This study confirmed the high P 1 velocity across the Nullabor Plain, and the reduced P1 velocity in the Adelaide Geosyncline. Few crustal phases were recorded as first arrivals for the large shot-receiver distances involved. They used an assumed two-layer crustal structure derived from the Geotraverse experiment in Western Australia (Mathur. 1974). Shackleford and Sutton (1981) conducted two long-range, unreversed refraction profiles in South Australia using quarry blasts at Leigh Creek and Iron Baron as the seismic source (LCK and lB in Fig. 2). The resulting refraction and wide-angle reflection travel time data were interpreted to yield a two-layer crustal model (Table 1). The 1, which is average crustal is 6.28 in kmearlier s comparable withvelocity that obtained studies. The P~velocityof 7.97 km s~,agrees with Finlayson’s result for Central Australia,

3. Crustal refraction profiles Figure 2 is a schematic diagram showing the location of crustal seismic profiles recorded in and around South Australia. The crustal models obtamed from these various studies are summarized in Table 1. In this section a brief description is given of the principal results, The first explosion seismic determinations of crustal structure in South Australia were made from the Maralinga atomic weapons tests during the 1950s. The blasts were recorded along two unreversed refraction profiles: a large scale 850 km survey west of Maralinga (MA2 in Fig. 2) across the Nullabor Plain and along the Perth railway (MAI line (Bolt et 2)al.,south-east 1958) and 600 km profile in Fig. of aMaralinga towards Adelaide (Doyle and Everingham, 1964) The crustal P-wave velocity of 6.3 km s ob-

327

EXPLOSION SEISMIC DETERMINATION. ADELAIDE

120°

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132° 134° I I I I NORTHERN TERRITORY

136° I

138°

140° QUEENSLAND

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2~01kmI 132°

MGR~ 134°

136°

-

36°

a

___

138°

140°

I 142°

Fig. 2. Location map of crustal seismic profiles in and around South Australia. For individual profile code identification and results. refer to Table 1.

Several unpublished student projects over the past decade or so have entailed the recording of Adelaide quarry blasts on portable seismic equipment, out to distances of 140 km. The resulting short-range refraction profiles have yielded upper crustal velocities of 5.1 to 5.75 km s~ (see Table 2). These values are lower than those obtained at greater range (Table 1), and exhibit an increase with average source-receiver distance, implying a velocity gradient in the upper crust. A composite “first-break” refraction travel time graph for Kanmantoo and Stonyfell quarry blasts is presented in Fig. 3. Slight curvature of the T—X (travel time—distance) curve is evident. The least

squares fit of the entire data set gives a P1 velocity of 5.69 km s~.This profile partly reverses Shackleford and Sutton’s (1981) Leigh Creek profile (LCK in Fig. 2), for which the P1 velocity was interpreted to be 5.95 km 51

4.

Basement velocities

Velocities of sedimentary basin rocks in South Australia have been measured extensively in connection with petroleum exploration seismic surveys, particularly in the Cooper Basin which occupies the north-east corner of the State (Fig.

328

S.A. GREENHALGH. ET AL.

TABLE I P-wave velocities and crustal thickness determined from explosion seismic profiles recorded in and around South Australia Locality Code (Fig. 2)

Seismic Velocity (km s~) P

Depths (km)

MA2

1 6.03

MA]

6.3

8.05

39

H39-H41

6.26

8.20

OD

6.0

8.27

12 Ocean basin 15 slope 40

—

6.2

6.5

8.1

20

39

KN MFI-MF2

6.2 6.2

6.7 6.7

8.23 7.98

20 20

34—40 40

6.12

6.6

8.3

19.5

31—49

6.1

6.7

7

24

40

BI B2

5.92 6.15

6.97 6.98

8.05 8.05

14.5 23.5

37 3]

1981

LCK, lB

5.94

6.46

7.97

16.19

39

Finlayson, 1983 — Eronanga Basin

E

5.9

7.0

8.1

25

37

Bolt et al., 1958 — Nullabor Plain Doyle and Everingham, 1964 — Maralinga Hawkins et al., 1965

Denham et al. 1972 — Central Aust. Simpson, 1973 — Central Aust. Finlayson et al. 1974 — Kunanalling-Island Lagoon — Mt. Fitton Mathur, 1974 West. Aust. Mathur, 1976 — Central Aust. Branson et al. 1976 — Broken Hill — Mildura Shackleford and Sutton, —

P2

1). The seismic velocities span a wide range, depending on the age, depth of burial and rock type. Average values lie in the range 3 to 4 km s”~. Far fewer measurements of basement velocity are available. Most work has been carried out by the South Australian Department of Mines and

P~ 8.21

Mid-crust

Moho 36

Energy which, in the 1950s and 1960s, undertook a systematic reconnaissance programme of basement refraction mapping, throughout various basins in South Australia. Table 3 summarises the basement velocities and depths. The basement is predominantly Pre-Cambrian meta-volcanics. The

TABLE 2 Upper crustal Pg velocities obtained from short-range quarry blast refraction profiles in the Adelaide Mt. Lofty region Quarry source

Adelaide Hills Kanmantoo Reynella-Linwood Stonyfell

No. of stations

138 25 9 8 13

Distance range

Average distance

Reference

(km)

Average apparent velocity (km)

(km) 1— 55 1— 44 25— 72 1— 50 6—114

12.5 22.0 48.0 25 60

5.08 ±0.39 5.51±0.14 5.75 5.6±0.10 5.38±0.17

Singh, 1985 Olsen, 1971 Smit, 1978 Williams, 1969

329

EXPLOSION SEISMIC DETERMINATION. ADELAIDE 30

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(3

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S

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0

20

I 40

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DISTANCE

I 100

(KILOMETRES)

I 120

I 140

I 160

180

Fig. 3. Travel-time graph of first arrivals obtained from quarry blasts at Kanmantoo and Stonyfell in the Adelaide region, and recorded on a profile running northwards, within the Adelaide Geosyncline.

average velocity value is around 5.5 km s~,which is similar to that obtained from quarry blasts (Table 2), and from some recent ultrasonic velocity measurements made at Sydney University on Proterozoic rock samples collected from the

Adelaide Geosyncline (D.W. Emerson, personal communication).

TABLE 3

5.1. Earthquakes

Basement velocities and depths determined from refraction surveys of sedimentary basins in South Australia *

_______________________________________________ Locality

Arckaringa Basin — Central — Western Eucla Basin Great Artesian Basin Murray Basin Officer Basin

Basement velocity (km s)

Basement depth (km)

5.94—6.40 5.70—5.91 5.46—6.19 5.51—5.90 5.185.94 5.50—6.10

1.4 0.8 1.7 2.3 2.7 1.5

5. Seismic investigations using state seismograph network

Crustal structure studies in South Australia, based on local and regional earthquake travel

TABLE 4

Pirie-Torrens Basin 5.00—6.31 0.7 Polda Basin 4.94—6.40 0.7 Stansbury Basin 6.21—6.8 2.6 Willochra Basin 4.21—5.6 0.3 Willunga Basin 5.18—5.94 0.3 * Data source: Numerous unpublished reports of the South Australian Department of Mines and Energy, Quart. Notes, Geol. Surv. S. Aust., Mineral Resource Rev. S. Aust.

Earthquake studies of crustal structure in South Australia Investigator P-wave S-wave Crustal velocities velocities thickness 1) (km ~_1) (km) (km s P 1 P S1 S n

Thomas, 1969 White, 1969 White, 1971 Stewart, 1972 Greenhalgh et al., 1989 * Upper crustal surveys

—

6.23 6.3 6.25 5.9—6.5 5.4—5.8 ° velocities

3.58 8.06 3.58 8.21 — 8.02 3.58 7.95 — — 2.7—3.2 ° deduced from —

4.56 37 4.63 38 — 39 4.56 37 — 26—40 —

—

microearthquake

330

S.A. ()REENHALGH, ET AL. 137°

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1989). The low time terms correpsond to high

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St. Vincent

crustal velocity while the high time terms reflect anomalously low crustal velocity. The residual component of the time terms can be explained in terms of variations in the depth of the Moho throughout the Geosyncline and surrounding region, of the order of 10 km. The prominent features of the tomographic velocity map are a north—south trending velocity low at the head of Spencer Gulf, a velocity high near Clare at the eastern edge of the Geosyncline, and a ridge of high velocity running east of Port Pine along the Flinders Ranges. The velocity low can be associated with a region of gravity low aligned parallel to the Torrens Hinge Zone, which marks the western boundary of the Geosyncline. The northern end of the high velocity ridge coincides with a region of positive Bouguer gravity. Both may indicate a dense mafic core of the Adelaide rift zone. However, no gravity correlations can be made for the velocity high near Clare.

Fig. 4. Contour map of upper-crustal P-wave velocity (km s~) obtained from tomographic analysis of local earthquake data (after Greenhalgh et al., 1989). 5.2.

times (and amplitudes), have been undertaken by Thomas (1969), White (1969, 1971), Stewart (1972) and Greenhalgh et al. (1988). The resulting velocity models are listed in Table 4. The P1 and P~ velocities are comparable to those determined by explosion seismic techniques (Table 1). However, no earthquake evidence for a mid-crustal discontinuity was found despite an intensive search. Lateral velocity variations in the South Australian crust were mapped by Greenhalgh et al. (1989) using a tomographic technique. The input data comprised 1340 local earthquakes Fecorded on the State seismograph network. They obtained the velocity inhomogeneity map of Fig 4. The quantity contoured is the average velocity of the upper 20 km of the crust. The area covered lies mainly within the Adelaide Geosyncline. There is a significant variation in crustal velocity from 5.9 to 6.5 km s~.The velocities correlate remarkably with P~time terms computed in a separate study of Moho refracted arrivals (Greenhalgh et al.,

Quarry blasts

The South Australian seismograph network regularly records large quarry blasts from the Middleback Iron Range and the Leigh Creek Coalfield, in addition to earthquakes (for quarry blast locations, see Fig. 5). These recordings can be used to determine the apparent velocity of the prominent arrival (principally “first-breaks”) from the known co-ordinates of the source and stations, and the relative arrival times on the various stalions which record the blast. It is necessary to restrict the analysis to those stations or distance ranges, where there is no confusion as to which phase has been recorded. (Strictly speaking, we require that the same, as yet unidentified, phase be recorded on each station). Table 5 lists the apparent velocity results for the various quarries investigated. The P-wave velocities have been sub-divided into P1, P2 and P~according to distance range. The values are similar to those reported in previous studies (Tables 1 and 4), and provide supporting evidence for a mid-crustal refractor with a P-wave velocity of about 6.6 km ~

331

EXPLOSION SEISMIC DETERMINATION. ADELAIDE

I

I I

SOUTH AUSTRALIA

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SK _________________

LEGEND

~

~I

Quarry blast site Seismograph station

I

Kangaroo Is

I

WKA I

I

_____________________________

QUARRY

BLAST SITE

1K IB

Iron Knob Iron Boron 151 Iron Monarch 1<

VICTORIA

CODE

MG Mount Gurison LC Leigh Creek S Storrytell

BFD~

MGR

Mount

Kanmontoo 0

I 135°E

I

50 100

200 km

SCALE 400S

lLOaE

Fig. 5. Location map of seismograph stations (permanent State network) and quarry blast sites used in crustal seismic investigations.

6. Reinterpretation of Leigh Creek and Iron Baron refraction profiles 6,1. Data

Figure 2 shows the locations of the two unreversed refraction and wide angle reflection profiles recorded by Shackleford and Sutton (1981). Leigh Creek quarry blasts were used for the north—south

profile, which runs within and parallel to the axis of the Adelaide Geosyncline. The Iron Baron profile, using blasts at Iron Baron mine, runs almost perpendicular to the Leigh Creek profile, and crosses the eastern and western boundaries of the Geosyncline. Portable analogue recording equipment was used, namely Kinemetrics PS-lA seismographs coupled with Ranger SS-1 seismometers. Forty two stations were occupied along the

332

S.A. GREENHALGI-I. PT AL

TABLES P wave velocities in South Australia from analysis of quarry blasts recorded on the South Australian Seismograph Networks Quarry site

No. of stations

Iron Baron

6 3 3 7 6 4 4 4

Iron Knob Iron Monarch Leigh Creek Mt. Gunson

Distance range (km)

5

Average distance (km)

Phase

38—172

127

P

107—178 209—358 25—185 80—185 120—180 73—173 249—399 112—228

142 283 139 118 150 153 296 155

Leigh Creek profile over a distance of 331 km. For the Iron Baron profile, fifty-eight records were obtained along a 365 km profile. From the profiles (Figs. 6 and 7) five P phases

Average Apparent velocity (km ~I)

1 P2 P,~ ? P1

6.08±0.25 6.75±1.11 7.83±0.69 6.42 ±0.60 6.10±0.11 6.53±0.12 6.10±0.11 8.42±0.63 6.93 ±0.29

P2

P1 P~ ?

were identified, corresponding to direct P1. refracted P2 and P~, and wide-angle reflected arrivals P2R and P0R, from the mid-crust (Conrad) and upper mantle (Moho). Using standard

a)

_________________________________

(a)

~

13,

E

-4 0 North

~

100 40

80

~ ~l

120

Distance A2001km) 160

200

240

300 283

320

South

-2

> a, a -4

360

I b)

Ill

2 C

P2 Pn

________

West

50

O

50

.c 20~

I

100 Distance 150 A 200 I km) 250 100

150

Lower crust

I

300

350 East

200

C

lb)

60

Distance A (km O

40

80

120

160

200

Distance A

240 O

~ 20 ~~er 40

___ ____

crust _____________ _________________

20 ±40 .~

c1J

____

a.

50

100

I

150

1km

200

250

300

350

______ ________

Mantle

a 60

Fig. 6. Reduced travel time graph of P arrivals (direct P1. refracted P2 and P~and reflected P2R and PAR) and final results after iterative ray tracing showing the Moho M and Conrad C discontinuities along the Leigh Creek seismic profile.

C

Fig. 7. Reduced travel time graph for direct (P1) and refracted (P2 and P~) P arrivals and iterative ray trace model for the Iron Baron seismic profile.

333

EXPLOSION SEISMIC DETERMINATION. ADELAIDE

TABLE 6 Velocities and layer thicknesses obtained by standard interpretation of the seismic data of Figs. 7 and 8 t) Thicknesses (km) Profile Method Velocities (km s” Data P 1

P2

~n

Upper Crust

Lower Crust

Leigh Creek Refraction 5.95 6.42 7.97 18.0 17.9 (N-S) Reflection 5.94 6.49 * — 18.7 19.2 Iron Baron Refraction 5.93 6.46 7.97 11.9 28.0 (E-W) 1 obtained for the Moho reflected arrivals. * This interval velocity was computed from the rev min~ velocity of 6.21 km s

least squares curve fitting, the data were interpreted to yield velocities and thicknesses for a 3horizontal layer model, as given in Table 6.

change of scale between b and c should be noted). For Fig. 6(c) the subsurface ray coverage is continuous in the distance range 20 to 180 km. The

6.2. Iterative ray tracing

depth to the interface from of a maximum of 19 km in the north increases to a minimum 16 km in

A more flexible approach to interpretation is to trace rays through this starting model, and then adjust the model to force the observed and cornputed data to agree. The procedure can be repeated iteratively until the correspondence between the observed and computed data is within some specified error tolerance (Scott, 1973). This approach permits lateral depth variations in each

the south. For the Moho, the ray coverage extends from 60 to 280 km distance range. The depth to the base of the crust decreases from an average 38 km in the distance range 60 to 110 km, to a minimum of 31 km at 270 km distance. This trend of the Moho deepening to the north, matches the trend of the mid-crustal boundary. Curiously the P 1 phase is not recorded in the distance range 130—170 km. White (1967) also reported a shadow zone for P1 arrivals from recordings of quarry blasts at some of the permanent stations of the South Australian seismic network. A possible explanation is the occurrence of a velocity reversal in the crust (Mueller and Landisman, 1966). Low velocity layers within the crust have been interpreted elsewhere in Australia, by matching with synthetic seismograms (Pinchin, 1980; Finlayson, 1982). The crustal model obtained by re-interpretation of the Leigh Creek profile is summarised in Fig. 8, along with the gravity and magnetic profiles. There are no obvious correlations between the seismic and the potential field data. If the Bouguer gravity was influenced by Moho topography, then one would expect to see an increasingly stronger positive anomaly towards the southern end of the profile where the crustal thickness is less. Similarly, if magnetic basement were to deepen to the north, then the magnetic field should increase towards the south. No such trends are evident. The explanation must lie in density and susceptibility variations within the sedimentary sequence forming the Geosyncline.

interface. In fact, the scatter in the travel times is interpreted in terms of the topography of each interface, Details of the ray tracing technique are given by Tapley (1984). The Leigh Creek and Iron Baron profiles were re-interpreted using this iterative ray tracing technique, to obtain the results shown in Figs. 6 and 7 Each discontinuity was treated separately, using the appropriate average (r.m.s.) velocities from those listed in Table 6. Reflections and refractions from both the mid-crustal boundary and the Moho were available for the Leigh Creek profile (Fig. 6). Only refraction data were available for the Iron Baron profile (Fig. 7). The original travel times are considered accurate to only 0.2 s, which corresponds to about 1 km error in depth. These errors did not warrant a ray spacing of less than 10 km. Any depth variations in the resulting model which are less than 1 km are within the error limits and so cannot be considered geologically significant. 6.3. Leigh Creek profile Ray diagrams for both the mid-crustal and Moho discontinuities are presented in Fig. 6 (the

334

S.A. GREENHAI,GH, PT AL.

6.4. Iron Baron profile

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Figure 7 displays the final ray diagrams for both the Conrad and the Moho boundaries, The crustal model resulting from the combination of these results is presented together with the gravity and aeromagnetic profiles in Fig. 9. The travel time data used to model the Conrad discontinuity were predominantly refraction arrivals, recorded in the distance range 104 to 195 km. The prominent second arrivals within 33 km of the source (Fig. 7(a)) have been tentatively

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identified as the reflected phase P 2R. The gap in the profile across Spencer Gulf hindered further recording of this phase. Refraction data (P2) yield a depth to the midcrustal boundary of 13 to 11.5 km in the distance range 80 to 160 km. Close to the source the interface depth is 9 km, on the basis of the P2R times. The lack of P2 observations in the distance range 33—105 km forced the analysis to assume a linear interface over the corresponding depth points, The shallow depth obtained west of Spencer Gulf is in agreement with the interpreted depth to magnetic basement, which is shallow beneath Eyre Peninsula, and deepens rapidly towards the east

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(Milton, 1982). The tectonic processes responsible for basement configuration may have produced a similar topography deeper in the crust. Gravity data along the western end of the profile are incomplete due to lack of observations across the Gulf. However, there does not appear to be any significant positive anomaly in the area, as would be expected if the depth to the higher density lower crustal layer, shallowed in the area. A crude estimation of the gravitational effect of a 3 km decrease in depth to the Conrad interface may be obtained by considemig the differences in mass per unit area of the two crustal columns. Densities may be assigned on the basis of the velocity-density relation of Talwani et is a!. 2~G~p (1959). The associated Thet.difference in massgravity per unitanomaly area is 0.i5—i0~g cc where G is the gravitational constant and ~p is the mass per unit area difference. Thus the effect is 18 mgal. Such an anomaly is not observed. Possible reasons are as follows: (1) the density contrast used is too large; (2) the gravity profile is insensitive to depth to the lower crust: (3) the Conrad boundary does not decrease in depth to the west; (4) the gravity anomaly is isostatically compensated at greater depths by an opposing mass distribution, or (5) any combination of the

335

EXPLOSION SEISMIC DETERMINATION. ADELAIDE

above. The presence of large gravity variations elsewhere along the line where the mid-crustal interface is flat implies that the gravity profile is more sensitive to near surface geological variations. In fact, the gravity low of 25 mgal over the mid-section of the line represents the decrease in density of sediments forming the Adelaide Geosyncline. Therefore the possibility that the Conrad layer dips to the east beneath Eyre Peninsula cannot be discounted on the basis of gravity information. Travel time data, for the Moho discontinuity, comprised refraction arrivals P~alone. As in the case of the Leigh Creek profile, the model could not be defined at the critical refraction depth point, due to insufficient data. The boundary is essentially horizontal over the distance range 70 to 240 km, at an average depth of 39 km. The Moho then shallows to the east, reaching a depth of 33 km at a distance of 300 km. The depth to magnetic basement also decreases east of the Adelaide Fold Belt (Milton, 1982). 6.5. Discussion

The iterative ray tracing interpretation of the Leigh Creek and Iron Baron seismic profile data has yielded a clear picture of lateral depth variability of the lower crust and upper mantle along the two profiles. A difference in Moho depth between the two profiles of about 4 km and the decrease in Moho depth towards the southern end of the Leigh Creek profile, both indicate a region of mantle uplift in the vicinity of Hallett (HTT in Figs. 2 and 5). The trend is a regional one of small amplitude and is more likely to be related to a region of isostatic compensation than a local feature. To the east of the Adelaide Fold Belt is the Murray Basin (Fig. 1). Crustal thickness in the tectonically quiet Murray Basin is likely to be less than the Adelaide Fold Belt, which has had a tectonically active history and has a rugged topography. The trend of decreasing Moho depth to the south-east indicated by the ray tracing results presumably reflects the crustal thinning towards the Murray Basin. The Moho represents a mineralogical change in the composition of the Earth at depth. However,

there is no corresponding widespread mineralogical change which can be related to the mid-crustal boundary. This Conrad discontinuity must be interpreted using geological and tectonic models. Cook et al. (1980) identified a mid-crustal layer at depths of 12 to 18 km beneath the Southern Appalachians using COCORP data. They interpreted the layer to be a basement, underlying a highly deformed upper crust, where thin-skinned thrusting has formed the present geology. It is likely that thin-skinned thrusting was a dominant tectonic process in the formation of the Adelaide Geosyncline (von der Borch, 1980). The South Australian mid-crustal boundary may therefore be a basement for this thin-skinned deformation, in a similar manner to the Southern Appalachian. However, such an interpretation is highly speculative and requires supporting evidence, which is not presently available. The ray tracing analysis implicitly assumed that the crustal velocities are essentially constant along each entire profile. Results of the geotomographic velocity study presented in Fig. 4 indicate that this assumption is not true. The P1 velocity is shown to increase towards the southern end of the Leigh Creek profile. This may account for some or all of the apparent decrease in the Moho and Conrad depths. Variations in the velocity of the lower crust and the upper mantle would also affect the results. The model generated by ray tracing can only be as good as the observed data. The identification of seismic phases and the recording of travel times is a subjective procedure and prone to error. By reversing the refraction profiles a more confident interpretation could be made.

7. Moralana seismic array experiment 7.1. Data aquisition

The crustal model described above was further investigated by means of a digital seismic array experiment carried out in the Southern Flinders Ranges. The array was located on the north-south profile of Shackleford and Sutton (1981), 119 km south of Leigh Creek in the vicinity of Moralana

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(Fig. 10). Quarry blasts from Leigh Creek again constituted the energy source. Recordings were made on one unreversed 10 km linear profile using digital seismic event recorders, designed and built inhouse (Parham and Greenhalgh, 1985). Twenty one station locations were accurately positioned at 0.5 km intervals on a horizontal plane at bearings of 184°. Factors which influence the choice of

array type and location were sufficient time separation between crustal seismic phases to allow identification, the number of available recorders, easy access to array stations, a suitable base to work from, a relatively low background noise level, and time and/or budgetary considerations. A maximum of four event recorders were available for fieldwork at any one time, the fifth unit

337

EXPLOSION SEISMIC DETERMINATION, ADELAIDE

being held for maintenance. The array was synthesized by multiple deployment of recorders, for numerous blasts. Available recorders were installed at consecutive stations along the profile and left to record one or more blasts over a period of about one week. (Blasting at the Leigh Creek quarry occurred about twice a week). After successfully recording one or more blasts at a site, the event recorders were moved down the array line until all sites had been occupied. However, it was later discovered that some elements of the array had none or only one useable seismo-

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338

S.A. (,REENHALGH. ET AL.

smoked paper recording, coupled to two prospecting geophones was used. The seismograph was activated some minutes before the explosion by the drill and blast engineer, with VNG radio signals providing the absolute time code. First arrival times from the blast were then read to an accuracy of 25 ms, within the sample rate of the digital recorders. A small correction to the zero times was made for propagation delay (distance between source and station) using nearsurface velocity of 2500 ms~ as determined from an earlier refraction survey of the coalfield (B. Taylor, personal communication). 7.2. Results

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Figure 11 shows the first nine seconds of data for the recovered 21 traces of the digital array after band pass filtering for 3 to 12 Hz. The traces have been arranged in order of increasing sourceto-receiver distance. The histogram on the top of the frame gives the amplitude normalization factors across the gather. It should be noted that the abcsissa does not correspond to an equal increment of 0.5 km distance as planned, due to source location variability (±2 km) within the coalfield. The trace numbers correspond to individual seismograms rather than the element numbers of the array, (e.g. traces 1, 4, 5 and 6 are from the first station of the array, but different source positions within the quarry). The P onsets (first break picks) are indicated by the short horizontal segments on the traces. A time-distance plot for the array element data and the six additional analogue stations is shown in Fig. 12. From least squares analysis of this data we obtained a P1 velocity of 5.88 km s~. The scatter in the plot is due to static errors associated with local source and station geologic effects, origin time errors and clock drift in the digital event recorders. To compensate for these static errors,tocorrections were appliedof so as km to align trace a first break velocity 5.88 s each ~.

All secondary crustal P arrivals of interest should occur within five seconds of P1 at the array offsets. No obvious correlation can be made from the traces displayed in Fig. 11. Phase identification is made difficult by source variation, the

Fig. 12. Time-distance graph of first arrivals for the Moralana digital array, supplemented by the six analogue station re. corders. The velocity for the direct (upper crustal) P 1 wave is 5.88 km s~

relatively poor signal to noise ratio and the static errors mentioned above. The extended nature of the source waveform (up to several seconds for a multiple delayed quarry blast) also make the phases interfere with one another, further cornplicating the situation. A more positive identification of arrivals was attempted in the frequency domain, by examining the various phase velocities. The analysis invloved a fast Fourier transformation of the time section (Fig. ii) from space-time to space-frequency and then a second slow transformation from space to phase velocity vs. frequency plane (March and Bailey, 1983). The slow distance transformation arises from the unequispaced nature of the traces (irregular source-receiver distance). Figure 13 is a contour representation of the result. Only the higher level contours (> 70%) have been plotted. Strong events are visible, centred around 9 Hz. The events with phase veloc1 are possibly ity of around 8.0 km s associated with 5.9 the and P 1 and P~phases respectively. The 8.3 km s arrival is likely to be a sub-Moho reflection. The strong event with phase velocity of 7.2 km ~ is interpreted to be the phase P~R,a super-critical reflection from the Moho. Reflections from the mid-crust are predicted to have

339

EXPLOSION SEISMIC DETERMINATION, ADELAIDE

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Fig. 13. Phase velocity vs. frequency plot for the Moralana digital array data, obtained by two-dimensional Fourier transformation of the traces of Fig. 11. The contour peaks, which represent the energy maxima, can be associated with direct (P 1) refracted (P2, P0) and super-critical Moho reflected (P0R) arrivals.

sediments on the Stuart Shelf in South Australia adjacent to the Adelaide Geosyncline (Fig. 2), has yielded strong reflections from various units within the basement complex, at depths of 0.5 to 3 km (Nelson and Greenhalgh, 1985). A representative

phase velocity of around 6.5 km s’. A possible association can be made in Fig. 12. However, the entire interpretation remains ambiguous. The variation in the seismic waveform generated by quarry blasts is a distinct disadvantage associated with their uses as a seismic source. The charge size varied from 1200 kg to 8300 kg. The number of shot holes ranged from 20 to 319 in patterns of up to 200 m length, with detonation times from 0.02 to 6.3 s. The source location varied over eight cuts within one man lobe of the coalfield. Blasting was confined to hanging walls in shales. One solution would have been to use only one blast and a large number of recorders. The limited availability and expense of the equipment make this impractical for large array. Another alternative would be to maintain a permanent recording station, at a fixed location for every blast recorded. In this way the variation of the source would be known exactly and used to deconvolve the various traces back to an equivalent single source signature.

section is shown in Fig. 14. The section has been verified by drilling to a depth of approximately 1500 m. There is good correlation of reflections with the base of the Adelaidean Pandurra Formation, a sequence of meta-sediments and volcanics, and a granitic brecca. A deep reflector, lying here at an estimated depth of 3 km, and as yet untested by drilling, was recorded consistently throughout the area as a strong reflection which stacked with a velocity of 5.6 km ~ It is suspected that this corresponds to the lower to middle Proterozoic unconformity. Expanded reflection spreads recorded elsewhere in the area yielded average velocities for the basement rocks of 5.5 km s~, which was subsequently confirmed in a well-shoot cxperiment. Intra-basement reflections have also been observed in a recent vibroseis survey of the Stansbury Basin (see Fig. 2), just west of the Torrens Hinge Zone. Figure 15 shows the final stacked section from a 10 km east-west profile. The prominent reflection at 0.3 s is the Proterozoic/ Cambrian unconformity. Within the basement there are strong unidentified reflections which have

.

340

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an apparent dip of about 30° to the west. Faulting which disrupts these units is evident in the vicinity of stations 1090 and 1170. Interval velocities obtained for the basement to below 0.8 s lies in the range 6 to 6.6 km s1. Velocity measurements carried out in the Stans-

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represent dipping Proterozoic strata of Adelaidean or, more likely, pre-Adelaidean age. The westerly component of dip of the reflectors and the easterly component of dip of the possible faults, is remeniscent of listric faulting related to the rifting stage of basin development (Mackenzie, 1978) with the axes of rifting of the Adelaide Geosyncline lying to the east. An alternative possibility is that the dipping reflectors represent layering within crystalline basement rocks. Further seismic work and drilling are planned to resolve the ambiguity.

Acknowledgements

9. Conclusions

References

The Adelaide Geosyncline is a major geologic feature of South Australia. It forms a meridional zone of upper Precambrian sediments which have been subjected to a complex history of tectonism, including basic volcanism, graben formation, folding, faulting and marginal basin development. The structure of the crust beneath the Geosyndine has been seismically investigated in only a very minor fashion over the last thirty years. Apart from studies based on local earthquake travel times, there have been only a handful of widelyspaced long-range refraction profiles (unreversed) recorded throughout the region, using mainly quarry blast sources. These investigations have inevitably led to very simple models of at the most two crustal layers, and an average Moho depth of 37 km. In this paper we have reviewed the available seismic data. Two of the refraction profiles have been re-interpreted using iterative ray tracing to yield depth variations to the mid-crustal and Moho boundaries. Refractions and reflections from these two interfaces have been tentatively indentified in a recently recorded, short-aperture digital seismic array experiment within the Flinders Ranges. Several reflection profiles have been obtained in regions adjacent to the Adelaide Geosyncline. The resulting seismic sections show evidence of complex upper crustal structure, with steeply dipping Proterozoic (intra-basement) reflectors, interrupted by listric faulting. perhaps associated with the rifting event. A detailed programme of deep reflection profiling and closely spaced refraction shooting is

needed to elucidate the fine velocity structure of the crust and upper mantle beneath the Adelaide Geosyncline.

This research was supported by a grant from the Australian Research Council. We are indebted to R. Singh, R. Nation and R. McDougall for assistance in field work and data reduction.

Bolt, BA., Doyle, HA. and Sutton, D.J. 1958. Seismic ohservations from the 1956 atomic explosions in Australia, Geophys JR. Astron, Soc., 1: 135—145 Branson, J.C., Moss, F.J. and Taylor, F.J.. 1976. Deep crustal reflection seismic test survey, Mildura. Victoria and Broken Hill. N.S.W. Aust. Bur, Miner. Resour. Geol. Geophys. Rep.. 193. Clear, J., 1973. Australian crustal structure. Tectonophysics. 20: 241—248. Cook. F.A.. Brown, L.D. and Oliver, J.F... 1980. The Southern Appalacheans and the growth of continents, Sci. Am,, Oct. 124—138. Denham, D., Simpson, OW.. Gregson, P.J. and Sutton. D.J.. 1972. Travel times and amplitudes from explosions in Northern Australia. Geophys. JR. Astron, Soc.. 28: 225-235. H. and Everingham. lB., 1964. Seismic velocitIes and crustal structure in South Australia, J. Geol. Soc. Aust.. 11. 141—50. Finlavson, D.M.. 1982. Geophysical difference in the lithosphere between Phanerozoic and Precambrian Australia. Doyle,

Tectonophysics. 84: 287—3 12. Finlayson, D.M.. 1983. The mid-crustal horizon under the Eromanga Basin, Eastern Australia. Tectonophysics. 100: 199—2 14.

D.M.. Cull, J.P. and Drummond, B.J., 1974. Upper mantle structure from the Trans. Australia Seismic Survey (TASS) and other seismic refraction data. J. Geol. Soc. Aust,. 21: 447—458. Finlayson, D.M., Collins, C.D.N. and Lock, J., 1984. P-wave velocity features of the lithosphere under the Eromanga Finlavson,

Basin. Eastern Australia, including a prominent mid-crustal Tectonophysics, 101: 267—291. S.A.. Tapley, D.J. and Singh. R., 1989. Crustal

discontinuity. Greenhalgh.

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in

South

Australia.

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