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Seismic reflection and refraction profiling across the Arunta Block and theNgalia and Amadeus BasinsB. R. Goleby ab; C. Wright a; C. D. N. Collins a; B. L. N. Kennett b

a Bureau of Mineral Resources, Canberra, ACT, Australia b Research School of Earth Sciences,Australian National University, Canberra, ACT, Australia

To cite this Article Goleby, B. R., Wright, C., Collins, C. D. N. and Kennett, B. L. N.(1988) 'Seismic reflection and refractionprofiling across the Arunta Block and the Ngalia and Amadeus Basins', Australian Journal of Earth Sciences, 35: 3, 275 —294To link to this Article: DOI: 10.1080/08120098808729447URL: http://dx.doi.org/10.1080/08120098808729447

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Australian Journal of Earth Sciences (1988) 35, 275-294

Seismic reflection and refraction profiling across the Arunta Blockand the Ngalia and Amadeus Basins

B. R. Goleby,1-2 C. Wright,1 C. D. N. Collins1 and B. L. N. Kennett2

1Bureau of Mineral Resources, GPO Box 378, Canberra, ACT 2601, Australia.2Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT

2601, Australia.

In order to investigate the tectonic evolution of the Arunta Block and the Ngalia and Amadeus Basins, aregional north-south seismic reflection line 420 km long from the Northern Arunta Province to thesouthern part of the Amadeus Basin, and an east-west refraction profile over 400 km within the AruntaBlock, were recorded by the Bureau of Mineral Resources in 1985. The most significant basement featuresobserved in the reflection data are prominent bands of northerly dipping reflections originating frombeneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s. In this region,reflected energy with frequencies as high as 100 Hz is present at two-way times of 5-6 s, implying that therocks have high Q to depths of at least 18 km. The character of the reflections changes markedly withvarying frequency, which suggests that they arise by interference phenomena, probably associated withlaterally varying lamellar structures. Deep crustal features on the reflection profiles from the CentralArunta Province are less clear, although the refraction data suggest an average crustal thickness of about55 km. Below the Southern Arunta Province there is a zone of northerly dipping reflectors at depthsbetween 21 and 30 km, which suggests deeply buried rocks of sedimentary origin. Beneath the southernpart of the Amadeus Basin, prominent bands of reflections, similar in character to those observed beneaththe Southern Arunta Province, occur at times between 6 and 10 s, but have an apparent dip to the south.The reflections from the sediments of the Ngalia and Amadeus Basins are generally weak, except for thosebelow the Missionary Plain in the northern part of the Amadeus Basin where strong, excellent-qualityreflections were obtained. Data from an expanding spread recorded in this area give well-constrainedvelocity estimates throughout the 10 km thick sedimentary sequence, thus enabling the local thickness ofthe basin to be accurately determined.

Key words: Amadeus Basin, Arunta Block, Ngalia Basin, seismic profiling.

INTRODUCTION

A programme of deep seismic reflection profilingwithin the Australian continent, initiated by thestate Lithospheric Transect Studies of the Aus-tralian Continent (LITSAC) groups (McElhinny1982), is currently being implemented by theBureau of Mineral Resources (BMR) and co-operating organizations. In 1985, BMR under-took a programme of seismic reflection andrefraction profiling, supplemented by aeromag-netic and gravity surveys, across the AruntaBlock and the Ngalia and Amadeus Basins incentral Australia (Fig. 1). This central Australianproject involves the first extensive reflectionprofiling within a hard-rock area in Australia.

The major purpose of this paper was to presentsome preliminary interpretations of the geo-physical data obtained from the fieldwork under-taken in 1985. At this stage, we can provide

detailed interpretations only for sedimentarystructures in the northern part of the AmadeusBasin and for basement structures within theArunta Block. However, ongoing research is con-centrating on the more difficult problems ofresolving deeper crustal structure beneath theAmadeus Basin and assessing the implicationsfor the tectonic evolution of the region.

PREVIOUS GEOLOGICAL ANDGEOPHYSICAL WORK

The Ngalia and Amadeus Basins are separated bya region of exposed Proterozoic crust, the AruntaBlock (Fig. 2), part of a major ensialic mobile beltin central Australia (Stewart et al 1984).

Downloaded By: [Australian National University] At: 05:21 22 February 2010

276 B. R. GOLEBY ET AL

| Officer Pasin

A WSTEANT AUSTRALIA

T

i

i OUt.HSlANO T .

L I . u:»:::7

0 300 km

Contour Interval 100 um/s2

Fig. 1 Map of the central Australian region, indicating the gravity anomalies and the location of the seismicprofiling undertaken in 1985. LI denotes the main north-south reflection profile. L2, L3 and L4 denote shortersupplementary profiles. Rl defines the route of the refraction profile within the Arunta Block.

The Arunta Block

The Arunta Block (Shaw et al 1984; Stewart et al1984) consists mainly of Precambrian meta-morphosed sedimentary and igneous rocks. Itconsists of three tectonic provinces with separatehistories of deformation and metamorphism(Fig. 2).

The complex tectonic history of the regioninvolved six cycles of extension and compressionthat commenced before 1800 Ma and culminatedin the Alice Springs Orogeny that took placebetween 400 and 300 Ma. Deformation of theNgalia Basin and northern part of the AmadeusBasin at this time was influenced by processesoperating in the underlying Arunta Block. Animportant function of the present seismicprofiling is to map faults throughout the crust todetermine the extent to which the most recentcompressional movements took place alongfaults or weaknesses generated by previoustectonic episodes.

The Northern Arunta Province (Fig. 2)consists largely of metamorphosed sedimentsintruded by granites. Rocks of the CentralProvince (Fig. 2) have generally been meta-morphosed to granulite and upper amphibolitefacies at depths of about 30 km during an event at1800-1750 Ma. The Southern Province (Fig. 2)consists of gneiss, unconformably overlain bymetasediments. Granites are widespread anddolerite dykes are locally abundant.

The Redbank Deformed Zone (RDZ) (Fig. 2) isa relatively narrow (7-10 km) easterly trendingregion of complex deformation located along thenorthern margin of the Southern Province. Thezone dips to the north at about 45° and is inter-preted as an overthrust towards the AmadeusBasin to the south, but more detailed inter-pretations in terms of thick- or thin-skinnedtectonic styles can only be tested by deep seismicprofiling. Many northerly dipping faults havebeen mapped throughout the Arunta Block.


SEISMIC PROFILING, CENTRAL AUSTRALIA 277

130°30'

ARUNTA BLOCK

CENTRAL PROVINCE

SOUTHERN PROVINCE^

134°00'22°00'

T AUSTMIIA

• " ' - \

24°30'

Fig. 2 Map of the Arunta Block and northern Amadeus Basin, giving the locations of expanding spreads (E1-E3)and other experimental profiles. L1-L4 are as in Fig. 1. Cl shows the location of eight large shots recorded on L2 atoffsets of 21-25 km.

The Ngalia Basin

The Ngalia Basin (Wells et al 1972; Wells & Moss1983a) covers much of the southern part of theNorthern Arunta Province. It contains LateProterozoic and Palaeozoic sediments, ofmaximum thickness in the west of about 6 km,that can be correlated with sediments in theAmadeus Basin. Thrusting of Arunta rocks fromthe north over the northern margin of the basinprobably took place during the Alice SpringsOrogeny. The mapping of both the basementshape and the bounding thrust faults of this basinwas a further objective of the seismic reflectionwork.

The Amadeus Basin

The Amadeus Basin (Wells et al 1970; Schroder& Gorter 1984; Lindsay & Korsch 1988) is aneast-west-trending intracratonic depression,800 km long, bounded on the north and south bythe Arunta and Musgrave Blocks, respectively(Fig. 1). Sedimentation in the basin was inter-

rupted by tectonic episodes at 1050-900 Ma andat about 600 Ma. Late Proterozoic sedimentationceased at around 600 Ma with uplift and foldingassociated with the Petermann Ranges Orogenyin the southwest of the basin. Early Palaeozoicdeposition was halted in the Late Ordovician toSilurian with folding and peneplanation in thenorth and east of the basin, followed by furtherdeposition during the Late Devonian. The AliceSprings Orogeny was the last major diastrophismto influence regionally the structure and petro-logical nature of the basin. Sediments reachthicknesses of about 14 km in the north, and thesub-basins in the north are separated from theplatform regions and sub-basins in the south byan east-west-trending ridge that is most probablya basement high.

Lindsay and Korsch (1988) argued that thebasin evolved in three stages, the first twoinvolving crustal extension and thermal sub-sidence, followed by a final stage of overthrustingcomprising the Alice Springs Orogeny. Anobjective of the present seismic profiling was todefine accurately the thicknesses of the sedi-


278 B. R. GOLEBY£T,1L

mentary sequence and underlying basement, andto determine any differences in the character ofbasement reflectors from those observed belowthe Arunta Block.

Gravity field and models of tectonic evolution

Some of the largest differences in the gravity fieldon any continent are present in central Australiawhere the total variation is about 1400 um/s2

(Fig. 1). In general, negative Bouguer anomaliescorrespond to the basins and positive anomaliescorrespond to the intervening areas of the Aruntaand Musgrave Blocks. Models of the crust toexplain features of the gravity field have beengiven by Anfiloff and Shaw (1973), Mathur(1976) and Wellman (1978), but none of theseauthors has attempted to explain the tectonicprocesses that could have resulted in theproposed models. The evolution of the basin andbasement regions comprising the Arunta Blockand the Ngalia and Amadeus Basins is not satis-factorily explained by conventional models ofbasin formation involving thermal or stretchingmechanisms, because the basins are not in localisostatic equilibrium and the basin margins arecharacterized by thrust structures.

A generalized mechanical model for the evolu-tion of the central Australian basins (Lambeck1983,1984) assumed that the crust has been pre-dominantly in compression for the last 1000 Ma;it gives a simplified description of the evolutionof the entire region, and has recently beenmodified to explain teleseismic travel-timeresiduals better (Lambeck et al 1988). Suddendecreases in crustal thickness of up to 20 kmacross the northern and southern margins of theAmadeus Basin due to the relative uplift of theArunta and Musgrave Blocks, respectively, arepredicted, and provide a satisfactory explanationof the gravity field. Features that the seismicwork may elucidate are: the amount of crustalthinning associated with the uplift and erosion ofthe Arunta Block; the location of any suddenchanges in crustal thickness; and the possiblepresence of underthrust upper crustal materialproducing local thickening of the crust.

GEOPHYSICAL FIELD SURVEYS ANDPROCEDURES

Seismic reflection profiling

The recording parameters used in the routinereflection profiling in central Australia are givenin Table 1, together with equivalent COCORP(Brown 1986) and DEKORP (DEKORPResearch Group 1985) data. The BMRparameters are similar to those of overseasgroups, and produce comparable results. Fordeep crustal profiling, the low fold does notappear to be a disadvantage, because, both incentral Australia and in southeast Queensland(Wake-Dyster et al 1987), stacking generally doesnot significantly improve the signal to noise ratioof reflected signals from the deep crust.

Figure 2 shows the locations of seismic reflec-tion lines in relation to the main tectonicelements of the region. The main north-southprofile (LI), about 420 km long, extends from theNorthern Arunta Province to the southern partof the Amadeus Basin; the field procedures usedare described by Goleby et al (1986). A shorteast-west line, about 15 km long, was recordedalong L3 in conjunction with the expanding-spread profiling described later. A north-southline (L2), 40 km long, east of LI and crossing thenorthern margin of the RDZ (Figs 2, 3), wasrecorded in order to map dipping fault structuresinto deeper regions of the crust. Eight additionalshots of 80 kg each were detonated on L2 at1.5 km intervals near the northern end (Fig. 2,Cl) and recorded at offsets between 21 and25 km. These larger shots were recorded to getadditional information on seismic velocities andto try to observe reflections or diffractions fromnortherly dipping faults extending into themiddle and lower crust. Eight portable refractioninstruments were also deployed in the northernportion of L2 to record the near-vertical in-cidence reflection shots and consequently toprovide information on lateral velocityvariations to depths of 1 km or more. L4 (Fig. 2),10 km long, was recorded in the northern part ofthe Amadeus Basin to tie existing industryseismic grids to LI.

Two expanding-spread reflection profiles withmaximum offsets of 36 km and common mid-points (Musgrave 1962) were recorded on LI andL3 (Fig. 4). They were located within the



Table 1 Recording parameters: COCORP (Mojave Survey 1982: Brown 1986), DEKORP (DEKORP 2-South:DEKORP Research Group 1985) and BMR (central Australia: Goleby et al 1986).

No. of recording channelsSample rate (ms)Record lengths (s) (correlated record

length for VIBROSEIS)*Spread location

Group interval (m)Geophone arrays

Source spacing (m)

Multiplicity (fold)

SourceHole depths (m)Charge size (kg)

Vibrators

Vibrator spacing (m)

COCORP

968

20

—

100.624 geophones,4.3 m spacing

100.6

48

VIBROSEIS*——

5 synchronouswith upsweep from

8 to 32 Hzover 32 s

15.2

DEKORP

2004

20

Off-end

80.080 m total length

320 (NVI)t1280 (WA)25 (NVI)6(WA)

Explosives3030

Some 5 (NVI)90

Some 60 (WA)

BMR

482

20 or 24, 39 forexpanding spreads

Symmetric orasymmetric split

spread83.3

16 geophones,5 m spacing

167or 333

6or 12

Explosives408

or 12,up to 168

for expandingspreads

*Registered trademark of Conoco Inc.tNVI, near-vertical incidence; WA, wide-angle.

20 km

• 21-25 km offset shot

southern part of the Central Province of theArunta Block and northern section of the RDZ,and were designed to obtain deep crustal reflec-tions to provide information on seismicvelocities in the deep crust. During the recordingof the expanding-spread shots, 16 portable re-fraction instruments were deployed along theline on which no shots or recording spread werelocated (Fig. 4), thus giving three-dimensionalcoverage. The objective of this experiment was touse refracted P and possibly S wave arrivals toquantify anisotropy or lateral velocity variations

Fig. 3 Map of the Milton Park region, showing thelocation of the reflection line, large offset shots and theinferred positions of four major faults (F). A and Bmark the northern and southern limits of the bedrockvelocity variations given in Fig. 10.


280 B. R. GOLEBY

0 20 kmI

O Expanding spread shots

Mount Heughlin

•n Helen"' •

Arunta

ar9in A

r. ANO. 20 Bore

ItrfcNo. 3 Bore

or

yOQ^*^ Mount Chappie

Mount Zeil ^ /̂ "̂ \r

Block

Fig. 4 Map of the Central and Southern AruntaProvince regions, showing the locations of expandingspreads. One, approximately north-south, lies on LIand consisted of 20 shots, and the other, on L3,consisted of 19 shots. The maximum shot receiveroffset in each case was 36 km. During the recording ofeach expanding spread, refraction recorders weredeployed on the other line to provide three-dimen-sional coverage.

using tomographic analysis techniques (Sugi-harto 1988).

A third expanding spread was recorded withinthe northern part of the Amadeus Basin, over asedimentary sequence about 10 km thick. Itsmain purpose was to provide accurate estimatesof seismic velocities at depths below 4 km, sincenear-vertical incidence reflection profiles givepoor velocity information for the deeper basinsediments because of the short (<2km) shot-receiver offsets.

Refraction profiling and supplementarygeophysical observations

A seismic refraction survey was undertakenwithin the Central Province of the Arunta Blockalong an east-west line about 400 km long (Rl,Fig. 1) with the major objective of obtaining

estimates of crustal velocities and thicknessalong the strike of the Arunta Block. The refrac-tion profile was located well away from themargins of the Amadeus or Ngalia Basins tominimize the possible effects of lateral variationsin seismic velocities and crustal thickness. Tenexplosions at five different shot sites wererecorded by 33 portable refraction recorders;these refraction instruments were deployed,depending on shot size and location, at 131different locations separated on average by2.5 km. Shot sizes varied between 0.5 and 3.01(Bracewell & Collins 1986).

Gravity readings were made at intervals of333 m (every fourth surveyed seismic recordingstation) along all reflection lines, and airbornemagnetics were recorded at altitudes of 150 mand 1000 m over LI, L2 and L3 of Fig. 1 (Golebyet al 1986).

RESULTS FROM NEAR-VERTICALINCIDENCE PROFILING

The main basement features have been inferredfrom examination of both the single-shot recordsand the stacked sections, and are shown sche-matically in Fig. 5. Some of these features arediscussed from north to south along LI.

Northern and Central Arunta Provinces

The band of northerly dipping reflections attimes of between 2 and 10 s beneath the NorthernArunta Province (CD of Fig. 5) is particularlystriking. These reflections probably correspondto a series of fault surfaces, the most northerly ofwhich is present just north of the Ngalia Basin.This interpretation is consistent with the patternof dipping faults within the Arunta Block ob-served in the surface geology (R. D. Shaw pers.comm. 1987; Goleby et al 1988), and is thefavoured explanation of persistent dippingreflectors, particularly at terrane boundaries, inother areas of Precambrian basement (Gibbs1986). The reflections die out north of the NgaliaBasin and are replaced by shallower, relativelyflat reflections. The persistent character of thedipping reflections suggests that the thrusting ofthe northern part of the Arunta Block towardsthe Ngalia Basin involved the entire crust.

Figure 6 shows a preliminary stack from thenorthern end of LI (Fig. 1) across the northern



o

,.-1200-

o

M

AMADEUS BASIN

Southern Platform Region

N

Gardiner MissionaryFault Plain

ARUNTA BLOCK

Northern Province

Ngalia Basin

'c D

100 kmI

Fig. 5 Diagram of the main deep crustal features observed along LI of the central Australian seismic experiment(Fig. 1) with the observed gravity profile. Regions of prominent reflections have been marked by thicker lines, andshading denotes zones of strong reflected energy. Vertical exaggeration is 2:1 for an average velocity of 6.0 km/s.CD, EF, GH and MN denote sections shown in later figures and discussed in the text.

margin of the Ngalia Basin covering about 40 km(CD of Fig. 5). It is possible to trace reflectionsfrom times of 1.5-1.9 s to the surface, at a pointcorresponding to the northern margin of theNgalia Basin. Beneath this is another group ofreflections from 3 to 5 s with southerly apparentdips at the northern end. However, the mainbands of reflections have an apparent northerlydip and are prominent in the middle of thesection. Features of the present section at thenorthern margin of the Ngalia Basin are some-what similar to the shallow thrust fault so clearlydenned in earlier seismic profiling west of thepresent line (Wells & Moss 1983b; Moss &Mathur 1986). Superimposed on this stacked sec-tion, slightly displaced, is a single-shot recorddisplayed to facilitate comparison of the sixfoldstack with unstacked data. The stack preservesthe main low-frequency features of the reflectedwave field, but much of the high-frequencyenergy is lost. In the northern part of the NgaliaBasin, there are significant statics problems,which also contribute to the degradation of thestack.

On the northern part of the profile (Fig. 6),reflections have unusually high frequencies;significant energy at a frequency of 100 Hz isreturned at times of 5-6 s from single 12 kgexplosive charges in shot holes 40 m deep. Inaddition, the dominant reflected energy occursin bands of fairly high-frequency arrivals. Ex-amples from a single shot in the Northern AruntaProvince are shown in Fig. 7.

Filter panels for the interval 3.5-6.5 sare givenfor passbands from 20 to 40 Hz (Fig. 7a) andfrom 40 to 60 Hz (Fig. 7b) to illustrate the rapidchange in character of the seismic reflectionswith frequency. For this time interval, there islittle seismic energy below 20 Hz. On Fig. 7a, b,the band of reflections starts at about 4 s, with aparticularly strong onset around 5.3 s with anapparent dip to the north. At higher frequenciesthe correlation distance of reflections is only ofthe order of five to ten traces (i.e. 416-833 m).This suggests that the signal is corrupted byrelatively rapid changes in the near-surface, sincereflections from 15 km depth would be expectedto show greater continuity. Figure 7c, d illustrate


282 B. R. GOLEBY ETAL

(a)S17150

Original record True posit ionShot 96 Shot 96

172501 17400 | 17550

^- *~^MjF*t£-~.~J

3-

9 -

12

6 km



48

Filter 40-60 HzChannel number. Shot 96

36 24 12 48

Filter 20-40 HzChannel number. Shot 96

36 24 12

••:: i • : . , s , : . : : : i : : : ; i : .

, i : S M V, ' i i s M a l ; 3 ; . , ; -

Fig. 7 Portion of shot record 96 (see Fig. 6 for position): (a) 20-40 Hz bandpass filtered; 3.5-6.5 s window;(b) 40-60 Hz bandpass filtered; 3.5-6.5 s window; (c) 20-40 Hz bandpass filtered; 8-11 s window; (d) 40-60 Hzbandpass filtered; 8-11 s window.

a deeper reflection band from 8 to 11 s, where thereflected energy is shifted to lower frequency,resulting in fewer coherent reflections in the40-60 Hz band but prominent low-frequencyevents below 20 Hz. The correlation betweenreflected events from adjacent shots is onlymoderate. Such features are difficult to modelwithout introducing a stochastic element into thedescription of the near-surface zone. The vari-ability in the character of the reflections acrossindividual shot gathers and between shot gathersmeans that these data do not meet the

assumptions built into conventional seismic dataprocessing schemes. Nevertheless, reasonableresults are obtained from common mid-pointstacking (see Fig. 6).

The most southerly band of reflections startingat times of 6 s below the Northern AruntaProvince becomes faint within the CentralProvince, suggesting that the reflection charactercan be used to define the boundary between thetectonic provinces to considerable depths. How-ever, beneath the Central Province, the linearextension of a prominent segment of seismic

Fig. 6 Portion of BMR line LI across the Arunta Block (CD on Fig. 5). Section is processed to brute stack stage. Anenhanced line drawing (a) is shown above. Shot record 96 is superimposed on both parts of the diagram to illustratethe effects of stacking.


284 B. R. GOLEBY

arrivals reaches the surface at the RDZ (F of Fig.5). These events are interpreted as reflectionswith a dip of about 42° after migration. It appearsthat this thrust (RDZ) observed at the surface(Shaw et al 1984; Stewart et al 1984) extendsthrough the crust to a depth of at least 30 km(Goleby et al 1988). The deep seismic data thusdelineate the boundary between the Central andSouthern Tectonic Provinces, and also provideevidence to support a thick-skinned model oftectonic evolution in which old zones of weak-ness were reactivated during the Alice SpringsOrogeny. However, the absence of evidence for asingle low-angle thrust on which faults sole out,suggests a different style of evolution from thatrecently proposed by Teyssier (1985).

Southern Arunta Province and NorthernAmadeus Basin

Prominent deep reflections beneath the SouthernProvince of the Arunta Block are observed fromlines LI and L2 (Fig. 2). Strong reflections attwo-way times of 6 s and a prominent band ofreflections between 8.5 and 10 s are present at thesouthern end of L2 beneath the RDZ. However,on LI the strongest band of reflections has aslight apparent dip to the north and commenceslater at about 6.8 s (EF of Fig. 5).

Figure 8 gives line diagrams of two short piecesof the seismic section below the northern part ofMissionary Plain, northern Amadeus Basin (Fig.8A), and below the Southern Arunta Province(Fig. 8B). In the Missionary Plain section (Fig.8A), the sedimentary sequence is about 10 kmthick and has reflections dipping gently to thenorth and rapidly dying out close to wheresurface geological observations suggest that theyare folded upwards (CDP 13850, Fig. 8A). Northof this, there is a gap of about 23 km with littlereflected energy. Beneath the Southern AruntaProvince (Fig. 8B), bands of reflectors extendfrom depths of roughly 21-30 km (about 7-10 s),again with an apparent dip to the north. Thereflections on the seismic sections of Fig. 8B arepossibly from sills of mafic rocks or from rocks ofsedimentary origin. A reasonable time correl-ation exists between reflections below the Mis-sionary Plain in the time interval 1.5-3.3 s, andbetween those below the Southern AruntaProvince in the time interval 7.0-8.7 s. Theapparent thickness in the time domain of the

deep section is about 6% less than its shallowcounterpart in the northern part of theMissionary Plain. If the bands of deep reflectorswere originally part of the Amadeus Basinsequence, the reflections at times of 7.0 s wouldcorrespond to the top of the Ordovician Lara-pin ta Group (Wells et al 1970; Schroder & Goiter1984). The deep reflectors below the SouthernArunta Province might also be the remnant ofanother Proterozoic or early Palaeozoic Basin, ormay be from westerly plunging metasedimentsbelonging to Division 3 rocks of the IwupatakaMetamorphic Complex (Stewart et al 1984). It isnot clear if near-surface heterogeneities make thedeep reflections appear less continuous than theyreally are, or if the rocks are less reflective thanthe Missionary Plain sedimentary sequence;further processing and numerical modelling mayclarify this situation.

Strong reflections from the sedimentarysequence are observed to depths greater than10 km below the Missionary Plain in the north-ern part of the Amadeus Basin (G of Fig. 5).Multiples generated from the strong reflectorswithin the basin sediments have so far preventedidentification of deep basement reflections. TheGardiner Fault (Fig. 5), with a vertical uplift ofthe southern section of more than 3 km, is clearlyobserved in the sections.

Southern platform region of the AmadeusBasin

Reflections from the sedimentary sectiontogether with some multiples remain clear for adistance of 40 km south of the Gardiner Fault(Fig. 5). Within the southern platform region ofthe Amadeus Basin (Fig. 5), shallow reflectionsbecome diffuse, and there are few clear reflec-tions from trie deep crust. The absence of strongshallow reflections continues throughout thesouthern part of the Amadeus Basin where thesedimentary cover appears thinner (generally lessthan 1.5 km) than earlier geological inter-pretations have suggested (Wells et al 1970;Lambeck 1984).

Strong reflections, similar in character to thoseobserved beneath the Southern Province of theArunta Block, are observed below the southernpart of the southern platform area at timesbetween 5 and 10 s, but have an apparent dip tothe south. The onset of these reflections is sharp



South North

Northern section Amadeus Basin

B

24 km

Deep reflectionsArunta Block

- 8 aE

CDP B

Fig. 8 Line diagram of short segments of seismic sections below the northern part of the Missionary Plain (GH ofFig. 5) and below the Southern Arunta Province (EF of Fig. 5). The lower part of the figure shows seismic sectionsbelow the northern part of the Missionary Plain (A) and the Southern Arunta Province (B). The vertical lines A and Bon the line drawing indicate the portion of section displayed below. Spot depths are given for both sections AandB.

over a distance of more than 40 km; this reflectedenergy starts at 5 s in the north and becomesprogressively later towards the south (Fig. 5). Abrute stack of part of the southern part of LI (MNof Fig. 5) shows particularly strong reflectedenergy between 5.8 and 6.0 s followed by bandsof reflections to 10 s (Fig. 9).

Possible improvements to the seismic sections

Attempts at signal enhancement by using im-proved statics corrections and refined seismicvelocity determinations are proving successful inimproving the quality of both the shallow anddeep seismic sections throughout the survey


286 B. R. GOLEBY ET AL

BMR 1 E SOUTHERN AMADEUSS

8400 8600 8800

N9000 COP

2 -

4 -

Fig. 9 Section processed to brute stack stage (MN of Fig. 5) from the southern part of the Amadeus Basin, showingstrong bands of reflected energy starting at 5.8-6.0 s.

region. In particular, better statics corrections areresulting from the use of the refracted arrivalsfrom the reflection shots to produce velocitymodels of the zone of variable weathering andoverburden thickness. P wave velocities inweathered and unweathered bedrock vary from3.0 to 6.5 km/s, while the averaged P wavevelocities in the overburden of the top 40 m varybetween 1.2 and 2.9 km/s over the survey region.The variable overburden thickness within theArunta Block and parts of the Amadeus Basin is aparticularly severe source of problems, because

of the large increase in seismic velocity of about4 km/s from overburden to unweatheredbedrock.

RESULTS FROM OTHER SEISMICPROFILING

Milton Park experimental profile

The early processing and interpretation of the• Milton Park line (Fig. 3 and L2 of Fig. 2) has con-



centrated on mapping P wave velocity variationsin the top 300 m of bedrock by using the refractedarrivals from the reflection shots. This shallow-velocity analysis has been undertaken because ofthe absence of clear shallow reflections and thecomplicated variations in signal waveforms andarrival times of the refracted arrivals. Localizedmeasurements of apparent velocity of therefracted arrivals that have penetrated tobedrock depths vary between 4 and 15 km/s.This emphasises the need for special techniquesto cope with the near-surface structural complex-ities that influence both the static and normalmoveout corrections. Seismic signals in the first4 s that may be associated with moderatelydipping fault structures could also be a form ofsignal-generated noise (Key 1967) and will be thesubject of a future investigation.

Deep reflections on L2 are faint except at thesouthern end of the line where strong reflectionsare observed at times of 5.8-6.0 s followed by aband of fainter reflections ending at 10 s. Farthernorth on L2, faint reflections are observed fromwithin the Central Arunta Province at times ofbetween 13 and 14 s. If these reflections are fromimmediately below the reflection spread, they

could be interpreted as indicating a crustalthickness of 42-45 km.

Wright (1988) has devised a method formapping shallow bedrock-velocity variationsto define lithological changes and zones ofextensive fracturing better by using refractedarrivals from the normal reflection shots; thismethod has been applied to a 15.4 km section ofL2, lying within the Central Arunta Province,and shown as the segment A-B on Fig. 3. Figure10 shows the results plotted as a contour map ofbedrock velocity variations from the base of theweathering (150 m) to depths of about 320 m.The measured velocities lie in the range 5.2-6.5km/s. The southern end is characterized byrelatively low velocities (approximately 5.6km/s), which increase steadily towards the north.North of location 2095 to about 2190 (~ 7.9 km),velocities are greater than 6.0 km/s at depths ofabout 200 m. These relatively high velocities aremost probably associated with mafic or felsicgranulites (Shaw et al 1984). The velocitiesdecrease rapidly north of location 2185 from 6.4to <5.6 km/s. The numerical and statisticaltechniques used to prepare Fig. 10 will causesome smearing out of relatively abrupt changes

South2150

I2200

I

North2250

I

200

300

Average overburden depth V < 2.5 km/s

Maximum weathering depth Fault• -f- -

5.2 5.4 5.6 5.8 6.0 6.2 6.4

V/H = 21:1 5 kmJ

Fig. 10 Contour map of bedrock velocity variations along a portion (A-B) of L2 of Fig. 1. The location of A-B isgiven in Fig. 3. V/H denotes vertical to horizontal exaggeration of diagram.



in seismic velocity. It is therefore suggested thatnorth of location 2185 a faulted contact betweengranulites and lower-velocity material is beingimaged and the apparent dip to the north is about15-20°. The more gradual change from high tolow velocity in the southern part of Fig. 10possibly results from an oblique crossing of anirregular granulite boundary (Wright 1988).Because of the complexity of the lithologicallayering in the Central Province of the Arunta(Shaw et al 1984), an unambiguous petrologicalexplanation of the lower-velocity material is notpossible. Granite or granitic gneiss are the mostlikely candidates.

Arunta Block tomographic experiment

Preliminary results from the refraction tomo-graphic survey (Fig. 4) suggest lateral hetero-geneity or anisotropy with P wave velocities inupper basement in the range 5.6-6.2 km/s, and Swave velocities of about 3.3 km/s on average(Sugiharto 1988). Because of the severity of thestatic correction problems in this region of the

Arunta Block, the results from the expandingspreads and coincident near-vertical incidenceprofiling are being used to remove the effects ofnear-surface heterogeneity and so enhance thetomographic analysis of the refracted P and Sarrivals.

Missionary Plain expanding spread

Figure 11 presents the seismograms from theexpanding spread recorded across the Mis-sionary Plain (E3 of Fig. 2) with receiver andleast-squares shot statics corrections applied(Wright & Taylor 1986). High-quality reflectionsare observed in the thick sedimentary section,but there are no clear deep-basement reflec-tions.

The first-break times have been used todetermine the velocity structure to a depth of4.3 km using the Herglotz-Wiechert inversionprocedure (Aki & Richards 1980, pp. 643-650).The slowness-distance relationship (Fig. 12) wasderived by summary value smoothing (Jeffreys1961; Bolt 1978), and shows a small increase in

MISSIONARY PLAIN EXPANDING SPREAD FILTERED 1 2 - 3 6 Hz

4 6 8 10 12 Shot

11I

Fig. 11 Missionary Plain expanding spread plotted with adjacent shots side by side and not in the alternatingmanner suggested by Musgrave (1962). The time displacement of the seismograms at the left margin of the figureoccurs because it was not possible to locate the recording spread correctly. S denotes refracted shear waves.



0.30

0.20

0.10

0.00

D a d< 0 11 s/degO * 0.11 < <f <0.18 S/degA A tt > 0.18 s/deg

Smoothed^slowness \

curve\

10 15Distance (km)

20 25

Fig. 12 Slowness-distance relationship derived byweighted summary value smoothing for the MissionaryPlain expanding spread (Jeffreys 1961; Bolt 1978). Thelines surrounding the summary value estimatesrepresent the calculated standard errors. The schematicslowness-distance relationship (continuous lines) inthe bottom left plot is for a low-velocity layer that givesrise to a small shadow zone. The broken line with alocal maximum indicates the form of a smoothedslowness curve constructed from scattered slownessmeasurements on the earliest detectable seismicenergy.

slowness with increasing distance, which had tobe removed in order to evaluate the Herglotz-Wiechert integral. A possible explanation of thesmall increase is the presence of a low-velocitylayer, which results in the earliest observedarrivals over a small distance range beingassociated with a cusp in the slowness-distancerelationship, as illustrated in the inset portion ofFig. 12 (Wright & Cleary 1972). The maximumvelocity of 6.02 km/s occurs at a depth of 4.3 km(Fig. 13). The inferred velocity inversion starting

Fig. 13 Velocity profile for the Missionary Plainsediments, derived from refracted arrivals by theHerglotz-Wiechert inversion method. Interval velo-cities inferred from the reflections from four keyhorizons are also plotted, together with the seismicstratigraphy (Schroder & Gorter 1984; Lindsay &Korsch 1988).

Velocity (km/s)2 0 3 0 4 0 5 0 6 0

41-

Brewer Conglomerate

Refraction Velocities*

Hermannsburg Sandstone

Parke Siltsione

Mereonio Sandstone

Carmichael SandstoneSlokos Siltstone

Stairway Sandstone

Horn Valley Siltstone

IntervalVelocities

Pacoota Sandstone

Goyder Formation

IntervalVelocities

Intra-Proterozoic Reflector

Bitter Springs Formation


290 B. R. GOLEBY£r^L

MISSIONARY PLAIN EXPANDING SPREADLocation

North

6750

ntra-ProterozoicReflector (7.0 km)

Pacoota S

Fig. 14 R.m.s. velocity variations for four key reflecting horizons plotted as a function of receiver location. Thedepth to each horizon is given after the formation name.

at depths of below 2 km adds significantuncertainty to the estimated maximum depthreached by the refracted energy, which can onlybe reduced by additional ray tracing and a studyof the fainter reflections on the expandingspread.

Reflection times for the four most prominentreflecting horizons have been picked for theinnermost 11 of the expanding-spread shots, andthe smoothed velocities with estimated errors areplotted in Fig. 14. The velocities are denned asthe square root of the slope of the T2 —X2

relationship derived for each shot, and thensmoothed as a function of receiver location. Thevelocity terms derived by this approach are verysensitive to reflector dip, and an account of theproblems associated with deriving a velocitymodel for the Missionary Plain sediments hasbeen given by Wright et al (1988). Estimates ofr.m.s. and interval velocities corrected for dip aregiven in Table 2 and the interval velocities areplotted in Fig. 13. The velocity inversion inferredfrom the refraction data is attributed mainly tothe Mereenie Sandstone (depths 3.0-3.9 km)because of its high porosity. Further velocity

inversions at depths below 4.3 and 7.0 km mayalso be present. The reflections used are from thebase of the Mereenie/Carmichael Sandstone,within the Pacoota Sandstone, an intra-Proterozoic boundary and the top of the BitterSprings Formation.

Table 2 Velocity estimates for sediments below theMissionary Plain, Amadeus Basin.

Reflectionhorizon

Vnns(km/s)

Two-waytime at

zero offset(s)

Vin,(km/s)

Depth(km)

A(base ofMereenieSandstone?)

B (withinPacootaSandstone)

C (Intra-Proterozoicreflector)

D (top of BitterSpringsFormation)

5.07

5.09

5.34

5.32

1.544

1.883

2.632

2.997

5.07

5.18

5.92

5.17

3.91

4.79

7.03

8.00



Clear S wave arrivals are observed on theexpanding-spread seismograms (Fig. 11); notethat a wave that has travelled for the major partof its propagation path as a shear wave is labelledS, but some P to S or S to P conversion may beinvolved (Wright & Finlayson 1988). Themeasured apparent velocities of S waves are inthe range 3.0-3.5 km/s, indicating P to S wavevelocity ratios of between 1.5 and 1.7.

Seismic refraction survey: Central AruntaProvince

The seismic refraction survey in the AruntaBlock (Rl of Fig. 1) yields P wave velocities nearthe surface that vary between 4.0 and 5.5 km/s,and increase rapidly to about 6.2 km/s at a depthof about 1 km. A weak refracted arrival with anapparent velocity of 7.2 km/s is present at offsetsgreater than 220 km, and is attributed to avelocity boundary at a depth of about 31 km (Fig.15). This boundary may correspond to the baseof the reflecting zone that ends at times of about10 s below parts of the Arunta Block (Fig. 5). Noarrivals with velocities greater than 8 km/s areobserved. Later arrivals at offsets between 200and 240 km may be wide-angle reflections; if thisis so, the crust-mantle boundary would be at adepth of about 55 km. Good S wave arrivals are

observed from most shots and yield apparentvelocities of between 3.4 and 3.9 km/s.

CONCLUSIONS

The seismic reflection profiling across the centralAustralian region has yielded strong deep crustalreflections below much of the Arunta Block and.the southern part of the Amadeus Basin. Resultsfrom the Amadeus Basin are varied, dependingon local surface structure, but more refinedstatics corrections using information on the firstarrival times is resulting in significant improve-ment in record quality. The base of the AmadeusBasin sediments is still poorly defined south ofthe Gardiner Fault.

The most interesting results have come fromthe Arunta Block and the southern platform areaof the Amadeus Basin. In the northern part of theArunta Block, dipping bands of reflected energymay well be generated by thrust faults, withapparent dips to the north of about 40°, some ofwhich may extend to depths greater than 30 km.The reflection character favours a thick-skinnedmodel of tectonic evolution involving lateral andvertical displacements of entire crustal blocks.The presence of bands of reflections below theSouthern Arunta Province and the southern part

WESTARUNTA BLOCK REFRACTION TRAVERSE

300 100

100 200 300

Fig. 15 Reversed refraction profiles recorded along the Arunta Block. Each branch has been marked with itsmeasured apparent velocity.


292 B. R. GOLEBY£T/1Z.

of the Amadeus Basin at depths greater than18 km needs to be explained by revised modelsfor the tectonic evolution of the region. Nowheredoes the reflection character clearly define theMoho, but this appears to be a characteristic ofPrecambrian regions (Brown et al 1987).

Many important attributes of the seismic datawill help constrain the petrological nature of thedeep crust. The variability of the frequencycontent of the reflection bands must represent aninterference phenomenon, possibly associatedwith laterally variable lamellar structures. Inbasement rocks of the Arunta Block and belowthe Amadeus Basin, there is no clear indicationof any depth interval being 'non-reflective', eventhough some reflection bands can be particularlyprominent. This is in sharp contrast to the crustbeneath the Phanerozoic basins of eastern Aus-tralia, which has a distinct non-reflective uppercrust (Moss & Mathur 1986; Wake-Dyster et al1987). The relatively short correlation distancesof the high-frequency energy require the develop-ment of alternatives to conventional stacking toexploit finer details in the reflections.

The relative weakness of deep crustal reflec-tions in the Central Arunta Province comparedwith the Northern Province appears to be correl-ated with the presence of granulites at the surface;faint reflections on L2 (Fig. 2) suggest that thethickness of the crust below the granulites is inthe range 42-45 km. The seismic refractionresults, mainly applicable to the Central AruntaProvince, also suggest a thick crust (approxi-mately 55 km) beneath much of the AruntaBlock. This is not easy to reconcile with thephysical model for the evolution of the centralAustralian region of Lambeck (1983, 1984) orwith the observed teleseismic travel-timeanomalies and gravity data (Lambeck & Penney1984; Lambeck et al 1988), which suggest arelatively thin crust beneath the Arunta, withoutintroducing some additional complications tomodels of the crust. The refraction data alsosuggest that there is little overall increase inseismic velocity within the reflective top 30 kmof the crust, and that the less-reflective regionbelow 30 km is not upper mantle material. Theexpanding-spread reflection profile recorded inthe northern part of the Amadeus Basin hasenabled seismic velocities to be estimatedaccurately for the lower part of the sedimentarysequence at depths greater than 4 km. However,for both this expanding spread and the two

recorded in the Arunta Block, the interpretationis complicated by the presence of significantlateral variations in seismic velocity belowweathering.

ACKNOWLEDGMENTS

We thank the numerous scientific, technical,administrative and field support staff of both theBureau of Mineral Resources and the AustralianSurvey Office who provided skilled assistance atvarious stages of the project. We also acknowl-edge the co-operation shown by the localproperty owners and Aboriginal communities.We also thank Chris Fitzgerald for drafting thefigures, and Drs B. J. Drummond and M. A.Etheridge for their comments and criticisms ofearlier versions of this paper. This paper is pub-lished with the permission of the Director,Bureau of Mineral Resources.

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(Received 4 August 1987; accepted 1 January 1988)


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