K.D. Wake-Dyster, M.J. Sexton, D.W. Johnstone, C. Wright and D.M. Finlayson, Division of Geophysics, Bureau ofmineral

Transcription

1 Geophys. J. R. astr. SOC. (1987). 89,42343 A deep seismic profile of 8 km length recorded in southern Queensland, Australia 1. Introduction. K.D. WakeDyster, M.J. Sexton, D.W. Johnstone, C. Wright and D.M. Finlayson, Division of Geophysics, Bureau ofmineral Resources, GPO Box 378, Canberra Cdy, ACTZ@I, Aurtralia. Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 8 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflectionhefraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as PtoSV or SVtoP converted reflections from a midcrustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide wellconstrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins. Between 198 and 1982, the Bureau of Mineral Resources (BMR) recorded 14 km of reflection data in the Eromanga Basin (Moss 8z Mathur 1986). In 1984, one traverse was extended eastwards across the Nebine Ridge and Surat Basin to the Kumbarilla Ridge, providing 67 km of new reflection data (WakeDyster et al. 1985) (Fig. 1). An additional line of 13 km length was recorded across the Clarence Moreton Basin to the Beenleigh Block (Johnstone et al. 1985). Gravity measurements and an aeromagnetic survey were undertaken along the lines. The main objective of the was: to obtain seismological data that would assist in 1) understanding the evolution of the Surat and Clarence Moreton Basins; 2) defining the role of the Thomson and New England Orogens in the development of eastern Australia, and 3) providing regional ties between petroleum exploration leases. This paper provides a brief summary of the tectonic problems that we are seeking to solve and gives some preliminary results for the Nebine Ridge and Clarence Moreton Basin areas.

2 424 K.D. WakeDyster et al. 144" 148' 152' 26" 28 Expandingspread locations ( 19884) BMR seismic traverses (19882) New EnglandYarrol Fold Belt Outcrop of Permo Triassic sediments BMR seismic traverses (1984) Concealed margin of Permo Triassic sediments Figure 1. Map showing the location of reflection profiles, expanding spreads and wideangle reflectionhefraction experiment in southeastern Queensland. The thin dotted line marks the concealed margin of the PermoTriassic sediments. The central part of the Surat Basin is underlain by the Bowen Basin. 2. The geological and tectonic features of the Nebine Ridge, Surat Basin and Clarence Moreton Basin The seismic profiles cross the Eromanga, Surat and Clarence Moreton basins (Fig. l), relatively undeformed Mesozoic basins, and also intersect three Palaeozoic infrabasins (Cooladdi and Westgate Troughs west of the Nebine Ridge and the Bowen Basin beneath the Surat Basin). Basement consists of Palaeozoic rocks of the Tasman Orogenic Zone (Murray & Kirkegaard 1978) that have been subdivided into the Thomson Orogen in the west and the New England Orogen in the east (Kirkegmd 1974). The Nebine Ridge separates the Eromanga Basin from the Surat Basin, and according to Senior (197 1) is a gentle rise in metamorphic rocks with acid intrusions. Basement rocks from boreholes in the vicinity of the Nebine Ridge consist of high grade schists and gneisses (C. Murray, personal comment). The Nebine Ridge has been interpreted as a fragment of Precambrian craton rifted from the old Australian continent before the formation of the Eromanga Basin and its infrabasins (Harrington 1974). It has also been postulated to have been an ancient island arc structure in the early Palaeozoic (Powell 1984). The Clarence Moreton Basin covers an area of about 23, km', and contains up to 3.5 km of Mesozoic sediments (Ties et al. 1985). It is bounded by exposed Palaeozoic rocks and exotic terrains of the Yarrol Fold Belt to the north (Yarraman and DAguilar Blocks), the Beenleigh Block to the east and the rocks of the New England Fold Belt (Texas Block) to the

3 Profile of 8 km length recorded in Southern Queensland, Australia 425 southwest. The prepermian tectonic elements of the Yarrol hovince are displaced by at least 2 km eastwards relative to their equivalents in the New England Province (Murray et al. 1986). To the west, the buried Kumbarilla Ridge, a prejurassic basement high, separates the Clarence Moreton from the Surat Basin. Recent discussions of the tectonic evolution of southeastern Queensland have been given by Harrington & Korsch (1985a,b) and Murray et al. (1986). 3. Nearvertical incidence reflection profiling The nearvertical incidence profiling was recorded 6 or 12 fold with 2s record lengths, except in the Surat Basin where 39 s were recorded. Explosive charges of 12 kg and 8 kg in 4 m deep shot holes were used west and east of the Nebine Ridge respectively. An account of the field operations has been given by WakeDyster & Johnstone (1985). The main features of line 14 (Fig. 1) are shown in Fig. 2 prepared from the stacked time sections. Evidence for faulting is seen in parts of the sedimentary section, and the nonreflecting upper basement overlies a more strongly reflecting lower crust. The general features of the crust are therefore similar to those found in the Eromanga Basin to the west (Mathur 1983). n Y n u c Y 2 ' I1 u a I EROMANGA 2 I I Permian Triassic sediments D? Probable diffractions Ordovician Carboniferous sediments Zones of deep seismic reflection events Figure 2. Interpretation diagram prepared from the stacked section across lines 14 (Fig. 1) showing the principal reflecting horizons. Thick lines denote faults.

4 426 KD. WakeDyster et al. I / I 3 L 16lG56111 Figure 3. Section across the Clarence Moreton Basin to indicate an interpretation of events observed on the stacked seismic sections in terms of a westerly dipping layered sequence (after Korsch et al. 1986). f n g 5 1 I= NEEINE RIDGE Production Shots Expanding Spread Shots I Figure 4. Expanding spread selsmograms that show PtoSV or SVtoP converted reflections, and single shot records that show nearvertical P reflections from the same boundary (depth 17 km). Field statics corrections have been applied to all records, and shot statics corrections for the expanding spread seismograms were calculated by the method of Wright & Taylor (1986). Adjacent shots are plotted side by side and not in the alternating manner suggested buy Musgrave (1962). The shots that supplement the usual Musgrave configuration are plotted on the right.

5 Profile of 8 km length recorded in Southern Queensland, Australia 427 The eastern part of line 16 (Fig. 1) shows some westerly dipping reflectors in the deep crust that extend to times of about 9 s (Fig. 3). Korsch et al. (1986) have provided a preliminary interpretation in terms of a thick layered sequence of sedimentary character that was telescoped by thrusting into a stack 2 km thick. They argue that the sequence is similar in character to that inferred by COCORP researchers below the Merrimack Synclinorium in New Hampshire (Ando et al. 1984). The Beenleigh terrane, a metamorphosed Palaeozoic accretionary wedge that overlies the deep, layered sequence is believed to be thinskinned with a maximum thickness of some 4.6 km (Korsch et al. 1986). 3. Other seismic profiling Expanding reflection spreads of 25 km length (Musgrave 1962) were recorded across the Nebine Ridge and within the Surat Basin, coincident with the main reflection profile (Fig. 1). The normal Musgrave configuration was supplemented by additional shots at offsets between 12 and 25 km to increase data redundancy (Wright & Barker 1986a). Another expanding spread with a maximum offset of 14 km was recorded parallel to line 14 near the western margin of the Swat Basin using 7 shots at a single location. During the recording of the Nebine Ridge reflection data, 16 remote recorders were deployed along the line to determine their usefulness in enhancing the expanding spread interpretation. Eight of the recorders were later deployed in the western part of the Surat Basin to ascertain the distance range over which the normal reflection shots could be recorded. Some of the results of the experiment with the remote recorders have been described by Wrigkt & Barker (1986b). A wideangle reflectiodrefraction survey to offsets of 192 km was also recorded across the Nebine Ridge, and located so that the expanding spread was 48 km from the western end. A preliminary interpretation (Finlayson & Collins 1986) indicates a crustal thickness in excess of 38 km, at which depth the P wave velocity is 7.7 km s'. There is no prominent midcrustal boundary at about 24 km depth like that found in the Eromanga Basin (Finlayson 1983), but an increase in velocity gradients at depths of about 15 km has been inferred in the Nebine Ridge area. The data from the Nebine Ridge expanding spread suggest significant lateral changes in uppermost basement velocities (Wright & Barker 1986a), and reflections from the deep crust at offsets of 9 to 19 km are interpreted as PtoSV or SVtoP converted reflections at a depth of about 17 km (Wright & Finlayson 1986). Corresponding P wave reflections are observed at 6.2 s travel time on the normal reflection shots (Fig. 4). Clear S wave arrivals are observed on 8 of the 19 shots, and integration of the wideangle reflectiodrefraction data with the expanding spread and nearvertical incidence data provides P and S wave velocity models of the upper crust. 4. Conclusions Preliminary analyses of the data from the seismic survey in southeastern Queensland in 1984 have focussed on deep crustal features beneath the Nebine Ridge and the eastern margin of the Clarence Moreton Basin. The deep crust below the Nebine Ridge differs from the surrounding Eromanga Basin and Surat Basins in having a less highly reflecting and possibly thicker lower crust. There is also a reflecting boundary at depths of about 17 km below the Nebine Ridge that gives both P and PtoSV or SVtoP converted arrivals, but there is no evidence for a deeper midcrustal boundary similar to that identified below the Eromanga Basin. The westerlydipping reflectors identified beneath the eastern part of the Clarence Moreton Basin have at present no clear petrological interpretation, and the search for an

6 428 KD. WakeDyster et al. unequivocal explanation may lead to a reassessment of current tectonic models for the evolution of eastern Australia. Acknowledgements This paper is published with the permission of the Director, Bureau of Mineral Resources. References Ando, C.J., Czuchra, B.L., Klemperer, S.L., Brown, L.D., Cheadle, M.J., Cook, F.A., Oliver, J.E., Kaufman, S., Walsh, T., Thompson, J.B., Jr, Lyons, J.B. & Rosenfeld, J.L., Crustal profile of mountain belt : COCORP deep seismic reflection profiling in New England Appalachians and implications for architecture of convergent mountain chains, Bull. Am. Ass. Petrol. Geol., 68, Finlayson, D.M., The midcrustal horizon under the Eromanga Basin, eastern Australia, Tectonophys., 1, Finlayson, D.M. & Collins, C.D.N Crustal differences between the Nebine Ridge and the Central Eromanga Basin from seismic data, (submitted to) Aust. J. Eurth Sci. Hamngton, H.J., The'Tasman Geosyncline in Australia, in The Tasman Geosyncline pp 38347, eds Denmead, A.L. Tweedale, G.W. & Wilson, A.F., Geol. SOC. Aust. Queensland Division. Hanington, H.J. & Korsch, R.J., 1985a. Tectonic model for the Devonian to middle Permian of the New England Orogen, Ausl. J. Eurth Sci., 32, Hamngton, H.J. & Korsch, R.J., 1985b. Late Permian to Cainozoic tectonics of the New England Orogen, Aust. J. Earth Sci., 32, Johnstone, D.W., Sexton, M.J. & WakeDyster, K.D., A geophysical transect across the ClarenceMoreton Basin, Exploration Geophysics, 16, Kirkegaard, A.G., Structural elements of the northern part of the Tasman Geosyncline, in The Tasman Geosyncline pp 4762, eds Denmead, A.K., Tweedale, G.W. &Wilson, A.F., Geol. Soc. Aust. Queensland Division. Korsch, R.J., Lindsay, J.F., O'Brien, P.E., Sexton, M.J. & WakeDyster, K.D., Results from BMR deep crustal seismic reflection profiling in eastern Australia: telescoping of the crust and a hidden, deep, layered, sedimentary sequence, (submitted to) Geology. Mathur, S.P., Deep crustal reflection results from the Central Eromanga Basin, Australia, Tectonophys., 1, Moss, F.J., & Mathur S.P., A review of continental reflection profiling in Australia, in Reflection Seismology: A Global Perspective, Geodynamics Series,Vol. 13, pp 6776, eds Barazangi, M. & Brown, L., American Geophysical Union, Washington, DC. Murray, C.G. & Kirkegaard, A.G., The Thomson Orogen of the Tasman Orogenic Zone, Tectonophys., 48, Murray, C.G., Fergusson, C.L., Hood, P.G., Whitaker, W.G. & Korsch, R.J., Plate tectonic model for the Carboniferous evolution of the New England Fold Belt, (submitted to) Aust. J. Eurrh Sci. Musgrave, A.W., Applications of the expanding reflection spread, Geophysics, 27, Powell, CMcA., Uluru and Adelaidian region Ordovician to earliest Silurian, in Phanerozoic firth History ofaustralia, pp 2939, ed. Veevers, J.J., Clarendon Press, London. Senior, B.R., Structural interpretation of the southern Nebine Ridge area, Queensland, Australasian Oil and Gas Review, 17,243. Ties, P., Shaw, R.D. & Geary, G.C., The petroleum prospectivity of the ClarenceMoreton Basin in New South Wales, J. Awl. Petrol. Explor. Ass., 25, WakeDyster, K.D. & Johnstone, D.W., Southeast Queensland seismic survey, 1984: Operational report, Bureau of Mineral Resources Record WakeDyster, K.D., Sexton, MJ. & Johnstone, D.W., Lithospheric transect study of southeastern Queensland, Exp. Geophys., 16, Wright, C. & Barker, M., 1986a. The use of an expanding spread reflection profile to determine shallow crustal structure and large scale static corrections, (submitted to) Geophys. Prosp. Wright, C., & Barker, M., 1986b. The interpretation of shots from a seismic reflection survey across the Nebine Ridge, southeastern Queensland, recorded on refraction recording instruments, (submitted to) Bur. Min. Res. J. Aust. Geol. Geophys. Wright, C. & Finlayson, D.M., Refracted and reflected shear wavesrecorded on an expanding spread reflection profile, (submitted to) J. geophys. Res.

7 Profile of 8 km length recorded in Southern Queensland, Australia 429 Wright, C. & Taylor, F.J., A note on shot statics for expanding spread reflection profiles, Exp. Geophysics, (in press).