Cuvier Basin: A product of ocean crust formation by Early Cretaceous rifting off western Australia

Earth and Planetary Science Letters, 45 (1979) 105-114 © Elsevier Scientific Pubhshmg Company, Amsterdam - Printed m The Netherlands

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CUVIER BASIN A PRODUCT OF OCEAN CRUST FORMATION BY EARLY CRETACEOUS RIFTING OFF WESTERN AUSTRALIA ROGER L LARSON, JOHN C MUTTER, JOHN B DIEBOLD, GEORGE B CARPENTER 1 Lamont Doherty Geological Observatory o f Columbia L nwersttv, Pahsades, N Y 10964 (U S A ) and PHILIP SYMONDS Bureau o f Mineral Resources, Geology and Geophystcs, P 0 Box 378, Canberpa Clt3, A C T 2601 (Austraha)

Recewed Yebruary 14, 1979 Revised version received May 23, 1979

t- our high-quality seismic refraction profiles were recorded parallel to the structural gram in the Cuvier Basin and adjacent Wharton Basra to study the nature ot the earth's crust m this area The principal result ot this experiment is that th~s area as generally floored w~th oceamc crust No transmonal velocity structure exists at the base ot the contmental slope Departures from a standard oceanic crustal section are observed on an intermediate profile that are attributed to structural complications on the flank of an abandoned spreading ridge Addltmnal magnetic anomaly profiles m the eastern Cuvier Basin are used to correlate the hneatlons in that area ~ lth Farly Cretaceous reversals M-5 to M-10 This correlanon dates the onset of plate separation at 120-125 m y , essentmlly contemporaneous with the opening of the Perth Basin to the south However, it leaves a 220-km gap between M-4 and M-5 in the Cuvier Basra that suggests a ridge jump of that nlagmtude occurred nominally at 118 m y Larly Cretaceous magnetic hneatlons northwest of the Exmouth Plateau suggest that spreading at the seaward edge ot the Exmouth Plateau began 120 m y ago, whde Late Jurassic marine sediments and fault structures landward ot the Exmouth Plateau suggest rifting m that area at 155 m y

1 Introduction T h e w e s t e r n m a r g i n o f A u s t r a h a Is c o m p o s e d o f several 5 0 0 0 - m - d e e p b a s i n s s e p a r a t e d b y irregulars h a p e d p l a t e a u s t h a t shoal a b o v e 2 5 0 0 m T h e area f o r m e d b y c o n t i n e n t a l rifting in t h e Late Mesozoic t h a t s e p a r a t e d A u s t r a l i a - A n t a r c t i c a f r o m t h e rest o f e a s t e r n G o n d w a n a l a n d L a r s o n [ 1] m a p p e d m a g n e t i c a n o m a l i e s M-0 t o M-4 m t h e W h a r t o n Basin n e a r t h e Wallaby a n d E x m o u t h P l a t e a u s t h a t d a t e d spreading in t h a t area t o have b e e n m m a t e d 120 m y ago in t h e E a r l y C r e t a c e o u s A slmtlar-aged p a t t e r n o f m a g n e t i c l l n e a t l o n s was m a p p e d in t h e P e r t h Basin t o t h e s o u t h b y Markl [2] T h u s , a c o n t i n u o u s b o u n d a r y o f plate s e p a r a U o n e x i s t e d o f f w e s t e r n Australia 120 m y ago 1 Now at U S Geological Survey, 1725 K Street, N W, Suite 20a, Washington, DC 20006, U S A

w i t h spreading c e n t e r s m t h e P e r t h and W h a r t o n Basins c o n n e c t e d b y a 1100-kin-long t r a n s f o r m fault k n o w n t o d a y as t h e Wallaby Plateau Scarp Markl [2] r e p o r t e d the oldest m a g n e t i c h n e a t a o n m t h e P e r t h Basra to b e M-11 t h a t h e s d i r e c t l y a d j a c e n t to t h e A u s t r a h a n c o n t i n e n t a l slope a n d dates t h e onset o f plate s e p a r a t i o n at 125 m y in t h a t basra T h e s i t u a t i o n n o r t h o f t h e Wallaby Plateaus is c o m p h c a t e d b y t h e Cuvier Basin, a deep r e - e n t r a n t in t h e Austrah a n m a r g i n b e t w e e n t h e Wallaby a n d E x m o u t h Plat e a u s (Fig 1) This b a s i n lies l a n d w a r d o f t h e Early C r e t a c e o u s m a g n e t i c l m e a t l o n s M-0 t o M-4 m a p p e d b y L a r s o n [1] in t h e W h a r t o n Basra, so t h e possibility exists t h a t t h e Cuvier Basin r e p r e s e n t s a n earher p e r i o d o f spreading o f f w e s t e r n A u s t r a h a I_arson [ 1 ] r e p o r t e d l o w e r - a m p l i t u d e m a g n e t i c h n e a t l o n s in t h e eastern Cuvier Basin t h a t parallel t h e M-0 to M-4 s e q u e n c e to t h e west in t h e W h a r t o n Basra These

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anomahes were labeled a, b, c, d because they could not be unambiguously identified, although Early Cretaceous reversals M-7 to M-10 and M-13 to M-15 were considered to be hkely posslblhtles The selsmtc nature of the earth's crust beneath the Cuvier Basra is also u n k n o w n and of considerable anterest The depth to a c o u m c basement on seismic reflection profiles and the presence of magnetic hneatlons suggest that the crust is oceanic m nature However, recent studies have shown that a transition zone often exists between continental and oceanic crust at rafted margins Off southern Austraha, this area just seaward of the continental slope is correlated with a

magnetic qmet zone and ts characterized by an lnhomogeneous amalgamation of various velocity structures [3] The structural history of the Cuvier Basra has been the subject of speculation Veevers and Johnstone [4] Interpret the Early Cretaceous sediments at DSDP Site 263 an the eastern Cuvier Basra to have been deposited in shallow water that lmphes a crustal subsidence since the Early Cretaceous of about 5 km Exon and Wfllcox [5] also accept the shallow-water ongm of these sediments, the lmphed amount of subsidence, and go on to suggest that "the eastern part of the Cuvier Basra is floored by subsided continental crust"

107 V3308-2, and were oriented along strike to minimize topographic lnhomogeneltles and other potentml cross-strike structural variations (Figs 1 and 2) U S Navy sonobuoys and our standard axrgun sound source were used to obtain sediment velocity determlnatlons Long-range sonobuoys (Select Industries model SB76) and TNT o f recent Austrahan manufacture were used to obtain the velocity structure to the base o f the earth's crust Standard shooting and recording techniques were used onboard shzp to obtain the raw data for the reversed seismic refraction profiles that are characterized by high shot density and overall high quality (Fig 3) The paper records from the oscdlograph camera recorder were read to the nearest 10 mdhseconds, and the travel t~mes for tile various acoustic and seismic arrivals were stored on fdes o f a PDP 11/70 computer Velocities and intercepts from the pairs of reversed refraction lines were reduced to true velocity and depth by a varla-

2 Seismic refraction results In order to test the various hypotheses of the velocity structure o f the crust beneath the Cuvier and adjacent Wharton Basins, and to obtain additional information on the magnetic anomahes o f the Cuvier Basin, part o f " V e m a " cruise 3405 was devoted to a geophysical study of that area The prime intent o f this cruise was to record well-controlled seismic refraction lines at various distances from the Austrahan continental slope in order to determine the crustal velocity structure and detect any possible differences as a function of distance from continental Austraha During this survey, additional magnetic anomaly data were obtained to estabhsh the identification o f sea-floor spreading magnetic hneatlons m the Cuvier Basra The seismic refraction measurements were made between previously acquired profiles V3308-1 and

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Flg 3 Travel time distance plots showing the first, and occasionally second, arrival events used in determining the seismic velocities and layer thicknesses shown m Fig 4 The slope of the thin line through the data points corresponds to the assocmted P-wave velocity, and allows an assessment of the data scatter for this velocity estimate tlon of the Adachl-Ocola method [6], which is itself a refinement of the method of Ewmg et al [7] The variation introduced here involves the breakdown of the inversion into two parts, the first, in which "true" velocities and dips are found from the velocity pairs, and the second, where thickness sections are found for each buoy separately, using the true velocities, but uncorrected intercept times It can be shown that errors of less than 1% will be thus introduced, even m the worst cases hkely to be encountered The advantages of this method are that layers which might be seen on only one of the two reversed lines can be taken into account more easily, and that a comparison can be made between dips calculated by velocity analysis and those geometrically required Discrepancies can point out areas where differences in intercept times are due more to structure than to dip Four crustal section estimates were measured at varying distances from the Australian continental slope (Figs 2, 3 and 4) Section 5-6 is located at the base of the continental slope on the flat, turbldlte

sediments of the eastern Cuvier Basin, and above the oldest (most landward) northeast trending magnenc hneatlon Section 3-4 is also located in the eastern Cuvier Basra in the midst of the magnetic lIneatlons and on magnetic anomaly C of Larson [1] Section 7-8 is located in the western Cuvier Basin in an area of no correlatable magnetic hneatlons just landward of magnetic anomaly M-4 Finally, section 9-10 is located on the well-substantiated magnetic anomaly M-1 of the Wharton Basin and was measured as a standard reference section of oceanic crust in this region The principal result of the seismic experiment is that the Cuvier and adjacent Wharton Basin are both generally floored with oceanic crust No transitional velocity structure such as was measured south of Australia by Talwam et al [3] is present in this area at the base of the continental slope (section 5-6) Considering the general velocity structure of this area from top to bottom, the uppermost sedimentary layer was measured to be 1 8 km/s, which is an aver-

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For clarity, the seismic refraction cross-sectmns are projected from their horizontal locations to the tront surlace of the block diagram This projection is done in a dtrectlon perpendicular to the front surface, not parallel to the refraction profiles or the magnetic hneatlons Although this projection clarlhes the relationships between adjacent profiles, features such as the seapeak that appears to occur in the middle of refraction profile 7-8 or the lower continental slope in profile 5-6 actually are located adjacent to these profiles, not on them age velocity of deep-sea pelagic sediments that are generally unconsolidated Underlying these sediments on sections 3-4 and 5-6 are additional, more consolidated sedimentary rocks with velocity of 3 0 km/s These probably correlate with the Cretaceous clays and claystones recovered from the eastern Cuvier Basin at DSDP Site 263 Layer 2 velocities vary from 5 3 to 6 3 kin/s, and thus would be categorized as layer 2B or 2C velocities according to the scheme of Houtz and Ewlng [8] They also closely approximate the average acoustic basement velocity measured on 1 2 0 - 1 4 0 - m y -old oceanic crust [8] The thickness of layer 2B in the Cuvier Basin averages about 1 5 km, slightly less than the average layer 2 thicknesses of 1 7 - 1 9 km (see Houtz and gwmg [8] and Raltt [91) Transitional layers are possibly present between layers 2 and 3 in the western Cuvier Basin and Whar-

ton Basin These possibly transitional layers do not exist near the Austrahan continental slope in the eastern Cuvier Basin It is difficult to distinguish between the thin 6-7-km/s layer and the underlying 6 9-km/s layer on section 9-10, and in any event, the entire unit should probably be considered to be layer 3 The 6 3-km/s layer on section 8-7 in the western Cuvier Basin could be categorized as layer 2C, or considered to be transitional and sandwiched between average layer 2 and layer 3 crust This IS the only section measured on crust not characterized by magnetic hneatlons, and this transitional crustal layer may represent a more complicated structural history As will be discussed below, it is likely that this area was subject to a ridge clest j u m p early in ItS history and may initially have been part of the Indian plate Layer 3 velocities range from 6 7 to 6 9 km/s and thus are equivalent to average "oceanic" layer veloci-

110 ties that are umque to and very unlfortn throughout normal ocean basins [8,9] Layer 3 here varies from 4 to 6 km in thickness with an apparent tendency to inlnimum values m the central part of the area Such a variation in thickness is not abnormal when considering ocean-wide averages but may be sigmficant on the scale o f this survey, which is only 500 km from end-to-end Only 65 km separates sections 3-4 and 5-6 that show a 1 8-kin thickening in layer 3 as the continental lnargm IS approached The existence o f a 4 6 - 5 0-km-thlck layer 3 located less than 100 km from the base o f the continental slope IS an unexpexted result The relative tlnckenlng o f layer 3 on profile 5-6 IS the only suggestion in this survey o f our proximity to continental crust Mantle velocity is uniformly 8 1 km/s, except tor the 7 9-km/s mantle velocity at section 8-7 which again points to structural complications in this area o f the western Cuvier Basra where layer 2 is either transmonal or abnormally thick, and layer 3 IS thinner than usual Total depth to mantle of 12 14 km and total thickness o f oceanic crust o f 6 8 km are average values Thus, except for structural colnphcations m the western Cuwer Basin at section 7-8 that we explain in terms o f Its tectonic evolution, the entire area is unambiguously floored with oceanic crust that is probably relatively unaltered since its formation except for a likely increase In layer 2 velocities as a result of crack, vein and void filling by secondary minerahzatlon In the extrusive pillow basalt sequences o f the upper oceanic crust

M-I 0 that nolnmally range In age from 118 to 122 m y These and all subsequent ages reter to the time scale o f Larson and Hllde [11 ] and are used simply for convenience without any Implication that the absolute ages are accurate to 1 m y A detailed magnetic anomaly chart o f the eastern Cuvier Basin (Fig 5) denronstrates the hnearity of these structures, and their comparison to a model anomaly profile Is reasonable support o f this identification The parameters o f the magnetic block model in Fig 4 are the same as those presented by Lalson [1] except that the half-spreading rate is 10% faster Subtle aspects o f the anomaly pattern Ill this area, such as the slightly wider and higher-amplitude M-9 present on all profiles, and the M-5, M-6, M-7 sequence on profiles V3308-2 and V3405-3 characterize this anolnaly sequence as M-5 to M-10 The identItIcatlon of M-5 to M-10 In the eastern Cuvier Basin places the age of initiation o f spreading there nominally at 122 m y (Fig 6A) coincident with the onset o f spreading in the Perth Basin to the south [2] The initial spreading rate in the Cuvier Basin was slightly faster, but probably these two

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3 Magnetic hneation results Larson [ 1 ] showed that the Wharton Basin in this area IS characterized by a well-hneated M-0 to M-4 magnetic anomaly sequence that is indicative of seafloor spreading from 108 to 118 m y Powel [10] demonstrated that this M-0 to M-4 pattern could be traced north across a fracture zone by correlating the B1971-1 profile shown in Fig 1 Additional anomalies parallel to this sequence In the eastern Cuvier Basin were also charted and suggested by Larson [1] as an older set o f Early Cretaceous reversals Additional magnetic and seismic reflection profiling o f the eastern Cuvier Basin supports the conclusion that these anomalies represent Early Cretaceous reversals M-5 to

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Fig 6 A Spreading begins In the Cuvier Basin with associated transform motion and continental break-up on the south and west sides of the E x m o u t h Plateau Rifting of the Rankin Platlorm area has ceased although subsidence continues B Spreading continues in the Wharton Basin after a ridge j u m p occurred In the Cuvier Basra and the Wallaby Plateau forms synchronously as a large, Early Cretaceous volcanic outpourlng

basins were formed by spreading centers at the same plate boundary, offset 1100 km along the Wallaby transform fault, now known as the Wallaby Plateau Scarp The Early Cretaceous spreading system that was initiated In the Cuvier Basin at 122 m y persisted only 4 m y until 118 m y when a reorganization o f plate boundaries took place (Fig 6A, B) At 118 m y , between M-5 and M-4 time, the spreading center in the newly-forming Cuvier Basin jumped 220 km to the west and re-estabhshed Itself at the base of the

Greater India subcontinent This spreading center jump established a part of what had been the Indian plate as the western Cuvier Basin Seismic reflecUon profiles (Fig 2) show this area to have a significantly rougher, and slightly elevated basement surface, and the refraction profile shows a thickened or transitional layer 2 and a thinned layer 3 No lmeated magnetic anomalies are present m this area, so the "mlrror-unage" of the M-5 to M-10 anomaly pattern that would confirm this hypothesis is not observed However, the western Cuvier Basin very closely approximates the area necessary to accommodate such a jump, and the 220-km gap between M-4 and M-5 strongly suggest the validity of this hypothesis At nearly the same time as the spreading center jump in the Cuvier Basin, sea-floor spreading began northwest of the Exmouth Plateau This is demonstrated by magnetic hneatlons M-0 to M-3, and perhaps hneatlons as old as M-6, correlated directly northwest of the Exmouth Plateau (Fig l) by Larson [1 ] and Powel [10] These hneatlons indicate seafloor spreading in that region from at least 115 to 108 m y This latter conclusion is at odds with that of Veevers and Cotterlll [ 12] who contend that continental break-up and sea-floor spreading were initiated northwest of the Exmouth Plateau at 160 m y This spreadlng system is viewed [ 12] to be synchronous with the Late Jurassic spreading system of the Argo Abyssal Plain and the deposition of marine sediments and normal faulting landward of the Exmouth Plateau in t h e v i c i n i t y of the Rankln Platform and the Dampier Sub-basin (Fig 6) This discrepancy apparently occurs because Veevers and Cotterfll [12] did not accept the magnetic llneatlons northwest o f the Exmouth Plateau Those correlations are not shown In their Fig 2 or mentioned In their discussion, although the hneatlons of the Cuvier and adjacent Wharton Basins are copied faithfully from I.arson [ 1] This discrimination between various parts of the Wharton Basin magnetic hneatlon pattern leads Veevers and Cotterlll [ 12] to the hypothesis that the Exmouth Plateau was initially bracketed by a double-rifted arch system in the m]d to late Jurassic, and that the oceanic extension of the Cape Range fracture zone (the fault bonding the southern edge of the Exmouth Plateau) is the Jurassic-Cretaceous fault boundary o f the Wharton Basin

112 We beheve that both these conclusions are unlikely mainly because we accept all the Wharton Basin magnetic correlations shown in Fig 1 [1,10] with equal veracity This correlation leads dnectly to the conclusion that the oceanic extension of the Cape Range fracture zone is not a large age discontinuity, but simply evidence o f a small ( ~ 1 0 0 kin) transform offset in the Early Cretaceous spreading pattern Certainly this age discontinuity separating Late Jurassic and Early Cretaceous spreading must exist off western Australia, but it is probably well north o f the Cape Range fracture zone In the Gascoyne Abyssal Plain characterized by high amphtude, but poorly hneated magnetic anomahes These anolnahes have been suggested to correlate with reversals M-9 to M-11 by Helrtzler et al [13] although these correlations have not yet been substantmted Until extremely detaded geophysical surveys are conducted in the Gascoyne Abyssal Plato, the exact location of the Jurassic-Cretaceous age discontinuity off western Austraha will remain an enigma The nature o f rtftmg o f the Exmouth Plateau is a more comphcated problem The existence of Early Cretaceous magnetic hneatlons directly northwest of the Exmouth Plateau does not unequivocally preclude Late Jurassic break-up and open marine sedimentation in this area, although it appears unhkely because the break-up unconformity and subsequent inarlne transgressions on the northwest shelf match very closely the age o f magnetic hneations in the Argo Abyssal Plain However, the thickness and extent o f the Late Jurassic sediments landward o f the Exmouth Plateau in the Danlpler sub-basra, and the taultlng and uplift o f the Rankm Platform strongly suggest that rifting did occm there in the Late Jurassic [12,14,15] The Dampler, along with the Exmouth, Barrow, and Beagle sub-basins form an elongate, synchnal basin parallel to, and just landward of, the Rankm Platform shown in Fig 6 [14--16] We suggest this rifting was associated with the rifting o f the northwest Australian margin and the subsequent spreading pattern an the Argo Abyssal Plain either as a direct, transformed extension of that rift, or as a separate rift that joined the one off northwest Austraha at a three-branch rift near the northern edge of the present-day Exmouth Plateau ( F g 7A) We believe the latter hypothesis to be more likely because it provides an explanation for the lack o f

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Fig 7 A Spreading takes place in the Argo Abyssal Plato terminating to the southwest at a triple junction near the I~xmouth Plateau The thud or "faded arm" of this triple junction sy stem mRlated rifting landward of the Exmouth Plateau m the Rankm Platform, Dampler sub-basra area B Spreading takes place m the Wharton and Perth Basins assomated w]th the general Gondwanaland fragmentation

113 break-up and continental drift landward o f the Exmouth Plateau Instead, this area was rifted by the third, and ultmaately, "faded arm" o f this rift system Such a rift could have had a negligible amount o f openmg, such as the Benue Raft or Eastern African Raft system that both die out along strike as the rotataonal pole for such a system as approached By thas hypothesas, the Late Jurassm sedaments that yield most o f the oil and gas o f the Dampler sub-basra [15] were d e p o m e d m this raft system The Early Cretaceous p a m a l unconformity seen an the Egret and Lambert wells [14] that are located m the Dampaer subbasin next to the landward fault scarp of the Rankin Platform correlates wath the change from thas rift system to the Early Cretaceous rlftmg pattern (Fag 7B), while the Barremlan to Hauterlwan "major regional marine transgressaon" [14] Is the result o f break-up and spreading m the Cuvier and Perth Basins that commenced at M-10 time Thus, the Exmouth Plateau was subjected to two, non-synchronous phases of rifting The inner, or Rankm Platform raft, occurred an the Late Jurassac in assocaatlon with rlftmg and subsequent sea-floor spreadmg to the north, whale the outer, or Wharton Basin rlft, initiated 35 m y later in the Early Cretaceous, as a part of the general Gondwanaland fragmentation

4 Conclusions (1) The Cuvier Basin and adjacent Wharton Basra are generally floored with oceanic crust (2) The velocity structure o f thas area can be characterized as follows a thin ( ~ 3 0 0 m) layer o f pelagm sediments overhes the entire area that as underlain by a significant ( ~ 3 0 0 - 4 0 0 m) accumulation of higher velomty, Early Cretaceous turbldltes m the eastern Cuvier Basin Layer 2 ranges from 5 3 to 6 3 km/s whale layer 3 ranges from 6 7 to 6 9 km/s with total layer 2 plus layer 3 thmknesses ranging from 6 1 to 8 1 km Mantle velocities range from 7 9 to 8 1 km/s at a total depth range o f 12 1 - 1 4 2 km (3) Varmtmns in the velocmes and thicknesses o f the crustal layers are found in the western Cuvier Basin which is assocmted with the flank o f an abandoned spreading ridge (4) A d d m o n a l magnetic anomaly profiles m the eastern Cuvier Basin are used to correlate the lmea-

tlons in that area with Early Cretaceous reversals M-5 to M-10 (5) The above correlation dates the onset of plate separation at 1 2 0 - 1 2 5 m y at a half-rate of 3 3 cm/yr, essentially contemporaneous with the Perth Basin to the south (6) A 220-km gap exists between M-4 and M-5 in the Cuvier Basin that suggests a ridge lump of that magnitude occurred nominally at 118 m y (7) Early Cretaceous llneattons also occur northwest o f the Exmouth Plateau, offset front the Cuvier Basin sequence by less than 100 km (8) These hneatlons suggest that the Exmouth Plateau was involved in the Early Cretaceous phase o f rifting (9) We conclude that the rifting history of the Exmouth Plateau consisted of an inner, or Rankln Platform rift, that occurred in the Late Jurassic m association with rafting and sea-floor spreading to the north, whde the outer, or Wharton Basra raft, Into. ated 35 m y later in the Early Cretaceous as a part of the general Gondwanaland fragmentation

Acknowledgements This research was supported by National Science Foundatxon grant NSF OCE 76 01434, Office of Naval Research Contract N00014-75-C-0210 Scope W, and a special grant from the Australian Bureau of Mineral Resources all to the Lamont-Doherty Geological Observatory We especially thank L C Noakes and other personnel of the Bureau of Mineral Resources for logistical support regarding explosives and David Holland o f Lamont who served as a special dectronlcs technician at sea Lamont-Doherty Geological Observatory Contribution No 2839

References 1 R L Larson, Early Cretaceous breakup of Gondwanaland offWestern Austraha, Geology 5 (1977) 57-60 2 R G Markl, Evidence for the breakup of eastern Gondwanaland by the Early Cretaceous, Nature 251 (1974) 196-200 3 M Talwanl, J Mutter, R Houtz and M Komg, The crustal structure and evolution of the area underlying the magnetm qmet zone on the margin south of Austraha, m

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