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42. South Tasman Basin and Borderlands: a Geophysical Summary R 42. SOUTH TASMAN BASIN AND BORDERLANDS: A GEOPHYSICAL SUMMARY R. E. Houtz, Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York ABSTRACT New profiler and sonobuoy data are presented to supplement the existing geophysical data in the Tasman area. The tectonically dis- turbed western edge of the Campbell Plateau extends southward as a deep-sea fracture zone that separates 80 m.y. old sea floor to the east from the 30 m.y. old Emerald Basin (Site 278). A 600-meter regional depth change occurs at the boundary. The effectiveness of bottom currents on sedimentation is shown on the profiler sections from the Campbell Plateau and the western Tasman Sea. A possible marginal rift, now buried, can be inferred from profiler data taken from near Sites 275 and 276. A widespread reflector on the plateau has been identified as being in the region of the Cenozoic/Mesozoic boun- dary. A normally faulted erosion surface on the South Tasman Rise corresponds with the Oligocene-late Eocene unconformity. A large east-west graben between the South Tasman Rise and Tasmania may represent an earlier rift that predates the rifted east- west margin drilled south of the rise (Site 280). Sonobuoy refraction data reveal a somewhat atypical structure section in the Central Tasman, where a 7.1- to 7.4-km/sec layer at a depth of 7.5 km supplants the typical 6.8-km/sec oceanic layer. INTRODUCTION complex structural relationships in these areas, but they certainly will provide useful constraints on further The most recent, complete summary of geophysical speculations. The data in Figure 2 show that most of results in the Tasman area is Volume 19, Antarctic the Leg 29 drill sites were located in areas where the age Research Series (Hayes, 1972). More recently new infor- of the sea floor is in doubt, especially at the Macquarie mation has become available such as the identification Ridge, and in the Emerald Basin. Similarly, the sites of magnetic anomalies that show an early opening of the west and south of Tasmania are located in areas where Tasman Sea (Hayes and Ringis, 1973), a sediment the sea floor is of an unknown age. isopach map of the area (Houtz et al., 1973) and the The New Zealand Plateau separated from Antarctica availability of previously unpublished sonobuoy and Australia about 80 m.y. ago (Hayes and Ringis, solutions and profiler data from a few selected ship 1973). The separation from Antarctica has continued up tracks. This summary utilizes all this information in the to the present time, but the separation of the Tasman discussions to follow. Sea Basin ceased about 60 m.y. ago. At that time, or The generalized bathymetry and geographic place slightly later, Australia detached from Antarctica; a names of the areas discussed appear in Figure 1, as well separation that has continued up to the present time. as marked profiler sections, and sonobuoy stations. The The two stages of separation have produced a boundary more reliable sonobuoy solutions from the regions are (shown as a heavy dashed line on Figure 2) in the listed in Table 1. Tasman Sea (Hayes and Ringis, 1973). Across this boundary an age difference of at least 30 m.y. exists. An BRIEF OUTLINE OF REGIONAL even larger time gap of about 50 m.y. exists between Site PLATE MOTIONS 278 and sea floor to the east. This requires a similar The history of plate motions in the Tasman Sea area boundary or fracture zone (Figure 2). can be inferred from the magnetic lineations mapped in Figure 2, which are the work of Pitman, Larsen, and SEDIMENT DISTRIBUTION Herron (in preparation). The separation of Australia Sediment distribution in the region of the Leg 29 drill from Antarctica was accompanied by extensive sites appears in Figure 3. Contours represent reflection transform faulting just to the west of Tasmania. The time in tenths of seconds to 'basement' whether con- faulting largely destroyed the magnetic record, and ap- tinental (e.g., Campbell Plateau) or oceanic basement. parently obscured the position of the mid-ocean ridge Due to the increase of velocity with depth in these to the south of this region. These factors reduce the sediments, a reflection time of 0.5 sec yields a thickness reliability of the reconstruction in the region of the of 0.44 km, whereas a reflection time of 3.0 sec would in- South Tasman Rise as developed by Weissel and Hayes dicate a thickness of 5.07 km (see fig. 2 and eq. 2 in (1972). The lack of coherent magnetic lineations from Houtz, Chapter 41, this volume). Apparently a 6-fold in- the Emerald Basin also leads to doubtful interpretations crease in reflection time would indicate nearly a 12-fold and reconstructions. Drilling results may not solve the increase in sediment thickness in this region. 1135 R. E. HOUTZ \ \ / J \ \ ^v*\ ^ \\ // / ^ \ / 60 \\ \ / 120 —T Vx , \ \ X \ / D / 45X4U&:'• ' • V— \ \ \ ^ ^r \ .38 46. \ \ ^ K \ β /35 \ ^^ rM^«<y25~~áδv \ rl • J// \ .. •'•&l\\ / "37 ^ jß/: • • • \ ^^"^^) / \ fJv ,33 y\ / >\> ‰ \ \ BATEAU \ //^^v •34 ^\ JB:• • • 27X31 50 L,^—^T 14h * TT^TS \ SlU \ _ ^\\ \ TASMANIA^ / c IL-^W ™ ‰^-. • \* (J V‰L| yf^••••'.'" MELBOURNE ;v1 130 Λ W?y..:.. > > \ j^-^ ?SP"::-:•• SONOBUOY STATIONS 0-40E52 11-19E43 22-28E53 32-36E53 42-41E53 pv . • 1-41E52 12-14E43 23-29E53 33-38E53 43-46E53 —^#K///ZiR W E Lf L1 N G T OM N^V- I?:-: • 2-43E52 13-12E43 24-33E52 34-37E53 44-47E53 l•• . 3-42E52 14-1E37 25-32E53 35-6E41 45-48E53 40 V\J\ \ Er: : •• ' 4-21E43 15-5E40 26-31E53 36-1E39 46-52E53 K • • . 5-22E53 16-1E53 27-14E37 37-5E41 47-51E53 6-23E43 17-4E53 28-15E37 38-39E53 48-54E53 7-15E43 18-.6E53 29-16E37 39-43E53 49-55E53 ^ \ΛN llv• •• • 8-16E43 19-8E53 30-13E37 40-44E53 50-56E53 /^T%L\liriri AIΛ VV\ lv.: 9-18E43 20-9E53 31-35E53 41-42E53 51-57E53 ^^ . 10-20E43 21-1E42 170 160 150 140 Figure 1. Generalized bathymetry of the South Tasman borderlands. Profiler sections are discussed in the text, and all sono- buoy stations are shown. Emerald Basin between 50°S, 150°E, and the southern tip of South The isopach map shows the thick accumulation of Island (Figure 3). The thicker sediments were laid down sediments within the Emerald Basin. These sediments on a sea floor that is as much as 30 m.y. older than that are densely stratified and for the most part, have a gen- to the south. The sediment distribution clearly reflects tle dip. Submarine canyons are prominent, so the the early opening of the Tasman Sea (Hayes and Ringis, sediments are at least partially composed of terrigenous 1973). This is enhanced by the prevalence of pelagic turbidites. A typical profiler section of these sediments deposition, whose continuity would have been disrupted appears in Section A-A', of Figure 4. Sediments that are if turbidite deposition has occurred. layered less densely have significant accumulations west Turbidites are rather scarce in this part of the world, and southeast of Macquarie Island. Reflectors in these and are confined to a narrow strip along-shore and west sediments can be absent (Section B-B', Figure 4), or of Tasmania, in the Emerald Basin, and in a small delicate and variable (Section C-C, Figure 4). The layer- abyssal plain in the central Tasman Sea. Elsewhere, ing tends to be nonhorizontal, and is occasionally pelagic sediments, modified by bottom current activity, parallel to basement. These isolated patches of thick are the predominant deep-sea sediment type. A region of (pelagic) sediment are probably the result of deposition pelagic deep-sea deposits, unaffected by bottom of very fine materials under the influence of bottom currents, is present about 300 km south of the South currents. Past changes in the speed or direction of the Tasman Rise. Profiler sections from this region (Figure currents near the Macquarie Ridge may account for the 5) show the remarkable parallelism of the sea floor and unconformities, and erosional outcrops seen near Site sediment section upon the rough basement surface 278 (Section C-C'1 in Figure 4). (morphologically an east-west ridge); however, the parallelism abruptly disappears (near the arrows shown in Figure 5). Although major changes in the sedimenta- Tasman Basin tion and current regimes must have occurred as the con- In the Tasman Basin, west of the Macquarie Ridge, a tinents separated, this region has clearly remained pronounced sediment thickening occurs north of a line isolated from significant bottom current activity. Any 1136 TABLE 1 Sonobuoy Solutions in Vicinity of Leg 29 Drill Sites km km/sec Sono- buoy V A V t LatS LongW 4E44 3.52 0.75 0.40 0.57 0.98 1.69 2.12 3.06* (4.00) (4.71) 50 00.1' 164 43.4' Solander Trough 7E42 4.64 0.46 0.22 1.82 1.74 (4.60)* 54°08.5' 163°05.2' 9E42 4.66 0.27 0.36 0.30 1.97* 3.06 3.05* 54°25.4' 163°05.0' Emerald Basin 10E42 4.71 0.46 0.40 0.91 1.82 2.38 2.90 54°33.6' 163°02.6' 14E42 5.35 0.37 1.42 (d-8)) (5.49) (6.67) 57°31.0' 170°30.2' 15E42 5.40 0.23 1.23 (d 8)) (5.38) (6.85) 57°31.3' 170°40.6' 16E42 5.22 0.50 1.53 1.94 (5.40) (6.62) 57°32.4' 170°00.5' South Pacific 9E43 4.91 0.95 1.82 2.41 (5.82) (6.61) 51°38.0' 178°27.2' 10E43 4.83 1.34 1.64 (5.41) 51°12.0' 177°32.9' 11E43 4.50 1.02 1.04 0.63 2.34 (5.60) (6.57) (7.17) 50°51.1' 176°54.5' 31E53 1.56 0.35 1.36 1.66 (5.34) (6.61) 47°58.1' 147°48.4' 32E53 1.61 0.31 0.82 0.78 1.11 1.42 1.60 2.56 3.28 (4.85) (5.80) (6.86) 47°58.8' 148°02.3' South Tasman 60 33E53 1.62 0.84 2.21 (5.69) 47°59.5' 148°45.2' Rise (continental) > 34E53 1.45 0.27 0.57 (d 8)) (4.70) (5.48) 47°55.1' 148°32.8' 27E53 4.20 0.53 (d 8)) (5.12) 51°09.8' 147°57.2' > 28E53 4.24 0.50 0.65 1.04 1.69 3.13 4.43 51°09.1' 147°40.6' σ Southeast Indian w 29E53 4.03 0.62 2.26 1.95 ((5.00)) 5.95 51°23.0' 147°49.7' o 30E53 4.14 1.05 1.94 2.67 (5.50) (6.23) 49° 04.7' 148°20.8' α 1E53 5.00 0.67 2.11 1.86 (5.15) (7.10) 43°07.0' 160°03.2' w 2E53 4.94 1.09 1.50 2.60 (4.65) (7.05) 43°09.6' 159°50.1' 4E53 4.93 0.37 0.46 43°19.6' 1.84 1.99 (4.90) Central Tasman 6E53 4.87 0.63 2.04 1.75 (5.19) (7.37) 44°02.5' 154°26.3' σ 8E53 4.71 0.52 0.40 1.73 ((2.20)) (5.50) 43°44.1' 154°04.3' 9E53 4.47 1.04 2.15 2.05 (4.80)* (6.20) 43°11.2' 155°13.5' > 2 2 a Note: * = poorly determined values; () = refraction velocities; (()) = assumed velocities; unbracketed velocities based on T /X data.
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