Observed magnetic anomaly


Computed magnetic anomaly upwelling basaltic magma

Fig. 5.4. The formation of linear magnetic anomalies at seafloor spreading centres, showing a comparison between the observed magnetic anomaly and a theoretical computed magnetic anomaly based on reversals in magnetization of the oceanic crust.

upwelling basaltic magma

Fig. 5.4. The formation of linear magnetic anomalies at seafloor spreading centres, showing a comparison between the observed magnetic anomaly and a theoretical computed magnetic anomaly based on reversals in magnetization of the oceanic crust.

of distinctive magnetic anomaly sequences from their shape and hence the age of the crust giving rise to them. Marine magnetic surveys, particularly by U.S.N.S. Eltanin in the late 1960s and early 1970s, have enabled the ages of a large portion of the oceanic crust of this region to be ascertained. The basic pattern of seafloor spreading in the area south of 45°S between South Australia and South America has been defined and described by many authors including Pitman et al. (1968), Herron and Hayes (1969), Herron (1971, 1972), Christoffel and Falconer (1972), Weissel and Hayes (1972), Hayes and Ringis (1973), Molnar et al. (1975), Handschumacher (1976), Herron and Tucholke (1976), Weissel et al. (1977), Cande et al.

(1982) and Stock and Molnar (1987). However, gaps in data exist, especially on the Antarctic plate north of Ross Sea and Marie Byrd Land.

Mesozoic Spreading

Although the Pacific margin of Gondwana has been a convergent oceanic-continental plate boundary for a large portion of time since the Precambrian (570 Ma B.P.) (Scotese, 1987), the present oceanic lithosphere of the Pacific Basin is largely 150 Ma or younger in age. To define the Early-Middle Cretaceous (90-135 Ma B.P.) development of the southern Pacific Ocean, the Early Cretaceous spreading patterns in the north and western Pacific (Larson and Chase, 1972) must be studied. As no magnetic anomalies of that age occur in the south, these Mesozoic spreading systems must have evolved into the later Cretaceous-Cenozoic system in the southern and eastern Pacific but details of this evolution are not obvious because of the lack of magnetic reversals recording the spreading pattern from 110-85 Ma B.P. (the Cretaceous Magnetic Quiet Zone). This was also a period of very rapid spreading at all ridges. The Mesozoic spreading pattern is interpreted in terms of a four plate system (Fig. 5.5), the Kula, Farallon, Phoenix and Pacific plates with two triple junctions (T, Fig. 5.5), one at the northern and one at the southern end of the Pacific-Farallon spreading centre. These triple junctions migrated north and south rapidly as a result of jumps in ridge (spreading centre) positions. The formation of new oceanic crust caused the southern spreading centres to migrate to the southeast and rotate anticlockwise to align with their Cenozoic counterparts. The southeastern ridge (Farallon/Phoenix) jumped to the west between 100 and 120 Ma B.P. to give the observed offset to the southwest of the Pacific/Farallon plate boundary (T1 to T2, Fig. 5.5). The Phoenix and Farallon plates were being continually subducted under South America and Antarctica with the Phoenix/Farallon spreading centre also being subducted, presumably at the South American margin. The Farallon plate broke up during the late Tertiary and the Nazca plate (Fig. 5.6) is the remaining fragment in the southern Pacific Ocean. The Aluk plate (Fig. 5.6) is another name for the last fragment of the Phoenix plate. As the Pacific-Phoenix spreading centre migrated southwards during the Cretaceous, possibly terminated to the west by the Eltanin Fracture Zone (EFZ, Fig. 5.5), it linked up with the Antarctic-New Zealand spreading centre which commenced spreading in the Late Cretaceous, about 85 Ma ago (Christoffel and Falconer, 1972).

Late Cretaceous-Cenozoic Spreading

Details of the initiation of the Late Cretaceous-Cenozoic (85 Ma to present) spreading episode are uncertain. If the Pacific-Phoenix spreading centre extended to the west of the Eltanin Fracture Zone (Fig. 5.5), subduction of the Phoenix plate would have occurred along the northern margin of the New Zealand block until this western portion of the spreading centre reached the subduction zone, at or just prior to anomaly 34 time (80 Ma), where it presumably aligned with the proto New Zealand-Antarctic rift zone (Larson et al., 1979). At this time,

Pacific Ocean Centre Magnet Shape NameAntarctic Australian Spreading Ridge

Fig. 5.5. The major plates in the Pacific Ocean and their development during the Cretaceous relative to a fixed Pacific plate (after Larson and Chase, 1972). T marks position of spreading ridge triple junctions. Anomaly ages (Ma) and position (dashed lines) are marked. EFZ = Eltanin Fracture Zone.

Eltanin Facture Zone

Fig. 5.5. The major plates in the Pacific Ocean and their development during the Cretaceous relative to a fixed Pacific plate (after Larson and Chase, 1972). T marks position of spreading ridge triple junctions. Anomaly ages (Ma) and position (dashed lines) are marked. EFZ = Eltanin Fracture Zone.

spreading ceased on this segment of the Pacific-Phoenix spreading centre which jumped southwards to between the New Zealand block and Antarctica, with the New Zealand block becoming incorporated into the Pacific plate and a major transcurrent boundary (1,000 km total offset (Larson et al., 1979)) lying along the Eltanin Fracture Zone. This boundary subsequently formed the southern boundary of the greater Antarctic Peninsula, with subduction occurring to the east but not the west (Fig. 5.6) (Barker, 1982).

The Late Cretaceous-Cenozoic development of the region is complex. Five spreading centres were operating for all or part of this period; the Pacific-Antarctic Ridge, the Chile (Antarctic-Nazca) Ridge, the Indian-Antarctic Ridge, the Central Tasman spreading centre and the Farallon-Phoenix (Aluk) Ridge (Fig. 5.6). Recent work (Stock and Molnar, 1987) indicates a possible sixth ridge,

Ocean Spreading Magnetic Anomaly Map
Fig. 5.6. The major plates in the Pacific Ocean and their development during the Cenozoic (after Weissel et al., 1977; Stock and Molnar, 1987). Magnetic anomaly number and ages in brackets shown at top of each diagram. (Polar Stereographic projection).

Bellingshausen-Antarctic Ridge, and an additional minor plate, the Bellingshausen plate, in the vicinity of Bellingshausen Sea (Fig. 5.6). The development of the present oceanic lithosphere on the Pacific Sector of the Antarctic plate commenced about 85 Ma ago with the onset of rifting of New Zealand (Pacific-Lord Howe plate(s)) from Australia and Antarctica (the Antarctic and Bellingshausen plates) along the spreading centres of the central Tasman Sea and the Pacific-Antarctic Ridge. Spreading continued along the existing Pacific-Phoenix and Farallon (Nazca)-Phoenix spreading centres. Very slow spreading was probably occurring between Australia and Antarctica (Cande and Mutter, 1982).

Southwestern Pacific Ocean

The Tasman Basin formed as a simple 2-plate spreading system (Weissel and Hayes, 1977). The anomaly lineations in the Tasman Sea are parallel to Lord Howe Rise but in the north are oblique to the east Australian margin where right lateral offsets in the spreading centre to the north are inferred (Fig. 5.7). The southern margin of the basin is a major fracture zone that separates the Tasman spreading episode from younger oceanic crust to the south. The part of this fracture zone to the east of the Tasman Ridge was a plate boundary between the Pacific plate and a "Lord Howe" plate in the Late Cretaceous to Palaeocene (Weissel et al., 1977).

About 55 Ma ago, spreading in the Tasman Sea ceased and was transferred to the Indo-Pacific Ridge south of Australia. This resulted in a rapid increase in movement of Australia away from Antarctica. At about this time, a plate boundary, largely transcurrent, developed through New Zealand separating the Pacific plate from the Indian plate and resulting in a triple junction south of New Zealand.

South of the triple junction, magnetic anomalies 1-18 (present to 45 Ma B.P.) mapped on the Antarctic plate were generated at the easternmost segment of the Indian-Antarctic Ridge (Fig. 5.7). Oceanic crust to the north of the Pacific-Antarctic plate boundary in this region was generated at the Pacific-Antarctic Ridge in the Late Cretaceous-Palaeocene (85-60 Ma). A change in crustal depth across this boundary reflects this difference in age of the oceanic crust. The trend of the older lineations in the south is NNE and intersects the Antarctic margin at a high angle suggesting that the northwestern Ross Sea margin was produced by strike slip action during the early stages of Antarctic- Australia rifting.

To the north of the triple junction, the Indian-Pacific plate boundary lies along the Macquarie Ridge complex. Here, the ENE-trending Cenozoic lineations on the Indian plate are apparently truncated at the present plate boundary.

Fig. 5.7. The southwestern Pacific Ocean showing the magnetic anomalies (solid line) in the Tasman Sea and the region of the Pacific-Indian-Antarctic triple junction (T). Fracture zones are marked by dotted lines, Fracture zone Z referred to in text. Plate boundaries marked by double line. 2,000 m bathymetry contours marked by dashed line. Anomaly numbers after Heirtzler et al. (1968) (Mercator projection).

Anomalies 5, 6, and 8 are shorter than their counterparts on the Antarctic plate indicating that part of the Indian plate younger than anomaly 10 is missing. It was either subducted at the Indian-Pacific plate boundary or the plate boundary jumped to the west. For the older lineations, anomalies 10 to 22, more oceanic crust is preserved on the Indian plate laying to the east of a fracture zone (Z in Fig. 5.7) than on the Antarctic plate. These Indian plate lineations are too far to the east to have been generated at the southeastern Indian Ridge and are therefore a remnant of one flank of a spreading system active when the Pacific-Indian boundary went through an extensional phase in the early to middle Tertiary. Corresponding lineations for anomalies 12 and 13 have been tentatively identified in Emerald Basin (Weissel et al., 1977).

Since these events, Australia and New Zealand have migrated steadily away from Antarctica with New Zealand undergoing steady deformation resulting from relative plate motion across the Indian-Pacific plate boundary which passes through New Zealand.

Southeastern Pacific Ocean

A major bight in the magnetic anomaly pattern of central southern Pacific Ocean indicates that a 3-plate system existed in Late Cretaceous time (Cande et al., 1982). The Pacific and Farallon plates were two of these, the third is unknown. Prior to anomaly 29 (65 Ma), the Antarctic plate apparently did not extend northeast of the Eltanin Fracture Zone, as no anomalies older than 29 occur on the Pacific plate in this region (Fig. 5.8). Any older crust northeast of the Eltanin Fracture Zone must have been subducted under West Antarctica. At anomaly 29 time, the Pacific-Phoenix spreading centre split into two slower spreading centres, a Pacific-Antarctic Ridge and a Phoenix (Aluk)-Antarctic Ridge, a split which has propagated eastwards with time reaching the Tula Fracture Zone about 64 Ma B.P. and the Pacific-Phoenix-Farallon triple junction at about 50 Ma B.P. Since then, it has propagated northwards splitting the Pacific-Farallon boundary and replacing the Antarctic-Pacific-Phoenix and Farallon-Pacific-Phoenix triple junctions by the Antarctic-Pacific-Farallon and Farallon-Antarctic-Phoenix triple junctions (Fig. 5.6) (Cande et al., 1982). Spreading between the Antarctic and Bellingshausen plates may have ceased by 51 Ma B.P. but had definitely stopped by 43 Ma B.P. when the two plates became a single (Antarctic) plate (Stock and Molnar, 1987). At anomaly 24 time (56 Ma B.P.), the spreading rates increased significantly across the Farallon-Pacific plate boundary and highly asymmetric spreading must have occurred across the Farallon-Antarctic boundary or an unknown plate, which has since been subducted under South America, must have existed to the east, between the Farallon and Antarctic plates.

The Farallon plate broke into three smaller plates (Nazca, Cocos and Gorda plates, the latter two occurring to the north) between the formation of anomalies 6 and 7 (27-21 Ma B.P.) and was caused by the impact in the north Pacific of the Pacific-Farallon spreading centre with the American plate (Handschumacher, 1976). This also resulted in the spreading centre to the north of the Antarctic plate rotating from a NNE direction to a north-south alignment. Northward jumps of the Pacific-Nazca-Antarctic triple junction have occurred several times during the past 20 Ma.

The northeast trending anomalies off west Antarctic become younger towards the Antarctic coast indicating that the other half of the oceanic crust formed at the

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