23. SOUTHWEST PACIFIC CENOZOIC PALEOGEOGRAPHY and an INTEGRATED NEOGENE PALEOCIRCULATION MODEL Anthony R
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23. SOUTHWEST PACIFIC CENOZOIC PALEOGEOGRAPHY AND AN INTEGRATED NEOGENE PALEOCIRCULATION MODEL Anthony R. Edwards, New Zealand Geological Survey, Lower Hutt, New Zealand INTRODUCTION New Zealand region were loosely attached to Antarc- tica. A broad Indo-Pacific deep-sea passage separated Our knowledge of the geological history of the northern Australasia (including New Guinea and southwest Pacific has recently been greatly expanded by Timor) from most of the Indonesian region. This deep-sea drilling, detailed sea-floor magnetic, and near- passage continued to widen throughout the Late shore petroleum exploration studies. Although this in- Cretaceous and into the earliest part of the Paleogene. formation will take a considerable time to assimilate, it Concurrently the New Zealand region separated from is now possible, providing several major assumptions the Antarctica-Australia continent. Details of this latter are made, to arrive at fair approximations of the development are still far from clear, but the first major Cenozoic paleogeography of the southwest Pacific and event appears to have been the splitting off from to develop a Neogene paleocirculation model. Such Australia of the Norfolk Ridge and consequent growth reconstructions can greatly assist our understanding of of the New Caledonia Basin. Then followed the con- the numerous geographic and stratigraphic variations current separation of the Lord Howe Rise from east evident in the lithologies and fossil contents of ancient Australia, and of the large Campbell Plateau from near sediments. the present Ross Sea area of Antarctica. These latter The inferred paleogeographies, given in terms of the events resulted in the growth probably as continuous ever-changing positions of continental blocks and their features, of the ancestral southwest Pacific, Pacific- coastlines, are heavily dependent not only on published Antarctic, and Tasman basins. geophysical and geological studies, but also on major In the mid Paleocene a major change occurred. The assumptions such as the presumed fixed Cenozoic previous tectonic trends appear to have been abruptly positions of Antarctica and most of Indonesia. The terminated and replaced by a pattern recognizably postulated paleocirculation model, given in terms of the similar to the present regime. The bonds previously inferred positions of the hydrological boundaries loosely attaching Australia to Antarctica were broken, separating major surface water masses, is dependent and Australia plus the now interlocked western part of upon the paleogeographies. The names adopted for the the New Zealand subcontinent commenced to move water masses and their boundaries are based on their northward. Eastern New Zealand, including the large assumed ancestral relationships to the present hydro- Campbell Plateau, also moved northward, but possibly logical features. Except in a very general sense, these accompanied by substantial concurrent counterclock- names are not indicative of climatic zones because sub- wise rotation. Major results of this new pattern were the sequent changes brought about by changes in paleo- slow growth of the now broad Australia-Antarctica geography must be taken into account. deep-sea passage, the continued slow growth, probably The maps presented in this report (Figures 1 to 11) are with a modified trend (see above), of the New Zealand- based on an oblique equidistant azimuthal projection Antarctica deep-sea passage, and the gradual construc- centered at 41°19'S, 174°46'E (New Zealand Mapping tion of the Indo-Pacific passage. The latter trend even- Service Map 47, 3rd ed., 1973). This unusual projection tually resulted in the collision of northern Australasia was selected for two reasons: first, there is little visual with most of Indonesia about mid Miocene time. distortion over a wide range of latitude and longitude, Numerous other as yet poorly understood developments and second, it fortuitously allows the Cenozoic paleo- occurred as a result of this major change in tectonic positions of Australasia to be reconstructed without tak- pattern. Early events include the growth of the South ing into account major changes in apparent size and Fiji Basin about Eocene or Oligocene time, of the Coral shape. The map base has been modified by adding both Sea Basin about early Eocene time, of the New Hebrides the areas underlain by continental and "island arc" Basin about mid Eocene time, and of the Emerald Basin crust (stipple pattern), and the positions of the high- in Oligocene time. Growth of the Emerald Basin standing mid-oceanic ridge crests (mottled pattern). The appears to have ceased when the Macquarie Ridge came boundary between continental and oceanic crust has into being in earliest Miocene time. Subsequent events been arbitrarily taken at either the base of the continen- include the still actively continuing very complex disrup- tal slope or, where such a slope is not prominent, at the tion of most of the northern and eastern rims of the 3000-meter isobath. southwest Pacific marginal seas. These essentially late Neogene disruptions have resulted in the formation of PALEOGEOGRAPHY many basins, notably the Pandora Basin (often called the Fiji Plateau), and the narrow but lengthy Lau and Paleopositions of Continental and Oceanic Crust Harve basins. The latter basins appear to have resulted from the separation of the Tonga and Kermadec ridges About Mid Cretaceous time Antarctica was about 15° from the Lau and Colville ridges. from its present position. Australia and what is now the 667 A. R. EDWARDS Figure 1. Present-day southwest Pacific bottom-water circulation pattern. 668 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL / ' 11111111 Illi Illl π•ii•jF•ii•iIB4,ailai,B|| s V: 206 "T-i• Af/d Tasman HII> w fl|1"" Convergence Mergence „„ 282 C°°V ' ».». m .„ 2£ fcr^1 - Figure 2. Present-day southwest Pacific surface-water circulation pattern (winter situation). 669 A. R. EDWARDS Figure 3. Late Miocene (7 m.y.) and early Pliocene southwest Pacific paleogeography and surface-water circulation pattern (winter situation). 670 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL Figure 4. Mid Miocene (13 m.y.) southwest Pacific paleogeography and surface-water circulation pattern (win ter situation). 671 A. R. EDWARDS Figure 5. Early Miocene (21 m.y.) southwest Pacific paleogeography and surface-water circulation pattern (winter situation). 672 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL Figure 6. Late (and mid) Oligocene (27 m.y.) southwest Pacific paleogeography and surface-water circulation pattern (winter situation). 673 A. R. EDWARDS y >^ A Figure 7. Early Oligocene (35 m.y.) southwest Pacific paleogeography and equatorial surface-water circulation pattern. 674 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL Figure 8. Late Eocene (42 m.y.) southwest Pacific paleogeography and equatorial surface-water circulation pattern. 675 A. R. EDWARDS Figure 9. Mid Eocene (47 m.y.) southwest Pacific paleogeography and equatorial surface-water circulation pattern. 676 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL Figure 10. Early Eocene (52 m.y.) southwest Pacific paleogeography. 611 A. R. EDWARDS Figure 11. Mid Paleocene (58 m.y.) southwest Pacific paleogeography. 678 CENOZOIC PALEOGEOGRAPHY AND NEOGENE PALEOCIRCULATION MODEL The above account is, with one exception, based on Coastline Paleopositions the information presented in a wide variety of The coastlines given in Figures 3 to 11 were modified geophysical and geological papers (see Additional from their present positions by compilation of the data Selected References under the subheading Paleo- and opinions in the papers listed in the Additional geography Continental and Oceanic Crust Paleoposi- Selected References under the subheading Paleo- tions). The exception relates to the formation of the geography (Coastline Paleopositions). The ages given in New Caledonia Basin which, the writer suggests, oc- these papers were usually accepted at face value. How- curred shortly prior to the formation and growth of the ever, wherever possible updated age assignments were Tasman Basin (80 m.y. to 60 m.y., Hayes and Ringis, made by utilizing the correlation data given in Horni- 1973). This inference is essentially based on the New brook and Edwards (1971) and Ludbrook and Unsay Caledonia Basin having a known minimum age of early (1967). Even so the coastline positions given are coarse Paleocene (ca 62 m.y.) at a sampling point which, approximations to the actual situation for two reasons. although 734 meters subbottom, is probably a con- First, they are based on correlations with subdivisions of siderable distance above basement (Burns, Andrews, et epochs, whereas the positions of the continental crust al., 1973). Thus, it is possible that the adjacent similarly areas are based on correlations with the "absolute" time trending New Caledonia and Tasman basins represent scale. Second, the accuracy of the correlations varies sequential expressions of the same tectonic trend. greatly. The information on which the above generalized ac- The coastlines of Antarctica, the Gulf of Carpentaria count of the tectonic history of the southwest Pacific is (northern Australia), New Caledonia, and most of the based can be converted into the basis for a series of Indonesia-Malyasia-Philippines region have not been paleogeographic maps provided several assumptions are modified from their present positions due to a lack of in- made. The paleogeographies given