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Migration of a pattern of plate motion
Earth and Planetary Science Letters, 21 (1974) 400-404
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©North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
MIGRATION
OF A PATTERN OF PLATE MOTION CARL BOWIN
Woods Hole Oceanographic Institution, Woods Hole, Mass. {USA)
Received November 12, 1973 Revised version received January 10, 1974 A pattern of plate motion is documented to have migrated from the South Pacific into the Indian Ocean during the Cretaceous and Early Tertiary, and then across the Indian Ocean during the Tertiary. The opening of the Gulf of Aden may be a more recent extension of this migration. The migration takes place by the episodic formation of new segments of sea-floor spreading having the new direction of plate motion; or by development of the new direction of motion in segments where sea-floor spreading had previously been active. In this manner plate motions can extend beyond their previously limiting plate boundaries.
1. Introduction The geometry of plates and their kinematics provides only indirect clues to the driving mechanism(s) of sea-floor spreading and plate tectonics. However, this kinematic information is presently the most indicative of what actual tectonic activity the earth's surface has undergone. Knowledge of this tectonic activity forms important constraints upon postulates of the processes active at depth. A corollary of the plate tectonics hypothesis requires that the total of instantaneous plate motions over the globe be such that there is no net change in the surface area of the earth. Attempts at making global inventories of the present plate motion under that assumption have had remarkable success in the accommodation of a variety of data sets [ 1]. Global patterns of sea-floor spreading change with time, but some patterns of plate motion appear to be relatively stable for a hundred million years or more (such as in the north and south Atlantic and eastern Pacific) while other patterns appear to be much less stable (such as in the Tasman [2], Labrador [3, 4], or Scotia Seas [5]. How the more and less stable patterns interact to provide a global consistency, and how a stable pattern may be changed is poorly defined. The purpose of this paper is to demonstrate that a pattern of plate motion in the southern Pacific migrated westward into the Indian Ocean during the
Late Cretaceous and Early Tertiary and then across the Indian Ocean during the Tertiary. The opening of the Gulf of Aden appears to be a more recent extension of this migration. The migration takes place by the episodic formation of new segments of sea-floor spreading, having the new direction of motion; or by development of the new direction of motion in segments where spreading had previously been active. It is not manifested at the surface as a continuous migration process as would be, for example, the longitudinal growth of a crack. In the wake of the migration, secondary adjustments in directions of plate motion appear to occur. For convenience of discussion, and for lack of more precise information, trends are given in terms of their orientation at the present time, but in the past they may have been much different. Ages of magnetic anomalies and geologic time periods as presented by Berggren [6] and Van Hinte [7] have been adopted for consistency. In summary, the Early CretaceousLate Cretaceous boundary occurs at 100 m.y. ago, the Cretaceous-Paleocene boundary occurs at 65 m.y. ago between magnetic anomalies 30 and 29, the Paleocene-Eocene boundary occurs about 53.6 m.y. ago between anomalies 23 and 22, and the E o c e n e Oligocene boundary occurs about 37.6 m.y. ago near anomaly 15. These ages differ from those presented by Heirtzler et al. [8].
MIGRATION OF A PATTERN OF PLATE MOTION
401 Eltanin Fracture Zone. Southwest of the Eltanin Fracture Zone, in the area southeast of New Zealand, magnetic anomaly 32 lies~rather close to the eastern escarpment of the New Zealand Plateau, and hence has been spreading since only a little more than 68 m.y. ago [ 1 0 - 1 2 ] . Anomaly 32 is at a greater distance from the edge of the New Zealand Plateau to the north than along the southern part of the Plateau. Hence the northern part (labeled 2a in Fig. 1) probably separated before the southern part (labeled 2b in Fig.l). At about the same time as the New Zealand Plateau and Antarctica appear to have started separating, the
2. Discussion The oldest oceanic crust along segments of the Pacific-Antarctic Ridge appears to show a progression in age from east of the Tonga Trench to the southeast branch in the Indian Ocean Ridge (Fig. 1). East of the trench there is considerable stretch of ocean floor between the trench and magnetic anomaly 32 (about 68 m.y. ago), and thus that portion of the sea floor was presumably created by sea-floor spreading much older than 70 m.y. ago [9]. The time of initiation of spreading is not known. This region is northeast of the I
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Fig.1. Diagram illustrating migration of a pattern of plate motion from the South Pacific to the western Indian Ocean. Active spreading centers are identified by double lines, extinct spreading centers by double lines with cross-hatching, and inferred extinct spreading centers have dashed double lines with cross-hatching. The cross-hatched extinct spreading center without bordering lines shown in northern Indonesia represents diagrammatically the spreading center that was offset by the Ninetyeast transform fault but has been lost through subduction beneath the Indonesian island arc. Areas with stippled pattern are oceanic crust that w~s formed by a former pattern of plate motion in the northern Indian Ocean. Major transform faults are indicated by heavy single lines, others are not shown but arrows indicate direction of spreading parallel to intraplate transform faults. Dotted lines show location of magnetic anomaly 32. Intermediate weight lines outline the approximate boundaries of oceanic crust formed during the westward migration of the proposed pattern of plate motion. The location of the boundary east of the Tonga Trench is uncertain. Numbers 1 through 5 refer to the order in which it is suggested that segments of these boundaries formed during the migration of the spreading center from the South Pacific to the Gulf of Aden. The (a) portions appear to have formed prior to the (b) portions. The dashed lines in the South Pacific outline approximate boundaries of crust formed following a younger adjustment in direction of plate motion. Sourceg of data are given in the text with exception of that for the stippled area which is from McKenzie and Sclater [14]. DSDP information [15-17], Bowin [18], and magnetic anomaly identifications in the Wharton Basin by J.G. Sclater and R.L. Fisher (personal communication, July, 1973).
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402 Tasman Sea began to open [2]. The initiation of spreading in the Tasman Sea is probably another indication of a reorganization of global spreading directions at that time. Spreading in the Tasman Sea ceased shortly after the time of magnetic anomaly 25 (59 m.y. ago). To the west of New Zealand Plateau and Macquarie Ridge the oldest magnetic anomalies identified with an east-west trend are 21 (?) at the southern limit of the Tasman Sea and 22 south of central Australia [3]. Thus about 5 9 - 5 5 m.y. ago Australia and Antarctica began to separate. This separation took place after the opening between the New Zealand Plateau and Antarctica occurred, and that opening, in turn, occurred after spreading had been active in the region west of the Tonga Trench. This evidence strongly suggests that a pattern of sea-floor spreading in the South Pacific migrated westward with time. Although the origin of Broken Ridge and the tectonic history of the south end of the Ninetyeast Ridge remains much in doubt, the oldest magnetic anomalies so far identified along the southeast branch of the Indian Ocean Ridge that are associated with the present spreading direction also become younger towards the west. South of westernmost Australia the oldest identified anomaly is 19 (44 m.y.), south of the west end of the Naturaliste Plateau it is anomaly 18 (43 m.y.), and south of Broken Ridge it is anomaly 17 (41 m.y.) [13, 14]. For the region of spreading just discussed between the New Zealand Plateau/Antarctica and the Broken Ridge/Kerguelan Plateau separations, the initiation of the new spreading pattern coincided with a new rifting apart of the lithosphere. Altough there had been active spreading in the Wharton Basin west of Australia prior to magnetic anomaly 17, it was about a center of spreading located near the north end of the Nine tyeast Ridge [15-18]. This spreading center now has largely been consumed by the Indonesian subduction zone. It is not yet resolved whether the spreading at the north end of the Ninetyeast Ridge transform fault ceased when spreading began south of Broken Ridge on the southeast Indian branch, or there was an overlap in time of their activity. I favor the former view. To the west of the south end of the Ninetyeast Ridge, however, the continued westward migration of the new pattern of plate motion has followed closely the general location of a former center of spreading
C. BOWIN (the previous direction of plate motion is indicated by arrows on the stippled crust of Fig. 1) but imposed upon it a new direction of motion. This change was to a direction more in accordance with that of the plate motion between Antarctica and Australia and appears to have similarly developed in segments that continued the migration of the new pattern westward across the Indian Ocean. Between longitude 90°E, and the Indian Ocean triple junction east of Madagascar, the present spreading direction is known to have commenced only sometime between magnetic anomaly 23 (55 m.y.) and anomaly 5 (9 m.y.) [14]. However, north of the triple junction, the spreading along the present Carlsberg Ridge was inferred by McKenzie and Sclater [14] to have commenced sometime after 35 m.y. ago (early Oligocene). This conclusion was supported by DSDP hole 238 [19], at the extreme northeast end of the Argo Fracture Zone, which suggests that the youngest date of possible separation of the Chagos/ Diego Garcia region from the Cargados Carajos/ Nazareth Bank is Early Oligocene (Rupelian, 33 m.y. ago). An early Oligocene age for initiation of the present spreading direction along the Carlsberg Ridge is compatible with a continuing westward migration with time of the plate motion direction related to the spreading center that separated Australia from Antarctica. The plate motion direction of the present Southeast Branch between 90°E longitude and the triple junction probably began either at the same time as that of the present Carlsberg Ridge spreading center, or, as here inferred more likely, a few million years earlier. Magnetic anomalies indicate that the Gulf of Aden has been undergoing sea-floor spreading at least since anomaly 5 (9 m.y.) [20]. Laughton [21] speculated that initial separation of Arabia and Somalia began about 20 m.y. ago. DSDP hole 231 [19] showed that the inception of marine sedimentation on the south edge of the Gulf of Aden occurred in Late Oligocene or Early Miocene time (about 23 m.y. ago). Prior to the beginning of opening of the Gulf of Aden, the Owen Fracture Zone may have been a transform fault along which a spreading center was offset to the region of an ancestral Persian Gulf, or perhaps it was the limiting boundary of the earlier northwestern Indian Ocean spreading pattern. The formation of the Gulf of Aden resulted from a new rifting apart of the
MIGRATIONOF A PATTERN OF PLATE MOTION lithosphere analogous to the earlier separation of New Zealand and Australia from Antarctica. The migration of a pattern of plate motion therefore presumably can either utilize pre-existing lithosphere accretionary boundaries or create new ones, presumably following a minimum work principle [4]. The spreading in the Gulf of Aden appears to be a young manifestation of the proposed westward migration of a pattern of plate motion that began in the South Pacific perhaps as much as 80 or more m.y. ago. As a major pattern of plate motion migrates away from a position along a new accretionary boundary, there is an increasing likelihood that secondary adjustments in spreading directions might occur. At times, and places, these adjustments are small and result in an apparent curvature to transform faults, such as south of Australia [13]. Other times the reorganisation is more pronounced, as in the eastern South Pacific [9] (Fig.l), where it seems that another major pattern of plate motion may be developing. There is some suggestion in the magnetic anomaly map of Pitman et al. [12] that, north of the Eltanin Fracture Zone, the adjustment may have begun earlier in the southern part of the East Pacific Rise and progressed northward with time. Several jumps in location of the spreading center are documented in the eastern South Pacific (Fig. 1 ; those in the northwestern Indian Ocean are only herein inferred as possible). The more recent change in direction of plate motion in the eastern South Pacific also may be related to the development of the Chile Ridge and Galapagos triple points. The present direction of motion in the northwestern Indian Ocean, between the Ninetyeast Ridge and the Owen Fracture Zone, is not very different from that on the Southeast Branch from south of Broken Ridge to south of Australia (Fig. 1). The probable poles of rotation for the relative motions in those two regions differ in location by only about 14 ° [22]. Within a 95% confidence ellipse, the poles of rotation for the present direction of relative motion for the northwestern Indian Ocean and the Gulf of Aden could be within 12° of each other [22[. The control of the Arabia/Africa rotation pole, however, is not well defined. The present poles of rotation for the Australia (India)/Antarctica spreading and the New Zealand (Pacific)/Antarctica spreading differ by 87 ° [22], a large difference. However, the directions of plate motion for these last two regions at the time of the proposed
403 migration of the plate motion pattern from south of New Zealand to south of Australia appear to have been similar, judging from the trends of the old magnetic anomalies and transform faults (see arrows for crustal sections 2b and 3a of Fig.l). The information pertaining to poles of rotation together with the dating of events is strong evidence for the proposed migration of a pattern of plate motion. From this study it is clear that plate motions have grown or extended beyond their previously limiting plate boundaries. Proposed explanations for the driving mechanisms must now also be able to acount for that observation. A similar migration of a pattern of plate motion appears to have occurred in the North Atlantic during the Mesozoic and earliest Tertiary ([23], Fig.2). The opening of the Labrador Sea [3, 4] and Bay of Biscay [24], also, may be analogous to the opening of the Tasman Sea.
3. Summary In summary, it is documented that a pattern of plate motion progressed westward from the South Pacific into the Indian Ocean during the Cretaceous and Early Tertiary, separating the New Zealand Plateau from Antarctica, and Australia from Antarctica, in the process. During the Tertiary, that pattern continued to migrate westward across the Indian Ocean, and the opening of the Gulf of Aden may be more recent extension of this migration. The advance of the pattern of plate motion appears to take place by the episodic establishment of segments having the new motion directions on either new or pre-existing rifts in the lithosphere.
Acknowledgements This work was supported by the U.S. Office of Naval Research. I thank J. Sclater, D. McKenzie and H.W. Menard for review comments.
References 1 X. Le Pichon, Sea-floor spreading and continental drift, J. Geophys. Res. 73 (1968) 3661.
404 2 D.E. Hayes, and J. Ringis, The early opening of the central Tasman Sea, Trans. Am. Geophys. Union 53 (1972)413 (abstract). 3 M.A. Mayhew, C.L. Drake and J.E. Nafe, Marine geophysics (Measurements on the continental margins of the Labrador Sea), Canadian J. Earth Sci. 7 (1910) 199. 4 P.R. Vogt, O.E. Avery, E.D. Schneider, C.N. Anderson and D.R. Bracey, Discontinuities in sea-floor spreading, Tectonophysics 8 (1969) 285. 5 P.F. Barker, and D.H. Griffiths, The evolution of the Scotia Ridge and Scotia Sea, Phil. Trans. R. Soc. Lond. A 271 (1972) 151. 6 W.A. Berggren, A Cenozoic time scale some implications for regional geology and paleobiogeography, Lethaia 5 (1972) 195. 7 J.E. van Hinte, Tile Cretaceous time scale and planktonicforaminiferal zones, Proc. Koninkl. Nederl. Akad. Wetenschappen, Amsterdam Ser. B 75 (1972) 1. 8 J.R. Heirtzler, G.O. Dickson, E.M. Herron, W.C. Pitman III and X. Le Pichon, Marine magnetic anomalies, geomagnetic field reversals and motions of the ocean floor and continents, J. Geophys. Res. 73 (1968) 2119. 9 E.M. Herron, Sea-floor spreading and the Cenozoic history of the eastern Central Pacific, Geol. Soc. Am. Bull. 83 (1972) 1671. 10 W.C. Pitman III, E.M. Herron and J.R. Heirtzler, Magnetic anomalies in the Pacific and sea-floor spreading, J. Geophys. Res. 73 (1968) 2069. 11 D.A. Christoffel and R.K.H. Falconer, Marine magnetic measurements in the southwest Pacific Ocean and the identification of new tectonic features, in: D.E. Hayes, ed., Antarctic Oceanology II, The Australian-New Zealand Sector, Antarctic Symp. Res. Ser. 19 (1972) 197. 12 W.C. Pitman III, R.L. Larson and E.M. Herron, The age of the oceans determined from magnetic anomaly lineations, Bull. Geol. Soc. Am. (1974) in press. 13 J.K. Weissel and D.E. Hayes, Magnetic anomalies in the southeast Indian Ocean, in: D.E. Hayes, ed. Antarctic Oceanology II, The Australian-New Zealand Sector, Antarctic Symp. Res. Ser. 19 (1972) 165.'
C. BOWIN 14 D. McKenzie and J.G. Sclater, The evolution of the Indian Ocean since the Late Cretaceous, Geophys. J.R. Astr. Soc. 25 (1971)437. 15 C.C. yon der Borch, J.G. Sclater, S. Gartner Jr., R. Hekinian, D.A. Johnson, B. McGowran, A.C. Pimm, R.W. Thompson and J.J. Veevers, Deep Sea Drilling Project, Leg 22.Geotimes 17 (1972) 15. 16 B.P. Luyendyk, T.A. Davies, K.S. Rodolfo, D.R.C. Kempe, B.C. McKelvey, R.D. Leidy, G.J. Horvath, R.D. Hyndman, H.R. Theirstien, E. Boltovskoy and P. Doyle, Across the southern Indian Ocean aboard Glomar Challenger, Geotimes 17 (1973) 16. 17 J.R. Heirtzler, J.J. Veevers, H.M. Bolle, A.N. Carter, P.J. Cook, V.A. Krasheninnikov, B.K. McKnight, F. Proto-Decima, G.W. Renz, P.T. Robinson, K. Rocker, Jr. and P.A. Thayer, Age of the floor of the eastern Indian Ocean, Science 180 (1973) 952. 18 C. Bowin, Origin of the Ninetyeast Ridge from studies near the Equator, J. Geophys. Res. 78 (1973) 6029. 19 R.L. Fisher, E.T. Brunce, P.J. Cernock, D.C. Clegg, D.S. Cronan, V.V. Damiani, L. Dmitriev, D.J.J. Kinsman, P.H. Roth, J. Thiede and E.S. Vincent, Deep Sea Drilling Project in Dodo land, Leg 24, Geotimes 16 (1972) 17. 20 A.S. Laughton, R.B. Whitmarsh and M.T. Jones, The evolution of the Gulf of Aden, Phil. Trans. R. Soc. Lond. A 267 (1970) 227. 21 A.S. Laughton, The Gulf of Aden, Phil. Trans. R. Soc. Lond. A 259 (1966) 150. 22 J.B. Minster, T.H. Jordan, P. Molnar and E. Haines, Numerical modeling of instantaneous plate tectonics, Geophys. J.R. Astr. Soc. (1974) in press. 23 W.C. Pitman III, and M. Talwani, Sea-floor spreading in the North Atlantic, Geol. Soc. Am. Bull. 83 (1972) 619. 24 X. Le Pichon, J. Bonnin, J. Francheteau and J.C. Sibuet, Une hypoth~se d'dvolution tectonique du Golfe de Gascogne, Symp. sur l'Histoire structurale du Golfe de Gascogne, 14-16 Dec. 1970, 2 (1971) VI. 11-1.
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©North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
MIGRATION
OF A PATTERN OF PLATE MOTION CARL BOWIN
Woods Hole Oceanographic Institution, Woods Hole, Mass. {USA)
Received November 12, 1973 Revised version received January 10, 1974 A pattern of plate motion is documented to have migrated from the South Pacific into the Indian Ocean during the Cretaceous and Early Tertiary, and then across the Indian Ocean during the Tertiary. The opening of the Gulf of Aden may be a more recent extension of this migration. The migration takes place by the episodic formation of new segments of sea-floor spreading having the new direction of plate motion; or by development of the new direction of motion in segments where sea-floor spreading had previously been active. In this manner plate motions can extend beyond their previously limiting plate boundaries.
1. Introduction The geometry of plates and their kinematics provides only indirect clues to the driving mechanism(s) of sea-floor spreading and plate tectonics. However, this kinematic information is presently the most indicative of what actual tectonic activity the earth's surface has undergone. Knowledge of this tectonic activity forms important constraints upon postulates of the processes active at depth. A corollary of the plate tectonics hypothesis requires that the total of instantaneous plate motions over the globe be such that there is no net change in the surface area of the earth. Attempts at making global inventories of the present plate motion under that assumption have had remarkable success in the accommodation of a variety of data sets [ 1]. Global patterns of sea-floor spreading change with time, but some patterns of plate motion appear to be relatively stable for a hundred million years or more (such as in the north and south Atlantic and eastern Pacific) while other patterns appear to be much less stable (such as in the Tasman [2], Labrador [3, 4], or Scotia Seas [5]. How the more and less stable patterns interact to provide a global consistency, and how a stable pattern may be changed is poorly defined. The purpose of this paper is to demonstrate that a pattern of plate motion in the southern Pacific migrated westward into the Indian Ocean during the
Late Cretaceous and Early Tertiary and then across the Indian Ocean during the Tertiary. The opening of the Gulf of Aden appears to be a more recent extension of this migration. The migration takes place by the episodic formation of new segments of sea-floor spreading, having the new direction of motion; or by development of the new direction of motion in segments where spreading had previously been active. It is not manifested at the surface as a continuous migration process as would be, for example, the longitudinal growth of a crack. In the wake of the migration, secondary adjustments in directions of plate motion appear to occur. For convenience of discussion, and for lack of more precise information, trends are given in terms of their orientation at the present time, but in the past they may have been much different. Ages of magnetic anomalies and geologic time periods as presented by Berggren [6] and Van Hinte [7] have been adopted for consistency. In summary, the Early CretaceousLate Cretaceous boundary occurs at 100 m.y. ago, the Cretaceous-Paleocene boundary occurs at 65 m.y. ago between magnetic anomalies 30 and 29, the Paleocene-Eocene boundary occurs about 53.6 m.y. ago between anomalies 23 and 22, and the E o c e n e Oligocene boundary occurs about 37.6 m.y. ago near anomaly 15. These ages differ from those presented by Heirtzler et al. [8].
MIGRATION OF A PATTERN OF PLATE MOTION
401 Eltanin Fracture Zone. Southwest of the Eltanin Fracture Zone, in the area southeast of New Zealand, magnetic anomaly 32 lies~rather close to the eastern escarpment of the New Zealand Plateau, and hence has been spreading since only a little more than 68 m.y. ago [ 1 0 - 1 2 ] . Anomaly 32 is at a greater distance from the edge of the New Zealand Plateau to the north than along the southern part of the Plateau. Hence the northern part (labeled 2a in Fig. 1) probably separated before the southern part (labeled 2b in Fig.l). At about the same time as the New Zealand Plateau and Antarctica appear to have started separating, the
2. Discussion The oldest oceanic crust along segments of the Pacific-Antarctic Ridge appears to show a progression in age from east of the Tonga Trench to the southeast branch in the Indian Ocean Ridge (Fig. 1). East of the trench there is considerable stretch of ocean floor between the trench and magnetic anomaly 32 (about 68 m.y. ago), and thus that portion of the sea floor was presumably created by sea-floor spreading much older than 70 m.y. ago [9]. The time of initiation of spreading is not known. This region is northeast of the I
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Fig.1. Diagram illustrating migration of a pattern of plate motion from the South Pacific to the western Indian Ocean. Active spreading centers are identified by double lines, extinct spreading centers by double lines with cross-hatching, and inferred extinct spreading centers have dashed double lines with cross-hatching. The cross-hatched extinct spreading center without bordering lines shown in northern Indonesia represents diagrammatically the spreading center that was offset by the Ninetyeast transform fault but has been lost through subduction beneath the Indonesian island arc. Areas with stippled pattern are oceanic crust that w~s formed by a former pattern of plate motion in the northern Indian Ocean. Major transform faults are indicated by heavy single lines, others are not shown but arrows indicate direction of spreading parallel to intraplate transform faults. Dotted lines show location of magnetic anomaly 32. Intermediate weight lines outline the approximate boundaries of oceanic crust formed during the westward migration of the proposed pattern of plate motion. The location of the boundary east of the Tonga Trench is uncertain. Numbers 1 through 5 refer to the order in which it is suggested that segments of these boundaries formed during the migration of the spreading center from the South Pacific to the Gulf of Aden. The (a) portions appear to have formed prior to the (b) portions. The dashed lines in the South Pacific outline approximate boundaries of crust formed following a younger adjustment in direction of plate motion. Sourceg of data are given in the text with exception of that for the stippled area which is from McKenzie and Sclater [14]. DSDP information [15-17], Bowin [18], and magnetic anomaly identifications in the Wharton Basin by J.G. Sclater and R.L. Fisher (personal communication, July, 1973).
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402 Tasman Sea began to open [2]. The initiation of spreading in the Tasman Sea is probably another indication of a reorganization of global spreading directions at that time. Spreading in the Tasman Sea ceased shortly after the time of magnetic anomaly 25 (59 m.y. ago). To the west of New Zealand Plateau and Macquarie Ridge the oldest magnetic anomalies identified with an east-west trend are 21 (?) at the southern limit of the Tasman Sea and 22 south of central Australia [3]. Thus about 5 9 - 5 5 m.y. ago Australia and Antarctica began to separate. This separation took place after the opening between the New Zealand Plateau and Antarctica occurred, and that opening, in turn, occurred after spreading had been active in the region west of the Tonga Trench. This evidence strongly suggests that a pattern of sea-floor spreading in the South Pacific migrated westward with time. Although the origin of Broken Ridge and the tectonic history of the south end of the Ninetyeast Ridge remains much in doubt, the oldest magnetic anomalies so far identified along the southeast branch of the Indian Ocean Ridge that are associated with the present spreading direction also become younger towards the west. South of westernmost Australia the oldest identified anomaly is 19 (44 m.y.), south of the west end of the Naturaliste Plateau it is anomaly 18 (43 m.y.), and south of Broken Ridge it is anomaly 17 (41 m.y.) [13, 14]. For the region of spreading just discussed between the New Zealand Plateau/Antarctica and the Broken Ridge/Kerguelan Plateau separations, the initiation of the new spreading pattern coincided with a new rifting apart of the lithosphere. Altough there had been active spreading in the Wharton Basin west of Australia prior to magnetic anomaly 17, it was about a center of spreading located near the north end of the Nine tyeast Ridge [15-18]. This spreading center now has largely been consumed by the Indonesian subduction zone. It is not yet resolved whether the spreading at the north end of the Ninetyeast Ridge transform fault ceased when spreading began south of Broken Ridge on the southeast Indian branch, or there was an overlap in time of their activity. I favor the former view. To the west of the south end of the Ninetyeast Ridge, however, the continued westward migration of the new pattern of plate motion has followed closely the general location of a former center of spreading
C. BOWIN (the previous direction of plate motion is indicated by arrows on the stippled crust of Fig. 1) but imposed upon it a new direction of motion. This change was to a direction more in accordance with that of the plate motion between Antarctica and Australia and appears to have similarly developed in segments that continued the migration of the new pattern westward across the Indian Ocean. Between longitude 90°E, and the Indian Ocean triple junction east of Madagascar, the present spreading direction is known to have commenced only sometime between magnetic anomaly 23 (55 m.y.) and anomaly 5 (9 m.y.) [14]. However, north of the triple junction, the spreading along the present Carlsberg Ridge was inferred by McKenzie and Sclater [14] to have commenced sometime after 35 m.y. ago (early Oligocene). This conclusion was supported by DSDP hole 238 [19], at the extreme northeast end of the Argo Fracture Zone, which suggests that the youngest date of possible separation of the Chagos/ Diego Garcia region from the Cargados Carajos/ Nazareth Bank is Early Oligocene (Rupelian, 33 m.y. ago). An early Oligocene age for initiation of the present spreading direction along the Carlsberg Ridge is compatible with a continuing westward migration with time of the plate motion direction related to the spreading center that separated Australia from Antarctica. The plate motion direction of the present Southeast Branch between 90°E longitude and the triple junction probably began either at the same time as that of the present Carlsberg Ridge spreading center, or, as here inferred more likely, a few million years earlier. Magnetic anomalies indicate that the Gulf of Aden has been undergoing sea-floor spreading at least since anomaly 5 (9 m.y.) [20]. Laughton [21] speculated that initial separation of Arabia and Somalia began about 20 m.y. ago. DSDP hole 231 [19] showed that the inception of marine sedimentation on the south edge of the Gulf of Aden occurred in Late Oligocene or Early Miocene time (about 23 m.y. ago). Prior to the beginning of opening of the Gulf of Aden, the Owen Fracture Zone may have been a transform fault along which a spreading center was offset to the region of an ancestral Persian Gulf, or perhaps it was the limiting boundary of the earlier northwestern Indian Ocean spreading pattern. The formation of the Gulf of Aden resulted from a new rifting apart of the
MIGRATIONOF A PATTERN OF PLATE MOTION lithosphere analogous to the earlier separation of New Zealand and Australia from Antarctica. The migration of a pattern of plate motion therefore presumably can either utilize pre-existing lithosphere accretionary boundaries or create new ones, presumably following a minimum work principle [4]. The spreading in the Gulf of Aden appears to be a young manifestation of the proposed westward migration of a pattern of plate motion that began in the South Pacific perhaps as much as 80 or more m.y. ago. As a major pattern of plate motion migrates away from a position along a new accretionary boundary, there is an increasing likelihood that secondary adjustments in spreading directions might occur. At times, and places, these adjustments are small and result in an apparent curvature to transform faults, such as south of Australia [13]. Other times the reorganisation is more pronounced, as in the eastern South Pacific [9] (Fig.l), where it seems that another major pattern of plate motion may be developing. There is some suggestion in the magnetic anomaly map of Pitman et al. [12] that, north of the Eltanin Fracture Zone, the adjustment may have begun earlier in the southern part of the East Pacific Rise and progressed northward with time. Several jumps in location of the spreading center are documented in the eastern South Pacific (Fig. 1 ; those in the northwestern Indian Ocean are only herein inferred as possible). The more recent change in direction of plate motion in the eastern South Pacific also may be related to the development of the Chile Ridge and Galapagos triple points. The present direction of motion in the northwestern Indian Ocean, between the Ninetyeast Ridge and the Owen Fracture Zone, is not very different from that on the Southeast Branch from south of Broken Ridge to south of Australia (Fig. 1). The probable poles of rotation for the relative motions in those two regions differ in location by only about 14 ° [22]. Within a 95% confidence ellipse, the poles of rotation for the present direction of relative motion for the northwestern Indian Ocean and the Gulf of Aden could be within 12° of each other [22[. The control of the Arabia/Africa rotation pole, however, is not well defined. The present poles of rotation for the Australia (India)/Antarctica spreading and the New Zealand (Pacific)/Antarctica spreading differ by 87 ° [22], a large difference. However, the directions of plate motion for these last two regions at the time of the proposed
403 migration of the plate motion pattern from south of New Zealand to south of Australia appear to have been similar, judging from the trends of the old magnetic anomalies and transform faults (see arrows for crustal sections 2b and 3a of Fig.l). The information pertaining to poles of rotation together with the dating of events is strong evidence for the proposed migration of a pattern of plate motion. From this study it is clear that plate motions have grown or extended beyond their previously limiting plate boundaries. Proposed explanations for the driving mechanisms must now also be able to acount for that observation. A similar migration of a pattern of plate motion appears to have occurred in the North Atlantic during the Mesozoic and earliest Tertiary ([23], Fig.2). The opening of the Labrador Sea [3, 4] and Bay of Biscay [24], also, may be analogous to the opening of the Tasman Sea.
3. Summary In summary, it is documented that a pattern of plate motion progressed westward from the South Pacific into the Indian Ocean during the Cretaceous and Early Tertiary, separating the New Zealand Plateau from Antarctica, and Australia from Antarctica, in the process. During the Tertiary, that pattern continued to migrate westward across the Indian Ocean, and the opening of the Gulf of Aden may be more recent extension of this migration. The advance of the pattern of plate motion appears to take place by the episodic establishment of segments having the new motion directions on either new or pre-existing rifts in the lithosphere.
Acknowledgements This work was supported by the U.S. Office of Naval Research. I thank J. Sclater, D. McKenzie and H.W. Menard for review comments.
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