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THE S E A - F L O O R S P R E A D I N G H I S T O R Y OF
THE E A S T E R N INDIAN O C EA N
T H O M A S S. P ( ) W E L L * and B R U C E P. I , U Y E N D Y K
Department of Geolo,eical Sciences, Unit'ersity of California, Santa Barbara, California 93106, U.S.A.
(Accepted 4 March, 1982~
Abstract. The geologic history of the eastern Indian Ocean between northwest Australia and the Java "French is known to involve two separate events of rifting and sea-floor spreading, l,ate Jurassic spreading in the Argo Abyssal Plain off northwest Australia was followed by Early Cretaceous spreading in the Cuvier and Perth Abyssal Plains off west Australia. However, the evolution and interaction of these events has not been clear. Mesozoic sea-floor spreading anomalies have been identified throughout the Argo Abyssal Plain that define a rifting event and subsequent northward spreading on the northwestern Australian margin at 155 m.y.b.p. Magnetic anomalies northwest of the Argo Abyssal Plain indicate a ridge jump to the south at about 130 m.y.b.p, that is approximately synchronous with east-west rifting along the southwestern Australian margin. The Joey Rise in the Argo Plain was probably formed by volcanism at the intersection of this new rift and the spreading ridge to the north. The southern and northern spreading systems were connected through the Exmouth Plateau which was stretched and faulted as spreading progressed. The RRR triple junction was formed at the intersection of the two spreading systems and appears to have migrated west along the northern edge of the Gascoyne Abyssal Plain. Spreading off northwest Australia cannot be easily related to simultaneous spreading in the west central Pacific via any simple tectonic scheme.
1, Introduction
The eastern Indian Ocean between the northwestern margin of Australia and the Java Trench is principally comprised of three distinct abyssal plains (Figure 1). The Gascoyne Abyssal Plain is separated from the Argo Abyssal Plain by the Roo and Joey Rises, and both are separated from the Cuiver Abyssal Plain by the Exmouth Plateau, a large marginal plateau adjoining the Australian north- west Shelf.
Recent studies of magnetic anomaly trends and identification have delineated two separate rifting events and directions of sea-floor for this region. An east-northeast trending set of magnetic anomalies in the eastern end of the Argo Abyssal Plain has been identified by Larson (1975) as being M-22 through M-25 of the Mesozoic reversal sequence of Larson and Hilde (1975). This identification requires a rifting age of about 155 m.y.b.p, for the northwest- ern margin. Further coverage of this area has since carried these identifications west to the Joey and Roo Rises (Heirtzler et al., 1978). Meanwhile, Markl (1974) identified north-northeast trending Mesozoic anomalies M-0 through M-7 in the
* Present address: Union Oil Geothermal, Box 6854, Santa Rosa, California 95496, U.S.A.
Marine Geophysical Researches 5 (1982) 225-247. 0025-3235/82/0053-0225503.45. CopyriRht ~ 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.
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SEA-FI .OOR SPREADING IN T H E INDIAN O( 'EAN 22"7
Cuvier and southern Gascoyne Abyssal Plains, supporting the rifting age of Veevers, et al. (1971) as Early Cretaceous (120-130m.y.b.p.) on the western margin.
The geologic history of the rifting event along the western margin has been studied by Veevers and Cotterill (1976, 1978) and by Johnson et al. (1980). They have been able to show that many of the features in the gross geometry and geology of rifting are analogous to present day rifting in East Africa (1976). They have also identified the nature of the continent to ocean transition along most of the margin's length (1978). Johnson et al. (1980) presented a history from the initial rifting at 125 m.y. to 90 m.y. when the spreading systems in the region reorganized.
It was the purpose of this study to detail the spreading events off northwestern Australia leading up to the rifting of the western margin. In our study we have used both magnetic anomaly and bathymetric data. This 'oceanic' picture pro- vides a separate and revealing perspective to the rifting events, with implications that help to explain the origin of many heretofore unexplained features. One main objective of our work was to map the Jurassic-Cretaceous boundary in ocean crust off northwest Australia. In some locations, this is clearly demon- strated, but mostly the details of the boundary remain obscure.
2. Identification of Magnetic Anomalies
Total field magnetic anomaly data were compiled from a number of different sources (see Powell, 1978) and are presented along ships' tracks in Figure 2.
Linear magnetic anomalies observed over oceanic areas are presumed to be of sea-floor spreading origin. Correlations and identifications are given with the data in Figure 3. In Figure 4 anomaly correlations are superimposed on the bathymetry. This study presents a more complete treatment of Argo Abyssal Plain anomaly correlations and identifications than that presented previously in Heirtzler et al. (1978), largely due to additional data. Although the major difference is in coverage, minor changes in interpretation do exist. The anomal- ies in the Argo Abyssal Plain are named in accordance with the Mesozoic reversal time scale of Larson and Hilde (1975). The oldest anomaly recognized is M-25 adjacent to the Australian continental rise. We could not locate the M-26, 27, 28 sequence of Cande et al. (1978).
Correlations in the eastern Argo Abyssal Plain (Figure 5) are reasonably good, owing to the high density of data there. The interpretation of this region is an expansion of Larson's (1975) original identifications along the two H.M.A.S. Moresby (1972) lines.
Anomalies along the Umitaka Maru (east) line in the central Argo Abyssal Plain present a problem in interpretation. Shapes along the northern end of this line do not match well with the model profile or with profiles on either side. The
228 T. S. POWELL AND B. P. LUYENDYK
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SEA-FLOOR SPREADING IN THE INDIAN OCEAN 229
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SEA-PI,OOR gPREADING IN T H t - I N D I A N CK 'EAN 231
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Fig. 5. Principle magnetic anomaly profiles projected and compared with the model profile for the Argo Abyssal Plain. The model layer magnetized sequence and associated reversal time scale are from Larson and Hilde (1975). Normally magnetized blocks are solid black. Magnetized layer is flat, 500 meters thick and 6 cm below the sea surface. Ambient field orientation used is inclination 45 ~ up, declination 2 ~ E from the U.S. Naval Oceanographic O~ce charts 1700 and t706, 8th ed., February 1966. Remanent magnetization parameters are inclination 76 ~ up, declination 30~ and intensity
0.01 emu/cc. Identifications along the two Moresby 1972 profiles are from Larson (1975).
232 T, S. P O W E L L A N D B. P. L U Y E N D Y K
JOEY
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Fig, 6, Principle magnetic anomaly profiles projected and compared with the model profile for the Joey Rise-Roo Rise area. Model parameters and information are identical to that given for Figure 5.
SEA-FI,OOR SPREADING IN I'HE INDIAN (~'EAN 233
southeast part of the line shows well developed M-22 and 23. The most compelling reason to correlate this line with the anomalies to the east is the
parallel trend and similar spacing of individual peaks and troughs. The profile can also be matched with anomalies to the west but with divergent anomaly trends. Both interpretations are presented in Figures 5 and 6, but the eastern correlation is favored because of a better match and a more reasonable position of a necessary fracture zone; anomalies M-23 through M-25 along the southern end of AII9314-8 (Figure 6) are offset to the north relative to the eastern sequence. This fracture zone is expressed bathymetrically as a notch in the continental rise near 116 ~ N and a trough bisecting the Roo Rise. Seismic profiles interpreted in Veevers et al. (1974) and Veevers and Heirtzler (1974a) show an abrupt deepening of acoustic basement just east of the Joey Rise, further supporting this location for a fracture zone.
Anomalies observed in the Joey Rise-Roo Rise area belong to two separate parts of the reversal sequence (Figure 6). The southern end of AII9314-8 continues a right offset M-23 through M-25 sequence from the east. These correlations degrade rapidly to the west over the Joey Rise and the trends of anomalies become inconsistent. The anomaly sequence north of the Joey Rise is best matched with anomalies M-13 and younger. The quality of these identifications is, in general, only fair but appears to improve in the younger more northern anomalies. Pronounced topography along many of these profiles undoubtedly affects their quality. Correlations from this sequence to two H.M.A.S. Moresby (1968) lines (Figure 2 and 3) located to the west of here are very difficult. These two lines do not match well with any of the models and do not even match well with each other. A general pattern of peaks and troughs can be correlated between the two lines and they roughly follow the northeast trends of Falvey (1972) in the north and a north-northeast trend in the south.
Anomaly identifications M-10 through M-0 made in the Cuvier and southern Gascoyne Abyssal Plains by Larson et al. (1979) can be extended north, with a northwestward offset, to the southern of two Bartlett (1971) lines (Figures 3 and 4). The anomaly sequence along the southern Bartlett line makes a good match with a model, extending identifications in this area back to M-6 (Figure 7). The trends of these anomalies are defined by intersecting profiles at M-0 and M-6 which show them to be parallel with trends to the south (Figure 3). M-6 along V2009 crosses the location of DSDP site 260, where the oldest sediments recovered were dated as Middle Albian (103 m.y.b.p.) by the Shipboard Scientific Party (1974). They interpreted the basalt below these sediments to be a sill, however, and propose that still older sediments lay below it. The identification of M-6 (119 m.y.b.p.) at this location is in agreement with this interpretation.
There is no visible bathymetric expression of the fracture zone that must separate this sequence found along the Bartlett profile from those to the south in the Cuvier Abyssal Plain. The fracture zone position in Figures 3 and 4 is
234 T. S. POWELL AND B. P. LUYENDYK
GASCOYNE AND CUVIER ABYSSAL PLAINS
IIOMY 115MY 120 MY I i ' n i l i �9 H i o ~oo
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Fig. 7. Principle magnetic anomaly profiles projected and compared with the model profile for the Gascoyne and Cuvier Abyssal Plains. Model information is identical to that given for Figure 5, except as noted. Model layer is 5.5 kilometers below the sea surface. Ambient field orientation is inclination 58* up and declination 2 ~ W. Remanent magnetization parameters are inclination 64 ~ up, declination 38~ and intensity 0.007 emu/cc. Identifications along Vema 3308-2 are from Larson et
al. (1979) and Johnson et al. (1980).
determined by the termination of the more southern anomaly sequence along V2009. An extension of this feature to the Exmouth Plateau shows that it may define the southern margin of the broad embaymen t in its western edge.
A small addition has been made to the reproduction of Larson ' s (1977)
identifications in Figures 3 and 4 by using profile AII9313. It concerns the fracture zone that offsets the two anomaly sequences nor thwest of the Exmouth
Plateau (Figure 4). As located by Larson, this fracture zone is slightly north of
an extension of the Cape Range Fracture Zone, which is evoked by Veevers and Cotterill (1978) to define the southern edge of the Exmouth Plateau. This might suggest that Larson ' s fracture zone is an extension of the Cape Range. It appears , however , that the eastern extension of Larson ' s fracture zone can be crossed by a prominent anomaly that is continuous between anomaly M-4 identified along V3308-2 and V3308-3 and what is identified here as M-4 along AII9313. Its northern position is in good agreement with the expected anomaly spacing, shape, and trend. Therefore , Larson ' s fracture zone is a feature formed after the rifting along the southwestern margin of the Exmouth Plateau and the Cape Range Fracture Zone does not extend beyond the plateau.
S E A - F L O O R S P R E A D I N G IN T H E INDIAN O C E A N
EAST
G o s c o y n e A b y s s a l P l a i n
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Fig. 8. Demonstration of the apparent variation in spreading rate along the various magnetic profiles in three areas. The horizontal axis is distance and the vertical axis is the Mesozoic time scale of
Larson and Hilde (1975). The slopes of the lines are spreading rates.
236 T S. P O W E L L A N D B. P. L U Y E N D Y K
3. Sea-floor Spreading Rates
Spreading half-rates were calculated according to the mapped positions of the identified anomalies. These rates are shown in Figure 8 for the three regions discussed above alongside the Larson and Hilde (1975) time scale.
At about 150 m.y.b.p, or earlier, spreading was proceeding at 4.0-5.0 cm yr -~1 in the Argo Abyssal Plain. The rate is highest in the east which suggests a relative motion pole located to the southwest. Around 148m.y.b.p. the rate here decreased to about 3.0 cm yr 1.
Spreading rates in the Joey Rise-Roo Rise region can be calculated between 130-120 m.y.b.p, and they range from 3.0 to 4.0 cm yr -1. Although some of the anomaly identifications are questionable, the UMI963-E profile evidently spans the period between 155 and 124 m.y.b.p. This profile indicates two changes of spreading rate in the Argo Abyssal Plain during this period; a decrease after 148 m.y.b.p, and an increase just prior to 130m.y.b.p. Anomalies M-5 and M-6 found on profile AII 9314-12/18 suggest another spreading rate increase to over 6 cm yr -1 after 118 m.y.b.p.
Anomalies in the Gascoyne Abyssal Plain were generated between a plate pair different from the plates which generated the Argo Abyssal Plain anomalies. Analysis of Bartlett (1971-1) profile shows that the rate northwest of the Exmouth Plateau was 4.6 cm yr -I from 120 to 116 m.y.b.p, when it decreased to half this value.
4. Discussion
Sea-floor spreading in the Argo Abyssal Plain appears to have taken place about a pole of spreading located towards the southwest, contrary to a pole near East Timor suggested by Veevers and Cotterill (1978). A major tectonic discontinuity arises in the Joey Rise-Roo Rise area. Magnetic anomalies on the northern and southeastern sides of the Joey Rise are separated by 20 m.y. in age and those over the rise itself do not appear to be of spreading center origin. A ridge jump to the south just before M-13 time is the most reasonable explanation of the abrupt age boundary, with the now-absent intervening crust carried away attached to the northern plate. The transform fault connecting the jumped ridge to the ridge segment to the northeast probably formed along the fracture zone just east of the Joey Rise (Figure 4).
The timing of this jump (130m.y.b.p.) is so close to the age of rifting (125 m.y.b.p.) suggested for western Australian margin (Veevers and Cotterill, 1978; Johnson et al., 1980), that it is undoubtedly related to that event. The proximity of the Joey Rise to the oldest recognizable anomalies in the jumped sequence is circumstantial evidence indicating its origin during this same event. This view is supported by the bathymetric trends on the rise, which resemble north-northeast, west-northwest trends of the western margin (Figure 1). Just
SEA FI+OOR S P R E A D I N G IN I H E INDIAN O( E A N 237
north of the Joey Rise, trends return to northeast, consistent with those of the
Argo Abyssal Plain. The implication is that the rise is the junction site of spreading in the Argo Abyssal Plain and rifting to the south off western Australia. The rise itself would be formed by volcanism associated with this junction.
A major requirement of this model, however+ is that rifting was connected from north to south through the Exmouth Plateau. Extensive block-faulting along a north-northeasterly trend has been observed in seismic profiles throughout most of the plateau (Willcox and Exon, 1976). The faulting cuts an unconformity surface (R4 of Veevers et al., 1974; F of Willcox and Exon, 1976)
and overlying sediments are fault ponded. The age of faulting, then, can be directly correlated with the age of the unconformity. Two wells drilled in the Rankin Platform, just east of the plateau give conflicting ages for the formation of this surface, (Veevers et al., (1974). Maximum ages range from Early to Late Cretaceous. The only age consistent with the range of both wells is Early Cretaceous (Neocomian), which is coincident with the time of rifting. It appears then, that the faulting is directly related to rifting events in the south, and that the Joey Rise region was the nexus of spreading in the northeast and rifting in the southwest.
The age of the ridge jump in the Argo Plain south to the Joey Rise region (130 m.y.b.p.) is not strictly consistent with rifting ages in the Perth and Cuvier Abyssal Plains to the south. Markl (1974) attempts to identify M-II at the eastern edge of the Perth Abyssal Plain, but the anomaly is located on the continental slope and the identification is poor. The next oldest identification is M-7, but it appears that M-8 can also be identified (Markl, 1974, Figure 2, Lines 0908-2 and EL48). This interpretation suggests an age of 120m.y.b.p. for the oldest sea-floor there,
Larson (1977) identified an isolated set of anomalies at the eastern edge of the Cuvier Abyssal Plain as being either M-7 through M-10 or M-13 through M-15. According to Larson, an M-7 through M-10 model makes the better match with anomalies. The other identification is a better fit with a 2.1 cm yr i extrapolation
to M-4 at the mouth of the abyssal plain, but the intervening sequence does not match the reversal time scale. Preferring the first identification, the entire anomaly profile makes a good match with a model if the reversal sequence is reversed at M-5 time ( l l7m.y.b.p. ; Larson et al., 1979; Johnson et al., 1980). This implies a cessation of spreading near the center of the plain and a ridge jump from this location to the plain's western edge just east of M-4. The age of oldest crust, using this model for the evolution of the Cuvier Abyssal Plain, would be M-10 or 122 m.y.b.p. This is in close agreement with the maximum age interpreted for the Perth Abyssal Plain (see Johnson et at., 1980), but is 8 m.y. younger than the ridge jump at Joey Rise.
The reason for this age discrepancy is not clear. It might indicate a prolonged period of crustal stretching in the north prior to spreading in the south. Evidence
238 T. S. P O W E L L A N D B. P. L U Y E N D Y K
of block-faulting during rifting has been observed in the Perth and Carnarvon Basins along the western margin (Jones and Pearson, 1972; Veevers, 1974). However, there does not appear to be evidence for enough stretching (about 200 km) to totally accommodate this time gap. A close, southern near pole of spreading for rifting would decrease the necessary stretching distance consider- ably, and might account for the younger age of the oldest crust in the Perth Abyssal Plain.
The stretching of the Exmouth Plateau appears to have been a prolonged event. The presence of M-4 at the western edge of the Exmouth Plateau (Larson, 1977, and this study, Figure 4) is interpreted as the age of its final rifting. If so, the westward ridge jump in the Cuvier Abyssal Plain postulated to have emerged at the mouth of the plain, also rifted the plateau. This jump occurred 5 to 8 m.y. after spreading was initiated in the Cuvier Abyssal Plain and 12 to 13 m.y. after the ridge jump in the Argo Abyssal Plain. The Exmouth Plateau is then inferred to have absorbed the spreading that took place during over 13 m.y. (see also a discussion in Larson et al., 1979).
As previously mentioned, the Exmouth Plateau is extensively blockfaulted in an orientation suggesting that the faulting was caused by the tensional forces of rifting. Interpretations of gravity survey data (in Willcox and Exon, 1976) conclude that the crust of the plateau "thins abruptly from typically continental to typically oceanic in thickness below the lower continental slope, and is roughly 19 km thick under the central Exmouth Plateau." Yet the deep seismic reflectors of the plateau indicate that its origin is continental (Veevers and Cotterill, 1978). The plateau must be a piece of continental crust that has been stretched to roughly one-half its original thickness during the course of plate separation. Since there is no information remaining about its original dimensions the exact amount of stretching cannot be quantified. Considering the severity of thinning and the extent to which it has been faulted, it is likely that it could have absorbed the necessary plate motions between about 130 and ll7 m.y.b.p.
An alternative explanation for the position of M-4 northwest of the Exmouth Plateau, is that the edge of the plateau was rifted earlier but that the northwest ridge jump rafted away the ocean crust which formed during the earlier rifting. This does not require as much time for stretch of the Exmouth Plateau but is difficult to reconcile with the termination of the Cape Range Fracture Zone at the western edge of the plateau. The Cuvier Abyssal Plain ridge jump would have broken across the Cape Range, yet not employed the already active transform fault in its new spreading configuration. Perhaps the site of the jump was in some way constrained to break the margins of both the Exmouth Plateau and the opposing continent when they were lined up at the mouth of the Cuvier Abyssal Plain. In either case, it appears that the plateau underwent a certain amount of stretching during rifting to become a low-lying continental fragment attached to the continental shelf.
A final observation on the magnetic anomalies is the more easterly trend of
S E A - F L O O R S P R E A D I N G IN T H E I N D I A N O C E A N 239
M-0 in the Gascoyne Abyssal Plain than that found to the south (Larson, 1977 and this study). The change is evident in two locations, both defined by two profiles, and separated by a fracture zone (Figures 3 and 4). The bathymetry shows two nearly parallel linear deeps (dashed) whose southern ends line up with, but are oblique to, two fracture zones in the Gascoyne Abyssal Plain which offset the M-0 to M-T sequence. The implication of these observations is that the spreading direction was changing at M-0 time and continued to change into the Cretaceous magnetic quiet period. The changes appear here to be discreet shifts of about 10 ~ swinging spreading to a more northerly direction. They probably occurred in the manner similar to that described by Menard and Atwater (1968), but they also appear to have propagated from the north, since the oblique trend is not visible in M-0 in the well-controlled southernmost identifications.
5. Reconstructions of Initial Rifting
The discussion here is centered around two graphic reconstructions of the area, each for a specific age (Figures 9 and 10). Since much of the material in these reconstructions is necessarily speculative, features supported by data are drawn with solid lines and features inferred are dashed.
Rifting events prior to spreading in the Argo Abyssal Plain have been documented by Veevers and Cotterill (1978). Spreading is estimated to have first started at 155m.y.b.p. (Larson, 1975); Figure 9 shows it 3 m.y. later. The rifted continental fragment is here tentatively identified as the Tarim Block of central Asia, as suggested by Larson (1975) but its true identity is still a mystery. This fragment is probably fairly minor, owing to the long history of Paleozoic marine transgressions along the northwest margin (Veevers, 1971, and Veevers and Heirtzler, 1974b).
The reconstruction of the Australian western margin in Figure 9 is that of Veevers et al. (1975) with Greater India the adjacent continental mass. Geologic evidence dictates that this margin was land-locked at this time (Veevers et al.,
1971), but reconstructions of Gondwanaland, assembled from constituent con- tinents, show a gap along the margin (Smith and Hallam, 1970). Veevers et al.
(1975) make a convincing argument that this missing continental mass has underthrust the Tibetan Plateau. In their reconstruction the northern edge of Greater India is marked by the Kun Lun-Astin Tagh-Nan Shah mountain front, the northern edge of the plateau. This boundary is reproduced in Figure 9. Coincidentally, the Astin Tagh boundary lies along strike with the spreading ridge active in the Argo Abyssal Plain.
Figure 10 carries the reconstruction to the time of the ridge jump in the Joey Rise-Roo Rise area, or what is interpreted to be just prior to the time of rifting along the western margin. Two new features have been added to this recon- struction: the Amsterdam-St. Paul and Kerguelen hot spots of Luyendyk and
240 r S. P( )WEI . I . A N D B. P. I U Y E N D Y K
ANOMALY M-25
152 MYBP
4 . , ' ~uu tYT4 INS
7
T E T H Y S '
AUSTRALIA
WHOLE SPREADING
"oA~E
CN/YR
GREATER INDIA
Fig. 9. Reconstruction of the northern margin of Australia and Greater India in a reconstruction of Gondwanaland at 152 m.y.b.p. (M-25). The figure is a Mercator projection of Australia in its present position; 2500 and 4500m bathymetric contours are added for reference. Spreading rate can be determined from the scale shown. Features supported by data are in solid lines, inferred features and
features not yet created are dashed.
Renn ick (1977). Their s tudy s h o w e d that the major a s e i s m i c ridges of the eastern
Indian Ocean could have been formed by the m o t i o n s of the cons t i tuent t ec ton ic
plates over these t w o hot spots , f ixed to their present pos i t ions relat ive to the
earth's spin axis . T h e y place the t w o hot spots very c lo se to the w e s t e r n margin of Austral ia in their 100 m.y .b .p , recons truc t ion ( L u y e n d y k and Rennick , 1977,
Figure 2b). T h e y sugges ted that the two hot spots might have been along the
SEA-FLOOR SPREADING IN T H E INDIAN OCEAN 241
ANOMALY M-13
150 MYBP
WHOLE SPREADING
RATE
""*'.. R{DGE " u \ \ l YR
Ag~%_ PLAI
,,'~ EXMOUTH / //~'~ ,-, /PLATEAU" /
AMSTERDAM SAINT PAUL I
AUSTRALIA
bt TETHYS
INDIA ~ / t o
AUSTRALIA
G R E A T E R INDIA v~CTOR C,RCU,T O,.GR~,.
KERGUELEN HOT SPOT
Fig. 10. Reconstruction for 130 m.y,b.p. (M-13). Triple junction velocity vector circuit diagram gives description of triple junction in velocity space. At this time a southward ridge jump occurs north of the Joey Rise in the Argo Abyssal Plain. This ridge connects with a rift trending northeast through the Exmouth Plateau. The disparity in spreading velocities northwest versus west of Australia requires a third northwest trending spreading center to be formed on the northeast edge of the Exmouth
Plateau.
western margin and initiated plate breakup. In this posit ion, the Amsterdam-St .
Paul hot spot coincides c lose ly with the Wal laby Plateau and the Kerguelen hot spot with the Naturaliste Plateau, both considered epiliths (oceanic volcanic
upgrowths) by Veevers and Cotterill (1978) and L u y e n d y k and Rennick (1977). The geometry and features of rifting along the western margin have been
242 T. S. POWELI. AND B. P. LUYENDYK
closely compared to those observed in East Africa (Veevers and Cotterill, 1976). Burke and Dewey (1973) argue that these same East African rifts are plume* generated. We suggest that the locations of the two western epiliths coincide with the positions of plumes in the triple junction of three rifts. The Wallaby Plateau is found at a 120 ~ bend in the continental-to-oceanic crust boundary defined by Veevers and Cotterill (1978), (solid lines in Figure 10). The Natural- iste Plateau is located at the junction of rifts between India, Australia, and Antarctica, which shows a similar geometry of a 120 ~ angle between the component plate boundaries (Smith and Hallam, 1970). Presumably, a failed rift (aulacogen) of the Wallaby rupture extended westward into Greater India. The inferred aulacogen between Australia and Antarctica would later be reactivated with rifting in the Eocene (Veevers et al., 1971), but evidence of its existence is lacking. These observations, however, make a strong permissive argument for hot spots at these two locations on the western Australia margin. Jones and Pearson (1972) propose a geologic history of the Perth Basin that shows an initial phase of rifting between the Lower Permian and the Lower Jurassic, separated from final rifting events by a period of stability in the Middle Jurassic. The hot spots seem to have reactivated an old rift, probably by coming to rest underneath it.
Further north, the tensional forces of rifting were transmitted through the Exmouth Plateau, causing block-faulting and thinning. North of the plateau, a segment of the spreading center in the Argo Abyssal Plain jumped to the south. The two spreading centers met at the Joey Rise causing disruption and volcanic outpouring. Figure 10 shows the rift that extends through the Exmouth Plateau offset northwest by a transform fault to the Joey Rise. There are bathymetric trends in this area consistent with this inferred feature (Figure i) and it is required by the geometry of stretching in the Exmouth Plateau. However, there appears to be no other evidence for its existence.
A third spreading ridge oriented northwest-southeast was formed north-west of the Joey Rise, as required by the triple junction geometry. Its orientation and spreading rate are determined by the spreading velocity geometry. This is assuming symmetric spreading, with the triple junction confined to move parallel to the direction of rifting in the south. The path of the triple junction is inferred as the zone where north-northeasterly magnetic anomaly trends of the Gascoyne Abyssal Plain meets the northeasterly trends of the Argo Abyssal Plain. It is indicated by a dashed line in Figures 3 and 4.
Johnson et al. (1980) have described the sea-floor spreading history off western Australia following rifting. They propose that spreading here proceeded about a single pole from 125 to 90 m.y.b.p, during which India moved northwestward
* 'Hol spo t ' is equ iva l en t to "plume' but il is the p re fe r red t e rm s ince it m a k e s no c o n n o t a t i o n as to
genes i s of these fea tu res .
SEA-FI.(X)R SPREADING IN THE INDIAN (X'EAN 243
away from Australia. At 90 m.y.b.p., a major reorganization of spreading sys- tems occurred after which India began its northward movement.
During this time period, we suggest that the two hot spots moved westward to underlie India. Spreading continued in the same direction off northwest Aus- tralia, possibly at a higher rate (Powell, 1978). The ridge northwest of the Exmouth Plateau changed orientation slightly to a more northeasterly trend. possibly due to unknown changes in the spreading systems to the north.
6. Relationship to the Mesozoic Pacific
It has been suggested (Larson, 1975; Hilde et al., 1976) that Mesozoic spreading systems that existed in the western Pacific might have extended into the Tethys Seaway. This connection seems probable, considering the configuration of the world's continents and oceans at that time. A preliminary analysis of this possibility shows some similarities between the Late Jurassic Argo Abyssal Plain spreading system and the western Pacific. Ultimately though, a recon- struction shows that the respective magnetic anomalies were probably not formed by the same spreading ridge.
Figure l l compares the spreading rates and timing of rate changes of the Phoenix (Larson, 1976) and Japanese (Hilde et al., 1975) lineations, and the Argo Abyssal Plain anomalies. While the Japanese spreading rates show no similarity to the Argo Abyssal Plain, the similarity of the Phoenix rates (Marshall, Gilberts and Nauru Basin) is striking enough to suggest a common spreading center. Had these two sets of anomalies been formed by the same spreading center, their spreading rates and relative locations would be compatible with a single pole of spreading. Larson (1976), by analysis of the variation of spreading rates within the Phoenix lineations, suggested a western near pole of spreading for the period between M-25 and M-15. The spreading rates in the Argo Abyssal Plain are only slightly less than for the Phoenix set for that time. Therefore, there can only be a connection between the two sets if the Argo Abyssal Plain was formed a short distance to the west of the Nauru Basin, the westernmost anomaly sequence of the Phoenix lineations. A geometric reconstruction using the paleolatitude and paleostrike for each anomaly set, and placing the two areas adjacent along a common spreading center was constructed to test the feasibility of a connection (Figure 12). For the Argo Abyssal Plain, paleolatitude (63 ~ and paleostrike (090 ~ were calculated using the 140 m.y.b.p, geomagnetic pole for Australia from Robertson (1963; 36~ 312 ~ E, c ~ = 13~ (Powell, 1978, appendix D). A paleolatitude of 10 ~ and a paleostrike of approximately 055 ~ were determined for the Phoenix lineations using the skewness approximations of Larson (1976), and the 112 to 122m.y.b.p. Pacific plate pole of Larson and Chase (1972), (50 ~ N, 330 ~ E). With these geometric constraints, the two areas can be connected along a common spreading system, offset by a very long (4000 km) transform fault.
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Fig. 12. Configuration of Argo Abyssal Plain-Phoenix spreading ridge at about 140-120 m.y. This is the closest possible position satisfying paleolatitudes and paleostrikes of anomalies in the two areas. A hyp~thetical relative motion pole is shown for the spreading system. Meridional stereographic
projection
The Phoenix lineations also lay almost due north of the Argo Abyssal Plain, and thus in the Tethyan Seaway of many contemporaneous continental recon- structions; (for example, Hilde et al., 1977; Smith et al., 1973). These two somewhat unreasonable consequences of the spreading reconstruction suggest that this type of connection was unlikely. In addition, Larson's (1976) skewness approximations suggest southward movement of the Pacific plate at this time, whereas paleomagnetic data for Australia (McElhinny, 1973) suggest northward movemement. This is an impossible situation for two plates so oriented, with a common spreading center between them. The spreading ridge that generated the Argo Abyssal Plain undoubtedly did extend into the Pacific region at this time, but it was the boundary between the Australian and Tethyean (?) plates, not the Pacific and Phoenix plates. Although the Argo and Phoenix spreading systems were separated by great distances of latitude in the Cretaceous, progressive northward movement of the Argo system, and southward jumping of the Phoenix system (Winterer, 1976) allow the possibility that the two systems 'joined' in the Early Tertiary.
246 T, S, P O W E L L AND B. P. L U Y E N D Y K
Acknowledgements
We would like to thank the following people for their contributions to this study: James Heirtzler for his assistance and direction as Chief Scientist of R/V Atlantis II Cruse 93, Leg 14; Hans Schouten for his help and guidance during T. Powell's stay at Woods Hole: Roger Larson for providing us with Lamont data and insights into magnetic anomalies in the study area; Allen Lowrie for graciously providing us with U.S.N.S. Bartlett data and valuable discussion; Franz Emmel for bathymetry used in Figures 1 and 4; Dave Crouch for his drafting; and Patty Beaman for her typing. Ken Macdonald reviewed and commented on the manuscript.
This research was supported by a grant from the National Science Foundation to Woods Hole Oceanographic Institution and a subcontract to the University of California at Santa Barbara.
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