Morphological reorganization within the Pacific-Antarctic Discordance

EPSL ELSEVIER

Earth and Planetary Science Letters 137 ( 1996) I57- I73

Morphological reorganization within the Pacific-Antarctic Discordance Mohamed Sahabi a, Louis Gkli a, Jean-Louis Olivet a, Laure Gilg-Capar b, Genevikve Roult ‘, H&ne Ondrkas a, Paul Beuzart a, Daniel Aslanian a*’ a Ijfemer, Marine Geosciences Depurtment, BP 70.29280 flourant?. Frunce b Uniuersite’ de Bretugne Occidentale, 6 avenue Le Gorgeu, 29200 Brest, France ’ Institut de Physique du Globe, 4 pluce Jussieu, 75252 Paris Cedex 05, France Received 25 June 1995; revised 29 September 1995; accepted 5 October 1995

Abstract Two domains with different satellite gravity signatures [l] appear along the axis and on the adjacent basins of the Pacific-Antarctic Ridge (PAR) between Udintsev FZ and 18O”W. One of these signatures is of rough and faulted seafloor, with a high density of apparent, well-marked fracture zones; the other is of smooth seafloor that is comparable with that of oceanic basins that are generally formed at fast-spreading centres. Between Udintsev FZ and 157”W these two domains are separated by a V-shaped structure that extends for more than 1000 km along the rise axis, whereas west of 157”W the boundary is more diffuse. The satellite gravity also reveals an abrupt change in the axial morphology of the PAR across FZ XII, despite the fact that the current spreading rate 121 is the same on both sides of the fracture zone (about 60 mm/yr, full rate). To interpret these features, we postulate that the domains with an apparently rough seafloor morphology have been created at a spreading centre with an axial valley, and that smooth morphology testifies to a spreading centre which was with or evolving into an axial high at the time the crust was formed. With this hypothesis, we show that, since An 210 time (ca. 48 Ma), the ridge morphology changed from an axial valley to an axial high wherever and whenever the spreading rate exceeded a given threshold value. We also show that there is no unique threshold value. Geophysical evidence suggests that the differences in spreading rate threshold values that we observe are probably related to upper mantle temperature heterogeneities below the axis of the PAR. Therefore, we conclude that changes in spreading rates, combined with changes in the upper mantle temperature, constitute the key process that has governed the morphological reorganization of the PAR between Udintsev FZ and 18O”W since An 210 time. The cause of upper mantle temperature heterogeneities, however, remains an open question. The 1000 km long ‘V’ south of Udintsev FZ reflects a change in axial morphology that progressively propagated southwards during the last 30 m.y., at a velocity of about 30 mm/yr. Thus, one tentative explanation for mantle heterogeneities which would also help understand the ‘V’ consists in postulating that the asthenosphere propagated below the PAR axis for the last 30 m.y., from a relatively ‘hot’ mantle province north of Udintsev FZ to a relatively ‘cold’ province south of the fracture zone. This flow model (originally proposed by Marks and Stock [3]) needs to be tested through further investigation.

’ Present address: Department of Earth Sciences, Bullard Laboratories, Madingley Road, Cambridge CB3 OEZ, UK 0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SD1 0012-821X(95)01 185-4

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1. Introduction

general rule are cold and hotspot-related ridges. The Reykjanes Ridge, for instance, is characterized by an axial high and a low-amplitude gravity high, although it spreads at a slow spreading rate (about 20 mm/yr) due to the proximity of the Iceland hotspot (e.g., [6]). The Southeast Indian Ridge (SEIR) spreads at a spreading rate of barely less than 80 mm/yr, but it exhibits a pronounced rift valley within the Australian-Antarctic Discordance (AAD), while east and west of the discordance the axial morphology is dome-shaped, although the spreading rate is of the same order of magnitude [7]. Geophysical studies have found that the upper mantle below the AAD is colder than below the SEIR outside the AAD ([8,9]), and that the crust is likely to be thinner [lo]. These exceptions with large upper mantle temperature variations suggest that the thermal structure at the ridge

Since the early observations of Menard [4] it has been widely accepted that the morphology at the mid-ocean ridge axes depends on the spreading rate. Slow-spreading ridges with full rates of less than about 25 mm/yr (we will subsequently always refer to full spreading rates) are generally characterized by a prominent axial valley of 1.5-3 km in depth and 20-30 km in width. The topography inherited at these ridges is preserved, generating a rough, faulted seafloor in the older basins [5]. Fast-spreading ridges with rates greater than about 80 mm/yr exhibit a smooth, triangular morphology with an axial high, the inherited topography in the older basins being significantly smoother than that inherited at slowspreading ridges. Well known exceptions to this 140-E

160-E

180’E

160-W

140’ W

12O”W

1OO”W

60’S

60’S

146’E

166%

lEttiE

Fig. I. Location of study area (shaded area). Thin lines plate boundaries. The Pacific-Antarctic Ridge separates CTJ = Chile Triple Junction; CP = Campbell Plateau; Heezen and the TharpFZs); UFZ = Udintsev FZ; EPR

16i)‘W

‘W

120-w

II

W

are ETOPOS bathymetric contours at the 2000 m contour interval. Heavy lines are the Pacific (PAC) and Antarctic (ANT) plates. MTJ = Macquarie Triple Junction; FZ XII = Pitman FZ; EFS = Eltanin Fracture System (the EFS is made up of the = East Pacific Rise; MBL = Marie Byrd Land.

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axis, which is primarily related to the spreading rate, may exert the ultimate control on ridge morphology

2. Kinematic Ridge

evolution

159

of the Pacific-Antarctic

(e.g., [11,121). Between the two end members (i.e., fast- and slow-spreading ridges) the case of intermediate spreading ridges is particularly significant, because the observational database and our current understanding of mid-ocean ridge processes suggest that the ridge axis morphology is controlled by a threshold phenomenon that is sensitive to relatively small changes in mantle temperature (e.g., 113,141). The high-resolution satellite gravity map of the world’s oceans [l] reveals a series of abrupt transitions from slow to fast spreading rate morphology occurring within a narrow range of spreading rates from 60 to 70 mm/yr ([ 13,15,16]). From the theoretical standpoint, steady-state numerical models of axial topographic and gravity variations with spreading rate indicate that some key parameters (such as viscosity) can be realistically chosen so that the transition from axial high to rift valley occurs abruptly at a given rate of say 60, 70 or 80 mm/yr [12]. Because large parts of the intermediate spreading ridges (e.g., the Southeast Indian (SEIR) and the Pacific-Antarctic (PAR) Ridges) are located in the remote southern oceans, shipboard observations of these structures are still rather sparse. Most published studies are of large-scale processes, and no focus is provided on the effect of the local conditions that control the transition from one type of morphology to the other (e.g., when both morphologies are nearly juxtaposed in a given area) 2. This work is a detailed study focussing on the PAR between 18O”W and 145”W (Fig. I), a 2000 km long ridge segment which may be of critical importance in our understanding of the axial morphology dependence on spreading rate and upper mantle temperature at intermediate spreading ridges. First, we present a detailed description of the kinematic and morphological evolution of the PAR since An 21 o time (ca. 48 Ma). Then we examine, on an observational basis, the interplay between the different parameters that control the axial morphology.

’ Note that while this paper was in review, new multibeam bathymetric data were collected on the crest of the SEIR between 90” and 118”E [17,18].

The plate boundary between the Pacific and the Antarctic plates (Fig. 1) extends from the Macquarie Triple Junction (160”E, 62”s) to the Chile Triple Junction (250”E, 35%). Spreading at the PAR initiated with the early breakup between the Campbell Plateau, southeast of New Zealand, and Mary Byrd Land, Antarctica, prior to An 34 time (83 Ma) and probably during the major Cretaceous tectonic reorganization of the Pacific 90 m.y. ago [ 191. The tectonic evolution prior to An 210 time (ca. 48 Ma) is not well constrained, due to the lack of magnetic data from the Antarctic plate (the synthetic trajectories of the plates that we have calculated using published parameters ([2,19]> between An 210 and the initial phase of spreading do not coincide with the trend of the fracture zones). Therefore, in the present work we will not deal with ages older than 48 Ma. This restriction is not really limiting because the morphological changes of the accreting plate boundary that we hereafter consider appeared only after An 2 1o time. Between the Chile Triple Junction and the Eltanin Fault System, the boundary between the Antarctic and Pacific plates consists, in the southernmost part of the East Pacific Rise CEPR), of an 1800 km long ridge crest with a remarkable uniform dome-shaped morphology [20] that is characterized by an accretion rate that increases northward, from present-day values of 84 mm/yr at 56”s to 100 mm/yr at 35”s [21]. South of Udintsev FZ, the ridge consists of two morphologically distinct segments, located east and west of 18O”W. West of 18O”W, a series of enechelon offsets that came into existence 5 m.y. ago steps northwest towards the Macquarie Triple Junction. Between 18O”W and Udintsev FZ, the PAR is segmented by a series of offsets. Some of these offsets have been given names [22]: Fracture Zone VII, VIII, IX, X, XI, XII (also called the Pitman FZ) and XIII (Figs. 2-4); other offsets and related fracture zones were revealed only recently by satellite gravimetry and are still unnamed. In the following, we will most particularly focus on this latter segment of the PAR, between Udintsev FZ and 18O”W. To describe the tectonic evolution of the PAR we use the plate motion parameters (pole and angles of

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finite rotation) published in [2] for the time periods between the following boundaries: An 2ay, An 3ay, An 4a, An .5a, An Sd, An 6c, An IOy, An 130, An 2Oy and An 210 (Tables 1 and 2). The work of Cande et al. [2] is a synthesis at the scale of the Pacific and Antarctic plate system, based on all the available magnetic anomaly data from the area, on the structural interpretation of the satellite gravimetry map, and on the underway geophysical data (multibeam bathymetry, gravimetry and magnetics) collected in 1992 on R.V. Maurice Ewing [23] along a 60 km wide, 600 km long corridor centred on FZ XII (Macario et al. 1241 have presented a detailed study of the flowline variations in abyssal hill morphology with spreading rate at 65”S, using all the different spreading phases that they could identify in the magnetic data (Fig. 6, top). In our study, we cannot take into account all these different phases with such a level of detail, because they have not been tested at the scale of the entire 2000 km long

Fig. 2. Satellite-derived

PAR segment that we consider south of Udintsev FZ.) Using the plate motion parameters for the different time periods (Tables 1 and 21, we have mapped a series of eleven equally spaced flowlines from Udintsev FZ to FZ XII. (Note that these flowlines are perfectly consistent with the trend of the fracture zones as they can be mapped from the world gravity map [l] and that this confirms the consistency of the parameters published in [2] at the scale of the plate boundary.) Along each different flowline, we have computed the average spreading rate corresponding to each different time period, using the timescale of Cande and Kent [25] and assuming that the spreading is symmetrical on each flank of the PAR. The resulting mapping of the spreading rate vs. age that we obtain (Figs. 5 and 6) allows the following observations: (1) For each period, the increase in the average spreading rate with increasing distance from the pole

gravity field of the Pacific-Antarctic

Ridge area after

[I].

Fig. 3. Map of maximum gravity slopes (this map was made by following the method explained in 1261, using the TRISMUS software developed at IFREMER). The satellite gravity grid (Fig. 2) was first converted into an equally spaced rectangular xy grid. In each node of the new grid, the gravity gradient was computed alon g eight different directions. The slope plotted in this map is the maximum gradient. Slopes greater than 10% are in red, those smaller than 10% are in blue. Indicated are the isochrons and variations in spreading rates that correspond to observed variations in seafloor morphology from rough to smooth. For instance, near 64’S, 175’W, isochron 3ay is represented; 23 and 28 stand for the half-spreading rate values before and after the time of An 3ay. In this case, the full spreading rate threshold value is between 46 and 56 mm/yr. CORI to COR5 are explained in the caption of Fig. 4. Pitman FZ = FZ XII. Fig. 4. Major structural features of the ocean floor derived from Figs. 2 and 3. Where it can be clearly located, the axis of the Pacific-Antarctic Ridge is represented with a thick black line where an axial high is present, and with a double thin line where an axial valley is present. Where it cannot be unambiguouly located, the ridge axis is represented by a dotted line. Major fracture zones are delineated with a thick red line indicating the corresponding gravity high and a thick blue line indicating the corresponding gravity trough. Fracture zones VIII, IX, X, XI, XII (Pitman FZ) and XIII were first identified in [22]. Other offsets are still unnamed. The 1000 km long V-shaped structure described in the text is shown as a thin dotted line that extends from Udintsev FZ to about 63’S, 156”W near FZ IX. The domains labelled respectively Rl, Rl’. S2, S2’, Sl and Sl’ are domains of apparent rough and smooth seafloor. South of FZ IX, a diffuse area of ‘intermediate roughness’ exists near the axis. The limit of this domain is not well defined. Between 639, 156”W and 65’S, l7O”W two minor (100-300 km long) V-shaped structures (Vl and V2, thin dotted lines) mark the boundaries between rough and smooth. Isochrons (thin green lines) are inferred from the plate motion parameters published in [2], and are indicated for An 3ay, 5a, 130 and 210. The labels CORl to COR5 indicate the corridors described in the text (the notations COR I, COR2 and COR3 were originally used in [3]). Fig. 5. Variation in spreading rate along eleven selected flowlines for the following time periods: Between An 210 and An 2Oy, between An 2Oy and An 130, between An 130 and An 10~. between An 1Oy and An 6c, between An 6c and An 5d, between An 5d and An 5a, between An 5a and An 4a, between An 4a and An 3ay, between An 3ay and An 2ay, and between An 2ay and the present day. The flowlines are numbered from north to south with the symbols fl to fll. Half-spreading rates are indicated on each flank. Along each flowline, the spreading rates are calculated using the plate parameters of [2] and the timescale of [25]. The lineations indicate the trace of the fracture zones. The colours indicate areas of different roughness apparent from the satellite gravity images; these areas are interpreted as differences in seafloor morphology. Red = area of smooth seafloor generated at the crest of a ridge with an axial high (thick black line). Brown = area of rough seafloor generated at an axial rift valley (double thin line). Yellow = diffuse transitional area of (apparently) smooth seafloor. The seafloor roughness appears to be more important in the yellow area than in the red area.

.F IA

Fig. 4

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Table I Finite poles of rotation of the Pacific plate relative to the Antarctic plate after 121.Time (CK) = age based on the timescale of [25]

Anomaly

Time (CK)

Latitude

Longitude

Angle

2aY 3aY 4a 5a M 6c 1OY 130 20Y 210

2.60 5.71 8.53 11.85 17.31 23.36 28.26 33.54 42.63 47.86

67.03 67.91 69.68 71.75 73.68 74.72 74.55 74.38 74.90 74.52

-73.72 -77.93 -77.06 -73.71 -69.85 -67.28 -67.38 -64.74 -51.31 -50.19

2.425 5.422 7.951 10.923 15.173 19.546 22.949 27.336 34.540 37.638

of rotation of the Pacific and Antarctic plates is relatively important (about 15-25 mm/yr>. For instance, between An 2ay and 3ay the spreading rate

varies from 56 mm/yr near FZ XII to 78 mm/yr near Udintsev FZ, and between An 210 and An 2Oy the variation ranges from 20 mm/yr to 36 mm/yr.

Table 2 Stage poles on the Antarctic plate (a) and the Pacific plate (b). See heading of Table I for further details

a) Pacific/Antarctic Epoch O-2ay Zay-3ay 3ay-43

4a-sa ?a-Sd 5d-6c 6c-1oy lOy-130 130-20~ 2oy-2 lo

Duration (Ma)

Latitude

2.60 3.11 2.81 3.32 5.46 6.05 4.90 5.28 9.09 5.23

67.03 68.47 73.50 77.18 78.63 78.49 73.53 73.89 73.65 71.10

Duration (Ma)

Latitude

2.60 3.11 2.81 3.32 5.46 6.05 4.90 5.28 9.09 5.23

67.03 68.60 73.38 76.39 77.51 77.42 73.59 72.29 67.57 69.46

Longitude

Angle

-73.72 -81.63 -15.52 -62.42 -57.30 -57.92 -67.23 -51.03 -1.55 -36.51

2.425 2.999 2.537 2.996 4.280 4.390 3.403 4.398 7.451 3.112

Longitude

Angle

-73.72 -81.48 -73.70 -58.60 -52.33 -53.09 -68.64 -53.75 -16.24 -45.34

-2.425 -2.999 -2.537 -2.996 -4.280 -4.390 -3.403 -4.398 -7.45 1 -3.112

b) Antarctic/Pacific Epoch 0-2ay 2ay-3ay 3ay4a 4a-5a 5a-5d Sd-6c bc-10~ lay-130 130-20~ 2oy-210

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(2) Along a given flowline the average spreading rate increases at An 130, An 5a, An 4a and An 3a and decreases slightly at An 2Oy (only in the south-

’

70 -I

em area, near FZ XII), An 1Oy (only in the northern area, near Udintsev FZ) and An 2ay. Major variations in spreading rates since the An 210 epoch are

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and Plmetury

increases, observed at the An 130, An 5a and An 3ay boundaries. For instance, for the flowline immediately south of Udintsev FZ (flowline 1, full line in Fig. 6, bottom), between An 210 and An 130, the average spreading rate is about 37 mm/yr. Between An 130 and An 5a this rate is about 50 mm/yr. For the period after 10 Ma the average spreading rate varies from 60 to 78 mm/yr. For the flowline immediately south of Pitman FZ (flowline 11, dotted line in Fig. 6, bottom), the average spreading rate between An 210 and An 130 is about 16 mm/yr. Between An 130 and An 5a, it is about 32 mm/yr. After Anomaly 5a, the average value varies from 39 to 55 mm/yr.

3. The morphology FZ

of the PAR south of Udintsev

In this section we use simultaneously a shaded relief satellite gravity image (Fig. 2) and a map of the maximum gravity gradients (Fig. 3). The first is based on the satellite gravity grid [l], and provides, to a first approximation, a picture of the ocean floor morphology (e.g., [ 151). The second is based on the original method of Cordell [26] (this method consists in mapping at each grid node the maximum gradient value computed along eight different directions) and is used to enhance the structural directions that can be seen with the naked eye on the satellite gravity image. These two maps reveal two notable features: (1) The axial morphology inferred from the gravity map apparently consists of an axial valley to the south of FZ XII and of an axial high to the north [15]. This change in morphology is not related to a change in spreading rate across the transform ([2,25]). The multibeam bathymetry data acquired along FZ

Science Letters 137 (19961 157-173

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XII indicates that the morphological change does not occur abruptly across the fault, but more than 35 km south of it [25]. Thus, it is not related to the presence of the fracture zone. (2) Two domains of apparently different gravity signature appear on the ridge and in the adjacent basins: one domain of rough and faulted seafloor, with a high density of apparent, well-marked fracture zones, and another domain of smooth seafloor comparable to the oceanic basins that are generally formed at fast-spreading centres. The boundaries between these two domains are outlined on the structural map that we have established based on both the gravity map and the map of maximum slopes (Fig. 5). For the sake of simplicity we have divided the area south of Udintsev FZ into five subregions (CORl to COR5 in Fig. 4, where COR stands for ‘corridor’, following the notation used in [3]): l Between Udintsev FZ and FZ IX (COR4 and COR5), a large-scale V-shaped structure extends for over more than 1000 km, separating two domains of different seafloor roughness. The two branches of the ‘V’ appear to be symmetric relative to the ridge axis. In COR4 and on both flanks of the PAR, the gravimetric signature of FZ VII, VIII and IX is clearly smoothed, if not interrupted, within the ‘V’. The interruption of the fracture zones thus provides a good means of delineating the ‘V’. The branch on the northwestern flank can be easily picked between 61”S, 156”W and 58%. 156”W, and that on the southeastern flank appears between 62.5”S, 153”W and 61”S, 146.5”W. In COR5, the existence of oblique structures allows the determination of the ‘V’ up to Udintsev FZ. The fracture zone is crossed by the northwestern branch near 52”S, 156”W and by the southeastern branch near 59’S, 131”W. Within the ‘V’, the morphology of the ocean floor appears

Fig. 6. (Top) Full spreading rate variations during the last 12 m.y. along a flow line paralleling FZ XII. The dotted line indicates the spreading rate values reproduced after [24], using as many as thirteen different phases of spreading. The full line represents the variation in spreading rate based on the plate motion parameters published in [2], considering only the major spreading phases (these phases are indicated with asterisks) that have been tested at the scale of the entire Pacific-Antarctic plate boundary. The simplification made in the present study (we consider only the major spreading phases) does not hide the major spreading events (such as, for example, the increase in spreading rate at An 3ay). (Bottom) Full spreading rates during the last 21 m.y. along the flowlines fl and fl I indicated in Fig. 5 (fl is located immediately south of Udintsev FZ; fl 1 is located near FZ XII). These variations were calculated using the plate motion parameters from [2]. Along each given flowline, the major variations in spreading rate occur at, respectively, the time of An 130, An 5 and An 3ay. For each given time period, the full spreading rate increases with increasing distance from the pole of rotation between the Pacific and the Antarctic plate. The increase ranges between 15 and 25 mm/yr.

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to be smooth. Outside the ‘V’, the morphology is rougher, although two areas of smooth seafloor morphology are present, symmetrically with respect to the ridge axis (see S2 and S2’ in Fig. 4). @Between FZ IX and XI (COR 3) the boundary between the two domains (of smooth vs. rough seafloor morphology) is very diffuse. l Between FZ XI and FZ XII (COR2) two Vshaped structures that are smaller than the structure described above are visible on the gravity map (Vl and V2). Extending for less than 150 km, from FZ XI to 63”3OS, 167”W, structure Vl has been described and tentatively interpreted by previous workers as the wake of a propagating rift [3,15,27]. The other, V2, extends for about 300 km, between FZ XI and FZ XII. Within V2, the seafloor appears to be smoother than outside V2. l Between FZ XII and FZ XIII (CORl) the boundary between the two domains (rough vs. smooth) is rather diffuse (i.e., similar to COR 2 and COR3), and it appears to be parallel to the axis of the PAR and coincident with the An 3ay isochron. As mentioned earlier, it is important to note that the crust younger than about 5 Ma has a structural fabric of fast-spreading ridges with no visible fracture zones, although the present-day spreading centre has an axial rift valley.

4. Morphology

vs. spreading

rate: Discussion

4.1. Working hypotheses In the following discussion, we consider that the morphology of the seafloor in the oceanic basins provides an indication of the ridge axial morphology at the time when the crust was formed. Our first working hypothesis is that the domain of rough and faulted seafloor that appears in Fig. 4 has been created at a spreading centre with an axial valley. This is justified by the fact that on the world satellite gravity map [l], all the oceanic domains which, visually, have a comparable roughness have been accreted at ridges with axial valleys. We are not aware of any examples where this is not the case. In addition, it is difficult to imagine a ridge axis with an axial high which would have been segmented by closely spaced fracture zones.

The second working hypothesis concerns the domain of smooth seafloor in Fig. 4. Generally, this domain could well be expected to have been generated at the crest of a spreading centre with an axial high, like, for instance, the EPR. However, south of FZ XII an axial valley appears to be present in a domain of visually smooth seafloor (see domains Sl and Sl’ in Fig. 4). Other regions of the world exhibit similar characteristics (e.g., on the Mid-Atlantic Ridge, south of the Azores near 32”N [28], or on the SEIR near 11O”E [17,18]). The existence of an axial valley in a domain of visually smooth morphology is thus a configuration that cannot be precluded, but we believe that is is most probably a transient stage. Our second working hypothesis is that, for steady-state configurations, smooth morphology most likely indicates seafloor that has been accreted at an axial high.

4.2. Mapping the ‘threshold’ spreading rate from rift ualIey to axial high In Figs. 3 and 6 we show the seafloor morphology inferred from the satellite gravity map and the spreading rate variations with age. This allows us to make the following observations: l Between Udintsev FZ and FZ VIII, the earlier change in seafloor morphology occurred at about the time of An 130 (Fig. 6). This is associated with a change in spreading rate, from 36 to 54 mm/yr along flowline fl, and from 30 to 50 mm/yr along flowline f4. This increase in spreading rate is likely to have been responsible for the creation of S2 and S2’, two domains of smooth seafloor morphology. The spreading rate is reported to decrease slightly at about the time of An 6c along fl , and at the time of An 1Oy along f4. This decrease in spreading rate (from 50 to 46 mm/yr along both flowlines) appears to be responsible for the creation of the two domains Rl and Rl’ of apparent rough seafloor morphology. Finally, the morphology changes, from rough to smooth, while the spreading rate changes from 46 to 50 mm/yr after An 5d along fl, and from 46 to 56 mm/yr after An 5a along f4. Therefore we conclude that between Udintsev FZ and FZ VIII the transition from rough to smooth seafloor morphology depends on whether the spreading rate was greater or smaller than a threshold value at the time the crust was

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formed. This threshold value falls between 30 and 50 mm/yr but may vary with age. @Between FZ VIII and FZ XI the only change in seafloor morphology is from rough to smooth, at An 3ay time (Fig. 6). This time boundary is associated with an increase in spreading rate, from 60 to 72 mm/yr along flowline f5, and from 56 to 66 mm/yr along flowline f8. This indicates that, in this area, the spreading rate threshold value is greater than about 60 mm/yr. l Between FZ XI and FZ XII the transition in seafloor morphology occurs at about the time of An 4a in COR2, and at the time of An 3a in CORI (Figs. 4 and 5). The transition is associated with an increase in spreading rate from 42 to 50 mm/yr along flowline f10, and from 46 to 56 mm/y along flowline fl 1, indicating that the threshold value for this area is likely to be about 50 mm/yr. Here also it appears that the threshold value may vary with age. Our analysis clearly indicates that there is a firstorder correlation between the spreading rate and the morphology of the seafloor accreted at the crest of the PAR between Udintsev FZ and 18O”W. For each area the morphology changed from rough to smooth whenever the spreading rate exceeded a given threshold value. There is, however, no unique threshold vaiue. but different values for each different area and each different period. This leads us to the conclusion that the morphological transitions along the PAR are not related to spreading rate only. Spreading rate is a predominant governing factor, but some other parameter must have controlled the morphology of the ridge axis also. Theoretical models [lo-12,29-331 suggest that, in addition to changes in spreading rate, variations in the upper mantle temperature are likely to be responsible for changes in the ridge axial structure. 4.3. Evidence for upper mantle temperature geneities below the PAR

hetero-

Three types of observations suggest that variations in the temperature of the upper mantle exist below the axis of the PAR, between 18O”W and Udintsev FZ: (1) Changes in geophysical characteristics north and south of FZ XII: The variations in mantle Bouguer anomaly (MBA) on either side of the FZ

169

XII show that the crust is thinner and/or the upper mantle is colder to the south than to the north of the fracture zone on the Pacific Plate [24]. This variation coincides with a well-resolved increase in the estimate of the average characteristic width (A) of the abyssal hills across the fracture zone. Such an increase indicates changes in the flexural rigidity of the lithosphere near the vicinity of the ridge crest, and by implication changes in the axial thermal structure [34]. (2) Analysis of subsidence data: By using all the shipboard data available prior to 1992, Marks and Stock [3] found that the subsidence rate (and consequently the mantle temperature) is significantly lower to the south than to the north of FZ XII. The observed difference in subsidence rate (226 m/m.y.“* in CORl vs. 373 m/m.y.‘/* in COR 2 and COR3) is so large that it cannot be reproduced using conductive models with reasonable changes in the physical parameters [35]. Instead, it may well be explained by a difference in the dynamic behaviour of the asthenosphere across FZ XII [36]. (3) Seismic tomography: In a recent review, Ritzwoller and Lavely [37] compared the long-wavelength seismic shear wave velocity at a depth of 200 km from a number of global models. Although short-wavelength details cannot be resolved from these models, two areas of relatively low velocity appear, one north of Udintsev FZ, and one west of 18O”W, indicating variations in the temperature of the upper mantle below the PAR in our study area. To improve the lateral resolution of three-dimensional seismic wave velocity models in Antarctica and the surrounding oceans, Roult et al. [38] analyzed direct earthquake-to-station Rayleigh wave data along 400 well-distributed paths. These data were recorded on ten GEOSCOPE stations, of which seven were located in the southern hemisphere; the remaining three were at equatorial latitudes. The geographical distribution of phase velocities and azimuthal anisotropy was then calculated with the tomographic inversion method without a-priori regionalization (see [39]). The results confirm the existence of mantle temperature variations below the PAR between Udintsev FZ and FZ XII. The differences in threshold values that we observe may thus well be explained by variations in temperature in the upper mantle below the axis of

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ANOMALY

( 11.8

Ia)

5a

My

j

Antarctic

Plate

M. Suhubi et ul./ Earth und Planetary Science Letters 137 (19961 157-173

the PAR. As suggested in [ 141, the lower the upper mantle temperature, the greater the spreading rate at which the transition occurs from rough to smooth seafloor morphology. 4.4. The Pacific-Antarctic phological reorganization

Discordance

and its mor-

The major V-shaped structure (Fig. 5) initiated shortly after An 130 time, about 30 m.y. ago. Considering our first working hypothesis, this ‘V’ reflects a change in axial morphology that progressively propagated southwards over geological time. The southern tip of the ‘V’ is presently about 1000 km from Udintsev FZ, suggesting an average velocity of about 30 km/m.y. for the last 30 m.y. The southward propagation of Vl started about 5 m.y. ago, shortly after An 3ay time. Smooth morphology within V2 appeared shortly before An 5 time, about 10 m.y. ago. Therefore, Vl and V2 propagated southward at a velocity of about 30 km/m.y. All the V-shaped structures that we consider propagate at the same velocity. By analogy with the Australian-Antarctic Discordance, let us call the Pacific-Antarctic Discordance (PAD) the portion of the PAR that is characterized by an axial valley and segmented by closely spaced pronounced fracture zones. Our analysis enables us to follow the evolution of the PAD with time: l At the time of An 130 (Fig. 7b), rough morphology was present between Udintsev FZ and 18O”W. On the basis of our first working hypothesis, the PAR throughout the length of this segment was characterized by an axial valley, while an axial high was present north of Udintsev FZ. The PAD was thus located between Udintsev FZ and 18O”W. l At the time of An 5a (Fig. 7a), the morphology at the axis of the spreading centre between Udintsev

171

FZ and the tip of the ‘V’ was either a high or a ‘transient valley’ evolving into a high. The PAD thus extended from 18O”W to the southern tip of the ‘V’. The structures Vl and V2, like the domains Sl and Sl’ south of FZ XII, actually appeared only after An 5a time. l Structure V2 appeared between An 5 and An 3, together with a diffuse area of smooth seafloor (yellow area in Fig. 5). The generation of V2 (at the locations indicated by the black points in Fig. 7a) appears to have been independent of the formation of the major ‘V’ structure, suggesting that V2 may have been initiated by the presence of hot mantle pockets in a cold mantle environment. 4.5. The asthenospheric

flow model

To explain the observed anomaly of subsidence north and south of FZ XII, Marks and Stock [3] hypothesized that some asthenospheric southward flow is presently supplying the accretion centre north of FZ XII. The observations reported in the present paper support this idea and provide additional constraints. When no significant changes in spreading rate are reported, the upper mantle is expected to be colder below the areas of rough seafloor than below the areas of smooth seafloor. This suggests that the mantle was likely to be colder to the south than to the north of Udintsev FZ prior to An 13 time. The subsequent temperature gradient across the fracture zone may have originated a mantle flow which propagated below the PAR axis for the last 30 m.y., from a relatively ‘hot’ mantle province north of Udintsev FZ to a relatively ‘cold’ province south of that fracture zone. While propagating, the flow contributed to increasing the mantle temperature below the ridge axis, allowing the transition from a rift valley to an axial high. The 1000 km long V-shaped

Fig. 7. (a) Reconstruction at An 5a (11.8 Ma). Plate motion parameters are from [2]. At this time only the major V-shaped structure was present, with an axial high throughout the length of the ridge crest between Udintsev FZ and the extremity of the ‘V’ as it was at this epoch. Near Udintsev FZ, the spreading rate averaged between An 130 and An 5a is estimated to be about 50 mm/yr. That at the southern extremity of the ‘V’ is about 44 mm/yr. South of the ‘V’, the average spreading rate was everywhere less than 44 mm/yr and a rift valley was present (double thin line). The Vl and V2 structures and the ‘band’ of smooth seafloor between FZ IX and FZ XI were initiated after 11.8 Ma. Dotted lines indicate the locations of flowlines fl, D, f5, f7, f10 and fl 1. (b) Reconstruction at An 130 (33.5 Ma). Plate motion parameters are from 121. Prior to this time a rift valley (double thin line) was present throughout the length of the PAR axis, and the full spreading rate was less than 36 mm/yr. The 1000 km long V-shaped structure immediately south of Udintsev FZ initiated shortly after An 13 time (ca. 30 Ma). Dotted lines indicate the locations of flowlines fl, 0, f5, t7, f10 and fl 1.

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structure may therefore simply be the surface expression of this jlow.

References

[II

5. Conclusion

We have established that the axial morphology at the crest of the Pacific-Antarctic Ridge (PAR) between Udintsev FZ and 18OW changed from rough to smooth whenever and wherever the spreading rate exceeded a given threshold value. We have also shown that there is no unique threshold value, but a different value for each different corridor, and that the threshold value may vary with age. These differences in threshold values may be related to upper mantle temperature heterogeneities below the axis of the PAR: changes in spreading rates, combined with changes in the upper mantle temperature, thus constitute the key process that has governed the morphological reorganization within the Pacific-Antarctic Discordance since An 210 time. The cause of upper mantle heterogeneities is not known. One possibility (which would help explain the presence of the 1000 km long V-shaped structure visible on the satellite gravity map south of Udintsev FZ) consists in postulating that the asthenosphere propagated southwestwards below the PAR axis for the last 30 m.y., but this needs to be tested through further investigation, both theoretical and observational. Also needed is a theoretical study of the time it takes for the ridge to respond to an external perturbation, such as a variation in spreading rate or a variation in mantle temperature. As far as we know, this has not yet been addressed in any theoretical paper.

Acknowledgements

We thank Steve Cande for providing unpublished results from the Pitman FZ and J.-C. Semp&& B.P. West, Will Wilcock, Don Forsyth, Marc Parmentier and Jason Phipps-Morgan for stimulating discussion. We also thank D. Sandwell and W. Smith for making available the satellite gravity digitized grid on which this work is based. Daniel Carre draughted Figs. 4 and 5. The other figures were drawn using the GMT software [401.[ PT I

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