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Analysis of propagators along the Pacific–Antarctic Ridge: evidence for triggering by kinematic changes
Earth and Planetary Science Letters 199 (2002) 415^428 www.elsevier.com/locate/epsl
Analysis of propagators along the Paci¢c^Antarctic Ridge: evidence for triggering by kinematic changes Anne Briais a; , Daniel Aslanian b , Louis Ge¤li b , He¤le'ne Ondre¤as b a
LEGOS-GRGS, CNRS UMR 5566, Obs. Midi-Pyre¤ne¤es, 18 Ave E. Belin, 31401 Toulouse Cedex 4, France b De¤partement des Ge¤osciences Marines, IFREMER, B.P. 70, 29280 Plouzane¤, France Received 5 February 2002; accepted 1 March 2002
Abstract We analyze the structure and evolution of two propagators along the Pacific^Antarctic Ridge (PAR) that we surveyed during the Pacantarctic cruise of the N/O L’Atalante. A large propagator at 63‡30PS, 167‡W shows a N50‡Etrending segment of the PAR propagating southwestward, while the adjacent, N45‡E-trending segment retreats. The propagating and doomed ridges are offset by about 43 km. They both curve into the offset to define an overlap zone about 25 km long. The inner pseudofault is juxtaposed to a series of E^W-trending ridges inferred to represent the failed axes. Their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure. The pseudofaults are 35 ? 5‡ oblique to the propagating ridge, which implies a rate of propagation of 44 ? 8 mm/yr, using a 62 mm/yr full spreading rate, comparable to that of other propagators with similar morphology. The age of the initiation of the propagation from the Heirtzler fracture zone is estimated to be 5^6 Ma, which coincides with the age of a clockwise change in spreading direction. A second, smaller, southwestward propagator is observed northeast of the major one, at 63‡15PS, 165‡10PW, with a morphology very similar to that of the larger one. It is inferred to have started near 1 Ma, again at the time of a clockwise change in spreading direction along the PAR. These two propagators are likely to have evolved from extensional relay zones which developed within the Heirtzler transform fault (TF) valley following clockwise changes in spreading direction. The present-day axial discontinuity is less than 40 km in offset and may not be a TF anymore. The development of propagators in this area of the PAR appears to be triggered by kinematic changes rather than by thermal gradients along the ridge. Other propagators that have left similar signatures on the flanks of the PAR, appear to have developed at similar spreading rates near 50^60 mm/yr full rate, as a result of kinematic changes. C 2002 Elsevier Science B.V. All rights reserved. Keywords: mid-ocean ridges; propagation; sea-£oor spreading; rates; kinematics
1. Introduction Most propagators along mid-ocean ridges are
* Corresponding author. Tel.: +33-5-61-33-29-37; Fax: +33-5-61-25-32-05. E-mail address: [email protected] (A. Briais).
characterized by a ridge segment (propagating rift, PR) growing at the expense of another (dying rift, DR). They were ¢rst suspected from their disruptive signature in magnetic anomaly maps, oblique with respect to both spreading and axis directions (e.g., [1^3]). They were then extensively surveyed and described tectonically and geophysically, for example on the Paci¢c^Juan de Fuca
0012-821X / 02 / $ ^ see front matter C 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 6 7 - 8
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Ridge [4], the Galapagos spreading center [5^9], the South-East Indian Ridge (SEIR) [10^12], the North Fiji and Lau Basin ridges (e.g., [13,14]), the East Paci¢c Rise (EPR) [15^19] or the Mid-Atlantic Ridge (MAR) [20^22]. Structural observations of propagators can be accounted for by kinematic models in which the propagating ridge cuts through the young lithosphere created at the DR, with a fairly continuous propagation, but episodic failure of the retreating ridge [1,3,5,23]. Although the kinematic evolution of rift propagation is now well described, the mechanisms triggering and driving the propagation are still debated. We describe two propagators near 167 and 165‡W on the Paci¢c^Antarctic Ridge (PAR) south of Udintsev fracture zone (FZ), and analyze the mechanical conditions for their development. We show that models implying a gravity gradient or a longitudinal £ow away from a hotspot do not apply in our study area. We present evidence that an axial reorganization of the PAR, implying the evolution of the Heirtzler TF into a series of smaller discontinuities, has triggered these propagators. We show that kinematic changes are also probably responsible for other propagators whose pseudofaults outline, in the gravity maps derived from satellite altimetry measurements, the V-shaped boundary between rough and smooth sea£oor generated at the slow- and fast-spreading PAR, respectively.
2. Tectonic setting The PAR is unique among the major spreading centers because the Euler pole of rotation has been very close to the ridge axis since the early opening between the Paci¢c and the Antarctic plates about 90 Myr ago. This situation implies that even small changes in the kinematic parameters (either the location of the pole or the velocity of the relative plate movement) have signi¢cant consequences on the geometry and morphology of the spreading plate boundary [24^26]. Changes in the location of the Euler pole of rotation are re£ected by changes in the curvature of FZs, clearly observed on the altimetry-derived gravity
anomaly map (Fig. 1; [27]). The spreading rate, which acts as a governing parameter controlling the axial morphology, increases rapidly with distance from the Euler pole, presently located at 64.25‡S, 100.94‡E [24]. Multibeam bathymetry and magnetic data collected with R/V L’Atalante in 1996 [25,26] indicate that south of the Pitman TF near 175‡W, the ridge is spreading slowly, with a half-rate of about 28 mm/yr, and is characterized by an axial valley. In the north, near the Udintsev TF, the ridge axis has a fast spreading rate of 38 mm/yr (half-rate), and is characterized by an axial high. A transition zone extends over 650 km along-axis between the two domains. It is characterized by a very variable axial morphology displaying a shallow axial valley, a rifted high or an axial dome, and by the presence of propagators [26]. Two domains of di¡erent roughness of the gravity anomaly derived from satellite altimetry measurements appear on the £anks of the PAR between Udintsev FZ and 180‡W: a rough sea£oor characterized by a high density of wellmarked discontinuity traces, and a smooth sea£oor comparable with the oceanic basins that have generally been formed at fast-spreading centers. Between the Udintsev and the Saint Exupery FZ, these two domains are separated by a large, V-shaped boundary pointing south, whose tip would meet the ridge axis near 62‡30PS, 157‡W, outlined by oblique structures located symmetrically with respect to the present-day ridge axis (Fig. 1; [28]). All the sea£oor created after Chron 3a, however, appears smooth on the gravity map, so that the very tip of the V is missing. The roughness contrast is interpreted to re£ect changes in axial morphology, segmentation and geometry which occur when the full spreading rate reaches a threshold of 50^60 mm/yr. As the angular velocity of the Paci¢c^Antarctic relative motion increased, this threshold in spreading rate propagated southward for more than 1000 km along the ridge axis, since Oligocene time (shortly before Chron 13o, about 30 Ma) [25,26,28]. Our study area is located about 400 km southwest of the broad V tip, and within the present-day axial morphology transition zone.
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Fig. 1. Shaded map of the free-air gravity anomalies estimated from satellite altimetry over the PAR area (from [27]). Illumination from North. White bold lines are routes of the Pacantarctic cruise of R/V L’Atalante. Bold black lines show PAR axis. Dotted black lines are inferred traces of other propagators on the ridge £anks. Red and yellow boxes show areas represented in Figs. 2 and 5, respectively. Inset: Location of study area. Black lines are plate boundaries. ANT: Antarctic plate; PAC: Paci¢c plate.
3. Morphology and structure of the PRs near 167 and 165‡W Most tectonic structures observed on the multibeam bathymetry at the 167‡W propagator on the PAR (Fig. 2) are comparable to those previously de¢ned on other propagators; therefore, we describe the interpreted structural elements using the terminology de¢ned for propagating ridge systems [5,7^9,23]. The propagating ridge is a 150 km-long ridge segment trending N50‡E (Fig. 2; [26]). The axis displays a 600^1000 m-deep, 6^14 km-wide axial valley. The southern tip of the propagator curves
from a N50‡E to an E^W orientation and terminates into an 8 km-wide, rhomboidal graben 500 m deeper than the rest of the axial valley, interpreted to re£ect amagmatic extension occurring at the very end of the propagating system. The central magnetic anomaly across the PR is anomalously narrow on the pro¢les PR6 and PR7 crossing its southern tip (Fig. 3). Away from the tip, the axial anomaly gets wider toward the northeast (Figs. 2 and 3), and the full, across-axis opening rate of 62 mm/yr is reached near 63‡20PS, 166‡W. This observation is consistent with the decrease in spreading rate towards the ends of the overlapping segments, as magmatic accretion is progres-
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Fig. 2. Top: Color-shaded relief map of the 167‡W propagator from multibeam bathymetry in the area covered by our survey. Illumination from North. Solid black lines are magnetic anomalies projected along track. Thin dotted lines are magnetic pro¢les. Black dashed lines are pseudofaults. Blue dashed lines are inferred failed rifts. Symbols are picked magnetic anomalies from [24] and this study: black symbols are magnetic picks, red symbols are conjugate picks after Euler rotation. Bold lines are isochrons inferred from the combination of magnetic anomalies and satellite gravity data. Bottom: Tectonic interpretation of the multibeam bathymetric data. Thin lines are ridge crests corresponding to abyssal hills. Barbed lines are major fault scarps. Ticks point downhill. Striped areas are tectonic reliefs and o¡-axis volcanoes. Solid double red line is present-day ridge axis. Dotted double red line is failing ridge. Green dashed lines are pseudofaults. 6
sively transferred from one segment to the other [5^7] (Fig. 3). The failing ridge overlaps the tip of the propagator by about 25 km. Its axial morphology, as well as that of the doomed ridge farther south, is characterized by an 8 km-wide, 300 m-high dome (Fig. 2). The orientation of the doomed ridge is N45‡E, implying a 5‡ change in spreading center azimuth between the propagating and the doomed ridges. The failing ridge tip curves toward the propagator in the overlap region to reach an E^W orientation. The propagating and the doomed ridges are o¡set by 43 km, creating a wide area where lithosphere generated by the doomed ridge on the Antarctic plate is transferred to the Paci¢c plate. This transfer zone is marked by a series of curved abyssal hills, bending from N50‡E to N^S, bounded by the overlapping spreading ridges. In map view, these abyssal hills are arranged in a fan near the PR, with one side of the fan parallel to the active spreading center. This arrangement of the ridges is explained partly by the decrease in spreading rate towards the tip of the overlapping spreading ridges, and partly by the rotation of the ridges about vertical axes in a set of blocks in the overlap zone, in a bookshelf faulting system [8,23,29]. The outer pseudofault zone is de¢ned as the boundary, on the Antarctic plate, between the lithosphere created at the doomed ridge and that created at the propagating ridge. It is marked by a series of graben bounded by N85‡E-trending normal faults, and can be followed between 63‡47PS, 167‡20PW and 63‡42PS, 165‡15PW, where the multibeam bathymetry coverage is complete, as well as on the swath near 63‡40PS, 164‡W, where the same N85‡E direction appears. The inner pseudofault zone is the boundary on the Pa-
ci¢c plate between lithospheres created at the doomed and propagating ridges. Near 63‡20PS, 166‡40PW, it is disrupted by two volcano-tectonic structures reaching a relief of 800 m above the surrounding sea£oor. West of the inner pseudofault, a zone of intense tectonic deformation about 30 km wide represents the o¡-axis trace of the zone of transferred lithosphere. We infer
Fig. 3. Identi¢cation of magnetic anomalies based on the kinematic parameters of [24] and the geomagnetic time scale of [50]. Gray area marks the location of the zone of transferred lithosphere.
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Fig. 4. Axial bathymetric pro¢le along the PAR, shown as a function of distance to westernmost axial point surveyed during Pacantarctic, at 65‡58PS, 175‡W. Arrows point to propagator tips.
that the N80^85‡E-trending ridges observed in this zone on the two bathymetric swaths are failed rifts representing relict segments of the retreating ridge (Fig. 4). Their nearly E^W orientation is the same as that of the failing ridge tip, re£ecting the rotation of the spreading axes in the overlap zone. Despite the incomplete bathymetric coverage, we suspect that the overlapping spreading center observed at 63‡15PS, 165‡10PW is a second, smaller propagator, because the tectonic structures associated with it are very similar to those described for the 167‡W propagator (Fig. 2). In particular, the deep basin which marks the end of the northeastern segment is characteristic of a propagator tip, and is much deeper than the end of the other segment. The failing rift of this second propagator is the other end of the same ridge segment as the larger, 167‡W propagator, so that a single ridge segment is propagating on one end and retreating on the other end. The two ridges of the 165‡W propagator are o¡set by 18 km and overlap by 25 km. The segment ends curve toward the overlap zone in map view. The transfer zone is marked by curved abyssal hills fanning away from the spreading ridges. The central magnetic anomaly along the pro¢le crossing the two overlapping branches exhibits a normal width, and is centered on the western, retreating branch. The modeling of this magnetic anomaly pro¢le indicates that extension at the propagating segment is mostly amagmatic, with the production of new crust mainly occurring at the retreating branch (Fig. 3).
4. Kinematics of the propagation The retreat of the doomed rift of the PAR 167‡W propagator is marked by a series of failed rifts oriented E^W, parallel to the present-day orientation of the failing ridge, suggesting that the evolution of the propagating system is not steady-state, but characterized by episodic jumps of the failing ridge, as commonly observed (e.g., [19]), and described by the model of overlapping rift propagation with cyclic rift failure proposed by Wilson [23] (Figs. 2 and 5). This kinematic model not only accounts for the presence, distribution and orientation of the failed rifts, but also for the observed curvature of the spreading axes. It is to be noted that the curving of the overlapping axes is similar to that of other large OSCs observed on the EPR and PAR (e.g., [19,30]). In particular, the curving of the individual failing ridges in map view is opposite to that described for the 95.5‡W Galapagos PR and predicted by a kinematic model of continuous propagation of McKenzie [31]. Within the framework of Wilson’s model [23], the distribution of the observed failed rifts suggests that the evolution of the propagating system involves long cycles between ridge jumps. Since the oldest lithosphere covered to the northwest of the area is about 2 Ma, the observation of two failed rifts implies cycles about 1 Myr long. The failed rifts, however, may be closer to one another than observed, since the bathymetric coverage is incom-
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Fig. 5. Top: Satellite altimetry-derived, 1 min-resolution gravity map of the Heirtzler TF area. Illumination from north. Black solid lines are topographic lineations identi¢ed from multibeam bathymetry (Fig. 2). Symbols and bold lines as in Fig. 2. White solid lines show ridge axis. Yellow dashed lines are propagator pseudofaults. Bottom: Map of magnetic isochrons, tectonic lineations identi¢ed from multibeam bathymetry (thin black lines), and simpli¢ed contours of gravity highs (red) or lows (blue).
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Fig. 6. Sketch of the evolution of the Heirtzler FZ showing development of the PRs within the transform valley following changes in spreading direction. Arrows mark kinematic changes derived from the Euler poles estimated by [24]. A change in spreading direction near Chron 3a resulted in the development of a short segment within the transform o¡set, which then started to propagate toward the south. Another change in spreading direction occured near Chron 2a. A more recent change in spreading direction near Chron 1o likely triggered a second propagator. Green, blue, and red lines show isochrons at Chrons 3a, 2a, and 1o, respectively. Thin black lines in bottom right frame are topographic lineations identi¢ed from multibeam bathymetry (Fig. 2). Inset: Variations of spreading direction and spreading rates in the last 10 Myr near the Heirtzler TF, from [24].
plete. In the latter case, the evolution would imply shorter cycles. The angle of the pseudofaults with the 167‡W propagating ridge is 35 ? 5‡, which implies a southwestward propagation rate of 44 ? 8 mm/yr, using a full spreading rate of 62 mm/yr.
5. Relationship between the propagators and the evolution of the Heirtzler transform The outer pseudofault of the 167‡W propagator is observed to meet the Heirtzler FZ near 163‡30PW, 63‡40PS, implying that the propagator
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initiated within the TF (Fig. 5). To estimate the age of origin of the propagators and understand the evolution of the plate boundary in this area, we analyze the magnetic anomalies identi¢ed on the few pro¢les crossing the area (Cande, written comm., 1999; [26]), and combine the magnetic picks to draw isochrons. The isochrons are also de¢ned using the interpretation of the altimetryderived gravity map, which permits the identi¢cation of the traces of TFs and non-transform discontinuities [26]. The age of inception of the 167‡W propagator is thus estimated to be about 5^6 Ma (shortly after Chron 3a). It is important to note that the intersections of the inner pseudofault and the outer limit of the transferred lithosphere with Heirtzler FZ are close to kinks in the FZ which re£ect a change in spreading direction at that time (Fig. 5). The present-day Heirtzler TF is ill-de¢ned on the gravity map derived from altimetry measurements [27] (Figs. 1 and 5). The ridge segment whose southern end, near 63‡15PS, 165‡10PW, was covered by bathymetry data during the Pacantarctic survey, appears to continue farther north and to terminate near 63‡S, 164‡20PW. Its axial valley can be followed on the satellite gravity map as a relatively linear low. The gravity high which characterizes the axial area just north of the Heirtzler FZ is also observed in continuation of the ridge segment south to 63‡15PS, 164‡W. The present-day Heirtzler o¡set is therefore likely to be shorter than about 40 km, and to correspond only to the linear, N135‡E-trending low in the gravity map (Fig. 5). The signature of the pseudofaults of the 165‡W propagator is di⁄cult to follow outside the shipboard bathymetric coverage (Figs. 2 and 5). The age of inception of this propagator could be about 1 Ma. In the last 5^6 Myr, the Heirtzler TF progressively evolved into a series of smallero¡set discontinuities, two of them propagating south (Fig. 6; see Section 6).
6. Discussion 6.1. Triggering mechanisms Three mechanisms have been suggested to ex-
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plain the triggering and evolution of PRs. (1) Authors suggested that PRs develop to allow for a ridge reorientation following a plate reorganization, especially in the case of the Juan de Fuca Ridge [2,4,32], or for the Easter and Juan Fernandez microplates on the EPR, where the propagators originate from within TFs [15]. (2) Phipps Morgan and Parmentier [33] present a quantitative model in which a PR propagates as a crack in an elastic material, in response to a stress concentration at its tip, resulting from excess gravity stresses in shallow regions of the spreading axis compared to deeper regions. (3) Brozena et al. [34] suggest that the existence of PRs along the southern MAR is due to mantle £ow away from hotspots. A similar mechanism, but implying a coupling between mantle £ow from hotspots and small-scale mantle £ow beneath oceanic ridges, is suggested by Rabinowicz and Briais [35,36] to explain the frequent ridge segment migrations away from hotspots. A mechanism intermediate between (2) and (3), where mantle temperature variations drive the propagation, is invoked by West et al. [11,37] for the propagators of the SEIR. The third model has been proposed for midocean ridge areas relatively close to hotspots, like the Reykjanes Ridge south of Iceland, the southern MAR near 9‡S (located 100^200 km west of the Ascension plume), and near 16‡S (located 400^600 km west of Santa Helena plume), and the EPR near Easter Island (e.g., [38]). In this model, the presence of nearby hotspots is expected to cause elevated asthenospheric mantle temperature and lateral mantle £ow, with disruption of the normally well-focused, vertical melt delivery processes at the ridge axis [34]. This model does not apply to the PAR propagators, since there is no evidence for the in£uence of hotspot on the PAR in this area. In particular, major and trace elements, and isotopic compositions of basalts from the PAR collected at 14 dredge sites south of Udintsev FZ, fall within the ¢eld of depleted MORBs of the EPR and are geochemically devoid of any hotspot in£uence [39,40]. Moreover, the along-axis bathymetric pro¢le only reveals local depth variations, but no long-wavelength trend (Fig. 4; [26]).
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In the second model, the PR propagates like a crack in the lithosphere in response to a stress concentration at its tip, characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. The forces driving ridge propagation and creating a positive contribution Kd to K are gravitational spreading forces due to isostatically compensated excess ridge topography along the growing ridge. For reasonable estimates of the parameters, the predicted force resisting propagation and creating a negative contribution Kr to K is approximately that required to produce the observed tip depression. The stress intensity factor is: K1 ¼ Kr þ Kd ¼
Z 2 L 1=2 L ð b l 3 b w ÞgN ðyÞ H dy H Z ðL2 3y2 Þ1=2 2 0 ð1Þ
for a symmetrically loaded crack of length 2L, and: Z 1 2 1=2 L ð b l 3 b w ÞgN ðyÞ H dy K2 ¼ Kr þ Kd ¼ H Z 2 y1=2 0 ð2Þ for a half-in¢nite crack in the lithosphere [41], where H is the plate thickness, bl and bw are densities for lithosphere and water, respectively, and d is the elevation change between the ridge axis and o¡-axis sea£oor. We estimated these stress intensity factors in the case of the propagating ridges observed in our study area. We ¢rst predicted the o¡-axis sea£oor topography from satellite gravity data and R/V L’Atalante multibeam data [42] using the method described by Smith and Sandwell [43]. We then used topographic pro¢les along the ridge axis and 20 km o¡-axis to estimate the anomalous gravity-spreading force acting on the PR segment. We ¢nd that K1 s 0 only if L s 450 km and K2 s 0 only if L s 800 km. Within an axial distance of 450 km to the northeast away from the tip of the 167‡W PR, the ridge axis is highly segmented and o¡set to the right by as much as 300 km, due to the cumulative e¡ects of Heirtzler, Le Renard and La Rose TFs. For the 167‡W and 165‡W propagators, it is therefore di⁄cult to invoke a mechanism
in which the triggering force is the gravity due to the along-axis topographic gradient [33]. A similar conclusion was reached by West et al. [11] for propagators along the SEIR. In addition to this estimation, it is also worth noting that, whereas most propagators tend to grow away from shallow sections of ridges [12], the propagator at 167‡W exhibits the lengthening of a deeper segment at the expense of a shallower one: the along-axis depth northeast of the propagator is fairly constant near 2300 m (except near large TFs), whereas the section between the propagator and Pitman FZ is as shallow as 2100 m (Fig. 4). Also, the propagating segment near 167‡W is characterized by an axial graben, whereas the doomed and failing ridges display an axial dome morphology (Fig. 2). These observations suggest that the initiation of rift propagation is not controlled by the robustness of magmatic supply and tend to preclude the hotspot model, as well as the crack model, to explain the driving mechanism of propagator initiation. Numerous observations lead authors to propose ridge axis rotation and propagation as mechanisms by which spreading centers can reorient following changes in the direction of sea£oor spreading [2,15,44]. In the case of the PAR, a major change in the relative plate motion occurred during the late Neogene [24], as evidenced by an abrupt, 8‡ clockwise rotation of the abyssal hills along the Pitman £owline near the young end of Anomaly 3a (5.9 Ma) [45]. This age approximately coincides with the age of inception of the 167‡W PR, shortly after Chron 3a, estimated from the age of the intersection of the PR pseudofaults with the Heirtzler FZ, based on the satellite-derived gravity map and the isochrons estimated by Ondre¤as et al. [26] (Fig. 5). In the gravity anomaly map, the propagator outer pseudofault is marked by a linear gravity low on the Antarctic £ank, and the failed rifts are marked by a series of highs and lows on the Paci¢c £ank of the PAR (Figs. 1 and 5). These signatures intersect Heirtzler FZ near a change in its direction likely to result from a change in the spreading direction (Fig. 5). Therefore, we suggest that the change in plate motion near Chron 3a triggered the rift propagation near 167‡W.
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6.2. From TF to PRs: evolution of the Heirtzler TF The evolution of the Heirtzler TF is marked by a series of kinks in the FZ, re£ecting sudden changes in the spreading direction (Figs. 1 and 5). One of the most prominent is observed near Chron 3a (5.9 Ma), and coincides with the inception of the 167‡W propagator. At that time a short segment seems to have developed within the 100 km-o¡set TF, in a transtensional basin, resulting in the split of the transform o¡set into two shorter o¡sets (Figs. 5 and 6). We infer that the formation of an extensive relay zone within the transform valley has developed into the propagation of the short accretionary segment. Spreading segments may develop when the rightstepping transform valley undergoes extension after a clockwise rotation of the opening direction. Such short, intra-transform segments are observed along many EPR TFs like the Siqueiros or Garrett TFs, or along PAR TFs like the Eltanin system, and inferred to result from the rotation of the spreading direction (e.g., [46^48]). Bird and Naar [15] suggest that the development of the Easter and Juan Fernandez microplates originated from intra-transform segment propagation following a change in the Paci¢c^Nazca relative plate motion. Examples of development of ridge segments near or within OSCs are also described along the southern EPR, with the ‘bisection’ of one OSC into two distinct ones [18]. In this case, a mechanism involving the propagation of a ridge segment due to excess relief cannot explain how new segments are spawned near or within OSCs, where the ridge axis is deeper [18]. 6.3. Fossil propagators on the £anks of the PAR. Implications for the gravity signature of PRs Ge¤li et al. and Ondre¤as et al. [25,26] have shown that the boundaries of the large V-shaped structure south of Udintsev FZ are underlined either by the fossil traces left on the sea£oor by numerous transitions from FZ to non-transform discontinuities, or by gravity lows marking pseu-
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dofaults related to ancient propagators. Indeed, pseudofaults, oblique to spreading, are observed on the Antarctic plate near 60‡S, 134‡W; 61‡S, 141‡W; 61‡30PS, 147‡W; and 64‡S, 151‡W, with their conjugate on the Paci¢c plate near 55‡S, 156‡W; 57‡S, 157‡W; 158‡W, 58‡30PS; and 61‡S, 161‡30PW, respectively (Fig. 1; [26]). Based on a detailed analysis of the evolution of spreading rates along the PAR since Oligocene times, Ondre¤as et al. [26] show that reorganizations in the geometry of the spreading boundaries are systematically recorded in lithosphere generated at full spreading rates of about 50^60 mm/yr, close to the threshold value between a slow-spreading ridge producing rough sea£oor and a fast-spreading ridge generating smooth sea£oor. The preferred observation of pseudofaults, in the gravity anomaly maps of the PAR derived from satellite altimetry measurements, at spreading rates close to that threshold, suggests that the crustal structure and deformation near the propagators are characteristic of these spreading rates. The pseudofault gravity signature probably results from anomalously thin or dense crust formed at the tip of the propagator, due to lithospheric stretching and low magmatic input at the rift tip [7,8,49]. The gravity signature of major pseudofaults associated with microplates in the Paci¢c is comparable to the signature of slow-spreading TFs, due to the decrease of spreading rate toward the tip of the propagator [49]. It is possible that propagators have formed in the fast-spreading sections of the PAR, but that they do not appear in the satellite altimetry-derived gravity map because the associated crustal structure is not signi¢cantly different from adjacent crust. This would occur if the age o¡set at the propagator is relatively small (1 Myr according to [49]), or if there is no overlap associated with the propagator, so that there is no signi¢cant decrease of the spreading rate toward the propagator tip. At spreading rates closer to the threshold, like in the present-day transition zone, the rheology of the young lithosphere allows for the development of propagators following kinematic changes, and the record of their pseudofaults on the £anks of the ridge.
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7. Summary Our analysis of bathymetry and magnetic data, combined with satellite altimetry data, reveals that the 167‡W propagator corresponds to a relatively large, 43 km o¡set of the PAR. Its evolution is marked by a series of failed rifts, the location of which may be predicted by a model of overlapping propagator with cyclic rift failure. We show that the 167‡W PR did not originate from a gravity slope, nor from the input of a hotspot, but that it was triggered by a change in spreading direction shortly after Chron 3a, between 6 and 5 Ma. The propagator likely evolved from the development of an extensional relay zone within the Heirtzler transform valley in response to the change in spreading direction. Kinematic changes also probably triggered other propagators whose pseudofaults outline, in the gravity maps derived from satellite altimetry measurements, the V-shaped boundary between rough and smooth sea£oor generated at the slow- and fast-spreading PAR, respectively. The signature of the propagators in the gravity maps appears to be speci¢c to these 50^60 mm/yr full spreading rates.
Acknowledgements We thank the captain, crew, and scienti¢c party of the Pacantarctic cruise of R/V L’Atalante. Most of the ¢gures were prepared using the GMT software developed by P. Wessel and W. Smith. The paper bene¢ted from constructive reviews by Martin Kleinrock and an anonymous reviewer. This work was supported by INSU, Centre National de la Recherche Scienti¢que.[AC] References [1] J. Shih, P. Molnar, Analysis and implications of the sequence of ridge jumps that eliminated the Surveyor transform fault, J. Geophys. Res. 80 (1975) 4815^ 4822. [2] R.N. Hey, A new class of pseudofaults and their bearing on plate tectonics: A propagating rift model, Earth Planet. Sci. Lett. 37 (1977) 321^325.
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[48] R.A. Pockalny, P.J. Fox, D.J. Fornari, K.C. Macdonald, M.R. Per¢t, Tectonic reconstruction of the Clipperton and Siqueiros fracture zones: evidence and consequences of plate motion changes for the last 3 Myr, J. Geophys. Res. 102 (1997) 3167^3181. [49] S.E. Kruse, S.F. Tebbens, D.F. Naar, Q. Lou, R.T. Bird, Comparisons of gravity anomalies at pseudofaults, frac-
ture zones, and nontransform discontinuities from fast to slow spreading areas, J. Geophys. Res. 105 (2000) 28399^ 28410. [50] S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic, J. Geophys. Res. 100 (1995) 6093^ 6095.
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Analysis of propagators along the Paci¢c^Antarctic Ridge: evidence for triggering by kinematic changes Anne Briais a; , Daniel Aslanian b , Louis Ge¤li b , He¤le'ne Ondre¤as b a
LEGOS-GRGS, CNRS UMR 5566, Obs. Midi-Pyre¤ne¤es, 18 Ave E. Belin, 31401 Toulouse Cedex 4, France b De¤partement des Ge¤osciences Marines, IFREMER, B.P. 70, 29280 Plouzane¤, France Received 5 February 2002; accepted 1 March 2002
Abstract We analyze the structure and evolution of two propagators along the Pacific^Antarctic Ridge (PAR) that we surveyed during the Pacantarctic cruise of the N/O L’Atalante. A large propagator at 63‡30PS, 167‡W shows a N50‡Etrending segment of the PAR propagating southwestward, while the adjacent, N45‡E-trending segment retreats. The propagating and doomed ridges are offset by about 43 km. They both curve into the offset to define an overlap zone about 25 km long. The inner pseudofault is juxtaposed to a series of E^W-trending ridges inferred to represent the failed axes. Their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure. The pseudofaults are 35 ? 5‡ oblique to the propagating ridge, which implies a rate of propagation of 44 ? 8 mm/yr, using a 62 mm/yr full spreading rate, comparable to that of other propagators with similar morphology. The age of the initiation of the propagation from the Heirtzler fracture zone is estimated to be 5^6 Ma, which coincides with the age of a clockwise change in spreading direction. A second, smaller, southwestward propagator is observed northeast of the major one, at 63‡15PS, 165‡10PW, with a morphology very similar to that of the larger one. It is inferred to have started near 1 Ma, again at the time of a clockwise change in spreading direction along the PAR. These two propagators are likely to have evolved from extensional relay zones which developed within the Heirtzler transform fault (TF) valley following clockwise changes in spreading direction. The present-day axial discontinuity is less than 40 km in offset and may not be a TF anymore. The development of propagators in this area of the PAR appears to be triggered by kinematic changes rather than by thermal gradients along the ridge. Other propagators that have left similar signatures on the flanks of the PAR, appear to have developed at similar spreading rates near 50^60 mm/yr full rate, as a result of kinematic changes. C 2002 Elsevier Science B.V. All rights reserved. Keywords: mid-ocean ridges; propagation; sea-£oor spreading; rates; kinematics
1. Introduction Most propagators along mid-ocean ridges are
* Corresponding author. Tel.: +33-5-61-33-29-37; Fax: +33-5-61-25-32-05. E-mail address: [email protected] (A. Briais).
characterized by a ridge segment (propagating rift, PR) growing at the expense of another (dying rift, DR). They were ¢rst suspected from their disruptive signature in magnetic anomaly maps, oblique with respect to both spreading and axis directions (e.g., [1^3]). They were then extensively surveyed and described tectonically and geophysically, for example on the Paci¢c^Juan de Fuca
0012-821X / 02 / $ ^ see front matter C 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 5 6 7 - 8
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Ridge [4], the Galapagos spreading center [5^9], the South-East Indian Ridge (SEIR) [10^12], the North Fiji and Lau Basin ridges (e.g., [13,14]), the East Paci¢c Rise (EPR) [15^19] or the Mid-Atlantic Ridge (MAR) [20^22]. Structural observations of propagators can be accounted for by kinematic models in which the propagating ridge cuts through the young lithosphere created at the DR, with a fairly continuous propagation, but episodic failure of the retreating ridge [1,3,5,23]. Although the kinematic evolution of rift propagation is now well described, the mechanisms triggering and driving the propagation are still debated. We describe two propagators near 167 and 165‡W on the Paci¢c^Antarctic Ridge (PAR) south of Udintsev fracture zone (FZ), and analyze the mechanical conditions for their development. We show that models implying a gravity gradient or a longitudinal £ow away from a hotspot do not apply in our study area. We present evidence that an axial reorganization of the PAR, implying the evolution of the Heirtzler TF into a series of smaller discontinuities, has triggered these propagators. We show that kinematic changes are also probably responsible for other propagators whose pseudofaults outline, in the gravity maps derived from satellite altimetry measurements, the V-shaped boundary between rough and smooth sea£oor generated at the slow- and fast-spreading PAR, respectively.
2. Tectonic setting The PAR is unique among the major spreading centers because the Euler pole of rotation has been very close to the ridge axis since the early opening between the Paci¢c and the Antarctic plates about 90 Myr ago. This situation implies that even small changes in the kinematic parameters (either the location of the pole or the velocity of the relative plate movement) have signi¢cant consequences on the geometry and morphology of the spreading plate boundary [24^26]. Changes in the location of the Euler pole of rotation are re£ected by changes in the curvature of FZs, clearly observed on the altimetry-derived gravity
anomaly map (Fig. 1; [27]). The spreading rate, which acts as a governing parameter controlling the axial morphology, increases rapidly with distance from the Euler pole, presently located at 64.25‡S, 100.94‡E [24]. Multibeam bathymetry and magnetic data collected with R/V L’Atalante in 1996 [25,26] indicate that south of the Pitman TF near 175‡W, the ridge is spreading slowly, with a half-rate of about 28 mm/yr, and is characterized by an axial valley. In the north, near the Udintsev TF, the ridge axis has a fast spreading rate of 38 mm/yr (half-rate), and is characterized by an axial high. A transition zone extends over 650 km along-axis between the two domains. It is characterized by a very variable axial morphology displaying a shallow axial valley, a rifted high or an axial dome, and by the presence of propagators [26]. Two domains of di¡erent roughness of the gravity anomaly derived from satellite altimetry measurements appear on the £anks of the PAR between Udintsev FZ and 180‡W: a rough sea£oor characterized by a high density of wellmarked discontinuity traces, and a smooth sea£oor comparable with the oceanic basins that have generally been formed at fast-spreading centers. Between the Udintsev and the Saint Exupery FZ, these two domains are separated by a large, V-shaped boundary pointing south, whose tip would meet the ridge axis near 62‡30PS, 157‡W, outlined by oblique structures located symmetrically with respect to the present-day ridge axis (Fig. 1; [28]). All the sea£oor created after Chron 3a, however, appears smooth on the gravity map, so that the very tip of the V is missing. The roughness contrast is interpreted to re£ect changes in axial morphology, segmentation and geometry which occur when the full spreading rate reaches a threshold of 50^60 mm/yr. As the angular velocity of the Paci¢c^Antarctic relative motion increased, this threshold in spreading rate propagated southward for more than 1000 km along the ridge axis, since Oligocene time (shortly before Chron 13o, about 30 Ma) [25,26,28]. Our study area is located about 400 km southwest of the broad V tip, and within the present-day axial morphology transition zone.
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Fig. 1. Shaded map of the free-air gravity anomalies estimated from satellite altimetry over the PAR area (from [27]). Illumination from North. White bold lines are routes of the Pacantarctic cruise of R/V L’Atalante. Bold black lines show PAR axis. Dotted black lines are inferred traces of other propagators on the ridge £anks. Red and yellow boxes show areas represented in Figs. 2 and 5, respectively. Inset: Location of study area. Black lines are plate boundaries. ANT: Antarctic plate; PAC: Paci¢c plate.
3. Morphology and structure of the PRs near 167 and 165‡W Most tectonic structures observed on the multibeam bathymetry at the 167‡W propagator on the PAR (Fig. 2) are comparable to those previously de¢ned on other propagators; therefore, we describe the interpreted structural elements using the terminology de¢ned for propagating ridge systems [5,7^9,23]. The propagating ridge is a 150 km-long ridge segment trending N50‡E (Fig. 2; [26]). The axis displays a 600^1000 m-deep, 6^14 km-wide axial valley. The southern tip of the propagator curves
from a N50‡E to an E^W orientation and terminates into an 8 km-wide, rhomboidal graben 500 m deeper than the rest of the axial valley, interpreted to re£ect amagmatic extension occurring at the very end of the propagating system. The central magnetic anomaly across the PR is anomalously narrow on the pro¢les PR6 and PR7 crossing its southern tip (Fig. 3). Away from the tip, the axial anomaly gets wider toward the northeast (Figs. 2 and 3), and the full, across-axis opening rate of 62 mm/yr is reached near 63‡20PS, 166‡W. This observation is consistent with the decrease in spreading rate towards the ends of the overlapping segments, as magmatic accretion is progres-
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Fig. 2. Top: Color-shaded relief map of the 167‡W propagator from multibeam bathymetry in the area covered by our survey. Illumination from North. Solid black lines are magnetic anomalies projected along track. Thin dotted lines are magnetic pro¢les. Black dashed lines are pseudofaults. Blue dashed lines are inferred failed rifts. Symbols are picked magnetic anomalies from [24] and this study: black symbols are magnetic picks, red symbols are conjugate picks after Euler rotation. Bold lines are isochrons inferred from the combination of magnetic anomalies and satellite gravity data. Bottom: Tectonic interpretation of the multibeam bathymetric data. Thin lines are ridge crests corresponding to abyssal hills. Barbed lines are major fault scarps. Ticks point downhill. Striped areas are tectonic reliefs and o¡-axis volcanoes. Solid double red line is present-day ridge axis. Dotted double red line is failing ridge. Green dashed lines are pseudofaults. 6
sively transferred from one segment to the other [5^7] (Fig. 3). The failing ridge overlaps the tip of the propagator by about 25 km. Its axial morphology, as well as that of the doomed ridge farther south, is characterized by an 8 km-wide, 300 m-high dome (Fig. 2). The orientation of the doomed ridge is N45‡E, implying a 5‡ change in spreading center azimuth between the propagating and the doomed ridges. The failing ridge tip curves toward the propagator in the overlap region to reach an E^W orientation. The propagating and the doomed ridges are o¡set by 43 km, creating a wide area where lithosphere generated by the doomed ridge on the Antarctic plate is transferred to the Paci¢c plate. This transfer zone is marked by a series of curved abyssal hills, bending from N50‡E to N^S, bounded by the overlapping spreading ridges. In map view, these abyssal hills are arranged in a fan near the PR, with one side of the fan parallel to the active spreading center. This arrangement of the ridges is explained partly by the decrease in spreading rate towards the tip of the overlapping spreading ridges, and partly by the rotation of the ridges about vertical axes in a set of blocks in the overlap zone, in a bookshelf faulting system [8,23,29]. The outer pseudofault zone is de¢ned as the boundary, on the Antarctic plate, between the lithosphere created at the doomed ridge and that created at the propagating ridge. It is marked by a series of graben bounded by N85‡E-trending normal faults, and can be followed between 63‡47PS, 167‡20PW and 63‡42PS, 165‡15PW, where the multibeam bathymetry coverage is complete, as well as on the swath near 63‡40PS, 164‡W, where the same N85‡E direction appears. The inner pseudofault zone is the boundary on the Pa-
ci¢c plate between lithospheres created at the doomed and propagating ridges. Near 63‡20PS, 166‡40PW, it is disrupted by two volcano-tectonic structures reaching a relief of 800 m above the surrounding sea£oor. West of the inner pseudofault, a zone of intense tectonic deformation about 30 km wide represents the o¡-axis trace of the zone of transferred lithosphere. We infer
Fig. 3. Identi¢cation of magnetic anomalies based on the kinematic parameters of [24] and the geomagnetic time scale of [50]. Gray area marks the location of the zone of transferred lithosphere.
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Fig. 4. Axial bathymetric pro¢le along the PAR, shown as a function of distance to westernmost axial point surveyed during Pacantarctic, at 65‡58PS, 175‡W. Arrows point to propagator tips.
that the N80^85‡E-trending ridges observed in this zone on the two bathymetric swaths are failed rifts representing relict segments of the retreating ridge (Fig. 4). Their nearly E^W orientation is the same as that of the failing ridge tip, re£ecting the rotation of the spreading axes in the overlap zone. Despite the incomplete bathymetric coverage, we suspect that the overlapping spreading center observed at 63‡15PS, 165‡10PW is a second, smaller propagator, because the tectonic structures associated with it are very similar to those described for the 167‡W propagator (Fig. 2). In particular, the deep basin which marks the end of the northeastern segment is characteristic of a propagator tip, and is much deeper than the end of the other segment. The failing rift of this second propagator is the other end of the same ridge segment as the larger, 167‡W propagator, so that a single ridge segment is propagating on one end and retreating on the other end. The two ridges of the 165‡W propagator are o¡set by 18 km and overlap by 25 km. The segment ends curve toward the overlap zone in map view. The transfer zone is marked by curved abyssal hills fanning away from the spreading ridges. The central magnetic anomaly along the pro¢le crossing the two overlapping branches exhibits a normal width, and is centered on the western, retreating branch. The modeling of this magnetic anomaly pro¢le indicates that extension at the propagating segment is mostly amagmatic, with the production of new crust mainly occurring at the retreating branch (Fig. 3).
4. Kinematics of the propagation The retreat of the doomed rift of the PAR 167‡W propagator is marked by a series of failed rifts oriented E^W, parallel to the present-day orientation of the failing ridge, suggesting that the evolution of the propagating system is not steady-state, but characterized by episodic jumps of the failing ridge, as commonly observed (e.g., [19]), and described by the model of overlapping rift propagation with cyclic rift failure proposed by Wilson [23] (Figs. 2 and 5). This kinematic model not only accounts for the presence, distribution and orientation of the failed rifts, but also for the observed curvature of the spreading axes. It is to be noted that the curving of the overlapping axes is similar to that of other large OSCs observed on the EPR and PAR (e.g., [19,30]). In particular, the curving of the individual failing ridges in map view is opposite to that described for the 95.5‡W Galapagos PR and predicted by a kinematic model of continuous propagation of McKenzie [31]. Within the framework of Wilson’s model [23], the distribution of the observed failed rifts suggests that the evolution of the propagating system involves long cycles between ridge jumps. Since the oldest lithosphere covered to the northwest of the area is about 2 Ma, the observation of two failed rifts implies cycles about 1 Myr long. The failed rifts, however, may be closer to one another than observed, since the bathymetric coverage is incom-
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Fig. 5. Top: Satellite altimetry-derived, 1 min-resolution gravity map of the Heirtzler TF area. Illumination from north. Black solid lines are topographic lineations identi¢ed from multibeam bathymetry (Fig. 2). Symbols and bold lines as in Fig. 2. White solid lines show ridge axis. Yellow dashed lines are propagator pseudofaults. Bottom: Map of magnetic isochrons, tectonic lineations identi¢ed from multibeam bathymetry (thin black lines), and simpli¢ed contours of gravity highs (red) or lows (blue).
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Fig. 6. Sketch of the evolution of the Heirtzler FZ showing development of the PRs within the transform valley following changes in spreading direction. Arrows mark kinematic changes derived from the Euler poles estimated by [24]. A change in spreading direction near Chron 3a resulted in the development of a short segment within the transform o¡set, which then started to propagate toward the south. Another change in spreading direction occured near Chron 2a. A more recent change in spreading direction near Chron 1o likely triggered a second propagator. Green, blue, and red lines show isochrons at Chrons 3a, 2a, and 1o, respectively. Thin black lines in bottom right frame are topographic lineations identi¢ed from multibeam bathymetry (Fig. 2). Inset: Variations of spreading direction and spreading rates in the last 10 Myr near the Heirtzler TF, from [24].
plete. In the latter case, the evolution would imply shorter cycles. The angle of the pseudofaults with the 167‡W propagating ridge is 35 ? 5‡, which implies a southwestward propagation rate of 44 ? 8 mm/yr, using a full spreading rate of 62 mm/yr.
5. Relationship between the propagators and the evolution of the Heirtzler transform The outer pseudofault of the 167‡W propagator is observed to meet the Heirtzler FZ near 163‡30PW, 63‡40PS, implying that the propagator
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initiated within the TF (Fig. 5). To estimate the age of origin of the propagators and understand the evolution of the plate boundary in this area, we analyze the magnetic anomalies identi¢ed on the few pro¢les crossing the area (Cande, written comm., 1999; [26]), and combine the magnetic picks to draw isochrons. The isochrons are also de¢ned using the interpretation of the altimetryderived gravity map, which permits the identi¢cation of the traces of TFs and non-transform discontinuities [26]. The age of inception of the 167‡W propagator is thus estimated to be about 5^6 Ma (shortly after Chron 3a). It is important to note that the intersections of the inner pseudofault and the outer limit of the transferred lithosphere with Heirtzler FZ are close to kinks in the FZ which re£ect a change in spreading direction at that time (Fig. 5). The present-day Heirtzler TF is ill-de¢ned on the gravity map derived from altimetry measurements [27] (Figs. 1 and 5). The ridge segment whose southern end, near 63‡15PS, 165‡10PW, was covered by bathymetry data during the Pacantarctic survey, appears to continue farther north and to terminate near 63‡S, 164‡20PW. Its axial valley can be followed on the satellite gravity map as a relatively linear low. The gravity high which characterizes the axial area just north of the Heirtzler FZ is also observed in continuation of the ridge segment south to 63‡15PS, 164‡W. The present-day Heirtzler o¡set is therefore likely to be shorter than about 40 km, and to correspond only to the linear, N135‡E-trending low in the gravity map (Fig. 5). The signature of the pseudofaults of the 165‡W propagator is di⁄cult to follow outside the shipboard bathymetric coverage (Figs. 2 and 5). The age of inception of this propagator could be about 1 Ma. In the last 5^6 Myr, the Heirtzler TF progressively evolved into a series of smallero¡set discontinuities, two of them propagating south (Fig. 6; see Section 6).
6. Discussion 6.1. Triggering mechanisms Three mechanisms have been suggested to ex-
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plain the triggering and evolution of PRs. (1) Authors suggested that PRs develop to allow for a ridge reorientation following a plate reorganization, especially in the case of the Juan de Fuca Ridge [2,4,32], or for the Easter and Juan Fernandez microplates on the EPR, where the propagators originate from within TFs [15]. (2) Phipps Morgan and Parmentier [33] present a quantitative model in which a PR propagates as a crack in an elastic material, in response to a stress concentration at its tip, resulting from excess gravity stresses in shallow regions of the spreading axis compared to deeper regions. (3) Brozena et al. [34] suggest that the existence of PRs along the southern MAR is due to mantle £ow away from hotspots. A similar mechanism, but implying a coupling between mantle £ow from hotspots and small-scale mantle £ow beneath oceanic ridges, is suggested by Rabinowicz and Briais [35,36] to explain the frequent ridge segment migrations away from hotspots. A mechanism intermediate between (2) and (3), where mantle temperature variations drive the propagation, is invoked by West et al. [11,37] for the propagators of the SEIR. The third model has been proposed for midocean ridge areas relatively close to hotspots, like the Reykjanes Ridge south of Iceland, the southern MAR near 9‡S (located 100^200 km west of the Ascension plume), and near 16‡S (located 400^600 km west of Santa Helena plume), and the EPR near Easter Island (e.g., [38]). In this model, the presence of nearby hotspots is expected to cause elevated asthenospheric mantle temperature and lateral mantle £ow, with disruption of the normally well-focused, vertical melt delivery processes at the ridge axis [34]. This model does not apply to the PAR propagators, since there is no evidence for the in£uence of hotspot on the PAR in this area. In particular, major and trace elements, and isotopic compositions of basalts from the PAR collected at 14 dredge sites south of Udintsev FZ, fall within the ¢eld of depleted MORBs of the EPR and are geochemically devoid of any hotspot in£uence [39,40]. Moreover, the along-axis bathymetric pro¢le only reveals local depth variations, but no long-wavelength trend (Fig. 4; [26]).
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In the second model, the PR propagates like a crack in the lithosphere in response to a stress concentration at its tip, characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. The forces driving ridge propagation and creating a positive contribution Kd to K are gravitational spreading forces due to isostatically compensated excess ridge topography along the growing ridge. For reasonable estimates of the parameters, the predicted force resisting propagation and creating a negative contribution Kr to K is approximately that required to produce the observed tip depression. The stress intensity factor is: K1 ¼ Kr þ Kd ¼
Z 2 L 1=2 L ð b l 3 b w ÞgN ðyÞ H dy H Z ðL2 3y2 Þ1=2 2 0 ð1Þ
for a symmetrically loaded crack of length 2L, and: Z 1 2 1=2 L ð b l 3 b w ÞgN ðyÞ H dy K2 ¼ Kr þ Kd ¼ H Z 2 y1=2 0 ð2Þ for a half-in¢nite crack in the lithosphere [41], where H is the plate thickness, bl and bw are densities for lithosphere and water, respectively, and d is the elevation change between the ridge axis and o¡-axis sea£oor. We estimated these stress intensity factors in the case of the propagating ridges observed in our study area. We ¢rst predicted the o¡-axis sea£oor topography from satellite gravity data and R/V L’Atalante multibeam data [42] using the method described by Smith and Sandwell [43]. We then used topographic pro¢les along the ridge axis and 20 km o¡-axis to estimate the anomalous gravity-spreading force acting on the PR segment. We ¢nd that K1 s 0 only if L s 450 km and K2 s 0 only if L s 800 km. Within an axial distance of 450 km to the northeast away from the tip of the 167‡W PR, the ridge axis is highly segmented and o¡set to the right by as much as 300 km, due to the cumulative e¡ects of Heirtzler, Le Renard and La Rose TFs. For the 167‡W and 165‡W propagators, it is therefore di⁄cult to invoke a mechanism
in which the triggering force is the gravity due to the along-axis topographic gradient [33]. A similar conclusion was reached by West et al. [11] for propagators along the SEIR. In addition to this estimation, it is also worth noting that, whereas most propagators tend to grow away from shallow sections of ridges [12], the propagator at 167‡W exhibits the lengthening of a deeper segment at the expense of a shallower one: the along-axis depth northeast of the propagator is fairly constant near 2300 m (except near large TFs), whereas the section between the propagator and Pitman FZ is as shallow as 2100 m (Fig. 4). Also, the propagating segment near 167‡W is characterized by an axial graben, whereas the doomed and failing ridges display an axial dome morphology (Fig. 2). These observations suggest that the initiation of rift propagation is not controlled by the robustness of magmatic supply and tend to preclude the hotspot model, as well as the crack model, to explain the driving mechanism of propagator initiation. Numerous observations lead authors to propose ridge axis rotation and propagation as mechanisms by which spreading centers can reorient following changes in the direction of sea£oor spreading [2,15,44]. In the case of the PAR, a major change in the relative plate motion occurred during the late Neogene [24], as evidenced by an abrupt, 8‡ clockwise rotation of the abyssal hills along the Pitman £owline near the young end of Anomaly 3a (5.9 Ma) [45]. This age approximately coincides with the age of inception of the 167‡W PR, shortly after Chron 3a, estimated from the age of the intersection of the PR pseudofaults with the Heirtzler FZ, based on the satellite-derived gravity map and the isochrons estimated by Ondre¤as et al. [26] (Fig. 5). In the gravity anomaly map, the propagator outer pseudofault is marked by a linear gravity low on the Antarctic £ank, and the failed rifts are marked by a series of highs and lows on the Paci¢c £ank of the PAR (Figs. 1 and 5). These signatures intersect Heirtzler FZ near a change in its direction likely to result from a change in the spreading direction (Fig. 5). Therefore, we suggest that the change in plate motion near Chron 3a triggered the rift propagation near 167‡W.
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6.2. From TF to PRs: evolution of the Heirtzler TF The evolution of the Heirtzler TF is marked by a series of kinks in the FZ, re£ecting sudden changes in the spreading direction (Figs. 1 and 5). One of the most prominent is observed near Chron 3a (5.9 Ma), and coincides with the inception of the 167‡W propagator. At that time a short segment seems to have developed within the 100 km-o¡set TF, in a transtensional basin, resulting in the split of the transform o¡set into two shorter o¡sets (Figs. 5 and 6). We infer that the formation of an extensive relay zone within the transform valley has developed into the propagation of the short accretionary segment. Spreading segments may develop when the rightstepping transform valley undergoes extension after a clockwise rotation of the opening direction. Such short, intra-transform segments are observed along many EPR TFs like the Siqueiros or Garrett TFs, or along PAR TFs like the Eltanin system, and inferred to result from the rotation of the spreading direction (e.g., [46^48]). Bird and Naar [15] suggest that the development of the Easter and Juan Fernandez microplates originated from intra-transform segment propagation following a change in the Paci¢c^Nazca relative plate motion. Examples of development of ridge segments near or within OSCs are also described along the southern EPR, with the ‘bisection’ of one OSC into two distinct ones [18]. In this case, a mechanism involving the propagation of a ridge segment due to excess relief cannot explain how new segments are spawned near or within OSCs, where the ridge axis is deeper [18]. 6.3. Fossil propagators on the £anks of the PAR. Implications for the gravity signature of PRs Ge¤li et al. and Ondre¤as et al. [25,26] have shown that the boundaries of the large V-shaped structure south of Udintsev FZ are underlined either by the fossil traces left on the sea£oor by numerous transitions from FZ to non-transform discontinuities, or by gravity lows marking pseu-
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dofaults related to ancient propagators. Indeed, pseudofaults, oblique to spreading, are observed on the Antarctic plate near 60‡S, 134‡W; 61‡S, 141‡W; 61‡30PS, 147‡W; and 64‡S, 151‡W, with their conjugate on the Paci¢c plate near 55‡S, 156‡W; 57‡S, 157‡W; 158‡W, 58‡30PS; and 61‡S, 161‡30PW, respectively (Fig. 1; [26]). Based on a detailed analysis of the evolution of spreading rates along the PAR since Oligocene times, Ondre¤as et al. [26] show that reorganizations in the geometry of the spreading boundaries are systematically recorded in lithosphere generated at full spreading rates of about 50^60 mm/yr, close to the threshold value between a slow-spreading ridge producing rough sea£oor and a fast-spreading ridge generating smooth sea£oor. The preferred observation of pseudofaults, in the gravity anomaly maps of the PAR derived from satellite altimetry measurements, at spreading rates close to that threshold, suggests that the crustal structure and deformation near the propagators are characteristic of these spreading rates. The pseudofault gravity signature probably results from anomalously thin or dense crust formed at the tip of the propagator, due to lithospheric stretching and low magmatic input at the rift tip [7,8,49]. The gravity signature of major pseudofaults associated with microplates in the Paci¢c is comparable to the signature of slow-spreading TFs, due to the decrease of spreading rate toward the tip of the propagator [49]. It is possible that propagators have formed in the fast-spreading sections of the PAR, but that they do not appear in the satellite altimetry-derived gravity map because the associated crustal structure is not signi¢cantly different from adjacent crust. This would occur if the age o¡set at the propagator is relatively small (1 Myr according to [49]), or if there is no overlap associated with the propagator, so that there is no signi¢cant decrease of the spreading rate toward the propagator tip. At spreading rates closer to the threshold, like in the present-day transition zone, the rheology of the young lithosphere allows for the development of propagators following kinematic changes, and the record of their pseudofaults on the £anks of the ridge.
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7. Summary Our analysis of bathymetry and magnetic data, combined with satellite altimetry data, reveals that the 167‡W propagator corresponds to a relatively large, 43 km o¡set of the PAR. Its evolution is marked by a series of failed rifts, the location of which may be predicted by a model of overlapping propagator with cyclic rift failure. We show that the 167‡W PR did not originate from a gravity slope, nor from the input of a hotspot, but that it was triggered by a change in spreading direction shortly after Chron 3a, between 6 and 5 Ma. The propagator likely evolved from the development of an extensional relay zone within the Heirtzler transform valley in response to the change in spreading direction. Kinematic changes also probably triggered other propagators whose pseudofaults outline, in the gravity maps derived from satellite altimetry measurements, the V-shaped boundary between rough and smooth sea£oor generated at the slow- and fast-spreading PAR, respectively. The signature of the propagators in the gravity maps appears to be speci¢c to these 50^60 mm/yr full spreading rates.
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