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Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific)
Earth and Planetary Sctence Letters', 89 (1988) 363-374 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific) R a p a n u i Scientific Party: J. F r a n c h e t e a u ~, P. Patriat ~, J. Segoufin t, R. A r m i j o ~, M. D o u c o u r e ~, A. Y e l l e s - C h a o u c h e t, j. Z u k i n t S. C a l m a n t 2 D.F. N a a r 3 and R.C. Searle 4 I Institut de Physique du GIohe, 4. Place Jussieu. 75252 Paris ('edex 05 (k)'ance) 2 CNES/GRGS, 18. Arenue Edouard Belin. 31055 Toulouse ('edex (l')'ance) 3 llawaii Institute of Geoph.vsics, 2525 ('orrea Road. ttonolulu, HI 96822 [ U. S. A.) 4 lnstttute of Oceanographic Sciences, 14'orml(T, 6"odahnmg. Surrey (ilJ'8 5 UB ( U. K 1 Received February 18, 1988: revised version accepted May 24. 1988 The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be criticalb, constrained by' improved relative motion and structural fabric data sets. During the January 1987 Rapanui expedition on the N / O "Jean Charcot" we conducted a Sea B e a m / m a g n e t i c s / gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern bounda W of the microplate is expressed as a series of parallel N W - S E trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift). The northern end of the Pito Rift merges with an E - W to 083 ° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates. The northern triple junction between the Easter (Rapanui). Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately F - W direction. The southv,'estern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately F.-W zone where shear must be predominant. The southern triple junction is a RRF junction with an overlapping ridge system. The structural data acquired during the survey provide strong constraints for kinematic m(×tels of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.
1. Introduction Herron [1] and Forsyth [2] suggested the existence of a small microplate between the Pacific and Nazca plates, near Easter Island, on the basis of earthquake distribution. Further work [3-9] has shown that the Easter microplate, which should be more appropriately named the Rapanui microplate (from the Polynesian name for Easter Island), has existed for the past 3 Myr and is bounded by seafloor spreading centers to the east and west. and transform fault zones to the north and south (Fig. 1). The Easter (Rapanui) microplate occurs where the fastest seafloor spreading on Earth is occurring today, near the center of a large shallow 0012-821X/88/$03.50
~' 1988 Elsevier .-Science Publishers B.V.
area characterized by a broad region of anomalously-low surface and shear wave velocities [10]. Together with the pattern of helium isotopes and light rare earth elements [6-11], these data indicate very intense mantle convection. Hey et al. [9] suggest that this correlation is unlikely to be coincidental, that the microplate is probably a transient feature caught between very large-scale propagating and failing rifts, and that it may be a modern analog of how long-distance rift jumps occur.
The Easter (Rapanui) microplate is a case example of a large dual spreading center system. Conflicting models have been proposed for the evolution of the microplate. Anderson et al. [3]
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suggested that it is a growing microplate b o u n d e d by stable triple j u n c t i o n s . In contrast, H a n d s c h u macher et al. [4] interpreted the microplate using a p r o p a g a t i n g rift model. Engeln and Stein [5] carried this analysis further a n d showed that the microplate could be described as a rigid plate existing temporarily between the p r o p a g a t i n g and dying spreading centers. Hey et al. [9] confirmed the existence of a transient microplate between two actively spreading segments of the East Pacific Rise which are p r o p a g a t i n g in opposite directions. Recent theoretical models of the tectonic evolu-
tion of dual spreading center systems [12] have explored the effects of shear a n d rigid rotation on the b o u n d a r i e s a n d internal structure of microplates b o u n d e d by such dual systems. The models must be critically c o n s t r a i n e d by improved relative m o t i o n a n d structural fabric data sets in order to d i s c r i m i n a t e between shear a n d rigid models [12]. T h o u g h the Easter ( R a p a n u i ) rnicroplate is the best d o c u m e n t e d of the mid-ocean ridge microplates, most of its b o u n d a r i e s are poorly known. D u r i n g the J a n u a r y 1987 R a p a n u i expedition on
Fig. 1. (a) Setting of the Easter (Rapanui) microplate [9]• (b) Boundaries of the Faster (Rapanui) microplate as deduced from the Rapanui N/O "Jean Charcot" cruise (this paper), and other workers [8.9,14]. Double lines: spreading center: single line: transform fault. Except for the small rift-transform intersection mapped by Marchig et al. [14] near 23 ° 28'S. 115 ° 30'W, the plate boundaries on the western edge of the rnicroplate have been purposely left out because the published data are too limited [9]. The dotted lines indicate the pseudo-faults of Naar and Hey [8]. Earthquake epicenters from the WWSSN network are shown• The boxes outline the three survey areas discussed in the text (A: Pito Deep, B: northern triple junction, C: southwestern microplate boundary). The plate boundaries from the Rapanui cruise result from a preliminary shipboard analysis.
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367 the N / O "Jean Charcot", we conducted a Sea B e a m / m a g n e t i c s / g r a v i t y survey of the northeast rift tip, the northern transform boundary, the northern triple junction, the southwest rift tip and the southern triple junction (Fig. 2). The method that was used was to fully map selected portions of the microplate boundaries. Although this approach is very time consuming with the rather narrow ( - 2 km) swath imaged by Sea Beam and the usual modest speed ( - 10 knots) of conventional oceanographic ships, we believe that it yields more fruitful results in terms of tectonic analysis than the broad brush surveys. The structural data resulting from our survey lead us to modify some of the prior models of the microplate. 2. Eastern boundao' (Easter-Nazca) The eastern boundary of the microplate is well defined between 26o50 ' and 25°S from prior work [4-9]. It consists of a rift zone (the Este Rift of Handschumacher et al. [4]) characterized by fast accretion (12 c m / y r ) and rapid (15 c m / y r ) northward propagation. Naar and Hey [7,8] and Sempere et al. [13] locate the tip of a well-defined fast propagating rift near 24°50'S, 112°20'W. North of the tip, the active rift is poorly defined with the present data, but Hey et al. [9] propose a set of dextral offsets of the axial zone up to 23 o S, I I I ° 3 0 ' W . One crossing from the Rapanui expedition tentatively locates the axis of the rift near 24°10'S, 111°43'W. Our profile at this location shows the axis to coincide with a 1000 m deep, 10 km wide valley, trending 354 °, and characterized by a 2 km wide inner floor which reaches a depth of 3800 m.
3. Pito Deep The East Rift can be followed north to its termination near 22°50'S, l 1 2 ° 1 0 ' W (Figs. 3 and 4). North of about 24°50'S, the East Rift is characterized by a median valley [8] with apparent right-lateral offsets. At 23°15'S the axial valley can be located along 111°35'W on the basis of its topographic signature, and further north there are a number of parallel N W - S E trending valleys, the westernmost and deepest of which we take as the axis. In this region, the depth increases dramatically from 3000 to 4500 m over less than 20 km,
reaching a maximum uncorrected depth of 5890 m at 23°00'S, 111°56.5'W. This is the deepest active rift axis mapped in the Pacific today. We propose to call this deep hole the Pito Deep (Pito is the Polynesian word for navel and is of significance in Rapanui mythology which considers the island to be the navel, or centre, of the world). The area surrounding the Pito Deep is characterized by a large number of normal fault scarps that trend generally northwest, but locally have quite variable strikes (Fig. 4). They dip both northeast and southwest, forming a series of nested grabens that enclose the Pito Deep. This deep lies at the foot of a northeast dipping scarp which is the northward continuation of the scarp identified as the inner pseudo-fault by Hey et al. [9]. To the east of the Pito Deep there are three major horst blocks or massifs, separated by shallower and narrower grabens which become more north-south oriented as one moves east. The southern portion of the westernmost block is partially rifted open by a small graben. Scarp heights of about 500 m are common, and they range up to 1500 m. At the northern end of the rift the scarps turn westward, and the grabens are closed off by west-trending, south-dipping faults, except in the east where the N - S grabens give way to a N N E - S S W fabric. Some of the west-trending faults continue westwards into what we interpret as a narrow (about 10 km) band of E N E - W S W striking, presumably dextral, strikeslip faults, which comprise the eastern part of the northern transform boundary. The 3.5 kHz seismic profiles show no significant sediment within the rifted area, except on top of some of the large massifs. Outside of the rifted area (i.e. to the west, north and east) the scafloor has a very different character. The topographic relief is much less (a few hundred meters), a significant veneer of sediment (a few meters) can be detected on the 3.5 kHz records, and the tectonic fabric is quite different. It consists predominantly of small N N E - S S W to N E - S W striking scarps, with heights generally less than 200 m and spacings of a few kilometers. Near the transform zone, and to the south of it, the trends are E N E - W S W . This "exterior" fabric is very similar to the usual tectonic fabric produced by the East Pacific Rise, except for the oblique orientation. There is a clear pattern be-
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twcen the two trends; the N N E - S S W trends become progressively more EN E - W S W as one moves from the east to the west along a line north of the rifted area (Fig. 4). The faults on the large blocks have a similar N N E - S S W trend and similar seafloor fabric. As one moves southward, some of the faults north of the rifted area have increasingly greater throws (up to 500 m), and curve progressively westward, merging with the transform zone trends. In addition, to the north and northeast of the rifted area there are a few minor faults striking N W - S E ; these have much less relief and are much less common than the other trends. We thus interpret these observations to indicate that the northern extension of the East Rift is a complex system, with one or more grabens which have cut into the older Nazca plate. It is unclear which rift within the rifted area is currently active, unless one assumes that only the deepest rift is active. The existence of the major rift grabens east
of the Pito Deep could be a result of westward or random jumping of active rifting sites over time, or joint parallel active rifting. The Pito Deep cannot have been the only active rift since the beginning of rifting because of what appears to be isolated blocks of pre-existing Nazca plate within the rifted area. and the highly asymmetric location of the Pito Deep with respect to the rifted area. Thus. the rifting process in this area has the potential for producing complex age relationships a n d / o r producing a complex history of spreading and stretching that would be difficult to unravel by conventional magnetic anomaly dating techniques. The pattern of the N N E - S S W faults becoming E N E - W S W near the transform zone suggests a few possible interpretations. The N N E - S S W faults may simply be responding to the shear of the transform zone, although this would imply that the transform zone extends further east than the Pito Deep. Alternatively, there could have been an
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earlier episode of East Rift propagation to the north and east of the present position of Pito Deep. This would have rotated the N N E - S S W abyssal hill fabric clockwise in the resulting overlap region between this rift and the East Pacific Rise north of the microplate. However, at present the northern end of the East Rift does not appear to be a simple propagating rift tip.
ridge with a relief in excess of 1700 m. Here the transform fault is still seen as a notch near the summit of the ridge. Our profiles establish the continuity of this ridge, akin to a median ridge typical of transform faults, from 113010 ' to 114°22'W, i.e. over a distance of 120 km. It is typically about 15-18 km wide, stands 1-2 km above the surrounding terrain, and trends roughly 260-265 °. The seafloor is consistently deeper to the south of the ridge by 500-750 m.
4. Northern boundary (Easter-Nazca) 5. Norlhern triple junction Near 22°48'S, 112°17'W, the northern end of the Pito Rift merges with an E - W to 083 ° narrow band of linear faults, interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates. We have mapped this shear zone (which we propose to name the Pito fracture zone) continuously to 22°49'S, 112°40'W. The 4 - 5 km wide principal shear zone is included within a 40-50 km wide domain, characterized by faults trending E - W to 0 7 0 ° i.e. grossly subparallel to the transform fault (Figs. 3 and 4). There is a marked asymmetry between the two limbs of the transform domain. In the north, the faults swing from a N N E (030°), to a N E (060°), and finally ENE (070 ° ) trend, between 112020 ' and 112°50'W, and end up merging with the transform fault west of 1 1 2 ° 5 0 ' W (Fig. 4). In the south, the faults have a single trend close to that of the Pito transform fault. Between 112o25 ' and 112°35'W, the transform domain shows subdued relief with a mean depth of approximately 3500 m. The transform fault appears as a 2 km wide valley in the middle of a 35 km wide raised massif. West of 112°40'W, the transform fault is better marked in the topography and shows as a 1 km wide valley cutting through a narrower (15 km) massif. At 112°47'W, the valley disappears and only a small notch remains on the flank of a narrow (8 km), 600-800 m high ridge. Each crossing of the transform fault between 112o20 ' and 1 1 2 ° 5 0 ' W is marked by a steep gradient of the magnetic anomaly, negative to the south, suggesting that crust of differing polarity is present on either side of the transform fault. The N / O "Jean Charcot" crossing of the Pito transform fault at 23°S, l 1 3 ° W shows a very high
West of 1 1 4 ° 2 0 ' W and up to 114°40'W, we renewed systematic mapping for a distance of 40 km in the region of the expected Easter-NazcaPacific northern triple junction. The region of the triple junction, located at 23°02.5'S, 114°31.5'W, is marked by an abrupt termination of the 002 ° axial ridge of the East Pacific Rise against a fault zone striking 088 ° (Fig. 5). The large ridge characterizing the transform fault west of Pito Rift disappears east of the triple junction near 1 1 4 ° 2 0 ' W , and gives way to a low ridge which continues to at least 114°32'W. Along the northern edge of this ridge we have identified the Nazca-Easter and Pacific-Easter transform faults, which appear to be colinear. The Pacific-Easter transform fault, at the triple junction, differs by about 10 ° from the azimuth of the transform fault (078 °) mapped by Marchig et al. [14], near 23°28'S, 1 1 5 ° 3 0 ' W . It also differs from the 081 ° motion predicted from the Euler poles of Engeln and Stein [5], and N a a r (unpublished). A large massif, 10 by 20 km, stands south of the linear ridge between 114°27 ' and 114°40'W. The massif is faulted on the northern side, with a 1000 m high, steep, north-dipping fault scarp. It appears, however, to bear an accretion fabric consisting of N - S ridges and troughs.
6. Southwestern boundary (Easter-Pacific) Prior to the Rapanui expedition, the southern boundary of the Easter (Rapanui) microplate was the least well defined of the microplate boundaries. In particular, the location and nature of the plate boundary east of 1 1 4 ° 3 0 ' W , and the location of
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the southern triple junction, were not resolved with existing data. The westernmost crossing of the southwestern boundary at 26°24'S, 1 1 5 ° 2 0 ' W shows that the axis of accretion, bctween the Easter (Rapanui) and Pacific plates, is in a small (6 km wide) valley which is perched in the middle 114°33
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Fig. 5. Bathymetry of the northern Easter-NaTca-Pacific triple junction showing the three active plate boundaries (box B in Fig. 1). The arca shown was totally insonified by Sea Beam. Contour interval 100 m (solid). 20 m and 50 m (dashed).
of a raised axial block about 30 km in width (Fig. 6). The axial block is itself lying in the middle of an 80 km wide valley, bordered by two linear ridges that stand 1500 m above the valley floor, the two southwestern pseudofaults of Hey et al. [9]. There is a distinct difference between the trend of the southern linear ridge (285 290 o ), and that of both the axis of accretion (320°), and the northern linear ridge (315°). South of the southern ridge, typical N - S accretion fabric in a scdimented terrain is apparent, evidently created by Pacific-Nazca accretion. The axis of Easter-Pacific accretion is marked by small en-echelon depressions (undetected by Hey et al. [9]), e.g. the 4 km deep at 1 1 5 ° 0 5 ' W , and turns progressively from a 320--325 ° trend at 26°35'S, ! 1 5 ° 0 5 ' W , to a 340 ° trend at 26°38'S, 1 1 4 ° 5 7 ' W . The depressions are typically oval in shape (10 km long and 4 km wide), are 400-800 m deeper than the surrounding terrain, and are linked by linear valleys trending 070-080 °. The morphological pattern is similar to that found in the Gulf of California. The last clear axis of accretion is found at 26°40'S, 1 1 4 ° 5 6 ' W : further southeast it lcads into a 2 km wide valley which turns progressively to a 295 ° trend at 26°45.5'S, 1 1 4 ° 5 0 ' W . This location corresponds to the first expression of a transform fault, marking the boundary between the Easter and Pacific plates east of 1 1 4 ° 5 0 ' W . We propose to namc this active zone the Orongo fracture zone, Orongo being the name of a sacred site in Rapanui mythology. Further east, the transform valley runs into an 85 km long, 5 - 1 0 km wide ridgc, which stands more than 2200 m above the surrounding terrain (reaching a minimum depth of 790 m at 26°46.5'S, 114°39'W), and is bordered to the south by the N--S Pacific-Nazca accretion fabric. This ridgc merges north of the plate boundary (near l 1 4 ° 1 0 ' W ) with another ridge of similar shape, which in turn represents the eastern prolongation of a ridgc previously discussed (near 26°10'S, l 1 5 ° W , Fig. 6). Thc transform fault coincides with this northern ridge between 114007 ' and I I 3 ' 1 0 ' W , i.e., over a distance of 100 km (Fig. 1). The 20 km wide ridgc diminishes rapidly in relief from over 2000 m west of 113°50'W, to 1000 m at 1 1 3 ° 3 0 ' W , and finally to less than 200 m at 1 1 3 ° 0 5 ' W , in close proximity to the triple junction. Ahhough there are locally curved segments,
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Fig. 6. Bathymetry of the southwestern boundary of the Easter (Rapanui) microplate (box C in Fig. 1). The active plate boundary is drawn on the figure. Depths greater than 2500 m and greater than 3500 m are shown with light and medium shading, respectively. Scarps bounding the rift zone are indicated. Sense of motion is marked on the Orongo transform fault.
the ridge, and thus the southern plate boundary, continues approximately along 26 o 50’s. 7. The southern triple junction The region of the southern triple junction was intensely surveyed between 112”56’-113”22’W and 26”40’-27”lO’S (Fig. 2). The axis of accretion between the Easter and Nazca plates is well known from the work of several authors [6,8,9,15,16]. It corresponds to a high-standing ridge, reaching a minimum depth of 2100 m at 26 o 30’S, 112 o 38/W, in a region characterized by a marked 3He enrichment [ll]. The ridge strikes 002” (and reaches 2100 m at 26”3O’S [9]). It plunges along strike both to the north, where it reaches a small overlap region at 25 o 55’S [8], and
to the south until 26”5O’S. Further south, this ridge progressively veers to the west until it reaches 112”5O’W at 27”17’S. The ridge is north-south between 27”17’ and 27”3O’S, where it is offset to the west, probably through a small overlap region. The ridge is rather subdued south of the shallow axis near 26 “30’S, and its recognition is based primarily on continuity in plan form and morphology of the summit region because the magnetic anomaly pattern in the area has not yet been unravelled. A second ridge, along 113”-113”05’W and about 20 km west of the Easter-Nazca axis, is present between 26O50’ and 27O37’S. It strikes N-S, is remarkably continuous, appears more developed than the axial ridge, and marks the eastern terminus of the E-W structures associated
372 with the Orongo fracture zone (Fig. 1). At 27 °21'S, 113°03'W, this ridge stands 300-500 m above the surrounding seafloor and 200 m above the more easterly ridge. The region of the triple junction is defined by the intersection of the Orongo E - W structures, and the N - S faults of the western ridge near 26°53'S, 1 1 3 ° 0 2 ' W (Figs. 1 and 2). 8. Discussion
The new observations reported above have important implications for growth processes of microplates and previously published tectonic analyses of the Easter (Rapanui) microplate. The pattern of normal faults and horsts and grabens in the Pito Deep area at the northern terminus of the East Rift and the relationship between the normal faults and the Pito fracture zone enable us to derive a well-constrained relative motion (70-75 ° ) at this location (Fig. 4). This direction is consistent with pure strike-slip on the eastern part of the Pito fracture zone which may thus behave as a true transform fault near its intersection with the Pito rift. This direction of slip is in conflict with that deduced from the Easter-Nazca pole calculated by Engeln and Stein [5]. With the latter pole, the opening direction would be along 100 ° across Pito Deep, and the Pito fracture zone: as mapped by us, would experience along its entire length a large component of compression along a direction varying between 120 ° and 140 ° . Although we are unable with our data to specify the slip vector along the Pito fracture zone west of l 1 3 ° W , the motion proposed by Engeln and Stein [5] is very unlikely in view of the topography along Pito fracture zone. In addition, for stability to be preserved the geometry of the triple junction requires pure strike-slip on the Pito fracture zone at the junction, along an approximately E - W direction (Fig. 5). In deriving the Euler pole for the Easter-Nazca motion, a large weight was given to the northward decrease in spreading rates observed by Handschumacher et al. [4] along the East Ridge [5]. N a a r and Hey [8] questioned the magnetic anomaly identifications of Handschumacher et al. [4] and thus the tectonic model of Engeln and Stein [5]. If we assume pure strike-slip at the northern triple
junction and along the easternmost segment of Pito transform fault at itsjunction with Pito Deep, slight compression occurs in the curved segment of Pito transform fault between 112o30 ' and l 1 3 ° W . It appears premature to compute an Easter-Nazca Euler pole at present without considering the match of magnetic isochrons across Pito Deep but it is clear that the true pole lies much further to the north than the pole of Engeln and Stein [5]. In contrast, the southwestern boundary, as mapped by us, with extension in the West giving way to shear further east up to the southern triple junction, shows good agreement with earthquake mechanisms and thus the relative motion vectors derived from the Easter-Pacific pole of Engeln and Stein [5]. The Sea Beam data are consistent with the southern boundary being a slowly spreading ridge, at least in the western portion, and the northern boundary being a transform fault. The geometry and nature of the microplatc boundaries together with the plate fabric in the vicinity of the boundaries agree with a rigid model for the growth of the microplate [12]. One cannot determine from the survey data in the Orongo fracture :,.one if the border ridges are volcanic or tectonic in origin, but the ridges may have built lines of islands in the past or constitute a modern analogue of how lines of islands were built. The setting of Pito Deep at an intersection between a spreading center and a transform fault is typical of nodal basins often found in regions of rift-transform intersections. The depth is too large, however, for the age (4 Ma) of the truncating edge of the lithosphere, according to the empirical relationship shown by Fox and Gallo [17] (although that relationship may only apply, however, to slow spreading centers). The great depth of the Pito Deep, a record for rift axes along the East Pacific Rise, may be explained by crustal denudation or crustal thinning. Removing the whole crust accounts for about I km of deepening if isostatic balance prevails. The additional 2 km of deepening may be due to lack of isostatic equilibrium. Pito Deep may thus represent an initial stage of rift evolution and be similar to the 5400 m Hess Deep at the rifting tip of the Cocos-Nazca spreading centre [18].
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9. Conclusion The N / O "Jean Charcot" geophysical survey of the Easter (Rapanui) microplate has focused on the poorly defined northern and southern boundaries. The northern end of the East Rift is marked by a series of deep N W - S E grabens, including the 5890 m Pito Deep, which curve into a transform fault that leads to the northern Easter-Nazca-Pacific triple junction. The great depth of the Pito Deep may be due to the tectonic setting of the deep at a rift-transform intersection and possible crustal denudation or abnormally thin crust. Our interpretation of the northern boundary of the Easter microplate is not compatible with the published Easter-Nazca Euler pole [5]. The pole must lie much further north than Engeln and Stein's [5] but a rigorous analysis of both slip vectors and magnetic isochrons across Pito Deep is needed in order to derive the Euler pole positions. The northern triple junction between the Easter (Rapanui), Nazca, and Pacific plates is a RFF type, with the two transform faults colinear along an approximately E - W direction. The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E - W zone where shear must be predominant. The high ridges which border the 80 km wide valley, enclosing the axis of rifting in the western portion of our southern survey (the Hey et al. [9] pseudofaults; Fig. 6), separate a triangular-shaped region where active rifting is occurring between older Pacific and Easter (Rapanui) crust to the south and north, respectively. Further east, in the Orongo fracture zone, the border ridges coalesce and the high relief becomes more subdued towards the southern triple junction. The southern triple junction between the Easter-Nazca-Pacific plates is a RRF junction with an overlapping ridge system, The geometry and nature of the Easter (Rapanui) microplate boundaries together with the plate fabric in the vicinity of the boundaries agree with a rigid model for the growth of the microplate.
Acknowledgements The Rapanui cruise on the N / O "Jean Charcot" was successful due to cooperation from the captain, officers and crew, and the expertise of the engineers and technicians who ran the geophysical systems. The hospitality of the Pascuans was greatly appreciated. We are particularly grateful to V. Marchig, H. Rask, and H. Backer for kindly providing Sea Beam bathymetric data from "Sonne" cruises (Geometep Project), prior to our cruise, and to R.N. Hey for discussions and sharing unpublished data and P. Choukroune for reviewing the manuscript. We thank S. Vappereau for editing the manuscript and G. Aveline for drafting the figures. The cruise was financed by IFREMER and INSU. This is IPGP Contribution No. 1028. References 1 E.M. Herron, Fwo small crustal plates in the South Pacific near Easter Island, Nature Phys. Sci, 240, 35- 37, 1972. 2 I).W. Forsyth, Mechanisms of earthquakes and plate motions in the East Pacific. Earth Planet. Sci. Lett. 17. 189- 193. 1972. 3 R.N. Anderson. I).W. Forsyth, P. Molnar and J. Mammerickx, Fault plane solutions of earthquakes on the Nazca plate boundaries and the Easter plate, Earth Planet. Sci. Lctt. 24, 188-202, 1974. 4 D.W. Handschumacher, R.tt. Pilger, J.A. Foreman and J.R. Campbell, Structure and evolution of the Easter plate. Geol. Soc. Am., Mere. 154. 63-76, 1981. 5 J.F. Engeln and S. Stein. Tectonics of the Easter plate. Earth Planet. Sci. l,cn. 68, 259-270, 1984. 6 J.G. Schilling. H. Sigurdsson, A.N. Davis and R.N. Hey. Faster microplate evolution. Nature 317, 325-331, 1985. 7 D.F. Naar and R.N. Hey, Possible models for the origin and evolution of the Easter microplate. EOS Trans. Am. Geophys. Union 66, 968, 1985. 8 D.F. Naar and R.N. Hey, Fast rift propagation along the t:~ast Pacific Rise near Easter Island. J. Geophys. Res. 91, 3425-3438. 1986. 9 R.N. Hey, D.F. Naar, M.C. Kleinrock, W.J. Phipps Morgan, E. Morales and J.G. Schilling, Microplate tectonics along a superfast seafloor spreading system near Easter Island, Nature 317. 320-325, 1985. 10 J.H. Woodhouse and A.M. Dziewonski, Mapping the upper mantle: three-dimensional modeling of earth structure by inversion of seismic waveforms, J. Geophys. Res. 89, 5953-5986. 1984. 11 H. Craig. K.R. Kim and W. Rison, Easter Island hotspot, I. Bathymetry, helium isotopes, and hydrothermal methane and helium, EOS Trans. Am. Geophys. Union 65, 1140, 1984.
374 12 J.F. Engeln. S. Stein, J. Werner and R.G. Gordon. Microplates and shear zones near overlapping ridge systems. J. Geophys. Res.. in press. 1988. 13 J.('. Sempere. I).F. Naar, J. Gee and R.N. Hey, in preparation. 14 V. Marchig. H. Gundlach and the Shipboard Scientific Party, Ore formation at rapidly diverging plate margins, Results of Cruise Geometep 4, BGR Circ. (Hannover) 4, 3-22, 1987. 15 J. Francheteau, A. Yelles-Chaouche and H. Craig, The Juan Fernandez microplate north of the Pacific-Na~'.ca-
Antarctic plate junction at 35°S, Earth Planet. Sci. Left. 86, 253- 268. 1987. 16 A. Yelles-Chaouche. J. Francheteau and P. Patriat, Evolulion of the Juan Fernandez microplate during the last three million years, Earth Planet. Sci. Left. 86, 269-286. 1987. 17 P.J. Fox and D.G. Gallo, A tectonic model for ridge-transform-ridge plate boundaries: implications for the structure of oceanic lithosphere, Tectonophysics 104, 205-242. 1984. 18 R.C. Searle and J. Francheteau. Morphology and tectonics of the Galapagos triple junction. Mar. Geophys. Res. 8. 95-129. 1986.
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[41
Pito and Orongo fracture zones: the northern and southern boundaries of the Easter microplate (southeast Pacific) R a p a n u i Scientific Party: J. F r a n c h e t e a u ~, P. Patriat ~, J. Segoufin t, R. A r m i j o ~, M. D o u c o u r e ~, A. Y e l l e s - C h a o u c h e t, j. Z u k i n t S. C a l m a n t 2 D.F. N a a r 3 and R.C. Searle 4 I Institut de Physique du GIohe, 4. Place Jussieu. 75252 Paris ('edex 05 (k)'ance) 2 CNES/GRGS, 18. Arenue Edouard Belin. 31055 Toulouse ('edex (l')'ance) 3 llawaii Institute of Geoph.vsics, 2525 ('orrea Road. ttonolulu, HI 96822 [ U. S. A.) 4 lnstttute of Oceanographic Sciences, 14'orml(T, 6"odahnmg. Surrey (ilJ'8 5 UB ( U. K 1 Received February 18, 1988: revised version accepted May 24. 1988 The Easter (Rapanui) microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on Earth is occurring today. Recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be criticalb, constrained by' improved relative motion and structural fabric data sets. During the January 1987 Rapanui expedition on the N / O "Jean Charcot" we conducted a Sea B e a m / m a g n e t i c s / gravity survey of a portion of the microplate boundaries. The method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships. The northern terminus of the East Rift or eastern bounda W of the microplate is expressed as a series of parallel N W - S E trending valleys including what appears to be, with 5890 m depth, the deepest active rift axis mapped in the Pacific today (Pito Rift). The northern end of the Pito Rift merges with an E - W to 083 ° narrow band of linear faults interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates. The northern triple junction between the Easter (Rapanui). Nazca and Pacific plates is a RFF type with the two transform faults colinear along an approximately F - W direction. The southv,'estern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately F.-W zone where shear must be predominant. The southern triple junction is a RRF junction with an overlapping ridge system. The structural data acquired during the survey provide strong constraints for kinematic m(×tels of the microplate. The structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate.
1. Introduction Herron [1] and Forsyth [2] suggested the existence of a small microplate between the Pacific and Nazca plates, near Easter Island, on the basis of earthquake distribution. Further work [3-9] has shown that the Easter microplate, which should be more appropriately named the Rapanui microplate (from the Polynesian name for Easter Island), has existed for the past 3 Myr and is bounded by seafloor spreading centers to the east and west. and transform fault zones to the north and south (Fig. 1). The Easter (Rapanui) microplate occurs where the fastest seafloor spreading on Earth is occurring today, near the center of a large shallow 0012-821X/88/$03.50
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area characterized by a broad region of anomalously-low surface and shear wave velocities [10]. Together with the pattern of helium isotopes and light rare earth elements [6-11], these data indicate very intense mantle convection. Hey et al. [9] suggest that this correlation is unlikely to be coincidental, that the microplate is probably a transient feature caught between very large-scale propagating and failing rifts, and that it may be a modern analog of how long-distance rift jumps occur.
The Easter (Rapanui) microplate is a case example of a large dual spreading center system. Conflicting models have been proposed for the evolution of the microplate. Anderson et al. [3]
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suggested that it is a growing microplate b o u n d e d by stable triple j u n c t i o n s . In contrast, H a n d s c h u macher et al. [4] interpreted the microplate using a p r o p a g a t i n g rift model. Engeln and Stein [5] carried this analysis further a n d showed that the microplate could be described as a rigid plate existing temporarily between the p r o p a g a t i n g and dying spreading centers. Hey et al. [9] confirmed the existence of a transient microplate between two actively spreading segments of the East Pacific Rise which are p r o p a g a t i n g in opposite directions. Recent theoretical models of the tectonic evolu-
tion of dual spreading center systems [12] have explored the effects of shear a n d rigid rotation on the b o u n d a r i e s a n d internal structure of microplates b o u n d e d by such dual systems. The models must be critically c o n s t r a i n e d by improved relative m o t i o n a n d structural fabric data sets in order to d i s c r i m i n a t e between shear a n d rigid models [12]. T h o u g h the Easter ( R a p a n u i ) rnicroplate is the best d o c u m e n t e d of the mid-ocean ridge microplates, most of its b o u n d a r i e s are poorly known. D u r i n g the J a n u a r y 1987 R a p a n u i expedition on
Fig. 1. (a) Setting of the Easter (Rapanui) microplate [9]• (b) Boundaries of the Faster (Rapanui) microplate as deduced from the Rapanui N/O "Jean Charcot" cruise (this paper), and other workers [8.9,14]. Double lines: spreading center: single line: transform fault. Except for the small rift-transform intersection mapped by Marchig et al. [14] near 23 ° 28'S. 115 ° 30'W, the plate boundaries on the western edge of the rnicroplate have been purposely left out because the published data are too limited [9]. The dotted lines indicate the pseudo-faults of Naar and Hey [8]. Earthquake epicenters from the WWSSN network are shown• The boxes outline the three survey areas discussed in the text (A: Pito Deep, B: northern triple junction, C: southwestern microplate boundary). The plate boundaries from the Rapanui cruise result from a preliminary shipboard analysis.
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367 the N / O "Jean Charcot", we conducted a Sea B e a m / m a g n e t i c s / g r a v i t y survey of the northeast rift tip, the northern transform boundary, the northern triple junction, the southwest rift tip and the southern triple junction (Fig. 2). The method that was used was to fully map selected portions of the microplate boundaries. Although this approach is very time consuming with the rather narrow ( - 2 km) swath imaged by Sea Beam and the usual modest speed ( - 10 knots) of conventional oceanographic ships, we believe that it yields more fruitful results in terms of tectonic analysis than the broad brush surveys. The structural data resulting from our survey lead us to modify some of the prior models of the microplate. 2. Eastern boundao' (Easter-Nazca) The eastern boundary of the microplate is well defined between 26o50 ' and 25°S from prior work [4-9]. It consists of a rift zone (the Este Rift of Handschumacher et al. [4]) characterized by fast accretion (12 c m / y r ) and rapid (15 c m / y r ) northward propagation. Naar and Hey [7,8] and Sempere et al. [13] locate the tip of a well-defined fast propagating rift near 24°50'S, 112°20'W. North of the tip, the active rift is poorly defined with the present data, but Hey et al. [9] propose a set of dextral offsets of the axial zone up to 23 o S, I I I ° 3 0 ' W . One crossing from the Rapanui expedition tentatively locates the axis of the rift near 24°10'S, 111°43'W. Our profile at this location shows the axis to coincide with a 1000 m deep, 10 km wide valley, trending 354 °, and characterized by a 2 km wide inner floor which reaches a depth of 3800 m.
3. Pito Deep The East Rift can be followed north to its termination near 22°50'S, l 1 2 ° 1 0 ' W (Figs. 3 and 4). North of about 24°50'S, the East Rift is characterized by a median valley [8] with apparent right-lateral offsets. At 23°15'S the axial valley can be located along 111°35'W on the basis of its topographic signature, and further north there are a number of parallel N W - S E trending valleys, the westernmost and deepest of which we take as the axis. In this region, the depth increases dramatically from 3000 to 4500 m over less than 20 km,
reaching a maximum uncorrected depth of 5890 m at 23°00'S, 111°56.5'W. This is the deepest active rift axis mapped in the Pacific today. We propose to call this deep hole the Pito Deep (Pito is the Polynesian word for navel and is of significance in Rapanui mythology which considers the island to be the navel, or centre, of the world). The area surrounding the Pito Deep is characterized by a large number of normal fault scarps that trend generally northwest, but locally have quite variable strikes (Fig. 4). They dip both northeast and southwest, forming a series of nested grabens that enclose the Pito Deep. This deep lies at the foot of a northeast dipping scarp which is the northward continuation of the scarp identified as the inner pseudo-fault by Hey et al. [9]. To the east of the Pito Deep there are three major horst blocks or massifs, separated by shallower and narrower grabens which become more north-south oriented as one moves east. The southern portion of the westernmost block is partially rifted open by a small graben. Scarp heights of about 500 m are common, and they range up to 1500 m. At the northern end of the rift the scarps turn westward, and the grabens are closed off by west-trending, south-dipping faults, except in the east where the N - S grabens give way to a N N E - S S W fabric. Some of the west-trending faults continue westwards into what we interpret as a narrow (about 10 km) band of E N E - W S W striking, presumably dextral, strikeslip faults, which comprise the eastern part of the northern transform boundary. The 3.5 kHz seismic profiles show no significant sediment within the rifted area, except on top of some of the large massifs. Outside of the rifted area (i.e. to the west, north and east) the scafloor has a very different character. The topographic relief is much less (a few hundred meters), a significant veneer of sediment (a few meters) can be detected on the 3.5 kHz records, and the tectonic fabric is quite different. It consists predominantly of small N N E - S S W to N E - S W striking scarps, with heights generally less than 200 m and spacings of a few kilometers. Near the transform zone, and to the south of it, the trends are E N E - W S W . This "exterior" fabric is very similar to the usual tectonic fabric produced by the East Pacific Rise, except for the oblique orientation. There is a clear pattern be-
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twcen the two trends; the N N E - S S W trends become progressively more EN E - W S W as one moves from the east to the west along a line north of the rifted area (Fig. 4). The faults on the large blocks have a similar N N E - S S W trend and similar seafloor fabric. As one moves southward, some of the faults north of the rifted area have increasingly greater throws (up to 500 m), and curve progressively westward, merging with the transform zone trends. In addition, to the north and northeast of the rifted area there are a few minor faults striking N W - S E ; these have much less relief and are much less common than the other trends. We thus interpret these observations to indicate that the northern extension of the East Rift is a complex system, with one or more grabens which have cut into the older Nazca plate. It is unclear which rift within the rifted area is currently active, unless one assumes that only the deepest rift is active. The existence of the major rift grabens east
of the Pito Deep could be a result of westward or random jumping of active rifting sites over time, or joint parallel active rifting. The Pito Deep cannot have been the only active rift since the beginning of rifting because of what appears to be isolated blocks of pre-existing Nazca plate within the rifted area. and the highly asymmetric location of the Pito Deep with respect to the rifted area. Thus. the rifting process in this area has the potential for producing complex age relationships a n d / o r producing a complex history of spreading and stretching that would be difficult to unravel by conventional magnetic anomaly dating techniques. The pattern of the N N E - S S W faults becoming E N E - W S W near the transform zone suggests a few possible interpretations. The N N E - S S W faults may simply be responding to the shear of the transform zone, although this would imply that the transform zone extends further east than the Pito Deep. Alternatively, there could have been an
369
earlier episode of East Rift propagation to the north and east of the present position of Pito Deep. This would have rotated the N N E - S S W abyssal hill fabric clockwise in the resulting overlap region between this rift and the East Pacific Rise north of the microplate. However, at present the northern end of the East Rift does not appear to be a simple propagating rift tip.
ridge with a relief in excess of 1700 m. Here the transform fault is still seen as a notch near the summit of the ridge. Our profiles establish the continuity of this ridge, akin to a median ridge typical of transform faults, from 113010 ' to 114°22'W, i.e. over a distance of 120 km. It is typically about 15-18 km wide, stands 1-2 km above the surrounding terrain, and trends roughly 260-265 °. The seafloor is consistently deeper to the south of the ridge by 500-750 m.
4. Northern boundary (Easter-Nazca) 5. Norlhern triple junction Near 22°48'S, 112°17'W, the northern end of the Pito Rift merges with an E - W to 083 ° narrow band of linear faults, interpreted to be a transform fault between the Nazca and Easter (Rapanui) plates. We have mapped this shear zone (which we propose to name the Pito fracture zone) continuously to 22°49'S, 112°40'W. The 4 - 5 km wide principal shear zone is included within a 40-50 km wide domain, characterized by faults trending E - W to 0 7 0 ° i.e. grossly subparallel to the transform fault (Figs. 3 and 4). There is a marked asymmetry between the two limbs of the transform domain. In the north, the faults swing from a N N E (030°), to a N E (060°), and finally ENE (070 ° ) trend, between 112020 ' and 112°50'W, and end up merging with the transform fault west of 1 1 2 ° 5 0 ' W (Fig. 4). In the south, the faults have a single trend close to that of the Pito transform fault. Between 112o25 ' and 112°35'W, the transform domain shows subdued relief with a mean depth of approximately 3500 m. The transform fault appears as a 2 km wide valley in the middle of a 35 km wide raised massif. West of 112°40'W, the transform fault is better marked in the topography and shows as a 1 km wide valley cutting through a narrower (15 km) massif. At 112°47'W, the valley disappears and only a small notch remains on the flank of a narrow (8 km), 600-800 m high ridge. Each crossing of the transform fault between 112o20 ' and 1 1 2 ° 5 0 ' W is marked by a steep gradient of the magnetic anomaly, negative to the south, suggesting that crust of differing polarity is present on either side of the transform fault. The N / O "Jean Charcot" crossing of the Pito transform fault at 23°S, l 1 3 ° W shows a very high
West of 1 1 4 ° 2 0 ' W and up to 114°40'W, we renewed systematic mapping for a distance of 40 km in the region of the expected Easter-NazcaPacific northern triple junction. The region of the triple junction, located at 23°02.5'S, 114°31.5'W, is marked by an abrupt termination of the 002 ° axial ridge of the East Pacific Rise against a fault zone striking 088 ° (Fig. 5). The large ridge characterizing the transform fault west of Pito Rift disappears east of the triple junction near 1 1 4 ° 2 0 ' W , and gives way to a low ridge which continues to at least 114°32'W. Along the northern edge of this ridge we have identified the Nazca-Easter and Pacific-Easter transform faults, which appear to be colinear. The Pacific-Easter transform fault, at the triple junction, differs by about 10 ° from the azimuth of the transform fault (078 °) mapped by Marchig et al. [14], near 23°28'S, 1 1 5 ° 3 0 ' W . It also differs from the 081 ° motion predicted from the Euler poles of Engeln and Stein [5], and N a a r (unpublished). A large massif, 10 by 20 km, stands south of the linear ridge between 114°27 ' and 114°40'W. The massif is faulted on the northern side, with a 1000 m high, steep, north-dipping fault scarp. It appears, however, to bear an accretion fabric consisting of N - S ridges and troughs.
6. Southwestern boundary (Easter-Pacific) Prior to the Rapanui expedition, the southern boundary of the Easter (Rapanui) microplate was the least well defined of the microplate boundaries. In particular, the location and nature of the plate boundary east of 1 1 4 ° 3 0 ' W , and the location of
370
the southern triple junction, were not resolved with existing data. The westernmost crossing of the southwestern boundary at 26°24'S, 1 1 5 ° 2 0 ' W shows that the axis of accretion, bctween the Easter (Rapanui) and Pacific plates, is in a small (6 km wide) valley which is perched in the middle 114°33
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23~07 114°29
Fig. 5. Bathymetry of the northern Easter-NaTca-Pacific triple junction showing the three active plate boundaries (box B in Fig. 1). The arca shown was totally insonified by Sea Beam. Contour interval 100 m (solid). 20 m and 50 m (dashed).
of a raised axial block about 30 km in width (Fig. 6). The axial block is itself lying in the middle of an 80 km wide valley, bordered by two linear ridges that stand 1500 m above the valley floor, the two southwestern pseudofaults of Hey et al. [9]. There is a distinct difference between the trend of the southern linear ridge (285 290 o ), and that of both the axis of accretion (320°), and the northern linear ridge (315°). South of the southern ridge, typical N - S accretion fabric in a scdimented terrain is apparent, evidently created by Pacific-Nazca accretion. The axis of Easter-Pacific accretion is marked by small en-echelon depressions (undetected by Hey et al. [9]), e.g. the 4 km deep at 1 1 5 ° 0 5 ' W , and turns progressively from a 320--325 ° trend at 26°35'S, ! 1 5 ° 0 5 ' W , to a 340 ° trend at 26°38'S, 1 1 4 ° 5 7 ' W . The depressions are typically oval in shape (10 km long and 4 km wide), are 400-800 m deeper than the surrounding terrain, and are linked by linear valleys trending 070-080 °. The morphological pattern is similar to that found in the Gulf of California. The last clear axis of accretion is found at 26°40'S, 1 1 4 ° 5 6 ' W : further southeast it lcads into a 2 km wide valley which turns progressively to a 295 ° trend at 26°45.5'S, 1 1 4 ° 5 0 ' W . This location corresponds to the first expression of a transform fault, marking the boundary between the Easter and Pacific plates east of 1 1 4 ° 5 0 ' W . We propose to namc this active zone the Orongo fracture zone, Orongo being the name of a sacred site in Rapanui mythology. Further east, the transform valley runs into an 85 km long, 5 - 1 0 km wide ridgc, which stands more than 2200 m above the surrounding terrain (reaching a minimum depth of 790 m at 26°46.5'S, 114°39'W), and is bordered to the south by the N--S Pacific-Nazca accretion fabric. This ridgc merges north of the plate boundary (near l 1 4 ° 1 0 ' W ) with another ridge of similar shape, which in turn represents the eastern prolongation of a ridgc previously discussed (near 26°10'S, l 1 5 ° W , Fig. 6). Thc transform fault coincides with this northern ridge between 114007 ' and I I 3 ' 1 0 ' W , i.e., over a distance of 100 km (Fig. 1). The 20 km wide ridgc diminishes rapidly in relief from over 2000 m west of 113°50'W, to 1000 m at 1 1 3 ° 3 0 ' W , and finally to less than 200 m at 1 1 3 ° 0 5 ' W , in close proximity to the triple junction. Ahhough there are locally curved segments,
371
26*
IO 26'
26O30
26'30'
27'10' 1
IO'
2790'
Fig. 6. Bathymetry of the southwestern boundary of the Easter (Rapanui) microplate (box C in Fig. 1). The active plate boundary is drawn on the figure. Depths greater than 2500 m and greater than 3500 m are shown with light and medium shading, respectively. Scarps bounding the rift zone are indicated. Sense of motion is marked on the Orongo transform fault.
the ridge, and thus the southern plate boundary, continues approximately along 26 o 50’s. 7. The southern triple junction The region of the southern triple junction was intensely surveyed between 112”56’-113”22’W and 26”40’-27”lO’S (Fig. 2). The axis of accretion between the Easter and Nazca plates is well known from the work of several authors [6,8,9,15,16]. It corresponds to a high-standing ridge, reaching a minimum depth of 2100 m at 26 o 30’S, 112 o 38/W, in a region characterized by a marked 3He enrichment [ll]. The ridge strikes 002” (and reaches 2100 m at 26”3O’S [9]). It plunges along strike both to the north, where it reaches a small overlap region at 25 o 55’S [8], and
to the south until 26”5O’S. Further south, this ridge progressively veers to the west until it reaches 112”5O’W at 27”17’S. The ridge is north-south between 27”17’ and 27”3O’S, where it is offset to the west, probably through a small overlap region. The ridge is rather subdued south of the shallow axis near 26 “30’S, and its recognition is based primarily on continuity in plan form and morphology of the summit region because the magnetic anomaly pattern in the area has not yet been unravelled. A second ridge, along 113”-113”05’W and about 20 km west of the Easter-Nazca axis, is present between 26O50’ and 27O37’S. It strikes N-S, is remarkably continuous, appears more developed than the axial ridge, and marks the eastern terminus of the E-W structures associated
372 with the Orongo fracture zone (Fig. 1). At 27 °21'S, 113°03'W, this ridge stands 300-500 m above the surrounding seafloor and 200 m above the more easterly ridge. The region of the triple junction is defined by the intersection of the Orongo E - W structures, and the N - S faults of the western ridge near 26°53'S, 1 1 3 ° 0 2 ' W (Figs. 1 and 2). 8. Discussion
The new observations reported above have important implications for growth processes of microplates and previously published tectonic analyses of the Easter (Rapanui) microplate. The pattern of normal faults and horsts and grabens in the Pito Deep area at the northern terminus of the East Rift and the relationship between the normal faults and the Pito fracture zone enable us to derive a well-constrained relative motion (70-75 ° ) at this location (Fig. 4). This direction is consistent with pure strike-slip on the eastern part of the Pito fracture zone which may thus behave as a true transform fault near its intersection with the Pito rift. This direction of slip is in conflict with that deduced from the Easter-Nazca pole calculated by Engeln and Stein [5]. With the latter pole, the opening direction would be along 100 ° across Pito Deep, and the Pito fracture zone: as mapped by us, would experience along its entire length a large component of compression along a direction varying between 120 ° and 140 ° . Although we are unable with our data to specify the slip vector along the Pito fracture zone west of l 1 3 ° W , the motion proposed by Engeln and Stein [5] is very unlikely in view of the topography along Pito fracture zone. In addition, for stability to be preserved the geometry of the triple junction requires pure strike-slip on the Pito fracture zone at the junction, along an approximately E - W direction (Fig. 5). In deriving the Euler pole for the Easter-Nazca motion, a large weight was given to the northward decrease in spreading rates observed by Handschumacher et al. [4] along the East Ridge [5]. N a a r and Hey [8] questioned the magnetic anomaly identifications of Handschumacher et al. [4] and thus the tectonic model of Engeln and Stein [5]. If we assume pure strike-slip at the northern triple
junction and along the easternmost segment of Pito transform fault at itsjunction with Pito Deep, slight compression occurs in the curved segment of Pito transform fault between 112o30 ' and l 1 3 ° W . It appears premature to compute an Easter-Nazca Euler pole at present without considering the match of magnetic isochrons across Pito Deep but it is clear that the true pole lies much further to the north than the pole of Engeln and Stein [5]. In contrast, the southwestern boundary, as mapped by us, with extension in the West giving way to shear further east up to the southern triple junction, shows good agreement with earthquake mechanisms and thus the relative motion vectors derived from the Easter-Pacific pole of Engeln and Stein [5]. The Sea Beam data are consistent with the southern boundary being a slowly spreading ridge, at least in the western portion, and the northern boundary being a transform fault. The geometry and nature of the microplatc boundaries together with the plate fabric in the vicinity of the boundaries agree with a rigid model for the growth of the microplate [12]. One cannot determine from the survey data in the Orongo fracture :,.one if the border ridges are volcanic or tectonic in origin, but the ridges may have built lines of islands in the past or constitute a modern analogue of how lines of islands were built. The setting of Pito Deep at an intersection between a spreading center and a transform fault is typical of nodal basins often found in regions of rift-transform intersections. The depth is too large, however, for the age (4 Ma) of the truncating edge of the lithosphere, according to the empirical relationship shown by Fox and Gallo [17] (although that relationship may only apply, however, to slow spreading centers). The great depth of the Pito Deep, a record for rift axes along the East Pacific Rise, may be explained by crustal denudation or crustal thinning. Removing the whole crust accounts for about I km of deepening if isostatic balance prevails. The additional 2 km of deepening may be due to lack of isostatic equilibrium. Pito Deep may thus represent an initial stage of rift evolution and be similar to the 5400 m Hess Deep at the rifting tip of the Cocos-Nazca spreading centre [18].
373
9. Conclusion The N / O "Jean Charcot" geophysical survey of the Easter (Rapanui) microplate has focused on the poorly defined northern and southern boundaries. The northern end of the East Rift is marked by a series of deep N W - S E grabens, including the 5890 m Pito Deep, which curve into a transform fault that leads to the northern Easter-Nazca-Pacific triple junction. The great depth of the Pito Deep may be due to the tectonic setting of the deep at a rift-transform intersection and possible crustal denudation or abnormally thin crust. Our interpretation of the northern boundary of the Easter microplate is not compatible with the published Easter-Nazca Euler pole [5]. The pole must lie much further north than Engeln and Stein's [5] but a rigorous analysis of both slip vectors and magnetic isochrons across Pito Deep is needed in order to derive the Euler pole positions. The northern triple junction between the Easter (Rapanui), Nazca, and Pacific plates is a RFF type, with the two transform faults colinear along an approximately E - W direction. The southwestern boundary of the Easter (Rapanui) microplate is marked by a series of en-echelon offsets, outlined by depressions, which merge into an approximately E - W zone where shear must be predominant. The high ridges which border the 80 km wide valley, enclosing the axis of rifting in the western portion of our southern survey (the Hey et al. [9] pseudofaults; Fig. 6), separate a triangular-shaped region where active rifting is occurring between older Pacific and Easter (Rapanui) crust to the south and north, respectively. Further east, in the Orongo fracture zone, the border ridges coalesce and the high relief becomes more subdued towards the southern triple junction. The southern triple junction between the Easter-Nazca-Pacific plates is a RRF junction with an overlapping ridge system, The geometry and nature of the Easter (Rapanui) microplate boundaries together with the plate fabric in the vicinity of the boundaries agree with a rigid model for the growth of the microplate.
Acknowledgements The Rapanui cruise on the N / O "Jean Charcot" was successful due to cooperation from the captain, officers and crew, and the expertise of the engineers and technicians who ran the geophysical systems. The hospitality of the Pascuans was greatly appreciated. We are particularly grateful to V. Marchig, H. Rask, and H. Backer for kindly providing Sea Beam bathymetric data from "Sonne" cruises (Geometep Project), prior to our cruise, and to R.N. Hey for discussions and sharing unpublished data and P. Choukroune for reviewing the manuscript. We thank S. Vappereau for editing the manuscript and G. Aveline for drafting the figures. The cruise was financed by IFREMER and INSU. This is IPGP Contribution No. 1028. References 1 E.M. Herron, Fwo small crustal plates in the South Pacific near Easter Island, Nature Phys. Sci, 240, 35- 37, 1972. 2 I).W. Forsyth, Mechanisms of earthquakes and plate motions in the East Pacific. Earth Planet. Sci. Lett. 17. 189- 193. 1972. 3 R.N. Anderson. I).W. Forsyth, P. Molnar and J. Mammerickx, Fault plane solutions of earthquakes on the Nazca plate boundaries and the Easter plate, Earth Planet. Sci. Lctt. 24, 188-202, 1974. 4 D.W. Handschumacher, R.tt. Pilger, J.A. Foreman and J.R. Campbell, Structure and evolution of the Easter plate. Geol. Soc. Am., Mere. 154. 63-76, 1981. 5 J.F. Engeln and S. Stein. Tectonics of the Easter plate. Earth Planet. Sci. l,cn. 68, 259-270, 1984. 6 J.G. Schilling. H. Sigurdsson, A.N. Davis and R.N. Hey. Faster microplate evolution. Nature 317, 325-331, 1985. 7 D.F. Naar and R.N. Hey, Possible models for the origin and evolution of the Easter microplate. EOS Trans. Am. Geophys. Union 66, 968, 1985. 8 D.F. Naar and R.N. Hey, Fast rift propagation along the t:~ast Pacific Rise near Easter Island. J. Geophys. Res. 91, 3425-3438. 1986. 9 R.N. Hey, D.F. Naar, M.C. Kleinrock, W.J. Phipps Morgan, E. Morales and J.G. Schilling, Microplate tectonics along a superfast seafloor spreading system near Easter Island, Nature 317. 320-325, 1985. 10 J.H. Woodhouse and A.M. Dziewonski, Mapping the upper mantle: three-dimensional modeling of earth structure by inversion of seismic waveforms, J. Geophys. Res. 89, 5953-5986. 1984. 11 H. Craig. K.R. Kim and W. Rison, Easter Island hotspot, I. Bathymetry, helium isotopes, and hydrothermal methane and helium, EOS Trans. Am. Geophys. Union 65, 1140, 1984.
374 12 J.F. Engeln. S. Stein, J. Werner and R.G. Gordon. Microplates and shear zones near overlapping ridge systems. J. Geophys. Res.. in press. 1988. 13 J.('. Sempere. I).F. Naar, J. Gee and R.N. Hey, in preparation. 14 V. Marchig. H. Gundlach and the Shipboard Scientific Party, Ore formation at rapidly diverging plate margins, Results of Cruise Geometep 4, BGR Circ. (Hannover) 4, 3-22, 1987. 15 J. Francheteau, A. Yelles-Chaouche and H. Craig, The Juan Fernandez microplate north of the Pacific-Na~'.ca-
Antarctic plate junction at 35°S, Earth Planet. Sci. Left. 86, 253- 268. 1987. 16 A. Yelles-Chaouche. J. Francheteau and P. Patriat, Evolulion of the Juan Fernandez microplate during the last three million years, Earth Planet. Sci. Left. 86, 269-286. 1987. 17 P.J. Fox and D.G. Gallo, A tectonic model for ridge-transform-ridge plate boundaries: implications for the structure of oceanic lithosphere, Tectonophysics 104, 205-242. 1984. 18 R.C. Searle and J. Francheteau. Morphology and tectonics of the Galapagos triple junction. Mar. Geophys. Res. 8. 95-129. 1986.