Formation of large summit troughs along the East Pacific

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B6, PAGES 12,971-12,988, JUNE 10, 1999 Formation of large summit troughs along the East Pacific Rise as collapse calderas: An evolutionary model Yves Lagabrielle Unit6 Mixte de Recherche 6538, Domaines Oc6aniques, Plouzan6, France Marie-H l ne Connier Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York Abstract. Summit troughs wider than 500 m and m deep are present along 15-20% of the fast spreading East Pacific Rise (EPR). They occur only along ridge segments with large cross-sectional areas, indicative of a time-averaged robust magma supply. Where available, seismic data confirm that these troughs are underlain by an axial magma chamber (AMC) km below the seafloor. Furthermore, detailed investigation of the large summit troughs which notch the EPR between 17ø56 ' and 18ø35'S indicates that the vertical relief of the troughs tends to be maximum where the AMC is shallowest. Both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension. Rather, we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated AMCs wanes or nearly ceases. Because seismic studies constrain the melt lens along the EPR to be only m thick, the proposed existence of collapsed calderas m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply. In particular, we suggesthat a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases. The resulting caldera will further widen with each subsequent dike intrusion. When an abundant melt supply finally resumes, the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera. 1. Introduction Large axial valleys a few kilometers to several tens of kilometers wide bounded by normal faults commonly mark the axes of mid-ocean ridges at slow to intermediate spreading rates (< 50 km/myr). These extensional grabens have dimensions controlled by the thickness of the brittle lithosphere, which is typically more than 6 km [Tapponier and Francheteau, 1978; Phipps Morgan et al., 1987; Lin and Parmentier, 1989; Shaw and Lin, 1996; Buck and Poliakov, 1998]. At fast spreading rates ( km/myr), a shallow magma chamber often underlies long sections of the ridge axis only km below seafloor [Derrick et al., 1987, 1993], and axial troughs wider than 2 km have not been observed. Instead, shipboard multibeam bathymetry generally indicates a smooth crestal plateau, although any trough narrower than 80 m or shallower than 10 m would escape detection. In fact, direct submersible observations and high-resolution deep-tow surveys along the crest of the EPR often reveal collapsed lava lakes 5-20 m deep and up to a few hundred meters wide, with arcuate boundaries and associated lava channels on their margins [Lonsdale, 1977; Renard et al., 1985; McConachy et al., 1986; Kastens et al., 1986; Haymon et al., 1991; Auzende Copyright 1999 by the American Geophysical Union. Paper number 1999JB /99/1999JB et al., 1996; Fornari et al., 1998]. The frequent occurrence of lava pillars, lava tube orifices, and partially collapsed lava lake roofs within these narrow troughs indicates that they form along primary eruptive fissures [e.g., Haymon et al., 1991; Fornari et al., 1998]. Macdonald and Fox [1988] noted that between 9øN and 13øN, summit troughs wide enough to be detected by shipboard multibeam bathymetry (> 80 m) consistently occur along ridge segments which have a large cross section and are undedain by an axial magma chamber (AMC). Conversely, summit troughs are not imaged by multibeam bathymetry where the axial high has a narrow cross section and an AMC is not detected seismically. Altogether, Macdonald and Fox [1988] estimated that summit troughs wider than -100 m are present along roughly 60% of the East Pacific Rise (EPR) between 9 ø and 13øN. However, the dimensions of summit troughs along the fast spreading section of the EPR (from 15ø30'N to 35øS) vary by as much as an order of magnitude. Summit troughs with intermediate dimensions ( m wide) display many of the characteristics of narrow collapse lava lakes, although their walls tend to be more linear because of repeated collapse and mass wasting [Fornari et al., 1998]. Only a few segments display summit troughs much larger than any observed collapsed lava lakes, m wide and m deep. These are unmistakably imaged by shipboard lnultibeam bathymetry and are termed "large summit troughs" throughout this paper. Large summit troughs have margins that are linear and clearly tectonized rather than scalloped [e.g., Lonsdale and Spiess, 1980; Gente et al., 1986; Cormier and Macdonald, 12,971

2 12,972 LAGABR]ELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 1994; Krasnov et al., 1997]. Escarpments and inner troughs occur within the main trough, suggesting multiple stages of development. They are comparatively rare features and only occur along 15-20% of the fast spreading EPR (Figures la and lb). From north to south, large summit troughs have been mapped along segments centered at 12ø45'N [Hdkinian et al., 1983; Gente et al., 1986; Macdonald and Fox, 1988; Crane et al., 1988; Krasnov et al., 1995], 10ø30'N [Kastens et al., 1986], 8ø45'N [Lonsda!e and Spies& 1980; Crane et al., 1988; Macdonald et al., 1992], 5ø10'N [Lonsdale, 1985a,b; Krasnov et al., 1995], 3ø10'N [Lonsdale, 1985b; Krasnov et al., 1995], 0ø10'N, 0ø55'S, and 2ø20'S [Lonsdale, 1989], 5øS [Lonsdale, 1983, 1989], 7ø45'S [Lonsdale, 1989; Cochran et al., 1993], 9ø15'S [Lonsdale, 1989], 18ø10'S and 18ø25'S [Renard et al., 1985; Lonsdale, 1989; Sinton et al., 1991; Cormier and Macdonald, 1994; Auzende et al., 1996], and 21ø30'S [Renard et al., 1985; Backer et al., 1985; Marcbig et al., 1988; Krasnov et al., 1995, 1997]. Their distribution in relation to the cross-sectional area of the ridge axis is consistent with the observations of Macdonald and Fox [1988]: Large summit troughs occur along robust, inflated segments which are most likely to be underlain by a magma chamber, and none are located along narrow, deflated ridge segments, such as north of the 20ø40'S overlapping spreading centers (OSC) or north of the Clipperton transform fault (Figures l a and lb). Different mechanisms have been proposed to explain the formation of the largest summit troughs at intermediate and fast spreading ridges. One model is rifting of the brittle crust during stages of amagmatic extension [Holler et al., 1990; Fornari et al., 1998]. An alternative model is that large summitroughs may initiate as elongate collapse calderas over a waning AMC [Lonsdale, 1977; Kappel and Ryan, 1986; Barone and Ryan, 1988; Macdonald and Fox, 1988; Hurst et al., 1994; Krasnov et al., 1995]. Large summit troughs may also form by cumulative graben subsidence over repeated shallow dike intrusions [Chadwick and Embley, 1998]. In order to evaluate the relative contributions of magmatic and tectonic processes to the formation of large summit troughs, we conducted a combined analysis of structural and geophysical data recently collected between ' and 18ø35'S along the ultrafast EPR, where axial troughs m wide are locally present. The results presented in this paper are most consistent with a model in which wide summit troughs result from collapse of the brittle crust over waning crustal magma reservoirs. The implications of this model are further discussed in terms of evolution of the magma reservoir. Latitude (degree north) I. / I I I I I..=,r.. %. I I I I I I -,,-/.:... p: r,...?;..<. :..,::.,. :: :.:: AMC likely i / I trouõha > S00 m wide '-- I I I I c {' f O... *:. ::: '"':' -2.8 (. I ',-- :::q : : :::::::i L :: a ' I I... I ' Latitude (degree north) :'":::'":'""':':: :':':':': Figure la. Magmatic budget versus occurrence of large summitroughs along the fast spreading section of the northern East Pacific Rise (EPR). (bottom) Axial depths versus latitude. Fracture zones are labeled, and thin vertical lines correspond to ridge offsets larger than 2 km. Thick solid segments above bottom graph indicate the intervals where summit troughs at least 500 m wide are present (see text for references). (top) The cross-sectional area of the axial high (a direct measure of its inflation) computed by Scheirer and Macdonald [1993]. These authors determined two threshold values (indicated by dashed lines) to predict the presence of an axial magma chamber reflector. Where the cross-sectional area is greater than 3.5 km 2 (as indicated with the thick solid segments below the top graph), they predicthe presence of an axial magma chambereflector with 90% confidence; where the cross-sectional area exceeds 2.5 km 2 (as indicated by the thick shaded segments below the top graph), the presence of an axial magma chamber (AMC) can be predicted with at least 75% confidence. Note that large axial troughs occur along relatively shallow, inflated segments that are likely to be underlain by a magma chamber.

3 LAGABRIELLE AND CORMIFR: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,973 _ Latitude (degree south) I I I I I I I I I I I I I I! :.,/., I. I, /.I,,11 III I II I... AMC likely troughs -I 1 > 500 m wide I I I I I ll r , ' ' I -3.2 o -4 a Latitude (degree south) Figure lb. Same as Figure la but for the ultrafast spreading southern EPR. 2. EPR Between ' and 18ø35'S A series of OSCs define ridge segments 25 to 300 km long between the Garrett Transform and Easter Microplate (Figure 2). Sinton et al. [1991] showed that this structural segmentation of the southern EPR generally corresponds to a magmatic segmentation defined by similar parental magma compositions and labeled the corresponding segments A through O. Segments I, J, and K are located along the shallowest, most inflated section of the ridge between 18o35 ' and 16ø30'S. Local spreading rates are 145 mm/yr [Cormlet and Macdonald, 1994], close to the maximum rates registered along the present mid-ocean ridge system (150 mm/yr, [Cormlet, 1997]). Segment I is only 25 km long and displays a markedly convex profile along axis (Figure lb), and for this reason is commonly known as the << hump. The three segments are separated by discontinuities only 3 km wide but display, nonetheless, very different morphologies (Figures 3 and 4). In order to investigate these differences, detailed geological observations were made in December 1993 during the Naudur expedition of R/V Nadir with 20 on-axis dives of the Nautile submersible [Auzende et al., 1996]. Numerous other surveys in the area using submersibles, deep-towed instruments, swath bathymetry systems, and geophysical methods provide the spatial and temporal contexts in which to interpret these dive observations [e.g., Renard et al., 1985; Backer et al., 1985; Macdonald et al., 1988; Lonsdale, 1989; Marcbig et al., 1988; Sinton et al., 1991; Detrick et al., 1993; Cormlet and Macdonald, 1994; Mutter et al., 1995; Fujioka et al., 1995; Urabe et al., 1995; Scheirer et al., 1996; Cormlet et al., 1996; MELT Seismic Team, 1998; Embley et al., 1998]. Between 16ø30'S and 17ø56'S, segment K retains a relatively constant depth (2630 _+ 50 m) for its entire length (Figure 5). On the basis of shipboard multibeam bathymetry data, its cross-sectional shape is a broad, smooth dome devoid of any significant summitrough (Figure 4). However, direct observations made during the Naudur expedition at 17o10 '- 17ø12'S and 17ø22'-17ø25'S reveal the presence along the axial summit of collapsed lava lakes ranging from 2 to 12 m in depth and from a few to several tens of meters in width [Auzende et al., 1996]. The axial region is covered with a combination of recent lobate flows and smooth or jumbled sheet flows. The only tectonic features observed on axis are occasional fissures, a few hundred meters long and at most a few meters wide [Auzende et al., 1996]. Similar observations were made in 1994 and 1997 during detailed investigations with the Shinkai 6500 submersible [Fujioka et al., 1995; Urabe et al., 1998; Embley et al., 1998]. In contrast, both segments I (18ø35'S-18ø22'S) and J (18ø35'S-17ø56'S) are notched by prominent axial troughs m wide (Figure 4). A subtle, ~0.5-km left-stepping offset of the axial trough is present at 18ø10'S (Figures 3 and 6). This minor discontinuity, or "deval"[langmuir et al., 1986], further divides segment J into two smaller segments, referred to as segment Jn (north) and segment Js (south). The floors of the troughs of segment I, Js, and Jn are intensely fissured and are generally dusted by several millimeters to a few centimeters of sediments [Auzende et al., 1996]. A narrower inner trough (< 100 m wide and < 15 m deep) has been detected along many dive transects of segments I and Js (segment Jn was not investigated by the Nautile submersible) [Auzende et al., 1996]. Recent, small-volume volcanism is confined to the narrow inner troughs, suggesting that they include primary eruptive fissures. An extremel young lobate flow has been observed to flood the trough of segment I near

4 12,974 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS -116 ø -114 ø.112 ø _110 ø flows, leveed lava channels, and draped or jumbled sheet flows. _14 ø _16 ø _18 ø _20 ø _22 ø _24 ø _26 ø NAZCA Garrett Transform Figure 2. Map view of the southern EPR, highlighting the segments studied in this paper (I, J, and K). Arrowheads point to axial discontinuities with offsets at least 2 km wide. Inset shows location relative to South America. 18ø33'S [Auzend et al., 1996]. Diffuse flows of shimmering water occurred over the entire surface of that unsedimented flow, and a temperature of-150øc was measuredirectly in a crack on its surface. Collapsed lava lakes with up to 10-to 15- m-high lava pillars mark the inner trough of segment Js. The chemistry of lava samples collected within this inner trough is nearly identical, consistent with them belonging to a single lava lake up to 18 km long [Auzende et al., 1996]. The outer flanks for both segments I and Js are largely unfaulted and similar to those of segment K, except for the presence of relatively uniform sedimentary cover. They display pillow 3. Morphology of Large Summit Troughs The dimensions of the summit troughs along segments I, Js, and Jn were measured directly from individual beam point profiles generated during each ping cycle of the SeaBeam 2000 multibeam bathymetric system (Figures 6, 7, and 8) [Cormier et al., 1997]. The 2 ø beam footprint diameter of SeaBeam 2000 is about 120 m in water depths of m; pings are typically separated by m along track, oversampling the footprint by about a factor of 2. Bathymetric profiles displayed in Figure 6 average two successive pings, and depth measurements made from these profiles indicate a background noise of +10 m (Figure 7), consistent with the estimated resolution of most multibeam bathymetric systems for these water depths [Kleinrock, 1992]. Direct comparison of bathymetric profiles obtained from SeaBeam 2000 and from the Nautile submersible also agree within +10 m, except for a systematic 25-to 30-m shift between the two data sets. The largest difference occurs for the narrow inner trough, which is not imaged by the multibeam data. In that respect, the maximum depth to the floor of the trough estimated from SeaBeam 2000 beam point profiles may be underestimated by up to m (Figure 7). Near each segment end, the trough wall located closest to the adjacent overlapping ridge tip tapers below the resolution of SeaBeam 2000 bathymetry records (Figure 6), and trough dimensions in the overlap region between segments could not be estimated reliably. We define the vertical relief of the trough as the maximum difference in depth between the trough floor and the two bounding walls. The trough width is estimated from the distance between the shallowest elevations east and west of the trough axis and represents upper bound values (Figures 6 and 8). In several other studies, the width of the summit trough is defined as the distance separating its bounding scarps [e.g., Haymon et al., 1991; Macdonald et al., 1992; Fornari et al., 1998; Chadwick and Embley, 1998]. This other method provides a lower bound on trough width, about a few hundred meters smaller than those estimated by our technique. However, both approacheshould produce the same patterns of along-axis variations. The advantage of our method is to provide an objective, automated method to estimate both the width and the vertical relief of large summit troughs. The most striking characteristics of the morphology of segments I, Js, and Jn are as follows (Figures 7 and 8) : (1) The floor of the summit trough is nearly flat (+ 15 m) within each segment. This may be interpreted to indicate that each 25-km-long segment corresponds to a single volcanic unit and/or that lava flows occasionally flood the entire floor of the summit troughs.. From segment I to segment Jn, the floor of the troughs deepens stepwise by --30 rn at each axial discontinuity. (2) The vertical relief of the trough is significantly larger for segment I (up to 110 m) than for segments Js and Jn (50-60 m). It clearly decreases toward the ends of segments I and Js. (3) The width of the troughs increases overall from segment I to segment Jn. It is significantly greater for segments Jn and Js (1000-!800 m) than for segment I ( m). (4) The top of the eastern wall is m shallower than that of the western wall along

5 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,975 center of segment K segment Jn segment Js segment I -17' 21' -17' 24' -17' 27' -17' 30' -17' 33' 18' 09' -17' 36' 18' 12' -113' 15' -113' 12' -113' 21' -113' 18' -113' 24' -113' 21' -113' 27' -113' 24' -113' 21' Figure 3. Shaded relief maps of four adjacent segments along the southern EPR. Illumination is from the east. Facing arrows point to the bounding scarps of the summit trough along segments I, Js, and Jn. Dashed line indicates the axis of accretion for segment K, which is devoid of any large summit trough. Bold lines indicate the approximate locations of the cross sections displayed in Figure 4. segments I and Jn (Figure 7). This asymmetrical pattern is best observed along segment Jn, where the western wall almost disappears at --18ø03'S. The origin of this asymmetry is unclear but may be related to a similarly asymmetrical distribution of hot, buoyant upper mantle beneath the axial region [Corrnier et al., 1996] and/or lateral variations in mantle viscosity [Eberle and Forsyth, 1998]. Other large summit troughs display characteristics similar to those of segments I, Js, and Jn. Along the northern EPR, a 0.5-to 1-km-wide trough is present near 12ø45'N along the shallowest part of a 230-km-long section of ridge [Gente et al., 1986]. Multichannel seismic reflection data indicate that this segment is underlain by a--1.3-km-deep magma chamber [Harding et al., 1989; Caress et al., 1992]. A summit trough km wide and m deep is also present along the segment centered at 21ø30'S [Renard et al., 1985; Krasnov et al., 1995, 1997]. Although seismic reflection data are not available to reveal the presence or absence of an AMC reflector, a shallow magma chamber is inferred to account for the numerous high-temperature hydrothermal fields along that segment. In both cases, the relief of the trough is a maximum near the center of the segment. Admittedly, the tectonized character and the scarcity of recent volcanism along segments I, Js, and Jn are consistent with either an extensional tectonic origin or with a collapse caldera origin for these large summit troughs. However, the absence of any large summit trough along sections of the EPR with very low magmatic budget, as well as the correlation described in section 4 between the dimensions of these troughs and the depth to the underlying AMC argues in favor of a collapse caldera mechanism. 4. Large Summit Troughs and Magma Chambers A shallow seismic reflector interpreted as the top of the AMC is detected along the southern EPR from multichannel seismic surveys [Derrick et al., 1993; Kent et al., 1994; Mutter et al., 1995; Hussenoeder et al., 1996; Tolstoy et al., 1997; Hooft et al., 1997]. The variations in depth of this AMC horizon are generally an order of magnitude greater than those of the overlying seafloor (Figure 5). On average, the AMC is shallowest near the segment midpoints and is a few to several hundred meters deeper near segment ends [Derrick et al., 1993; Tolstoy et al., 1997; Hooft et al., 1997]. Within individual segments, the AMC horizon arches upward by over 200 m at intervals of km (for instance, between ' and 17ø29'S, see Figure 5). These shoalings may reflect magma bodies that are rising into the preexisting dike section [Mutter et al., 1995]. Along segment K, the AMC reflector is among the shallowest observed [Derrick et al., 1993]. In contrast to the nearly flat summit of that segment, the AMC horizon shoals to 800 m below seafloor at 17ø25'S (Figure 5). Mutter et al. [1995] suggest that this spike of the AMC reflects a magma lens that was erupting at the time of the survey (February 1991). Accordingly, very fresh basalt samples were recovered with Nautile at this site in December 1993 [Auzende et al.,

6 - _ 12,976 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS ' a I I I I I I I 17ø23'S segment K 18ø03,S segment Jn 18ø26'S segment I I DISTANCE (krn) Figure 4. Selected cross sections of the ridge crest extracted from raw Hydrosweep [Detrick et al., 1993] and SeaBeam 2000 bathymetry data [Cormier et al., 1997]. Each profile averages two successive ensonification "pings." Individual frames contrast sections characteristic of segments I, Js, Jn, and K. Within each frame, the bottom profile is shown at its correct depth, and successive profiles are offset vertically by 50 m. About 120 m along axis separate successive profiles. Along the shorter notched segments (I, Js, and Jn), the AMC reflector lays at a generally greater depth than for the surrounding segments (Figure 5). Some of this increase in depth may only be apparent and may reflect instead mean seismic velocities that are lower than assumed, as may result from the intense fissuring of the floor of the troughs. However, independent geological evidence indicates that magmatism along these segments is waning, consistent with a greater depth to the AMC. Lava samples for segments I and Js are less homogeneous than for segment K and extend to substantially more differentiated compositions, suggesting that they tapped a magma reservoir which has not received volumetrically significant, recent replenishment from the mantle [Sinton et al., 1991; Auzende et al., 1996; Caroff et al., 1997]. Furthermore, active and abandoned hydrothermal sites are very abundant along segments I and Js, many of them displaying sizable chimneys and mound characteristics of mature sites [Backer et al., 1985; Renard et al., 1985; Urabe et al., 1995; Auzende et al., 1996; Charlou et al., 1996], which suggests that the upper crust is being efficiently cooled by hydrothermal circulation at these localities. Figures 8 and 9 compare the dimensions of the summit troughs with the depth to the top of the AMC. These two observables vary on different time scales, which probably accounts for some of the' scatter of the data points Figure 9. At any given location, the depth to the AMC may fluctuate by hundreds of meters over ~100 years [Hooft et al., 1997]. In contrast, the summit troughs may have formed as long ago as a few thousand years. This age is consistent with the several millimeters of sediment accumulation within the troughs [Auzende et al., 1996] and the local sedimentation rate of 3-26 mm/1000 yr [Marchig et al., 1986; Dekov and Kuptsov, 1992]. Yet there exists some overall correlation between AMC depth and trough dimensions. Along segment I, the summit trough is narrower and less deep near segment ends where the AMC tends to deepen (Figure 8). In contrast, along segments Js and Jn where the AMC is generally deeper than 1.5 km, the summit trough displays less variability in width and vertical relief. Overall, segment I defines a separate correlation trend from segments Js and Jn (Figure 9). Its summit trough is systematically deeper and narrower than those along segments Js and Jn, suggesting that different or additional factors affect the morphology of segment I and those of segments Js and Jn. Despite these differences, there exists an overall correlation between summit trough dimensions and depth to the AMC for all three segments combined. Where the AMC is shallower than 1500 m, the summit trough displays a vertical relief of 60 m or more; conversely, the minimum vertical relief tends to correspond with the deeper AMC encountered near segment ends. However, the width of the summit trough and the depth to the AMC do not appear clearly correlated (Figure 9). 5. Different Models for the Origin of Large Summit Troughs Because large summit troughs are elongated features bounded by linear scarps, they have been commonly described as axial summit grabens [e.g., Macdonald et al., 1984]. Models for their formation include not only extensional rifting of the brittle upper crust, as is suggested implicitly by the term << graben, >> but also initiation as linear collapse calderas over waning magma reservoirs [Lonsdale, 1977; Macdonald et al., 1984; Kappel and Ryan, 1986; Kastens et al., 1986; Macdonald and Fox, 1988; Barone and Ryan, 1988; Hurst et al., 1994]. The accumulation over the last decade of high-resolution near-bottom observations has revived the debate as to the exact origin of these summit troughs. Fornari et al. [1998] propose that large summit troughs along the fast spreading EPR are tectonic grabens that form

7 , LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,977 ' t Latitude (degree south) ) , I i J i ' segments G H I Js Jn K OO ) Latitude (degree south) Figure 5. Variability of axial characteristics with latitude. Vertical lines mark the position of the axial discontinuities; segments are labeled between the two graphs. (top) Minimal axial depth. The depth to the floor of the summitroughs is displayed as a finer line for notched segments I, Js, and Jn, (bottom) Depth below seafloor to the reflector interpreted as the top of the axial magma chamber [after Hooft et al., 1997]. during phases of relative magmatic quiescence. According to their model, narrow (< 200 m wide) summit troughs represent the shallow collapse of lava flow surfaces over primary eruptive conduits, and the larger summit troughs (> 200 m wide) form subsequently over a period of a few thousand years by amagmatic extension of the narrow summit troughs. They further suggesthat large axial summit grabens are bounded by opposing normal faults which nucleate either at the transition zone between dikes and extrusives ( km below seafloor) or at a zone of weakness just above the magma chamber ( km below seafloor). However, the observed correlations between trough dimension and AMC depth and the absence of large troughs along deflated sections of the ridge axis are the opposite of what would be expected if extensional tectonism accounted for the formation of large summit troughs. The dimensions of grabens that form by remote tectonic extension increase with the thickness of the brittle lithosphere [e.g., Tapponnier and Francheteau, 1978; Phipps Morgan et al., 1987; Lin and Parrnentier, 1990; Atlernand and Brun, 1991; Shaw and Lin, 1996; Buck and Poliakov, 1998]. The melt supply along mid-ocean ridges is generally enhanced near the center of segments [Macdonald et al., 1988; Lin et al., 1990], and consequently, the depth to the brittle/ductile transition is expected to increase toward segment ends. Along the slow spreading Mid-Atlantic Ridge, many axial rift valleys systematically deepen toward ridge discontinuities, and in plan view, many segments display an "hour glass" shape, widening toward segment ends [Sernpdrd et al., 1993]. Along the EPR, the AMC tends to deepen toward segment ends [Hooft et al., 1997], also implying some thickening of the brittle upper crust. If the formation of large summit troughs were controlled entirely by extensional tectonic processes, they should, on average, be widest and deepest near segment ends. Furthermore, they should preferentially occur and be most prominent where AMC reflectors are absent altogether (e.g., north of the 20ø40'S OSC or north of the Clipperton transform fault, Figure 1). However, the opposite trends are observed (Figures 1 and 9). Furthermore, detailed near-bottom observations and side scan data collected along the EPR document that fissures dominate in the axial region and that large scale normal faulting does not typically initiate until 1-2 km off axis [e.g., Edwards et al., 1991]. Carbotte and Macdonald [1994] interpret this to mean that the brittle layer is insufficiently thick in the neovolcanic zone to support shear failure. They estimate that tensile failure rather than shear failure will predominate in oceanic crust over depths of km, a thickness comparable to that of the brittle plus ductile lid overlying the AMC, where present. Along the intermediate spreading Juan de Fuca Ridge, Chadwick and Ernbley [1998] documentwo cases in which narrow grabens (< 100 m wide) have formed directly over eruptive dikes as they were intruding toward the surface. They propose that narrow summit troughs along the fast spreading EPR form in a similar manner and represent dike-induced grabens which have become partially buried by subsequent eruptions. Their model may be extended further to suggesthat repeated shallow dike injections combined with a lack of surface eruption could eventually produce a large graben

8 12,978 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS -18' 2O' < OSC > < OSC > -18' 36' 2õ00-18' 38' -113'26' -113'24' -113'22' I I I I I I ' 26' -113' 24' -113' 22' a Figure 6. Map views of the three notched segments: (a) I, (b) Js, and (c) Jn. Left graphs display the bathymetry with 20 m contour interval. Dive tracks from the Nautile submersible [Auzendet al., 1996] are indicated with bold solid lines. Right graphs represent series of axial cross sections displayed in their correct geographical positions and projected northward. Profiles are obtained from SeaBeam 2000 bathymetry data collected parallel to the ridge crest. They average two successive SeaBeam 2000 pings and are spaced approximately every 120 m along strike. Dots indicate the automated picks for the minimum depths of the two bounding walls of the summit trough and for the maximum trough depths w.hich are displayed in Figure 7. structure at the axis of the EPR. The formation of fault- these numerical models can be used to estimate the dimensions bounded grabens above dike intrusions has been well of dike-induced grabens that would form in a fast spreading documented on land, and numerical modeling is often used to environment. Observations at Hess Deep [Francheteau et al., estimate the dimensions of underlying dikes from the observed 1992] and in the Oman Ophiolite [MacLeod and Rothery, surface deformation [Mastin and Pollard, 1988; Rubin, 1992; 1992] suggest that dikes along fast spreading ridges are Head et al., 1996; Bonafede and Danesi, 1997]. Inversely, typically 1 m wide. An upper limit on dike height is provided

9 LAGABR]ELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,979-18' 08' < deval > < OSC > -18' 22' km km 0 I 2 0 I 2-18' 24' -113' 24' -113' 22' -113' 20' I I I I I I I '24' -113'22' -113'20' b Figure 6. (continued) by the depth to the top of the AMC, which varies between 0.8 and 2 km along the southern EPR [Hooft et at., 1997]. Although published studies investigate surface deformation produced by dikes at least twice these sizes [Mastin and Pollard, 1988; Rubin, 1992; Head et at., 1996; Bonafede and Danesi, 1997], they consistently suggest that an individual diking event along the EPR is unlikely to produce more than a few tens of centimeters of elastic vertical displacement, accommodated in part by a broad uplift of the seafloor on each side of the dike plane. The width of the-resulting trough is predicted to be 2 to 3 times the depth to the top of the dike. To produce a trough with the dimensions observed within our study area ( km wide and m deep) would require the intrusions of several hundred dikes. At spreading rates of mm/yr, one 1-m-dike is injected on average every 7-11 years, implying that large summit troughs would form over several thousand years. Because lava flows would tend to infill the topography created by previous dike injections, this dikeinduced graben mechanism also requires that few of the dikes intruded over this time frame produce surface flows. While dike injection probably contributes to widen summit troughs, this mechanism could not explain why large summit troughs

10 12,980 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS -17 ø 54' < OSC >... ;' krn < deval > km..:: o ø 12' -113 ø 22' -113 ø 20' -113 ø 18' -113 ø 16' -113 ø 22' ø 20' -113 ø 18' -113 ø 16' c Figure 6. (continued) only occur along segments with time-averaged robust magmatic budgets. That fact alone suggests that a dikeinduced graben mechanism is not likely to be the dominant process leading to the formation of large summit troughs. In summary, we consider that several lines of evidence are inconsistent with large summit troughs originating as extensional grabens during phases of amagmatic seafloor spreading. (1) They occur along segments characterized by an overall robust magmatic budget. (2) Conversely, they are conspicuously absent along magma-starved segments where extensional tectonic processes are expected to dominate (e.g., north of 20ø40'S OSC, near 15ø55'S OSC, and north of the Clipperton transform). (3) Their maximum vertical relief correlates with areas where the magma chamber reaches its shallowest levels. (4) Within the study area, the summit trough with the maximum vertical relief (along segment I) is also narrower overall. These observations suggest instead that the physiography of large summit troughs is dominantly controlled by the geometry and evolution of the underlying magma reservoir. We propose that large summit troughs along fast spreading ridges initiate as collapse structures above partially drained magma reservoirs. Seismic studies constrain the size of the melt lens below the southern EPR to be km wide [Kent

11 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,981 Floor W wall E wall ;'. -, L..:.i... ' 5- L,,.7.,.:...' segment I o > o segment Js = segment Jn ' i i ' i LATITUDE Figure 7. Topographic variations along the three notched segments (I, Js, and Jn), as measured from the SeaBeam 2000 data displayed in Figure 6. The three parameters measured are the maximum depth to the trough floor (solid curve), the minimum depth of the western wall (dotted curve), and the minimum depth of the eastern wall (dashed curve). et al., 1994] and continuous for tens of kilometers along axis [Sinton and Detrick, 1992; Detrick et al., 1993; Kent et al., 1994; Mutter et al., 1995; Tolstoy et al., 1997]. These areal activity. Collapse events would involve the poorly organized to chaotic subsidence of upper crustal blocks. Collapse events may be separated by intervals of about 100,000 years. dimensions are comparable to those of the large summit troughs present along segments I, Js, and Jn, as would be predicted if these troughs resulted from collapse of the brittle upper crust over a partially drained AMC. Numerical modeling of the elastic deformation resulting from draining a melt sill of varying dimensions and depths indicates that surface displacements increase with proximity of the sill [Ryan et al., 1983], consistent with the observations in our study area. 6. Conditions for Caldera Development Several studies have recognized that there exists a continuum from smooth axial highs to axial highs notched by a summit trough and have proposed that these are part of a cyclic evolution of the EPR [Lonsdale, 1977; Gente et al., 1986; Macdonald and Fox, 1988; Holler et al., 1990; Fornari While numerical modeling also indicates that the area affected et al., 1998]. However, the occurrence of large summit by vertical displacement increases with sill depth, the associated amplitude and lateral gradient of the vertical displacement becomes quickly negligible with increasing sill troughs exclusively along inflated, robust sections of the fast spreading EPR suggests that they do not represent a systematic stage of development of the axial region, and we depth [Ryan et al., 1983]. Analog experiments that simulate propose that several factors influence the formation of largethe fracturing of the brittle lithosphere in response to the draining of an underlying magma reservoir have been conducted using inflatable balloons buried in a medium of fused alumina powder. They documenthat a caldera has an scale collapse structures along the EPR. Because large summit troughs only occur along segments benefiting from a time-averaged robust magma budget, their formation may be triggered by a decreasing melt supply outline similar to that of the underlying magma chamber, and following a phase of paroxysmal volcanism. This is that its surface area (as defined by the outermost bounding consistent with the fact that the outer flanks of large summit faults) increases slightly as the magma chamber shallows troughs are draped by lobate flows, leveed lava channels, and [Marti et al., 1994]. jumbled sheet flows [Auzende et al., 1996], which are Structural data independently support a model of episodic characteristics of high lava effusion rates [Gregg and Fink, axial collapse along the fast spreading EPR. Dikes dipping moderately to steeply outward from the present EPR are exposed in the walls of the Hess Deep Rift [Francheteau et al., 1995]. Conversely, the fact that large summit troughs do not occur along starved segments may simply indicate that the underlying magma reservoirs are too small to ever produce 1992], and cross cutting vertical dikes suggest that substantial noticeable collapse structures [Macdonald and Fox, 1988]. tectonic rotations occurred at or very near to the EPR axis The presence of small-volume magma reservoirs along starved [Hurst et al., 1994]. Measurements of remanent magnetic sections of the EPR is consistent with basalt geochemistry. inclination on oriented samples taken from these dikes indicate ø of cumulative rotation about a horizontal axis On the basis of samples collected in the axial region of the northern EPR, Batiza et al. [1996] document large temporal toward the ridge axis. To account for these observations, changes in the temperature of the magma chamber along a Hurst et al. [1994] propose that the ridge axis episodically magmatically starved ridge segment. They propose that it inflates and collapses because of variations in magmatic reflects the presence of a small composite AMC, which is

12 , 12,982 LATITUDE (degree S) ß ß ",- -' 1500 ",,,. ' '[; '...o.. :' -..; i. i i I I ' I I o o '-' O segment I I segment Js I I segment Jn I I E :::) 100- O I-- O 0 LL m 50-5ore, LU 0 > segment I u (:: segment Js. segment Jn LATITUDE (degree S) Figure 8. Morphological characteristics of the axial summit trough and magma chamber depth as a function of latitude. Trough width and vertical relief are estimated from the data displayed in Figures 6 and 7 (see text for explanation). The depth below seafloor to the top of the AMC is after Hooft et al. [1997]. Solid curves are obtained by applying a median filter to the data (dots). 0LU 0.10 I, I N=250 R=0.74, I, I, I ' o.8 o ' AMC Depth (km) 0.2 AMC Depth (km) Figure 9. (left) Vertical relief of the trough versus depth to the top of the AMC. (right) Width of the summit trough versus depth to the top of the AMC. Values are resampled from the filtered curves displayed in Figure 8 at latitudinal intervals of 0.02 ø (-220 m). Solid circles correspond to segment I, open triangles correspond to segment Js, and open squares correspond to segment Jn. Error bars correspond to the standard deviations between the data and the median filtered curves fitted through them and displayed in Figure 8 and are at least as large as the estimated measurement errors. -1.2

13 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12,983 thermally vulnerable to changes in melt supply. We interpret the lack of evidence for amagmatic extension along the most starved sections of the fast spreading EPR to indicate that melt supply always remains sufficient to keep pace with the spreading rate. The length of a ridge segment may be another factor influencing the formation of large collapse calderas. Segments with large summit troughs tend to be relatively short (20-60 km) in contrast to a typical segment length of km along the EPR (Figures la and lb). However, it is unclear whether this is a consequence of or a factor in caldera formation or whether it is entirely fortuitous. It is conceivable that for the longer segments, the underlying magma plumbing would allow along-axis melt transport within the hot upper mantle and lower crust to compensate for any localized waning in melt supply [Bell and Buck, 1992; Wang and Cochran, 1993; Cormier et al., 1995] thus reducing the likelihood of caldera formation. In contrast, short segments may be more vulnerable to small fluctuations in their melt supply, and hence display greater morphological and compositional variations. Although large summit troughs are commonly observed along axial highs of the intermediate spreading ridges, the precise fact that they are ubiquitous suggests that the thermal structure of the axial domain is the main factor controlling the interplay between magmatism and extensional tectonism. Magma reservoirs are predicted to be thermally unstable at spreading rates lower than 60 mm/yr [Phipps Morgan and Chen, 1993], and calderas may form episodically (rather than occasionally) when a phase of robust magmatism is followed by a declining melt supply [Kappel and Ryan, 1986; Barone and Ryan, 1988]. With prolonged cooling of the magma reservoir by hydrothermal circulation, the brittle crust may thicken to the extent that it can support shear failure, and the caldera will enlarge by normal faulting processes [Kappel and Ryan, 1986; Barone and Ryan, 1988; Carbotte and Macdonald, 1994; Chadwick and Embley, 1998]. 7. From Smooth Dome to Large Summit Trough: A Model for the Evolution of the Crestal Region Along Segments With a Robust Magmatic Budget Although the crestal morphologies of segments I, Js, Jn, and K are diverse, all four segments are shallow and have similar broad, inflated cross sections (Figure 4). These crestal morphologies display a continuum from one to the next, and may represent different evolutionary stages in inflated areas of the EPR. On the basis of the observations made with the Nautile submersible [Auzende et al., 1996], the following scenario for the evolution of a large summit trough has been proposed [Lagabrielle et al., 1996]. It consists of four stages (A through D in Figure 10) Stage A The axis has a broad, smooth dome structure, with very active volcanism and very thin to absent sedimentary cover, as observed along segment K. An alignment of small collapsed lava lakes ( < 100 m wide) mark the axis of accretion Stage B The axial high consists of a broad dome with a deep, 0.5- to 1-km-wide summit depression, as observed in segment I. Small, unsedimented lava lakes and fresh lobate flows occur discontinuously along the trough floor Stage C The axial high is a broad dome with a relatively wide trough (1-1.5 km) and an associated narrow inner trough where recent volcanism is confined, as observed along segment Js Stage D The axial high has a broad dome, with a wide, low-relief axial trough, similar to that found in segment Jn. Observations at the northern end of segment Js indicate a trough that is intensely fissured, mostly sedimented, and devoid of recent volcanism. Although no dives were carried out along segment Jn, we anticipate similar features along its length, except for the possibility of recent flows near 18ø03 '- 18ø07'S where the floor of the trough rises by about m (Figure 7) and seismic data indicate a pronounced shallowing of the AMC (Figure 8). We propose that the above evolutionary stages of large summit troughs are closely related to changes in the underlying magma reservoir, as described in the following scenario. The formation of a broad, smooth axial high corresponds to very active magmatism and most likely to a shallow axial melt lens (< 1200 m), as observed along segment K (stage A). When the replenishment to the magma chamber wanes slightly, surface flows will become less frequent and most fissures will remain exposed on the seafloor [Haymon et al., 1991], favoring an efficient hydrothermal circulation. As a result, the roof of the magma chamber will progressively deepen [Phipps Morgan and Chen, 1993], and the dike layer will thicken noticeably, as schematically illustrated in Figure 10. With prolonged magmatic quiescence, the brittle layer (extrusives and upper dike sections) may thicken to the extent that it can no longer be supported by the waning magma reservoir (melt lens plus underlying crystal mush), leading to its collapse along the preexisting fissures (stage B). The residual melt may exude from the fissures of the collapsed roof, explaining the occurrence of lava lakes on the young caldera floor. However, most of the expelled magma probably resides below the seafloor as intrusions. Investigations of sub aerial collapse calderas indicate that their volume can be much greater than that of the magma erupted during their formation, suggesting that sub-surface movements of magma are more important causes of collapse than are eruptions that drain support from the substructure [Williams and McBirney, 1978]. There is little direct evidence for subsurface magma movements other than shallow dike intrusions along mid-ocean ridges, and a possible scenario is presented section 8. However, any mechanism leading to the crustal collapse of the neovolcanic zone would result in the pervasive fracturing of the neovolcanic zone, an enhanced hydrothermal circulation favoring the establishment of high temperature vent fields along the main bounding scarps, as observed along segment I. The young caldera will further mature toward a morphology like that of segment Js, as prolonged reduction of the magma supply and further thickening of the brittle layer will cause outward collapse of the AMC roof (stage C). Sub surface dike intrusions will also contribute to widen the axial depression. At fast spreading rates ( mrn/yr), 500 m of new seafloor is accreted in a few thousand years, a distance which could account for the different trough widths along segment Js (stage C) and

14 ,, 12,984 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS smooth, axial dome segment K stage A volcanics transition zone L /..:".':'... :...:.::."::,'.":i:i:i :.'.' ':."."-'.".':':.::i:i:.-': :.:: : i:i:i:i:i:i':: :'.'.::i::'.';: : :!: : : :!:!:.:: :i:i:i:i:; :i. -':.. ':':i::':i::./:' '.::':':' gabbros narrow axial caldera 500m 2700m dikes segment I stage B volcanics km " r wide axial caldera,1000m-1500m, dikes transition zone 2700m- segment JS stage C km smoothed axial caldera ', 1500m-2000m ', volcanics stage D dikes km.....,....,..,.,.,.,.,.,.... ß......,.....:.:..:.:.:.:.:.:.:.x....:.:.:.:.:.:.:...:.:.:.:.:...:....,....,,....,... x+:o:,: 0 i km no vertical exaggeration transition zone Figure 10. Possible scenario relating the morphological evolution of large summit troughs to the varying magma supply and to the inferred position of the magma lens overlying the crystal mush (see text for detailed discussion). Stages A, B, C, and D would find their equivalent in the morphology of segments K, I, Js, and Jn, respectively. Stage A was drawn according to an original sketch by Sinton and Detrick [1992]. segment I (stage B) (Figure 8). A few thousand years of contribute to the smoothing of the caldera topography (stage relative volcanic quiescence along segment Js is suggested by D). The renewed replenishment of the magma reservoir is the several millimeters of sediments blanketing its floor as well as the presence of numerous mature hydrothermal sites [Auzende et al., 1996]. Finally, the collapse caldera probably also widens by mass wasting along the main bounding scarps, as indicated by the numerous fresh talus deposits observed during the 1993 Nautile dives at the foot of the bounding walls expected to produce a shallowing of the AMC roof (similar to that observed along segment Jn near 18ø04'-18ø06'S), which probably occurs by stoping of the base of the dike layer. With renewed eruptive volcanism, lava flows will gradually infill the trough. With the full resumption of the robust melt supply, the axial region will again display a domal cross- [Auzende et al., 1996]. Eventually, several factors would section (stage A).

15 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS 12, A Scenario for Subsurface Magma Movements Along the EPR Seismic analysis indicates that the melt lens along the southern EPR is m thick [Hussenoeder et al., 1996], and the hypothetical draining and collapse of such a thin lens would not be sufficient to explain the formation of a summit trough with m of vertical relief. Additional mechanisms for sub surface magma withdrawal need to occur if large summit troughs do represent elongated collapse calderas, and we suggest that these include the compaction and the "necking" of the gabbros below the melt lens (Figure 11). If the melt that migrates from the composite magma reservoir (melt lens plus ci'ystal mush) is not being replenished, the 2-4 km of crystal mush may compact accordingly. Seismic tomography results are generally interpreted to indicate the presence of about 5% of interstitial melt within the lower gabbros [e.g., Caress et al., 1992; Wilcock et al., 1992]. However, numerical modeling of the seismic properties of partially molten gabbros that take into account their anisotropy suggests that seismic velocities allow for 15-50% melt [Mainprice, 1997], a result more consistent with detailed investigations of the Oman ophiolites [Nicolas and Boudier, 1995]. Whichever model applies, the extraction of 1% melt from a 2- to 4-km-thick gabbro section may result in a maximum compaction of m at depth. The extraction of as little as 2-3% melt from the gabbro mush, if not replenished for an extended period of time, may produce enough compaction in the lower crust to contribute appreciably to surface deformation. In particular, gabbro compaction may be facilitated by ductile or magmatic flow. Although the origin of the layered texture of the lower gabbros in the Oman Ophiolite is controversial, recent models suggest that it results from lower crustal flow. One model involves ductile deformation driven by passive seafloor spreading [Phipps Morgan and Chen, 1993], and another model involves magmatic flow driven by active mantle upwelling at the ridge axis [Nicolas and Boudier, 1995]. Either model could provide a mechanism for the crystal mush to deform by stretching and thinning in response to steady state spreading during a prolonged interval of waning magma supply (3,000-5,000 years). This proposed "necking" of the lower crust may contribute to depressing the seafloor along the neovolcanic zone where a large volume of hot gabbro mush underlies the ridge axis (Figure 11). However, where the magmatic budget of the EPR remains modest for extensive periods (~100,000 1 Km 0 No vertical exageration Figure 11. Schematic model for the formation and the evolution of large summit calderas of the EPR in relation to the depth variations to the top of the AMC. This model implies stretching of the crystal mush in response to steady state extension at the ridge axis during stages of reduced magma supply. Caldera width varies from 500 to 2000 m, while the roof of the AMC varies in depth from m to m below seafloor.

16 12,986 LAGABRIELLE AND CORMIER: ORIGIN OF LARGE EPR SUMMIT TROUGHS years), the volume of the gabbro mush beneath the melt lens may be too restricted to neck significantly. This scenario is consistent with the lack of large summit trough along the starved section of the EPR and with other observations presented in this paper. It remains to be evaluated against structural, petrological, and geochemical data from the EPR and the Oman ophiolite. Buck, W.R., and A.N.B. Poliakov, Abyssal hills formed by stretching 9. Conclusions oceanic lithosphere, Nature, 392, , Carbotte, S.M., and K.C. Macdonald, The axial topographic high at Axial troughs along the crest of fast spreading (> 90 mm/yr) intermediate and fast spreading ridges, Earth Planet. Sci. Lett., 128, 85-97, ridges that are wider than 500 m occur only along inflated, Caress, D.W., M.S. Burnett, and J.A. Orcutt, Tomographic image of the magmatically robust sections of the ridge and are probably axial low-velocity zone at 12ø50'N on the East Pacific Rise, J. always underlain by a shallow magma chamber. Conversely, Geophys. Res., 97, , large summit troughs are conspicuously absent along the most Caroff, M., Y. Lagabrielle, P. Spadea, and J.M. Auzende, A magmatically starved sections of the EPR. Between ' geochemical modelling of non-steady state magma chambers: A and 18ø35'S, three short ridge segments are notched by such case study from an ultrafast spreading ridge: East Pacific'Rise, 17 ø to 19øS (Naudur cruise, 1993), Geochim. Cosmochim. Acta, 61, large summit troughs. Overall, their vertical relief is 4374, maximum where the magma chamber is shallowest; furthermore, the narrowestrough displays the greater vertical relief. These lines of evidence suggesthat large summit Chadwick, W.W., and R.W. Embley, Graben formation associated with recent dike intrusions and volcanic eruptions on the mid-ocean ridge, J. Geophys. Res., 103, , Charlou, J.-L., Y. Fouquet, P.-P. Donval, J.-M. Auzende, P. Jeantroughs are not rifted features resulting from stages of Baptiste, M. Stievenard, and S. Michel, Mineral and gas chemistry dominantly amagmatic extension. Rather, we suggesthat of hydrothermal fluids on an ultrafast spreading ridge: East Pacific they represent elongate calderas forming by collapse of the Rise, 17 ø to 19øS (Naudur cruise, 1993) phase separation processes brittle upper crust in response to a decreasing melt supply to a controlled by volcanic and tectonic activity, J. Geophys. Res., 101, 15,899-15,919, formerly robust magma chamber. The proposed origin of large Cochran, J.R., J.A. Goff, A. Malinverno, D.J. Fornari, C. Keeley, and X. axial troughs as collapsed calderas has significant Wang, Morphology of a "superfast" mid-ocean ridge crest and implications with regard to the structure of the underlying flanks: The East Pacific Rise 7ø-9øS, Mar. Geophys. Res., 15, 65-75, magma reservoir. Seismic studies suggesthat the melt lens along the EPR is about m thick. Even if such a melt lens were entirely drained, it would be unlikely to result in a Cormier, M.-H., The ultrafast East Pacific Rise: Instability of the plate boundary and implications for accretionary processes, Philos. Trans. R. Soc. London, $er. A, 355, , collapse structure m deep at the seafloor. We therefore Cormier, M.-H., and K.C. Macdonald, East Pacific Rise 18ø-19øS: propose that along segments with a time-averaged robust melt Asymmetric spreading and ridge reorientation by ultra-fast migration supply, the thick crystal mush zone underlying the melt sill of axial discontinuities, J. Geophys. Res., 99, , will deform and compact during intervals of waning magma supply. This model also implies that along inflated sections of the EPR, the base of the dike layer is a dynamic boundary which thickens during phases of reduced volcanism because of the deep penetration of hydrothermal circulation and thins by stoping at its base during phases of renewed magmatism. Acknowledgments. We are indebted to J.-M. Auzende, who led the Naudur expedition during which the submersible observations discussed in this paper were collected. We thank the scientific parties and crew of surveys Naudur and Sojourn-l, A. Nicolas, R. Batiza, V. Ballu, J. Dyment, P. Gente, R. Haymon, K. Macdonald, P. P6zard, W.B.F. Ryan, D. Scheirer, J. Sinton, S. White, and L. Ximenis for fruitful discussions and W. Chadwick, D. Fornari, E. Hooft, and an anonymous reviewer for thorough reviews. Y.L. benefited from grants from Institut National des Sciences de l'univers, CNRS (G6osciences Marines). 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