* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Oceanic corrugated surfaces and the strength of the axial
Document related concepts
Earth and Planetary Science Letters 288 (2009) 174–183 Contents lists available at ScienceDirect Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l Oceanic corrugated surfaces and the strength of the axial lithosphere at slow spreading ridges Mathilde Cannat a,⁎, Daniel Sauter b, Javier Escartín a, Luc Lavier c, Suzanne Picazo a a b c Equipe de Géosciences Marines, CNRS-UMR7154, Institut de Physique du Globe, 4 place Jussieu 75252 Paris cedex 05 France Ecole et Institut de Physique du Globe, CNRS-UMR 7516 5 rue Descartes 67084 Strasbourg cedex France University of Texas, Institute for Geophysics, Jackson School of Geosciences, Austin, Texas 78759 USA a r t i c l e i n f o Article history: Received 24 February 2009 Received in revised form 29 August 2009 Accepted 9 September 2009 Available online 15 October 2009 Editor: R.D. van der Hilst Keywords: mid-ocean ridges detachment fault topography corrugated surface lithosphere strength a b s t r a c t We analyse the topography and gravity signature of 39 corrugated surfaces formed over the past 26 myrs in the footwall of axial detachment faults at the eastern Southwest Indian Ridge. These corrugated surfaces appear to have formed at a melt supply about half the global melt supply average for mid-ocean ridges, and we ﬁnd that their presently elevated topography, relative to adjacent non-corrugated seaﬂoor, was mostly acquired at the end of their formation, at the “termination stage”. This conﬁguration, which also characterizes many off-axis corrugated surfaces in other oceans, suggests that the plate ﬂexural rigidity was very low during the formation of the corrugated surface, and increased signiﬁcantly at the termination stage. Following Buck (1988), we hypothesize that stresses related to bending of the plate cause internal deformation and damage in the footwall of the fault, which is associated with weakening. As a possible mechanism for enhanced footwall weakening while corrugated surfaces form, we propose the formation of weak shear zones coated with hydrous minerals such as talc, amphibole, chlorite and serpentine, in mantle-derived ultramaﬁcs next to gabbro intrusions. If this hypothesis is correct, the amount of footwall weakening and roll-over along axial detachment faults at slow spreading ridges may be controlled both by access to hydrothermal ﬂuids in the footwall of the detachment, and by the abundance and distribution of gabbros intrusions in exhumed ultramaﬁcs. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Oceanic corrugated surfaces are domal features characterized by spreading-parallel undulations of the seaﬂoor. They were ﬁrst discovered in the Atlantic (Cann et al., 1997), and interpreted as the exposed inactive footwall of large offset axial normal faults, also called detachment faults (Cann et al., 1997; Tucholke et al., 1998). Corrugated surfaces have since been dredged, cored and sampled with submersibles at a number of near-axis and off-axis locations, yielding variably deformed rock suites, including gabbros, serpentinized peridotites, and lesser volumes of basalt (MacLeod et al., 2002; Karson et al., 2006; Ildefonse et al., 2007; Dick et al., 2008). Deformed samples from the corrugated fault zone typically display low temperature to greenschist facies syntectonic minerals, and fabrics are dominantly brittle, with intervals of plastically deformed talc, amphibole, chlorite and serpentine (Escartin et al., 2003; Schroeder and John, 2004). Higher grade sheared gabbro and peridotite mylonites have also been sampled below exposed detachment surfaces (Cannat et al., 1991; Dick et al., 2000 ; Schroeder and John, 2004). The presence of gabbros conﬁrms that, in spite of a thin crust inferred from seismic and gravity-data and suggesting reduced melt ⁎ Corresponding author. E-mail address: [email protected] (M. Cannat). 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.09.020 supply (Blackman et al., 1998; Canales et al., 2004), corrugated surfaces form during magmatically active stages of ridge spreading (Buck et al., 2005; Ildefonse et al., 2007). Recent observations at 13°N in the Atlantic have shown corrugated domes originating in the footwall of active axial normal faults, with volcanic ridges in the hanging wall (Smith et al., 2006, 2008). Assuming fault dips of 40° or more at depth (e.g., (deMartin et al., 2007)), roll-over to dips less than 5° occurs within 5 km of footwall emergence (Smith et al., 2006), indicating a very low footwall ﬂexural rigidity (Smith et al., 2006, 2008). Corrugated surfaces have now been identiﬁed near most intermediate to ultraslow spreading ridges, predominantly, but not exclusively (Escartín and Cannat, 1999; Smith et al., 2008), in ridgetransform inside-corner settings. Their domal shape has been reproduced in numerical models by ﬂexure of the footwall of large offset normal faults (Lavier et al., 1999). Numerical models of symmetrical spreading predict that such faults are most likely to form when about 50% of total extension is accommodated by magmatic accretion in the hanging wall (Buck et al., 2005; Tucholke et al., 2008). Corrugations, however, are not always present in the footwall of large offset axial normal faults. Near-axis domal structures devoid of corrugations, such as at the Trans Atlantic Geotraverse (TAG) (deMartin et al., 2007), are also interpreted as the footwall of detachments. It has been proposed that axial detachments, forming M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 corrugated surfaces or not, are active along nearly 50% of the length of slow-spreading ridges (Escartin et al., 2008b). This is consistent with the estimated surface proportion of ultramaﬁc and gabbroic seaﬂoor exposures in the Atlantic (~25% ;(Cannat et al., 1995)), because these exposures form asymmetrically about the axis in the exhumed footwall of these detachments. In this paper, we present a detailed description of the corrugated surfaces identiﬁed in ultraslow-spreading seaﬂoor of the Southwest Indian Ridge (SWIR), east of the Melville Fracture Zone (Fig. 1). Our study area covers 630 km of ridge axis, and extends to ages of 28 myrs on both plates. Seismic data suggest that this eastern part of the SWIR has a very low average melt supply, equivalent to a 3 to 4 km-thick magmatic layer (Muller et al., 1999; Minshull et al., 2006). This is about half of the average melt supply estimated for the global midocean ridge system (Chen, 1992). Corrugated surfaces occur throughout the mapped area (Fig. 1), although they cover only about 4% of the total surface (Cannat et al., 2006). This region of the SWIR also displays large expanses (~40% of the mapped area; Fig. 1) of non- 175 corrugated «smooth seaﬂoor», which have been interpreted as mostly mantle-derived ultramaﬁc rocks exposed in the footwall of axial detachments (Cannat et al., 2006). The rest of the seaﬂoor (“volcanic seaﬂoor” in Fig. 1) displays volcanic cones, and spreading-perpendicular scarps. Many of these scarps have steep outward-facing slopes and may represent tectonically-rotated volcanic surfaces, as proposed by (Smith et al., 2008) for similar scarps formed by axial detachments in the 14°N area of the Mid-Atlantic Ridge (MAR). The prevalence of detachments in accretion processes at the eastern SWIR is thus probably greater than at the MAR (Cannat et al., 2006). Previous work in the eastern SWIR has shown that smooth seaﬂoor forms at minimal melt supply, and that the seaﬂoor opposite the corrugated surfaces and across the ridge axis (conjugate seaﬂoor) is primarily volcanic (Cannat et al., 2006). The eastern SWIR therefore has two assets for a study of how corrugated surfaces form: (1) it contains a large number of these surfaces in a relatively well deﬁned spreading and regional melt supply context; and (2) some of these corrugated surfaces transition into seaﬂoor that was formed at a Fig. 1. Shaded bathymetric map of the eastern Southwest Indian Ridge, showing the location of 39 corrugated surfaces (modiﬁed after (Cannat et al., 2006). Red lines are corrugation trends, white lines are calculated spreading vectors for 2 myrs-worth of plate spreading (plate rotation parameters as in (Cannat et al., 2006)). White contours separate seaﬂoor with numerous scarps and volcanic cones (volcanic seaﬂoor) from seaﬂoor with broad hills and no volcanic features (smooth seaﬂoor; (Cannat et al., 2006). Thick dashed line: Central Magnetic Anomaly. Thinner dashed lines are other magnetic isochrons (Sauter et al., 2008). 176 M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 minimal melt supply, with little to no volcanism. This is not the case in the Atlantic, where corrugated surfaces are surrounded by seaﬂoor with morphological evidence for a volcanic carapace (e.g. (Smith et al., 2006, 2008)). Based primarily on the eastern SWIR data, we speciﬁcally discuss the topography and the conditions of termination of oceanic corrugated surfaces. We then propose a conceptual model in which apparent variations of the rigidity of the axial plate during and after the formation of corrugated surfaces, are explained by variable degrees of mechanical weakening of the footwall of axial detachment faults. 2. Size, age and map distribution of eastern SWIR corrugated surfaces Bathymetric data used for this study were acquired with a Thomson TMS 5265B multibeam system, and have a horizontal resolution of 1 to 2% of seaﬂoor depth. At depths of 4000 m and more, these data therefore do not properly image seaﬂoor structures less than 100 m across. They are sufﬁcient, however, to map the corrugated surfaces, and to derive some basic characteristics of their transition to non-corrugated seaﬂoor. Individual corrugated surfaces in the eastern SWIR cover areas up to 880 km2 (Fig. 2a; Table 1). Many surfaces have similar along- and across-axis lengths, yet a few large surfaces extend substantially more along-axis (up to 74 km; Fig. 2b), than across-axis (up to 31 km; Fig. 2b). Most surfaces appear to have formed in less than 2 myrs (Fig. 2c), and corrugations are parallel to the spreading vectors (Fig. 1) calculated using the rotation parameters listed in (Cannat et al., 2006), which are within errors of the more recent regional plate velocity model of (Patriat et al., 2008). Signiﬁcant differences between corrugations and calculated spreading vectors are only observed for corrugated surfaces between magnetic anomalies 6 (A6) and 6C (A6C). These differences are likely due to local variations in spreading direction with respect to the regional plate velocity model. Based on the magnetic anomaly pattern, the ages of corrugated surfaces in Fig. 1 range between recent (Fuji Dome; (Searle et al., 2003; Searle and Bralee, 2007), and older than 25 myrs. They are more abundant, and commonly larger, in seaﬂoor older than A6 (Fig. 2a). This may relate to a decrease in spreading rate with time, from 30 km/ myr prior to A6C, to ~16 km/myr between A6 and A6C, and to ~14 km/myr since A6 (Sauter et al., 2008; Patriat et al., 2008). The two largest groups of corrugated surfaces (numbers 29 to 37, and 9 to 13; Fig. 1) initiated on opposite sides of the ridge soon after the onset of SWIR spreading at the Rodriguez Triple Junction. At that time the spreading rate was ~ 30 km/myr (Patriat et al., 2008). Surfaces younger than 10 myrs (magnetic anomaly 5) are small (<50 km2; Table 1), with the exception of surfaces number 5, 14 and 21. Finer resolution Towed Ocean Bottom Instrument (TOBI) data on surfaces 15 (Fuji Dome; (Searle et al., 2003)) and 16, show that they represent two portions of a single, larger, corrugated surface (Searle and Bralee, 2007). Surfaces 17 and 18, on the northern axial valley wall, may also belong to a single, faintly corrugated surface. Surfaces 25 to 28 delineate corrugated areas on the southern ﬂank of a blocky topographic high (Fig. 1), previously described as the fossil footwall of a large axial fault (fault block h9 in (Cannat et al., 2003); also discussed in (Searle and Bralee, 2007)). 3. Gravity and topographic signature of eastern SWIR corrugated surfaces Processing of the gravity data and calculation of residual topography for the study area are presented in (Cannat et al., 2006). Modelling of both residual seaﬂoor depths and gravity anomalies involves a correction for plate cooling with age, which is most pronounced and probably more inaccurate, in near-axis regions. Because the near axis region in Fig. 1 also comprises few corrugated surfaces, we have limited our study to the gravity and topographic signature of crust formed prior to magnetic anomaly 3A (Figs. 1 and 2a). The residual Fig. 2. Characteristics of the 39 eastern Southwest Indian Ridge corrugated surfaces shown in Fig. 1 (Table 1). Open circles : corrugated surfaces accreted since the time of magnetic anomaly 3A. Grey circles : corrugated surface with area <200 km2; black circles : area > 200 km2. Ages for magnetic anomalies 3A, 6, 6C and 8 in (a) as in (Cande and Kent, 1995). Estimated duration in (c) is maximum across-axis extension of corrugated surface divided by half-spreading rate at time of formation. It is a maximum because it assumes symmetrical spreading during the formation of corrugated surfaces, which may not be the case (Searle and Bralee, 2007; Baines et al., 2008; Dick et al., 2008). Residual topography in (d) is calculated by difference with the square root of age subsidence model also used for gravity data correction (Cannat et al., 2006). Crustal thickness in (d) is modelled from residual mantle Bouguer gravity anomalies (RMBA), with a reference crustal thickness of 3.4 km, and constant density crust and mantle (Cannat et al., 2006). This crustal thickness model is consistent with available seismic refraction data (Minshull et al., 2006). Line is (d) is best linear ﬁt for corrugated surfaces >200 km2; it has a slope of 0.43 (with a correlation coefﬁcient R2 of 0.42). The predicted slope for isostatic balance (for crustal density of 2700 kg/m3, and mantle density of 3300 km/m3) would be 0.26. Dashed arrows in (d) link corrugated surfaces formed along the same ﬂow line as shown in Fig. 4. 177 M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 Table 1 Characteristics of the 39 eastern Southwest Indian Ridge corrugated surfaces shown in Fig. 1. Number Area Length Width Azimuth (km2) (km) (km) (°) 31 66 130 511 450 105 700 358 280 198 880 156 31 180 39 35 46 16 132 37 134 41 169 13 26 40 46 9 58 36 70 302 289 542 828 265 67 32 25 4 8 14 23 20 13 20 29 12 17 31 13 4 23 6 9 9 6 13 8 15 6 11 5 7 13 8 4 7 5 9 20 7 10 18 9 3 5 6 7 9 13 24 25 14 58 14 23 14 45 19 10 8 8 4 8 4 11 7 12 5 22 4 4 5 7 3 12 8 14 19 40 42 74 35 20 8 7 189 183 187 185 183 183 181 187 186 184 188 185 181 175 179 179 179 180 180 182 180 178 181 182 180 183 180 180 188 181 183 182 181 181 189 185 184 184 179 Corrug. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Longitude Latitude Res. Crustal Term. E S Topo Thickness Fault av. Heave (myr) av. av. (m) (km) (m) Before/after West/east 26.6 25.3 28.0 20.1 8.4 25.3 24.1 26.6 22.8 20.1 20.1 22.8 24.7 9.3 2.6 2.6 1.3 1.3 10.9 9.3 8.4 10.9 14.6 14.6 9.3 9.3 9.3 9.3 28.5 28.0 28.0 26.6 26.6 25.1 24.1 22.1 22.1 17.8 2.9 61.261 61.534 62.537 62.115 62.261 61.510 61.698 62.514 64.718 64.614 65.038 64.190 64.107 65.300 63.782 63.671 63.362 63.422 64.592 64.551 64.676 63.727 63.818 63.611 64.150 64.215 63.955 64.042 63.251 63.413 63.563 63.304 63.905 63.577 63.629 64.176 63.853 65.138 65.484 − 30.935 − 30.720 − 30.423 − 29.837 − 28.079 − 27.414 − 27.524 − 26.701 − 26.375 − 26.645 − 26.610 − 26.476 − 26.348 − 28.345 − 28.015 − 28.023 − 28.006 − 27.953 − 27.245 − 27.350 − 27.434 − 28.537 − 28.753 − 28.783 − 28.448 − 28.469 − 28.417 −28.429 − 30.171 − 30.029 − 29.991 − 29.761 − 29.725 − 29.564 − 29.412 − 29.251 − 29.293 − 26.712 − 27.852 587 436 724 961 826 − 268 738 454 388 1137 354 723 352 924 – – – – 114 409 1279 269 689 109 629 134 668 1121 − 111 62 85 568 185 103 159 225 135 233 – 2.9 3.7 2.7 3.8 3.8 2.8 3.3 3.8 3.4 4.3 3.5 4.5 3.9 3.3 – – – – 3.6 3.2 4.1 3.1 3.2 3.0 3.0 3.7 3.8 3.3 2.2 2.9 2.8 2.8 2.6 3.2 3.0 2.9 3.3 2.7 – 0 100 1435 996 1417 0 1332 869 700 1642 727 886 0 0 0 0 0 0 0 1213 1005 615 1801 285 0 0 0 0 808 1019 804 1294 824 785 788 416 336 0 815 ? vv vv sv vv vv vv vv sv vv sv sv sv vs vv vv vs vs vv vv vs vv vv vv vv vv vv vv vv vv ? ? ss vv vv ss ss vv vv ss vv v? ss ss ?v ?s vv vs vv vs vs vs vv sv ss vs ss vv ss ss vv vs vv vv vs vv vv ? ss ss ?s sv ?s vs ss ss ss vv Estimated age Transition Length is along-axis extension, width is across-axis extension. Longitude and latitude are mean values for each surface. Age is estimated using the plate rotation parameters in (Cannat et al., 2006), which differ only marginally from those of Patriat et al. (2008). Residual topography and crustal thickness (from gravity-derived model in (Cannat et al., 2006) are averages for all grid points which belong to the surface. Standard deviation values range between 100 m and 500 m for residual topography, and between 50 m and 500 m for crustal thickness. Termination fault heave is maximum value measured along termination scarp. Last two columns correspond to types of seaﬂoor (v : volcanic ; s : smooth) accreted along the same ﬂow line before and after the corrugated surface, or along the same isochron to the west and east of the surface. Question marks correspond to ambiguous morphologies in older, sediment-covered seaﬂoor. topography is the difference between actual topography, and the square root of age subsidence model also used for gravity data correction (Cannat et al., 2006). Our gravity-derived crustal thickness model is consistent with available seismic refraction data (Minshull et al., 2006). Collectively, corrugated surfaces shown in Fig. 1 have a positive average gravity signature, and an anomalously elevated residual topography. The conjugate corrugated-volcanic seaﬂoor pairs (Cannat et al., 2006) are characterized by extreme topographic and gravimetric asymmetry (Fig. 3). Corrugated surfaces stand on average 500 m above the predicted thermal subsidence level, while volcanic seaﬂoor accreted simultaneously in the opposite plate has near zero average residual topography (Fig. 3b). Higher mantle Bouguer gravity anomalies on average (Fig. 3a) also suggest thinner average crustal thickness for corrugated surfaces than for the conjugate volcanic seaﬂoor. However, the mean gravity signature for the corrugatedvolcanic conjugate pairs is close to the regional average (Fig. 3a). Seismic data suggest that this regional average is equivalent to a 3 to 4 km-thick magmatic layer (Muller et al., 1999; Minshull et al., 2006). Corrugated surfaces of the eastern SWIR thus appear to have formed at a melt supply about half the global melt supply average for midocean ridges (Chen, 1992). Gravity anomalies in the eastern SWIR also suggest that melt supply to the ridge was less focused during the formation of the older seaﬂoor of our study area (Cannat et al., 2006), resulting in a larger proportion of the seaﬂoor being formed at the average regional melt supply. This is consistent with the observation that corrugated surfaces are more common in this older seaﬂoor (Fig. 2a). Individually, corrugated surfaces show a wide range of topographic and gravity signatures (Fig. 2d). Corrugated surfaces have gravityderived crustal thickness ranging between 2.1 km and 4.4 km (Fig. 2d) and many have a gravity-derived crustal thickness lower than the reference value of 3.4 km used for the inversion (Cannat et al., 2006). Mean residual topography, by contrast, is positive in all surfaces but two, with values up to 1200 m, and shows a weak overall positive correlation with gravity-derived crustal thickness (Fig. 2d). In four pairs of corrugated surfaces which formed on the same ﬂow line within 2 to 5 myrs (numbers 6 and 7, 9 and 10, 13 and 12, and 19 and 178 M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 surfaces do culminate near the termination, with outward-facing slopes up to 12° (Fig. 6). This is very similar to the outward-facing topography of many corrugated surfaces in the Atlantic (Tucholke et al., 1997; Escartin et al., 2003; Smith et al., 2008); Fig. 7). This suggests that uplift was, for a good part at least, achieved at the termination stage, and that the corrugated surface exposed at the seaﬂoor has been ﬂexurally tilted away from the axis. Heave values vary substantially along each termination scarp (as can be seen in the examples shown in Fig. 6), which probably explains the large scatter in Fig. 5 (in which only maximum heave is shown for each scarp). 4. Transition of eastern SWIR corrugated surfaces to adjacent volcanic and smooth seaﬂoor Fig. 3. Gravity and topographic signature for volcanic–volcanic, corrugated–volcanic, smooth–volcanic, and smooth–smooth pairs of conjugate eastern Southwest Indian Ridge seaﬂoor (seaﬂoor inferred to have formed simultaneously on each side of the axial valley ; (Cannat et al., 2006). Grey line links mean values for conjugate pairs, and vertical dashed lines link corresponding mean values for each type of seaﬂoor. Plots are restricted to crust accreted in the area of Fig. 1, prior to magnetic anomaly 3A (5.89 myr ; (Cande and Kent, 1995). (a) Mean residual mantle Bouguer anomaly (RMBA). The average RMBA value over the study area is 20 mGal. The crustal thickness used as a reference for gravity modelling based on available seismic data (Muller et al., 1999) is 3.4 km (Cannat et al., 2006). (b) Mean residual topography (see caption for Fig. 2). Standard deviation values are large in both diagrams :~8 mGal in (a), and ~500 m in (b). 21; Fig. 1), we also observe that the most recent surface has a thicker gravity-derived crust and a higher residual topography (Fig. 2d). Most corrugated surfaces described in the Atlantic and other oceans are characterized by one or several isochron-parallel ridges which deﬁne the breakaway zone, where the fault initiated, and by a ‘scarp and valley’ structure, also isochron-parallel in most cases, at the termination of the fault (Tucholke et al., 1998). Terminations of some Atlantic corrugated surfaces, however, are not associated with a scarp, but the detachment simply plunges into the seaﬂoor. This is the case for the northern part of the Atlantis Fracture Zone core complex (Cann et al., 1997; Blackman et al., 1998). By contrast, the southern part of this core complex is elevated more than 3000 m above the adjacent rift valley ﬂoor by a large termination fault scarp (Cann et al., 1997; Blackman et al., 1998 ). Corrugated surfaces that presently form at 13°N at the MAR have sharp breakaway ridges, interpreted as rotated extrusives (Smith et al., 2006). Ridges similar to these breakaway structures, but that do not necessarily occur at breakaway, have been interpreted as tilted blocks of hanging wall material (mostly volcanics) rafted on the footwall. This is demonstrably the case for the volcanic ridge identiﬁed at the SWIR Fuji Dome (Searle et al., 2003), and, at the MAR, along part of the Atlantis Fracture Zone detachment (Cann et al., 1997). Many corrugated surfaces in the eastern SWIR do not have a breakaway ridge (in Fig. 4 it is for example present only in corrugated surface number 19), and thirteen surfaces (Table 1) do not show a termination scarp (for example surfaces number 6 and 19 in Fig. 4). Most of these thirteen surfaces are small (<50 km2; Table 1), four are very recent and located on, or near the walls of the axial valley (numbers 15, 16, 17 and 18), and four (numbers 25 to 28) are interpreted as corrugated portions of a single fault block (Cannat et al., 2003; Searle and Bralee, 2007). Their residual topography is variable (Fig. 5). Among those corrugated surfaces that do have a termination scarp (26 out of 39; Table 1), we observe a correlation between the maximum heave measured along this scarp, and the surface's mean residual topography (Fig. 5). In map view, it is apparent that these Corrugated surfaces typically form at the transition between volcanic domains and smooth seaﬂoor domains. Fig. 1 shows that only seven corrugated surfaces (number 22, 24-28 and 39) occur within wide expanses of volcanic seaﬂoor, and none within a wide expanse of smooth seaﬂoor. This is consistent with the intermediate gravity signature of corrugated-volcanic conjugate pairs (Fig. 3a), which suggests that they formed at intermediate melt supply, in the low regional melt supply context of the eastern SWIR (Cannat et al., 2006). Seismic data (Muller et al., 1999; Minshull et al., 2006) suggest that this intermediate melt supply corresponds to about half the global melt supply average for mid-ocean ridges (Chen, 1992). Along ﬂow lines, almost two thirds (24 out of 39) of the corrugated surfaces transition to and from volcanic seaﬂoor, while transitions to and from smooth seaﬂoor are observed in only 3 cases (Table 1). Along isochrons, the (lateral) transition occurs in equivalent proportions to volcanic or to smooth seaﬂoor (Table 1). Spreadingperpendicular scarps and volcanic ridges in volcanic seaﬂoor domains about the edges of corrugated surfaces, curving in some cases but not in a systematic manner (see corrugated surfaces number 10 and 19 in Fig. 4). Similar relations are observed in the Atlantic (Smith et al., 2006). By contrast, the lateral transition to smooth seaﬂoor is commonly diffuse: large corrugations (with a wavelength of 500 m or more) disappear, but there are hints of smaller wavelength undulations, which are not resolvable in the shipboard bathymetric data. Another common characteristic feature is that corrugated surfaces with no termination scarp, or portions of corrugated surfaces located away from the termination scarps, lie at a greater depth than adjacent contemporaneous seaﬂoor, be it volcanic, or smooth. This can be seen in Fig. 4c, where corrugated surface number 19 lies 300 to 600 m below the volcanic seaﬂoor to the east and west, and in Fig. 4d, where the eastern part of corrugated surface number 23 lies 700 m below contemporaneous smooth seaﬂoor to the east. 5. Discussion 5.1. Fault topography and the rigidity of the axial plate Dome-shaped corrugated surfaces that form presently at 13°N in the Mid Atlantic Ridge show evidence for a large rotation of the detachment fault footwall (at least 35°) over remarkably short distances (less than 5 km; Smith et al., 2006). This requires a very low ﬂexural rigidity (equivalent to an elastic thickness <1 km; (Smith et al., 2006, 2008)). Very low rigidity is also indicated by the subdued topography of some off-axis corrugated surfaces in our SWIR study area, relative to surrounding volcanic or smooth seaﬂoor (Fig. 4c and d). We interpret this adjacent uncorrugated (volcanic or smooth) seaﬂoor as fault blocks or exhumed fault surfaces, that have undergone a lesser roll-over and record higher fault uplift. Termination scarps of the SWIR corrugated surfaces accommodate substantial uplift, have an overall dip of 15° to 22°, and cut the adjacent corrugated surfaces at angles of 15° or more (Fig. 6). These M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 179 Fig. 4. Detail maps of residual topography for selected eastern Southwest Indian Ridge corrugated surfaces, numbered as in Fig. 1. Arrows point toward ridge axis. Detail maps of corrugated surfaces number 14, 32, 34 and 35 may be found in (Cannat et al., 2006). Outlines of corrugated surfaces (dashed lines) are conservative : because ship tracks also trend north south, faint corrugations could be artefacts in the multibeam data and have not been considered. termination scarps show evidence for mass-wasting in the form of cuspate landslides head scarps (visible in the bathymetric maps of Fig. 4). The dip of the termination fault as it emerged from the axial valley seaﬂoor was therefore larger than the present dip of these scarps. Corrugated surfaces of the MAR commonly show a similar relationship between the ﬂat, or outward-facing slopes of old corrugated surfaces, and the steeper, inward-facing termination scarps with evidence for mass wasting (Figs. 7, and 1 in (MacLeod et al., 2002)). This indicates that axial normal faults at the termination stage proceeded with less footwall ﬂexure, creating a larger topography than during the formation of the corrugated surface. From this, we deduce that the rigidity of the plate increased at the termination stage. Spatial and temporal changes in axial plate rigidity at slow spreading ridges may be due to changes in the overall thermal structure, related to along-axis or temporal variations in melt supply (Chen and Morgan, 1990; Shaw and Lin, 1996). However, a thin thermal plate and hot thermal regime is inconsistent with observa- tions made at the actively forming corrugated surfaces of the MAR. These MAR surfaces are associated with high levels of seismicity (Smith et al., 2006, 2008), indicating that the axial brittle plate has a substantial thickness. In addition, earthquakes as deep as 8 km below the seaﬂoor have been detected at the Trans Atlantic Geotraverse (TAG) detachment, which does not expose clear corrugations, but appears to undergo a large roll-over (~50° ; (deMartin et al., 2007)). The alternative to a thin thermal plate, as proposed by (Buck, 1988), is that stresses related to bending of the detachment footwall cause internal deformation, forming a weaker domain in an otherwise thick mechanical lithosphere. Very large roll-over and subdued topography would then be consistent with maximum weakening of the footwall at detachments that form corrugated surfaces (Fig. 8a). Incidentaly, this very weak footwall domain could also be able to cast the irregularities of the hanging wall in the form of corrugations, as proposed by (Spencer, 1999). We now use available observations, from this and previous studies, to discuss what mechanisms could enhance the mechanical weakening of the 180 M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 Fig. 5. Maximum heave of termination scarps as a function of average residual topography (Table 1) for eastern Southwest Indian Ridge corrugated surfaces formed prior to magnetic anomaly 3A (Fig. 1). Same symbols as in Fig. 3. Dashed line is best linear ﬁt for corrugated surfaces of area >200 km2 (slope is 1.47, with a correlation coefﬁcient R2 of 0.52). Dashed arrows as in Fig. 2d. footwall of axial detachments during the formation of corrugated surfaces. 5.2. Enhanced mechanical weakening of the detachment footwall Our study of the eastern SWIR indicates that corrugated surfaces there do not form when the melt supply is so low that there is no, or very little axial volcanic activity (Cannat et al., 2006). In such very low melt supply conditions, the new seaﬂoor is not corrugated, but smooth, and structured in axis-parallel fault blocks (Fig. 4d) with substantial relief (>500 m) suggesting that the detachment's footwall had a somewhat larger rigidity than during the formation of corrugated surfaces. Although sampling of these smooth seaﬂoor domains is still scarce, the proportion of gabbros in each available dredge is less than 10%, with over 90% serpentinized ultramaﬁcs (Seyler et al., 2003). This observation, from the low melt supply end of the mid-ocean ridge spectrum, points to a link between the weakening mechanism we are trying to identify for detachments that do form corrugations, and a signiﬁcant volume of gabbros in the footwall of the detachment fault. Such a link is also indicated by sampling evidence for kilometer-sized gabbro bodies beneath MAR corrugated surfaces, next to outcrops of serpentinized peridotites (Ildefonse et al., 2007; Dick et al., 2008). Atlantis Bank at the SWIR, where 2 km of gabbro have been drilled (ODP site 735) next to exhumed mantle-derived ultramaﬁcs, may also have once had a corrugated upper surface, now eroded (Dick et al., 2000). Another observation is that, in 29 out of 39 eastern SWIR corrugated surfaces, the termination coincides with a transition to a volcanic mode of spreading (Table 1). In this mode, seaﬂoor with volcanic ediﬁces is accreted to the two diverging plates, and gravity suggests systematically thicker crust than in the other two main modes of spreading identiﬁed in this ridge region (corrugatedvolcanic and smooth-smooth ; (Cannat et al., 2006)). This is consistent with the observation made by (Tucholke et al., 1998, 2008) that gravity-derived crustal thickness appears to increase at the termination of many MAR corrugated surfaces. (Canales et al., 2008) recently conﬁrmed this with seismic data for a subset of corrugated surfaces near the Kane Fracture Zone. Moreover, the study of over 40 corrugated surfaces in the 13°N region of the Atlantic indicates that they also generally have a thinner gravity-derived crust than the surrounding volcanic seaﬂoor (Smith et al., 2008). Thus, despite the fact that gravity-derived crustal thickness models are non-unique and do not in most cases have the horizontal resolution to study small Fig. 6. Flowline-parallel bathymetric sections for selected Southwest Indian Ridge corrugated surfaces. Sections are located in Fig. 4. Arrows point toward ridge axis. Bold lines above topography : corrugated surfaces. Bold topography : termination scarps. Scale is the same in all sections, with a vertical exaggeration of 4.5. Values in meter unit refer to height of the principal scarps, values in degree unit refer to slopes. scale variations at and around corrugated surfaces, observations converge to indicate that most corrugated surfaces do terminate in a thicker crust and presumably higher melt supply regime. The weakening mechanism we are looking for is therefore probably not that melt-rich zones, or hot and ductile gabbros, deform easily (as demonstrated for example by the ductile shear zones drilled in gabbros at the ODP Site 735B (Cannat et al., 1991; Dick et al., 2000)) because it would then be difﬁcult to explain why this deformation should be less extensive at termination, when it occurs in conditions of higher melt supply. The third observation that we view as relevant concerns the prevalence of talc, chlorite, serpentine and amphibole-bearing ultramaﬁc schists in sheared horizons which have been sampled so far at, and below, corrugated surfaces (Escartin et al., 2003; Schroeder and John, 2004; Boschi et al., 2006; Dick et al., 2008; Boschi et al., 2008). Spinel relicts in these schists shows that their protolith is at least partially mantle-derived, while talc, amphibole and lesser chlorite reveal the addition of silica, calcium, and aluminium. This addition is thought to be controlled by hydrothermal ﬂuids derived through seawater–gabbro interactions (Escartin et al., 2003; Schroeder and John, 2004; Boschi et al., 2006; McCaig et al., 2007; Boschi et al., 2008). Talc, chlorite, serpentine and tremolite–actinolite schists M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 181 Fig. 7. Bathymetric map of the Mid-Atlantic Ridge in the 13°N area (from (Smith et al., 2006), with ﬂowline-parallel sections and location of Achadze hydrothermal vents. Sections are drawn at scale similar to that in Fig. 5. Values in meter unit refer to height of the principal scarps, values in degree unit refer to slopes. Sections 1 and 2 cut through the actively forming corrugated surfaces described by (Smith et al., 2006). Fault uplift varies along-strike, from virtually no topography in Section 1, to about 700 m in Section 2. Section 3 cuts through an older corrugated surface, now uplifted by a total of about 1800 m along the termination scarp. NVR : Neovolcanic Ridge. have also been described next to gabbro veins, in mid-ocean ridge ultramaﬁc outcrops located away from major faults (Cannat and Casey, 1995; Karson et al., 2006; Boschi et al., 2006, 2008; Dick et al., 2008). This spatial association indicates that the growth of these alteration products is directly related to the distribution of gabbroic intrusions in exhumed ultramaﬁcs. Fig. 8. Conceptual model showing the two contrasted axial conﬁgurations inferred from our study of corrugated surfaces. Across-axis sections. (a) A corrugated surface forms. A very weak domain (grey) develops in the detachment footwall. The roll-over is large, and the fault uplift is small. The geological interpretation we propose for this conﬁguration involves gabbros (darker grey) as 1 to 2 km-sized bodies, and as dikes and veins (dashed lines) intrusive into ultramaﬁc rocks. (b) Termination. Rock formations in the upper part of the footwall plate have become stronger, the roll-over is less, and the fault uplift is greater. The geology proposed for the footwall at this stage is not sketched. It could comprise either more, or less gabbros than in stage a (discussed in text). Seaﬂoor topography is drawn to scale in (a) from an active corrugated surface at the axis of the Mid Atlantic Ridge (Section 1 in Fig. 7), and in (b) from Southwest Indian Ridge (SWIR) corrugated surface #7 (Section a-1 in Fig. 5). Base of brittle lithosphere (inferred to correspond to 750 °C isotherm ; (Hirth et al., 1998)) on axis is drawn at about 8 km depth, from seismicity studies at the SWIR (Yamada et al., 2002) and Mid-Atlantic Ridge (Toomey et al., 1988; Wolfe et al., 1995; deMartin et al., 2007). The base of the brittle lithosphere off-axis, and of the ductile lithosphere, are inferred. We have used recent seismic constraints of (Canales et al., 2007), and (deMartin et al., 2007) on the TAG (Trans-Atlantic Geotraverse) region to sketch the detachment fault in (a). Our inferred very weak footwall domain is sketched to coincide with their upper domain of low seismicity. 182 M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 Talc is a very weak material, with a strength and coefﬁcient of friction which are lower than those of lizardite (Morrow et al., 2000; Escartin et al., 2008a). Chlorite displays mechanical properties that are intermediate between those of talc and serpentinite (Murrell and Ismail, 1976; Moore and Lockner, 2004). The mechanical properties of tremolite have not been properly studied experimentally, but the intense deformation of tremolite schists, as well as deformation microstructures similar to those of talc and serpentinite schists, also suggests a weak nature that promotes strain localization. Furthermore, fault rocks are commonly composed of a mixture and intergrowth of several of these minerals (e.g., (Escartin et al., 2003; Schroeder and John, 2004; Hirose and Hayman, 2008). Consequently, the overall rheology of such fault or alteration zones are likely to be controlled by the weakest mineral, even if present in small quantities. Our preferred mechanism for footwall weakening during the formation of corrugated surfaces is therefore the following: we propose that weakening of the detachment footwall is primarily achieved with the presence of sheeted minerals (mainly phyllosilicates) that result from alteration and ﬂuid circulation in mantlederived ultramaﬁcs next to gabbro intrusions (Fig. 8a). After they have cooled into the brittle regime, these gabbro intrusions are likely to be rheologically stronger than serpentinized ultramaﬁcs, and the contrast in rheology may help localize footwall deformation along lithological boundaries (Ildefonse et al., 2007), where the growth of weak hydrous minerals will be most favored. The domain in which this would occur would be controlled by access to hydrothermal ﬂuids. In Fig. 8a, we have tentativelly drawn it as 2 to 3 km-thick, and parallel to the main detachment. As sketched in Fig. 8a, this weak footwall domain could be equivalent to the low seismicity domain detected by (deMartin et al., 2007) in the footwall of the TAG detachment fault. We hypothesize that successive gabbro intrusions are progressively exhumed through this weak domain. Each intrusion is inferred to be surrounded by veined ultramaﬁcs, in which hydrothermal alteration will favor serpentinization, and the local crystallization of talc, amphibole and chlorite, which will in turn facilitate deformation. 5.3. Termination Termination of a corrugated surface probably follows a change of the mechanical behavior of the master detachment fault and may occur in two conﬁgurations, which have recently been explored by (Smith et al., 2008): 1) A new master fault initiates, which is most likely to cut inward into the hanging wall plate (Buck, 1988 ; Tucholke et al., 1998; Smith et al., 2008). This new master fault cuts into material which would for the most part not have been deformed and mechanically weaken by bending stresses yet and would thus initially have a higher rigidity. And 2) the detachment continues on the same master fault, but the roll over is less, and the main fault cuts at a steeper angle through the upper crust, capturing a small wedge of the hanging wall plate. This second conﬁguration is represented in Fig. 8b, in which we assume that this captured wedge of dominantly volcanic rocks is dismantled by mass wasting of the termination scarp. In this sketch we propose that the alteration of the mechanical behavior of the master detachment fault which leads to termination can occur: 1) if gabbroic intrusions in the footwall are so frequent that they form a coherent skeleton of strong brittle rocks ; and 2) if gabbroic intrusions are so rare that shear zones coated by talc, amphibole, chlorite and serpentine become too scarce and disconnected to effectively weaken the footwall. More generally, we would infer that the strength of the footwall at mid-ocean ridge detachments, the extent of roll over, and the fault topography, would be inﬂuenced by the abundance of ultramaﬁc rocks in the hydrothermally altered domain of the footwall, and by the size and distribution of gabbroic intrusions in these ultramaﬁc rocks. We would predict that corrugated surfaces would have the optimum proportion of gabbros to insure that most faults and fractures that accommodate footwall ﬂexure may be coated by weak assemblages of talc, chlorite, serpentine and amphibole. 6. Concluding remarks In this paper, we document the location, size, approximate age, gravity signature and residual topography of 39 corrugated surfaces in the melt-poor eastern region of the SWIR. We ﬁnd that these corrugated surfaces formed at a melt supply about half the global melt supply average for mid-ocean ridges (Chen, 1992). This is consistent with the model of (Buck et al., 2005), which was recently reﬁned by (Tucholke et al., 2008). In this model, which does not address asymmetric spreading during the formation of corrugated surfaces (Searle and Bralee, 2007; Baines et al., 2008; Dick et al., 2008), diking in the upper plate accommodates half of the plate separation, the other half being accommodated by displacement along the detachment fault. Our main focus in the discussion is to understand why axial detachment faults at slow and ultraslow ridges appear to accommodate very large footwall roll-over during the formation of corrugated surfaces, and why the roll-over appears to be less, in most cases, after these corrugated surfaces terminate. We conclude that the formation of corrugated surfaces requires enhanced weakening of the detachment footwall. The mechanism we propose as a way to achieve such enhanced weakening is the coating of cracks and faults zones associated with footwall ﬂexure with weak hydrous minerals (talc, amphibole, chlorite, and serpentine). This hypothesis, although consistent with available observations on rock samples from slow and ultraslow ridges, and with geophysical constraints on the melt supply to the ridge during the formation of corrugated surfaces, is not tested at this point, and should be viewed as a working hypothesis for future investigations. Acknowledgments We warmly thank our three reviewers for their extremely helpfull comments. Most data we used are from the «SWIR 61–63» cruise of RV Marion Dufresne (2003), and processing was partly supported by a research grant from CNRS-INSU. This is IPGP publication number 2545. References Baines, A., Cheadle, M., John, B., Schwartz, J., 2008. The rate of oceanic detachment faulting at Atlantis Bank, SW Indian Ridge. Earth Planet. Sci. Lett. 273, 105–114. Blackman, D.K., Cann, J.R., Janssen, B., Smith, D.K., 1998. Origin of extensional core complexes: evidence from the Mid-Atlantic Ridge at Atlantis Fracture Zone. J. Geophys. Res. 103, 21315–21333. Boschi, C., Fruh-Green, G.L., Escartin, J., 2006. Occurrence and signiﬁcance of serpentinite-hosted, talc- and amphibole-rich fault rocks in modern oceanic settings and ophiolite complexes: an overview. Oﬁoliti 31, 129–140. Boschi, C., Dini, A., Fruh-Green, G.L., Kelley, D.S., 2008. Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30 degrees N): insights from B and Sr isotope data. Geochim. Cosmochim. Acta 72, 1801–1823. Buck, W.R., 1988. Flexural rotation of normal faults. Tectonics 7, 959–973. Buck, W.R., Lavier, L., Poliakov, A., 2005. Modes of faulting at mid-ocean ridges. Nature 434, 719–723. Canales, J.P., Tucholke, B.E., Collins, J.A., 2004. Seismic reﬂection imaging of an oceanic detachment fault: Atlantis megamullion (Mid-Atlantic Ridge, 30 degrees 10 ′ N):. Earth Planet. Sci. Lett. 222, 543–560. Canales, J.P., Sohn, R.A., Demartin, B.J., 2007. Crustal structure of the Trans-Atlantic Geotraverse (TAG) segment (Mid-Atlantic Ridge, 26 degrees 10-Minutes-N): Implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges. Geochem. Geophys. Geosyst. 8. Canales, J.P., Tucholke, B.E., Xu, M., Collins, J.A., DuBois, D.L., 2008. Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes. Geochem. Geophys. Geosyst. 9. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and the Cenozoic. J. Geophys. Res. 100, 6093–6095. Cann, J.R., Blackman, D.K., Smith, D.K., McAllister, E., Janssen, B., Mello, S., Avgerinos, E., Pascoe, A.R., Escartín, J., 1997. Corrugated slip surfaces formed at North Atlantic ridge-transform intersections:. Nature 385, 329–332. Cannat, M., Casey, J.F., 1995. An ultramaﬁc lift at the Mid-Atlantic Ridge: successive stages of magmatism in serpentinized peridotites from the 15°N region. In: Vissers, M. Cannat et al. / Earth and Planetary Science Letters 288 (2009) 174–183 R.L.M., Nicolas, A. (Eds.), Mantle and lower crust exposed in oceanic ridges and in ophiolites. Kluwer Academic publishers, Netherlands, pp. 5–34. Cannat, M., Mevel, C., Stakes, D., 1991. Stretching of the deep crust at the slowspreading Southwest Indian Ridge:. Tectonophysics 190, 73–94. Cannat, M., Mével, C., Maia, M., Deplus, C., Durand, C., Gente, P., Agrinier, P., Belarouchi, A., Dubuisson, G., Humler, E., Reynolds, J., 1995. Thin crust, ultramaﬁc exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22–24 N):. Geology 23, 49–52. Cannat, M., Rommevaux-Jestin, C., Fujimoto, H., 2003. Melt supply variations to a magma-poor ultra-slow spreading ridge (Southwest Indian Ridge 61° to 69°E). Geochem. Geophys. Geosyst. 4 2002GC000480. Cannat, M., Sauter, D., Mendel, V., Ruellan, E., Okino, K., Escartin, J., Combier, V., Baala, M., 2006. Modes of seaﬂoor generation at a melt-poor ultra-slow-spreading ridge. Geology 34, 605–608. Chen, Y.J., 1992. Oceanic crustal thickness versus spreading rate. Geophys. Res. Lett. 19, 753–756. Chen, Y., Morgan, W.J., 1990. A nonlinear rheology model for mid-ocean ridge axis topography:. J. Geophys. Res. 95, 17583–17604. deMartin, B.J., Sohn, R.A., Canales, J.P., Humphris, S., 2007. Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal ﬁeld on the Mid-Atlantic Ridge:. Geology 35, 711–714. Dick, H.J.B., Natland, J.H., Alt, J.C., Bach, W., Bideau, D., Gee, J.S., Haggas, S., Hertogen, J.G.H., Hirth, G., Holm, P.M., Ildefonse, B., Iturrino, G.J., John, B.E., Kelley, D.S., Kikawa, E., Kindom, A., LeRoux, P.J., Maeda, J., Meyer, P.S., Miller, D.J., Naslund, H.R., Niu, Y.-L., Robinson, P.T., Snow, J., Stephen, R.A., Trimby, P.W., Worm, H.-U., Yoshinobu, A.S., 2000. A long in situ section of the lower oceanic crust: results of ODP leg 176 drilling at the Southwest Indian Ridge. Earth Planet. Sci. Lett. 179, 31–51. Dick, H.J.B., Tivey, M.A., Tucholke, B.E., 2008. Plutonic foundation of a slow-spreading ridge segment: oceanic core complex at Kane Megamullion, 23 degrees 30′ N, 45 degrees 20′ W. Geochem. Geophys. Geosyst. 9. Escartín, J., Cannat, M., 1999. Ultramaﬁc exposures and the gravity signature of the lithosphere near the Fifteen-Twenty Fracture Zone (Mid-Atlantic Ridge, 14°– 16.5°N). Earth Planet. Sci. Lett. 171, 411–424. Escartin, J., Mével, C., MacLeod, C.J., McCaig, A.M., 2003. Constraints on deformation conditions and the origin of oceanic detachments: the Mid-Atlantic Ridge core complex at 15°45′N. Geochem. Geophys. Geosyst. 4 2002GC000472. Escartin, J., Andreani, M., Hirth, G., Evans, B., 2008a. Relationships between the microstructural evolution and the rheology of talc at elevated pressures and temperatures. Earth Planet. Sci. Lett. 268, 463–475. Escartin, J., Smith, D.K., Cann, J., Schouten, H., Langmuir, C.H., Escrig, S., 2008b. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere. Nature 455, 790-U5. Hirose, T., Hayman, N.W., 2008. Structure, permeability, and strength of a fault zone in the footwall of an oceanic core complex, the Central Dome of the Atlantis Massif, Mid-Atlantic Ridge, 30 degrees N. J. Struct. Geol. 30, 1060–1071. Hirth, G., Escartín, J., Lin, J., 1998. The rheology of the lower oceanic crust: implications for lithospheric deformation at Mid-Ocean ridges. In: Buck, W.R., Delaney, P.T., Karson, J.A., Lagabrielle, Y. (Eds.), Faulting and magmatism at mid-ocean ridges. : Geophysical Monograph Series, vol. 106. AGU, Washington D. C, pp. 291–303. Ildefonse, B., Blackman, D.K., John, B.E., Ohara, Y., Miller, D.J., MacLeod, C.J., 2007. Oceanic core complexes and crustal accretion at slow-spreading ridges:. Geology 35, 623–626. Karson, J.A., Fruh-Green, G.L., Kelley, D.S., Williams, E.A., Yoerger, D.R., Jakuba, M., 2006. Detachment shear zone of the Atlantis Massif core complex, Mid-Atlantic Ridge, 30 degrees N. Geochem. Geophys. Geosyst. 7. Lavier, L., Buck, W.R., Poliakov, A.N.B., 1999. Self-consistent rolling-hinge model for the evolution of large-offset low-angle normal faults:. Geology 27, 1127–1130. MacLeod, C.J., Escartín, J., Banerji, D., Banks, G.J., Gleeson, M., Irving, D.H.B., Lilly, R.M., McCaig, A., Niu, Y.-L., Allerton, S., Smith, D.K., 2002. Direct geological evidence for oceanic detachment fauling: the Mid-Atlantic Ridge, 15 45′N. Geology 30, 279–282. 183 McCaig, A.M., Cliff, R.A., Escartin, J., Fallick, A.E., MacLeod, C.J., 2007. Oceanic detachment faults focus very large volumes of black smoker ﬂuids. Geology 35, 935–938. Minshull, T.A., Muller, M.R., White, R.S., 2006. Crustal structure of the Southwest Indian Ridge at 66 degrees E: seismic constraints. Geophys. J. Int. 166, 135–147. Moore, D.E., Lockner, D.A., 2004. Crystallographic controls on the frictional behavior of dry and water-saturated sheet structure minerals. J. Geophys. Res.-Solid Earth 109. Morrow, C.A., Moore, D.E., Lockner, D.A., 2000. The effect of mineral bond strength and adsorbed water on fault gouge frictional strength. Geophys. Res. Lett. 27, 815–818. Muller, M.R., Minshull, T.A., White, R.S., 1999. Segmentation and melt supply at the Southwest Indian Ridge. Geology 27, 867–870. Murrell, S.A.F., Ismail, I.A.H., 1976. The effect of decomposition of hydrous minerals on the mechanical properties of rocks at high pressures and temperatures. Tectonophysics 31, 207–258. Patriat, P., Sloan, H., Sauter, D., 2008. From slow to ultraslow: a previously undetected event at the Southwest Indian Ridge at ca. 24 Ma. Geology 36, 207–210. Sauter, D., Cannat, M., Mendel, V., 2008. Magnetization of 0–26.5 Ma seaﬂoor at the ultraslow spreading Southwest Indian Ridge, 61 degrees–67 degrees E. Geochem. Geophys. Geosyst. 9. Schroeder, T., John, B.E., 2004. Strain localization on an oceanic detachment fault system, Atlantis Massif, 30 degrees N, Mid-Atlantic Ridge. Geochem. Geophys. Geosyst. 5. Searle, R.C., Bralee, A.V., 2007. Asymmetric generation of oceanic crust at the ultra-slow spreading Southwest Indian Ridge, 64 degrees E. Geochem. Geophys. Geosyst. 8. Searle, R., Cannat, M., Fujioka, K., Mével, C., Fujimoto, H., Bralee, A., Parson, L., 2003. Fuji Dome: a large detachment fault near 64°E on the very slow-spreading southwest Indian Ridge. Geochem. Geophys. Geosyst. 4 2003GC000519. Seyler, M., Cannat, M., Mével, C., 2003. Evidence for major element heterogeneity in the mantle source of abyssal peridotites from the southwest Indian Ridge (52 to 68° East). Geochem. Geophys. Geosyst. 4 2002GC000305. Shaw, W.J., Lin, J., 1996. Models of ocean ridge lithospheric deformation: dependence on crustal thickness, spreading rate, and segmentation. J. Geophys. Res. 101, 17977–17993. Smith, D.K., Cann, J.R., Escartin, J., 2006. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge. Nature 442, 440–443. Smith, D.K., Escartin, J., Schouten, H., Cann, J.R., 2008. Fault rotation and core complex formation: Signiﬁcant processes in seaﬂoor formation at slow-spreading midocean ridges (Mid-Atlantic Ridge, 13 degrees–15 degrees N). Geochem. Geophys. Geosyst. 9. Spencer, J.E., 1999. Geologic continuous casting below continental and deep-sea detachment faults and the striated extrusion of Sacsayhuamán, Perú. Geology 27, 327–330. Toomey, D.R., Purdy, G.M., Solomon, S.C., 1988. Microearthquakes beneath the median valley of the Mid-Atlantic ridge near 23°N: tomography and tectonics. J. Geophys. Res. 93, 9093–9112. Tucholke, B.E., Lin, J., Kleinrock, M.C., Tivey, M., Reed, T.B., Goff, J., Jaroslow, G.E., 1997. Segmentation and crustal structure of the western Mid-Atlantic Ridge ﬂank, 25°25′–27°10′N and 0–29 m.y. J. Geophys. Res. 102, 10203–10223. Tucholke, B.E., Lin, J., Kleinrock, M.C., 1998. Megamullions and mullion structure deﬁning oceanic metamorphic core complexes on the Mid-Atlantic Ridge:. J. Geophys. Res. 103, 9857–9866. Tucholke, B., Behn, M.D., Buck, R., Lin, J., 2008. Role of melt supply in oceanic detachment faulting and formation of megamullions. Geology 36, 455–458. Wolfe, C., Purdy, G.M., Toomey, D.R., Solomon, S.C., 1995. Microearthquake characteristics and crustal velocity structure at 29 N of the Mid-Atlantic Ridge: the architecture of a slow-spreading segment. J. Geophys. Res. 100, 24449–24472. Yamada, T., Tamaki, K., Fujimoto, H., Kanazawa, T., 2002. Microearthquakes characteristics at the Jourdanes segment: seismic experiment at 64°E (SWIR). (abstract), InterRidge South West Indian Ridge Workshop: Southampton, U.K.