RESEARCH ARTICLE Distinctive Seafloor Fabric Produced Near Western Versus 10.1029/2018GC008101 Eastern Ridge‐Transform Intersections of the Northern Key Points: ‐ fl • Distinctive seafloor fabrics are Mid Atlantic Ridge: Possible In uence of produced at western versus eastern ridge‐transform intersections of the Ridge Migration ‐ northern Mid Atlantic Ridge Marie‐Helene Cormier1 and Heather Sloan2 • This morphological asymmetry may be a consequence of the westward 1Graduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA, 2Department of Earth, migration of the Mid‐Atlantic Ridge • High migration rates and slow Environmental and Geospatial Sciences, Lehman College, City University of New York, New York, NY, USA spreading rates in the northern Atlantic may contribute to the common occurrence of oceanic core Abstract Multibeam bathymetry compiled along fracture zones of the northern Atlantic reveals a complexes striking morphological asymmetry. Seafloor fabric produced at western ridge‐transform intersections (RTIs) tends to consist of linear ridges, small in amplitude, and regularly spaced, and oceanic core complexes (OCCs) occur infrequently. In contrast, seafloor fabric produced at eastern RTIs is more irregular and Correspondence to: blocky and displays characteristics usually associated with melt‐poor accretion: These include more than ‐ M. H. Cormier, double the occurrence of OCCs as well as greater seafloor depths, even where seafloor is younger than that [email protected] on the opposite side of the fracture zone. We propose that this asymmetry is a consequence of the westward migration of the Mid‐Atlantic Ridge. Such migration is expected to result in an enhanced melt supply at Citation: leading (western) RTIs compared to trailing (eastern) RTIs. The morphological asymmetry is observed for Cormier, M.‐H., & Sloan, H. (2019). Distinctive seafloor fabric produced ridge offsets of ~40 to ~200 km, a range that may be related to the width of melting regimes that supply ridge near western versus eastern segments. At slow spreading ridges, contrasting melt supplies across fracture zones may therefore be best ‐ ridge transform intersections of the expressed in the distinct seafloor fabrics preserved beyond the dynamically maintained relief of the axial rift northern Mid‐Atlantic Ridge: Possible influence of ridge migration. valley and walls rather than in the contrasting axial depths documented for faster spreading ridges. Geochemistry, Geophysics, Geosystems, Although OCCs appear to form preferentially at spreading rates lower than 30 mm/year, their common 20. https://doi.org/10.1029/ occurrence along the northern Mid‐Atlantic Ridge may also reflect the significantly higher rates at which it 2018GC008101 is migrating compared to the southern Mid‐Atlantic Ridge. Received 20 NOV 2018 The young seafloor fabric consists of elongate abyssal hills aligned Accepted 22 FEB 2019 Plain Language Summary Accepted article online 27 FEB 2019 parallel to spreading centers. Bathymetry compiled along fracture zones of the northern Atlantic reveals a subtle morphological asymmetry in that pattern. Seafloor fabric produced at western ridge‐transform intersections (RTIs) consists of small, linear, and regularly spaced ridges. In contrast, fabric produced at eastern RTIs is irregular and blocky and lies generally deeper than the opposite side of the fracture zone. It also displays over twice as many oceanic core complexes, prominent highs resulting from detachment faulting. We propose that this asymmetry results from the westward migration of the Mid‐Atlantic Ridge. Such migration is expected to produce an enhanced melt supply at leading (western) RTIs compared to trailing (eastern) RTIs. At slow spreading ridges, contrasting melt supplies across fracture zones may therefore be expressed as distinctive seafloor fabrics rather than in axial depth variations, as has been documented for faster spreading ridges. Although oceanic core complexes appear to form preferentially at spreading rates lower than 30 mm/year, their common occurrence along the northern Mid‐Atlantic Ridge may also reflect the significantly higher rates at which it is migrating compared to the southern Mid‐ Atlantic Ridge.

1. Introduction Because the absolute motions of two diverging plates are rarely equal and opposite, most mid‐ocean ridges are migrating relative to the stable deep mantle (Stein et al., 1977). Various studies have investigated how ridge migration may affect mid‐ocean ridge processes. Davis and Karsten (1986) proposed that ridge migra- tion could account for the asymmetric distribution of seamounts about the Juan de Fuca Ridge. Small and Danyushevsky (2003) proposed that ridge migration accounts for the prominent discontinuity in both axial fi fi ‐ ©2019. American Geophysical Union. depth and geochemistry that exists between the East Paci c Rise and the Paci c Antarctic Ridge. Doglioni All Rights Reserved. et al. (2003) suggested that ridge migration could explain why the eastern flanks of mid‐ocean ridges tend

CORMIER AND SLOAN 1 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

to be shallower than their western flanks, and Chalot‐Prat et al. (2017) further proposed that ridge migration leads to asymmetric upper mantle differentiation. Carbotte et al. (2004) and Katz et al. (2004) proposed that another consequence of ridge migration is a con- trasting melt supply at the extremities of two ridge segments offset by a transform fault or a nontransform discontinuity (NTD) and, consequently, predictably differing axial morphologies on opposing sides of trans- form faults and NTDs. Specifically, kinematic models of passive mantle upwelling (Davis & Karsten, 1986; Schouten et al., 1987; Katz et al., 2004) predict rates of upwelling, and thus rates of melt supply, that are higher beneath the plate that moves faster over the stable deep mantle (the leading plate) compared to the slower‐moving plate (the trailing plate; Figure 1). Such kinematic models imply that differing amounts of melt should be supplied to the extremities of ridge segments offset by a transform fault or an NTD: Melt is thought to be tapped not only from the underlying upwelling zone but also from the overlapping upwelling zone of the adjacent segment (Figure 2). Therefore, faster upwelling rates beneath the leading plate should result in more melt being tapped at segment ends that abut that leading plate compared to segment ends that abut the trailing plate. In other words, the extremities of leading segments are expected to display more melt‐ rich characteristics compared to the extremities of trailing segments. Observed axial depth variations along fast spreading and intermediate spreading ridges provide solid evi- dence for more melt‐rich accretion at the extremities of leading ridge segments (Carbotte et al., 2004). At these spreading rates, the axial region is characterized by an axial high, with shallower and broader ridge segments thought to reflect enhanced melt supply while deeper and narrower segments are thought to be more melt‐starved (e.g., Langmuir et al., 1986; Macdonald et al., 1988; Scheirer & Macdonald, 1993). Accordingly, in the vicinity of transform faults and NTDs of the East Pacific Rise, Juan de Fuca Ridge, and Southeast Indian Ridge, leading ridge segments are shallower than trailing segments in about 75% of cases (Carbotte et al., 2004). However, convincing evidence for similarly asymmetric axial depths across transform faults and NTDs at slow spreading ridges is lacking. Supak et al. (2007) estimated that leading segments are shallower across only 60% of discontinuities of the Mid‐Atlantic Ridge and sug- gested that this poor correlation may reflect a more limited entrainment of melt from across slow spread- ing discontinuities due to the predominance of three‐dimensional upwelling and melt focusing to segment centers. In the following sections, we document a previously unreported asymmetry in the seafloor fabric on oppos- ing sides of fracture zones of the northern Mid‐Atlantic Ridge, as well as generally greater depths on the sides that were accreted at trailing segment extremities. We interpret these observations to reflect a contrasting melt supply at leading versus trailing segment extremities. We then propose that at slow spreading rates, the seafloor fabric preserved off‐axis near ridge‐transform intersections (RTIs) and along fracture zones is a better proxy for the time‐averaged melt supply than is axial depth.

2. The Westward Migration of the Mid‐Atlantic Ridge A number of models exist for present‐day absolute plate motion (e.g., Becker et al., 2015; Doglioni et al., 2015; Doubrouvine et al., 2012; Morgan & Phipps Morgan, 2007; Wang et al., 2018) and the precise directions in which ridges are migrating are therefore open to debate (see Figure 3). Nonetheless, the majority of abso- lute plate motion models agree about which of any two diverging plates is the faster moving plate, and thus about the general direction of migration of a mid‐ocean ridge with respect to the deeper mantle. In the case of the slow spreading northern Mid‐Atlantic Ridge, the North American and South American plates are leading the W to SW migrating spreading center, and the Eurasian and Nubian plates are trailing it (Figure 3). Accordingly, we will refer to ridge segments at western RTIs as leading segments, western RTIs as leading RTIs, and likewise, we will refer to ridge segments at eastern RTIs as trailing segments and eastern RTIs as trailing RTIs (Figure 1). Furthermore, we will refer to the side of a fracture zone that was accreted at a trailing RTI as the trailing side, and the side of a fracture zone that was accreted at a leading RTI as the leading side. We note that regardless of the sense of offset of a transform fault (right stepping or left stepping), the trailing side corresponds to the older side of the fracture zone on the west flank of the Mid‐ Atlantic Ridge and to the younger side of the fracture zone on the east flank, and conversely for the leading side (Figure 1).

CORMIER AND SLOAN 2 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 1. (Top) Schematic map‐view representation of a mid‐ocean ridge (double solid lines), active transform faults (sin- gle solid lines), and their aseismic fracture zones (dashed line). Thin dashed arrows mark the trajectories followed by the migrating ridge. Here the plate on the left is leading the migrating ridge, and the plate on the right is trailing it. Accordingly, ridge segments displaced to the left by the transform faults are leading segments, and those displaced to the right are trailing segments. (Bottom) Schematic cross section of the migrating ridge. The thick gray layer represents the upper diverging plates. Vertical arrows illustrate the asymmetric rates of mantle upwelling beneath the migrating ridge axis. RTI = ridge‐transform intersection.

Figure 2. Three‐dimensional schematic representation of melt trajectories near a transform fault at slow spreading rate, after Bai and Montési (2015). The blue‐white‐red step pattern at top marks the ridge‐transform‐ridge plate boundary within the upper plate; black arrows indicate spreading directions. The gray surface below represents the approximate base of the lithosphere, often presumed to be a melt permeability barrier. Thin gray lines represent depth contours. The dashed line indicates the projection onto that surface of the transform fault and its fracture zone extension. Curved arrows represent trajectories that the buoyant melt follows along the underside of the permeability barrier on its upward migration toward the ridge axis. Blue (red) arrows correspond to melt trajectories issued from the melting regime asso- ciated with the ridge segment displaced to the left (right) of the figure. Note how melt extracted from the melting regime associated with one ridge segment may be diverted to the adjacent ridge segment in the vicinity of ridge‐transform intersections.

CORMIER AND SLOAN 3 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 3. Tectonic context of the northern Mid‐Atlantic Ridge. Plate boundaries are shown as red lines (after Bird, 2003). The inset locates the displayed area relative to adjacent continents. Regions shaded in orange are shallower than 1,500 m and highlight current and former hot spots. Fine black lines mark small circles, followed by the westward‐ to south- westward‐migrating Mid‐Atlantic Ridge based on the spreading‐aligned reference frame of Becker et al. (2015). Migration rates are labeled along each small circle, and black arrows show the direction of ridge migration. Fine dotted lines indicate small circles followed by the migrating Mid‐Atlantic Ridge based on MM07‐M, an alternate hot spot reference frame from Becker et al. (2015). Local spreading rates from the MORVEL56 relative plate motion model (Argus et al., 2011) are indicated in parenthesis next to the named transform faults.

For discussion purposes, we further adopt the “spreading aligned” absolute plate motion model of Becker et al. (2015), a model that optimizes the alignment of spreading directions with that of absolute plate motions. Unlike other models, the spreading aligned model closely fits several of the parameters commonly used to constrain absolute plate motions. In particular, it provides a good match to global hot spot tracks as well as to the azimuthal seismic anisotropy thought to result from shearing of the asthenosphere underneath lithospheric plates.

CORMIER AND SLOAN 4 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 4. Key morphologic features of North Atlantic transform faults and their aseismic extension. (Top) Multibeam bathymetric map of the Atlantis transform fault (after Zervas et al., 1995; Pariso et al., 1995; Cann et al., 1997; Blackman et al., 1998). (Bottom) Interpretative sketch. The red line marks the plate boundary. Thin black lines highlight the trend of the abyssal hills. IC = inside corner. OC = outside corner. Shaded areas are color coded as follows: Blue, flat‐ bottom sediment ponds; orange, oceanic core complexes.

3. Previously Unreported Asymmetries in Seafloor Fabric and in Seafloor Depths across Fracture Zones of the North Central Atlantic Few oceanic fracture zones have been imaged with high‐resolution multibeam bathymetric sonars beyond their active transform domains. The northern Mid‐Atlantic Ridge (Figure 3) is an exception: The majority of large fracture zones located between 10°N to 35°N have been surveyed over distances of tens to hun- dreds of kilometers from the ridge axis (e.g., Blackman et al., 1998; Cann et al., 1997; Escartín & Cannat, 1999; Fujiwara et al., 2003; Fujiwara & Fujimoto, 1998; Gente et al., 1995; Kastens et al., 1998; Pockalny et al., 1988; Rabain et al., 2001; Smith et al., 2008; Zervas et al., 1995). These existing bathymetric data document a previously unrecognized asymmetry in the seafloor fabric across fracture zones, including the asymmetric distribution of oceanic core complexes (OCCs), a feature that sometimes develops at RTIs at slow spreading rates. They also reveal that differences in seafloor depths across north Atlantic fracture zones are not as predicted by classic models of differential subsidence (e.g., Sandwell & Schubert, 1982; Wessel & Haxby, 1990). Prominent topographic highs typically occur at the inside corner of RTIs (Figure 4), that corner delineated by the ridge segment and the active transform fault (Severinghaus & Macdonald, 1988). Sometimes, inside corner highs correspond to OCCs, long‐lived detachment faults that can accommodate as much as 60% of the divergent plate motion during episodes of diminished melt supply (e.g., Cann et al., 1997; Escartín et al., 2008; Smith et al., 2008; Tucholke et al., 1998; Tucholke et al., 2008). These detachment faults produce vertical relief that may exceed a thousand meters, and their slip surfaces may be marked by subtle ridge‐ perpendicular striations with a wavelength of hundreds of meters and vertical amplitude of tens of meters (e.g., Figures 5, 6c, and 6e). As the young lithosphere is transported beyond the rift mountains, OCCs become embedded in the seafloor fabric and are thus preserved, for the most part, on the older side of the fracture zone. Of the 34 OCCs reported along Atlantic fracture zones and appearing in Table 1, 68% occur on the wes- tern flank of the Mid‐Atlantic Ridge, a trend also apparent in Figures 5 and 6. A few OCCs occur in seafloor generated at the outer corner of the Fifteen‐Twenty fracture zone (the corner delimited by the ridge segment and the inactive fracture zone), and both segments adjacent to that transform fault also display OCCs along their entire length (Fujiwara et al., 2003; MacLeod et al., 2009; Smith et al., 2008). These anomalous occur- rences are thought to reflect the more limited magma supply associated with the presence in that area of the diffuse triple junction between the North America, South America, and Nubia plates (e.g., Escartín et al.,

CORMIER AND SLOAN 5 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 5. (a) Seafloor fabric produced across the oceanographer transform fault. The middle panel displays the multi- beam bathymetry published in Rabain et al. (2001). The thick black lines mark the ridge axis, and the dashed black line marks the transform fault and its aseismic fracture zone extension. Blue and red lines locate the depth profiles extracted 3 km South and North from the fault trace and displayed in matching colors below and above the map. Arrows pointing to the axial valley indicate whether the segment is leading or trailing. Fine black lines represent small circles followed by the westward migrating Mid‐Atlantic Ridge, based on the spreading‐aligned reference frame of Becker et al. (2015). Thick magenta lines correspond to magnetic chron 4A (Merkouriev & DeMets, 2014). That chron, dated at ~9.1 Ma (Hilgen et al., 2012), approximately corresponds to the onset of a 25% decrease in spreading rates and a 3–8° clockwise change in spreading direction (Gente et al., 1995; Sempéré et al., 1995; Sloan & Patriat, 1992). Black circles mark the locations of oceanic core complexes (see Table 1). Boxes outline the areas enlarged in Figures 6a and 6b. For direct comparison, the profile corresponding to the trailing side is overlain as a thin red line on the profile corresponding to the leading side (blue), and seafloor ages are labeled in mega annum with like colors along the horizontal axis (after Merkouriev & DeMets, 2014). Black dots above the depth profiles mark peaks with local relief >100 m, and the number of such peaks (N) is noted above each profile. Arrows indicate where the profiles intersect the ridge axis. (b) The middle panel displays the multibeam bathymetry of the Atlantis transform. The bathymetric compilation is mostly derived from data published in Zervas et al. (1995), Pariso et al. (1995), Cann et al. (1997), and Blackman et al. (1998); it is comple- mented with other data available from the National Geophysical Data Center. Thick magenta lines correspond to mag- netic chron 4A (9.1 Ma), after Sempéré et al. (1995). Other details as described in Figure 5a. (c) The middle panel displays the multibeam bathymetry of the Kane transform. The bathymetric compilation is derived from data published in Pockalny et al. (1988), Gente et al. (1995), and Fujimoto et al. (1996); it is complemented with a few swaths available from the National Geophysical Data Center. The data from Fujimoto et al. (1996) mostly cover the area west of 46°W and are available as a grid with coarser resolution than the other data. Depth profiles extracted from that coarser bathymetric grid west of ~46°W are displayed as thin red and blue lines. Other details as described in Figure 5a. (d) The middle panel displays the multibeam bathymetry of a southward‐propagating offset, after Gente et al. (1995). This ~40 km transform located south of Kane was stable until about ~6 Ma. The V‐shaped discordant zone that resulted from its southward propagation and that of an older ridge propagation event at SE corner are labeled “DZ.” Thick magenta lines correspond to magnetic chron 4A (9.1 Ma), after Gente et al. (1995). Other details as described in Figure 5a. (e) The middle panel displays the multibeam bathymetry of the Marathon transform fault, after Smith et al. (2008). Other details as in Figure 5a, except for this area being close to the magnetic equator, magnetic anomalies cannot be reliably identified. Instead, seafloor ages labeled in mega annum above the depth profiles in corresponding colors are derived from relative plate motion model MORVEL56 (Argus et al., 2011). Ages are likely overestimated for seafloor older than ~9 Ma, when spreading rates were ~50% faster (Sloan & Patriat, 1992).

CORMIER AND SLOAN 6 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

c

Figure 5. (continued)

2003; Müller & Smith, 1993; Smith et al., 2008). When the 10 OCCs along the Fifteen‐Twenty transform fault (a triple junction), as well as three OCCs that formed at short (<25 km) intratransform segments of the St Paul transform and Ascencion transform are excluded from the compilation, 71% of OCCs identified along Atlantic transform faults occur on the western flank. Because OCCs tend to form preferentially at the inside corners of RTIs, their more frequent occurrence (more than double) on the western flank of the Mid‐Atlantic

CORMIER AND SLOAN 7 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

e

Figure 5. (continued)

Ridge, regardless of the sense of offset of the transform fault at which they formed, indicates that OCCs form preferentially at eastern RTIs, that is, at trailing ridge segments. Figure 6 highlights how the physical trace of north Atlantic fracture zones generally corresponds to a sur- prisingly narrow zone about 1 km wide. For the most part, these aseismic extensions of the plate

CORMIER AND SLOAN 8 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 6. Bathymetric details of five northern Atlantic fracture zones. Maps (a) through (h) are located in Figures 5a–5e. Thin black or white lines are bathymetric contours selected to highlight flat‐bottomed sediment ponds, although no single depth contour does a good job of accurately highlighting every such pond. The side of a fracture zone that was accreted at leading or trailing ridge‐transform intersection is labeled accordingly at the map edges, with their relative ages in parenthesis. Black circles locate known oceanic core complexes (see Table 1.). Red lines mark the ridge axis. Note how oceanic core complexes and sediment ponds dominantly (but not exclusively) occur on the sides accreted at trailing ridge segments (see text). Data sources as described in the captions of Figures 5a–5e.

boundary coincide with the sharp juxtaposition of closely spaced regular ridges on the leading side, against flat‐bottom sediment ponds interspersed with ridges, and occasionally OCCs, on the trailing side. In map view, the linear ridges that abut the fracture zone on the leading side extend parallel to the ridge axis for tens of kilometers and are spaced a few kilometers apart. Their local relief may be as low as 50 m, and they are therefore much smaller features than the longer wavelength and higher relief (10‐ to 30‐km‐wide and up to 1‐km‐high) abyssal hills that typically flank slow spreading ridges (e.g., Cormier & Sloan, 2017). In contrast, the seafloor fabric on the trailing side tends to be blockier and irregular, whether OCCs are present or absent. There, flat‐bottom sediment ponds are interspersed between blocky abyssal hills, producing a distinctive crenulated pattern along the trailing side. While some of the ridges extend to and abut the fracture zones, others terminate up to 10–20 km away from the fracture zones. In a few places, smaller ridges terminate progressively, seemingly diving into the flat‐bottom sediment ponds (e.g., Figure 6g). However, many display blunt extremities with relief in excess of 500 m (Figures 6c, 6d, 6f, and 6h), indicating that the irregular pattern does not merely reflect some higher sediment accumulation near

CORMIER AND SLOAN 9 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Table 1 Thirty‐Four Core Complexes Identified Along Atlantic Fracture Zones Offset Lat. Long. Ridge Leading/ Transform (km) (°N) (°W) flank Comments trailing IC/OC References

Oceanographer 120 35.100 35.917 E Near Azores hotspot Leading IC Rabain et al. (2001) Oceanographer 120 35.133 35.033 W Near Azores hotspot Trailing IC Lagabrielle et al. (1998) Hayes 80 33.600 38.067 E Short (15 km) Trailing Short segment Escartín et al. (2008) segment N of Hayes. Atlantis 70 30.283 42.970 W WOCC Trailing IC Cann et al. (1997); Tucholke et al. (1998) Atlantis 70 30.167 42.533 W Keystone Trailing IC Blackman et al. (2008) Atlantis 70 30.133 42.117 W Atlantis Massif, RTI Trailing IC Blackman et al. (2008); Cann et al. (1997); Smith et al. (2006); Tucholke et al. (1998) Atlantis 70 29.917 42.500 E SOCC Leading IC Blackman et al. (2008); Cann et al. (1997); Smith et al. (2006); Tucholke et al. (1998) Kane 150 23.683 45.750 W Trailing IC Cann et al. (2015) Kane 150 23.667 46.117 W Trailing IC Cann et al. (2015) Kane 150 23.667 46.933 W Trailing IC Cann et al. (2015) Kane 150 23.617 45.550 W Trailing IC Cann et al. (2015); Tucholke et al. (1998) Kane 150 23.583 45.260 W Babel Dome Trailing IC Cann et al. (2015); Dick et al. (2008); Tucholke et al. (1998) Kane 150 23.517 45.050 W RTI Trailing IC Cann et al. (2015) Relict fracture zone 40 22.117 46.267 W Trailing IC Cann et al. (2015) south of Kane Relict fracture zone 40 22.017 45.813 W Trailing IC Cann et al. (2015) south of Kane Relict fracture zone 40 22.050 46.408 W Trailing IC Cann et al. (2015) south of Kane Fifteen‐Twenty 190 15.500 46.500 E RTI; triple junction Leading IC Escartín et al. (2008) Fifteen‐Twenty 190 15.125 45.479 W Triple junction Trailing IC Smith et al. (2008) Fifteen‐Twenty 190 15.092 45.358 W Triple junction Trailing IC Smith et al. (2008) Fifteen‐Twenty 190 15.113 45.265 W Triple junction Trailing IC Smith et al. (2008) Fifteen‐Twenty 190 15.116 45.112 W Triple junction Trailing IC Smith et al. (2008) Fifteen‐Twenty 190 15.092 45.044 W Triple junction Trailing IC Smith et al. (2008) Fifteen‐Twenty 190 15.050 44.095 W RTI; triple junction Trailing IC Escartín et al. (2008) Fifteen‐Twenty 190 15.044 44.656 E Triple junction Trailing OC Smith et al. (2008) Fifteen‐Twenty 190 15.011 44.523 E Triple junction Trailing OC Smith et al. (2008) Fifteen‐Twenty 190 14.969 44.222 E Triple junction Trailing OC Smith et al. (2008) Marathon 90 12.781 44.771 E RTI Leading IC Escartín et al. (2008); Smith et al. (2008) Marathon 90 12.541 44.555 W Trailing IC Smith et al. (2008) St Paul 550 0.667 27.000 W Multiple transform Central Intra‐transform Maia et al. (2014, 2016) segment St Paul 550 0.500 26.500 W Multiple transform Southern Intra‐transform Maia et al. (2016) segment 5°S 80 −5.167 11.700 W Trailing IC Planert et al. (2009); Planert et al. (2010); Reston et al. (2002) Ascencion 260 −7.450 13.333 E Multiple transform Leading IC Bialas et al. (2015) Ascencion 260 −7.000 12.250 E Multiple transform Intra‐ Intratransform Ohara et al. (2011) transform Cardno 90 −14.000 14.250 E Multiple transform Leading IC Li et al. (2014) Note. IC = inside corner; OC = outside corner; RTI = ridge‐transform intersection.

fracture zones. The morphological contrast between leading and trailing sides applies to fracture zones both east and west of the Mid‐Atlantic Ridge, regardless of which side of the fracture zone is older or younger. Depth profiles that extend parallel (Figures 5a–5e) to the fracture zones help quantify this asymmetry. Profiles were extracted 3 km north and south of each fracture zone, on seafloor accreted close to a RTI but clear of the fracture zone trace. A detection algorithm was applied to these profiles to objectively identify any peak with a local relief greater than 100 m. The peaks are highlighted with black dots above each profile, and their cumulative number is indicated by the parameter “N.” Although some ridges have local relief as low as 50 m, a threshold value of 100 m was selected in order to avoid possible data artifacts. Indeed,

CORMIER AND SLOAN 10 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Table 2 Distribution of Ridges Abutting Five North Atlantic Fracture Zones Leading side Trailing side Profile length Fracture zone (km) N Ridges per 100 km Mean ridge spacing (km) N Ridges per 100 km Mean ridge spacing (km)

Oceanographer 347 42 12.1 8.3 17 4.9 20.4 Atlantis 482 68 14.1 7.1 38 7.9 12.7 Kane 436 50 11.5 8.7 33 7.6 13.2 South of Kane 170 14 8.2 12.1 12 7.0 14.2 Marathon 178 34 19.1 5.2 22 12.4 8.1 Note. Measurements were performed on the depth profiles displayed in Figures 5a–5e. N indicates the number of ridges with local relief >100 m (see text). Leading and trailing sides as in Figure 1.

multibeam bathymetric sonars have an accuracy that ranges from ~0.5% of the water depth for the older systems (De Moustier & Matsumoto, 1993; Goff & Kleinrock, 1991) to ~0.2% of the water depth for the modern ones. This translates into a vertical accuracy of 10–30 m for the water depths typical of Atlantic fracture zones—an accuracy that may further degrade after gridding. Spreading‐parallel profiles (Figure 5) document that for each of the five transform faults, ridges locally higher than 100 m are more numerous along leading sides than trailing sides of the fracture zones, a relationship summarized in Table 2. The average spacing of small ridges ranges from 5 to 12 km along the leading side and from 8 to 20 km along the trailing side. In turn, flat‐bottom sediment ponds occur almost exclusively along the trailing sides. Considered in isolation, this more common occurrence of sediment ponds indicates a seafloor that lies generally deeper along the trailing sides compared to leading sides. The superimposition of the depth profiles for each of the five fracture zones confirms this depth differential (Figure 5): Outside of the active transform faults domain (delimited by the vertical arrows) and beyond the dynamically supported relief of the rift mountains, the trailing sides (red) lie deeper overall by hundreds of meters compared to the leading sides (blue). In fact, these profiles underestimate the actual depth contrast between oceanic basement on either side of the fracture zones since the ponded sediments are smoothing out the deepest areas. We note that these differential depths holds true regardless of which side of the fracture zone is older or younger. The Kane fracture zone is an exception, with few sediment ponds present along its length and with both profiles displaying similar depths on the west flank. The generally greater depths along the Kane fracture zone may place it just below the CCD, which hovers at 4.5–5 km in the North Atlantic since the Early Miocene (Tucholke & Mountain, 1986), thus explaining the apparently low rate of sediment accumulation. Furthermore, the greater length (170 km) and the left‐stepping sense of offset of the Kane transform may account for the observed depth pattern across the fracture zone: During the clockwise change in relative plate motion at or after ~9 Ma (Sloan & Patriat, 1992), this transform would have been subjected to transpressive stresses, an event that might have induced flexural uplift of the older sides of the fracture zone (Pockalny et al., 1996). A series of 40‐km‐long depth profiles extracted perpendicular to Atlantis fracture zone (Figure 7) further illustrate the more blocky, irregular character of the seafloor fabric along the trailing side. Overall, depths across the leading side (blue) increase progressively toward the fracture zone. In contrast, depths across the trailing side (red) increase in step‐like fashion toward the fracture zone, with steps ranging from a few hundred meters to >1 km. Many of the profiles on the trailing side terminate against the fracture zone trace in a flat‐bottom sediment pond, resulting in the underestimation of the actual depth of the igneous basement.

4. Discussion In the following, we discuss the possible origin of the observed asymmetry in seafloor fabric and unexpected differential depths across north Atlantic fracture zones (regular vs. crenulated fabric, distribution of OCCs, and deeper trailing sides) in terms of melt supply, propose a model that could explain that asymmetry, and explore some of the implications of that model. Admittedly, five fracture zones represent a small sample from which to derive a model. However, the morphological relations described above are generally consis- tent across all five fracture zones, suggesting that they have a common origin.

CORMIER AND SLOAN 11 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 7. Contrasting seafloor fabrics across the Atlantis fracture zone. Areas shown at the bottom are located in Figure 5b. Blue and red lines correspond to the depth profiles displayed in matching colors above the maps, from west (bottom) to east (top). Black circle indicates an oceanic core complex.

4.1. Differing Seafloor Fabrics and Seafloor Depths Across North Atlantic Fracture Zones as Indicators of Melt‐Poor Versus Melt‐Rich Conditions Typical abyssal hills at slow spreading rates have amplitudes >500 m and range in width from 6 to 20 km (e.g., Cormier & Sloan, 2017; Goff et al., 1995). Their relief tends to reach a maximum at inside corners where it sometimes culminates as OCCs, both near transform faults (Goff et al., 1995; Severinghaus & Macdonald, 1988) and nontransform discontinuities (Sloan & Patriat, 2004). Two other, more subtle types of seafloor fab- rics have been recognized along the Mid‐Atlantic Ridge (Cann et al., 2015; Smith et al., 2008). The first type, termed “volcanic terrain,” consists of closely spaced (2–10 km), elongated ridges running parallel to the spreading axis. The ridges that define this lineated terrain are bounded by steep, small fault scarps on the side facing the spreading axis, with throws ranging from <100 up to 500 m, and by gentler, occasionally faulted volcanic slopes on the other side. Smith et al. (2008) presented the area north of the Marathon trans- form fault (Figures 5e and 6h) as an example of such volcanic terrain and proposed that its accretion is indi- cative of relatively melt‐rich conditions. The second type of seafloor fabric, which Cann et al. (2015) termed “extended terrain” or “detachment fault dominated terrain,” has an irregular and blocky appearance and is generated by large‐offset normal faults (with displacement >3 km). As tectonic extension continues, a large fault may evolve into the formation of an OCC or into the formation of consecutive large‐offset faults. Fujiwara et al. (2003) and Smith et al. (2008) described a blocky and irregular seafloor fabric near the Fifteen‐Twenty fracture zone, and Smith et al. (2008) noted that this region is undergoing large amounts

CORMIER AND SLOAN 12 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

of tectonic extension on individual faults. Cann et al. (2015) cited the area south and west of the Kane frac- ture zone (Figures 5c and 6e) as an example of extended terrain and concluded that a weaker input of magma at the spreading axis has a general control on the formation of such terrain, although they caution that this relationship is not strong. The high density of small ridges encountered along leading sides at each of the five fracture zones (Table 2) fits the description of “volcanic terrain” introduced by Smith et al. (2008) and Cann et al. (2015). We propose that the distinct seafloor fabrics accreted on opposite sides of fracture zones in the northern Atlantic (Figures 6 and 7) correspond to the same two types of terrains: A lineated volcanic terrain indicative of melt‐rich conditions is generally accreted at leading RTIs and rafted off to the leading sides of fracture zones, while an irregular and blocky extended terrain or detachment fault dominated terrain indi- cative of more melt‐poor conditions tend to be accreted at trailing RTIs and rafted off to the trailing sides of fracture zones. In addition to the distinct fabrics present along leading sides versus trailing sides, two other observations support the existence of contrasting melt supplies at leading RTIs versus trailing RTIs. First, the unexpected depth relation along the eastern branches of the fracture zones (Figures 5 and 7), whereby younger seafloor along the trailing side lies deeper than adjacent older seafloor along the leading side, is compatible with the accretion of a thinner crust at trailing RTIs—as discussed below. Second, the fact that OCCs occur more than twice as frequently along trailing sides compared to leading sides (Section 3 and Table 1) indicates that long‐ lived detachment faults, rather than magmatism, accommodate plate divergence more than twice as often at trailing RTIs compared to leading RTIs. Indeed, consistent with the common exposure of gabbros across their summits, geophysical studies have confirmed that a thinner crust, and thus diminished magmatism, is associated with OCCs (e.g., Blackman et al., 2008). Simple half‐space cooling models document how seafloor depths increase linearly with the square root of age (Parsons & Sclater, 1977). The coefficient relating depth and square root of age is about 350 m/Myr1/2, although it varies locally between ~200 and ~450 m/Myr1/2 for the Atlantic basins (Crosby et al., 2006; Kane & Hayes, 1994; Marty & Cazenave, 1989; Stein & Stein, 1992). Leading and trailing sides juxtaposed across fracture zones are therefore expected to subside at different rates based on their respective ages, with the younger side remaining shallower than the older side. The five fracture zones considered here juxtapose seafloors with age contrasts ranging from ~3 Ma (offset south of Kane) to ~9 Ma (Kane). Assuming a linear coefficient of 350 m/Myr1/2,a5‐Ma lithosphere should therefore be lying ~200 m shallower than adjacent 8‐Ma lithosphere across the offset south of Kane, and ~530 m shallower than adjacent 14‐Ma lithosphere across the Kane fracture zone, with intermediate values applying to the other three fracture zones. While this depth relationship is roughly applying across the western branches of the fracture zones where flat sediment ponds are absent, it clearly does not apply to their eastern branches where the opposite relation is observed (except for the Kane fracture zone, as discussed earlier.) There, the younger trailing sides lay hundreds of meters (often >500 m) deeper than expected compared to the adjacent older leading sides (Figures 5–7). The accretion of a thinner crust at trailing RTIs could explain this unexpected depth relation. More melt‐poor conditions should result in the accretion of a crustal section that is thinner than typical, and, in turn, this thinner crust should reach isostatic equilibrium at greater seafloor depths than typical. For example, simple Airy isostasy implies that the top of a 4‐km‐thick crust should lie about 450 m deeper below the sea surface than a typical 6‐km‐thick oceanic crust (e.g., Turcotte & Schubert, 2014). The presence of a thinner crust along the trailing side is, in fact, supported by geophysical data. Early seismic refraction studies as well as gravity surveys have documented the common occurrence of a thinner crust beneath north Atlantic fracture zones (e.g., White et al., 1984; Lin et al., 1990), with crustal thicknesses as low as 2–3 km. Although these studies often lacked the resolution to determine how this thinner crust is positioned relative to the fracture zone trace, more detailed analyses of refraction data that derive crustal structures from ray tracing rather than from classic travel‐time analysis indicate that the thinnest crust underlies the sediment ponds and basins present along fracture zones. With the hindsight of high‐resolution multibeam bathymetric data, the thinnest crust therefore appears to be positioned beneath the trailing sides. Such ray‐tracing studies have investigated the Oceanographer fracture zone near 37°W and 35°25′W (Ambos & Hussong, 1986; Sinha & Louden, 1983), the Kane fracture zone near 44°W and 43°30′W (Abrams et al., 1988), and the Marathon fracture zone near 45°W (Peirce et al., 2019). An assumption implicit to this model, where the deeper trailing sides reflect the isostatic adjustment of a thinner crustal section, is that fracture

CORMIER AND SLOAN 13 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

zones do not become locked at RTIs immediately upon transform‐to‐fracture zone transition. This assump- tion is supported by a recent seismic study that indicates vertical movement occurs across the Marathon frac- ture zone at least until ~1 Myr after transition from transform fault to fracture zone (Peirce et al., 2019). It challenges the common view that fracture zones are locked and differential subsidence across them is accommodated by nonisostatic lithospheric flexure instead of vertical slip (e.g., Sandwell & Schubert, 1982). 4.2. Influence of Ridge Migration on Seafloor Fabric Near Ridge‐Transform Intersections Melt supplied to the mid‐ocean ridge follows complex pathways near RTIs. Mantle upwelling beneath the ridge results in decompression melting within a roughly triangular cross section called the melting regime, a region thought to range from 25 km to several hundred kilometers across depending on spreading rates (e.g., Langmuir & Forsyth, 2007). A number of models have been proposed to explain how melt extracted from this broad melting regime is focused toward a narrow (<5 km) neovolcanic zone at the ridge axis. An early model proposed that melt is drawn toward the ridge axis through a combination of flow‐induced pressure gradient and strain‐induced permeability anisotropy (e.g., Phipps Morgan, 1987). More recent mod- els suggest that buoyant melt rises vertically after segregating from the upwelling mantle until it reaches a sloping low‐permeability barrier near the base of the lithosphere. Such permeability barrier may result from melt crystallization (Gregg et al., 2012; Hébert & Montési, 2010; Sparks & Parmentier, 1991, 1994; Spiegelman, 1993) or an upward decrease in grain size (Turner et al., 2017). Melt is then thought to migrate upslope toward the ridge axis by following the underside of that permeability barrier (Figures 2, 8c, and 8d). Permeability barrier models predict a surface that deepens progressively on either side of the ridge axis, but also away from the end of a ridge segment at RTIs, thus slopping down beneath the adjacent older litho- sphere (Figure 2). Therefore, at RTIs, melt would be supplied not only from either side of the ridge axis but also diverted from beneath the older lithosphere beyond the ridge segment extremity (Bai & Montési, 2015; Gregg et al., 2009; Hébert & Montési, 2011; Katz et al., 2004; Magde & Sparks, 1997). This model assumes that the length of the transform fault is short enough that the melting regime associated with the adjacent RTI extends across most of the ridge offset (Figures 2, 8a, 8c, and 8e). Ridge migration introduces further complexity to this model. Numerical models suggest that mantle upwelling must be faster beneath the leading plate than the trailing plate (Davis & Karsten, 1986; Schouten et al., 1987; Small & Danyushevsky, 2003; Katz et al., 2004) and that the faster the ridge is migrat- ing relative to its spreading rate, the more pronounced the difference in mantle upwelling rates (and thus melt production) between leading and trailing flanks becomes. Ridge migration thus results in an asym- metric melting regime, with enhanced melt production beneath the leading plate compared to the trailing plate (Figures 8c and 8d). As a consequence of this asymmetry, melt at leading RTIs is drawn from the rela- tively melt‐rich regime that extends beneath the adjacent leading plate, and conversely, melt at trailing RTIs is drawn from the relatively depleted, melt‐poor regime that extends beneath the adjacent trailing plate (Katz et al., 2004). Thus, segments near leading RTIs are predicted to benefit from a comparatively more robust melt supply than segments near trailing RTIs. Katz et al. (2004) predict a difference in crustal thick- ness of about 1 km between leading and trailing RTIs, corresponding to ~10–20% difference in melt supply. Because the Mid‐Atlantic Ridge migrates westward overall, western RTIs are predicted to experience rela- tively melt‐rich conditions, and eastern RTIs are predicted to experience relatively melt‐poor conditions. These predictions are consistent with the observed preferential occurrence of OCCs and greater depths of seafloor accreted at trailing RTIs. Accordingly, we propose that the observed tendency for closely spaced linear ridges to occur in crust that was accreted at leading RTIs and the tendency for both OCCs and greater seafloor depths to occur within crust that was accreted at trailing RTIs are a direct consequence of the west- ward migration of the Mid‐Atlantic Ridge (Cormier & Sloan, 2013). An additional factor may also contribute to the variability of melt supply at leading versus trailing RTIs. The lateral migration of a mid‐ocean ridge over the asthenosphere is thought to result in a compositional asymmetry in the upper mantle relative to the ridge axis, whereby the upper mantle beneath the trailing plate would be systematically depleted compared to the upper mantle beneath the leading plate (Chalot‐ Prat et al., 2017; Cormier & Sloan, 2013; Doglioni et al., 2003). As a result, leading RTIs would be expected to sample a relatively undepleted mantle from the melting regime that underlies the adjacent leading plate. Conversely, the melt supplied to trailing RTIs would be partly derived from the already depleted melting regime underlying the trailing plate, and a more melt‐poor accretion would be expected.

CORMIER AND SLOAN 14 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 8. Schematics of melting regimes at transform faults of the migrating Mid‐Atlantic Ridge, in map view and in cross section. Map views for left‐stepping transform faults with (a) shorter offset (40 to 150 km) and (b) larger offset (>150 km); (c) and (d) corresponding cross‐sectional views; right‐stepping transform faults (e) short offset (40 to <150 km) and (f) large offset (>150 km). Solid arrows in all panels depict pathways followed by the melt out of the melting regime and along a hypothetical permeability barrier near the base of the lithosphere for the leading (blue) and trailing (red) segments. Colored triangles represent the melting regime beneath the leading (blue) and trailing (red) segments. Dashed vertical arrows depict the upwelling mantle near the base of the melting regime, with lengths schematically proportional to upwelling rates. Brown horizontal bars correspond to the crustal section.

4.3. Influence of Transform Fault Length on Melt Supply at Leading Versus Trailing Ridge Segments Available multibeam bathymetric data acquired near transform faults with lengths greater than ~200 km, such as the Vema and Romanche transform faults (e.g., Kastens et al., 1998; Ligi et al., 2002), do not show any clear asymmetry of leading versus trailing segment seafloor fabric. Similarly, seafloor fabrics generated near nontransform offsets do not reveal any obvious asymmetry (e.g., see Figure 5d for the presently migrat- ing offset at 21°30′N). Nontransform offsets are typically less than 40 km in length (Grindlay et al., 1991). A lack of clear asymmetry in seafloor fabric for both longer and shorter ridge offsets is consistent with the model we propose in which ridge migration results in asymmetric melt supply across ridge offsets. Melting regimes at slow spreading centers are thought to extend less than 100 km on either side of the ridge axis (e.g., Katz et al., 2004; Langmuir & Forsyth, 2007; Ligi et al., 2005; Phipps Morgan, 1987) and may be as narrow as 50 km (Hébert & Montési, 2010). In fact, numerical modeling and geochemical analysis indicate

CORMIER AND SLOAN 15 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

that only melt formed within ~25 km of the axis actually gets focused there, even when the melting regime is wider (e.g., Behn & Grove, 2015; Keller et al., 2017). Thus, melting regimes at opposite ends of transform faults longer than about 200 km should not overlap significantly, with the consequence that melt sources for leading and trailing RTIs should be largely independent of each other and should not be predictably different (Figures 8b, 8d, and 8f). In contrast, at shorter nontransform ridge offsets (<40 km), there may be too much communication or overlap between the melting regimes near leading and trailing RTIs to expect systematically dissimilar melt supplies (Figures 8a, 8c, and 8e). The above model is not inconsistent with the formation of long‐lived OCCs at short intratransform spreading segments, such as along the equatorial Atlantic transform faults. Indeed, OCCs mapped along such segmen- ted transform faults do not display any preferential occurrence for either leading or trailing RTIs (Table 1). Examples include two segmented transform faults of the Central Atlantic, the St Paul transform fault (Maia et al., 2014, 2016) and the Ascencion transform fault (Brozena & White, 1990; Ohara et al., 2011). OCCs also commonly occur along the series of short ridge segments and long transform faults of the Central Indian Ridge where spreading rates are ~35 mm/year (Kamesh Raju et al., 2012). Because intratransform spreading segments are bounded on both sides by thick, cold lithospheric plates, the resulting “transform fault effect” is expected to depress the height of the melting regime (e.g., Langmuir & Forsyth, 2007) as well as its width (e.g., Ligi et al., 2005) along the entire length of these short spreading segments. As a result, intratransform segments may be chronically melt starved, an effect that Ohara et al. (2011) described as the “transform sandwich effect.” 4.4. About a Lack of a Clear Relation Between Ridge Migration and Axial Depths Across Transform Faults at Slow Spreading Rates Across ridge offsets of intermediate‐ and fast spreading ridges, leading segments tend to be significantly shallower than trailing segments, a difference thought to reflect a systematically more robust melt supply at leading segments (Carbotte et al., 2004; Katz et al., 2004). Across ridge offsets of the slow spreading Mid‐Atlantic Ridge, however, Supak et al. (2007) did not detect any clearly predictable elevation differences between leading and trailing segment ends, a result that they suggest reflects a more limited entrainment of melt from across transform faults at slow spreading ridges. We propose that this lack of predictably different axial depths reflects instead the episodic interplay of volcanic and tectonic activity within the axial rift valley. Both the development of the axial volcanic ridges that mark the axis of accretion and the growth of the normal faults that define the rift valley walls occur on timescales of several tens of thousands of years (e.g., Briais et al., 2000; Searle et al., 2010) to a few million years (e.g., Tucholke et al., 1998). At such time- scales, the constructional relief of an axial volcanic ridge can vary by 100–400 m, while slip on normal faults that form the rift valley walls may lead to subsidence of the valley floor by several hundreds of meters, as has been documented in the Afar Rift (e.g., De Chabalier & Avouac, 1994; Stein et al., 1991) and in Iceland (e.g., Bull et al., 2003). These two processes would overprint any potential axial depth variations resulting from differences in melt supply, and, as a result, the axial depth of slow spreading ridges is unlikely to be a good indicator of the time‐averaged melt supply. Instead, contrasting melt supplies across transform faults may be better reflected in the distinctive seafloor fabrics between trailing and leading sides and greater seafloor depths along trailing sides. 4.5. Is There a Speed Limit for Oceanic Core Complexes? Tucholke et al. (2008) documented that OCCs are most common at slow spreading rates (14–32 mm/year), suggesting that some critical spreading rate may exist above which OCCs rarely develop. Indeed, OCCs frequently occur along both the northern Mid‐Atlantic Ridge (e.g., Cann et al., 1997; Escartín et al., 2008; Macleod et al., 2009; Smith et al., 2008; Tucholke & Lin, 1994) and the Central Indian Ridge where spreading rates are up to ~33 mm/year (Kamesh Raju et al., 2012). A critical spreading rate of about 30 mm/year is further supported by this study. Limited multibeam bathymetric data for the northern Mid‐Atlantic Ridge suggest that OCCs occur in greater abundance for seafloor younger than magnetic Chron 4A (Figures 5a–5d). Chron 4A is dated at about 9.1 Ma (Hilgen et al., 2012) and marks the onset of a 25% decrease in spreading rate, from ~32 to ~24 mm/year (Gente et al., 1995; Sempéré et al., 1995; Sloan & Patriat, 1992). This correlation between the appearance of OCCs and a sharp decrease in spreading rates is consistent with formation of OCCs at spreading rates lower than about 30 mm/year. This hypothesis is also compatible with the fact that less than a handful of OCCs have been mapped along the southern

CORMIER AND SLOAN 16 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Figure 9. Spreading rates (red) and ridge migration rates (blue and purple) as a function of latitude along the Mid‐Atlantic Ridge. All rates are calculated assuming the plate boundary geometry of Bird (2003). Spreading rates are from the MORVEL56 relative plate motion model (Argus et al., 2011). Migration rates are displayed for two absolute plate motion models from Becker et al. (2015): the “spreading aligned” model (blue) and MM07M (purple). The dashed blue and purple lines are the spreading‐parallel (ridge‐perpendicular) components of ridge migration, as calculated for the “spreading aligned” model and MM07M model, respectively. Named fracture zones are labeled on the left at their latitudinal position. Dashed black lines mark the triple junctions. Negative rates indicate an eastward ridge migration.

Mid‐Atlantic Ridge where spreading rates range from 30 to 34 mm/year (Table 1; Figure 9). Among this small number of OCCs are one occurring near the trailing RTI of the 5°S transform fault (Reston et al., 2002); a long‐lived, 25 km‐long OCC formed at the intratransform segment of the Ascension multiple transform fault at 7°S (Brozena & White, 1990; Ohara et al., 2011), and one at 14°S near the leading RTI of the Cardno transform fault (Li et al., 2014). Another factor may contribute to the apparent paucity of OCCs along the southern Mid‐Atlantic Ridge: OCCs may form in greater abundance at RTIs when the ridge migration rate is comparable to or exceeds the full spreading rate. Figure 9 illustrates that this condition occurs for the northern and central

CORMIER AND SLOAN 17 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Mid‐Atlantic Ridge, but not for the southern Mid‐Atlantic Ridge. Numerical models for ridge migration (Davis & Karsten, 1986; Katz et al., 2004) suggest that the faster the ridge migration rate is for a given spread- ing rate, the stronger the asymmetry in mantle upwelling across the ridge axis will be, and therefore the more likely that melt‐poor conditions will develop at trailing RTIs. Furthermore, Schouten et al. (1987) and Small and Danyushevsky (2003) propose that a lower ratio of spreading rate to ridge migration rate implies a weaker consumption rate of the asthenosphere at a spreading center, which they predict will result in a lower degree of mantle melting. Admittedly, giant OCCs (longer than 100 km in a spreading‐parallel direction) have been reported for inter- mediate spreading rates. However, in each case, their formation is associated with anomalous melt‐poor conditions that persisted for a long period. The Parece Vela Basin on the Philippine Sea Plate contained a back‐arc spreading center with an intermediate rate of ~70 mm/year when spreading ceased at around 12 Ma (Ohara et al., 2001). This extinct spreading center displays the axial rift valley typical of slow spreading ridges, is anomalously deep (>6,000 m), consists of a series of short (<60 km) segments separated by trans- form faults, and is the site of Godzilla Megamullion, one of the largest OCCs ever observed (125 km long and 55 km wide). Ohara et al. (2001) suggest that a highly variable melt supply may be characteristic of back‐arc spreading centers because of the episodic nature of induced mantle flow above subducting plates, thus accounting for the occasional formation of giant OCCs. Alternatively, the formation of the Godzilla Megamullion may be tied to a strongly reduced melt supply during the waning stage of accretion, as the spreading center became extinct. Another giant OCC has been mapped at the Australian‐Antarctic Discordance along the Southeast Indian Ridge, where the spreading rate is 72 mm/year (Okino et al., 2004). Here, too, the anomalously deep seafloor, the presence of an axial rift valley more typical of slow spreading ridges, and a host of other observations all support the presence of an anomalously cold mantle beneath the ridge axis, and thus the occurrence of persistent melt‐poor conditions (e.g., Sempéré et al., 1991).

5. Conclusions High‐resolution bathymetric data document striking asymmetries in the seafloor fabric formed near RTIs along the northern Mid‐Atlantic Ridge: (a) OCCs, commonly assumed to result from melt‐poor conditions, form predominantly at trailing RTIs, resulting in their preferential occurrence on the western ridge flank; (b) low‐amplitude, closely spaced ridges that extend all the way to the fracture zone tend to form at leading RTIs, with only rare occurrence of OCCs; (c) the trailing sides of fracture zones is generally deeper than the adjacent leading side, regardless of whether it is younger or older. These observations are best explained as follows: 1) The preferential occurrence of OCCs on the older trailing sides of fracture zones suggests that melt‐poor conditions episodically recur at trailing RTIs and, conversely, that melt‐poor conditions do not develop nearly as often at leading RTIs. 2) The generally deeper seafloor present along the trailing sides of fracture zones, regardless of the existing age contrast with the adjacent leading sides, is compatible with the accretion of a significantly thinner crust at trailing RTIs, and therefore with more melt‐poor accretion. 3) The overall westward migration of the Mid‐Atlantic Ridge may cause melt‐poor conditions at trailing RTIs. At the end of a trailing ridge segment, melt is tapped not only from the underlying melting regime but also from the relatively unproductive melting regime that extends beneath the adjacent trailing plate. Conversely, at leading RTIs, melt is tapped from the comparatively more productive melting regime that extends beneath the adjacent leading plate. 4) As reported previously, significantly fewer OCCs form where spreading rates are higher than about 30 mm/year, such as along the southern Mid‐Atlantic Ridge or prior to 8 Ma. This observation may indi- cate that a critical threshold in melt supply rate is reached above ~30 mm/year whereby melt‐poor con- ditions rarely develop. Alternatively, and not exclusively, the prevalence of OCCs along the central and northern Mid‐Atlantic Ridge may be associated with the significantly higher rates at which this section of the ridge is migrating compared to the southern Mid‐Atlantic Ridge. 5) The asymmetric melting regime that develops across a migrating ridge can also account for the absence of a morphological asymmetry near offsets shorter than ~40 km or longer than ~200 km. In the case of the shorter nontransform offsets, melting regimes supplying leading or trailing segments may overlap

CORMIER AND SLOAN 18 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

too much to expect dissimilar melt supplies at opposing segment ends. In the case of offsets longer than about 200 km, melting regimes do not overlap, and no melt is available to be tapped from beneath the adjacent older plate across RTIs. 6) We propose that the episodic interplay of tectonic and magmatic processes within the slow spreading axial rift valley explains the absence of systematic differences in axial depth between leading and trailing segments. On timescales of tens of thousands to a few millions years, axial depths may increase or decrease by hundreds of meters during a tectono‐magmatic episode and thus overshadow any depth variation that may result from contrasting melt supply. The pattern of abyssal hill terminations preserved off‐axis along the northern Mid‐Atlantic Ridge as well as the greater seafloor depths encoun- tered along the trailing sides of fracture zones appear to be a more faithful recorder of the melt supply averaged over the timescales of tectono‐magmatic episodes.

Acknowledgments References Constructive criticisms from the editor and three anonymous reviewers Abrams, L. J., Detrick, R. S., & Fox, P. J. (1988). Morphology and crustal structure of the Kane fracture zone transverse ridge. Journal of – improved an earlier submission of this Geophysical Research, 93(B4), 3195 3210. https://doi.org/10.1029/JB093iB04p03195 manuscript. We are grateful for the free, Ambos, E. L., & Hussong, D. M. (1986). Oceanographer transform fault structure compared to that of surrounding oceanic crust: Results – ‐ ‐ public availability of multibeam from seismic refraction data analysis. Journal of Geodynamics, 5(1), 79 102. https://doi.org/10.1016/0264 3707(86)90024 4 ‐ ‐ bathymetric data through the National Argus, D. F., Gordon, R. G., & DeMets, C. (2011). Geologically current motion of 56 plates relative to the no net rotation reference frame. Geophysical Data Center (https://www. Geochemistry, Geophysics, Geosystems, 12, Q11001. https://doi.org/10.1029/2011GC003751 ‐ ‐ ngdc.noaa.gov/mgg/bathymetry/ Bai, H., & Montési, L. (2015). Slip rate dependent melt extraction at oceanic transform faults. Geochemistry, Geophysics, Geosystems, 16, – multibeam.html) and GeoMapapp 401 419. https://doi.org/10.1002/2014GC005579 (http://www.geomapapp.org). Becker, T. W., Schaeffer, A. J., Lebedev, S., & Conrad, C. P. (2015). Toward a generalized plate motion reference frame. Geophysical – Bathymetric data have been processed Research Letters, 42,3188 3196. https://doi.org/10.1002/2015GL063695 ‐ ‐ with MB‐System and figures have been Behn, M. D., & Grove, T. L. (2015). Melting systematics in mid ocean ridge basalts: Application of a plagioclase spinel melting model to – generated with GMT, two freely global variations in major element chemistry and crustal thickness. Journal of Geophysical Research: Solid Earth, 120, 4863 4886. available software packages (Caress https://doi.org/10.1002/2015JB011885 et al., 2015; Wessel et al., 2013). In Bialas, J., Dannowski, A., & Reston, T. J. (2015). Interplay between magmatic accretion, spreading asymmetry and detachment faulting at a – addition, we thank the following segment end: Crustal structure south of the Ascension Fracture Zone. Earth and Planetary Science Letters, 432,84 94. https://doi.org/ scientists for making additional 10.1016/j.epsl.2015.09.027 bathymetric data available: Toshiya Bird, P. (2003). An updated digital model of plate boundaries. Geochemistry, Geophysics, Geosystems, 4(3), B05418.. https://doi.org/10.1029/ Fujiwara provided a bathymetric grid of 2001GC000252 ‐ the Kane Fracture zone compiled from Blackman, D. K., Cann, J. R., Janssen, B., & Smith, D. K. (1998). Origin of extensional core complex: Evidence from the Mid Atlantic Ridge – data acquired during the 1994 at Atlantis fracture zone. Journal of Geophysical Research, 103(B9), 21,315 21,333. https://doi.org/10.1029/98JB01756 ‐ expedition MODE'94 of the M/S Blackman, D. K., Karner, G. D., & Searle, R. C. (2008). Three dimensional structure of oceanic core complexes: Effects on gravity signature fl ‐ Yokosuka; Mathilde Cannat provided a and ridge ank morphology, Mid Atlantic Ridge, 30°N. Geochemistry, Geophysics, Geosystems, 9, L24303. https://doi.org/10.1029/ bathymetric grid of the Oceanographer 2008GC001951 ‐ ‐ fracture zone compiled from data Briais, A., Sloan, H., Parson, L. M., & Murton, B. J. (2000). Accretionary processes in the axial valley of the Mid Atlantic Ridge 27°N 30°N ‐ – acquired with the N/O L'Atalante from TOBI side scan sonar images. Marine Geophysical Researches, 21(1/2), 87 119. https://doi.org/10.1023/A:1004722213652 – during the 1998 expedition Brozena, J. M., & White, R. S. (1990). Ridge jumps and propagations in the South Atlantic Ocean. Nature, 348(6297), 149 152. https://doi. SUDACORES; Pascal Gente released a org/10.1038/348149a0 high quality bathymetric grid of the Bull, J. M., Minshull, T. A., Mitchell, N. C., Thors, K., Dix, J. K., & Best, A. I. (2003). Fault and magmatic interaction within Iceland's – ‐ area south of the Kane fracture zone via western rift over the last 9 kyr. Geophysical Journal International, 154(1), F1 F8. https://doi.org/10.1046/j.1365 246X.2003.01990.x ‐ GeoMapApp. Lastly, two high school Cann, J. R., Blackman, D. K., Smith, D. K., McAllister, E., Janssen, B., Mello, S. L. M., et al. (1997). Corrugated slip surfaces formed at ridge ‐ – students, Dale Storti and Daniel Thach, transform intersections on the Mid Atlantic Ridge. Nature, 385(6614), 329 332. https://doi.org/10.1038/385329a0 ‐ — helped with data compilation at the Cann, J. R., Smith, D. K., Escartin, J., & Schouten, H. (2015). Tectonic evolution of 200 km of Mid Atlantic Ridge over 10 million years – early stage of this study. Interplay of volcanism and faulting. Geochemistry, Geophysics, Geosystems, 16, 2303 2321. https://doi.org/10.1002/2015GC005797 Carbotte, S. M., Small, C., & Donnelly, K. E. (2004). The influence of ridge migration on the magmatic segmentation of mid‐ocean ridge. Nature, 429(6993), 743–746. https://doi.org/10.1038/nature02652 Caress, D.W., Chayes, D. N., & dos Santos Ferreira, C. (2015). MB‐system seafloor mapping software—Processing and display of swath sonar data. Retrieved from http://www.mbari.org/data/mbsystem and http://www.ldeo.columbia.edu/res/pi/MB‐System Chalot‐Prat, F., Doglioni, C., & Falloon, T. (2017). Westward migration of ocean ridges and related asymmetric upper mantle differentia- tion. Lithos, 268‐271, 163–173. https://doi.org/10.1016/j.lithos.2016.10.036 Cormier, M.H., & Sloan, H. (2013). Effects of ridge migration on the distribution of oceanic core complexes along the Mid‐Atlantic Ridge, AGU Meeting of the Americas, Cancun (Mexico), Abstract T32A‐03. Cormier, M. H., & Sloan, H. (2017). Abyssal hills and abyssal plains. In A. Micallef, S. Krastel, & A. Savini (Eds.), Submarine geomorphology, Chapter 20 (pp. 389–408). Springer. https://doi.org/10.1007/978‐3‐319‐57852‐1_20 Crosby, A. G., McKenzie, D. P., & Sclater, J. G. (2006). The relationship between depth, age and gravity in the oceans. Geophysical Journal International, 166(2), 553–573. https://doi.org/10.1111/j.1365‐246X.2006.03015.x Davis, E. E., & Karsten, J. L. (1986). On the cause of the asymmetric distribution of seamounts about the Juan de Fuca ridge: Ridge‐crest migration over a heterogeneous asthenosphere. Earth and Planetary Science Letters., 79(3‐4), 385–396. https://doi.org/10.1016/0012‐ 821X(86)90194‐9 De Chabalier, J. B., & Avouac, J. P. (1994). Kinematics of the Asal Rift (Djibouti) determined from the deformation of Fieale Volcano. Science, 265(5179), 1677–1681. https://doi.org/10.1126/science.265.5179.1677 De Moustier, C., & Matsumoto, H. (1993). Seafloor acoustic remote sensing with multibeam echo‐sounders and bathymetric sidescan sonar systems. Marine Geophysical Researches, 15(1), 27–42. https://doi.org/10.1007/BF01204150 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°30′N, 45°20′W. Geochemistry, Geophysics, Geosystems, 9, Q05014. https://doi.org/10.1029/2007GC001645

CORMIER AND SLOAN 19 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Doglioni, C., Carminati, E., & Bonatti, E. (2003). Rift asymmetry and continental uplift. Tectonics, 22(3), 2214. https://doi.org/10.1029/ 2002TC001459 Doglioni, C., Carminati, E., Crespi, M., Cuffaro, M., Penati, M., & Riguzzi, F. (2015). Tectonically asymmetric Earth: From net rotation to polarized westward drift of the lithosphere. Geoscience Frontiers, 6(3), 401–418. https://doi.org/10.1016/j.gsf.2014.02.001 Doubrouvine, P. V., Steinberger, B., & Trond, T. H. (2012). Absolute plate motions in a reference frame defined by moving hot spots in the Pacific, Atlantic, and Indian oceans. Journal of Geophysical Research, 117, B09101. https://doi.org/10.1029/2011JB009072 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 and Planetary Science Letters, 171(3), 411–424. https://doi.org/10.1016/S0012‐821X(99)00169‐7 Escartín, J., Smith, D. K., Cann, J. R., Schouten, H., Langmuir, C. H., & Escrig, S. (2008). Central role of detachment faults in accretion of slow‐spreading oceanic lithosphere. Nature, 455(7214), 790–794. https://doi.org/10.1038/nature07333 Escartín, J., Smith, D. K., & Cannat, M. (2003). Parallel bands of seismicity at the Mid‐Atlantic Ridge. Geophysical Research Letters, 30(12), 1620. https://doi.org/10.1029/2003GL017226 Fujimoto, H., Seama, N., Lin, L., Matsumoto, T., Tanaka, T., & Fujioka, K. (1996). Gravity anomalies of the Mid‐Atlantic Ridge north of the Kane fracture zone. Geophysical Research Letters, 23(23), 3431–3434. https://doi.org/10.1029/96GL02080 Fujiwara, T., & Fujimoto, H. (1998). Seafloor geomagnetic vector anomaly of the intersection of the Mid‐Atlantic Ridge and the Kane Transform Fault: Implications for magnetization of the oceanic crust. Journal of Geophysical Research, 103(B12), 30,335–30,349. https:// doi.org/10.1029/1998JB900015 Fujiwara, T., Lin, J., Matsumoto, T., Kelemen, P. B., Tucholke, B. E., & Casey, J. F. (2003). Crustal evolution of the Mid‐Atlantic Ridge near the Fifteen‐Twenty fracture zone in the last 5 Ma. Geochemistry, Geophysics, Geosystems, 4(3), 1024. https://doi.org/10.1029/ 2002GC000364 Gente, P., Pockalny, R. A., Durand, C., Deplus, C., Maia, M., Ceuleneer, G., et al. (1995). Characteristics and evolution of the segmentation of the Mid‐Atlantic Ridge between 20°N ands 24°N during the last 10 million years. Earth and Planetary Science Letters, 129(1‐4), 55–71. https://doi.org/10.1016/0012‐821X(94)00233‐O Goff, J. A., & Kleinrock, M. C. (1991). Quantitative comparison of bathymetric survey systems. Geophysical Research Letters, 18, 1253–1256. Goff, J. A., Tucholke, B. E., Lin, J., Jaroslow, G. E., & Kleinrock, M. C. (1995). Quantitative analysis of abyssal hills in the Atlantic Ocean: A correlation between inferred crustal thickness and extensional faulting. Journal of Geophysical Research, 100(B11), 22,509–22,522. https://doi.org/10.1029/95JB02510 Gregg, P. M., Behn, M. D., Lin, J., & Grove, T. L. (2009). Melt generation, crystallization, and extraction beneath segmented oceanic transform faults. Journal of Geophysical Research, 114, B11102. https://doi.org/10.1029/2008JB006100 Gregg, P. M., Hébert, L. B., Montési, L. G. J., & Katz, R. F. (2012). Geodynamic models of melt generation and extraction at mid‐ocean ridges. Oceanography, 25(1), 78–88. https://doi.org/10.5670/oceanog.2012.05 Grindlay, N. R., Fox, P. J., & Macdonald, K. C. (1991). Second‐order ridge axis discontinuities in the south Atlantic: Morphology, structure, evolution and significance. Marine Geophysical Researches, 13(1), 21–49. https://doi.org/10.1007/BF02428194 Hébert, L. B., & Montési, L. G. J. (2010). Generation of permeability barriers during melt extraction at mid‐ocean ridges. Geochemistry, Geophysics, Geosystems, 11, Q12008. https://doi.org/10.1029/2010GC003270 Hébert, L. B., & Montési, L. G. J. (2011). Melt extraction pathways at segmented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform. Geophysical Research Letters, 38, L11306. https://doi.org/10.1029/2011GL047206 Hilgen, F. J., Lourens, L. J., & Van Dam, J. A. (2012). The Neogene period. In F. M. Gradstein, J. G. Ogg, M. Schmitz, & G. Ogg (Eds.), The geologic time scale 2012 (pp. 947–1002). Elsevier Science B.V. https://doi.org/10.1016/B978‐0‐444‐59425‐9.00029‐9 Kamesh Raju, K. A., Samudrala, K., Droliam, R. K., Amarnath, D., Ramachandran, R., & Mudholkar, A. (2012). Segmentation and mor- phology of the Central Indian Ridge between 3°S and 11°S Indian Ocean. Tectonophysics, 554‐557, 114–126. https://doi.org/10.1016/j. tecto.2012.06.001 Kane, K. A., & Hayes, D. E. (1994). A new relationship between subsidence rate and zero‐age depth. Journal of Geophysical Research, 99, 21,759–21,777. https://doi.org/10.1029/94JB01747 Kastens, K. A., Bonatti, E., Caress, D. W., Carrara, G., Dauteuil, O., Früh‐Green, G. L., et al. (1998). The Vema Transverse Ridge (Central Atlantic). Marine Geophysical Researches, 20(6), 533–556. https://doi.org/10.1023/A:1004745127999 Katz, R. F., Spiegelman, M., & Carbotte, S. M. (2004). Ridge migration, asthenospheric flow and the origin of magmatic segmentation in the global mid‐ocean ridge system. Geophysical Research Letters, 31, L15605, https://doi.org/10.1029/2004GL020388 Keller, T., Katz, R. F., & Hirschmann, M. M. (2017). Volatiles beneath mid‐ocean ridges: Deep melting, channelised transport, focusing, and metasomatism. Earth and Planetary Science Letters, 464,55–68. https://doi.org/10.1016/j.epsl.2017.02.006 Lagabrielle, Y., Bideau, D., Cannat, M., Karson, J. A., & Mével, C. (1998). Ultramafic‐mafic plutonic suites exposed along the Mid‐Atlantic Ridge (10°–30°N). Symmetrical‐asymmetrical distribution and implications for seafloor spreading processes. In Faulting and magma- tism at mid‐ocean ridges (Vol. 106, pp. 153–176). American Geophysical Union Monograph. Langmuir, C. H., Bender, J. F. F., & Batiza, R. (1986). Petrological and tectonic segmentation of the East Pacific Rise, 5°30′‐14°30'N. Nature, 322(6078), 422–429. https://doi.org/10.1038/322422a0 Langmuir, C. H., & Forsyth, D. W. (2007). Mantle melting beneath mid‐ocean ridges. Oceanography, 20(1), 78–89. https://doi.org/10.5670/ oceanog.2007.82 Li, B., Yang, Y., Shi, X., Ye, J., Gao, J., Zhu, A., & Shao, M. (2014). Characteristics of a ridge‐transform inside corner intersection and associated mafic‐hosted seafloor hydrothermal field (14.0°S, Mid‐Atlantic Ridge). Marine Geophysical Researches, 35(1), 55–68. https:// doi.org/10.1007/s11001‐013‐9209‐1 Ligi, M., Bonatti, E., Cipriani, A., & Ottolini, L. (2005). Water‐rich basalts at mid‐ocean‐ridge cold spots. Nature, 434(7029), 66–69. https:// doi.org/10.1038/nature03264 Ligi, M., Bonatti, E., Gasperini, L., & Poliakov, A. N. B. (2002). Oceanic broad multifault transform plate boundaries. Geology, 30(1), 11–14. https://doi.org/10.1130/0091‐7613(2002)030<0011:OBMTPB>2.0.CO;2 Lin, J., Purdy, G. M., Schouten, H., Sempéré, J.‐C., & Zervas, C. (1990). Evidence from gravity data for focused magmatic accretion along the Mid‐Atlantic Ridge. Nature, 344(6267), 627–632. https://doi.org/10.1038/344627a0 Macdonald, K. C., Fox, J. P., Perram, L. P., Eisen, M. F., Haymon, R. M., Miller, S. P., et al. (1988). A new view of the mid‐ocean ridge from the behavior of ridge‐axis discontinuities. Nature, 335(6187), 217–225. https://doi.org/10.1038/335217a0 MacLeod, C. J., Searle, R. C., Murton, B. J., Casey, J. F., Mallows, C., Unsworth, S. C., et al. (2009). Life cycle of oceanic core complexes. Earth and Planetary Science Letters, 287(3‐4), 333–344. https://doi.org/10.1016/j.epsl.2009.08.016 Magde, L. S., & Sparks, D. W. (1997). Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges. Journal of Geophysical Research, 102, 20,571–20,583. https://doi.org/10.1029/97JB01278

CORMIER AND SLOAN 20 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Maia, M., Birot, D., Brachet, C., Brehme, I., Briais, A., Brunelli, D., et al. (2014). Equatorial Atlantic, Recife, January 24 ‐ Recife, February 28, 2013, InterRIDGE News, 22, 52–56. Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., et al. (2016). Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), 619–623. https://doi.org/10.1038/ngeo2759 Marty, J. C., & Cazenave, A. (1989). Regional variations in subsidence rate of oceanic plates: A global analysis. Earth and Planetary Science Letters, 94(3‐4), 301–315. https://doi.org/10.1016/0012‐821X(89)90148‐9 Merkouriev, S. A., & DeMets, C. (2014). High‐resolution Neogene reconstructions of Eurasia‐North America Plate motion. Geophysical Journal International, 198(1), 366–384. https://doi.org/10.1093/gji/ggu142 Morgan, W. J., & Phipps Morgan, J. (2007). Plate velocities in the hotspot reference frame. In G. R. Foulger & D. M. Jurdy (Eds.), Plates, plumes, and planetary processes (Vol. 430, pp. 65–78). GSA Special Paper. Müller, R. D., & Smith, W. H. F. (1993). Deformation of the oceanic crust between the North American and South American plates. Journal of Geophysical Research, 98, 8275–8291. https://doi.org/10.1029/92JB02863 Ohara, Y., Okino, K., & Snow, J. E. (2011). Tectonics of unusual crustal accretion in the Parece Vela Basin, in Accretionary Prisms and Convergent margin Tectonic in the Northwestern Pacific Basin. In Y. Ogawa et al. (Eds.), Modern approaches in solid earth sciences (pp. 149–168). Springer. Ohara, Y., Yoshida, T., Kato, Y., & Kasuga, S. (2001). Giant megamullion in the Parece Vela backarc basin. Marine Geophysical Researches, 22(1), 47–61. https://doi.org/10.1023/A:1004818225642 Okino, K., Matsuda, K., Christie, D. M., Nogi, Y., & Koizumi, K. (2004). Development of oceanic detachment and asymmetric spreading at the Australian‐Antarctic discordance. Geochemistry, Geophysics, Geosystems, 5, Q12012. https://doi.org/10.1029/2004GC000793 Pariso, J. E., Sempéré, J. C., & Rommevaux, C. (1995). Temporal and spatial variations in crustal accretion along the Mid‐Atlantic Ridge (29°–31°30′N) over the last 10 m.y.: Implications from a three‐dimensional gravity study. Journal of Geophysical Research, 100(B9), 17,781–17,794. https://doi.org/10.1029/95JB01146 Parsons, B., & Sclater, J. G. (1977). An analysis of the variation of ocean floor bathymetry and heat flow with age. Journal of Geophysical Research, 82, 803–827. https://doi.org/10.1029/JB082i005p00803 Peirce, C., Reveley, G., Robinson, A. H., Funnell, M. J., Searle, R. C., Simao, N. M., et al. (2019). Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid‐Atlantic Ridge. Geophysical Journal International, 217(2), 988–1010. https://doi.org/ 10.1093/gji/ggz074 Phipps Morgan, J. (1987). Melt migration beneath mid‐ocean spreading centers. Geophysical Research Letters, 14(12), 1238–1241. https:// doi.org/10.1029/GL014i012p01238 Planert, L., Flueh, E. R., & Reston, T. J. (2009). Along‐ and across‐axis variations in crustal thickness and structure at the Mid‐Atlantic Ridge at 5°S obtained from wide‐angle seismic tomography: Implications for ridge segmentation. Journal of Geophysical Research, 114, B09102. https://doi.org/10.1029/2008JB006103 Planert, L., Flueh, E. R., Tilmann, F. J., Grevemeyer, I., & Reston, T. J. (2010). Crustal structure of a rifted oceanic core complex and its conjugate side at the MAR at 5°S: Implications for melt extraction during detachment faulting and core complex formation. Geophysical Journal International, 181(1), 113–126. https://doi.org/10.1111/j.1365‐246X.2010.04504.x Pockalny, R. A., Detrick, R. S., & Fox, P. J. (1988). Morphology and tectonics of the Kane transform from SeaBeam bathymetry data. Journal of Geophysical Research, 93(B4), 3179–3193. https://doi.org/10.1029/JB093iB04p03179 Pockalny, R. A., Gente, P., & Buck, W. R. (1996). Oceanic transverse ridges: A flexural response to fracture‐zone‐normal extension. Geology, 24(1), 71–74. https://doi.org/10.1130/0091‐7613(1996)024<0071:OTRAFR>2.3.CO;2 Rabain, A., Cannat, M., Escartín, J., Pouliquen, G., Deplus, C., & Rommevaux‐Jestin, C. (2001). Focused volcanism and growth of a slow spreading segment (Mid‐Atlantic Ridge, 35°N). Earth and Planetary Science Letters, 185(1‐2), 211–224. https://doi.org/10.1016/S0012‐ 821X(00)00371‐X Reston, T. J., Weinrebe, W., Grevemeyer, I., Flueh, E. R., Mitchell, N. C., Kirstein, L., et al. (2002). A rifted inside corner massif on the Mid‐ Atlantic Ridge at 5°S. Earth and Planetary Science Letters, 200(3‐4), 255–269. https://doi.org/10.1016/S0012‐821X(02)00636‐2 Sandwell, D., & Schubert, G. (1982). Lithospheric flexure at fracture zones. Journal of Geophysical Research, 87(B6), 4657–4667. https://doi. org/10.1029/JB087iB06p04657 Scheirer, D. S., & Macdonald, K. C. (1993). Variations in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center. Journal of Geophysical Research, 98, 7871–7885. https://doi.org/10.1029/93JB00015 Schouten, H., Dick, H. J. B., & Klitgord, K. D. (1987). Migration of mid-ocean-ridge volcanic segments. Nature, 326,835–839. https://doi. org/10.1038/326835a0 Searle, R. C., Murton, B. J., Achenbach, K. L., LeBas, T. P., Tivey, M. A., Yeo, I. A., et al. (2010). Structure and development of an axial volcanic ridge: Mid‐Atlantic Ridge, 45°N. Earth and Planetary Science Letters, 299(1‐2), 228–241. https://doi.org/10.1016/j.epsl.2010.09.003 Sempéré, J. C., Blondel, P., Briais, A., Fujiwara, T., Géli, L., Isezaka, N., et al. (1995). The Mid‐Atlantic Ridge between 29°N and 31°30′Nin the last 10 Ma. Earth and Planetary Science Letters, 130(1‐4), 45–55. https://doi.org/10.1016/0012‐821X(94)00259‐2 Sempéré, J. C., Palmer, J., Christie, D., Phipps Morgan, J., & Shor, A. (1991). Australian‐Antarctic discordance. Geology, 19(5), 429–432. https://doi.org/10.1130/0091‐7613(1991)019<0429:AAD>2.3.CO;2 Severinghaus, J. P., & Macdonald, K. C. (1988). High inside corners at ridge‐transform intersections. Marine Geophysical Researches, 9(4), 353–367. https://doi.org/10.1007/BF00315005 Sinha, M. C., & Louden, K. E. (1983). The oceanographer fracture zone—I. Crustal structure from seismic refraction studies. Geophysical Journal of the Royal and Astronomical Society, 75(3), 713–736. https://doi.org/10.1111/j.1365‐246X.1983.tb05007.x Sloan, H., & Patriat, P. (1992). Kinematics of the North American‐African plate boundary between 28° and 29°N during the last 10 Ma: Evolution of the axial geometry and spreading rate and direction. Earth and Planetary Science Letters, 113(3), 323–341. https://doi.org/ 10.1016/0012‐821X(92)90137‐K Sloan, H., & Patriat, P. (2004). Reconstruction of the flanks of the Mid‐Atlantic Ridge, 28° to 29°N: Implications for evolution of young oceanic lithosphere at slow‐spreading centers. Geochemistry, Geophysics, Geosystems, 5, GC003850. https://doi.org/10.1029/ 2004GC000727 Small, C., & Danyushevsky, L. V. (2003). Plate‐kinematic explanation for mid‐ocean ridge depth discontinuities. Geology, 31(5), 399–402. https://doi.org/10.1130/0091‐7613(2003)031<0399:PEFMDD>2.0.CO;2 Smith, D. K., Cann, J. R., & Escartín, J. (2006). Widespread active detachment faulting and core complex formation near 13° N on the Mid‐Atlantic Ridge. Nature, 442(7101), 440–443. https://doi.org/10.1038/nature04950 Smith, D. K., Escartín, J., Schouten, H., & Cann, J. R. (2008). Fault rotation and core complex formation: Significant processes in seafloor formation at slow‐spreading mid‐ocean ridges (Mid‐Atlantic Ridge, 13°–15°N). Geochemistry, Geophysics, Geosystems, 9, Q03003. https:// doi.org/10.1029/2007GC001699

CORMIER AND SLOAN 21 Geochemistry, Geophysics, Geosystems 10.1029/2018GC008101

Sparks, D. W., & Parmentier, E. M. (1991). Melt extraction from the mantle beneath spreading centers. Earth and Planetary Science Letters, 105(4), 368–3377. https://doi.org/10.1016/0012‐821X(91)90178‐K Sparks, D. W., & Parmentier, E. M. (1994). The generation and migration of partial melt beneath oceanic spreading centers. In M. P. Ryan (Ed.), Magmatic systems (pp. 55–75). Spiegelman, M. (1993). Physics of melt extraction: Theory, implications and applications. Philosophical Transactions of the Royal Astronomical Society, 342A,23–41. Stein, C. A., & Stein, S. (1992). A model for the global variation in oceanic depth and heat flow with lithospheric age. Nature, 359(6391), 123–129. https://doi.org/10.1038/359123a0 Stein, R. S., Briole, P., Ruegg, J. C., Tapponier, P., & Gasse, F. (1991). Contemporary, holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges. Journal of Geophysical Research, 96(B13), 21,789–21,806. https:// doi.org/10.1029/91JB02118 Stein, S., Melosh, H. J., & Minster, J. B. (1977). Ridge migration and asymmetric seafloor spreading. Earth and Planetary Science Letters, 36(1), 51–62. https://doi.org/10.1016/0012‐821X(77)90187‐X Supak, S., Carbotte, S. M., & Macdonald, K. C. (2007). Influence of ridge migration and proximity to hot spots on the morphology of slow‐ and intermediate‐spreading centers. Geocheistry, Geophysics, Geosystems, 8, Q01010. https://doi.org/10.1029/2006GC001387 Tucholke, B. E., Behn, M. D., Buck, W. R., & Lin, J. (2008). Role of melt supply in oceanic detachment faulting and formation of mega- mullions. Geology, 36(6), 455–458. https://doi.org/10.1130/G24639A.1 Tucholke, B. E., & Lin, J. (1994). A geological model for the structure of ridge segments in slow spreading ocean crust. Journal of Geophysical Research, 99(B6), 11,937–11,958. https://doi.org/10.1029/94JB00338 Tucholke, B. E., Lin, J., & Kleinrock, M. C. (1998). Megamullions and mullion structures defining oceanic metamorphic core complexes on the Mid‐Atlantic Ridge. Journal of Geophysical Research, 103(B5), 9857–9866. https://doi.org/10.1029/98JB00167 Tucholke, B. E., & Mountain, G. S. (1986). Tertiary paleoceanography of the western North Atlantic Ocean. In P. R. Vogt & B. E. Tucholke (Eds.), The geology of North America, volume M, the western North Atlantic Region (pp. 631–650). Boulder, CO: Geological Society of America. Turcotte, D. L., & Schubert, G. (2014). Geodynamics (3rd ed.). Cambridge, UK: Cambridge University Press. Turner, A. J., Katz, R. F., Behn, M. D., & Keller, T. (2017). Magmatic focusing to mid‐ocean ridges: The role of grain‐size variability and non‐ newtonian viscosity. Geochemistry, Geophysics, Geosystems, 18, 4342–4355. https://doi.org/10.1002/2017GC007048 Wang, S., Yu, H., Zhang, Q., & Zhao, Y. (2018). Absolute plate motions relative to deep mantle plumes. Earth and Planetary Science Letters., 490,88–99. https://doi.org/10.1016/j.epsl.2018.03.021 Wessel, P., & Haxby, W. F. (1990). Thermal stresses, differential subsidence, and flexure at oceanic fracture zones. Journal of Geophysical Research, 95(B1), 375–391. https://doi.org/10.1029/JB095iB01p00375 Wessel, P., Smith, W. H. F., Scharroo, R., Luis, J., & Wobbe, F. (2013). Generic mapping tools: Improved version released. Eos, Transactions American Geophysical Union, 94(45), 409–410. https://doi.org/10.1002/2013EO450001 White, R. S., Detrick, R. S., Sinha, M. S., & Cormier, M. H. (1984). Anomalous seismic structure of oceanic fracture zones. Geophysical Journal of the Roydal Astronomical Society, 79, 779–798. https://doi.org/10.1111/j.1365-246X.1984.tb02868.x Zervas, C. E., Sempéré, J. C., & Lin, J. (1995). Morphology and crustal structure of a small transform fault along the Mid‐Atlantic Ridge: The Atlantis fracture zone. Marine Geophysical Researches, 17(3), 275–300. https://doi.org/10.1007/BF01203466

CORMIER AND SLOAN 22