A Global Review and Digital Database of Large-Scale Extinct Spreading Centers GEOSPHERE

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GEOSPHERE A global review and digital database of large-scale extinct GEOSPHERE; v. 13, no. 3 spreading centers doi:10.1130/GES01379.1 Sarah J. MacLeod, Simon E. Williams, Kara J. Matthews, R. Dietmar Müller, and Xiaodong Qin EarthByte Group, School of Geosciences, University of Sydney, Camperdown, New South Wales 2006, Australia 9 figures; 4 tables; 2 supplemental files

CORRESPONDENCE: sarah.macleod@sydney ABSTRACT into which proposed locations are more likely to have been former spreading .edu.au centers, and our analysis further leads to the discovery of several previously Extinct mid-ocean ridges record past plate boundary reorganizations, and unidentified structures in the south of the West Philippine Basin that likely CITATION: MacLeod, S.J., Williams, S.E., Matthews, K.J., Müller, R.D., and Qin, X.D., 2017, A global review identifying their locations is crucial to developing a better understanding of represent extinct ridges and a possible extinct ridge in the western South At- and digital database of large-scale extinct spread- the drivers of plate tectonics and oceanic crustal accretion. Frequently, extinct lantic. We make available our global compilation of data and analyses of indi- ing centers: Geosphere, v. 13, no. 3, p. 911–949, ridges cannot be easily identified within existing geophysical data sets, and vidual ridges in a global extinct ridge data set at the GPlates Portal webpage1. doi:10.1130/GES01379.1. there are many controversial examples that are poorly constrained. We analyze the axial morphology and gravity signal of 29 well-constrained, global, Received 17 June 2016 Revision received 29 November 2016 large-scale extinct ridges that are digitized from global data sets, to describe INTRODUCTION Accepted 15 March 2017 their key characteristics. Additionally, the characteristics of a representative Published online 21 April 2017 collection of active spreading centers are analyzed to review the present-day Extinct spreading centers are tectonic structures in the seafloor that pre- variation in the bathymetry and gravity signal of ridges in different tectonic serve the terminal location of a past divergent plate boundary, after it has settings such as backarc basin ridges, microplate ridges, and large-scale plate ceased both magmatic and tectonic accretion, leading the crust at the bound- boundaries with varied spreading rates. Uncertain extinct ridge-like structures ary to become joined as a single plate. They are valuable sources of infor- are evaluated in comparison with the signals of well-defined extinct ridges, mation regarding accretionary mechanisms and present a natural laboratory and we assess whether their morphology and gravity signals are within the for investigation of spreading ridge characteristics and behavior, without the range seen at extinct (or active) ridges. There is significant variability in ex- additional complexity of a thermal anomaly (Jonas et al., 1991; Livermore tinct ridge morphology; yet we find that the majority of well-defined extinct et al., 2000). Although some extinct spreading centers have a clear signature ridges have a trough form and a negative free-air gravity anomaly. We com- in bathymetry or gravity maps, there are many proposed extinct ridges that pile available data on the spreading characteristics of extinct ridges prior to are uncertain in location and/or origin. In some areas, asymmetric ocean floor cessation, such as their spreading rates and duration of spreading, and find (Müller et al., 2008), magnetic anomalies, or tectonic reconstructions are used significant differences between ridge subtypes and between oceans. Large- to infer the location of an extinct ridge; yet there may be no obvious structure scale extinct mid-ocean ridges persist much longer than extinct microplate that can be interpreted as an extinct ridge. This difficulty is compounded in spreading ridges and extinct backarc basin spreading ridges before cessation. remote regions where marine ship-track data and sampling are incomplete Extinct fragmented plate and microplate spreading centers have the highest or of poor quality and when ocean crust was formed during magnetic “quiet pre-extinction spreading rates, and they have greater median relief at their zones.” Therefore, we seek to better describe the general characteristics of ex- axial segments, suggesting that different crustal accretion styles could lead tinct ridges, in order to test whether these can be used to provide an alterna- to different morphology after spreading cessation. Backarc basin ridges have tive means of assessing ridge-like tectonic structures within ocean basins that more pronounced relief when they have been active for longer before cessa- have ambiguous or uncertain modes of formation. Locating extinct ridges is tion, which supports theories of reduced magmatic supply as the basin width essential to the development of accurate regional and global reconstructions increases. Extinct ridges in the Atlantic Ocean have the lowest spreading rates that can assist in understanding the evolution of plate boundaries and the geo- prior to cessation and tend to persist for twice as long as those in the Pacific dynamic processes that control these changes. before extinction. There are a larger number of extinct ridges preserved within 1 marginal basins than expected for their combined area; these ridges may re- Online database of the digitized extinct ridge (and analyzed active ridge) locations is in the GPlates Portal: http://portal .gplates .org /cesium /?view =ExRidges. Reference IDs for extinct ridges For permission to copy, contact Copyright late to the complexity of the plate boundaries in these regions. Our review are listed on the index page, http://portal .gplates .org /portal /ExRidges/, and these link to collated Permissions, GSA, or [email protected]. of a large number of controversial extinct ridge locations offers some insight data and summaries of key studies for each ridge that can be cross-referenced with Tables 1A–1C.

To improve identification of these structures we catalogue large-scale tec- smaller magnitude gravity lows of 5 mGal, with wavelengths of 10–15 km, for tonic structures that have been proposed as extinct ridges previously (Fig. 1; centers with faster spreading before cessation. We seek to increase our under Tables 1A–1C) and rank these with respect to how well studied and constrained standing of extinct ridges by investigating a global, more extensive data set these features are. To better understand the variability of extinct ridges, we using high-resolution satellite gravity (Sandwell et al., 2014) and global bath review the characteristics of a collection of active spreading centers that rep- ymetry (Weatherall et al., 2015) data. We consider whether there is a charac- resent different tectonic settings (Table 1D), to permit appropriate comparison teristic morphology and gravity signal at extinct ridge axes by quantifying the of extinct examples of various subtypes. We analyze the characteristics of the across-axis relief and gravity anomaly at individual segments of extinct ridges. “well-defined” extinct ridges, particularly their axial signature, using two-dimensional profiles from global satellite-derived bathymetry data (Weatherall et al., 2015; GEBCO_2014_1D, version 20141103) and gravity data (Sandwell Classification of Spreading Ridge Subtypes et al., 2014; V23.1 gridded global gravity data). We combine these data with a compilation of information on potential controlling factors for the evolution of Global studies of active spreading centers have investigated specific each ridge, such as the spreading rate of the system prior to cessation, the type features of the global mid-ocean ridge (MOR) system, such as the variabil- of spreading center, the duration of activity at the ridge system, and the time ity in morphology related to spreading rates (Small, 1998), abyssal hill form since spreading cessation (i.e., age of crust formed at the extinct axis). From (Goff, 1991) and segmentation (Schouten et al., 1985; Batiza, 1996; Carbotte the characteristics of well-resolved extinct ridges, we revisit ridge-like features et al., 2015). As yet, however, there has been no synthesis of the variability of with more uncertain origin and further evaluate several oceanic features that spreading ridges relative to different ridge subtypes. Therefore, we first review may represent previously unreported extinct ridges. the different tectonic settings in which spreading centers occur. We consider microplate spreading ridges, backarc basin spreading ridges, and large-scale mid-ocean ridges as distinct subtypes of spreading centers and a small class BACKGROUND of transient spreading ridges that evolve in the context of plate fragmentation. This evaluation provides useful information regarding active spreading ridge Physical Characteristics of Extinct Ridges variability that permits a more meaningful comparison for extinct ridges of different subtypes. Extinct ridges have experienced deceleration of their spreading rate to zero In a previous review of “failed rifts,” Batiza (1989) identified mid-ocean (Mammerickx and Sandwell, 1986); so it is expected that their morphology will ridges that had jumped to a new location over 400 km away after cessation be typical of that seen at slow- to ultraslow-spreading centers, and these char- as examples of the largest scale of extinct ridge. Batiza (1989) included within acteristics have indeed been reported at a number of extinct spreading cen- this group ridges that did not have a subsequent spreading center within the ters (Mammerickx and Sandwell, 1986; Jung and Vogt, 1997; Livermore et al., region. Within the large-scale structures, we define extinct mid-ocean ridges 2000). Yet some variability is also noted, with Okino and Fujioka (2003) report- (XMORs) as a subgroup of extinct ridges that at the time of spreading were ing that several segments of the Central Basin extinct spreading center in the situated between two major plates (Fig. 2A). Active examples of large-scale West Philippine Basin were unusual in that they have fast spreading character- mid-ocean ridges include the Mid-Atlantic Ridge, the Southwest and South- istics such as smooth topography, compared with other segments of the ridge. east Indian ridges, the Pacific-Antarctic ridge, and the East Pacific Rise. They attribute this variation to the influence of a proximal hotspot in the west The spreading centers within active microplates, such as the Easter and of the basin that was limited in its regional influence (Okino and Fujioka, 2003). Juan Fernandez microplates (Searle et al., 1993), have significantly different Previous investigations have reported a characteristic negative free-air morphology, structural characteristics, and crustal accretion style than large- gravity anomaly at extinct ridges (Jonas et al., 1991; Louden et al., 1996; Liv- scale mid-ocean ridges; therefore, we separate extinct microplate spreading ermore et al., 2000; Greenhalgh and Kusznir, 2007). Observation of the anom- centers (Fig. 2B) into a second subgroup (extinct microplate spreading ridges alous gravity signal permitted Matthews et al. (2011) to refine the locations of [XMPRs]). Microplates have been defined as oceanic plates that are typically 26 extinct ridges using the minima of their gravity signal using global vertical less than 500 km in diameter (Bird, 2003) and that move relative to a local pole gravity gradient (VGG) data. Jonas et al. (1991) compared the gravity signature of rotation (Naar and Hey, 1991; Schouten et al., 1993), such as the well-stud- of the Labrador Sea, Coral Sea, Magellan Trough, and West Philippine Basin ex- ied, presently active Easter microplate (Naar and Hey, 1991). They form by tinct spreading centers and tested various potential explanations for the gravity ridge propagation into existing ocean floor, a process that detaches a piece anomalies observed in these locations. They found that the gravity anomaly of the larger oceanic plate. Dual spreading at both the preexisting spreading across the extinct ridge axis varied according to the paleospreading rate. Spe- center and the new spreading center results in the growth of the microplate cifically, they reported free air gravity lows of 25–45 mGal, with wavelengths by accretion of new crust to the core of the microplate—the captured crust 20–45 km, at extinct spreading centers with low paleospreading rates and (Engeln et al., 1988; Naar and Hey, 1991; Hey, 2004; Matthews et al., 2016).

120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W 80°W A 60°N 60°N KOM Figure 1 (on this and following four pages). KUL Locations of the extinct ridges evaluated in this study are shown on the vertical CH JDF gravity gradient (VGG) grid of Sandwell SoJ et al. (2014). Well-defined (primary-tier) HKT ridges are shown in red, controversial 40°N MON 40°N (secondary-tier) ridges in pink and poorly EMP constrained (excluded) ridge locations shown in yellow. Active mid-ocean ridges LIL GUA PVS GOM are shown in light blue or in thick, orange SHI lines where segments were assessed for comparison with extinct ridges. (A) Pro- MGD RIV posed extinct ridges in the Pacific Ocean 20°N WPH 20°N and neighboring marginal basins (white labels): AD—Adare Trough; BA—Bauer; SCS COC—Cocos; CRN—Carnegie; ELB—Ellice Basin; EMP—Emperor fracture zone; MT MTH PAL MAG TE COC FRI—Friday; HUD—Hudson; GL—Gallego; CT GLP—Galapagos Rise; GUA—Guadalupe; MAT SAN HKT—Hokkaido Trough; KUL—Kula ridge; CAR KT WMG EPR CEL NCT LIL—Liliuokalani ridge; MTH—Mathemati- 0° CBW NZC MAP 0° cian; MAG—Magellan Trough; MAP—Mal- CRN pelo; MEN—Mendoza; MGD—Magdalena; OJ PEN GL MNH—Manihiki; MON—Monterey; NCT— DAM ELB Nova Canton Trough; NZC—Nazca propa- SRT GLP gator; OJ—Ontong Java; OSB—Osbourn; PEN—Penrhyn; SAN—Sandra; SEL—Sel- MNH BA kirk; SHI—Shirley; TE—Tehuantepec; TO— COR NFB Tongareva; WMG—West Magellan Trough. 20°S MEN 20°S Marginal basin extinct ridges (pink labels): OSB CAR—Caroline Basin ridge-jump (C10); DMP SFB EAS CBW—Caroline Basin west ridge-jump (chron C11); CEL—Celebes; COR—Coral SEL Sea; CT—Caroline Trough; DAM—Damar TO JF Basin; DMP—Dampier Ridge; GOM—Gulf TH of Mexico; KOM—Komanorsky; KT—Kils- gaard Trough; MAT—Mati; PAL—Palau; 40°S FRI 40°S PVS—Parece Vela-Shikoku; SCS—South TAS China Sea; SoJ—Sea of Japan; SRT— South Rennell Trough; SFB—South Fiji Ba- sin; TAS—Tasman Sea; TH—Three Kings Ridge; WPH—West Philippine. Active ridges (orange labels): EAS—Easter microplate; EPR—East Pacific Rise; JDF—Juan 60°S HUD 60°S de Fuca; JF—Juan Fernandez microplate; MT—Mariana Trough; NFB—North Fiji PAC-ANT Basin; PAC-ANT—Pacific Antarctic ridge; RIV—Rivera. AD

120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W 80°W

GEOSPHERE | Volume 13 | Number 3 MacLeod et al. | Global review and digital database of large-scale extinct spreading centers Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/3/911/2539132/911.pdf 913 by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/3/911/2539132/911.pdf Research Paper Basin; GRE—Grenada Basin; NF—North Falkland; SAO—Sao Paulo; VE—Vema microplate. Active ridge (orange label): S.MAR—Southern Mid-Atlantic ridge. Figure 1 ( continued 10°N 10°S 20°N 20°S B 30°N 30°S 0° 40°N 40°S 50°S 70°W 70°W ). (B) Proposed extinct ridges in the Atlantic Ocean. Abbreviations: AB—Abimael Ridge; AG—Agulhas Ridge; ANG—Angolan Basin; BI—Bay of Biscay; LIG—Ligurian GRE 60°W 60°W 50°W 50°W AB VE 40°W 40°W SA NF O 30°W 30°W 20°W 20°W S.MAR 10°W 10°W BI 0° 0° ANG 10°E LI G 10°E 20°E 20°E AG 50°S 40°N 40°S 30°N 30°S 20°S 20°N 10°S 10°N 0°

GEOSPHERE | Volume 13 | Number 3 MacLeod et al. | Global review and digital database of large-scale extinct spreading centers 914 Research Paper

C 40°E 60°E 80°E 100°E 20°N 20°N GOP LAX

EFE

0° WH 0° WSB Figure 1 (continued). (C) Proposed extinct ridges in the Indian Ocean. Abbre- viations: BR—Bruce Rise; CON—Con- rad–Del Cano; COS—South Conrad; CUV—Cuvier Abyssal Plain; DHR— Dirk Hartog Ridge; DIA—Diamantina GAS fracture zone; EFE—Eighty Five East Ridge; END—Enderby Basin; GAS— MAM Gascoyne Abyssal Plain; GOP—Gop SOJ SON Basin; GNT—Gonneville Triangle; 20°S 20°S LAX—Laxmi Basin; MAM—Mammer- WA ickx; MSC—Mascarene Basin; NTB— CUV NTB MSC Northern Natal Basin; SON—Sonne Perth AP 1 Ridge; SOJ—Sonja Ridge; WA— Wallaby Ridge; Perth AP—Perth Abys- 4 2 sal Plain, four proposed placements (numbered 1–4); WH—Wharton Ba- DHR 3 sin; WSB—West Somali Basin. Active ridges (orange labels): SEIR—South- GNT east Indian ridge; SWIR—Southwest DIA Indian ridge.

40°S SWIR 40°S

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Figure 1 (continued). (D) Proposed extinct ridges in the Scotia Sea and neighboring marginal basins. Abbreviations: DOV—Dove Basin; JB—Jane Basin; PHX—Phoenix (Pacific Ocean); POW— Powell Basin; PR—Protector Ridge; WE—Weddell Sea; WSR—West Scotia Ridge.

Dual spreading also results in independent rotation of the microplate, with a Investigations of present-day active microplates suggest that they are likely number of studies relating this motion to drag that develops between the major to be short-lived tectonic features that may play a role in accommodating plate and the microplate (Engeln et al., 1988; Naar and Hey, 1991; Schouten et al., changes in relative plate motions between major plates (Engeln et al., 1988; 1993). If spreading ceases at the preexisting ridge, the microplate, along with its Schouten et al., 1993; Cuffaro and Jurdy, 2006) or to facilitate major ridge- extinct ridge, will be captured by the neighboring plate, and spreading continues jumps (Naar and Hey, 1991). Well-developed microplate spreading centers at the alternative bounding ridge, which now forms a continuous, large-scale frequently demonstrate fanning, mostly symmetric, spreading about a central plate boundary for the major plate (Tebbens and Cande, 1997; Hey, 2004). For spreading system and a clear spreading axis may be evident (e.g., the Magel- this reason, extinct microplates typically preserve only one spreading center. lan Trough, Tamaki and Larson 1988; the Galapagos Rise/Bauer microplate,

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Figure 1 (continued). (E) Proposed extinct ridges in the Arctic. Abbreviations: AEG—Aegir Trough; BF—Baffin Bay; CAN—Canada Basin; KOM—Komandorsky; LAB—Labrador Sea.

Eakins and Lonsdale, 2003; and the Mathematician ridge, Batiza and Vanko, spreading ridge and the Juan de Fuca spreading ridge, while extinct examples 1985). Microplate spreading centers can be extremely complex and evolve rap- include the Magdalena (Michaud et al., 2006), Guadalupe (Mammerickx and idly (Naar and Hey, 1991); therefore, identification of their extinct spreading Klitgord, 1982; Batiza and Vanko, 1985; Michaud et al., 2006), and Monterey axes can be more complex than for large-scale spreading systems. (Lonsdale, 1991) ridges, all situated offshore the western margin of the North We also identify a class of short-lived spreading centers that are associated America plate. with smaller plates (less than 1000 km in width) that are proposed to have Although several published regional reviews of extinct spreading centers formed by the mechanism of plate fragmentation as a downgoing oceanic (Mammerickx et al., 1980; Mammerickx, 1981; Batiza, 1989; Lonsdale, 2005) plate enters a subduction zone at an oblique angle (Lonsdale, 1991, 2005). We have not included extinct backarc basin spreading ridges, there are several refer to these ridges as extinct fragmented plate spreading ridges (XFPR). Pres- well-defined preserved examples. Stern and Dickinson (2010) summarized ent-day examples of fragmented plate ridges (FPR) include the Rivera plate active and extinct backarc basins suggesting there are 18 extinct examples.

TABLE 1A. WELL-DEFINED PRIMARY- TIER EXTINCT RIDGES Half-spreading rate Duration active ID# Ridge nameOcean Ridge type (mm yr–1) (m.y.)Reported time of cessation, youngest identified magnetic anomaly, and reference 1.01 Aegir Ridge Atlantic XMOR 8–13* 14 28.2 Ma, chron C10n (Gernigon et al., 2012) 1.02 Baffin Bay Ridge Atlantic XMOR 226 33.5 Ma, chron C13 (Jackson et al., 1979) 1.03 Labrador Sea ridge Atlantic XMOR 2.7 48 33.5 Ma, chron C13 (Srivastava, 1978) 1.10 Agulhas Ridge Atlantic XMOR 15 22 Chron C27o (Marks and Stock, 2001) 2.01 West Somali Basin ridge Indian XMOR 22 39 Either M0 (120.6 Ma) (Cochran, 1988) or M10 (ca. 130 Ma) (Rabinowitz et al., 1983) 2.03 Mascarene Basin Indian XMOR 80 23 62 Ma, chron 27n (Bernard and Munschy, 2000) 2.10 Wallaby Ridge Indian XMOR 32 24 Possibly ca. 100 Ma (Mihut and Müller, 1998) 2.12 Wharton Ridge Indian XMOR 20 37 38 Ma, chron C18 (Hébert et al., 1999) 3.01 Adare Trough Pacific XMOR 613 27 Ma, chron C9n (Cande et al., 2000) 3.03 Osbourn Trough Pacific XMOR 45 34 Estimated at 86 Ma, during Cretaceous Normal Superchron (Chandler et al., 2012) 3.15 Magellan Trough Pacific XMPR 11–45* 15 Ca. 128 Ma, after M9 (Tamaki and Larson, 1988) 3.16 West Magellan Ridge Pacific XMPR 10.5 9 Ca. 134 Ma, M14 (Nakanishi and Winterer, 1998) 3.18 Guadalupe microplate ridge Pacific XMPR 50 4 11 Ma, chron C5AB (Michaud et al., 2006) 3.20 Mathematician microplatePacific XMPR 35–50* 10 Between 5 and 3.5 Ma, chron C3n4n (Batiza and Vanko, 1985) 3.22 Galapagos Rise Pacific XMPR 30–40* 13 Ca. 6 Ma, after chron 3A (Eakins and Lonsdale, 2003) 3.24.1 Selkirk microplatePacific XMPR 20 5 20.8 Ma, anomaly C6A.r (Blais et al., 2002) 3.26 Sandra Ridge (Panama Basin) Pacific XMOR 12 10 Ca. 8 Ma, C4 (Lonsdale, 2005) 3.30 Malpelo Ridge Pacific XBABR 15 5 9 Ma, C5n (Lonsdale, 2005) 3.32 Phoenix RidgePacific XMOR 13 15 3.3 Ma, C2A (Livermore et al., 2000) 3.35 Mendoza ridge-jumpPacific XMOR 72 25 Estimated at 18.5, after anomaly C7 (Mammerickx et al., 1980) 4.01 South China Sea ridge Marginal XMOR 18 15 Ca. 15 Ma, after C5c (Briais et al., 1993) 4.02 West Philippine Sea ridge Marginal XBABR 60 28 36 Ma, C16r (Sasaki et al., 2014) 4.05 Parece Vela–Shikoku RidgeMarginal XBABR 3.5–7 13 18 Ma, after C5Dy (Sdrolias et al., 2004) 4.07 Kilsgaard Trough Marginal XBABR 55 9 26 Ma, after C8r (Gaina and Müller, 2007) 4.12 Canada Basin Marginal XMOR 13 33 Ca. 127 Ma, M12 (Taylor et al., 1981) 4.14 Tasman Sea ridge Marginal XMOR 16 33 52 Ma, C24 (Gaina et al., 1998) 4.15 Coral Sea ridge Marginal XBABR 24 9 Ca. 52 Ma, C24o (Gaina et al., 1999) 4.16 West Scotia Ridge Marginal XMOR 16 19 Ca. 6 Ma, after C3A (Maldonado et al., 2000; Lodolo et al., 2006) 4.22 South Fiji Basin ridge Marginal XBABR 26 7 After 24 Ma, C7no (Sdrolias et al., 2003) Notes: #ID numbers refer to the reference used in accompanying Supplemental Materials (see text footnote 2) provided in the GPlates Portal website (text footnote 1). Abbreviations: XBABR—extinct backarc spreading ridge; XMOR—extinct large-scale mid-ocean ridge; XMPR—extinct microplate spreading ridge. *A range of spreading rates is shown where the spreading rate varied along the length of the spreading axis.

However, a number of these extinct backarc basins have little data available Outstanding Problems and Questions (Stern and Dickinson, 2010), and in some cases, they do not have a preserved spreading center, particularly where much of the crust has been subsequently In addition to considering if a characteristic extinct spreading center mor- destroyed at a proximal subduction zone (Schellart et al., 2006). The most phology and gravity signal can be described, our classification of different closely studied extinct backarc basin spreading center (XBABR) is the Parece spreading center subtypes allows us to evaluate how the tectonic setting of Vela–Shikoku Ridge (Fig. 2C-i), which extends the length of the Shikoku and a spreading center influences its physical characteristics, for both active and Parece Vela basins for ~2500 km and is thought to have ceased spreading at extinct examples. This permits a comparison between these two groups and ca. 15 Ma, after chron C5En (Mrozowski and Hayes, 1979; Chamot-Rooke et al., can offer an insight as to whether the final stages of spreading are likely to be 1987; Sdrolias et al., 2004). Other examples are noted close to the Antarctic primarily magmatic or tectonically driven. Further, in cataloguing all known Peninsula (Jane Basin, Powell, Dove, and Protector Basins), in the South Fiji examples of large-scale extinct spreading centers, we evaluate the spatial and Basin (Fig. 2C-ii), and in the West Philippine Basin (Fig. 2). We review the char- temporal distribution of ridge-jumps to consider why some regions have a acteristics of preserved backarc basin spreading centers and consider how greater or lesser number of ridge reorganizations. For example, why are there these may differ from other ridge subtypes. so many extinct ridges in the Pacific Ocean but few reported in the Atlantic?

TABLE 1B. SECONDARY- TIER EXTINCT RIDGES Pre-extinction Duration Ridge half-spread rate active ID# Ridge nameOcean subtype (mm yr–1) (m.y.)Reported time of cessation, youngest identified magnetic anomaly, and reference 1.05 Bay of BiscayAtlantic XMOR 532 Ca. 73 Ma, chron C32n.2n (Williams, 1975) 1.06 Angolan Basin, West Africa Atlantic XMOR ––Not constrained by available data (Sandwell et al., 2014) 1.07 Vema microplateAtlantic XMPR –11 92.5 Ma, after chron C34 (Pérez-Díaz and Eagles, 2014) 1.08 Abimael Ridge Atlantic XMOR –22 110 Ma, after chron C34 (Scotchman et al., 2010) 1.08 Sao Paulo Atlantic XMOR –10 Either 120.6 or 124 Ma (Cande and Rabinowitz, 1978) 1.09 North Falkland Atlantic XMOR ––Not constrained but possibly ca. 90 Ma 2.02 N. Natal Basin Indian XMOR 13 16 Around the time of M2, ca. 124 Ma (Tikku et al., 2002) 2.04 Conrad-Del Cano Indian XMOR –27 Ca. 79 Ma, formed after C33n.o (based on anomalies identified by Segoufin et al., 2004) 2.05 South Conrad Rise Indian XMOR –8Not constrained by available data 2.07 Perth AP–1* Indian XMOR –25 Ca. 105 Ma, during CTQZ (Mihut and Müller, 1998) 2.07 Perth AP–2* Indian XMOR 24 28 Close to 102 Ma (Watson et al., 2016) 2.07 Perth AP–3* Indian XMOR 35 9 120 Ma, M0 (Williams et al., 2013) 2.07 Perth AP–4* Indian XMOR 21 50 77 Ma, (Markl, 1974) 2.07 Dirk Hartog Ridge Indian XMOR 24 28 Ca. 102 Ma, during CTQZ (Watson et al., 2016; Whittaker et al., 2013) 2.08 Gonneville Tr iangle Indian XMOR –22 Possibly ca. 90 Ma, during the CNS 2.08 Diamantina fracture zone Indian XMOR 25 54 Proposed as 64 Ma (Munschy, 1998) 2.09 Sonne Ridge Indian XMOR 40 9 126 Ma, M4 (Mihut and Müller, 1998) 2.09 Sonja Ridge Indian XMOR 30 6 124 Ma, M3 (Robb et al., 2005) 2.09 Cuvier Abyssal Plain (Gibbons placement) Indian XMOR 35 7 125 Ma, M3 (Gibbons et al., 2012) 2.11 Gascoyne AP (Robb) Indian XMOR 38 11 Proposed at 121 Ma, M0 (Robb et al., 2005) 2.11 Gascoyne AP (Fullerton)* Indian XMOR 35 5 Proposed at 127 Ma, M5 (Fullerton et al., 1989) 2.14 Laxmi Basin Indian XMOR 219 63 Ma, anomaly 28 (Bhattacharya et al., 1994) 2.15 Gop Basin Indian XMOR 614 56 Ma, chron C25r (Yatheesh et al., 2009) 2.16 Mammerickx microplate Indian XMPR ––After anomaly C21n, around 47 Ma (Matthews et al., 2016) 3.05 Ellice BasinPacific XMOR 42 9 Estimated at 86 Ma, during CNS (Chandler et al., 2012) 3.10 Hokkaido Trough Pacific XMOR 40 3 Late in CTQZ, possibly ca. 84 Ma (Mammerickx and Sharman, 1988) 3.12 Chinook Trough Pacific XMOR 940 Ca. 79 Ma, C33 (Mammerickx and Sharman, 1988) 3.13 Liliuokalani Ridge Pacific XMOR –21 Late CTQZ (Atwater et al., 1993) 3.19 Magdalena Ridge–option 1*Pacific XMPR 51 5 Ca. 10 Ma, chron C4 (Lonsdale, 1991) 3.19 Magdalena Ridge–option 2*Pacific XMPR 68Ca. 7 Ma (Michaud et al., 2006) 3.21 Tehuantepec Pacific XMPR 60 7 After C5, ca. 7 Ma (Manea et al., 2005) 3.23 Early EPR, west of Bauer Pacific XMOR 26 11 After 6 Ma, C3An1n (Eakins and Lonsdale, 2003) 3.24 Selkirk Trough Pacific XMPR –7Estimated at 23 Ma, chron 6C (Tebbens and Cande, 1997) 3.29 Nazca propagator Pacific XMOR –1–2 After 2 Ma, C2n (Klein et al., 2005) 3.31 Friday microplatePacific XMPR 26 13 12.6 Ma, C5A (Tebbens and Cande, 1997) 4.04 Mati Ridge Marginal XBABR ––Not constrained by available data 4.04 East Palau Basin Marginal XBABR ––Not constrained by available data 4.06 Caroline Trough Marginal XBABR 35 14 26 Ma, after C8r (Gaina and Müller, 2007) 4.06 Caroline Basin (C10 jump) Marginal XBABR 35 11 29 Ma, after C10 (Gaina and Müller, 2007) 4.09 Gulf of Mexico ridge Marginal XMOR –25 145 Ma, no anomalies identified (Ross and Scotese, 1988) 4.17 Protector Basin Atlantic XBABR –10 Controversial, ~C7 or C5 (Hill and Barker, 1980) or C11 (ca. 30 Ma) or C19 (Eagles et al., 2006) 4.18 Dove Basin Atlantic XBABR 321 Possibly 34.7 Ma, chron C15 (Eagles et al., 2006) 4.19 Weddell Sea Atlantic XMOR 50 5 After 130 Ma, M10Nn.1n (Kovacs et al., 2002) Controversial, possibly 23 Ma (King and Barker, 1988), 29 Ma (King et al., 1997), or 21. 8, 4.20 Powell BasinAtlantic XBABR –8chron C6AA (Eagles and Livermore, 2002) 4.21 Jane Basin Atlantic XBABR –3Ca. 15 Ma (Maldonado et al., 1998) 4.24 South Rennell Trough Marginal XMOR ––Ca. 28 Ma (Mortimer et al., 2014) Notes: #ID numbers refer to the reference used in accompanying Supplemental Materials (see text footnote 2) provided in the GPlates Portal website (see text footnote 1). *Several proposed locations for extinct ridge axes are assessed from the Perth Abyssal Plain and for the Magdalena spreading center. Abbreviations: AP—abyssal plain; CNS—Cretaceous Normal Superchron; CTQZ—Cretaceous Quiet Zone; EPR—East Pacific Rise; XBABR—extinct backarc spreading ridge; XMOR—extinct large-scale mid-ocean ridge; XMPR—extinct microplate spreading ridge; – information not available.

TABLE 1C. POORLY CONSTRAINED EXTINCT RIDGES (EXCLUDED FROM STATISTICAL ANALYSES) Half-spread rate Duration active ID# Ridge nameOcean Ridge type (mm yr–1) (m.y.) Reported time of cessation, youngest identified magnetic anomaly, and reference 2.06 Enderby Basin Indian XMOR 30 10 Ca. 120 Ma, M0 (Gibbons et al., 2012) or 124 Ma, M2 (Gaina et al., 2007) 2.06.2 Bruce Rise Indian XMOR 30 8 Ca. 120 Ma, M0 (Gibbons et al., 2012) or 124 Ma, M2 (Gaina et al., 2007) 2.13 Eighty Five East Ridge Indian XMOR ––Uncertain, possibly ca. 120 Ma (Mishra, 1991) 3.02 Dampier RidgePacific XMOR –0No estimate of age provided (van der Linden, 1969) 3.06.1 Nova Canton Trough Pacific XMOR –37 Estimated at 110 Ma, after anomaly M1 (Winterer, 1991) 3.06.3 Ontong JavaPacific XMOR –40 Estimated at 86 Ma, during CNS (Chandler et al., 2012) 3.07 Penrhyn Basin Pacific XMOR –10 Before 105 Ma (Larson et al., 2002) 3.08 Manihiki Pacific XMOR –10 Estimated age, 83 Ma (Müller et al., 2016) 3.09 TongarevaPacific XMOR –34 Proposed as 86 Ma (Seton et al., 2012) 3.11 Kula Ridge Pacific XMOR 22 57 Ca. 40 Ma, anomaly C18r (Lonsdale, 1988) 3.14 Emperor fracture zone Pacific XMOR –28 Unlikely to be an extinct ridge, formed during CNS 3.25 Shirley ridge-jumpPacific XMPR –425 Ma, C7A (Lonsdale, 2005) 3.27 Carnegie RidgePacific MOR 30 6 2 Ma, C2n (Anderson et al., 1976) 3.28 Cocos RidgePacific MOR 20 8 14.7 Ma, C5ADr (Meschede et al., 1998) 3.33 Hudson platePacific XMPR –10 46 Ma, C21 (Cande et al., 1982) 3.34 Monterey fracture zone ridge Pacific XMPR –520 Ma, after anomaly C6 (Lonsdale, 1991) 3.36 Gallego ridge-jumpPacific XMOR 70 15 Estimated 15 Ma (Okal and Bergeal, 1983) 3.37 Roggeveen ridge-jumpPacific XMOR ––Poorly constrained (Okal and Bergeal, 1983) 4.01.2 N. South China Sea ridge-jumpMarginal XMOR 22 7 Ca. 28 Ma, after C10 (Briais et al., 1993) 4.03 Celebes Sea Marginal XBABR 20 9 Ca. 39 Ma (Beiersdorf et al., 1997) 4.06.3 Caroline Basin West (C11 jump) Marginal XBABR 35 7 33 Ma, after C11 (Gaina and Müller, 2007) 4.08 Damar Basin Marginal XBABR 28 4 3 Ma, C2An3n (Hinschberger et al., 2001) 4.10 Grenada Basin Marginal XBABR 30 21 Poorly constrained, ca. 35 Ma (Speed and Walker, 1991) 4.11 Komandorsky Ridge Marginal XBABR –17 Estimated to be 25 m.y. old (Baranov et al., 1991) 4.23 Three Kings Ridge Marginal Not applicable ––Unlikely to be an extinct ridge, probably formed ca. 35 Ma (Sdrolias et al., 2003) 4.25 Sea of Japan Marginal XBABR –?13 Poorly constrained, thought to have been active 25–12 Ma (Kimura et al., 2005) 4.26 Ligurian Basin Marginal XBABR –15 Poorly constrained (proposed by Rollet et al., 2002) Notes: #ID numbers refer to the reference used in accompanying Supplemental Materials (see text footnote 2) provided in the GPlates Portal website (text footnote 1). Abbreviations: CNS—Cretaceous Normal Superchron; XBABR—extinct backarc spreading ridge; XMOR—extinct large-scale mid-ocean ridge; XMPR—extinct microplate spreading ridge.

What factors determine the frequency of ridge reorganizations? Is the number Guadalupe Island (Batiza, 1977; Batiza and Vanko, 1985), the eastern South China of extinct ridges proportional to ocean floor area? By compiling the spread- Sea ridge (Pautot et al., 1990), the Styx volcano on the Wharton Ridge (Hébert ing rate at extinct ridges prior to cessation, there is also potential to investi- et al., 1999), the Phoenix Ridge (Haase et al., 2011a), the Socorro Islands on the gate the relationship between regional spreading rates and the likelihood of Mathematician Ridge (Favela and Anderson, 2000), and the Central Ridge of ridge-jumps. the Galapagos Rise (Haase et al., 2011b). At many of these ridges, the volcanic We consider what factors drive the variability in behavior and life span of edifice was formed after cessation of spreading. Where postcessation volcanic spreading-ridge subtypes. Stern and Dickinson (2010) observed that generally rocks from seamounts seated on extinct ridge segments have been analyzed, backarc basin ridges are short-lived relative to large-scale MORs and proposed they have been found to be alkali basalts or alkali-olivine basalts (Batiza, 1977; that as the backarc basin widens, the availability of melt sourced from the hy- Batiza and Vanko, 1985; Pautot et al., 1990; Haase et al., 2011a) that are sugges- drated mantle wedge to the spreading axis is reduced and that this leads to tive of fertile mantle parental magmas produced with a low degree of melting. eventual death of the ridge. By reviewing all reported examples of both types Thinned crust in the region of an extinct ridge has been proposed to present of extinct spreading centers and active examples, we evaluate if there is any a conduit for volcanism (Favela and Anderson, 2000) due to locally elevated morphological or geological evidence to support this proposal. mantle temperatures and potentially hydrated mantle conditions where hydro- Finally, we investigate the occurrence of late-stage and postcessation vol thermal activity has penetrated the upper mantle. This could effectively drive canism that has been observed at a number of extinct ridge segments. The postcessation volcanism but is expected to be limited to a short time frame after best studied examples of volcanic ridges or seamounts at extinct ridge axes are cessation and would likely require a tensional environment (Favela and Ander-

TABLE 1D. ACTIVE SPREADING RIDGES ASSESSED FOR COMPARISON Half-spread Estimated duration rate Anomaly used to calculate active thus far Oldest identified magnetic Ref. ID# Ridge nameOcean Ridge-type^ (mm yr–1) spreading rate and reference (m.y.) anomaly from spreading center@ AR-1Southern Mid-Atlantic ridge Atlantic MOR 16.6 ± 3.2 Chron A2A, 3.16 Ma, DeMets et al. (2010) 131. 91 M11 An 1n AR-2Southwest Indian ridge Indian MOR 7.1 ± 1.5 Chron A2A, 3.16 Ma, DeMets et al. (2010) 64.75 C29n (in present configuration) AR-3Southeast Indian ridge Indian MOR 35 ± 5.8 Chron A1n 0.78 Ma, DeMets et al. (2010) 40.0 C18n 2n AR-4Pacific-Antarctic ridge* Pacific MOR 35* ± 12.7 Chron A1n 0.78 Ma, DeMets et al. (2010) >83C34n AR-5East Pacific Rise (northern section)* Pacific MOR 50* ± 20.4 Chron A1n 0.78 Ma, DeMets et al. (2010) 10.949 C5n 2n AR-6Juan de Fuca ridge* Pacific FPR 26 ± 2.7 Chron A1n 0.78 Ma, DeMets et al. (2010) 6.6C3An 2n AR-7Rivera plate ridge* Pacific FPR 30* ± 13.5 Chron A1n 0.78 Ma, DeMets et al. (2010) 9.92 C5n 2n AR-8Easter microplate* Pacific MPR 40* ± 25 Naar and Hey (1991) 3.87 C2A AR-9Juan Fernandez* Pacific MPR 38* ± 28 Pardee et al. (1998) 3.87 C2A AR-10 Mariana Trough* Marginal BABR 18* ± 8.5 Taylor and Martinez (2003)9.0 Seton et al. (2012) AR-11 North Fiji Basin—Central ridge* Marginal BABR 33* ± 16.5 Gràcia and Escartin (1999) 3.5 Gràcia and Escartin (1999) AR-12 North Fiji Basin—South Pandora ridge Marginal BABR 8Gràcia and Escartin (1999) 7Gràcia and Escartin (1999) AR-13 West Fiji ridge Marginal BABR 17.5 Auzende et al. (1994) 1. 5C1n, Auzende et al. (1994) AR-14 South Sandwich ridge Marginal BABR 33 ± 5.2 Chron A1n 0.78 Ma, DeMets et al. (2010) 15 Cn5B, Livermore (2003) #ID numbers provide the reference used in accompanying Supplemental Materials (.ZIP File S5 that contains the maps and profiles generated for active ridge segments; see text footnote 2) and the index on the GPlates Portal website (text footnote 1). *Indicates high variability in spreading rates between ridge segments (>10 mm difference), and the spreading rate given here is an average value. For our quantitative assessments, we use different spreading rates for individual segments of these systems, and these are provided in Table S1B (text footnote 2). ^Abbreviations used for ridge subtypes: BABR—backarc basin spreading ridge; FPR—fragmented plate spreading ridge; MOR—large-scale mid-ocean ridge; MPR—microplate spreading ridge. @Magnetic anomalies are identified in Seton et al. (2014) digital dataset, unless otherwise stated.

son, 2000). We evaluate whether there are any factors in common between the version 20141103) and global gravity and vertical gravity gradient (VGG) data examples of late-stage and postcessation volcanism and what insights can be sets (Sandwell et al., 2014). These data sets have been used in numerous pre- gained by a wider comparison with active spreading ridges. vious investigations of seafloor tectonic structures (e.g., Matthews et al., 2011; Sandwell et al., 2014), and global gravity grids can resolve features as small as 6 km in size (Sandwell et al., 2014). The GEBCO 2014 bathymetry data set is METHODS compiled from a combination of available depth soundings, ship-track data, and satellite-altimetry–derived data and is estimated to be able to resolve fea- Assessment of Axial Physical Characteristics tures of 12 km width at a depth of 4000 m (Weatherall et al., 2015), with resolution decreasing with increased depth. Global data sets were used in preference We compile data from extinct ridges that have been proposed previously, to ship-track data because it is possible to extract a greater number of profiles including moderate to large-scale tectonic features but excluding ridge-jumps for each ridge segment from the gridded data and to have a more consistent of less than 150 km in distance. This cut-off distance is employed to focus on approach between areas with highly variable coverage by marine surveys. A the larger-scale migration events that are most important for incorporation in compilation of magnetic anomaly identifications (Seton et al., 2014) was used regional and global reconstructions, within the bounds of the resolution of the to confirm that symmetric magnetic anomalies have been identified about gridded data sets that we use and as a practical limit for the study, given the extinct ridge axes. To better understand the characteristics of the axial struc- large number of proposed extinct ridges (~100 examples globally). We group ture of spreading centers, spreading ridge axes were digitized in continuous the ridges into three tiers based on how well defined they are, as outlined in segments, bounded by offsets of more than 10 km length. The morphology detail in Table 2A. The well-defined extinct ridges in the primary group (Table (bathymetric relief) and gravity signal of extinct ridges are evaluated using 1A) are used to define the characteristic signal of the extinct ridges. Ages are two-dimensional profiles across ridge axes (Fig. 3). We provide an online data given based on the timescale by Gee and Kent (2007). base of the digitized extinct ridge locations and profiles in the GPlates Portal Extinct ridges were digitized from published studies, and locations were re- (see footnote 1), along with summaries of our compiled data and key studies fined using updated bathymetry data (Weatherall et al., 2015, GEBCO_2014_1D, in these areas.

A10°E 15°E 20°E 60°W 55°W 50°W 45°W 40°W 155°E 160°E 50°E 55°E 60°E 10°S 40°S 40°S

60°N 15°S

45°S 45°S 20°S

55°N 25°S

i. Agulhas Ridge ii. Labrador Sea extinct ridge iii. South Tasman Sea ridge iv. Mascarene Basin ridge

−16 −12 −8 −4 0481216 B Vertical Gravity Gradient (Eötvös) 120°W 118°W 116°W 175°E 180° 115°W 110°W 128°W 126°W 124°W 122°W 120°W 30°N 15°N

20°N 32°S

34°S 28°N 10°N

15°N 36°S

26°N 5°N 38°S i. Guadalupe extinct ii. West Magellan microplate iii. Mathematician microplate iv. Selkirk Microplate fragmented plate spreading spreading ridge spreading ridge spreading ridge ridge

−16 −12 −8 −4 0481216 Vertical Gravity Gradient (Eötvös)

Figure 2 (on this and following page). Selected examples of well-defined extinct spreading centers are shown with inferred positions of axial segments as thin red lines on maps of the vertical gravity gradient (VGG) grid, version 23.1 of Sandwell et al. (2014). (A) Type examples of large-scale extinct mid-ocean ridges (XMORs). (B) Type example of an extinct fragmented plate spreading ridge (XFPR) (i) and extinct oceanic microplate spreading ridges (XMPR) (ii–iv).

C 135°E 140°E 170°E 175°E 180° 125°E 130°E 135°E

20°S

20°N

25°S 20°N

15°N 30°S

15°N

35°S i. Parece Vela-Shikoku Basin ii. South Fiji Basin iii. West Philippine Sea extinct spreading ridge extinct spreading ridge extinct spreading ridge

−16 −12 −8 −4 0481216 Vertical Gravity Gradient (Eötvös)

Figure 2 (continued). (C) Type examples of extinct backarc basin spreading ridges (XBABR).

TALE 2A. CRITERIA SED TO RANK EXTINCT RIDGES Tier Primary: well-defined Secondary: moderately well constrained Excluded: poorly constrained Magnetic anomalies Symmetric magnetic anomalies identified either side of ridge, with Anomalies identified on one flan only or in No identified magnetic anomalies in the region of the youngest anomaly at center. surrounding region. proposed extinct ridge. athymetric and Linear trend of the former axis was visible in surface, global gridded Ridge-lie structure evident in global datasets Ridge morphology is not evident. gravity signals datasets ICO-ICH GECO20141D, 1D; Sandwell et al., 2014, yet not uneuivocally interpreted as an extinct Or: Gravity 23.1 and GG 23.1. ridge. No linear gravity signal present. Fracture one trends are consistent with the interpretation of an extinct Or: ridge oriented perpendicular to the ridge, parallel to the inferred Structure inconsistent with trend of fracture ones, for direction of spreading. inferred direction of spreading. Symmetric gravity anomaly evident in profile. OtherConsensus of published studies of formation as a former spreading Alternative modes of formation have been Published studies have discounted extinct-ridge formation. center. proposed for example, pseudo-fault or Or: transform fault origin. Oceanic structures are thought to be ridge-umps of less than 150 m in distance.

A 50°E 52°E 54°E 56°E B 50°E 52°E 54°E 56°E Figure 3. Illustration of the methodology used to assess axial character- 22°S 22°S istics of extinct spreading center segments, showing an example for a southern segment of the Mascarene Basin extinct ridge. (A) Shows the coordinates used to define the end points of the spreading segment axis Cross-profiles (black-lines) (red circles). The paleospreading direction is indicated by black arrows in the direction of opening and is inferred from fracture zone orientation. (B) The axial-segment is defined (red solid line) between the end points (red circles). Cross profiles were generated perpendicular to the axial seg- Representative profile ment and are spaced every 10 km along axis (black lines). Bathymetry and 24°S AxialAxial 24°S (pink(pink linee) gravity data sets are sampled every 1 km along the profiles and the total sesegmentgment profile length is 120 km for display purposes in C and D. The representative profile (pink solid line) is used to calculate the axial signal within DefinedDefined segmsegmentent DefinedDefined axial 30 km of the ridge axis. (C) The bathymetry data set (GEBCO_2014_1D) end-point segment (red(red linee)) is sampled by all cross profiles, and profiles are stacked with the median (r(reded circcircle)lee)) value displayed as the solid black line and the envelope of deviation from Paleo-spreadingPaleo-spreading the median enclosed in the region colored in red. Relief in the region of ddirectionirection ddeterminedetermined frfromom ffractureracture zozonesnes the axis is assessed from the representative profile, within 30 km either 26°S 26°S side of the axis, indicated by the region bounded by dashed orange lines at distances –30 km and 30 km. The minimum depth is given by a white Bathymetry (GEBCO_2014_1D) stacked profiles Gravity (Sandwell et al. 2014,. v.23.1) stacked profiles filled circle labeled “MIN” and the maximum depth is given by a filled yel- C across axial segment D across axial segment low circle labeled “MAX.” The difference between the minimum and maxi −4000 MAX 40 MAX mum values gives the measure of bathymetric relief in the axial region 30

and the distance between these two points is taken to represent the half- ) −4500 20 width of the axial structure. (D) Similar to panel C; however, the profiles 10 sample the gravity data set (Sandwell et al., 2014), and the peak-to-trough −5000 Relief 0 −10 Peak to trough

gravity signal is calculated from the difference between the minimum and Depth (m signal −5500 MIN −20 MIN maximum values, with the distance between these points taken to repre- Half-width −30 Half-wavelength sent the half-wavelength of the gravity signal. −40

−6000 Gravity anomaly (mGal ) −60−40 −200 20 40 60 −60−40 −200 20 40 60 Distance from ridge (km) Distance from ridge (km) Profile Legend: Median stacked profile Envelope of deviation from the median value Details of the spreading rate calculaon (from Ridge Segment ID Ridge name Segment Name Time of cessaon and reference Tier Ocean referenced publicaon) subtype (1.4826 x median-absolute deviation) 1_1AbimaelAbimael Ridge segment 1 110 Ma, a er chron C34 (Scotchman et al., 2010) -SECONDARY AtlancXMOR 2_1Adare Adare Trough segment 1 27 Ma, chron C9n (Cande et al., 2000) PRIMARYPacificXMOR Spreading rate modelled between anomaly Representative profile (total length 60 km) 2_2Adare Adare Trough segment 2 27 Ma, chron C9n (Cande et al., 2000) PRIMARYPacificXMOR C17 to C9 (13 Myr mespan) 2_3Adare Adare Trough segment 3 27 Ma, chron C9n (Cande et al., 2000) PRIMARYPacificXMOR 3_1Aegir Aegir Ridge segment 1 28.2 Ma, chron C10n (Gernigon et al., 2012) PRIMARYAtlanc XMOR Pre-exncon spreading rate calculated from Vertical lines bound the length of the profile that is 3_2Aegir Aegir Ridge segment 2 28.2 Ma, chron C10n (Gernigon et al., 2012) PRIMARYAtlanc XMOR C13 to C10 (mespan 5.7 Myr) 3_3Aegir Aegir Ridge segment 3 28.2 Ma, chron C10n (Gernigon et al., 2012) PRIMARYAtlanc XMOR Minimum value within 30 km of axis 5_1 Agulhas Agulhas Ridge segment 1 Chron C27o (Marks and Stock, 2001) PRIMARYAtlanc XMOR 5_2 Agulhas Agulhas Ridge segment 2 Chron C27o (Marks and Stock, 2001) PRIMARYAtlanc XMOR 5_3 Agulhas Agulhas Ridge segment 3 Chron C27o (Marks and Stock, 2001) Pre-exncon spreading rate calculated from PRIMARYAtlanc XMOR used to assess the axial signal. 5_4 Agulhas Agulhas Ridge segment 4 Chron C27o (Marks and Stock, 2001) C31o to C27y (mespan 7.5 Myr) PRIMARYAtlanc XMOR 5_5 Agulhas Agulhas Ridge segment 5 Chron C27o (Marks and Stock, 2001) PRIMARYAtlanc XMOR 5_6 Agulhas Agulhas Ridge segment 6 Chron C27o (Marks and Stock, 2001) PRIMARYAtlanc XMOR Maximum value within 30 km of axis 6_1BaffinBaffin Bay ridge segment 1 33.5 Ma, chron C13 (Jackson et al., 1979) PRIMARYAtlanc XMOR Visual representation of measured distance/signal. 6_2BaffinBaffin Bay ridge segment 2 33.5 Ma, chron C13 (Jackson et al., 1979) Pre-exncon spreading rate calculated from PRIMARYAtlanc XMOR 6_3BaffinBaffin Bay ridge segment 3 33.5 Ma, chron C13 (Jackson et al., 1979) C21 to C13 (mespan 14.9 Myr) PRIMARYAtlanc XMOR 6_4 Baffin Baffin Bay ridge segment 4 33.5 Ma, chron C13 (Jackson et al., 1979) PRIMARYAtlanc XMOR 7_1Bauer Bauer Scarp segment 1 A er 6 Ma, C3An1n (Eakins and Lonsdale, 2003) SECONDARYPacificXMOR Pre-exncon spreading rate calculated from 7_2Bauer Bauer Scarp segment 2 A er 6 Ma, C3An1n (Eakins and Lonsdale, 2003) SECONDARYPacificXMOR CA4A to C3An1n (mespan 3.4 Myr) 7_3Bauer Bauer Scarp segment 3 A er 6 Ma, C3An1n (Eakins and Lonsdale, 2003) SECONDARYPacificXMOR 8_1BiscayBay of Biscay segment 1 79 Ma, chron C33n.o (Williams, 1975) SECONDARYAtlanc XMOR Pre-exncon spreading rate calculated from 8_2BiscayBay of Biscay segment 2 79 Ma, chron C33n.o (Williams, 1975) SECONDARYAtlanc XMOR C33 y to C32n.2n (mespan 0.6 Myr) 9_1Biscay alt. Bay of Biscay (alternave placement79 Ma, ch ron C33n.o (Williams, 1975) SECONDARYAtlanc XMOR 10_1BruceRise Bruce Rise segment 1 124 Ma, M2 (Gaina et al., 2007) EXCLUDED Indian XMOR 10_2BruceRise Bruce Rise segment 2 124 Ma, M2 (Gaina et al., 2007) Pre-exncon spreading rate calculated from EXCLUDED Indian XMOR 10_3BruceRise Bruce Rise segment 3 124 Ma, M2 (Gaina et al., 2007) M4o to M2y (mespan 2.6 Myr) EXCLUDED Indian XMOR 10_4BruceRise Bruce Rise segment 4 124 Ma, M2 (Gaina et al., 2007) EXCLUDED Indian XMOR 11_1CanadaBasinCanada Basin segment 1 ~127 Ma, M12 Taylor et al., 1981). A single spreading rate is inferred for PRIMARYMarginalXMOR 11_2CanadaBasinCanada Basin segment 2 ~127 Ma, M12 (Taylor et al., 1981). anomaly sequence M25 - M12 (mespan 21.2 PRIMARYMarginalXMOR 2 11_3CanadaBasinCanada Basin segment 3 ~127 Ma, M12 (Taylor et al., 1981). Myr) . PRIMARYMarginalXMOR Following the same methodology, the axial characteristics of 14 active Materials . However, for statistical analyses, we compared a representative 12_1CarnegieCarnegie Ridge segment 1 2 Ma, C2n (Anderson et al., 1976) EXCLUDED Pacific XMOR 12_2CarnegieCarnegie Ridge segment 2 2 Ma, C2n (Anderson et al., 1976) Pre-ridge jump spreading rate calculated EXCLUDED Pacific XMOR 12_3CarnegieCarnegie Ridge segment 3 4 Ma, C3 (Anderson et al., 1976) from C4 to C2n (mespan 5.5 Myr) EXCLUDED Pacific XMOR 12_4CarnegieCarnegie Ridge segment 4 2 Ma, C2n (Anderson et al., 1976) EXCLUDED Pacific XMOR 13_1Caroline_C10Caroline Basin (C10) segment 1 29 Ma, a er C10 (Gaina and Muller 2007) SECONDARYMarginalXBABR spreading ridges are evaluated, to provide a representative sample of ridge sub- cross profile from each ridge segment, for 30 km each side of the inferred Pre-exncon spreading rate calculated from 13_2Caroline_C10Caroline Basin (C10) segment 2 29 Ma, a er C10 (Gaina and Muller 2007) SECONDARYMarginalXBABR C16r to C8r (mespan ~9.8 Myr) 13_3Caroline_C10Caroline Basin (C10) segment 3 29 Ma, a er C10 (Gaina and Muller 2007) SECONDARYMarginalXBABR 14_1Caroline_TroughC8 Caroline Trough segment 1 26 Ma, a er C8r (Gaina and Muller 2007) SECONDARYMarginalXBABR Pre-exncon spreading rate calculated from 14_2Caroline_TroughC8 Caroline Trough segment 1 26 Ma, a er C8r (Gaina and Muller 2007) SECONDARYMarginalXBABR C16r to C8r (mespan ~9.8 Myr) 14_3Caroline_TroughC8 Caroline Trough segment 1 26 Ma, a er C8r (Gaina and Muller 2007) SECONDARYMarginalXBABR types and spreading rates for comparison with extinct spreading ridges (Table location of the ridge axis (Fig. 3). A representative profile was chosen that was 15_1Caroline_WestC11Caroline Basin (C11) segment 1 33 Ma, a er C11 (Gaina and Muller 2007) EXCLUDED Marginal XBABR Pre-exncon spreading rate calculated from 15_2Caroline_WestC11Caroline Basin (C11) segment 2 33 Ma, a er C11 (Gaina and Muller 2007) EXCLUDED Marginal XBABR C16r to C8r (mespan ~9.8 Myr) 15_3Caroline_WestC11Caroline Basin (C11) segment 3 33 Ma, a er C11 (Gaina and Muller 2007) EXCLUDED Marginal XBABR 1D). Active spreading-ridge locations are refined from Bird’s (2003) digitized within the envelope of the median absolute deviation for the stacked profiles 2Supplemental Materials. Table S1A: Summary of plate boundaries, using the VGG data set to better locate axial segments. The for that ridge segment, was similar to the median stacked profile, and did not data collected and calculated at extinct ridges and active spreading ridges of the Easter microplate are based on the spreading seg- include anomalous features, such as ridge flank seamounts or intersecting Table S1B the data for active ridges. .ZIP File S2: Digi tized locations of extinct ridge axial segments and ments identified by Bird and Naar (1994), and the spreading ridges of the Juan fracture zones. The location of the representative profile is shown on the maps active ridges analyzed here. ZIP File S3: Bathymetry, Fernandez microplate follow the segments identified by Searle et al. (1993). provided in the Supplemental Materials (see footnote 2) and online database gravity, and magnetic anomaly maps displaying in- The morphology and gravity signal of extinct and active ridges are eval- (footnote 1). Bathymetric relief in the region of the ridge axis was determined ferred topographic locations of extinct ridge axial uated using two-dimensional profiles across their axes (Fig. 3), sampling the from the difference between the minimum and maximum depth within 30 km segments. .ZIP File S4: Plots of profiles generated across extinct ridge axial segments through global GEBCO bathymetry grid with 30 arc-second resolution (Weatherall et al., 2015, of the ridge axis (Fig. 3C). The peak-to-trough gravity signal was measured bathymetry and gravity grids. .ZIP File S5: Maps and GEBCO_2014_1D, version 20141103), and the V23.1 free-air gravity grid with from the difference between minimum and maximum gravity within 30 km profiles for active spreading ridges analyzed. Please a 1 min resolution (Sandwell et al., 2014). We stack profiles across the axes of the ridge axis (Fig. 3D). The horizontal distance between the minimum and visit http://doi .org /10 .1130/GES01379 .S1 or the full- text article on www.gsapubs.org to view the Supple- and plot the median profile and range of values between the upper and lower maximum depth or gravity anomaly values, reflects the half-width of the axial mental Materials. confidence interval (Fig. 3), with these plots provided in our Supplemental valley (or ridge) and the half-wavelength of the gravity signal, respectively. In

the case of ridge morphology at the extinct ridge axis, the bathymetric relief Evaluation of Secondary-Tier and Controversial Extinct Ridges is positive (for example, Fig. S4-1-1 in .ZIP File S4 of Supplemental Materials [footnote 2], South China Sea segment 3), and in the case of trough morphol- We review several previously unreported or poorly known extinct ridges ogy, the relief is negative (Fig. 3). that were identified from the global bathymetry and gravity data sets (Fig. 4). We also describe the physical characteristics of all secondary-tier extinct Review of Spreading Characteristics spreading ridges, including various alternative placements in controversial areas (noted in Table 1C) and evaluate how they compare with well-defined The time of commencement and cessation of extinct ridges and their extinct ridges. The criteria applied for assessment of secondary-tier ridges is pre-extinction spreading rates are extracted from published studies (Table 1). outlined in Table 2B. A maximum score of 6 is given if the proposed extinct The precision with which pre-extinction spreading rates have been reported ridge meets all of the criteria. Proposed locations with a score >4 are consid- varies significantly, according to the amount of data available regionally, ered likely to have been a former spreading center. Centers with a score be- whether marine data was acquired at an optimal orientation to the spreading tween 3 and 4 are considered “possible” extinct ridges, and scores <3 indicate direction and the timing of cessation. The timespans of magnetic reversals are that the feature is unlikely to be an extinct ridge. Supporting evidence from highly variable and can differ by two orders of magnitude (Gee and Kent, 2007), additional literature review is given a score between 0 and 2 (Table 2B). meaning that the longer the timespan over which the pre-extinction spreading rate is calculated, the lower the resolution of the estimate. At lower resolution, the calculated spreading rate is less likely to indicate the latest, pre-extinction RESULTS spreading rate, and the time of cessation may be less well constrained. Ideally, a consistent measure would be chosen to assess the pre-extinction spreading A total of 102 extinct ridge locations are assessed and digitized in 317 in- rate for all extinct ridges, such as the spreading rate between the last recorded dividual segments. The Pacific Ocean has the greatest number of proposed reversal and the time of cessation, or a specific time range such as the past 2 or extinct spreading center locations (38 ridges, with two alternative placements 5 m.y. of seafloor spreading. Unfortunately, due to the variation in timespans considered), followed by the Indian Ocean (16 ridges, with nine alternative of isochrons, this is not possible, and we therefore provide details of the mag- placements suggested), and then the Atlantic Ocean (ten ridges, with one al- netic isochrons used to infer the pre-extinction spreading rate and the time ternative placement suggested). Marginal basins also account for a significant span over which the rate was calculated (Table S1A in Supplemental Materials number (27 ridges), with many of these situated in Southeast Asia and in the [footnote 2]). For active ridges, most spreading rates are taken from DeMets northwest Pacific Ocean. et al. (2010) and are calculated from anomaly A1n for fast spreading ridges and We rank 29 ridges in the primary tier (Table 1A, sections 4.2 and 4.3) from A2A for slow spreading ridges, with exceptions noted in Table S1B in for use in describing the characteristics of well-defined ridges, with a Supplemental Materials (footnote 2). total of 129 primary-tier segments. The most commonly occurring extinct ridge subtype is XMOR (17 ridges; 71 segments). This is followed by Descriptive and Exploratory Statistics XBABR (six ridges; 32 segments), XMPR (five ridges; 24 segments), and finally, the Guadalupe Trough is the only primary-tier example of a XFPR Primary-tier ridge characteristics are described using binned histograms (two segments). Although the northern segment of the Guadalupe extinct and box plots. The correlation between ridge physical characteristics in- ridge features a large seamount, because this is believed to have formed cludes bathymetric relief, the maximum depth in the axial region, the width postspreading cessation, a profile was chosen that sampled the axial trough of the axial structure, the peak-to-trough gravity signal, the half-wavelength at the south of the segment. of the gravity signal, and spreading characteristics, including the time of ces- We define 48 ridges in the secondary tier that have some uncertainty as sation, the spreading rate prior to cessation, and duration of spreading. Cor- to their location or their mode of formation (Table 1B, section 4.4). Twenty-six relations are calculated in R using the Spearman’s rank correlation method suggested ridge locations (Table 1C) had insufficient constraints on their lo- (Upton and Cook, 2014), unless otherwise stated, and the rank correlation co- cation, a very weak signal, or were unlikely to represent former spreading efficient “rho” is given for these calculations. The coefficient of determination centers; thus, they were excluded from statistical analyses. However, our Sup- (r2) represents the shared variance of the ranked variables, and along with the plemental Materials (see footnote 2) and online database (footnote 1) provide p-value, indicates the probability that the correlation is meaningful and is re- details of key studies in these areas and profile observations. Physical char- ported when rho > 0.5. The p-value gives the probability of obtaining a result at acteristics of extinct and active ridges are displayed in Figures 5A and 5B, re- least as extreme as the one that was actually observed, assuming that the null spectively; their spreading characteristics are shown in Figures 6A and 6B; and hypothesis is true. Traditionally, a p-value of 0.05 or lower is required to reject correlations between physical and spreading characteristics are presented in the null hypothesis (Upton and Cook, 2014). Figures 7A and 7B, respectively.

126°E 128°E 130°E 46°W 44°W 42°W 40°W 38°W 42°E 44°E 46°E 48°E 50°E 44°S

8°N 48°S

46°S

6°N 50°S FZ 48°S discontinuity

4°N 50°S 52°S i. Possible ‘Mati’ extinct spreading ii. Southwest Atlantic possible ridge iii. Conrad−Del Cano probable extinct ridge ridge 130°E 132°E 134°E 136°E 46°E 48°E 50°E 52°E 12°N 0° Legend: Inferred extinct ridge axial location 10°N 2°S Fracture zone trends (Matthews et al., 2011) Inferred paleo-spreading direction 8°N 4°S Crust formed by later spreading −16−12 −8 −4 0481216 (post-cessation) Vertical Gravity Gradient (Eötvös)

6°N 6°S iv. ‘Palau’ possible extinct ridge segment v. West Somali Basin extinct ridge (eastern segment)

Figure 4. Speculative extinct spreading center locations in: the Palau Basin, the West Philippine Sea (i and iv); the southern Atlantic to the north of South Georgia (ii); and between the Conrad and Del Cano Rises in the Indian Ocean (iii). The eastern segment of the West Somali Basin extinct ridge (v) is shown for comparison with the Palau possible extinct ridge. Fracture zones (Matthews et al., 2011) are white lines, and arrows indicate the paleospreading direction (inferred from fracture zone orientation). Inferred extinct ridge axes are shown as thin red lines.

TALE 2. CRITERIA SED TO ASSESS SECONDAR- TIER EXTINCT RIDGES AFTER ANALSIS OF AXIAL PROFILES Tier Liely to be an extinct ridge Possibly an extinct ridge nliely to be an extinct ridge A. Morphology: athymetric relief within 1 standard deviation of the primary-tier mean athymetric relief within range of the bathymetric Either: Strongly asymmetric form typical of oceanic for bathymetric relief. Score 1 relief of the primary-tier extinct ridges. transform fault and/or fracture one. Score 0 Width of axial trough or ridge within 1 standard deviation of the Score 0.5 Or: primary-tier mean width. Score 0.5 athymetric relief outside range of primary-tier bathymetric relief. Score 0 . Gravity signal: Gravity signal within 1 standard deviation of the primary-tier mean for Gravity signal within range of the gravity signal of Either: Strongly asymmetric form typical of oceanic gravity signal. Score 2 the primary-tier extinct ridges. Score 0.5 transform fault and/or fracture one. Score 0 Or: Half-wavelength of gravity signal within 1 standard deviation of the Gravity signal outside range of primary-tier extinct ridges. primary-tier mean half-wavelength. Score 0.5 Score 0 C. Other Additional data from further literature review e.g. absolute dating, Some evidence supporting extinct ridge Reviewed studies discount extinct-ridge formation geochemical evidence support formation as an extinct ridge. formation, yet no consensus reached. for example, geochemical or geochronological evidence Score 2 Score 1 that is incompatible with a spreading ridge. Score 0

Morphology of Well-Defined Extinct Ridge and cessation (rho 0.22; p-value < 0.05); however, this reflects a greater diversity Active Ridge Axial Segments in relief for segments that have ceased spreading more recently, and relief is relatively stable for ridges that ceased before 40 Ma (Fig. 7A-V). For example, Large variations in relief (Fig. 5A-I) reflect extremely variable along-axis a small number of ridges that became extinct more than 100 m.y. ago retain morphology for many of the well-defined extinct ridges. There are also over 1000 m relief between valley floor and flanks (e.g., segments of the Wal- significant morphological differences between segments within individual laby Ridge, Magellan Trough, and West Magellan extinct spreading centers). ridge systems, such as an elevated ridge where the other segments may Although for all primary-tier extinct segments there is a weak correlation be- be deep troughs, with eight ridge segments in the primary tier. The clear- tween the duration that the ridge was active and relief at the ridge axis (rho est examples of these axial ridges are those previously discussed, such 0.19, p < 0.05), for XMORs, these variables are moderately correlated (rho = as the Socorro Island on the Mathematician ridge, the central ridge of the 0.50, r2 = 0.27, p-value < 0.001, Fig. 7A-VI). No meaningful correlation is found Galapagos Rise, the eastern segment of the South China Sea ridge, and between bathymetric relief and the published pre-extinction spreading rates Guadalupe Island. The majority of well-constrained extinct ridges (67 seg- (rho < 0.01, p-value = 0.99). ments) have trough morphology in bathymetric profile, although these are Active MOR and BABR have similar mean relief (Fig. 5B-I) to their corre- highly variable in width and depth (Table 3A), and several are juxtaposed on sponding extinct subtypes (Fig. 5A-I), although median values are weaker for larger-scale ridge structures. Of the primary-tier ridges, 26 axial segments extinct ridges (Tables 3A-I and 3A-II). In contrast, active MPR and FPR are less have no expression in bathymetric profile, and a further 28 segments have likely to have the prominent negative relief that is seen at extinct examples. irregular profiles that cannot be categorized as having either a trough or Amongst active spreading ridges, BABRs have greater negative mean relief ridge-like morphology. (mean –972 m, SD = 878) than other spreading types; although when median The mean bathymetric relief at primary-tier extinct ridge axes was found values are considered active, MORs (median –1198 m) are slightly greater to be –772 m (standard deviation [SD] = 816 m). Extinct FPR and XMPR exhibit than BABR (–1101 m) (Table 3A-III). As expected, extinct ridges are situated at greater negative bathymetric relief at their axes than large-scale XMORs (Fig. greater depth (Figs. 5A-II and 5A-III) than active spreading ridges (Figs. 5B-II 5A-I; Table 3A-I), with median relief of –1075 at XMPR and –1473 at XFPR, which and 5B-III). Extinct and active subtypes have similar relative maximum depth is considerably more than at XMOR (median –641) and XBABR (median –774). (Figs. 5A-II and 5B-II); for example, XBABR and BABR tend to have greater However, XMPRs have the widest range, with a number of segments that have maximum depth; yet the minimum depth of active ridges is very similar be- positive relief, and this leads to the mean relief that is closer to the XMOR tween all subtypes (Fig. 5B-III), while extinct subtypes vary by up to 1400 m mean relief (Fig. 5A-I). The maximum depth in the region of the ridge axis (Fig. 5A-III). Overall, the median width of the axial structure of extinct ridges increases with the time since cessation (rho –0.65; r2 = 0.42; p-value < 0.001), is similar (mean 25 km, SD = 14 km) to the width of active examples (mean as expected due to the subsidence of oceanic crust with increasing age. There 22 km, SD = 10 km), although there are some variations between subtypes is a weak correlation between bathymetric relief and the time of spreading (Figs. 5A-IV and 5B-IV).

Figure 5 (on this and following page). (A) Physical characteristics of extinct spreading ridge segments (primary tier). Plots show the range of values for bathymetric relief (I), maximum (II) and minimum depth (III), and half-width of the axial valley or ridge (IV), the peak-to-trough gravity signal (V), minimum (VI) and maximum gravity values (VII), and half-wavelength of the gravity signal (VIII) for ridge subtypes: extinct backarc basin spreading center (XBABR), extinct microplate spreading center (XMPR), extinct fragmented plate spreading ridge (XFPR), and large-scale extinct mid-ocean ridge (XMOR). Boxes show the range of values for each subgroup, with the box enclosing the first to third quartile and the median value shown as a thick black line within the box. Thin black lines extending from the box show the minimum and maximum values; outliers are dots above or below. The mean value is shown with an open diamond. Summary tables below each plot give the mean value and standard deviation for each subgroup. Box widths are scaled according to the number of samples in each subgroup.

Peak-to-Trough Gravity Signal at Well-Defined mean and standard deviation of the gravity signal for each ridge subtype are Extinct Ridge and Active Ridge Axes summarized in Table 3A and Figure 5A-V. The half-wavelength of the anomalies (estimated from the lateral distance between the minimum and maximum The peak-to-trough gravity signals for primary-tier extinct ridge segments gravity value within 30 km of the ridge axis) ranges from 12 km to 52 km, with are strongly correlated with segment axial relief morphology (rho 0.64, r2 = a mean of 24 km (SD = 7 km) (Fig 5A-VIII), which is the same value as the 0.40, p-value < 0.001, Fig. 7A-IV). A characteristic negative gravity signal at active spreading ridges (mean 24 km, SD = 8 km; Fig. 5B-VIII; Table 3A-II). The extinct ridge axes is evident on many profiles, as seen in Figures 3 and 5A-V. wavelength of the gravity anomaly is moderately negatively correlated with The gravity signal ranges from –146 mGal to 76 mGal, yet the mean signal is the pre-extinction spreading rate (rho –0.42, r2 = 0.17, p-value < 0.001). For all moderately negative, with a value of –39 mGal (SD = 30 mGal) (Fig. 5A-V). The primary-tier segments, there is a very weak correlation between the peak-to-

Figure 5 (continued). (B) Physical characteristics of active spreading ridge segments. Plots show the range of values for bathymetric relief (I), maximum (II) and minimum depth (III) and axial valley or ridge half-width (IV), the peak-to-trough gravity signal (V), minimum (VI) and maximum gravity values (VII) and half-wavelength of the gravity signal (VIII) for ridge subtypes: active backarc basin spreading ridge (BABR), active microplate spreading ridge (MPR), active fragmented plate spreading ridge (FPR), and large-scale active mid-ocean ridge (MOR). Plots are presented in the same way as described for Figure 5A.

trough gravity signal and pre-extinction spreading rate (rho = 0.15, p-value < maximum (Figs. 5A-VII and 5B-VII) gravity anomalies near the ridge axis are 0.1) (Fig. 7A-I), with a weak to moderate correlation found for XMOR (rho = very different when active and extinct ridges are compared, the subtypes tend 0.30, p < 0.05). to plot in similar fields relative to each other. For example, FPRs tend to have The correlation of bathymetric relief and gravity signal is even stronger more negative mean minimum and maximum gravity anomalies in the axial from active ridges (rho 0.92, r2 = 0.85, p-value < 0.001) than extinct exam- region, and anomalies at BABR tend to be less negative, and, in many cases, ples (Figs. 7A-IV and 7B-III). The peak-to-trough gravity signal at active MOR they have positive gravity anomalies near the ridge axis. Large-scale MORs (mean = –45 mGal, SD = 53 mGal) and BABR (mean = –33, SD = 34; Fig. 5B- have the broadest range of values for active and extinct examples, but mean V) subtypes are quite similar to the corresponding extinct subtypes, XMOR values for the minimum and maximum gravity anomalies near the ridge axis (mean = 42 mGal, SD = 26 mGal) and XBABR (mean = –35 mGal, SD = 28 mGal) tend to be higher than microplate and fragmented plate ridges and lower than (Fig. 5A-V). Although mean values for the minimum (Figs. 5A-VI and 5B-VI) and backarc basin ridges.

Figure 6 (on this and following page). (A) Spreading characteristics of extinct spreading ridge segments (primary tier). The half-spreading rate (I and II), duration of spreading (III and IV), and time of cessation (V and VI) are displayed according to the ridge subtype in the top row (I, III, and V, respectively) and the ocean in which the ridge is located in the bottom row (II, IV, and VI, respectively). Plots use the same conventions and abbreviations as explained in Figure 5. The “spreading rate” is the half-spreading rate that is given as the final rate at the spreading ridge before the time of cessation from the published studies referenced in Table 1. “Duration” is the estimated length of time that the spreading ridge was active, from published studies and review of magnetic anomaly compilations (Seton et al., 2014). The “time ceased” refers to the time recorded in published studies of the time of spreading cessation.

Spreading Rate, Time of Cessation, and Duration of Spreading similar to that of active ridge subtypes (Fig. 6B-I), with the exception of the XFPR, but there is not a representative sample of this group with only one We summarize the collated spreading characteristics of the primary-tier ridge in the primary tier. Active MPR and FPR have higher mean spreading extinct ridges in Table 3B-I and Figure 6A and active ridges in Table 3B-II and rates, although fragmented plates have a larger standard deviation than all Figure 6B. Pre-extinction half-spreading rates range between 2 and 80 mm other groups. yr–1, with a mean of 25 mm yr–1 (SD = 20 mm yr–1) (Table 3B-I). The variation Extinct ridges have strong variations in pre-extinction spreading rates in pre-extinction spreading rates for extinct ridge subtypes (Fig. 6A-I) is very within different ocean basins (Fig. 6A-II). Pre-extinction half-spreading rates

Figure 6 (continued). (B) Spreading characteristics of active spreading ridge segments. The half-spreading rate (I and II), duration of spreading thus far (III and IV) are displayed according to the ridge subtype in the top row (I and III, respectively) and the ocean in which the ridge is located in the bottom row (II and IV, respectively). Plots use the same conventions and abbreviations as explained in Figures 5A and 6A. Abbreviations: BABR—backarc basin spreading ridge; FPR—fragmented plate spreading ridge; MOR—mid-ocean ridge; MPR—microplate spreading ridge; XBABR—extinct backarc basin spreading ridge; XFPR—extinct fragmented plate spreading ridge; XMOR—extinct mid- ocean ridge; XMPR—extinct microplate spreading ridge.

were slowest in the Atlantic Ocean (median 7 mm yr –1, range 2–15 mm yr–1), highest mean spreading rates are in the Pacific, the lowest in the Atlantic; with marginal basins generally in the slow to intermediate range (median and in the Indian Ocean and marginal basins, spreading rates are in the 16 mm yr–1, range 3.5–60 mm yr–1), with faster spreading rates in the Pacific intermediate range. (median 25 mm yr–1, range 6–72 mm yr –1) and Indian Ocean (median 27 mm Extinct ridge duration of spreading ranged from a minimum of 4 m.y. at yr–1, range 20–80 mm yr –1) (Fig. 6A-II). The variability of spreading rates the Guadalupe microplate to a maximum of 50 m.y. at the Labrador Sea Ridge between different oceans is fairly consistent with present-day spreading (Figs. 6A-III and 6A-IV). Ridge subtype strongly influences the duration of ac- rates at active ridges (Fig. 6B-II). The median half-spreading rates for active tivity, with XMPR and XBABR having shorter durations compared with XMOR ridges are between ~8–15 mm yr –1 faster than extinct ridges in the corre- (Fig. 6A-III). Extinct ridges in the Atlantic and Indian oceans had longer dura- sponding regions (Figs. 6A-II and 6B-II), with the exception of the Indian tions before cessation than those in the Pacific Ocean and marginal basins (Fig. Ocean where the median spreading rate is lower than extinct examples 6A-IV). Ridge extinction events in our data set have occurred regularly over the (median 21 mm yr–1, range 7–35 mm yr–1). At the active ridges reviewed, the past 150 m.y. with the time of cessation ranging from 3 to 136 Ma (Figs. 6A-V

Figure 7 (on this and following page). (A) Relationship between physical and spreading characteristics for extinct ridges. Correlations between variables investigated in this study showing linear fit for each subgroup, with the associated 95% confidence interval for the linear regression model. Abbreviations: XBABR—extinct backarc basin spreading ridge; XFPR—extinct fragmented plate spreading ridge; XMOR—extinct mid-ocean ridge; XMPR—extinct microplate spreading ridge.

and 6A-VI); although a greater number of recently ceased extinct ridges are Extinct ridge spreading characteristics are compared with the physical preserved, proportionate to the area of younger aged oceanic crust in global characteristics of primary-tier extinct ridges (Fig. 7A). Gravity anomalies are a oceans. The duration of spreading of active spreading ridges cannot be di- little more subdued for older oceanic crust (rho 0.24, p-value < 0.01). There is rectly compared with extinct ridges, because the length of time that the active a weak correlation between the pre-extinction spreading rate and the magni- ridge will continue to spread is not constrained. However, the time of com- tude of the gravity signal at the ridge axis for XMOR (rho 0.30, p-value < 0.05) mencement gives an approximate measure of the longevity of active spread- and no significant correlation found for other subtypes (Fig. 7A-V). There is ing ridges of different subtypes (Fig. 6B-III) and for different oceans (6B-IV), also no meaningful correlation between bathymetric relief at the axis and and results are broadly similar to those from extinct examples (Figs. 6A-III and the pre-extinction spreading rate for extinct ridges. This contrasts with active 6A-IV). In contrast to the weak positive correlation between duration and axial spreading centers, at which the spreading rate is strongly correlated with the relief for extinct ridges (rho 0.19, p-value < 0.05; Fig. 7A-VI), there is a weak bathymetric relief (rho = 0.72, r2 = 0.52, p-value < 0.001) and the peak-to-trough negative correlation between the time of commencement for active ridges and gravity signal (rho = 0.76, r2 = 0.58, p-value < 0.001) (Fig. 7B-I), as has been bathymetric relief at the axis (rho –0.24, p-value < 0.05), which is moderate for reported elsewhere. At extinct ridges, there is a weak negative correlation be- MOR (rho –0.44, p-value < 0.01) (Fig. 7B-IV). tween the minimum depth in the axial region and the pre-extinction spreading

Figure 7 (conitnued). (A) Relationship between physical and spreading characteristics for extinct ridges. Correlations between variables investigated in this study showing linear fit for each subgroup, with the associated 95% confidence interval for the linear regression model. Abbreviations: XBABR—extinct backarc basin spreading ridge; XFPR—extinct fragmented plate spreading ridge; XMOR—extinct mid-ocean ridge; XMPR—extinct microplate spreading ridge. (B) Relationship between physical and spreading characteristics for active ridges. Abbreviations: BAB—backarc basin spreading ridge; FPR—fragmented plate spreading ridge; MOR—mid-ocean ridge; MPR—microplate spreading ridge.

rate (rho –0.21, p-value < 0.05), although a moderate to strong negative cor- their segments (Sao Paulo, Conrad Rise South, and the Angolan basin sug- relation is found for XMOR (rho –0.63, p < 0.001) (Fig. 7A-II). Active ridges have gested extinct ridges). From the new locations we propose, these ridges score a stronger negative correlation between spreading rate and minimum depth above 3 out of a maximum of 4 points for their physical characteristics, which (rho –0.58, p-value < 0.001), and this is more pronounced for MOR and MPR are within a standard deviation of primary-tier extinct ridges; however, they (rho 0.67, both p-values < 0.005) (Fig. 7B-II). receive no score for criteria C due to no supporting studies. Therefore, they are categorized as possible extinct ridges. For alternative locations considered in controversial regions (Table 4B), only three of 20 locations achieved a score Evaluation of Secondary-Tier Ridges greater than 4 for 50% or more of their defined segments. In some cases, such as the proposed locations on the Perth Abyssal Plain, two possible placements The locations of secondary-tier ridges are shown in Figure 1, and refer- scored in the range of probable extinct ridge. ences for published studies are listed in Table 1B, alongside collected data. Additional discussion of the evidence reviewed in our assessment of con- The degree to which secondary-tier ridges match the characteristics of well- troversial and uncertain ridge locations is contained in the summary pages defined ridges is evaluated and scored in Tables 4A and 4B, following the cri- linked to our online database of extinct ridges (Fig. 8; footnote 1). Overall, the teria detailed in Table 2B. Of the uncertain ridges listed in Table 4A, there were secondary tier extinct ridge axes have weaker peak-to-trough gravity signals 19 of 22 locations that achieved a score greater than 4 for 50% or more of (mean –22 mGal, SD = 37 mGal) than that of the primary-tier ridges (mean their defined segments and are, therefore, deemed “likely” to represent for- –39 mGal, SD = 30 mGal), which may contribute to the difficulty in identifying mer spreading centers. Only three ridges scored below 4 for the majority of these features in global data sets.

TALE 3A-I. PHSICAL CHARACTERISTICS OF PRIMAR- TIER EXTINCT RIDGES Maximum Minimum Axial Half-width of Minimum Maximum Pea-to-trough Half-wavelength depth depth bathymetric axial region gravity gravity gravity of gravity signal m m relief m mGal mGal mGal m All primary-tier extinct ridge segments n 129 Minimum –6971 –5788 –31014 –99–22 –146 12 Median –4662 –3946 –858 21 –1921–35 23 Maximum –856 –649 2068 62 34 80 76 52 Mean –4639 –3708 –772 25 –2122–39 24 Standard deviation 1065 1012 816142321307 Large-scale extinct mid-ocean ridge segments n 71 Minimum –6541 –5477 –31014 –85–22 –136 12 Median –4558 –3753 –641 23 –1919–35 25 Maximum –856 –649 1049 62 34 80 37 38 Mean –4422 –3624 –711 28 –2221–42 25 Standard deviation 1114 1047 690152226266 Extinct microplate spreading ridge segments n 24 Minimum –6465 –5788 –30716 –99–13 –146 12 Median –4367 –2991 –107517–27 14 –3718 Maximum –3205 –2259 2068 39 –3 51 76 32 Mean –4801 –3484 –733 18 –3117–36 20 Standard deviation 1062 1227 1265 81917436 Extinct fragmented plate spreading ridge segments n 2 Minimum –4337 –3056 –166413–60 –5 –5514 Median –4205 –2732 –147316–44 1–46 16 Maximum –4073 –2408 –128119–29 8–37 18 Mean –4205 –2732 –147316–44 1–46 16 Standard deviation 187 458 2714 22 9133 Extinct bacarc basin spreading ridge segments n 32 Minimum –6971 –5423 –22918 –828–9514 Median –4785 –4255 –774 20 –2 26 –3022 Maximum –3298 –2642 70 56 15 52 27 52 Mean –5024 –4124 –892 23 –1127–35 25 Standard deviation 868 580 655122211289

DISCUSSION variable precision of calculated spreading rates for the final stages of spreading, spreading rates for extinct examples are not accurate enough to obtain a Comparison with Previous Studies of Extinct and Active Ridges reliable result. Alternatively, if the final stages of spreading at the dying ridge are primarily magmatic rather than tectonic, the morphology and gravity signal The magnitude of gravity anomalies at extinct spreading ridge axes within of the ridge axis is less likely to be influenced by the final spreading rate. our global analysis are consistent with trends observed at four extinct spreading Extinct microplate spreading ridges and XFPR segments have higher- centers investigated by Jonas et al. (1991). We see a weak spreading rate depen- amplitude negative anomalies at their axes than their active counterparts, dence of the axial gravity anomaly for all ridge subtypes, with higher spreading suggesting that an additional component contributes to the gravity signal for rates prior to cessation correlated with a lower amplitude gravity anomaly (Fig. these extinct ridge subtypes, such as a low-density body situated within the 7F). Yet there are many outliers, and this relationship is considerably weaker crust, as has been suggested by Jonas et al. (1991). Significant hydrothermal than the correlation between the gravity anomaly at active spreading segments alteration to depth, caused by deep fracturing, or alternatively late-stage mag- and spreading rate, which has coefficient of determination r2 = 0.58. There are matic emplacement, could both generate significant density contrasts that are several possible explanations for this finding. The first may be that due to the likely to be maintained with minimal attenuation over time.

TALE 3A-II. PHSICAL CHARACTERISTICS OF SELECTED ACTIE SPREADING RIDGES Maximum Minimum Axial Half-width of Minimum Maximum Pea-to-trough Half-wavelength depth depth bathymetric axial region gravity gravity gravity of gravity signal m m relief m mGal mGal mGal m All included active spreading ridge segments n 91 Minimum –4898 –3435 –28354 –57–8–139 12 Median –3419 –2390 –108421–743–38 23 Maximum –2777 –1629 9256144115 34 44 Mean –3488 –2377 –739 22 –6 40 –3524 Standard deviation 555 365 1049 10 22 28 46 8 Active large-scale mid-ocean ridge segments n 37 Minimum –4898 –2784 –28354 –576–139 13 Median –3349 –2320 –119823–15 43 –6227 Maximum –2778 –1629 7423914115 22 40 Mean –3444 –2243 –794 23 –1343–45 26 Standard deviation 553 309 1155 71628537 Active microplate spreading ridge segments n 10 Minimum –4780 –2684 –20979 –458–7612 Median –3195 –2402 –216 15 –1114–616 Maximum –2844 –2209 60029331 18 30 Mean –3370 –2434 –432 17 –1616–15 19 Standard deviation 597 164 1045 616735 7 Active fragmented plate spreading ridge segments n Minimum –3542 –2739 –13218 –40–9–78 12 Median –3035 –2398 –7 22 –170–3 23 Maximum –2788 –2045 54847–7381344 Mean –3152 –2427 –317 25 –196–1625 Standard deviation 276 254 8461413633 11 Active bacarc basin spreading ridge segments n 19 Minimum –4633 –3436 –25606 –1742–80 13 Median –3859 –2509 –1101162563–37 20 Maximum –2811 –1763 9256144103 34 40 Mean –3778 –2587 –972 21 19 61 –3621 Standard deviation 527 473 878131713307

TALE 3A-III. PHSICAL CHARACTERISTICS OF SECONDAR- TIER EXTINCT RIDGES for reference n 122 Maximum Minimum Axial Half-width of Minimum Maximum Pea-to-trough Half-wavelength depth depth bathymetric axial region gravity gravity gravity of gravity signal m m relief m mGal mGal mGal m Minimum –6693 –5759 –28276–103 –52–120 12 Median –4930 –3937 –567 22 –238–2225 Maximum –1476 –1062 2238 62 53 106124 62 Mean –4848 –3887 –436 27 –2212–22 27 Standard deviation 995 967 1064 15 28 29 37 11

TALE 3-I. SPREADING CHARACTERISTICS TALE 3-II. SPREADING CHARACTERISTICS OF PRIMAR-TIER EXTINCT RIDGES OF ACTIE SPREADING RIDGES Half-spreading Time of Duration of Half-spreading Time of rate cessation spreading rate commencement mm yr–1 Ma m.y. mm yr –1 Ma All primary-tier ridges n 29: All active ridge segments n 14 Minimum 2 3.3 4 Minimum 71.5 Median 18 28 15 Median 31 9 Maximum 80 134 50 Maximum 53 132 Mean 25 44.3 20 Mean 28 28 Standard deviation 20 41 12 Standard deviation 12 39 Large-scale extinct mid-ocean ridges n 17 Large-scale mid-ocean ridges n 5 Minimum 2 3.3 10 Minimum 712 Median 16 36 25 Median 35 65 Maximum 80 127 50 Maximum 52 132 Mean 23 49 27 Mean 29 66 Standard deviation 22 40 12 Standard deviation 18 45 Extinct microplate spreading ridges n 5 Active microplate spreading ridges n 2 Minimum 10.5 55Minimum 32 Median 32 21 10 Median 36 Maximum 40 134 13 Maximum 40 Ca. 4 Ma Mean 27 59 9.8 Mean 36 Standard deviation 12 66 3 Standard deviation 6 Extinct fragmented plate spreading ridges n 1 Active fragmented plate spreading ridges n 2 alues 50 11 4 Minimum 26 7 Extinct bacarc basin spreading ridges n 6 Median 28 8 Maximum 30 10 Minimum 3.5 10 5 Mean 28 8 Median 24 24 9 Standard deviation 32 Maximum 60 52 26 Mean 27 26 12 Active bacarc basin spreading ridges n 5 Standard deviation 22 14 7 Minimum 82 Median 30 7 Maximum 34 15 Influence of Ridge Subtype on Ridge Characteristics Mean 25 7 Standard deviation 11 5

Our results indicate that spreading centers in different tectonic settings may vary considerably in their morphology and the magnitude of their gravity signal, but that this variation is not consistent between active and extinct spreading Easter microplate (depth 5890 m), in close proximity to the shallowest point on ridges. For example, although XMPR and XFPR have more pronounced nega- the East Pacific Rise (depth 2050 m), located in the south of the eastern rift of tive relief than other extinct spreading ridges (their median relief is 300–400 m the Easter microplate (Naar and Hey, 1991). This results in almost 4 km of depth and 700–800 m greater than XBABR and XMOR, respectively), the median relief change over a distance of less than 500 km. We propose that the combination at active examples of these ridge types is about 1000 m less than that seen of two processes that are intrinsic to microplate spreading systems may be at active BABR and MOR segments (Fig. 5B-I; Table 3A-II). This could suggest responsible for the formation of greater relief at their axial segments. Firstly, that the processes related to spreading cessation at microplate and fragmented microplate crustal accretion is dominated by the process of ridge propagation plate spreading centers are fundamentally different than in other settings, lead- into existing ocean floor, resulting in capture of the major plate crust, rather ing to a significant morphological change in the final stages of spreading. than by magmatic accretion (Naar and Hey, 1991; Schouten et al., 1993; Teb- Active microplate ridges have unusual depth profiles, as observed by Naar bens and Cande, 1997; Hey, 2004). Propagating ridge segments are observed and Hey (1991), who found that the deepest point on the East Pacific Rise was to develop away from shallow regions (Hey, 2004), forming “flow-induced situated at the northernmost point of the active spreading eastern rift of the depressions” at the segment tip (Morgan and Parmentier, 1985, p. 8603).

TABLE 4A. COMPARISON OF UNCERTAIN OR SPECULATIVE SECONDARY- TIER RIDGES WITH GRAVITY AND BATHYMETRIC STATISTICS FROM PRIMARY- TIER RIDGES Bathymetric relief at axis Half-width of axial valley and/or ridge Peak-to-trough gravity signal at ridge axis Half-wavelength of gravity anomaly Primary-tier segment statistics (m) (km) (mGal) (km) Taken to Minimum –3101 4–146 12 represent the Median –858 21 –3523 “characteristic” Maximum 2068 62 76 52 signal at a well- defined extinct Mean –772 25 –3924 spreading center Standard deviation 816 14 30 7 segment. 1st standard deviation LowerUpper LowerUpper LowerUpper LowerUpper –1588 44 11 39 –69–91731

COMPARISON OF UNCERTAIN SECONDARY-TIER RIDGES WITH PRIMARY- TIER EXTINCT RIDGE CHARACTERISTICS Bathymetric Comparison Half-width of Comparison Peak-to-trough Comparison Half-wavelength Comparison Assessment of likelihood of extinct ridge formation relief with axial valley with gravity signal with primary- of gravity with primary- (score >4 reflects a “likely” extinct ridge) at axis primary-tier and/or ridge primary-tier at ridge axis tier mean signal tier mean Segment name (m) mean relief (km) mean width (mGal) gravity signal (km) wavelength ABCTotal Abimael Ridge (Santos Basin) Abimael1 –1254 <1 SD 62 Within range –42 <0.25 SD 25 <0.5 SD 131 5 Bauer microplate west and Early East Pacific Rise Bauer1 –1118 <0.25 SD 22 <0.5 SD –33 <0.5 SD 41 Within range1.5 214.5 Bauer2 –516 <1 SD 33 <1 SD –21 <1 SD 26 <1 SD 1.52.5 1 5 Bauer3 1025 Within range 20 <0.5 SD 25 Within range51Outside range0.5 0.51 2 Chinook Trough Chinook1 –1627 Within range 25 <0.25 SD –64 <1 SD 25 <0.25 SD 0.52.5 1 4 Chinook2 –1419 <1 SD 16 <1 SD –33 <0.5 SD 28 <1 SD 1.52.5 1 5 Chinook3 –1281 <1 SD 19 <0.5 SD –47 <0.5 SD 23 <0.25 SD 1.52.5 1 5 Chinook4 –987 <0.25 SD 17 <1 SD –30 <0.5 SD 19 <1 SD 1.52.5 1 5 Conrad–Del Cano Conrad1 –740 <0.25 SD 31 <1 SD –45 <0.25 SD 30 <1 SD 1.52.5 1.5 5.5 Conrad2 –558 <0.5 SD 13 <1 SD –20 <1 SD 13 Within range1.5 21.5 5 Conrad3 –612 <0.25 SD 19 <0.5 SD –26 <0.5 SD 18 <1 SD 1.52.5 1.5 5.5 Conrad4 –684 <0.25 SD 22 <0.5 SD –31 <0.25 SD 18 <1 SD 1.52.5 1.5 5.5 Conrad Rise (South) ConradSouth1 –458 <1 SD 16 <1 SD –17 <1 SD 19 <1 SD 1.52.5 0.5 4.5 ConradSouth2 –509 <1 SD 13 <1 SD –9 <1 SD 16 Within range1.5 20.5 4 ConradSouth3 –84 <1 SD 45 Within range –8 <1 SD 54 Outside range1 20.5 3.5 ConradSouth4 –72 <1 SD 56 Within range –11 <1 SD 13 Within range1 20.5 3.5 Dove Basin Dove1 1501 Within range 10 Within range –29 <0.5 SD 34 Within range0.5 224.5 Ellice Basin Ellice1 –736 <0.25 SD 45 Within range –20 <1 SD 48 Within range1 225 Ellice2 –938 <0.25 SD 37 <1 SD –32 <0.25 SD 28 <1 SD 1.52.5 2 6 Ellice3 –989 <0.25 SD 21 <0.5 SD –32 <0.25 SD 19 <1 SD 1.52.5 2 6 Ellice4 955 Within range 51 Within range –33 <0.25 SD 36 Within range0.5 224.5 Friday microplate Friday1 –549 <1 SD 13 <1 SD –18 <1 SD 15 Within range1.5 214.5 Friday2 –1580 <1 SD 18 <1 SD –38 <0.25 SD 16 Within range1.5 214.5 Friday3 –859 <0.25 SD 20 <0.5 SD –26 <0.5 SD 17 Within range1.5 214.5 (continued)

TABLE 4A. COMPARISON OF UNCERTAIN SECONDARY-TIER RIDGES WITH PRIMARY- TIER EXTINCT RIDGE CHARACTERISTICS (continued) Bathymetric Comparison Half-width of Comparison Peak-to-trough Comparison Half-wavelength Comparison Assessment of likelihood of extinct ridge formation relief with axial valley with gravity signal with primary- of gravity with primary- (score >4 reflects a “likely” extinct ridge) at axis primary-tier and/or ridge primary-tier at ridge axis tier mean signal tier mean Segment name (m) mean relief (km) mean width (mGal) gravity signal (km) wavelength ABCTotal Gop Basin Gop1 –143 <1 SD 62 Within range –13 <1 SD 26 <0.25 SD 12.5 2 5.5 Gop2 358 Within range 20 <0.5 SD 24 Within range59Outside range0.5 0.52 3 Gulf of Mexico GulfofMexico1 –81 < 1 SD 62 Within range –19 <1 SD 62 Outside range1 214 GulfofMexico2 –249 < 1 SD 62 Within range –16 <1 SD 32 Within range1 214 GulfofMexico3 –255 < 1 SD 62 Within range 15 Within range52Within range1 0.51 2.5 GulfofMexico4 –357 < 1 SD 59 Within range –8 Within range32Within range1 0.51 2.5 GulfofMexico5 –411 <1 SD 62 Within range –12 <1 SD 23 <1 SD 12.5 1 4.5 GulfofMexico6 –165 <1 SD 12 < 1 SD –12 <1 SD 37 Within range1.5 214.5 GulfofMexico7 –179 <1 SD 32 < 1 SD –14 <1 SD 37 Within range1.5 214.5 GulfofMexico8 207 Within range 47 Within range 8Within range 61 Outside range0.5 0.51 2 Hokkaido Hokkaido1 –967 <0.25 SD 21 <0.5 SD –37 <0.25 SD 23 <0.5 SD 1.52.5 1 5 Hokkaido2 –1038 <0.5 SD 26 <0.5 SD –24 <0.5 SD 24 <0.25 SD 1.52.5 1 5 Hokkaido3 –1376 <1 SD 36 <1 SD –31 <0.25 SD 24 <0.25 SD 1.52.5 1 5 Hokkaido4 –824 <0.25 SD 6Within range –22 <1 SD 20 <1 SD 12.5 1 4.5 Jane Basin Jane1 1029 Within range 62 Within range –15 <1 SD 43 Within range0.5 1.52 4 Jane2 –860 <0.5 SD 52 Within range –17 <1 SD 20 <1 SD 12.5 2 5.5 Laxmi Basin Laxmi1 –186 <1 SD 19 <1 SD –13 <1 SD 41 Within range1.5 225.5 Northern Natal Basin Natal1 414 Within range 50 Within range –15 <1 SD 28 <1 SD 0.52.5 2 5 Powell Basin Powell1 –86 <1 SD 24 <0.25 SD –30 <0.5 SD 32 Within range1.5 225.5 Protector Basin Protector1 –336 <1 SD 19 <0.5 SD –27 <0.5 SD 20 <1 SD 1.52.5 2 6 Sao Paulo SaoPaulo1 163 Within range 50 Within range 19 Within range25 <0.25 SD 0.51 12.5 SaoPaulo2 439 Within range 51 Within range –18 <1 SD 21 <0.5 SD 0.52.5 14 South Rennell Trough SouthRennell1 –2313 Within range 15 <1 SD –89Within range18 <1 SD 1124 SouthRennell2 –2208 Within range 25 <0.25 SD –104 Within range24 <0.25 SD 1124 SouthRennell3 –1642 Within range 15 <1 SD –64 <1 SD 20 <1 SD 0.52.5 2 5 SouthRennell4 –1325 <1 SD 27 <0.25 SD –59 <1 SD 24 <0.25 SD 1.52.5 2 6 Tehuantepec possible microplate Tehuantepec1 –480 <1 SD 15 <1 SD –22 <1 SD 27 <0.25 SD 1.52.5 1 5 Tehuantepec2 –901 <0.25 SD 21 <0.5 SD –23 <1 SD 14 Within range1.5 214.5 Tehuantepec3 –496 <0.5 SD 15 <1 SD –19 <1 SD 19 <1 SD 1.52.5 1 5 (continued)

COMPARISON OF SPECULATIVE SECONDARY-TIER RIDGES WITH PRIMARY- TIER EXTINCT RIDGE CHARACTERISTICS

Bathymetric Comparison Half-width of Comparison Peak-to-trough Comparison Half-wavelength Comparison Assessment of likelihood of extinct ridge formation relief with axial valley with gravity signal with of gravity with (Table 2B) at axis primary-tier and/or ridge primary-tier at ridge axis primary-tier anomaly primary-tier Segment name (m) mean (km) mean (mGal) mean (km) mean ABCTotal Mati, Southern West Philippine Sea Mati1 –1714 Within range 23 <0.5 SD –44 <0.25 SD 20 <1 SD 12.5 –3.5 Mati2 –1308 <1 SD 19 <0.5 SD –39 <0.25 SD 20 <1 SD 1.52.5 –4 Mati3 –721 <0.25 SD 15 <1 SD –23 <1 SD 21 <1 SD 1.52.5 –4 Mati4 –1074 <0.5 SD 26 <0.25 SD –21 <1 SD 28 <1 SD 1.52.5 –4 Palau, Southeast West Philippine Sea Palau1 –1443 <1 SD 17 <1 SD –44 <0.5 SD 46 Within range1.5 2– 3.5 North of Falkland fracture zone and South Georgia, South Atlantic NorthFalkland1 –342 <1 SD 25 <1 SD –15 <1 SD 30 <1 SD 1.52.5 –4 Notes: Alternative locations that are less than one standard deviation of the mean for primary-tier segments are shown in bold. Scores of >4 indicate the feature is a “probable extinct ridge”; scores between 3 and 4 indicate a possible extinct ridge; and scores <3 indicate the feature is unlikely to be an extinct ridge (or was heavily modified by volcanism or deformation after spreading cessation). Where no data were available on spreading characteristics from published studies, the cell is marked “–.” If there have been no published studies for a proposed extinct ridge location, the criteria C is marked “–.”

The tips of propagating segments have been reported in some cases to be found that XMPRs have higher spreading rates prior to cessation and predict- deep, narrow troughs (Kleinrock and Hey, 1989; Hey, 2004), and lower-crustal ably shorter durations of spreading relative to large-scale XMORs (Fig. 6A-I). and even upper-mantle rocks can be exposed within these structures (such Lifespans of XBABR and XMPR are less than 15 m.y. in general; whereas large as at the Hess Deep, on the Cocos-Nazca spreading center; Gillis et al., 2014; XMORs are more likely to persist for ~25 m.y. before failure (Fig. 6B) but are Hekinian, 2014). Therefore, the crustal accretion style is likely to be a key in- significantly more variable. The life span of spreading centers created by plate fluence in the development of relief at MPRs. Additionally, the rotation of fragmentation appears to be usually limited to less than 10 m.y., although in microplate can generate high-relief transpressional (Rusby and Searle, 1993) the case of the Cocos–Nazca Ridge, after several ridge-jumps, the present-day and compressional ridges (Searle et al., 1993), where convergence occurs be- ridge appears to have established a stable configuration. This observation tween the microplate crust and the bounding major plate (Rusby and Searle, may suggest that crustal production initiated by plate fracturing cannot be 1993). We suggest that the combination of these two processes explains the maintained unless an additional force (such as slab-pull or anomalous upwell- observed high relief on XMPR. It is possible that active microplate ridges will ing) is available to the spreading system. develop greater relief with continued rotation and become more similar to Extinct backarc basin ridges show a markedly different trend to the other their extinct counterparts. ridge subtypes when the length of activity is compared with relief at the ridge In other respects, the spreading characteristics of extinct ridge subtypes axis (Fig. 7-VI). While XMORs have lower relief after longer durations of spread- vary in a similar way to those at active spreading centers. For example, we ing, the opposite trend is seen for extinct backarc spreading centers (Fig. 8C).

TALE 4. COMPARISON OF ALTERNATIE SGGESTED LOCATIONS OF EXTINCT RIDGES IN CONTROERSIAL REGIONS athymetric relief Half-width of axial Pea-to-trough gravity Half-wavelength of Primary-tier segment statistics at axis valley and/or ridge signal at ridge axis gravity anomaly nits mm mGalm Minimum –31014–146 12 Median –858 21 –3523 Taen to represent the characteristic signal at a well-defined extinct Maximum 2068 62 76 52 spreading center segment. Mean –772 25 –3924 Standard deviation 81614307 1st standard Lowerpper Lowerpper Lowerpper Lowerpper deviation –1588441139–69 –9 17 31

COMPARISON OF PROPOSED ALTERNATIE LOCATIONS OF SECONDAR- TIER RIDGES WITH PRIMAR- TIER EXTINCT RIDGE CHARACTERISTICS Assessment of lielihood athymetric Comparison Half-width Comparison Pea-to-trough Comparison Half- Comparison of extinct ridge formation relief with of axial with gravity signal with wavelength of with Criteria, Table 2 at axis primary-tier valley/ridge primary-tier at ridge axis primary-tier gravity signal primary-tier m mean m mean mGal mean m mean ACTotal Cuvier Abyssal Plain 1. Cuvier Gibbons et al., 2012CuvierGibb1 137 Within range 12 <1 SD 6Within range 49 Outside range1 0.51 2.5 CuvierGibb2 533 Within range 57 Within range11Within range22 <0.25 SD 0.51 12.5 2. Sona Ridge Robb et al., Sona1 1770 Within range 24 <0.5 SD 58 Within range20 <0.25 SD 1113 2005 3. Sonne Ridge Mihut and Sonne1 1556 Within range 28 <0.5 SD 53 Within range28 <1 SD 1113 Müller, 1997 Sonne2 1588 Within range 17 <1 SD 53 Within range22 <1 SD 1113 Perth Abyssal Plain AP 1. Dir Hartog Ridge Whittaer DirHartog1 –781 < 0.25 SD 14 <1 SD –19 < 1 SD 14 Outside range1.5 214.5 et al., 2013 DirHartog2 2238 Outside range 37 <1 SD 124Outside range38Outside range0.5 011.5 2. Perth AP1 Mihut and Müller, PerthAP1 –576 < 0.5 SD 21 <0.5 SD –18 <1 SD 19 <0.25 SD 1.52.5 0.54.5 199 3. Perth AP2 inferred from PerthAP21 –868 <0.25 SD 32 <1 SD –28 <0.5 SD 28 <1 SD 1.52.5 –4 vertical gravity gradient grid PerthAP22 –764 <0.25 SD 11 Within range–25 <1 SD 22 <0.5 SD 1.52.5 –4 PerthAP23 –886 <0.25 SD 16 <1 SD –31 <0.5 SD 20 <1 SD 1.52.5 –4 PerthAP24 –1192 <1 SD 16 <1 SD –48 <0.5 SD 19 < 1 SD 1.52.5 –4 4. Perth AP3 Williams et al., PerthAP31 –198 <1 SD 31 <1 SD –8 WIthin range15Within range1.5 0.51 3 2013 5. Perth AP4 Marl, 1974PerthMarl1 –488 <1 SD 17 <1 SD –18 <1 SD 33 Within range1.5 20.5 4 ay of iscay alternatives 1. ay of iscayiscay1 –2190 Within range 15 <1 SD –76Within range29 <1 SD 1113 2. ay of iscay alternative iscayAlt1 –941 <0.5 SD 26 <0.5 SD –56 <0.5 SD 21 <0.5 SD 1.52.5 15 location, segment 1 Gascoyne Abyssal Plain 1. Gascoyne Robb et al., 2005 Gascoyne1 –268 <1 SD 10 Within range6Within range 40 Within range1 113 Gascoyne2 192 Within range 14 <1 SD 7Within range 13 Within range1 113 2. Gascoyne Fullerton et al., GascoyneFull1 606 Within range 55 Within range–9 <1 SD 62 Outside range0.5 0.51 2 199 Liliuoalani alternatives 1. Liliuoalani Liliuoalani1 –1557 <1 SD 22 <0.5 SD –46 <0.5 SD 26 <0.5 SD 1.52.5 15 2. Liliuoalani alternative LiliualaniAlt1 –811 <0.25 SD 20 <0.5 SD –17 <1 SD 19 <1 SD 1.52.5 04 location continued

TABLE 4B. COMPARISON OF PROPOSED ALTERNATIVE LOCATIONS OF SECONDARY- TIER RIDGES WITH PRIMARY- TIER EXTINCT RIDGE CHARACTERISTICS (continued) Assessment of likelihood Bathymetric Comparison Half-width Comparison Peak-to-trough Comparison Half- Comparison of extinct ridge formation relief with of axial with gravity signal with wavelength of with (Criteria, Table 2B) at axis primary-tier valley/ridge primary-tier at ridge axis primary-tier gravity signal primary-tier (m) mean (km) mean (mGal) mean (km) mean ABCTotal Magdalena alternatives 1. Magdalena (Lonsdale, 1991) Magdalena1 663 Within range 46 Within range19Within range42Within range0.5 112.5 Magdalena2 935 Within range 26 <1 SD –52 <0.5 SD 38 Within range1 214 Magdalena3 896 Within range 17 <1 SD 32 Within range17 <1 SD 1113 Magdalena4 –1128 <1 SD 11 <1 SD –35 <0.25 SD 15 Within range1.5 214.5 Magdalena5 –477 <1 SD 18 <0.5 SD –16 <1 SD 30 <1 SD 1.52.5 15 2. Magdalena Michaud et al., MagdaNew1 1288 Within range 22 <1 SD 59 Within range31 <1 SD 1113 2006 MagdaNew2 1074 Within range 7Within range –34 <0.5 SD 22 <0.5 SD 0.52.5 14 MagdaNew3 2140 Outside range 29 <1 SD –61 <1 SD 28 <1 SD 0.52.5 14 MagdaNew4 1191 Within range 14 <1 SD 28 Within range34Within range1 113 MagdaNew5 –1119 <1 SD 9Within range 34 Within range18 <1 SD 11.5 13.5 MagdaNew6 –1164 <1 SD 13 <1 SD 35 Within range14Within range1.5 113.5 Selir alternatives 1. Selir microplate lais Selir1 –1698 Within range 7Within range –46 <0.5 SD 14 Within range0.5 224.5 et al., 2002 Selir2 2068 Within range 29 <1 SD 76 Within range17Within range1 0.52 3.5 2. Selir Trough Selir21 –733 <0.25 SD 9Within range –17 <1 SD 12 Within range1 20.5 3.5 Gonneville Tr iangle region 1. Diamantina fracture one F DiamantinaF1 –2827 Within range 35 <1 SD –96Within range35Within range1 0.51 2.5 DiamantinaF2 –2825 Within range 38 <1 SD –120 Within range27 <0.5 SD 10.5 12.5 DiamantinaF3 –2010 Within range 35 <1 SD –100 Within range33Within range1 0.51 2.5 DiamantinaF4 –1587 <1 SD 18 <1 SD –72Within range20 <1 SD 1.51 13.5 DiamantinaF5 –1097 <0.5 SD 16 <1 SD –44 <0.5 SD 20 <1 SD 1.52.5 15 DiamantinaF6 –1917 Within range 18 <1 SD –95Within range25 <0.25 SD 1113 2. Gonneville Tr iangle SouthPAP1 –692 <0.5 SD 31 <1 SD –36 <0.25 SD 27 <0.5 SD 1.52.5 –4 alternative location SouthPAP2 –561 <0.5 SD 24 <0.25 SD –24 <1 SD 28 <1 SD 1.52.5 –4 Notes: Alternative locations that are less than one standard deviation of the mean for primary-tier segments are shown in bold. Scores of 4 indicate the feature is a probable extinct ridge; scores between 3 and 4 indicate a possible extinct ridge; and scores 3 indicate the feature is unliely to be an extinct ridge or was heavily modified by volcanism or deformation after spreading cessation. Where no data were available on spreading characteristics from published studies, the cell is mared –. If there have been no published studies for a proposed extinct ridge location, the criteria C is mared –. More detailed review of the evidence in support or against interpretation of the proposed locations is presented in our extinct ridge database on the GPlates Portal see text footnote 1 using reference IDs that are cross-referenced with Table 1.

Increasing relief in the backarc basin spreading ridges after a longer duration crust, such as on passive margins or at extinct island arcs. Thinner crust and of spreading provides some physical evidence to support the cessation mech- a higher temperature regime within the backarc environment have also been anism proposed by Stern and Dickinson (2010). They proposed that melts de- argued to promote subduction polarity reversals, given that the strength of the rived from the hydrated mantle wedge may become inaccessible as the backarc lithosphere is reduced and is more prone to “rupture” (Stern, 2004, p. 280). basin width increases over time (Stern and Dickinson, 2010). Reduced magmatic There is therefore a geodynamic explanation for the lower rates of preservation supply is likely to result in a similar axial morphology to a slow-spreading axis. of backarc-basin–formed crust and their extinct ridges over geological periods. The lack of any examples of backarc basin ridges greater than Cenozoic age, despite much older preserved Cretaceous microplates and large-scale Evaluation of Controversial and Previously XMORs (Fig. 6C), illustrates the lower preservation potential for backarc basin Unreported Possible Extinct Ridges oceanic crust. Several extinct backarc basins are reported to have been partially or fully subducted a relatively short time after their formation (for example, the By determining the character of well-defined extinct ridges, it is possible to South Loyalty, Solomon, and Santa Cruz Basins; Schellart et al., 2006; Stern and estimate the probability of some enigmatic, poorly mapped seafloor features Dickinson, 2010). Previous studies (Karig, 1982; Faccenna et al., 1999) have sug- being former spreading centers. For this purpose, the magnitude of the gravity gested that subduction may initiate at the interface of oceanic and continental anomaly at the ridge axis appears to be the most reliable indicator for evaluat-

Figure 8. Screenshot of an example from our extinct ridges database hosted in the GPlates Portal (see footnote 1). Ex- tinct ridges are colored according to their assigned ranking, as described in Figure 1, and compiled data can be accessed on the Web site by clicking a feature or choosing it from the index. Each digitized extinct ridge segment has its spreading characteristics and physical properties summarized in the floating window shown. Further information on the individual ridge systems are available by clicking the link within the summary table, which opens to a new page containing maps and profiles across the segments and a summary of key works undertaken at each ridge.

ing suspect features (Tables 4A and 4B). The bathymetric relief is more likely to -DelCano_Ridge .html] and associated map and profile files available in .ZIP be modified by later sedimentation; yet the gravity anomalies of some extinct Files S3 [file name S2-4a_Maps _Conrad .jpg] and S4 [file names S2-4_Plots ridge subtypes are in a similar range to that of active subtype examples, par- _Bathymetry_Conrad .png and S2-4_Plots _Grav _Conrad -DelCano .png] of Sup- ticularly XMOR and XBABR. plemental Materials [footnote 2]) in the southern Indian Ocean (Cross et al., A proposed ridge between the Conrad Rise and Del Cano Rise (Fig. 4iii; 2011; Nogi et al., 2011) has a morphology and gravity anomaly that strongly Table 4A; combined materials on GPlates Portal [footnote 1; http://portal resembles other extinct spreading centers such as the West Philippine Basin .gplates.org/static/html/ExRidges/ExRidges_HTML_pages/2-04_Conrad ridge. Recovery of “granitic” rocks (Kobayashi et al., 2013, abstract) and meta

morphic rocks from dredges on the Ob Seamount (Nogi et al., 2011) strongly structure is scored as a possible extinct ridge in the absence of any additional suggests the presence of a continental fragment within the Conrad Rise; this data but presents a possible target for future surveys. fragment requires a ridge-jump and extinct ridge in this location. The extinct Of the ambiguous oceanic structures formed during the Cretaceous Nor- ridge is also supported by the marked asymmetry to the north of the Enderby mal Superchron in the northwest Pacific Ocean that we reviewed, the Hok- Basin between Madagascar and Antarctica (Müller et al., 2008). Therefore, this kaido Trough (Mammerickx and Sharman, 1988) most closely resembles the feature is considered a probable extinct spreading center. well-defined extinct ridges in profile (Table 4A; combined materials on GPlates We evaluate the origin of several structures that have not been previously Portal [footnote 1; http://portal.gplates.org/static/html/ExRidges/ExRidges classified. Of these, the “Mati” ridge (Fig. 4i), in the western Palau Basin of _HTML_pages /3-10 _Hokkaido _trough .html] and associated map and pro- the southern West Philippine Sea, bears the closest resemblance to an extinct file files are in .ZIP File S3 [footnote 2; file names S3-10_Plots _Bathymetry ridge in map view (Fig. 4i), profile (combined materials on GPlates Portal [foot- _HokkaidoTrough.png and S3-10_Plots _Grav _HokkaidoTrough .png]) and is note 1; http://portal.gplates.org/static/html/ExRidges/ExRidges_HTML_pages thought to preserve a piece of the mostly subducted Izanagi plate. In contrast, /4-04_Mati _Palau .html] and associated profile files available in .ZIP File S4 the Emperor Trough has an asymmetric morphology with the eastern flank [footnote 2; file names S4-4_Plots _Bathymetry _Mati .png and S4-4_Plots _Grav consistently more elevated by ~1000 m relative to the western flank (com- _Mati.png]), and by the magnitude of the gravity anomalies at axial segments, bined materials on GPlates Portal [footnote 1; http://portal .gplates .org /static which are consistent with those of well-defined ridges. However, with limited /html/ExRidges /ExRidges _HTML _pages /3-14 _Emperor .html] and profile files in magnetic anomaly identifications in the western Palau Basin, or other age con- .ZIP File S4 [footnote 2; file names S3-14_Plots _Bathymetry _Emperor .png and straints on the crust in the region of the proposed extinct ridge, additional S3-14_Plots _Grav _Emperor .png]). The observed asymmetry is characteristic data are needed to evaluate this structure. The “Palau” feature, to the east of an oceanic transform fault and is usually generated by differential oceanic of the Palau Basin, is less characteristic of an extinct ridge in map view (Fig. crustal age either side of the structure, and therefore we conclude that the 4-iv; combined materials on GPlates Portal [footnote 1; http://portal.gplates .org trough is unlikely to be an extinct ridge as was previously suggested (Mam- /static/html /ExRidges /ExRidges _HTML _pages /4-04 _Mati _Palau .html] and asso- merickx and Sharman, 1988). ciated map file in .ZIP File S3 [footnote 2; file name S4-4-2a_Maps_Palau .jpg]), Evaluation of alternative ridge locations in controversial regions (Tables 4A although it bears some resemblance to the most easterly segment of the West and 4B) offers some insight as to the probability of uncertain structures being Somali Basin extinct ridge. The inferred axial location is a bathymetric trough; extinct ridges. In several locations, a proposed axial location that is closer to however, the morphology is complex and highly variable, making it difficult to the mean gravity signal, width, and bathymetric relief of the primary-tier ex- conclude that the segment is an extinct ridge segment. The “Palau” ridge is tinct ridges can be argued to be more likely to represent a former spreading an intriguing structure for its similarity to the West Somali Basin segment and center. However, the comparison is less helpful where large volcanic ridges therefore would be an interesting target for further investigation. and edifices are present at proposed extinct ridge locations that are likely to A possible extinct ridge is observed in the eastern Argentine Basin, south- be the result of postextinction overprinting. It is also possible that a proportion west Atlantic Ocean. It is located north of South Georgia and the Falkland- of the secondary and tertiary ridges investigated here, but not included in sta- Agulhas fracture zone and is a subdued feature (Fig. 4-ii) that has minimal tistical analyses for reasons discussed above, experienced a different mech- expression in bathymetry and gravity cross profiles (Table 4A; combined mate- anism of cessation. These may not have developed obvious “extinct ridge- rials on GPlates Portal [footnote 1; http://portal .gplates .org /static /html /ExRidges like” trough structures, potentially due to a higher spreading rate and rapid /ExRidges_HTML _pages /1-09 _North _Falkand _possible _ex _ridge .html] and transfer to the new location. Candidates in this subset include the Gallego and profile files in .ZIP File S4 [footnote 2; file names S1-9_Plots _Bathymetry Roggeveen ridge-jumps that are proposed in the eastern Pacific (Mammerickx _NorthFalkland.png and S1-9_Plots _Grav _NorthFalkland .png). Discontinuous et al., 1980; Okal and Bergeal, 1983), and that are required by significant asym- fracture zones to the east of the suspected extinct ridge suggest a plate reor- metry of oceanic crust (Müller et al., 2008), yet have left no significant axial ganization (Fig. 4-ii), and a ridge-jump would address asymmetry of ~250 km signature. The Roggeveen Rise, in particular, is located in a region of chaotic on the South American plate. The age of the extinct ridge in this location is seafloor fabric that is proposed to have resulted from successive formation of estimated to be older than chron C33o (79.9 Ma), which has been identified overlapping spreading centers and ridge-crest microplates due to high spread- to the east of the possible axis (Granot et al., 2012; Granot and Dyment, 2015). ing rates (Searle et al., 1995; Matthews et al., 2011). Three other extinct ridges are proposed in the South Atlantic to the north including the Vema (Pérez-Díaz and Eagles, 2014), Abimael (Scotchman et al., Characteristic versus Atypical Ridges 2010), and Angolan Basin (Sandwell et al., 2014) extinct ridges. Two of these ridges involved similar eastward migration of the spreading center, and there- While complex three-dimensional structures may be present, this review fore there is significant evidence for a complex opening of the South Atlantic demonstrates that the majority of established extinct ridges exhibit the char- (Heine et al., 2013; Pérez-Díaz and Eagles, 2014; Granot and Dyment, 2015). This acteristics of slow-spreading centers. They are defined by prominent axial

valleys and uplifted flanks, which are accompanied by negative gravity anom- to greater complexity in the east of the ocean. It has the greatest relative pro- alies that have a symmetric signature. The modest bathymetric relief found portion of subduction zones along its boundaries, and the Pacific plate has ex- between extinct rift flanks and axial valleys (Fig. 5A) is similar in morphology perienced frequent changes in the pole of rotation (Wessel and Kroenke, 2008). to active ridges with full spreading rates of less than 80 mm yr–1 (see for exam- It may be that a higher proportion of hotspots in the Pacific Ocean also contrib- ple, Small, 1998; Fig. 5). Yet, a small number of ridges do not have the charac- utes to the propensity for ridge reorganizations because hotspots have been teristics of slow-spreading centers and instead are characterized by high-relief suggested to facilitate ridge-jumps (Nakanishi et al., 1999; Müller et al., 2001). volcanic ridges and positive gravity anomalies. The observation of a subgroup The increased frequency of large-scale ridge reorganizations in the Pa- of “atypical” extinct ridges suggests that several uncertain volcanic structures, cific is broadly consistent with the observation of higher rates of small-scale such as the Sonne, Sonja, or Dirk Hartog ridges (Mihut and Müller, 1998; Robb spreading system migration in the Pacific, relative to lower levels in the Atlan- et al., 2005; Watson et al., 2016) could potentially be explained as extinct ridges, tic Ocean (Whittaker et al., 2015). It is also consistent with increased levels of although alternative mechanisms for formation are not excluded. skew in the Pacific plate spreading directions in the past, represented by an angular mismatch between the paleospreading direction and absolute plate Regional Distribution of Extinct Ridges motion, compared with the Indian and Atlantic oceans (Williams et al., 2016). It may be that the combined influence of numerous, active and proximal sub- The Pacific Ocean contains a greater number of extinct spreading centers duction zones and a greater number of oceanic hotspots contributes to the fre- (38), than the Indian (16) and Atlantic (10) oceans combined total of 26 ridges, quency of ridge reorganizations in the Pacific. We infer that the greater number despite the present area of the Pacific being of comparable area to the other of extinct ridges in marginal basins relates to their smaller size and that they two major ocean basins (Fig. 9A). We note that extinct ridges in the Atlantic are shorter-lived (Fig. 6A-III) and thus represent a greater number of spreading and Indian oceans were typically active for longer prior to cessation than those systems than the major oceans, despite the fact that many of the marginal in the Pacific Ocean and marginal basins (Fig. 6A-V), which also suggests a basins are bounded by active, destructive plate margins. regional influence on the longer-term stability of an oceanic ridge. The greater number of ridge reorganizations in the Pacific may be due to the greater influence of time-varying slab pull forces and/or the higher proportion of oceanic CONCLUSIONS crust of younger age in the Pacific (Fig. 9B), since extinct ridges are more likely to be preserved in younger aged crust. However, marginal basins preserve a Well-defined extinct ridges often have trough morphology, with mean relief large number of extinct ridges (27) compared to their present-day area (~53 –772 m (SD = 816 m) and a negative peak-to-trough free-air gravity signal of million square km) and do not have a higher proportion of younger crust than ~39 mGal (SD = 30), with half-wavelength ~24 km (SD = 7). This is slightly more the other ocean basins (Fig. 9B). prominent than the peak-to-trough signal at active spreading ridges (mean = We observe that the number of extinct ridges in each major ocean in- –35 mGal, SD = 46 mGal) and likely reflects a crustal low-density body, gen- creases relative to the increasing complexity of the geometry of the plate sys- erated by extensive alteration of the seafloor or by emplacement of late-stage tem and as the oceans have become increasingly dominated by active rather magmatic bodies. There is considerable variation within individual spreading than passive margins. For example, the Atlantic Ocean has the lowest number systems and axial characteristics differ according to ridge subtype, with in- of proposed extinct ridges and for the majority of its evolution has essentially creased relief observed at axial segments of extinct fragmented plate ridges been controlled by a two-plate system, stable Euler poles, and is surrounded and microplate ridges (but not their active counterparts) and more pronounced by passive margins. The Indian Ocean is observed to have a greater number axial relief at extinct backarc basin ridges that were active longer before ces- of extinct ridges than the Atlantic despite its smaller size, but significantly less sation. Regional differences are evident in the numbers of extinct ridges than the Pacific Ocean. At the initiation of spreading in the Indian Ocean, a sim- present, with the Pacific Ocean and marginal basins recording more extinct ple two-plate system was present between Africa and Antarctica (Jokat et al., ridges; these ridges are likely to be a result of a higher proportion of younger 2003), and the spreading system later increased in complexity. The ocean has aged crust and more complex plate boundaries. Extinct backarc basin ridges generally been dominated by either single or double triple-junction spreading and extinct microplate spreading ridges have shorter durations of spreading systems, with a lengthy active destructive margin in the north (Gibbons et al., and faster spreading rates than large-scale mid-ocean ridges. Evaluation of 2013). The Pacific Ocean is composed of a number of oceanic plates, including uncertain extinct ridges by comparison with well-defined examples assists in the Pacific plate, Nazca and Cocos plates, and the remnant Juan de Fuca and identifying the structures more likely to have been former spreading centers Rivera plates (Bird, 2003). During the Mesozoic, two synchronous triple-junc- and can improve regional reconstructions. Our catalogue of global large-scale tion systems were active in the Pacific (Izanagi-Pacific-Farallon and Pacific- extinct ridge locations provides a resource that improves access to the many Farallon-Phoenix; Winterer, 1991; Nakanishi et al., 1999), and more recently, individual studies that have been completed at extinct ridges and summarizes the fragmentation of the Farallon plate (Lonsdale, 1991, 2005) has contributed quantitative data about the axial characteristics of extinct spreading centers.

ATLANTIC MARGINAL ~74 x 106 6 square km ~53 x 10 square km PACIFIC ~134 x 106 square km INDIAN ~69 x 106 square km

Figure 9. (A) Approximate surface area of B the ocean basins defined in this study. (B) Histograms of the crustal age of ocean floor for each of the oceans defined in this Age of ocean crust in global ocean basins study, at present day.

300,000 Crustal age (Ma) 0–20 280,000 20–40 40–60 260,000 60–80 240,000 80–100 100–120 220,000 120–140 140–160 200,000 160–180 180,000 180–400 )

160,000

140,000

120,000

100,000 rea (square kilometres A

80,000

60,000

40,000

20,000

INDIAN MARGINAL PACIFIC ATLANTIC

Our review has identified several new oceanic structures that are possible ex- Blais, A., Gente, P., Maia, M., and Naar, D.F., 2002, A history of the Selkirk paleomicroplate: Tec- tinct ridges and represent interesting targets for further study. These include tonophysics, v. 359, no. 1, p. 157–169, doi:10.1016/S0040-1951(02)00509-7. Briais, A., Patriat, P., and Taponnier, P., 1993, Updated Interpretation of Magnetic Anomalies and the Mati Ridge in the western Palau Basin, West Philippine Sea, and the North Seafloor Spreading Stages in the South China Sea: Implications for the Tertiary Tectonics of Falkland structure in the eastern Argentine Basin, southwest Atlantic. Southeast Asia: Journal of Geophysical Research, v. 98, no. B4, p. 6299–6328, doi:10 .1029 /92JB02280. Cande, S.C., and Rabinowitz, P.D., 1978, Mesozoic seafloor spreading bordering conjugate continental margins of Angola and Brazil: Offshore Technology Conference, p. 1869–1872, doi: ACKNOWLEDGMENTS 10.4043/3268-MS. S.J.M. was supported by the Australia-India Strategic Research Fund and the University of Sydney Cande, S.C., Herron, E.M., and Hall, B.R., 1982, The early Cenozoic tectonic history of the southeast Postgraduate Award. K.J.M. and R.D.M. were supported by ARC Discovery Project DP13010946. Pacific: Earth and Planetary Science Letters, v. 57, p. 63–74, doi:10 .1016 /0012 -821X (82)90173 -X. S.E.W. was supported by the Science Industry Endowment Fund (RP 04-174) Big Data Knowledge Cande, S.C., Stock, J.M., Müller, R.D., and Ishihara, T., 2000, Cenozoic motion between East and Discovery Project. We are grateful for helpful discussions with Dr. Yatheesh Vaddakkeyakath and West Antarctica: Nature, v. 404, no. 6774, p. 145–150, doi:10.1038/35004501. Dr. Joanne Whittaker in the early stages of this project. We are grateful to William Sager and an Carbotte, S.M., Smith, D.K., Cannat, M., and Klein, E., 2015, Tectonic and magmatic segmentation anonymous reviewer for their thorough reading of the text and for constructive suggestions that of the Global Ocean Ridge System: A synthesis of observations, in Wright, T.J., Ayele, A., allowed us to improve the manuscript. We thank Laurent Gernigon and an anonymous reviewer Ferguson, D.J., Kidane, T., and Vye-Brown, C., eds., Magmatic Rifting and Active Volcanism: for their helpful feedback and comments on an earlier version of this work. Geological Society of London Special Publication 420, no. 1, p. 249, doi:10.1144/SP420.5. Chamot-Rooke, N., Renard, V., and Le Pichon, X., 1987, Magnetic anomalies in the Shikoku Basin: A new interpretation: Earth and Planetary Science Letters, v. 83, p. 214–228, doi:10.1016/0012 REFERENCES CITED -821X(87)90067-7. Anderson, R.N., Moore, G.F., Schilt, S.S., Cardwell, R.C., Tréhu, A., and Vacquier, V., 1976, Heat Chandler, M.T., Wessel, P., Taylor, B., Seton, M., Kim, S., and Hyeong, K., 2012, Reconstructing flow near a fossil ridge on the north flank of the Galapagos Spreading Center: Journal of Ontong Java Nui: Implications for Pacific absolute plate motion, hotspot drift and true polar Geophysical Research, v. 81, no. 11, p. 1828–1838, doi:10 .1029/JB081i011p01828. wander: Earth and Planetary Science Letters, v. 331–332, p. 140–151, doi:10 .1016/j .epsl.2012 Atwater, T., Sclater, J., Sandwell, D., Severinghaus, J., and Marlow, M.S., 1993, Fracture zone .03.017. traces across the North Pacific Cretaceous quiet zone and their tectonic implications,in Cochran, J., 1988, Somali Basin, Chain Ridge, and origin of the Northern Somali Basin grav- Pringle, M.S., Sager, W.W., Sliter, W.V., and Stein, S., eds., The Mesozoic Pacific: Geology, Tec- ity and geoid low: Journal of Geophysical Research: Solid Earth, v. 93, no. B10, p. 11,985– tonics, and Volcanism: American Geophysical Union, p. 137–154, doi:10.1029 /GM077p0137 . 12,008, doi:10.1029/JB093iB10p11985. Auzende, J.M., Pelletier, B., and Lafoy, Y., 1994, Twin active spreading ridges in the North Fiji Cross, A., Kusznir, N., and Alvey, A., 2011, Structure and origin of the Crozet Plateau and Conrad Basin (southwest Pacific): Geology, v. 22, no. 1, p. 63–66, 10doi: .1130/0091-7613(1994)022 Rise, SW Indian Ocean: Insights from crustal thickness mapping using 3-D satellite gravity <0063:TASRIT>2.3.CO;2. inversion: American Geophysical Union, Fall Meeting 2011, abstract #V51D-2544. Baranov, B.V., Seliverstov, N.I., Murav’ev, A.V., and Muzurov, E.L., 1991, The Komandorsky Ba- Cuffaro, M., and Jurdy, D.M., 2006, Microplate motions in the hotspot reference frame: Terra sin as a product of spreading behind a transform plate boundary: Tectonophysics, v. 199, Nova, v. 18, no. 4, p. 276–281, doi:10.1111/j.1365-3121.2006.00690.x. p. 237–269, doi:10.1016/0040-1951(91)90174-Q. DeMets, C., and Stein, S., 1990, Present-day kinematics of the Rivera plate and implications Batiza, R., 1977, Petrology and chemistry of Guadalupe Island: An alkalic seamount on a fossil for tectonics in southwestern Mexico: Journal of Geophysical Research: Solid Earth, v. 95, ridge crest: Geology, v. 5, p. 760–764, doi:10.1130/0091-7613(1977)5<760:PACOGI>2.0.CO;2. no. B13, p. 21,931–21,948, doi:10.1029/JB095iB13p21931. Batiza, R., 1989, Failed rifts, in Winterer, E.L., Hussong, D.M., and Decker, R.W., eds., The Eastern Eagles, G., and Livermore, R.A., 2002, Opening history of Powell Basin, Antarctic Peninsula: Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, The Geology Marine Geology, v. 185, no. 3–4, p. 195–205, doi:10.1016/S0025-3227(02)00191-3. of North America, v. N, p. 177–186. Eagles, G., Livermore, R., and Morris, P., 2006, Small basins in the Scotia Sea: The Eocene Drake Batiza, R., 1996, Magmatic segmentation of mid-ocean ridges: A review: Geological Society of Passage gateway: Earth and Planetary Science Letters, v. 242, no. 3–4, p. 343–353, doi:10 London, Special Publications, v. 118, no. 1, p. 103–130, doi:10.1144/GSL.SP.1996.118.01.06. .1016/j.epsl.2005.11.060. Batiza, R., and Vanko, D.A., 1985, Petrologic evolution of large failed rifts in the Eastern Pacific: Eakins, B.W., and Lonsdale, P.F., 2003, Structural patterns and tectonic history of the Bauer micro Petrology of volcanic and plutonic rocks from the Mathematician Ridge area and the Guada plate, Eastern Tropical Pacific: Marine Geophysical Researches, v. 24, no. 3–4, p. 171–205, lupe Trough: Journal of Petrology, v. 26, no. 3, p. 564–602, doi:10.1093/petrology/26.3.564. doi:10.1007/s11001-004-5882-4. Beiersdorf, H., Bach, W., Delisle, G., Faber, E., Gerling, P., and Hinz, K., 1997, Age and possible Engeln, J.F., Stein, S., Werner, J., and Gordon, R., 1988, Microplate and shear zone models for modes of formation of the Celebes Sea basement and thermal regimes within the accretion- oceanic spreading center reorganizations: Journal of Geophysical Research, v. 93, no. B4, ary complexes off SW Mindanao and N. Sulawesi, in Dheeradilok, P., et al., eds., Proceedings p. 2839–2856, doi:10.1029/JB093iB04p02839. of the International Conference on Stratigraphic and Tectonic Evolution of Southeast Asia Faccenna, C., Giardini, D., Davy, P., and Argentieri, A., 1999, Initiation of subduction at Atlan- and the South Pacific, Integrated Ocean Drilling Program, p. 369–387. tic-type margins: Insights from laboratory experiments: Journal of Geophysical Research, Bernard, A. and Munschy, M., 2000, Le bassin des Mascareignes et le bassin de Laxmi (océan v. 104, no. B2, p. 2749–2766, doi:10.1029/1998JB900072. Indien occidental) se sont-ils formés à l’axe d’un même centre d’expansion? (Were the Mas- Favela, J., and Anderson, D.L., 2000, Extensional Tectonics and Global Volcanism, in Boschi, carene and Laxmi Basins (western Indian Ocean) formed at the same spreading centre?): E., Ekstrom, G., and Morelli, A., eds., Problems in Geophysics for the New Millennium: Comptes Rendus de l’Académie des Sciences, Series IIA, Earth and Planetary Science, v. 330, Bologna, Italy, Conference Proceedings (Editrice Compositori), p. 463–498. no.11, p. 777–783. Fullerton, L.G., Sager, W.W., and Handmuscher, D., 1989, Late Jurassic–Early Cretaceous evo- Bhattacharya, G.C., Chaubey, A.K., Murty, G.P.S., Srinivas, K., Sarma, K.V.L.N.S., Subrah- lution of the eastern Indian Ocean adjacent to northwest Australia: Journal of Geophysical manyam, V., and Krishna, K.S., 1994, Evidence for seafloor spreading in the Laxmi Basin, Research, v. 94, no. B3, p. 2937–2953, doi:10.1029/JB094iB03p02937. northeastern Arabian Sea: Earth and Planetary Science Letters, v. 125, p. 211–220, doi:10 Gaina, C., and Müller, R.D., 2007, Cenozoic tectonic and depth/age evolution of the Indonesian .1016/0012-821X(94)90216-X. gateway and associated backarc basins: Earth-Science Reviews, v. 83, no. 3–4, p. 177–203, Bird, R.T., and Naar, D.F., 1994, Intratransform origins of mid-ocean ridge microplates: Geology, doi:10.1016/j.earscirev.2007.04.004. v. 22, p. 987–990, doi:10.1130/0091-7613(1994)022<0987:IOOMOR>2.3.CO;2. Gaina, C., Müller, D.R., Royer, J.Y., Stock, J., Hardebeck, J., and Symonds, P., 1998, The tectonic Bird, P., 2003, An updated digital model of plate boundaries: Geochemistry Geophysics Geosys- history of the Tasman Sea: A puzzle with 13 pieces: Journal of Geophysical Research, v. 103, tems, v. 4, no. 3, doi:10.1029/2001GC000252. no. B6, p. 12,413–12,433, doi:10.1029/98JB00386.

Gaina, C., Müller, R.D., Royer, J.-Y., and Symonds, P., 1999, Evolution of the Louisiade triple Jackson, H.R., Keen, C.E., Falconer, R.K.H., and Appleton, K.P., 1979, New geophysical evidence junction: Journal of Geophysical Research, v. 104, no. B6, p. 12,927–12,939, doi:10 .1029 for sea-floor spreading in central Baffin Bay: Canadian Journal of Earth Sciences, v. 16, /1999JB900038. p. 2122–2135, doi:10.1139/e79-200. Gaina, C., Müller, R.D., Brown, B., Ishihara, T., and Ivanov, S., 2007, Breakup and early seafloor Jokat, W., Boebel, T., König, M., and Meyer, U., 2013, Timing and geometry of early Gondwana spreading between India and Antarctica: Geophysical Journal International, v. 170, no. 1, breakup: Journal of Geophysical Research, v. 108, 2428, doi:10.1029/2002JB001802. p. 151–169, doi:10.1111/j.1365-246X.2007.03450.x. Jonas, J., Hall, S., and Casey, J.F., 1991, Gravity anomalies over extinct spreading centers: A test Gee, J.S., and Kent, D., 2007, Source of oceanic magnetic anomalies and the geomagnetic polar- of gravity models of active centers: Journal of Geophysical Research, v. 96, no. B7, p. 11,759– ity timescale: Treatise on Geophysics, v. 5, p. 455–507, doi:10 .1016 /B978 -044452748 -6 /00097 -3 . 11,777, doi:10.1029/91JB00617. Gernigon, L., Gaina, C., Olesen, O., Ball, P.J., Péron-Pinvidic, G., and Yamasaki, T., 2012, The Jung, W., and Vogt, P.R., 1997, A gravity and magnetic anomaly study of the extinct Aegir Ridge, Norway Basin revisited: From continental breakup to spreading ridge extinction: Marine and Norwegian Sea: Journal of Geophysical Research, v. 102, no. B3, p. 5065–5089, doi:10.1029 Petroleum Geology, v. 35, no. 1, p. 1–19, doi:10.1016/j.marpetgeo.2012.02.015. /96JB03915. Gibbons, A.D., Barckhausen, U., van den Bogaard, P., Hoernle, K., Werner, R., Whittaker, J.M., Karig, D.E., 1982, Initiation of subduction zones: Implications for arc evolution and ophiolite and Müller, R.D., 2012, Constraining the Jurassic extent of Greater India: Tectonic evolution development, in Leggett, J.K., ed., Trench-Forearc Geology: Sedimentation and Tectonics on of the West Australian margin: Geochemistry Geophysics Geosystems, v. 13, no. 5, Q05W13, Modern and Ancient Active Plate Margins: Geological Society of London Special Publication doi:10.1029/2011GC003919. 10, p. 563–576, doi:10.1144/GSL.SP.1982.010.01.37. Gibbons, A.D., Whittaker, J.M., and Müller, R.D., 2013, The breakup of East Gondwana: assimi- Kimura, J.-I., Stern, R.J., and Yoshida, T., 2005, Reinitiation of subduction and magmatic relating constraints from Cretaceous ocean basins around India into a best-fit tectonic model: sponses in SW Japan during the Neogene time: Geological Society of America Bulletin, Journal of Geophysical Research: Solid Earth, v. 118, no. 3, p. 808–822, doi:10 .1002/jgrb v. 117, p. 969–986, doi:10.1130/B25565.1. .50079. King, E.C., and Barker, P.F., 1988, The margins of the South Orkney microcontinent: Journal of the Gillis, K.M., Snow, J.E., Klaus, A., Abe, N., Adrião, Á.B., Akizawa, N., Ceuleneer, G., Cheadle, Geological Society of London, v. 145, no. 2, p. 317–331, doi:10.1144/gsjgs.145.2.0317. M.J., Faak, K., Falloon, T.J., Friedman, S.A., Godard, M., Guerin, G., Harigane, Y., Horst, A.J., King, E.C., Leitchenkov, G., Galindo-Zaldívar, J., Maldonado, A., and Lodolo, E., 1997, Crustal Hoshide, T., Ildefonse, B., Jean, M.M., John, B.E., Koepke, J., Machi, S., Maeda, J., Marks, structure and sedimentation in Powell Basin, in Barker, P.F., and Cooper, A.K., eds., Geology N.E., McCaig, A.M., Meyer, R., Morris, A., Nozaka, T., Python, M., Saha, A., and Wintsch, and Seismic Stratigraphy of the Antarctic Margin (Part 2): American Geophysical Union, R.P., 2014, Primitive layered gabbros from fast-spreading lower oceanic crust: Nature, v. 505, p. 75–93, doi:10.1029/AR071p0075. no. 7482, p. 204–207, doi:10.1038/nature12778. Klein, E.M., Smith, D.K., Williams, C.M., and Schouten, H., 2005, Counter-rotating microplates at Goff, J.A., 1991, A global and regional stochastic analysis of near-ridge abyssal hill morphology: the Galapagos triple junction: Nature, v. 433, p. 855–858, doi:10.1038/nature03262. Journal of Geophysical Research: Solid Earth, v. 96, no. B13, p. 21,713–21,737, doi:10.1029 Kleinrock, M.C., and Hey, R.N., 1989, Detailed tectonics near the tip of the Galapagos 95.5 W /91JB02275. propagator: How the lithosphere tears and a spreading axis develops: Journal of Geophysi- Gràcia, E., and Escartín, J., 1999, Crustal accretion at mid-ocean ridges and backarc spreading cal Research, v. 94, n. B10, p. 13,801–13,838, doi:10 .1029/JB094iB10p13801. centers: Insights from the Mid-Atlantic Ridge, the Bransfield Basin, and the North Fiji Basin: Kobayashi, K., Ishuzuka, H., Nogi, Y., Sato, H., Nakano, N., Adachi, T., and Osanai, Y., 2013, Contributions in Science, v. 1, no. 2, p. 175–192. Petrography and provenance of granitic and sedimentary rocks dredged from the Conrad Granot, R., and Dyment, J., 2015, The Cretaceous opening of the South Atlantic Ocean: Earth and Rise in the Southern Indian Ocean: Fourth Symposium on Polar Science, Abstract, OG-P13. Planetary Science Letters, v. 414, p. 156–163, doi:10.1016/j.epsl.2015.01.015. Kovacs, L.C., Morris, P., Brozena, J., and Tikku, A., 2002, Seafloor spreading in the Weddell Sea Granot, R., Dyment, J., and Gallet, Y., 2012, Geomagnetic field variability during the Cretaceous from magnetic and gravity data: Tectonophysics, v. 347, p. 43–64, doi:10.1016/S0040-1951 Normal Superchron: Nature Geoscience, v. 5, no. 3, p. 220–223, doi:10.1038/ngeo1404. (01)00237-2. Greenhalgh, E.E., and Kusznir, N.J., 2007, Evidence for thin oceanic crust on the extinct Aegir Larson, R.L., Pockalny, R., Viso, R., Erba, E., Abrams, L.J., Luyendyk, B.P., Stock, J.M., and Clay- Ridge, Norwegian Basin, NE Atlantic derived from satellite gravity inversion: Geophysical ton, R.W., 2002, Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the Research Letters, v. 34, no. 6, L06305, doi:10 .1029/2007GL029440. southwestern Pacific Basin: Geology, v. 30, no. 1, p. 67–70, doi:10 .1130/0091-7613(2002)030 Haase, K.M., Beier, C., Fretzdorff, S., Leat, P.T., Livermore, R.A., Barry, T.L., Pearce, J.A., and Hauff, <0067:MCTEOT>2.0.CO;2. F., 2011a, Magmatic evolution of a dying spreading axis: Evidence for the interaction of tec- Livermore, R., 2003, Backarc spreading and mantle flow in the East Scotia Sea,in Larter, R.D., and tonics and mantle heterogeneity from the fossil Phoenix Ridge, Drake Passage: Chemical Leat, P.T., eds., Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes: Geologi Geology, v. 280, no. 1–2, p. 115–125, doi:10.1016/j.chemgeo.2010.11.002. cal Society of London Special Publication 219, p. 315–331, doi:10 .1144 /GSL .SP .2003 .219 .01 .15 . Haase, K.M., Regelous, M., Duncan, R.A., Brandl, P.A., Stroncik, N., and Grevemeyer, I., 2011b, Livermore, R., Balanyá, J.C., Maldonado, A., Martinez, J.M., Rodriguez-Fernandez, J., Sanz de Insights into mantle composition and mantle melting beneath mid-ocean ridges from Galdeano, C., Galindo-Zaldívar, J., Jabaloy, A., Barnolas, A., Somoza, L., Hernández-Molina, postspreading volcanism on the fossil Galapagos Rise: Geochemistry Geophysics Geosys- F.J., Surinach, E., and Viseras, C., 2000, Autopsy on a dead spreading center: The Phoenix tems, v. 12, no. 5, Q0AC11, doi:10.1029/2010GC003482. Ridge, Drake Passage, Antarctica: Geology, v. 28, no. 7, p. 607–610, doi:10.1130/0091-7613 Hébert, H., Villemant, B., Deplus, C., and Diament, M., 1999, Contrasting geophysical and geo- (2000)28<607:AOADSC>2.0.CO;2. chemical signatures of a volcano at the axis of the Wharton Fossil Ridge (N-E Indian Ocean): Lodolo, E., Donda, F., and Tassone, A., 2006, Western Scotia Sea margins: Improved constraints Geophysical Research Letters, v. 26, no. 8, p. 1053–1056, doi:10.1029/1999GL900160. on the opening of the Drake Passage: Journal of Geophysical Research, v. 111, no. B06101, Heine, C., Zoethout, J., and Müller, R.D., 2013, Kinematics of the South Atlantic rift: Solid Earth, doi:10.1029/2006JB004361. v. 4, no. 2, p. 215–253, doi:10.5194/se-4-215-2013. Lonsdale, P., 1988, Paleogene history of the Kula plate: Offshore evidence and onshore impli- Hekinian, R., 2014, Sea Floor Exploration: Scientific Adventures Diving into the Abyss: Heidel- cations: Geological Society of America Bulletin, v. 100, p. 733–754, doi:10.1130/0016-7606 berg, Springer, 370 p., doi:10.1007/978-3-319-03203-0. (1988)100<0733:PHOTKP>2.3.CO;2. Hey, R.N., 2004, Propagating rifts and microplates at mid-ocean ridges, in Selley, R.C., et al., eds., Lonsdale, P., 1991, Structural patterns of the Pacific floor offshore of peninsular California,in Encyclopedia of Geology: London, Academic Press, p. 396–405. Dauphin, J.P. and Simoneit, B.R.T., eds., The Gulf and Peninsular Province of the Californias: Hill, I.A., and Barker, P.F., 1980, Evidence for Miocene backarc spreading in the central Scotia Tulsa, American Association of Petroleum Geologists Bulletin, v. 47, p. 87–125. Sea: Geophysical Journal International, v. 63, no. 2, p. 427–440, doi:10 .1111/j.1365-246X.1980 Lonsdale, P., 2005, Creation of the Cocos and Nazca plates by fission of the Farallon plate: Tec- .tb02630.x. tonophysics, v. 404, no. 3–4, p. 237–264, doi:10.1016/j.tecto.2005.05.011. Hinschberger, F., Malod, J.-A., Dyment, J., Honthaas, C., and Réhault, J.-P., 2001, Magnetic linea- Louden, K.E., Osler, J.C., Srivastava, S.P., and Keen, C.E., 1996, Formation of oceanic crust at tions constraints for the backarc opening of the Late Neogene South Banda Basin (eastern slow-spreading rates: New constraints from an extinct spreading center in the Labrador Sea: Indonesia): Tectonophysics, v. 333, p. 47–59, doi:10.1016/S0040-1951(00)00266-3. Geology, v. 24, no. 9, p. 771–774, doi:10.1130/0091-7613(1996)024<0771:FOOCAS>2.3.CO;2.

Maldonado, A., Zitellini, N., Leitchenkov, G., Balanya, J.C., Coren, F., Galindo-Zaldívar, J., Lodolo, Munschy, M., 1998, La zone de Diamantine, témoin de la separation de l’Australie et de l’Artarctique: E., Jabaloy, A., Zanolla, C., Rodriguez-Fernandez, J., and Vinnikovskaya, O., 1998, Small Arguments géophysiques (The Diamantina Zone as the result of rifting between Australia ocean basin development along the Scotia-Antarctica plate boundary and in the northern and Antarctica: Geophysical constraints) [Earth and Planetary Science]: Comptes Rendus de Weddell Sea: Tectonophysics, v. 296, p. 371–402, doi:10.1016/S0040-1951(98)00153-X. l’Académie des Sciences. Série 2. Sciences de la Terre et des Planètes, v. 8, no. 327, p. 533–540. Maldonado, A., Balanyá, J.C., Barnolas, A., Galindo-Zaldívar, J., Hernández, J., Jabaloy, A., Liver- Naar, D.F., and Hey, R.N., 1991, Tectonic evolution of the Easter microplate: Journal of Geophysi more, R., Martínez-Martínez, J.M., Rodríguez-Fernández, J., De Galdeano, C.S., and Somoza, cal Research, v. 96, no. B5, p. 7961–7993, doi:10 .1029/90JB02398. L., 2000, Tectonics of an extinct ridge-transform intersection, Drake Passage (Antarctica): Nakanishi, M., and Winterer, E.L., 1998, Tectonic history of the Pacific-Farallon-Phoenix triple Marine Geophysical Researches, v. 21, no. 1–2, p. 43–68, doi:10.1023/A:1004762311398. junction from Late Jurassic to Early Cretaceous: An abandoned Mesozoic spreading system Mammerickx, J., 1981, Depth anomalies in the Pacific: Active, fossil and precursor: Earth and in the Central Pacific Basin: Journal of Geophysical Research, v. 103, p. 12,453–12,468. Planetary Science Letters, v. 53, p. 147–157, doi:10.1016/0012-821X(81)90150-3. Nakanishi, M., Sager, W., and Klaus, W., 1999, Magnetic lineations within Shatsky Rise, northwest Mammerickx, J., and Klitgord, K.D., 1982, Northern East Pacific Rise: Evolution from 25 m.y. BP Pacific Ocean: Implications for hot spot-triple junction interaction and oceanic plateau forma- to the present: Journal of Geophysical Research, v. 87, no. 6, p. 751–756. tion: Journal of Geophysical Research, v. 104, no. B4, p. 7539–7556, doi:10 .1029 /1999JB900002 . Mammerickx, J., and Sandwell, D., 1986, Rifting of old oceanic lithosphere: Journal of Geophysi Nogi, Y., Sato, H., Sato, T., Hnayu, T., Kobayashi, S., and Ishuzuka, H., 2011, Seafloor spreading cal Research, v. 91, no. B2, p. 1975–1988, doi:10.1029/JB091iB02p01975. history South of the Conrad Rise, the Southern Indian Ocean: International Symposium on Mammerickx, J., and Sharman, G.F., 1988, Tectonic evolution of the North Pacific during the Antarctic Earth Sciences, 11, abstract, p. 562. Cretaceous quiet period: Journal of Geophysical Research, v. 93, no. B4, p. 3009–3024, doi: Okal, E.A., and Bergeal, J.M., 1983, Mapping the Miocene Farallon ridge jump on the Pacific 10.1029/JB093iB04p03009. plate: A seismic line of weakness: Earth and Planetary Science Letters, v. 63, no. 1, p. 113– Mammerickx, J., Herron, E., and Dorman, L., 1980, Evidence for two fossil spreading ridges in 122, doi:10.1016/0012-821X(83)90027-4. the southeast Pacific: Geological Society of America Bulletin, v. 91, Part 1, p. 263–271, doi:10 Okino, K., and Fujioka, K., 2003, The Central Basin Spreading Center in the Philippine Sea: Struc- .1130/0016-7606(1980)91<263:EFTFSR>2.0.CO;2. ture of an extinct spreading center and implications for marginal basin formation: Journal Manea, M., Manea, V.C., Ferrari, L., Kostoglodov, V., and Bandy, W.L., 2005, Tectonic evolution of Geophysical Research, v. 108, no. B1, 2040, doi:10.1029/2001JB001095. of the Tehuantepec Ridge: Earth and Planetary Science Letters, v. 238, no. 1, p. 64–77, doi:10 Pardee, D.R., Hey, R.N., and Martinez, F., 1998, Cross-sectional areas of the mid-ocean ridge .1016/j.epsl.2005.06.060. axes bounding the Easter and Juan Fernandez microplates: Marine Geophysical Researches, Markl, R.G., 1974, Evidence for the breakup of eastern Gondwanaland by the early Cretaceous: v. 20, p. 517–531, doi:10.1023/A:1004768716893. Nature, v. 251, p. 196–200, doi:10.1038/251196a0. Pautot, G., Rangin, C., Briais, A., Wu, J., Han, S., Li, H., Lu, Y., and Zhao, J., 1990, The axial ridge Marks, K., and Stock, J.M., 2001, Evolution of the Malvinas Plate south of Africa: Marine Geo- of the South China Sea: A seabeam and geophysical survey: Oceanologica Acta, v. 13, no. 2, physical Researches, v. 22, p. 289–302, doi:10.1023/A:1014638325616. p. 129–143. Matthews, K.J., Müller, R.D., Wessel, P., and Whittaker, J.M., 2011, The tectonic fabric of the ocean Pérez-Díaz, L., and Eagles, G., 2014, Constraining South Atlantic growth with seafloor spreading basins: Journal of Geophysical Research, v. 116, no. B12, doi:10.1029/2011JB008413. data: Tectonics, v. 33, p. 1848–1873, doi:10.1002/2014TC003644. Matthews, K.J., Müller, R.D., and Sandwell, D.T., 2016, Oceanic microplate formation records the Rabinowitz, P.D., Coffin, M.F., and Falvey, D., 1983, The separation of Madagascar and Africa: onset of India-Eurasia collision: Earth and Planetary Science Letters, v. 433, p. 204–214, doi: Science, v. 220, no. 4592, p. 67–69, doi:10.1126/science.220.4592.67. 10.1016/j.epsl.2015.10.040. Robb, M.S., Taylor, B., and Goodliffe, A.M., 2005, Re-examination of the magnetic lineations of Meschede, M., Barckhausen, U., and Worm, H.U., 1998, Extinct spreading on the Cocos Ridge: the Gascoyne and Cuvier Abyssal Plains, off NW Australia: Geophysical Journal Interna- Terra Nova, v. 10, no. 4, p. 211–216, doi:10.1046/j.1365-3121.1998.00195.x. tional, v. 163, no. 1, p. 42–55, doi:10.1111/j.1365-246X.2005.02727.x. Michaud, F., Royer, J.Y., Bourgois, J., Dyment, J., Calmus, T., Bandy, W., Sosson, M., Mortera- Rollet, N., Déverchere, J., Beslier, M.O., Guennoc, P., Réhault, J.P., Sosson, M., and Truffert, C., Gutiérrez, C., Sichler, B., Rebolledo-Viera, M., and Pontoise, B., 2006, Oceanic-ridge sub- 2002, Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western duction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental Mediterranean: Tectonics, v. 21, p. 6-1–6-23, doi:10.1029/2001TC900027. margin since 15 Ma: Geology, v. 34, no. 1, p. 13–16, doi:10.1130/g22050.1. Ross, M.I., and Scotese, C.R., 1988, A hierarchical tectonic model of the Gulf of Mexico and Carib- Mihut, D., and Müller, R.D., 1998, Volcanic margin formation and Mesozoic rift propagators in the bean region: Tectonophysics, v. 155, p. 139–168, doi:10.1016/0040-1951(88)90263-6. Cuvier Abyssal Plain off Western Australia: Journal of Geophysical Research: Solid Earth, Rusby, R.I., and Searle, R.C., 1993, Intraplate thrusting near the Easter microplate: Geology, v. 21, v. 103, no. B11, p. 27,135–27,149, doi:10.1029/97JB02672. p. 311–314, doi:10.1130/0091-7613(1993)021<0311:ITNTEM>2.3.CO;2. Mishra, D.C., 1991, Magnetic crust in the Bay of Bengal: Marine Geology, v. 99, no. 1, p. 257–261, Sandwell, D.T., Müller, R.D., Smith, W.H.F., Garcia, E.S., and Francis, R., 2014, New global marine doi:10.1016/0025-3227(91)90095-L. gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure: Science, v. 346, Morgan, J.P., and Parmentier, E.M., 1985, Causes and rate-limiting mechanisms of ridge prop- p. 65–67, doi:10.1126/science.1258213. agation: A fracture mechanics model: Journal of Geophysical Research: Solid Earth, v. 90, Sasaki, T., Yamazaki, T., and Ishizuka, O., 2014, A revised spreading model of the West Philippine no. B.10, p. 8603–8612. Basin: Earth, Planets, and Space, v. 66, p. 83, doi:10.1186/1880-5981-66-83. Mortimer, N., Gans, P.B., Palin, J.M., Herzer, R.H., Pelletier, B., and Monzier, M., 2014, Eocene and Schellart, W.P., Lister, G., and Toy, V.G., 2006, A Late Cretaceous and Cenozoic reconstruction Oligocene basins and ridges of the Coral Sea–New Caledonia region: Tectonic link between of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback pro- Melanesia, Fiji, and Zealandia: Tectonics, v. 33, p. 1386–1407, doi:10.1002/2014TC003598. cesses: Earth-Science Reviews, v. 76, p. 191–233, doi:10.1016/j.earscirev.2006.01.002. Mrozowski, C.L., and Hayes, D.E., 1979, The evolution of the Parece Vela Basin, eastern Philippine Schouten, H., Klitgord, K.D., and Whitehead, J.A., 1985, Segmentation of mid-ocean ridges: Na- Sea: Earth and Planetary Science Letters, v. 46, p. 49–67, doi:10.1016/0012-821X(79)90065-7. ture, v. 317, no. 6034, p. 225–229, doi:10.1038/317225a0. Müller, R.D., Gaina, C., Roest, W., and Hansen, D.L., 2001, A recipe for microcontinent formation: Schouten, H., Klitgord, K.D., and Gallo, D.G., 1993, Edge-driven microplate kinematics: Journal Geology, v. 29, no. 3, p. 203–206, doi:10.1130/0091-7613. of Geophysical Research, v. 98, p. 6689–6701, doi:10.1029/92JB02749. Müller, R.D., Sdrolias, M., Gaina, C., and Roest, W.R., 2008, Age, spreading rates, and spreading Scotchman, I.C., Gilchrist, G., Kusznir, N.J., Roberts, A.M., and Fletcher, R., 2010, The breakup of asymmetry of the world’s ocean crust: Geochemistry Geophysics Geosystems, v. 9, no. 4, the South Atlantic Ocean: Formation of failed spreading axes and blocks of thinned conti- Q04006, doi:10.1029/2007GC001743. nental crust in the Santos Basin, Brazil and its consequences for petroleum system develop- Müller, R.D., Seton, M., Zahirovic, S., Williams, S.E., Matthews, K.J., Wright, N.M., Shephard, G.E., ment, in Vining, B.A. and Pickering, S.C., eds., Petroleum Geology: From Mature Basins to Maloney, K., Barnett-Moore, N., Hosseinpour, M., and Bower, D.J., 2016, Ocean Basin Evo- New Frontiers: London, Proceedings of the 7th Petroleum Geology Conference, Geological lution and Global-Scale Plate Reorganization Events Since Pangea Breakup: Annual Review Society of London, p. 855–866. of Earth and Planetary Sciences, v. 44, no. 1, p. 107–138, doi:10.1146 /annurev -earth-060115 Sdrolias, M., Müller, R.D., and Gaina, C., 2003, Tectonic evolution of the southwest Pacific using -012211. constraints from backarc basins, in Hillis, R.R., and Müller, R.D., Evolution and Dynamics of

the Australian Plate: Geological Society of America Special Publication 372, p. 343–359, doi: Tebbens, S.F., and Cande, S.C., 1997, Southeast Pacific tectonic evolution from early Oligocene 10.1130/0-8137-2372-8.343. to present: Journal of Geophysical Research, v. 102, no. B6, p. 12,061–12,084, doi:10.1029 Sdrolias, M., Roest, W.R., and Müller, R.D., 2004, An expression of Philippine Sea plate rotation: /96JB02582. The Parece Vela and Shikoku basins: Tectonophysics, v. 394, p. 69–86, doi:10.1016/j.tecto Tikku, A., Marks, K., and Kovacs, L.C., 2002, An Early Cretaceous extinct spreading center in the .2004.07.061. northern Natal Valley: Tectonophysics, v. 347, p. 87–108, doi:10.1016/S0040-1951(01)00239-6. Searle, R.C., Bird, R.T., Rusby, R.I., and Naar, D.F., 1993, The development of two oceanic micro- Upton, G., and Cook, I., 2014, A Dictionary of Statistics (third edition): New York, Oxford Univer- plates: Easter and Juan Fernandez microplates, East Pacific Rise: Journal of the Geological sity Press.doi:10.1093/acref/9780199679188.001.0001. Society of London, v. 150, no. 5, p. 965–976, doi:10.1144/gsjgs.150.5.0965. van der Linden, W.J.M., 1969, Extinct mid-ocean ridges in the Tasman Sea and in the West- Searle, R.C., Francheteau, J., and Cornaglia, B., 1995, New observations on mid-plate volcanism ern Pacific: Earth and Planetary Science Letters, v. 6, p. 483–490, doi:10.1016/0012-821X and the tectonic history of the Pacific plate, Tahiti to Easter microplate: Earth and Planetary (69)90120-4. Science Letters, v. 131, no. 3, p. 395–421, doi:10.1016/0012-821X(95)00018-8. Watson, S.J., Whittaker, J.M., Halpin, J.A., Williams, S.E., Milan, L.A., Daczko, N., and Wyman, Segoufin, J., M., Munschy, P., Bouysse, and V. Mendel, V., 2004, Map of the Indian Ocean: Com- D.A., 2016, Tectonic drivers and the influence of the Kerguelen plume on seafloor spreading mission for the Geological Map of the World, scale 1:20,000,000, 2 sheets. during formation of the Early Indian Ocean: Gondwana Research, v. 35, p. 97–114, doi:10 Seton, M., Müller, R.D., Zahirovic, S., Gaina, C., Torsvik, T., Shephard, G., Talsma, A., Gurnis, .1016/j.gr.2016.03.009. M., Turner, M., Maus, S., and Chandler, M., 2012, Global continental and ocean basin re- Weatherall, P., Marks, K.M., Jakobsson, M., Schmitt, T., Tani, S., Arndt, J.E., Rovere, M., Chayes, constructions since 200 Ma: Earth-Science Reviews, v. 113, no. 3, p. 212–270, doi:10.1016/j D., Ferrini, V., and Wigley, R., 2015, A new digital bathymetric model of the world’s oceans: .earscirev.2012.03.002. Earth and Space Science, v. 2, p. 331–345, doi:10.1002/2015EA000107. Seton, M., Whittaker, J.M., Wessel, P., Müller, R.D., DeMets, C., Merkouriev, S., Cande, S., Gaina, Wessel, P., and Kroenke, L.W., 2008, Pacific absolute plate motion since 145 Ma: An assessment C., Eagles, G., Granot, R., Stock, J., Wright, N., and Williams, S.E., 2014, Community infra of the fixed hotspot hypothesis: Journal of Geophysical Research, v. 113, no. B6, p. 2156– structure and repository for marine magnetic identifications: Geochemistry Geophysics 2202, doi:10.1029/2007JB005499. Geosystems, v. 15, no. 4, p. 1629–1641, doi:10.1002/2013GC005176. Whittaker, J.M., Williams, S.E., and Müller, R.D., 2013, Revised tectonic evolution of the Eastern Small, C., 1998, Global systematics of mid-ocean ridge morphology, in Buck, W.R., Delaney, P.T., Indian Ocean: Geochemistry Geophysics Geosystems, v. 14, no. 6, p. 1891–1909, doi:10 .1002 Karson, J.A., and Lagabrielle, Y., eds., Faulting and Magmatism at Mid-Ocean Ridges: American /ggge.20120. Geophysical Union Geophysical Monograph Series, v. 106, p. 1–25, doi:10 .1029 /GM106p0001 . Whittaker, J.M., Afonso, J.C., Masterton, S., Müller, R.D., Wessel, P., Williams, S.E., and Seton, Speed, R.C., and Walker, J.A., 1991, Oceanic crust of the Grenada Basin in the southern Lesser M., 2015, Long-term interaction between mid-ocean ridges and mantle plumes: Nature Geo- Antilles arc platform: Journal of Geophysical Research: Solid Earth, v. 96, no. B3, p. 3835– science, v. 8, no. 6, p. 479–483, doi:10.1038/NGEO2437. 3851, doi:10.1029/90JB02558. Williams, C.A., 1975, Sea-floor spreading in the Bay of Biscay and its relationship to the North Srivastava, S.P., 1978, Evolution of the Labrador Sea and its bearing on the early evolution of the Atlantic: Earth and Planetary Science Letters, v. 24, p. 440–456, doi:10.1016/0012 -821X North Atlantic: Geophysical Journal of the Royal Astronomical Society, v. 52, p. 313–357, doi: (75)90151-X. 10.1111/j.1365-246X.1978.tb04235.x. Williams, S.E., Whittaker, J.M., Granot, R., and Müller, R.D., 2013, Early India-Australia spread- Stern, R. J., 2004, Subduction initiation: Spontaneous and induced: Earth and Planetary Science ing history revealed by newly detected Mesozoic magnetic anomalies in the Perth Abyssal Letters, v. 226, no. 3-4, p. 275–292, doi:10.1016/j.epsl.2004.08.007. Plain: Journal of Geophysical Research: Solid Earth, v. 118, no. 7, p. 3275–3284, doi:10 .1002 Stern, R.J., and Dickinson, W.R., 2010, The Gulf of Mexico is a Jurassic backarc basin: Geosphere, /jgrb.50239. v. 6, no. 6, p. 739–754, doi:10.1130/GES00585.1. Williams, S.E., Flament, N., and Müller, R.D., 2016, Alignment between seafloor spreading di- Tamaki, K., and Larson, R.L., 1988, The Mesozoic tectonic history of the Magellan microplate in rections and absolute plate motions through time: Geophysical Research Letters, v. 43, the western Central Pacific: Journal of Geophysical Research, v. 93, no. B4, p. 2857–2874, p. 1472–1480, doi:10.1002/2015GL067155. doi:10.1029/JB093iB04p02857. Winterer, E.L., 1991, The Tethyan Pacific during Late Jurassic and Cretaceous times: Palaeo- Taylor, B., and Martinez, F., 2003, Backarc basin basalt systematics: Earth and Planetary Science geography, Palaeoclimatology, Palaeoecology, v. 87, p. 253–265, doi:10 .1016 /0031 -0182 Letters, v. 210, no. 3, p. 481–497, doi:10.1016/S0012-821X(03)00167-5. (91)90138-H. Taylor, P.T., Kovacs, L.C., Vogt, P.R., and Johnson, G.L., 1981, Detailed aeromagnetic investigation Yatheesh, V., Bhattacharya, G.C., and Dyment, J., 2009, Early oceanic opening off Western of the Arctic Basin: 2: Journal of Geophysical Research: Solid Earth, v. 86, B06101, doi:10 India-Pakistan margin: The Gop Basin revisited: Earth and Planetary Science Letters, v. 284, .1029/JB086iB07p06323. p. 399–408, doi:10.1016/j.epsl.2009.04.044.