Research Paper 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 ana- lyze 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. © 2017 Geological Society of America 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 911 by guest on 29 September 2021 Research Paper 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-di- mensional 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 micro plate 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