articles An ultraslow-spreading class of ocean ridge Henry J. B. Dick, Jian Lin & Hans Schouten Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA ........................................................................................................................................................................................................................... New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr21, though their characteristics are commonly found at rates up to approximately 20 mm yr21. The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change. Ocean ridges spreading at less than ,20 mm yr21 comprise from fast- to slow-spreading and slow- to ultraslow-spreading ,20,000 km of the ,55,000 km global ridge system1, but until tectonics (Fig. 1b). recently have been little studied. On the basis of studies at faster Magmatic accretionary ridge segments are second-order seg- spreading rates, ocean ridges have been divided into fast-, intermedi- ments that link together between transform faults to make up ate-, and slow-spreading, each with distinctive morphologic charac- first-order segments16. They are considered the basic unit of mag- teristics (Fig. 1a). Slow-spreading ridges (less than ,55 mm yr21) matic accretion, and at slow-spreading ridges often have circular have deep rift valleys with highly variable relief from ,400 to negative mantle Bouguer anomalies (MBAs) centred on them, 2,500 m (ref. 2) and rough rift mountain topography weakly suggesting focused melt and/or mantle flow resulting in thicker correlated to spreading rate3. Fast-spreading ridges (,80– crust at their centres17–20. As there are no transform faults defining a 180 mm yr21) have low (,400 m) axial highs4, sometimes with first-order segmentation at ultraslow rates, we use the term ‘super- small linear depressions (less than ,100 m wide and less than segment’ to describe distinct physiographic regions consisting of ,10 m deep, for example5) at their crests, and minimal rift moun- linked magmatic and amagmatic ridge segments. tain topography uncorrelated to spreading rate. Intermediate- Magmatic segments form sub-perpendicularly to the least prin- spreading ridges (,55–70 mm yr21; ref. 2) have long alternating cipal compressive stress, at an angle intermediate to the ridge trend sections with either slow- or fast-spreading ridge morphology6–8. and orthogonal to the spreading direction, for example21,22. Mor- Slow-spreading ridges near mantle hotspots that have an axial rise phologically, they consist of linear axial highs or troughs with saddle (for example, the Reykjanes ridge), and abrupt changes in mor- points at the segment centres and rift valley walls consisting of phology at intermediate-spreading-rate ridges, show that ridge staircase normal boundary faults. In the absence of transform faults, morphology is not strictly dependent on spreading rate, but may these structures occur in sets overlapping en echelon or connect by reflect variations in ridge geometry, melt supply, and mantle small non-transform discontinuities to accommodate plate bound- composition and thermal structure. ary trends that are oblique to the spreading direction. Where the But this classification also does not take into account variations ridge trend is highly oblique (for example, the Mohns ridge), intra- in crustal thickness with spreading rate. Although seismic crustal rift faults and axial ridges often have sigmoidal traces where they thickness shows little dependency on spreading rate down to merge with rift valley bounding faults that run at higher angles sub- 20 mm yr21, it then drops off rapidly (Fig. 1b). Models for buoyant parallel to the ridge trend21. The close interaction of these two fault and passive plate-driven mantle flow attribute this to conductive sets shows that they are part of an integrated system accommo- cooling from above depressing the top of the mantle melting dating oblique extension during brittle faulting in the shallow crust column and limiting melt production9–12. Thus, changes in ridge (for example, ref. 22). geometry, mantle composition, flow and thermal structure that at Amagmatic accretionary ridge segments are a key component of faster rates would have a minor effect, dramatically affect crustal ultraslow-spreading ridges, replacing transform faults while extend- production and tectonics at very slow spreading rates13,14, creating a ing the zone of lithospheric accretion. In contrast to magmatic class of ridges that is distinct from previous categories. segments, they assume any orientation relative to the spreading Just as the East Pacific Rise is the only fast-spreading ridge, the direction, and are usually marked by an axial trough rarely more only ultraslow-spreading ridge is the Gakkel ridge in the Arctic than a kilometre deep that may extend 50 km or more. They have Ocean: an 1,800-km-long ridge without transform faults spreading only scattered volcanics or a thin basalt carapace, virtually no 8to13mmyr21 (ref.15).Thevery-slow-spreading(14– seismic layer 3 (for example, ref. 23), expose abundant mantle 16 mm yr21) Southwest Indian ridge (SWIR) and the remaining peridotite, and have weak magnetization and a relatively positive Arctic ridge system (13–18 mm yr21) are transitional between slow MBA. The basic accretion unit appears to be mantle horst blocks and ultraslow. Just as intermediate-rate ridges alternate between fast rising up through the rift valley floor along an axis running parallel and slow morphology, these ridges alternately exhibit slow or ultra- to the plate boundary trend. These are subsequently uplifted to slow ridge characteristics over long sections. Thus, there are two create the axial trough walls—often creating single ,148–208 low- intermediate-ridge classes rather than one, representing transitions angle fault surfaces bounding lenticular peridotite ridges. NATURE | VOL 426 | 27 NOVEMBER 2003 | www.nature.com/nature © 2003 Nature Publishing Group 405 articles The SWIR from 98 to 258 E East of Mayes seamount, there is a 180-km-long, 4,200-m-deep 0 0 The SWIR has a very oblique trend—typically ,608 from the oblique amagmatic segment from 118 30 to 148 24 E, oriented 328 spreading direction—and is an example of a transitional ridge from the spreading direction. It has an axial trough with local deep between slow and ultraslow. Long sections are similar to slow- areas to 4,700 m. Twenty-two dredges (2,574 kg) recovered 46.2% spreading ocean ridges (that is, second-order magmatic segments serpentinized peridotites, 2.6% dunite, 14.2% pillow basalt, 19.9% linked by transform faults). Other sections lack transforms and basalt and diabase, and 12.6% hydrothermal silica, sepiolite, and have the character of ultraslow ridges, consisting of sub-orthogonal massive sulphides from the trough walls and floor. As even a thin ,50-km-long magmatic segments linked by amagmatic oblique basalt flow effectively covers peridotite emplaced to the seafloor, the troughs (for example, ref. 24). This slow to ultraslow transitional dredge statistics and lack of gabbro show that igneous crust is thin behaviour is probably largely due to the influence of ridge geometry. or absent here and that the terrain is largely created by faulting. As obliquity increases ridge length, mantle upwelling rate must slow Although the trough floor is rough, with ,500-m relief, there is no proportionately (Fig. 2b, inset). This is expressed by the spreading morphologic evidence of a neovolcanic zone. Rather, long irregular component perpendicular to the ridge trend: the effective spreading axial horst blocks rise up from the valley floor. Dredging, physio- rate for mantle upwelling (ESR). At very slow spreading rates less graphy, weak magnetization, and positive MBA suggest these blocks than ,20 mm yr21, varying ESR (and mantle composition and temperature) creates a region in the spreading-rate spectrum where ridges can assume either a slow-spreading or an ultraslow-spreading morphology. a 1 The two sharply contrasting supersegments along the 1,000 km of the SWIR between the Shaka fracture zone at 98 E and the Prince Rise Edward fracture zone at 258 E, provides a case in point (Fig. 2). The 0 eastern ‘orthogonal’ supersegment, mapped in ref. 25, extends 600 km from 168 to 258 E and has 330- and 191-km-long subsections oriented approximately perpendicularly to the spreading direction Rift –1 linked by a 150-km slightly oblique (,108) section (Fig. 2b). From 178 E, the ridge curves smoothly to the southwest, and thus the ESR steadily decreases to the west. At 168