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Home , Central Indian Ridge, Gakkel Ridge, Hydrothermal circulation, Seafloor spreading, Southwest Indian Ridge

or collective redistirbution other or collective or means this reposting, of machine, is by photocopy article only anypermitted of with portion the approval Th of society. send allcorrespondence to: e [email protected] or UsA. md 20849-1931, society, rockville, Po box 1931, e oceanography Th Th in published been has is article sPeciAl issUe FeAtUre Oceanography

Ultraslow-spreading ridges , Volume 1, a quarterly 20, Number Th journal of society. oceanography copyright e 2007 by society. ePermissionreserved. oceanography Allrights is grantedTh use in to copy teaching research. and for article this republication, systemmaticreproduction, rapid Paradigm changes

by JoNAthAN e. sNow ANd heNriettA N. edmoNds

Ultraslow-spreading ridges (< 20 mm yr-1 full rate) represent largely on their spreading rate: “fast” (> 60 mm yr-1 full rate) a major departure from the style of crustal accretion seen in and “slow” (< 60 mm yr-1 full rate). An “intermediate” type is the rest of the basins. Since the 1960s, observations of often placed between them. Both types of ridges share certain fast- and slow-spreading mid-ocean ridges of the Pacifi c and characteristics: (1) they have roughly the same crustal thickness Atlantic , combined with those of (pieces of (6–7 km—see Figure 2), (2) in plan view they have a charac- oceanic that have been thrust onto land through plate- teristic stair-step geometry of volcanic separated by per- tectonic processes), were used to defi ne the conceptual struc- pendicular transform offsets, and (3) they generate a charac- tural, tectonic, and hydrothermal architecture of . teristic outcrop pattern of elongate, -bounded abyssal hills Over the last 15 years, studies of ultraslow-spreading ridges trending normal to the spreading direction. Their differences have identifi ed several anomalies that cannot be explained lie primarily in their across-axis morphology: fast-spreading by the standard model of oceanic crustal formation. Thus, in ridges have an axial rise with a very narrow summit graben that recent years, ultraslow-spreading ridges have become recog- is the locus for most volcanic and tectonic activity, whereas nized as a class unto themselves. Their “anomalous” charac- slow-spreading ridges have rugged mountains enclosing a teristics in fact provide key information about many of the broad axial valley. Fast-spreading ridges tend to be dominated underlying processes that govern crustal accretion at all spread- by , while the morphology of slow-spreading ridges ing rates. Ultraslow ridges include the is dominated more by . This distinction was widely (between Africa and Antarctica), the (which accepted for many years as a basic principle by the worldwide bisects the ), and several smaller ridges (Figure 1). geologic community. It was taught to undergraduates as a fun- Since the advent of plate-tectonic theory, mid-ocean ridges damental characteristic of seafl oor geology, and its causes were have been classifi ed based on their structural, morphological, debated as one of the not-completely understood corners of and volcanologic characteristics into two major types based plate-tectonic theory.

90 Oceanography Vol. 20, No. 1 Figure 1. global plate boundaries (gray) and oce- LT GR anic spreading ridge segments as defi ned KR by bird (2003). green indicates fast- MR spreading ridges (full spreading rate ≥ 60 mm yr-1). red indicates ultraslow-spreading ridges. All other ridge segments are indicated in yellow. gr = gakkel ridge, lt = lena trough, CT Kr = Knipovich ridge, mr = mohns ridge, by JoNAthAN e. sNow ANd heNriettA N. edmoNds ct = cayman trough, AAr = America- Antarctic ridge, and swir = southwest indian ridge. Note that sections of the Kolbeinsey and reykjanes ridges SWIR north and south of iceland, which spread at < 20 mm yr-1, AAR are not indicated in red because the infl uence of the iceland hotspot causes these ridges to behave diff erently than other ultraslow-spreading ridges.

10

9 “Normal” Oceanic Crust 8 Figure 2. crustal thickness, determined by seismic 7 refraction, as a function of spreading rate. Normal oce- 6 anic crust has a mean thick- ness of 6 km at all spreading 5 rates above 20 mm yr-1. Modifi ed after Bown and ustal ickness (km) 4 White (1994) 3 ismic Cr ismic

Se 2

1 Ultraslow Cutoff 20 mm/yr 0 0 20 40 60 80 100 120 140 160 Full Spreading Rate (mm/yr)

Oceanography march 2007 91 The characteristics of both slow- and gabbro, and (see Figure 3) in a Problems with the fast-spreading ridges fit well with the thickness and proportion consistent with Penrose Models model for the formation of the seismic structure of both fast- and Outcrop of mantle ultramafic rocks on oceanic crust, which entered the geologi- slow-spreading crust. These characteris- the ocean floor was first described at cal canon at the 1972 Penrose Confer- tics have been an important cornerstone the slow-spreading Mid-Atlantic Ridge ence on Ophiolites and Ocean Crust of plate-tectonic theory for the past (Aumento and Loubat, 1970). In the (Conference Participants, 1972). This 35 years and have continuously proven Penrose model, a 6-km layer of basaltic model, based on geologic mapping on useful in helping to understand the rock covers the mantle; thus, ultramafic land in ophiolites, calls for a layered most inaccessible parts of Earth’s crust rocks at the seafloor should be rare. Their structure of pillow , sheeted dikes, (Nicolas, 1995). emplacement to the ocean floor requires

Figure 3. Models of oceanic crustal structure (Nicolas, 1995). The harzburgite-type model describes crust similar to “normal” Penrose-style mid-ocean ridge crust. The origin of lherzolite-type crust was debated for many years, but is now correlated with nonvolcanic rifted margins and to ultraslow- spreading ridges.

92 Oceanography Vol. 20, No. 1 mechanisms that would seem implau- (Cannat et al., 1999; le Roex et al., 1992; ness across the entire range of spread- sible, such as faults with a minimum of Patriat et al., 1997) ing rates (see Figure 2) began to unravel 6-km displacement (though such faults Another major change to ideas of as well. While it remained true at most were in fact later found), or serpentinite crustal accretion came through discov- spreading ridges that the seismically diapirism when the serpentinites them- eries at ridge-transform intersections. determined crustal thickness was nearly selves have densities hardly less than the Dredging results showed that the elevat- constant, at the slowest spreading rates, basaltic rocks through which they must ed inside-corner-high sections of trans- notably in a seismic study done through rise. Also, there is no indication how form faults contained abundant rocks the ice of the Arctic Ocean, the oceanic water might penetrate through many from the lower crust and upper mantle. crust seemed to be dramatically thinner kilometers of oceanic crust to create ser- While the faults bounding the inside than along the rest of the global mid- pentinites in the mantle. Early on, these problems were explained by the idea that great transform faults (Morgan, 1968), which offset ridge segments, provided a pathway for water to enter the mantle Ultraslow-spreading ridges have and for serpentine diapirs to rise to the recently emerged as a unique, new class surface. Most abyssal peridotites were of mid-ocean ridge spreading centers. recovered from the walls of large-offset transform faults, or near them. On the Southwest Indian Ridge (SWIR), peri- dotites were even more common (Dick, 1989; Engel and Fisher, 1975). corner high were large, with as much ocean ridge system (Figure 2). The crust By the mid-1990s, unusual ridge seg- as 500 m of obvious vertical displace- was so thin, in fact, that the very concept ments were also found on the SWIR that ment, this was nowhere near enough to of a “crust” had to be called into ques- were anomalously deep and not per- bring up mantle rocks from beneath a tion, as the seismic structures found pendicular to the spreading direction, full section of oceanic crust (Dick, 1989; could easily be satisfied by a thin layer as Penrose-style volcanic rifts are sup- Karson and Dick, 1983). Subsequently, of serpentinite overlying bare mantle posed to be. Neither were they transform improved bathymetric imaging showed (Bown and White, 1994; Jackson et al., faults, which are geometrically required obvious signs that this faulting was along 1982; Reid and Jackson, 1981). to be parallel to the spreading direction. many tens of kilometers of displace- The walls of these rifts instead trend at ment (Cann et al., 1997; Tucholke et al., Evidence from the Frozen a highly oblique angle to the prevail- 1998). Nearby dredging of anomalously North: AMORE 2001 ing spreading direction. These oblique deep regions of the ridge (so-called zero- A consensus thus evolved during the rift segments frequently contain mantle offset fracture zones, or nontransform 1990s that while the Penrose model still peridotites as well as basalts of unusual discontinuities) also recovered lower held for most mid-ocean ridge systems, composition. Although these types of crustal and upper mantle rocks, far from were already known from ocean the effects of transforms (Cannat et al., Jonathan E. Snow ([email protected]) is islands and ridges near major hotspots, 1995). At this point, one was forced to Assistant Professor, Department of Geo- their presence in these anomalous rifts, wonder, even on slow-spreading ridges, sciences, University of Houston, Houston, often called “leaky transforms,” could just how much of the crust could be TX, USA. Henrietta N. Edmonds is not easily be reconciled with prevail- considered “normal.” Associate Professor, The University of ing ideas about crustal accretion or the Almost from the beginning, the at Austin, Marine Science Institute, Port generation of oceanic basaltic observation of a constant crustal thick- Aransas, TX, USA.

Oceanography March 2007 93 the very slowest spreading ridges were Cochran, 1998; Cochran et al., 2003; despite its relatively thin crust, with quite different, and thus might illustrate Edwards et al., 2001). no outcrops of mantle peridotite. On extreme forms of the fundamental geo- AMORE 2001, the international map- either side of this western volcanic zone, dynamic forcing functions driving mid- ping and sampling program that left however, were long rift segments where ocean ridge construction everywhere. A Tromsø, Norway, in July 2001, was an apparently no magmatic crustal con- major effort then evolved to investigate unprecedented success (Edmonds et al., struction occurred at all (Michael et al., the world’s slowest-spreading mid- 2003; Jokat et al., 2003; Michael et al., 2003; Snow and Petrology Group ARK- ocean ridge—the Gakkel Ridge in the 2003). Encountering good ice conditions, XX-2, in press), and to the east, volca- Arctic Ocean (InterRidge Arctic Working scientists aboard PFS Polarstern and the nism increased in discrete volcanic cen- ters separated by amagmatic basins. The SWIR and Gakkel Ridge together ...studies of ultraslow-spreading ridges thus required a new entry in the table of mid-ocean ridge types (Dick et al., have identified several anomalies that 2003). At ultraslow-spreading ridges, the cannot be explained by the standard seismic crust is thinner (~ 1–4 km) than at other ridges (see Figure 2) (Bown and model of oceanic crustal formation. White, 1994; Jokat et al., 2003; Reid and Jackson, 1981). The changeover from slow- to ultraslow-spreading character- Group, 1998). Gakkel Ridge is the north- new US Coast Guard Cutter Healy were istics is not strictly a function of spread- ern continuation of the Mid-Atlantic able to create a detailed bathymetric ing rate. Rather, a number of factors can Ridge, and extends over 2000 km from map of much of the of Gakkel influence the style of crustal accretion: the northeast tip of Greenland to the Ridge (Figure 4), carry out over 200 rock plate-boundary geometry, mantle mag- Laptev in . sampling stations, record a dozen seis- matic productivity, and mantle potential The full spreading rate varies from about mic crustal thickness measurements, and temperature (Dick et al., 2003). 14 mm yr-1 at the Greenland end to less conduct continuous seismic-reflection Ultraslow-spreading ridges uniquely than 8 mm yr-1 at its Laptev Sea termina- transects across both the Nansen and possess amagmatic rifts that expose tion (Reid and Jackson, 1981; Vogt et al., Amundsen basins. The two ships reached mantle peridotite directly on the sea- 1979). Thus, at its fastest end, the Gak- the North Pole together on Septem- floor, with only scattered basalt and kel Ridge spreads more slowly than any ber 6, 2001 (Jokat et al., 2003; Michael gabbro (Dick et al., 2003). Along with other mid-ocean ridge. Pack-ice cover et al., 2003). In 2004, PFS Polarstern magmatic rifts, transforms, and subduc- had prevented Arctic hard-rock dredge mapped and sampled the Lena Trough tion zones, amagmatic rifts form a new, operations until 1999 when the icebreak- and returned to Gakkel Ridge for addi- fourth class of plate-boundary structure. er PFS Polarstern ( Insti- tional mapping and sampling (Snow and Figure 5 shows an amagmatic segment tute, Bremerhaven, Germany) recovered Petrology Group ARK-XX-2, in press). on the SWIR (Dick et al., 2003); similar the first peridotite, basalt, and hydro- segments also occur on the Gakkel Ridge thermal rocks from the Lena Trough, just A “New” Class of Mid- (Michael et al., 2003) and Lena Trough to the south (Snow et al., 2001). At the Ocean Ridge (Snow and Petrology Group ARK-XX-2, same time, a US Navy-civilian science The new observations made along in press). Unlike magmatic segments, cooperation used civilian instruments Gakkel Ridge were confusing. There they have mantle peridotite walls formed on a US nuclear submarine to produce was significantly more volcanism than by long sloping scarps or irregular uplifts the first bathymetric and sidescan sonar had been expected. The western part of rather than the basaltic-block, normal images of Gakkel Ridge (Coakley and Gakkel Ridge was magmatically robust, faulting of Penrose crust (Dick et al.,

94 Oceanography Vol. 20, No. 1 6 10.8 6 11.2

5 70°E

88°N

24 11.6

5 60°E 23 20 5 Amundsen Basin 87°N 12.0 20 6 50°E

12.2

86°N 17

12.6 Nansen Basin

40°E

85°N 12.8

Gakkel Ridge 84°N 13.4

17 20 30°E km 83°N 14.0 0 50 12 100 22 0° 10°E 20°E

6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 Depth (m) Figure 4. Gakkel Ridge based on AMORE 2001 and International of the Ocean data sets. Full spreading rates in each identified section are given in mm yr-1. Red lines show magnetic lineation picks. Modified from Figure 1, Jokat et al. (2003)

Oceanography March 2007 95 -51˚ 30'

10˚ Peridotite 6.2 mm/y Peridotite & Basalt 3.9 r r Basalt Empty mm/y -52˚ 00' Normal Fault Earthquake Strike-slip Earthquake 11˚ Z.Z. F.F. -52˚ 30' ShakaShaka 12˚

-53˚ 00'

SW Indian Ridge Oblique -53˚ 30' 13˚ Supersegment (10°-16° E)

Depth (m) 100 km 1000 2000 3000 4000 5000 -54˚ 00' 10˚ 11˚ 12˚ 13˚ 14˚ 15˚ 16˚ 17˚

Figure 5. Oblique amagmatic accretion on the Southwest Indian Ridge (Dick et al., 2003). Filled circles show sampling points, giving rock type coded by color as shown in key. Note that the spreading direction (red arrows) is not at a right angle to the strike of the rift. This type of spreading, far from being anomalous, is characteristic of ultraslow-spreading ridges.

2003; Michael et al., 2003; Okino et al., of spreading, the amagmatic style of nearly direct access to Earth’s mantle. 2002). Amagmatic segments can also spreading is enhanced such that oblique Analyses of these rocks have led to the assume any orientation to the spreading amagmatic segments can form at higher conclusion that there is primary mineral- direction, sometimes forming oblique spreading rates than would otherwise be ogic variability in the upper mantle that rifts (Dick et al., 2003; Snow et al., 2001), possible (Dick et al., 2003). affects partial melting (Dick et al., 1984), and they can produce a unique “smooth” The more variable and alkalic nature that mid-ocean ridge melting occurs seafloor (Cannat et al., 2006). Magmatic of basalts from ultraslow-spreading by a process of near-fractional melt- accretion at ultraslow-spreading rates ridges (le Roex et al., 1992; Meyzen et al., ing (Johnson et al., 1990), and that deep can either be robustly magmatic, as at 2003; Snow et al., 2001) can be explained melts formed in the presence of garnet the orthogonal supersegment of the by an overall lower degree of partial are a component of mid-ocean ridge ba- SWIR (Dick et al., 2003) or the western melting in the mantle (which produces salt (Hellebrand and Snow, 2003). Gakkel Ridge (Michael et al., 2003), or it rich in alkali elements relative to may be confined to highly focused, short silica), and the presence of mantle geo- Insights on Continental magmatic segments (producing crust chemical components not observed at Rifting and Ophiolites with orthogonal structures) linked by slow- and fast-spreading ridges (Hart, Ultraslow-spreading ridges have increas- amagmatic segments. Where the plate 1984). Mantle peridotites from ultra- ingly become a model for understand- boundary runs oblique to the direction slow-spreading ridges afford unique, ing some of the more puzzling aspects

96 Oceanography Vol. 20, No. 1 of ophiolites and continental rifts. The displaced by the Penrose ophiolite model mid-ocean ridges. This combination of Penrose model had always met with shown in Figure 3. Alpine rock types was exactly the one difficulty in explaining some aspects Then, in the 1990s, the Ocean Drill- whose tectonic significance Harry Hess of the tectonic setting and structure of ing Program completed a series of deep- noticed 50 years ago. In fact, the west- ophiolites. Far too many ophiolites (the sea drill holes off the coast of Portugal ern Alpine outcrops involved were the “lherzolite-type” in Figure 3 and Nicolas, that showed the existence of serpenti- same ones where Steinmann originally 1995) did not conform to the 6-km-thick nite-floored oceanic crust on the Euro- described his unique rock association crust observed nearly everywhere in pean (Whitmarsh 100 years ago (Steinmann, 1905). the oceans. Proponents of the ophiolite et al., 1996; Whitmarsh et al., 2001; paradigm concluded that the incom- Whitmarsh et al., 1993). At the same Hydrothermal Activity plete ophiolites had been dismembered time, sedimentary sequences in the west- on Ultraslow-Spreading during the process of obduction onto ern Alps were recognized as the margin Ridges Ultraslow-spreading ridges have also provided surprises and new insights in research. Before observations made on these ridges the first high-temperature venting was observed on the Mid-Atlantic Ridge have brought tremendous advances in (Rona et al., 1986), many scientists had understanding the tectonics, magmatic been skeptical about whether even slow- architecture, and hydrothermal evolution spreading ridges were capable of sup- porting such venting due to their colder of oceanic crust at all spreading rates, thermal structure. By the 1990s, a lin- and are continuing to do so. ear relationship was proposed between hydrothermal venting and spreading rate, largely based on results from the faster-spreading ridges (Baker, 1996; the . Among the incomplete of a rifted continent (Froitzheim and Baker et al., 1994). German and Par- lherzolite-type ophiolites are the origi- Manatschal, 1996; Manatschal, 2004). son (1998), in extending this work to nal ones, those where the association of The basement of this fossil continen- the slow-spreading Mid-Atlantic Ridge, volcanic rocks, serpentinite, and deep- tal rifted margin changes, just as on pointed out that the length scale over sea sediment was first recognized and the present-day Iberia margin, from which survey results are considered may called “ophiolite” in 1905 (Steinmann, continental basement to mantle rocks, lead to substantial variability, particu- 1905). These ophiolites contained little sometimes with a thin cover of basalt, larly on slower-spreading ridges, where middle crust, consisting largely of a thin and covered by deep-sea sediments. The tectonic fabric plays a stronger role in layer of pillow basalt erupted directly on rock types, structures, and ultimately controlling peridotite and covered by deep-sea sedi- the mechanisms of exhumation of deep and can also limit the dispersal of hydro- ments. Harry Hess, a Princeton miner- crustal and mantle rocks in both of these thermal plumes. All of these studies pre- alogist and early mentor to the founders locations bear striking similarity to that dicted that hydrothermal activity would of plate-tectonic theory, first recognized occurring today at ultraslow-spreading be very low at ultraslow-spreading rates, their significance as the trace of ancient ridges. Thus, on some continental mar- and the perception persisted that the oc- plate boundaries (Hess, 1962). His idea, gins, the rifting that creates new ocean currence of high-temperature venting however, that the ocean floor consisted basins occurs in an amagmatic “Hess- might drop to near zero. At the Gakkel entirely of serpentinite was ultimately type” manner analogous to ultraslow Ridge, for example, vent-site frequency

Oceanography March 2007 97 was predicted to be on the order of one (Bach et al., 2002; Baker et al., 2004), ing ridges. Hydrothermal circulation in site per 200–300 km. and the Gakkel Ridge from 8°W to 85°E peridotite-hosted systems can be driven It is thus perhaps ironic that ultra- (Edmonds et al., 2003). by the exothermic serpentinization re- slow-spreading ridges have now been The results of these surveys, along actions, rather than by magmatic heat. surveyed more thoroughly for hydro- with isolated discoveries on the Mohns These reactions result in colder and less thermal activity than ridges in most and Knipovich Ridges (Connelly and metal-rich vent fluids than “traditional” other spreading-rate classes (Baker and German, 2002; Pedersen et al., 2005) and basalt-hosted systems and do not lead German, 2004). This discrepancy is the in the Lena Trough (Snow et al., 2001), to formation of a particle-rich plume. result of the coincidence between the be- demonstrate that high-temperature A case in point is the 40–75°C fluid tem- ginning of the international ridge com- hydrothermal venting is ubiquitous on peratures at the Lost City ultramafic munity’s focus on ultraslow-spreading ultraslow-spreading ridges. The three hydrothermal field in the ridges and the development of strategies densely surveyed sections yield a site (Kelley et al., 2001). The only water- for systematic large-scale mapping of frequency of about one site per 100 km. column signal that ultramafic-hosted hydrothermal plumes (e.g., Baker and Recent normalization of site frequen- systems might consistently generate is el- Milburn, 1997). Such mapping relies on cies to the ridge magma-production rate evated methane, but as yet there is no re- the plumes’ optical backscatter, a mea- suggests that ultraslow-spreading ridges liable, robust sensor for in situ detection sure of suspended particles in the water may be two to four times as efficient as of methane at the expected levels. Such column that result from the mixture of fast-spreading ridges at converting the “non-traditional” hydrothermal systems metal-rich, high-temperature hydrother- available magmatic heat into vent fields may be especially important because mal fluid and . The deployment (Baker et al., 2004; Baker and German, of their unique contributions to global of optical sensors during rock sampling 2004). With the exception of the Mohns geochemical cycles. High-temperature and deep-towed geophysical operations Ridge sites (Pedersen et al., 2005), which systems that involve magmatic heat as enables much more efficient detection of are located in about 500-m water depth, well as ultramafic rocks (such as Rain- bow and Logatchev on the northern Mid-Atlantic Ridge) also exhibit elevated precious-metal concentrations and the ...there is a growing realization that abiogenic production of complex (multi- ultraslow-spreading ridges provide an analog carbon) organic molecules. Further- more, hydrothermal systems on slow- to processes that govern the breakup of and ultraslow- spreading ridges may be along nonvolcanic rifted margins. active for longer time periods and lead to the deposition of larger ore bodies due to their localization on long-lived fault structures. plumes than earlier exploration strate- all of the sites are in deep water and none Ultraslow-spreading ridges also gies that relied on “spot” sampling by have yet been surveyed or sampled with occupy regions of particular interest for conductivity-temperature-depth (CTD) deep submergence technology. vent biogeographic studies (Tyler and instruments. Three sections of ultraslow- Ultraslow-spreading ridges may have Young, 2003; Van Dover et al., 2002). spreading ridges, totaling ~ 2250 km, even more hydrothermal activity than The deep Arctic ridges are isolated from have now been thoroughly surveyed outlined above. Optical plume map- the rest of the mid-ocean ridge system (Figure 6), including the SWIR from ping does not detect some forms of due to the presence of Iceland astride 58° to 60°E and 63° to 66°E (German et hydrothermal activity likely to be more the Mid-Atlantic Ridge, and to the shal- al., 1998), the SWIR from 10° to 23°E common on slow- and ultraslow-spread- low connections between the Arctic and

98 Oceanography Vol. 20, No. 1 a

20˚ 40˚ 60˚ b 30˚ 30˚

40˚ 40˚

50˚ 50˚

20˚ 40˚ 60˚

Figure 6. Summary of known hydrothermal activity on ultraslow-spreading ridges. (a) Map of the Arctic ridges indicating the area thoroughly surveyed on the AMORE 2001 cruise (box). Red stars mark vent sites identified during AMORE. Vent sites identified on other cruises show as purple stars: Lena Trough (Snow et al., 2001), Knipovich Ridge (Connelly and German, 2002), and Mohns Ridge (Pedersen et al., 2005). (b) Map of the Southwest Indian Ridge in- dicating the three areas that have been systematically surveyed for hydrothermal plumes (boxes); red stars locate identified plumes. For (b), the authors indicate all possible plume sites; estimates range from three to eight plumes in the 10–23°E survey area.

the Pacific Oceans (through the Bering ary path quite separate from any other The communities Strait) and between the Arctic and the yet studied. A return expedition planned exhibit affinities with both the Atlantic Atlantic Oceans (through the passages of for 2007 should answer some of these in- and the western Pacific biogeographic the Canadian Arctic Archipelago and the triguing questions for the Gakkel Ridge. provinces but are most similar overall sills between Greenland and Scotland). The SWIR, meanwhile, occupies a to the western Pacific (Van Dover et al., At no time since the inception of spread- crucial gap between known vent biogeo- 2001). The exploration of vent fields ing on these northern ridges has there graphic provinces: no vent sites have yet on the southern Mid-Atlantic Ridge been a deep (thousands of meters) con- been characterized in terms of their fau- (e.g., German et al., 2005; Haase and nection between the Arctic basins and nal communities between the Rodriguez M64/1 Scientific Party, 2005) and the the North Atlantic, and thus it is quite (southern Central In- SWIR thus holds promise for exciting possible that endemic vent fauna of the dian Ridge, Edmond and Kairei fields) new discoveries regarding vent organism Arctic ridges have followed an evolution- and the northern Mid-Atlantic Ridge. ecology and dispersal.

Oceanography March 2007 99 Summary Meeting and Workshop, Sestri Levante, patterns at the Mid-Atlantic Ridge (22°–24° N). Ultraslow-spreading ridges have recently Italy). The authors thank the conference Geology 23:49–52. Cannat, M., C. Rommevaux-Jestin, D. Sauter, C. emerged as a unique, new class of mid- participants for their contributions, and Deplus, and V. Mendel. 1999. Formation of the ocean ridge spreading center. Observa- H. Dick, W. Jokat, and P. Michael for axial relief at the very slow spreading Southwest Indian Ridge (49° to 69°E). Journal of Geophysi- tions made on these ridges have brought constructive reviews of this article. cal Research 104(B10):22,825–22,843. tremendous advances in understanding Cannat, M., D. Sauter, V. Mendel, E. Ruellan, K. the tectonics, magmatic architecture, and References Okino, J. Escarin, V. Combier, and B. Baala. 2006. Modes of seafloor generation at a melt- hydrothermal evolution of oceanic crust Aumento, F., and H. Loubat. 1970. The mid-Atlan- tic ridge near 45°N: Serpentinized ultramafic poor ultraslow-spreading ridge. Geology at all spreading rates, and are continuing intrusions. Canadian Journal of Earth Sciences 34(7):605-608. to do so. In addition, there is a grow- 8:631–663. Coakley, B., and J. Cochran. 1998. Gravity evidence of very thin crust at the Gakkel Ridge (Arctic ing realization that ultraslow-spreading Bach, W., N.R. Banerjee, H.J.B. Dick, and E.T. Baker. 2002. Discovery of ancient and active Ocean). Earth and Planetary Science Letters ridges provide an analog to processes hydrothermal systems along the ultra-slow 162:81–95. that govern the breakup of continents spreading Southwest Indian Ridge 10°–16°E. Cochran, J.R., G.J. Kurras, M.E. Edwards, and B.J. Coakley. 2003. The Gakkel Ridge: Bathymetry, along nonvolcanic rifted margins. The Geochemistry, , Geosystems 3(7):1044, doi:10.1029/2001GC000279. gravity anomalies, and crustal accretion at ultraslow-spreading ridge community Baker, E.T. 1996. Geological indexes of hydrother- extremely slow spreading rates. Journal of Geo- has set several near-term goals on which mal venting. Journal of Geophysical Research physical Research 108(B2):2116, doi:10.1029/ 101:13,741-13,753. 2002JB001830. to focus its activities, beginning with two Baker, E.T., H.N. Edmonds, P.J. Michael, W. Bach, Conference Participants. 1972. Penrose Field Con- workshops/conferences held in Europe H.J.B. Dick, J.E. Snow, S.L. Walker, N.R. Baner- ference: Ophiolites. Geotimes 17:24–25. Connelly, D.P., and C. German. 2002. Geochemical in the fall of 2006. There will be a major jee, and C.H. Langmuir. 2004. 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