Marine Geophysical Researches21: 43–68, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Tectonics of an extinct ridge-transform intersection, Drake Passage(Antarctica)

Andres Maldonado1, Juan Carlos Balanya1,2, Antonio Barnolas3, Jesus Galindo-Zaldıvar2,Javier Hernandez4, Antonio Jabaloy2, Roy Livermore5, Jose Miguel Martınez-Martınez1,2, JoseRodrıguez-Fernandez1, Carlos Sanz de Galdeano1, Luis Somoza3, Emma Suriñach6 & CesarViseras71Instituto Andaluz Ciencias de la Tierra, CSIC/Universidad de Granada, Facultad de Ciencias, 18002 Granada,Spain;2Departamento de Geodin´amica, Universidad de Granada, 18071 Granada, Spain;3Instituto Tecnol´ogicoGeominero de España, R´ıos Rosas, 23, 28003 Madrid, Spain;4Facultad de Ciencias del Mar, 11510 Puerto Real,Cadiz, Spain;5British Antarctic Survey, Madingley Road, CB3 0ET Cambridge, UK;6Departament de GeologiaDinamica i Geof´ısica, Universitat de Barcelona, 08028 Barcelona, Spain;7Departamento de Estratigraf´ıa yPaleontolog´ıa, Universidad de Granada, 18071 Granada, Spain

Key words:Drake Passage, oceanic crust stratigraphy, ridge-transform intersection, tectonics

Abstract

New swath bathymetric, multichannel seismic and magnetic data reveal the complexity of the intersection between the extinctWest Scotia Ridge (WSR) and the Shackleton Fracture Zone (SFZ), a first-order NW-SE trending high-relief ridge cuttingacross the Drake Passage. The SFZ is composed of shallow, ridge segments and depressions, largely parallel to the fracturezone with an ‘en echelon’ pattern in plan view. These features are bounded by tectonic lineaments, interpreted as faults. Theaxial valley of the spreading center intersects the fracture zone in a complex area of deformation, where N120◦ E lineamentsand E–W faults anastomose on both sides of the intersection. The fracture zone developed within an extensional regime, whichfacilitated the formation of oceanic transverse ridges parallel to the fracture zone and depressions attributed to pull-apart basins,bounded by normal and strike-slip faults.

On the multichannel seismic (MCS) profiles, the igneous crust is well stratified, with numerous discontinuous high-amplitude reflectors and many irregular diffractions at the top, and a thicker layer below. The latter has sparse and weakreflectors, although it locally contains strong, dipping reflections. A bright, slightly undulating reflector observed below thespreading center axial valley at about 0.75 s (twt) depth in the igneous crust is interpreted as an indication of the relict axialmagma chamber. Deep, high-amplitude subhorizontal and slightly dipping reflections are observed between 1.8 and 3.2 s (twt)below sea floor, but are preferentially located at about 2.8–3.0 s (twt) depth. Where these reflections are more continuousthey may represent the Mohorovicic seismic discontinuity. More locally, short (2–3 km long), very high-amplitude reflectionsobserved at 3.6 and 4.3 s (twt) depth below sea floor are attributed to an interlayered upper mantle transition zone. The MCSprofiles also show a pattern of regularly spaced, steep-inclined reflectors, which cut across layers 2 and 3 of the oceaniccrust. These reflectors are attributed to deformation under a transpressional regime that developed along the SFZ, shortly afterspreading ceased at the WSR. Magnetic anomalies 5 to 5 E may be confidently identified on the flanks of the WSR. Ourspreading model assumes slow rates (ca. 10–20 mm/yr), with slight asymmetries favoring the southeastern flank between 5Cand 5, and the northwestern flank between 5 and extinction. The spreading rate asymmetry means that accretion was slowerduring formation of the steeper, shallower, southeastern flank than of the northwestern flank.

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Introduction

The Shackleton Fracture Zone (SFZ) is a first-orderNW-SE trending high-relief ridge cutting across theDrake Passage. It has tectonic and geophysical charac-teristics typical of long-offset, strike-slip faults (BAS,1985; Pelayo and Wiens, 1989; Cunningham et al.,1995; Aldaya and Maldonado, 1996; Klepeis andLawver, 1996). This fracture zone intersects two ex-tinct spreading centers, the West Scotia Ridge (WSR)and the Phoenix-Antarctic Ridge (PAR), betweenwhich it formerly acted as a ridge-to-ridge transformfault when both spreading centers were active (Fig-ure 1). The analysis of transform faults and fracturezones is important as they are sensitive to plate move-ments. The SFZ is still a tectonically active boundarybetween the Scotia and Antarctic plates, accommo-dating, in conjunction with the South Scotia Ridge,the relative motion between the Scotia and Antarcticplates (Barker et al., 1991; Livermore et al., 1994;Galindo-Zaldívar et al., 1996).

Studies of the oceanic crust during the last twodecades have modified the earlier hypothesis of a uni-form and simple three-layer sequence of sediment, pil-low basalt diabase and thick gabbro section overlyingthe upper mantle, with the petrological crust/mantleboundary corresponding to the Mohorovicic seismicdiscontinuity (Fryer et al., 1989; Francheteau et al.,1992; Solomon and Toomey, 1992; Purdy et al.,1992; Cannat et al., 1995; Dick et al., 1999). Crustalthickness, in addition, remains approximately con-stant in fracture zones related to fast-spreading cen-ters, whereas intermediate and slow-spreading centersshow crustal thinning (Solomon and Toomey, 1992;Mutter and Mutter, 1993; Barth, 1994; Bown andWhite, 1994; Minshull and White, 1996). Moreover,models for the internal stratigraphy of the lower crusthave become more complex due to the knowledge ofnarrow zones of mineral cumulates, ephemeral magmachambers and variations in thermal structure (Swiftand Stephen, 1992; Sleep and Barth, 1994; Dick,1996). At present, the internal stratigraphic structureof the crust is thought to be governed by mechanismsof hydrothermal alteration and tectonism, superim-posed on the igneous processes (Sinton and Detrick,1992; Carbotte and MacDonald, 1994; Dick et al.,1991, 1999).

In this article we discuss swath bathymetry, multi-channel seismic (MCS) profiles, and magnetic anom-alies collected in a marine geophysical survey of theDrake Passage during the 1997–98 season. Several

tectonic problems are addressed, including: (1) what isthe stratigraphy and structure of the crust in the vicin-ity of the ridge-fracture zone intersection? (2) whatstructures, if any, may be detected at the extinct WSR?and (3) how does the present structure of the SFZreflect past plate motions, in particular the end ofspreading on the ridges? In order to answer thesequestions, we have elaborated a structural map of thespreading center/fracture zone intersection, analyzedthe major tectonic features, the seismic stratigraphyand age of the oceanic crust. The absence of majorsedimentary cover, due to the strong bottom currentsactive in the Drake Passage (Pudsey and Howe, 1998),and the combination of several different techniques,allow a detailed study of the tectonics and nature ofthe oceanic crust near the SFZ.

Methodology

During the ANTPAC 97/98 cruise with the Span-ish vessel B/O HESPERIDES, multichannel seismic(MCS), multibeam echosounder and magnetometryprofiles were obtained along several transects crossingthe main morphological trends of the area (Figure 2).Three MCS profiles were made orthogonal (SM06,SM08, SM10) and one parallel to the SFZ (SM09).A further profile was made orthogonal to the WSR(SM07). Additional measurements parallel to the SFZto fully cover the area were obtained with swathbathymetry, gravimetric and magnetic profiles. Thevessel was positioned by GPS satellite, with almostcomplete daily coverage during the cruise.

The MCS system consisted of a five BOLT airgun tuned array with a total volume of 22.4 l and ashot interval of 50 m. The lines were recorded witha TELEDYNE analogue 96-channel streamer with ac-tive sections of 25 m. A record length of 10 s and asampling interval of 2 ms were chosen, using a DFSV system. Profiles were processed with a standard se-quence, including migration, using a DISCO/FOCUSsystem. The processed profiles have a fold of 24.

The swath bathymetry data were obtained with aSIMRAD EM12 system and postprocessed onboardwith contour lines and bottom reflectivity using theMB system (Caress and Chayes, 1996). The swathbathymetry was visualized with the FLEDERMAUSsystem at the University of Oregon.

Scalar magnetic field measurements were madeduring the survey, using a Geometrics G-876 magne-tometer, with a sampling interval of 2 s. These data

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Figure 1. Geological setting of the study area. 1, active subduction zone; 2, inactive subduction zone; 3, transpressive fault zone; 4, faultzone; 5, active spreading axis; 6, inactive spreading axis; 7, active extensional zone; 8, active transtensional fault zone; 9, continental crust;10, oceanic crust; 11, continental-oceanic crust boundary. BS, Brandsfield Strait; PAR, Phoenix/Antarctic Ridge; HFZ, Hero Fracture Zone;SFZ, Shackleton Fracture Zone; SSB, South Shetland Block; SST, South Shetland Trench; SSR, South Scotia Ridge; STC, South Chile Trench;WSR, West Scotia Ridge. Other symbols, magnetic anomalies. Magnetic anomalies from BAS (1985) and Livermore and Woollett (1993).

were recorded every 5 s, together with the UTC time,and integrated with ship navigation. The data wereedited visually to eliminate spikes and obvious errors,and filtered using a running mean to obtain data everyminute. Magnetic anomalies were calculated using theIGRF 1997 (IAGA, 1996), after corrections for diurnalvariations and magnetic disturbances, using land dataobtained at the Spanish geomagnetic station (Tortaet al., 1997, 1998) at Livingston Island (62◦39′44′′ S,60◦23′41′′ W and 19.4 m amsl).

Tectonic setting

The Drake Passage between South America and thenorthern Antarctic Peninsula reveals a complex mo-

saic of plates and bathymetric features that formedas a result of the fragmentation and separation ofSouth America and Antarctica (Figure 1). The bound-ary between the two major plates in the area, theScotia and Antarctic plates, is a left-lateral trans-pressive fault zone located along the South ScotiaRidge. This boundary extends northwestward acrossthe Drake Passage as a prominent submarine ridge,called the Shackleton Fracture Zone (SFZ), betweenthe Southern Chile Trench and the South ShetlandBlock (Pelayo and Wiens, 1989; Barker et al., 1991;Cunningham et al., 1995; Aldaya and Maldonado,1996; Klepeis and Lawver, 1996; Lodolo et al., 1997).The tectonic evolution of the central sector of the SFZis complex, since it entailed several episodes of trans-

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Figure 2. Top: bathymetry predicted from satellite-derived free-air anomaly field (Sandwell and Smith, 1997) and location chart of the ANTPAC97/98 cruise with B/O Hesperides track lines. Thick lines represent the location of the MCS profiles. Contour interval 250 m (depth in meters).Bottom: simplified map from the GEOSAT gravimetric anomaly map recently released by the US Navy (Sandwell and Smith, 1997). Stars:earthquake epicenters (1973–1978), USGS data base.

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Figure 3. Sketches of three tectonic stages of the central Drake Passage during the Cenozoic showing the changing scenario along theShackleton Fracture Zone. Legend: BS, Brandsfield Strait; PANT, Phoenix/Antarctic/Nazca triple junction; PAR, Phoenix/Antarctic Ridge;PNR, Phoenix/Nazca Ridge; SFZ, Shackleton Fracture Zone; SSR, South Scotia Ridge; SST, South Shetland Trench; STC, South Chile Trench;WSR, West Scotia Ridge. Explanation in the text.

form faulting, contemporaneous with the opening ofthe western Scotia Sea after 30 My (Figure 1). Marinemagnetic anomalies on both flanks of the PAR pro-vide, moreover, a record of spreading, extending backto about 23 My (BAS, 1985; Larter and Barker, 1991).

When sea-floor spreading was active at the WSRand the Phoenix-Nazca Ridge (PNR), the SFZ acted asa transform fault that connected these two slow to in-termediate spreading centers, while part of the move-ment was transferred to the Southern Chile Trench andSouth Shetland Trench (Figure 3 A). The PNR wassubducted in the southern Chile Trench due to theeastward migration of the Phoenix-Antarctic-Nazcatriple junction and a ridge/transform intersection ex-isted between the Phoenix-Antarctic Ridge (PAR) andthe SFZ (Larter and Barker, 1991; Forsythe and Prior,1992). Sinistral transform motion was maintained inthe central segment of the SFZ due to spreading at thePAR and the WSR (Figure 3). The PAR shows aver-age apparent full spreading rates of about 52 mm/yrand both plates reveal rather similar half-spreadingrates, although during the last phase of spreading vari-ations between<10 and>60 mm/yr are observed onthe PAR (Larter and Barker, 1991; Maldonado et al.,1994; Livermore et al., 2000). The WSR reveals,in contrast, slower average spreading rates of about35 mm/yr (Table 1).

It is proposed that spreading of the WSR endedafter magnetic anomaly 5A, due to the shifting ofthe spreading center to the eastern Scotia Sea (Barkeret al., 1991; Livermore et al., 1994; Larter et al., 1998).The tectonic evolution of the SFZ then became largelyinfluenced by spreading on the PAR (Figure 3B).Phoenix Plate ocean floor was subducted beneath theAntarctic continental lithosphere at the South ShetlandTrench, while farther northwest new ocean floor wascreated at the PAR, which migrated southeastward.

Probably due to reorganization of seafloor spread-ing in the western Scotia Sea, the stress field wasmodified across the SFZ and spreading on the lastthree remaining segments of the PAR ended at sometime during the Pliocene (Figure 3C). Displacementalong the SFZ has been maintained, however, sincethe relative motion between the South American andAntarctic plates is distributed along the North ScotiaRidge, South Scotia Ridge and the SFZ (Pelayo andWeins, 1991; Galindo-Zaldívar et al., 1996).

Nature of the ocean crust

Swath bathymetry and ocean-floor fabric

The morphology of the central Drake Passage in thevicinity of the WSR and SFZ intersection is illustratedby the swath bathymetry map and shaded relief im-

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Figure 4. (pull-out map pp. 49–50) Shaded relief map and line drawing interpretation of the West Scotia Ridge spreading center and ShackletonFracture Zone junction area (Drake Passage). Shaded areas on line-drawing map indicate sectors of more than 4000 m water depth, and blackareas correspond to nodal basins. Shaded Simrad EM-12 swath bathymetry mosaic is included in GEOSAT predicted bathymetry from free-airgravimetric anomalies. Legend: PHP, Phoenix Plate; SFZ, Shackleton Fracture Zone; WSP, West Scotia Plate; WSR, West Scotia Ridge.

Table 1 Average Apparent Half Spreading Rates (mm/yr) on the Antarctic-Phoenix(from Larter and Barker, 1991) and West Scotia (this paper and BAS, 1985) spreadingcenters in the proximity of the Shackleton Fracture Zone

Age (Ma) Phoenix/Antarctic Ridge West Scotia Ridge

Phoenix Plate Northern flank Southern flank

28–23 29 28

23–16 21 20 20

16–10 25–30 10 12

10–6 25–34 12 9

6–3.3 22 ? ?

Total average 26 18 17

age views (Figures 4 and 5). The main morphologicfeatures differentiated in the area, from west to eastinclude: (1) the former Phoenix plate; (2) the SFZ,and (3) the west Scotia plate, containing the WSR. TheWSR intersects the SFZ in a complex area of deforma-tion, which allows us to differentiate, in addition, fourregional domains (Figure 4, line drawing).

The SFZ is composed of ridge segments and de-pressions with an ‘en echelon’ pattern in plan view.These features are bounded by tectonic lineamentstrending N120◦ E, which are interpreted as the sur-face trace of faults. The most continuous lineamentis located along the western flank of the morpholog-ical ridge axis (Figures 4 and 5). Southwestward ofthis fault is another fault that bounds the deformedfracture zone and the undeformed ocean floor of theformer Phoenix plate, which is characterized by a reg-ular N35◦ E trending oceanic fabric. Between thesetwo faults, an area of oceanic crust with a clearN35◦ E trending fabric is also recognized within thefracture zone. The main N120◦ E lineaments are inter-sected by subsidiary W-E faults, which are located inelongated depressions parallel to the N120◦ E trend-ing ridge. The largest depression is located in thesouthwestern domain, between longitude 60◦30′ Wand 61◦45′ W. Several narrower depressions are ob-served in the northwestern domain (Figure 4). Allthese depressions seem to represent pull-apart basinsdeveloped under a transtensional regime.

Northeastward of the N120◦ E axial fault, thestructure seems to be more complex (Figures 4 and

5). The main morphological lineaments become frag-mented and irregular at both sides of the intersectionbetween the SFZ and the rift valley of the WSR.N120◦ E lineaments and E-W faults anastomose atboth sides of the intersection (Figures 4 and 5). Theroughly N40◦ E trending fabric of the westernmostend of the WSR is prolonged into the central area ofthe fracture zone. This extension of the fabric of thespreading center is bounded by NE–SW faults and anE–W fault.

North of the intersection the ridge associated withthe SFZ is composed of one long irregular elevationand two smaller elevations separated by a narrow val-ley (Figure 5). In the area of the intersection the ridgeis deeper and crossed by perpendicular, morphologiclineaments, extending from the northern and southernflanks of the spreading center. Nodal basins have de-veloped in this area at both corners of the ridge-faultintersection. South of this depressed area, a large domethat protrudes into the Scotia crust occupies the inter-section between the SFZ and the spreading center ofthe WSR.

Several smaller rhomboid depressions are ob-served on the Scotia ocean floor northwest of theintersection, on the northeastern flank of the fracturezone, between 62◦00′ W and 63◦30′W. These depres-sions may represent relict nodal basins that developedat the ridge-transform intersection, as described forthe Kane and Atlantis transforms of the Mid AtlanticRidge (Karson and Dick, 1983; Pockalny et al., 1988;Escartin and Lin, 1995).

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Figure 5. Shaded relief images (Fledermaus) oblique views of the West Scotia Ridge and Shackleton Fracture Zone junction. Top, view towardthe north. Bottom, view toward the southeast. Length of area is about 220 km.

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Figure 6. Selected segment of MCS profiles showing the structure of the relict spreading center of the WSR (A) and the deeply inclinedreflectors of the crust in the proximity of the boundary between the SFZ and the Scotia plate (B). Layers 2 and 3 of the igneous crust and theMoho seismic discontinuity are depicted in the line drawing interpretations. The reflectors (AMC) observed below the rift valley of the WSR(A′) may indicate petrological discontinuities developed in the relict axial magma chamber. See discussion in the text and Figure 2 for location.(Vertical exaggeration: 4.2)

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Figure 6B.

The WSR spreading center trends N40◦ E and thevalley floor becomes progressively wider and flatterthe further from the dome at the intersection with the

SFZ. An asymmetric and fault-bounded median valleyrepresents the WSR axis, which tapers from a widthof 20 km in the NE to 0 km at the intersection with

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Figure 7. Representative fragment of MCS profiles illustrating the structure of the West Scotia Ridge (profile PRSM 07) and ShackletonFracture Zone (profiles PRSM 06 and PRSM 10). See Figure 2 for location. (Vertical exageration: 6.6)

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the SFZ. The valley floor is flat and smooth, reflect-ing the presence of sediments. Abyssal hills on bothflanks trend approximately N30◦ E, sub-parallel tothe ridge axis, suggesting strong modification of themorphology by tectonics in the proximity of the for-mer junction, as observed in the MCS profiles (seenext section). The trend of the ridge axis shown bythe GEOSAT gravimetric anomaly map and the swathbathymetry map, changes abruptly near 58◦20′ S61◦20′ W, near the limit of the swath bathymetry cov-erage, from N60◦ E to N35◦ E (Figure 4). The changeis reflected in some of the younger off-axis abyssal hillfabrics. The two flanks of the ridge axis are asymmet-ric: the southern flank is characterized by very steepwalls, whereas the northern flank displays a moresubdued relief.

The sea floor of the Scotia plate exhibits a patternof NE–SW parallel, narrow linear ridges and depres-sions that developed when ocean floor accreted at thespreading center (Figures 4 and 5). These lineationsare very regular near the spreading center and pro-gressively more irregular and subdued away from thespreading center. In the distal sectors, the bathymetryshows an increasing proportion of elongated depres-sions and intervening, narrow and irregular morpho-logical highs. The floor of the Phoenix plate shows amore regular sea floor fabric of parallel lineations.

Tectonic expression in MCS profiles

The MCS profile orthogonal to the WSR (LinePRSM07, see location on Figure 2) shows that therift valley floor of the spreading center is asymmetri-cal and ponded with a depositional sequence 1 s thick(two way travel time, twt) above the acoustic base-ment (Figures 6A, 7). The valley is fault-bounded onboth flanks, with low- and high-angle normal faultson the northern and southern flanks respectively. Thebasement of the sediment fill is tilted southward andfault-bounded in the southern side (Figure 6A). Thetop of the igneous crust is also disrupted by conju-gate normal faults, which bound basement blocks. Asoutheastward-dipping reverse fault affecting the low-ermost depositional units and the acoustic basementis observed along the axis of the depression in thevicinity of the southern wall.

The igneous crust shows a rough, high-amplitudereflection at the top and numerous discontinuous high-amplitude reflectors and irregular diffractions, some ofwhich may be out of plane, in a layer 0.2–0.8 s thick(twt). Below this is a thicker layer with sparse and

weak reflectors, although it locally contains strong,dipping reflections (Figures 6 and 7). Several subhor-izontal and slightly tilted reflectors are observed at1.8 to 2.5 s (twt) depth below sea floor near the axisof the rift valley of the WSR and over a distance ofabout 4.5 km (Figure 6A). A bright, slightly undulat-ing reflector observed at about 0.75 s (twt) below thetop of the igneous crust (7.4–7.6 s below sea level)is interpreted as a sill perhaps indicative of a relictmagma chamber (AMC). Converting travel times todepth, using a velocity of 1.6–2.5 km s−1 for the sed-iments and 3.5–6.1 km s−1 for the igneous crust, weobtain average of the relict AMC of 2.1 to 2.5 km.Average depths are comparable to those for AMC re-flectors observed in active intermediate-rate spreadingcenters ranging from 2 to 3 km (Kent et al., 1990;Detrick et al., 1993; Hooft et al., 1997; Carbotteet al., 1998). It is important to note, however, thatspreading ceased in the WSR about 6 Ma (see Mag-netic Anomalies section) and that these reflectors, inconsequence, may not be attributed to an active axialmagma chamber. The reflectors observed near the axisof the extinct WSR spreading center may indicate, inconsequence, discontinuities that developed during thecooling process, probably by crystallization of a meltmush below the upper crust, as proposed in severalexamples of modern magma chambers (e.g., Sintonand Detrick, 1992).

The margins of the valley on both sides of thespreading center are also characterized by strongasymmetry (Figures 6A, 7, 8). The northern flank ex-hibits a subdued relief and is ponded with sedimentsin the area proximal to the rift valley. The southernflank, in contrast, is bounded by a steep, faulted walland has prominent and irregular relief, which becomessmoother and less pronounced away from the spread-ing center. The sedimentary cover is up to 0.5 s (twt)thick in the northern area (Figure 7, profile PRSM 07,shot points 2550–2700) and is sparse in the southernsector, except in a small perched basin (Figure 7, pro-file PRSM 07, shot points 3150–3200). The scarcityof sediments may be attributed to the activity of thestrong circum-polar current system in this area sincethe opening of the Drake Passage (Lawver et al., 1992;Pudsey and Howe, 1998).

Deep crustal dipping reflectors are well expressedin the profiles perpendicular to the SFZ (Figures 6Band 7; profiles PRSM 06, PRSM 10). Most of thesereflectors are tilted southwest and are observed in thecrust of both the Scotia and Antarctic plates, and be-neath the SFZ. Dipping reflectors are less abundant in

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Figure 8. Line-drawing interpretations of the MCS profiles in Figure 7. See Figure 2 for location. (vertical exaggeration: 6.6).

the profile orthogonal to the WSR (Figures 7 and 8;profile PRSM 07), which seems to indicate a preferen-tial southwestward dip of the crustal reflectors.

The upper seismic unit of the igneous crust, withdiscontinuous high-amplitude reflections in all theMCS profiles may represent the top of oceanic crustlayer 2, composed of extrusives underlain by sheeteddiabase dykes (Francheteau et al., 1992; Mével et al.,1993; Cannat et al., 1995). The underlying, moretransparent seismic unit is attributed to the isotropiclayer 3 gabbros (Figures 6–8). The transition betweenthese two layers is not well shown in the profiles.

Deep, high-amplitude subhorizontal and slightlytilted reflections are observed at several locations inthe profiles (Figures 7 and 8). These reflectors arevery discontinuous and are distributed between 1.8and 3.2 s (twt) below sea floor, but preferentially lo-cated at about 2.8–3.0 s (twt) depth. Where thesereflections are more continuous they may represent theMohorovicic seismic discontinuity (Moho), which issupported by two-dimensional gravity models of thefree-air anomaly (Maldonado et al., 1999). The shal-

lower reflections could be attributed, in contrast, todiscontinuities within layer 3 of the oceanic crust.

More locally, short (2–3 km long), very high-amplitude reflections are seen between 3.6 and 4.3 s(twt) depth below sea floor in the profiles orthogo-nal to the WSR (Figures 6 and 7). The MCS profilesorthogonal to the SFZ also show deep subhorizon-tal reflectors at 3.6–4.3 s (twt), which coincide withthe deeper reflections of the profile orthogonal to thespreading center (Figures 7 and 8). These reflectionsare probably too deep to represent the crust/mantleboundary and it is most likely that the deep reflectionsobserved in the Drake Passage profiles correspond toan interlayered upper-mantle transition zone.

The MCS profiles orthogonal to the SFZ show aprominent ridge of 1000 m to 2000 m relief abovethe ocean floor (Figures 7 and 8). This ridge is asym-metrical and contains a small irregular depression onthe southwestern slope. The profiles show numerousinternal reflectors and discontinuities, some of whichare attributed to faults (Figures 6B, 7, 8). The north-eastern flank of the ridge is transpressive above the

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Scotia oceanic crust, as revealed by low-angle, high-amplitude reflectors dipping southwestward below theridge (Figures 6B, 7, 8; profile PRSM 06, shots50–250, profile PRSM 10, shots 8500–8700).

Magnetic anomalies

In the study area, magnetic anomalies 5 and older havepreviously been recognized on the flanks of the WSR(Barker and Burrell 1977, BAS 1985). Figure 9 sum-marises all available magnetic profiles in the vicinityof the SFZ, including the new profiles, superimposedupon the satellite-derived marine gravity field. Cov-erage of the westernmost ridge segment is reasonablygood, with several complete crossings of the axis andboth flanks. The new data also improve coverage ofthe younger anomalies in the zone closest to the SFZ.

We have modeled the anomalies using a smoothedversion of the observed bathymetry and a 1 km thicksource layer with a susceptibility of 0.005 SI (Fig-ure 10). Although anomalies 5 to 5E may be confi-dently identified, younger anomalies are, as observedby Barker and Burrell (1977), more irregular and vari-able, and their amplitudes are reduced. Our modelassumes slow rates (ca. 10–20 mm/yr), with slightasymmetries favoring the southeastern flank between5C and 5, and the northwestern flank between 5 andextinction. The spreading rate asymmetry means thataccretion was slower during formation of the steeper,southeastern flank than of the northwestern flank. Theextinction time in our model is the same as in themodel of Barker and Burrell (1977), i.e., 6 Ma, al-though on the newer timescale of Cande and Kent(1995) this correlates with anomaly 3A rather than4. However, the poor agreement with observed axialanomalies means that we can say only that spreadingceased some time between magnetic chron 3A and 2(6.6 to 1.8 Ma), since it had to have stopped prior tothe Brunhes.

The oceanic crust of the extinct Phoenix platein the study area contains magnetic anomalies cre-ated on the southern flank of the Phoenix-Antarcticspreading center (Figure 1). These magnetic anom-alies have been analyzed in detail by Larter and Barker(1991) and more recently by Livermore et al. (2000).This area contains magnetic anomalies, interpreted byLarter and Barker (1991) as indicating extinction onthe northern segment closest to the SFZ (P1), at chron3 (3.3 Ma on the timescale of LaBrecque et al. 1977),and on the southern segment, closer to the Hero Frac-ture Zone at chron 2A (2.2 Ma). New data from the

middle segment show that spreading ceased simulta-neously on all three segments at the time of anomaly2 A (3.3 Ma) (Livermore et al., 2000).

The map in Figure 11 was elaborated with datafrom the new profiles combined with all availablemagnetic anomaly data. Although the pattern of anom-aly lineations is not well resolved in detail, theirN30–40◦ E trend is clear on both flanks of the western-most segment, even where the axis is highly oblique.It thus appears that spreading became oblique shortlybefore extinction.

Discussion and conclusions

Stratigraphy and tectonics of the crust

Lower crustal inclined reflectors in MCS profiles ofthe Mesozoic North Atlantic crust were initially as-cribed to mafic-ultramafic cumulate layers reflectingrelict magma chamber structures (McCarthy et al.,1988). More recent studies of the Central North At-lantic show that the Atlantic oceanic crust containsmiddle to lower crustal reflectors that generally diptoward the spreading center and also toward nearbyfracture zones (White et al., 1990; Morris et al., 1993).Mutter and Karson (1992) interpreted these reflectorsas major detachment faults that penetrated the entirethickness of the crust and correlated them with the ma-jor detachment fault structures of the ridge axis wallsof the central Mid-Atlantic Ridge. The dipping reflec-tors observed in the vicinity of this slow-spreadingridge suggest major faults interpreted as lateral rampslinking detachments in a complex three-dimensionalextensional system (Mutter and Karson, 1992; Col-lier et al., 1997). A possible interpretation of thelower-crustal dipping reflectors is that they representlithological layering (Banda et al., 1992; Ranero et al.,1997), following the model proposed by Nicolas et al.(1988) to explain magmatic foliations observed in thelayered gabbro section of the Oman ophiolite.

The overall reflectivity patterns in the oceanic crustof spreading centers suggest a different mechanical be-havior for the upper (brittle domain) and lower crusts(ductile domain). These domains are separated by thepresence of a widespread seismic reflection bound-ary, related perhaps to the base of sheeted dikes, tothe maximum depth of hydrothermal circulation or tomajor structural discontinuities (Ranero et al., 1997).Normal faults (listric in cross-section) play a dominantrole in the morphology and internal structure of crust

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Figure 9. Wiggle plot of magnetic profiles in the vicinity of the study area, superimposed on shaded relief of satellite gravity field (Sandwelland Smith, 1997). Positive anomalies on new profiles from ANTPAC 97/98 shaded in black, other profiles in grey.

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Figure 10. Modeled magnetic profiles over the West Scotia Ridge, adjacent to the Shackleton Fracture Zone. TJ2C, TJ6C and TJ7C are profilesacquired in 1997 by B/O Hesperides; NBP93 is part of a profile obtained by R/V Nathaniel B. Palmer in 1993. The observed bathymetry alongprofile TJ6C has been used as the top of the magnetized layer in the axial region; elsewhere, a flat sea floor is used.

formed at slow-spreading mid-ocean ridges (Minshulland Hall, 1997). Studies of slow-spreading centersindicate that the interplay of magmatic constructionand mechanical extension varies significantly both re-gionally and locally (e.g., Cannat et al., 1995). TheMCS profiles of the oceanic crust of the central DrakePassage also show a pattern of steeply-inclined reflec-tors. These reflectors dip toward the SFZ and trendobliquely to both the WSR and the flow lines of theextinct Phoenix plate. Since these reflectors cut acrosslayers 2 and 3 of the oceanic crust, but do not seemto affect the uppermost deposits of sedimentary layer1, they most likely were developed contemporaneouswith spreading or shortly after spreading ceased at theWSR. When spreading was active at the WSR the tec-tonics along the SFZ indicate an extensional regime,

which favored the development of pull-apart basinsand shallow oceanic ridges parallel to the fracturezone (see next section). The transpressive characterof the northern flank of the SFZ reveals, in contrast,a change in the tectonic regime after the cessation ofspreading at the WSR, which may have facilitated thedevelopment of these reflectors.

Alternatively, these reflectors could be interpretedas evidence of sills, perhaps indicative of extinctmagma chambers. Although large local variations areobserved in the depth and width of the axial magmachamber, studies suggest that the development ofmagma chambers is generally restricted to a narrowzone or to lenses, which may show some variationsin depth within the upper crust (Sinton and Detrick,1992; Detrick et al., 1993; Carbotte et al., 1998). In

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Figure 11. Shaded relief plot of marine magnetic anomaly field,computed from all available profiles. Shading is derived from ma-rine free-air gravity field, illuminated from the north. Positiveanomalies shown in black.

addition, the dipping reflectors observed in the DrakePassage extend throughout most of the crust and areoblique or at right angles to the spreading centers (Fig-ures 6B and 8). We prefer, in consequence, a structuralinterpretation for the origin of the inclined reflectors inthe Scotia Plate.

At Hole 735B of the Ocean Drilling Program in thesouthwest Indian Ridge, three different models wereproposed to explain the stratigraphy of the lower oceancrust (Dick et al., 1991, 1999). One model consid-ers that the crust is made of relatively small gabbroicbodies cutting each other discordantly and a partiallyserpentinized upper mantle. A second interpretation isthat the lower crust comprises an assembly of smalldiscordant gabbroic bodies that pass abruptly into theupper mantle, as suggested by Smith and Cann (1993)for the Mid-Atlantic Ridge. The third model, derivedfrom ophiolite studies, suggests that the discordantgabbroic bodies occur only in the upper crust, and thengrade downward into large layered gabbro intrusions,which have a sharp magmatic contact with upper man-tle peridotite. During the most recent leg of the ODPto the Southwest Indian Ridge, a thick sequence wascored down to 1500 m depth in the oceanic crust, con-sisting of gabbros and olivine gabbros, but withoutreaching the crust-mantle boundary or demonstratingwhether the Moho is an alteration front rather thanthe crust-mantle boundary (Dick et al., 1999). The

exceptional abundance of serpentinized peridotite insamples from the walls of rift valleys in fracture zonesand in the rift mountains away from fracture zonessuggests, in addition, the possibility that serpentinitecould be a component of oceanic layer 3 (Francheteauet al., 1992; Cannat, 1993).

The Moho may not correspond everywhere in theoceans to the boundary between the oceanic crust andthe mantle, but to an alteration front. If this is so,perhaps corresponding to the depth of downward cir-culating seawater, the subhorizontal deep reflectors inthe MCS profiles of the central Drake Passage mayalso represent alteration fronts within the lowermostmagmatic crust or the upper mantle. The deeper re-flections observed between 3.6 and 4.3 s (twt) depthbelow sea floor may represent, in addition, the lowerboundary of a layered zone between the upper crustand the mantle.

Ancient examples of the ocean crust exposed indeep crustal sections illustrate, moreover, a large vari-ability of ophiolites. Studies of the complexes in theBay of Islands ophiolite show the presence of a com-plexly interlayered transition zone as much as 3 kmthick in the crust/mantle transition (Karson et al.,1984). The Trinity ophiolites, considered to representa slow-spreading environment, are characterized bythe lack of well-developed layering and small intru-sions, whereas the Oman ophiolites, attributed to afast-spreading center, show well-defined, planar andcontinuous igneous layering in the lower two-thirdsof the sequence (e.g., Nicolas et al., 1996). Althoughophiolite sequences may not represent typical oceaniclithosphere, the deeper reflections observed in ourprofiles could also be attributed to layered cumulateultramafic rocks between the crust and the mantle. Alaminated upper mantle is a plausible explanation forthese deep reflections in the Drake Passage. Our MCSprofiles thus confirm a complex three-dimensionalstructure of the deep ocean crust, resulting from bothmagmatic and tectonic processes, and influenced bythe spreading rates and proximity to fracture zones.

Tectonic evolution along the SFZ

The SFZ is a major tectonic and morphologic fea-ture cutting across the Drake Passage, which separatesoceanic crust of varying age and thickness formed atdifferent spreading rates (Figures 1, 3, 12; Table 1).The most significant features of this fracture zone ob-served in the orthogonal seismic profiles are the strongstructural asymmetry and the significant width (Fig-

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Figure 12. Interpretative tectonic diagram of the West Scotia Ridge-Shackleton Fracture Zone intersection based on the swath bathymetrymosaic and the MCS profiles.

ure 8). The SFZ is also very variable along the axis(Figures 4 and 5). The segment of the fracture zonenorthwest of the intersection with the WSR is about34 km wide (Figure 8, profile PRSM 06) This seg-ment represents a typical ‘transform’ fault betweentwo spreading centers (Figure 3). It shows a promi-nent ridge on the northeastern flank (2400 m waterdepth), a depression (3800 m depth) and a large irreg-ular plateau (about 3500 m depth) on the southwesternflank (Figures 5 and 8; profile PRSM 06). The south-eastern segment of the SFZ, from the WSR to theSouth Shetland Trench, appears to correspond to a‘non-transform’ segment of a fracture zone, since it

does not link two spreading centers. However, the en-tire fault was active during the evolution of the fracturezone, from the Oligocene to the present (Figure 3).The fracture zone is narrower, about 26 km wide,and is characterized by long, constricted, irregular de-pressions parallel to it. The northeastern segment ofthe ridge has high relief (1875 m depth), bounded tothe southwest by a depression (4500 m depth) andan irregular plateau with ridges and depressions atabout 3750 m mean water depth. The spreading center-fracture zone intersection is marked by deep sub-circular depressions that may be attributed to nodalbasins in the ‘transform’ segment of the fault, defining

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Figure 13. Average Apparent Spreading Rate (ASR) on the Antarc-tic-Phoenix (from Larter and Barker, 1991) and West Scotia Ridge(calculated from the magnetic anomalies of BAS, 1985) spreadingcenters in the proximity of the Shackleton Fracture Zone.

the former positions of the intersection (Figure 12).Additional nodal basins are observed on the northeast-ern domain of the Scotia crust near the SFZ (Figures 4and 12).

These features are different from the Kane Frac-ture Zone on the slow-spreading (10–14 mm/yr) Mid-Atlantic Ridge, which is characterized by a rathersymmetric, deep transform valley and a slightly asym-metrical and deep non-transform valley (Karson andDick, 1983). An intermediate, relatively low reliefridge is also observed near the axis of the transformvalley, in the proximity of the ridge/transform inter-sections (Pockalny et al., 1988). It is also differentfrom the Atlantis II Fracture Zone mapped in detailby Dick et al. (1991). This transform is marked bya 6480 m deep valley, with high flanking transverseridges shoaling to as little as 700 m. The valley wallsare extremely steep for large distances and the floorof the transform has a thick (>500 m) sequence ofdeposits and is bisected by a 1.5-km high interme-diate tectonic ridge. The physiographic contrast anddistribution of plutonic and volcanic rocks at this

fracture zone were attributed to a crustal weld thatperiodically formed between the shallow levels of theocean crust and the old cold lithospheric plate at theridge/transform intersection (Dick et al., 1991). Thisweld forced the shallow levels of the new ocean crustto spread with the older plate and favored the devel-opment of detachment faults. A similar model wassuggested to explain the asymmetric physiography anddistribution of plutonic and volcanic rocks at the KaneFracture Zone in the North Atlantic, where the surfaceof the detachment fault has been observed by sub-mersibles (Karson and Dick, 1983; Mével and Cannat,1991). It has been proposed that similar detachmentfaults may also occur periodically within rift valleys(Harper, 1985; Karson, 1991).

The SFZ segments investigated may also be com-pared with the Clipperton transform along the EastPacific Rise (Macdonald et al., 1988). Spreading inthis region of the East Pacific Rise is at an interme-diate to high-intermediate rate (70–80 mm/yr and 45–55 mm/yr, north and south respectively of the trans-form zone), which is more similar to the spreading rateour study area, at 52 mm/yr for the PAR and 35 mm/yrfor the WSR (Table 1, Figure 13). In contrast tothe Clipperton transform, however, the SFZ is morecomplex in the ‘non-transform’ zone (the southeasternsector of the SFZ between the WSR and the SouthShetland Trench). This region contains, in additionto a high relief ridge, deep elongated basins, whichreflect the complex tectonic evolution of this fracturezone (Figures 4 and 12). Oceanic transverse ridges,parallel to fracture zones, are characteristic features ofsea-floor morphology, attributed to modest amountsof extension along low-angle faults (Pockalny et al.,1996). In this extensional regime, the depressions areattributed to pull-apart basins, bounded by normal andstrike-slip faults due to the left-lateral motion betweenthe Scotia and Antarctic plates. The structure of theSFZ, in addition, is further complicated by the changein the tectonic regime after cessation of spreading inthe WSR, which is recorded by the transpressive char-acter of the northeastern flank of the fracture zone(Figures 6B and 8).

Implications of the spreading rates

The significant differences in spreading rates betweenthe WSR and the PAR imply that the SFZ acted alongalmost all its length as a transform fault when thesespreading centers were active during the Cenozoic(Figure 13, Tables 1 and 2). The differential mo-

64

Figure 14. Sketches of plate tectonic reconstructions of the central sector of the Drake Passage from the late Miocene to present showing thetectonics along the Shackleton Fracture Zone. Discussion in the text.

65

Table 2. Average spreading rates and relative motions (mm/yr) between plates along the Shackleton Fracture Zone

Age (Ma) Full spreading rate Half spreading rate West Scotia Ridge Relative motion

Phoenix Antarctic Ridge Northern Flank Southern Flank Transform segment Non-transform segment

20 48 20 20 68 28

12 58 10 12 68 46

9 59 12 9 71 50

7 59 12 9 71 50

4 <22 – – <22 <22

tions between the Phoenix-Antarctic plates and thetwo flanks of the West Scotia plate were compen-sated by subduction at the Southern Chile Trench andthe South Shetland Trench, whereas relative motionwas maintained on the SFZ. Plate tectonic evolu-tion of the Pacific margin of the Antarctic Peninsulatook place in a simple tectonic setting, since Phoenixplate ocean floor was subducted at the South Shet-land Trench, while new ocean floor was accreted atthe PAR (Figure 1). Southward of the SFZ the north-western, trailing flank of the ridge and the overridinglithosphere at the South Shetland trench were both partof the Antarctic plate, with the oceanic Phoenix platebeing accreted and subducted between the spreadingcenter and the trench (Larter and Barker, 1991; Mal-donado et al., 1994). A significant consequence of thisplate distribution is that the Phoenix plate is movingtoward the trench at a rate equal to the full spread-ing rate at the ridge crest. The plate tectonics of thewest Scotia Sea is also simple, with oceanic crust ac-creted at the WSR as the South American plate driftednorthwestward from the Antarctic plate (Figure 3).

This simple plate geometry and the magneticanomalies on both flanks of the PAR and WSR allowreconstruction of the evolution in the vicinity of theSFZ. The tectonic history of the central Drake Passageis well constrained between about magnetic anomalies8 and 3A for the WSR (BAS, 1985; Figure 9) andmagnetic anomalies 6B and 2A for the PAR (Larterand Barker, 1991; Livermore et al., 2000). Ocean floorof the Phoenix plate older than magnatic chron 6A wassubducted at the South Shetland trench and the spread-ing history at the ridge is only preserved on the trailingflank of the ridge crest. In fact, much of the ocean floorsubducted beneath the Antarctic Peninsula during theCenozoic was produced at the Nazca-Phoenix ridges(Figures 1 and 3).

Several sketches of the plate tectonic configurationnortheast of the Hero Fracture Zone and in the prox-

imity of the SFZ are shown in Figure 14. At chron5A (12 Ma), the full spreading rate at the PAR wasabout 58 mm/yr, whereas half-spreading rates of about10 and 12 mm/yr were recorded for the northern andsouthern flank of the WSR respectively (Figure 14A).The motion between the two plates in the transformsegment of the SFZ was about 68 mm/yr, with the twoplates moving in opposite directions. However, the‘non-transform’ segment of the fracture zone was alsoactive, with a left-lateral motion of about 46 mm/yrand both plates moving in the same southeastward di-rection relative to the Antarctic plate. Table 2 showsthe average spreading rates and relative motions alongthe SFZ at several time periods (BAS, 1985; Larterand Barker, 1991, this paper). It is significant that therelative motion in the ‘transform’ segment of the SFZuntil cessation of spreading in the WSR was ratheruniform at about 68–71 mm/yr, whereas the ‘non-transform’ segment of the fracture zone shows ratesof 28 to 50 mm/yr. Between cessation of spreading atthe WSR and the PAR, the relative motion along theentire length of the SFZ, southeastward from the PARintersection was less than 22 mm/yr (Table 2).

After cessation of spreading at the PAR at anom-aly 2A time (3.3 Ma) (Livermore et al., 2000), thekinematics along the SFZ is less precisely constrained.The present stress field deduced from focal mecha-nisms (Pelayo and Wiens, 1989) and the right-dihedradiagrams (Galindo-Zaldívar et al., 1996) show thatfor the entire Scotia Sea region there is predominantSW-NE subhorizontal compression and SE-NW sub-horizontal extension. The SFZ shows, however, aperturbation of the general stresses. The maximumcompression is WNW-ESE subhorizontal, while theextension is steeply inclined (Galindo-Zaldívar et al.,1996). These stresses and the sinistral transpressivecharacter of the plate boundary are recorded by thereverse faults observed in our MCS seismic profiles(Figures 6–8). This tectonic evolution implies that the

66

SFZ has always maintained since the beginning ofspreading at the WSR a sinistral motion along the‘transform’ and ‘non-transform’ segments, southeastof the intersection with the PAR or the PNR (Fig-ure 3). Significant differences are observed, however,in the rate of relative motion between plates (Table 2).Thus, a sharp decrease in the relative motion betweenplates is recorded in both the ‘transform’ and the ‘non-transform’ segments of the fracture zone from about71 and 50 mm/yr, respectively, to less than 22 mm/yrafter cessation of spreading at the WSR. Another de-crease in the rate of relative motion between the platesalong the SFZ should have occurred after cessationof spreading at the PAR, since the present transcur-rent motion is only due to the tectonics between theAntarctic-South American and Antarctic-Scotia plates(Pelayo and Wiens, 1989; Galindo-Zaldívar et al.,1996).

Acknowledgements

We acknowledge with gratitude the cooperation of theCommander B/O HESPERIDES C.F. L. M. Nuchedel Rivero, the officers and crew of the vessel whomade it possible to obtain these data under some-times very severe sea conditions. We also thank thetechnicians participating in the cruise for their ded-ication and help. The diligence and expertise of en-gineers E. Litcheva and J. Maldonado who processedthe MCS data and swath bathymetry is appreciated.We are indebted to Chris Goldfinger, University ofOregon, for facilitating the use of the program FLE-DERMAUS to visualize the swath bathymetry, andto two anonymous reviewers for helpful suggestionswhich improved the original manuscript. The Span-ish Comisión Interministerial de Ciencia y Tecnología(CYCIT) provided financial support through ResearchProject ANT99-0817.

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