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Cooper, A. K., P. J. Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National Academies Press. Antarcticaâs Continent-Ocean Transitions: Consequences for Tectonic Reconstructions K. Gohl1 ABSTRACT arctic Peninsula, and 400 km are of active convergent type. The structure and composition of continental margins, in Antarctica was the centerpiece of the Gondwana supercon- particular those of rifted margins, can be used to elucidate the tinent. About 13,900 km of Antarcticaâs 15,900-km-long geodynamic processes of continental dispersion and accre- continental margins (87 percent) are of rifted divergent type, tion. The margins of Antarctica have mostly been subject to 1600 km (10 percent) were converted from a subduction regional studies mainly in areas near research stations in the type to a passive margin after ridge-trench collision along Ross Sea, Prydz Bay, Weddell Sea, and along the Antarctic the Paciï¬c side of the Antarctic Peninsula, and 400 km (3 Peninsula near national research facilities. In recent yearsâ percent) are of active convergent type. In recent years the mainly motivated by the United Nations Convention on the volume of geophysical data along the continental margin of Law of the Seaâlarge volumes of new offshore geophysical Antarctica has increased substantially, which allows differ- data have been collected, primarily along the East Antarctic entiation of the crustal characteristics of its continent-ocean margin. For the ï¬rst time this provides the opportunity to boundaries and transitions (COB/COT). These data and geo- make a comprehensive analysis of the development of these dynamic modeling indicate that the cause, style, and process continental margins over large tracts of extended continental of breakup and separation were quite different along the crust that were previously unknown. The coverage of circum- Antarctic margins. A circum-Antarctic map shows the crustal Antarctic multichannel seismic lines from the Antarctic styles of the margins and the location and geophysical char- Seismic Data Library System for Cooperative Research acteristics of the COT. The data indicate that only a quarter of (SDLS) of the Scientiï¬c Committee on Antarctic Research the rifted margins are of volcanic type. About 70 percent of (SCAR) (Wardell et al., 2007) (Figure 1) is, with the excep- the rifted passive margins contain extended continental crust tion of some areas in the central Weddell Sea, off western stretching between 50 and 300 km oceanward of the shelf Marie Byrd Land and off the Ross Sea shelf, dense enough edge. Deï¬nitions of the COT and an understanding of its for quantifying basement types and volcanic and nonvolca- process of formation has consequences for plate-kinematic nic characteristics of the margins. The track map (Figure 1) reconstructions and geodynamic syntheses. shows that deep crustal seismic data, necessary for a com- plete and accurate characterization of the marginal crust to INTRODUCTION upper mantle level, are still absent over most margins. In this paper I ï¬rst present a compilation of the struc- About 13,900 km of the 15,900-km-long continental margins tural types of the circum-Antarctic continental margins of the Antarctic plate are of rifted divergent type, 1600 km based on a review of relevant published data of diverse types were converted from a subduction-type to a passive margin together with new data. Then I contemplate implications of after ridge-trench collision along the Paciï¬c side of the Ant- the knowledge of margin crustal types and properties for plate-kinematic and paleobathymetric reconstructions and 1 Alfred Wegener Institute for Polar and Marine Research, Postbox for isostatic response models. 120161, 27515 Bremerhaven, Germany (email@example.com). 29
30 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD FIGURE 1 Overview map of Antarctica and surrounding ocean ï¬oor using a com- bined BEDMAP (bedrock topography with ice sheet removed) (Lythe et al., 2001) and satellite-derived bathymetry grid (McAdoo and Laxon, 1997). White areas on the con- tinent are below sea level if the ice sheet is removed (without isostatic compensation). Yellow lines mark boundaries between the six crustal breakup sectors of respective conjugate continents in this paper. Red lines show the tracks of offshore multichan- nel seismic proï¬les of the SCAR Seismic Data Library System (SDLS) (compiled by M. Breitzke, AWI). Black lines mark the locations of offshore deep crustal seismic refraction proï¬les. WS = Weddell Sea sec- tor; DML = Dronning Maud Land sector; EL = Ellsworth-Lambert Rift sector; WL = Wilkes Land sector; MBL = Marie Byrd Land sector; APE = Antarctic Peninsula- Ellsworth Land sector. STRETCHING AND BREAKING: at about 147 Ma. It is not clear, however, whether some of PASSIVE MARGIN TYPES the crustal extension is also associated with this early Wed- dell Sea opening. The crust between the northern end of Weddell Sea (WS) Sector Conjugate to South America the large positive gravity anomaly and the magnetic Orion Anomaly and Andenes Anomaly (Figure 2) is interpreted The complex tectonic development of the Weddell Sea as a COT with the Orion Anomaly suggested to represent sector (Figure 2) has more recently been reconstructed by an extensive zone of volcanics that erupted during the ï¬nal HÃ¼bscher et al. (1996a), Jokat et al. (1996, 1997, 2003, breakup between South America and Antarctica (KÃ¶nig and 2004), Leitchenkov et al. (1996), Ghidella and LaBrecque Jokat, 2006). Deep crustal seismic refraction data across the (1997), Golynsky and Aleshkova (2000), Ghidella et al. Orion and Andenes anomalies and the assumed COT south of (2002), Rogenhagen and Jokat (2002), and KÃ¶nig and Jokat it are needed in order to constrain their crustal composition (2006). Deciphering of the crustal types in the central Wed- and type. Although the Orion Anomaly may provide a hint dell Sea is still hampered by the lack of deep crustal seismic toward a volcanic-type margin, the few seismic data do not data. In the southern Weddell Sea seismic refraction data allow a complete characterization of the COT in the southern reveal a thinned continental crust of about 20 km thick- Weddell Sea. Identiï¬ed magnetic spreading anomalies (old- ness beneath the northern edge of the Filchner-Ronne ice est is M17) show evidence that oceanic crust exists north shelf (HÃ¼bscher et al., 1996a). It can be assumed that this of the Orion and Andenes anomalies with the prominent T- thinned crust extends northward to a boundary marked by Anomaly marking supposedly the changeover from slow to the northern limit of a large positive gravity anomaly (Figure ultraslow spreading-type crust (KÃ¶nig and Jokat, 2006). 2). KÃ¶nig and Jokat (2006) associate an east-west rifting The Weddell Sea margin along the east coast of the Ant- of this crust (stretching factor of 2.5) with the motion of arctic Peninsula is still rather enigmatic due to missing data. the Antarctic Peninsula from East Antarctica as the earliest KÃ¶nig and Jokat (2006) show that it rifted from the western event in the Weddell Sea plate circuit at about 167 Ma prior Patagonian margin as part of the earliest plate motion in the to the early Weddell Sea opening in a north-south direction Weddell Sea region at about 167 Ma. They follow Ghidella
GOHL 31 FIGURE 2 Weddell Sea sector with satellite-derived gravity ï¬eld (McAdoo and Laxon, 1997) and continental mar- gin features. Red lines show the tracks of offshore multichannel seismic pro- files of SDLS. Dark blue lines mark the locations of offshore deep crustal seismic refraction proï¬les. COT = con- tinent-ocean boundary; GRAV = positive gravity anomaly marking northern limit of thinned continental crust; FREC = Filchner-Ronne extended crust; EE = Explora Escarpment; EW = Explora Wedge; AP = Andenes Plateau; WR = Weddell Sea Rift; T-A = T-Anomaly; OA = Orion Anomaly; AA = Andenes Anomaly. and LaBrecqueâs (1997) argument for a nonvolcanic margin Lazarev Sea margin is characterized by a COT consisting based on low-amplitude magnetic anomalies and a charac- of a broad stretched continental crust and up to 6-km-thick teristic bathymetry. volcanic wedges clearly identiï¬ed by SDR sequences. Deep The margin of the eastern Weddell Sea along the western crustal seismic data show that the crust thins in two steps Dronning Maud Land coast is more clearly characterized by from 23 km to about 10 km thickness over a distance of 180 the prominent bathymetric expression of the Explora Escarp- km (KÃ¶nig and Jokat, 2006). Their velocity-depth model ment and the massive volcanic ï¬ows along the Explora reveals high seismic P-wave velocities in the lower crust of Wedge (Figure 2), identiï¬ed by the abundance of seaward this COT, suggesting voluminous underplating and intru- dipping reï¬ectors (SDRs) in the seismic reï¬ection data. sion of magmatic material. A coast-parallel strong positive A number of deep crustal seismic refraction proï¬les cross and negative magnetic anomaly pair marks the northern the Explora Wedge and the Explora Escarpment and allow limit of the COT and is interpreted as the onset of the ï¬rst models showing a 70-km to 90-km-wide transitional crust oceanic crust generated by spreading processes at chron thinning from about 20 km thickness to 10 km thickness M12 (136 Ma). The volcanic characteristics of the eastern toward the north (e.g., Jokat et al., 2004). Relatively high Lazarev Sea segment is very likely to be related to the same P-wave velocities in the lower crust and in the upper crustal magmatic events leading to the Early Cretaceous crustal section of the SDRs (Jokat et al., 2004) are evidence for a accretion of a Large Igneous Province (LIP) consisting of volcanic-type continental margin. the separated oceanic plateaus Maud Rise, Agulhas Plateau, and Northeast Georgia Rise (Gohl and Uenzelmann-Neben, 2001) and to which also parts of the Mozambique Ridge Dronning Maud Land (DML) Sector Conjugate to Africa may have belonged. Data and syntheses of the central and eastern Dronning At the Riiser-Larsen Sea margin east of the Astrid Ridge, Maud Land (DML) margin stem from work by Hinz and the outer limit of the COB is constrained by densely spaced Krause (1982), HÃ¼bscher et al. (1996b), Roeser et al. aeromagnetic data revealing spreading anomalies up to M24 (1996), Jokat et al. (2003, 2004), Hinz et al. (2004), and (155 Ma) (Jokat et al., 2003). This is so far the oldest mag- KÃ¶nig and Jokat (2006). The recent plate-kinematic recon- netic seaï¬oor spreading anomaly observed along any of the structions by Jokat et al. (2003) and KÃ¶nig and Jokat (2006) circum-Antarctic margins. Seismic reï¬ection data (Hinz et show that southeast Africa was conjugate to the DML mar- al., 2004) indicate a COT of stretched continental crust that gin (Figure 3) from just east of the Explora Escarpment to is with 50 km width much narrower compared with the COT the Gunnerus Ridge. However, this DML margin has two of the Lazarev Sea margin. However, the data do not seem distinct parts, separated by the Astrid Ridge. The eastern to indicate a strong magmatic inï¬uence of the COT as major
32 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD FIGURE 3 Dronning Maud Land sec- tor with satellite-derived gravity field (McAdoo and Laxon, 1997) and conti- nental margin features. Red lines show the tracks of offshore multichannel seis- mic proï¬les of SDLS. The dark blue line marks the location of an offshore deep crustal seismic refraction proï¬le. COT = continent-ocean boundary; EW = Explora Wedge. SDRs are missing (Hinz et al., 2004). Deep crustal seismic and may be biased by the structure and signal of crossing refraction data do not exist to better characterize this part of fracture zones. The eastern Enderby margin zone has more the DML margin and its COT or COB. of a normal rifted margin setting with a COB up to 300 km north of the shelf edge (Stagg et al., 2004). The prominent magnetic Mac Robertson Coastal Anomaly (MCA) corre- Enderby Land to Lambert Rift (EL) Sector Conjugate to India lates with the northern limit of the COT in this eastern zone. Most of the crustal and sedimentary structures of the conti- Ocean-bottom seismograph data along two seismic refrac- nental margins off Enderby Land (east of Gunnerus Ridge), tion proï¬les were recently acquired in the eastern Enderby between Prydz Bay and the Kerguelen Plateau, off Wilhelm Basin and across the Princess Elizabeth Trough (PET) II Land and Queen Mary Land (Figure 4) have been revealed between the Kerguelen Plateau and Princess Elizabeth Land by large seismic datasets acquired by Russian and Austra- as part of a German-Russian cooperation project (Gohl et al., lian surveys (e.g., Stagg et al., 2004, 2005; Guseva et al., 2007a) (Figure 4). The western proï¬le conï¬rms an extremely 2007; Leitchenkov et al., 2007a; Solli et al., 2007). Gaina stretched crystalline continental crust, which thins to 7 km et al. (2007) developed a breakup model between India thickness (plus 4 km sediments on top), from the shelf edge and this East Antarctic sector based on compiled magnetic to the location of the MCA. It is interesting to note that apart data of the southernmost Indian Ocean and the structures from a few scattered observations close to the marked COBs, of the continental margin interpreted from the seismic data. major SDR sequences do not seem to exist on the Enderby Despite the large amount of high-quality seismic reï¬ection Land margin (Stagg et al., 2004, 2005), suggesting the lack data, the deï¬nition of the COB or COT is equivocal due of a mantle plume at the time of breakup at about 130 Ma to the lack of deep crustal seismic refraction data with the (Gaina et al., 2007). exception of nonreversed sonobuoy data. Stagg et al. (2004, The characteristics of the margin off central and eastern 2005) deï¬ned the COB as the boundary to a zone by which Prydz Bay is affected by both the inherited structure of the the ï¬rst purely oceanic crust was accreted and which shows Paleozoic-Mesozoic Lambert Rift system as well as by mag- a changeover from faulted basement geometry of stretched matic events of the Kerguelen Plateau LIP, probably postdat- continental crust to a relatively smoother basement of ocean ing the initial India-Antarctica breakup by about 10 million crust. This boundary is often accompanied by a basement to 15 million years (Gaina et al., 2007). Guseva et al. (2007) ridge or trough. proposed a direct connection between the volcanic Southern The Enderby margin can be divided into two zones of Kerguelen Plateau crust and stretched continental crust of distinct character. West of about 58Â°W the ocean fractures Wilhelm II Land. The recent deep crustal seismic refraction zone terminates in an oblique sense at the margin, thus giving data and a helicopter-magnetic survey, however, provides it a mixed rift-transform setting (Stagg et al., 2004). Their constraints that the central part of the PET consists of oce- deï¬ned COB lies between 100 km and 170 km oceanward anic crust, possibly affected by the LIP event (Gohl et al., of the shelf edge. Gaina et al. (2007) identiï¬ed spreading 2007a). This result is used to draw a narrow zone of stretched anomalies from M0 to M9 east of Gunnerus Ridge from rela- continental crust that widens eastward along the margin of tively sparse shipborne magnetic data. Most of both magnetic the Davis Sea (Figure 4), where it reaches the width of the and seismic proï¬les do not parallel the spreading ï¬ow lines COT in the area of Bruce Rise as suggested by Guseva et
GOHL 33 FIGURE 4 Enderby Land-Lambert Rift sector with satellite-derived gravity ï¬eld (McAdoo and Laxon, 1997) and continental margin features. Red lines show the tracks of offshore multichannel seismic proï¬les of SDLS. Dark blue lines mark the locations of offshore deep crustal seismic refraction proï¬les. COT = continent-ocean boundary; PET = Princess Elizabeth Trough; GR = Gunnerus Ridge; MCA = Mac Robertson Coastal Anomaly. al. (2007). Similar to observations along the Enderby Land (2007b) identiï¬ed a COT consisting primarily of strongly margin, major SDR sequences are not observed on the Ant- stretched and faulted continental crust with embedded arctic margin of the Davis Sea but only around the margins magmatic segments following linear trends parallel to the of the Southern Kerguelen Plateau. However, SDRs appear margin. The interpretation of the COB along parts of the as strongly reï¬ecting sequences at Bruce Rise (Guseva et margin is debatable, as mainly basement characteristics al., 2007). were used for the differentiation of crustal types. It cannot be completely excluded that the magnetic anomalies inter- preted as magmatic components are actually true seaï¬oor Wilkes Land (WL) Sector Conjugate to Australia spreading anomalies C33y and C34y, which would move A vast amount of seismic reï¬ection, gravity, and magnetic the COB farther south. However, this seems unlikely based data as well as nonreversed sonobuoy refraction data col- on the results of Colwell et al. (2006), Direen et al. (2007), lected by Australian and Russian scientists in the last few and Sayers et al. (2001) in a comparison of the conjugate years allows a characterization of the Wilkes Land and Terre magnetic anomalies as well as new theoretical and analogue AdÃ©lie Coast margin east of Bruce Rise (Figure 5) (Stagg et examples published in Sibuet et al. (2007). Reversed deep al., 2005; Colwell et al., 2006; Leitchenkov et al., 2007b). crustal refraction data would provide better constraints on Along most of this margin the shelf slope is underlain by the composition of the crustal units but are missing anywhere a marginal rift zone of stretched and faulted basement. along this margin. However, in the absence of better data From the northern limit of this marginal rift zone to 90-180 I adopted the interpretation by Colwell et al. (2006) and km farther oceanward, Eittreim et al. (1985), Eittreim and Leitchenkov et al. (2007b) of an up-to-300-km-wide zone Smith (1987), Colwell et al. (2006), and Leitchenkov et al. of stretched continental crust (Figure 5). FIGURE 5 Wilkes Land sector with satellite-derived gravity ï¬eld (McAdoo and Laxon, 1997) and continental margin fea- tures. Red lines show the tracks of offshore multichannel seismic profiles of SDLS. The dark blue line marks the location of an offshore deep crustal seismic refraction proï¬le. NVR = Northern Victoria Land; AR = Adare Rift.
34 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD Off the Terre AdÃ©lie Coast margin the AdÃ©lie Rift Block, Seismic refraction data from a proï¬le across the Ross Sea which is interpreted as a continental crustal block, is part of shelf by Cooper et al. (1997) and Trey et al. (1997) show a the stretched marginal crust. Major sequences of SDRs are crust of 17-km to 24-km thickness with the thinnest parts not observed along the margin of the Wilkes Land sector, beneath two troughs in the eastern and western region. It although Eittreim et al. (1985), Eittreim and Smith (1987), can be assumed that this type of stretched continental crust and Colwell et al. (2006) describe signiï¬cant volumes of extends to the shelf edge at least. Whether crustal thinning is maï¬c intrusions within the sedimentary sequences of the a result of the New Zealand-Antarctic breakup or extensional landward edge of the AdÃ©lie Rift Block. The margin east of processes of the West Antarctic Rift System or both cannot about 155Â°E toward the Ross Sea is characterized by promi- be answered due to insufï¬cient data in this area. It is also nent oblique fracture zones (e.g., Balleny FZ) reaching close not known whether magmatic events affected the structure to the shelf edge. This is similar to the rift-transform setting of the marginal crust. as observed in the western Enderby Basin but in an opposite Structural models of the continental margin of western directional sense. Stock and Cande (2002) and Damaske et Marie Byrd Land also suffer from the lack of seismic and al. (2007) suggest that a broad zone of distributed deforma- other geophysical data. The only assumption for a sharp tion was active at the margin, affecting the continental crustal breakup structure and a very narrow transitional crust is blocks of Northern Victoria Land even after the initiation of derived from the close ï¬t of the steep southeastern Camp- ocean spreading. bell Plateau margin to the western MBL margin and coastal The continental margin in the western Ross Sea under- gravity anomaly (e.g., Mayes et al., 1990; Sutherland, 1999; went a rather complex development that makes a clear Larter et al., 2002; Eagles et al., 2004). The same approach delineation of the COB difï¬cult. A major proportion of the was applied for the closest ï¬t between Chatham Rise and Ross Sea crustal extension is associated with a 180 km plate eastern MBL. However, new deep crustal seismic data from separation between East and West Antarctica when the Adare the Amundsen Sea indicate that the inner to middle shelf Trough was formed in Eocene and Oligocene time (Cande et of the Amundsen Sea Embayment consists of crust thinned al., 2000; Stock and Cande, 2002). The southward extension to about 21- to 23-km thickness with a pre- or syn-breakup of this rift dissects the continental shelf region of the western failed rift structure (Gohl et al., 2007b). Here the continental Ross Sea (Davey et al., 2006). margin structure is rather complex due to the propagating and rotating rifting processes between the breakup of Chatham Rise from the western Thurston Island block at about 90 Marie Byrd Land (MBL) Sector Conjugate to Zealandia Ma (Eagles et al., 2004), the opening of the Bounty Trough The eastern Ross Sea margin (Figure 6) is conjugate to the between Chatham Rise and Campbell Plateau (Eagles et al., southwesternmost margin of the Campbell Plateau of New 2004; Grobys et al., 2007), and the initiation of the breakup Zealand. Unlike the western Ross Sea margin, this margin of Campbell Plateau from MBL at about 84-83 Ma (e.g., can be considered a typical rifted continental margin with the Larter et al., 2002; Eagles et al., 2004). The analysis of a oldest identiï¬ed shelf-edge parallel spreading anomaly being crustal seismic refraction proï¬le (Gohl et al., 2007b) and a C33y just east of the Iselin Rift (Stock and Cande, 2002). seismic reï¬ection dataset (Gohl et al., 1997b) between the FIGURE 6 Marie Byrd Land sector with satellite-derived gravity field (McAdoo and Laxon, 1997) and continental margin features. Red lines show the tracks of offshore multichannel seismic proï¬les of SDLS. Dark blue lines mark the locations of offshore deep crustal seismic refraction proï¬les. ASE = Amundsen Sea Embay- ment; PIB = Pine Island Bay; TI = Thurston Island.
GOHL 35 FIGURE 7 Antarctic Peninsula-Ellsworth Land sector with satellite-derived gravity ï¬eld (McAdoo and Laxon, 1997) and con- tinental margin features. Red lines show the tracks of offshore multichannel seismic proï¬les of SDLS. Dark blue lines mark the locations of offshore deep crustal seismic refraction proï¬les. AI = Alexander Island; TI = Thurston Island; PI = Peter Island; DGS = De Gerlache Seamounts; DGGA = De Gerlache Gravity Anomaly; BGA = Bellingshausen Gravity Anomaly; SFZ = Shackleton Fracture Zone. shelf edge and the Marie Byrd Seamount province reveals to 30 km beneath the middle and outer continental shelf. The that the crust in this corridor is 12-18 km thick, highly frac- remains of the collided ridge segments cannot be resolved tured, and volcanically overprinted. Lower crustal velocities by the models. Data are also not sufï¬cient to estimate any suggest a continental afï¬nity, which implies highly stretched possible crustal extension of the western Antarctic Peninsula continental crust or continental fragments, possibly even into due to stress release after subduction ceded. the area of the seamount province. SDRs are not observed in the few existing seismic reï¬ection proï¬les crossing the shelf GEODYNAMIC AND PLATE-TECTONIC IMPLICATIONS and margin (Gohl et al., 1997a,b, 2007b; Nitsche et al., 2000; AND COMPLICATIONS Cunningham et al., 2002; Uenzelmann-Neben et al., 2007). The compilation of circum-Antarctic margin characteristics indicates that only a quarter (3400 km) of the rifted margins SUBDUCTION TO RIDGE COLLISIONS: is of volcanic type, although uncertainties on volcanic and THE CONVERTED ACTIVE-TO-PASSIVE MARGIN nonvolcanic afï¬nity exist for about 4900 km (35 percent) of the rifted margins due to the lack of data in the Weddell Sea Antarctic Peninsula to Ellsworth Land (APE) Sectorâ and along the MBL and Ross Sea margins. Only the east- Proto-Paciï¬c Margin ern WL and western DML sectors as well as some isolated Subduction of the Phoenix plate and subsequent collision of areas such as Bruce Rise of the EL sector and the AdÃ©lie the Phoenix-Paciï¬c spreading ridge resulted in a converted Rift Block in the WL sector show well-observed magmatic active to passive nonrifted margin along the Ellsworth Land characteristics that are explained by syn-rift mantle plumes and western Antarctic Peninsula margin between Thurston or other smaller-scale magmatic events. Most other margins Island and the Hero Fracture Zone (e.g., Barker, 1982; Larter seem to have been formed by processes similar to those and Barker, 1991) (see Figure 7). The age for the oldest proposed for other nonvolcanic margins, such as the Iberian- ridge-trench collision in the western Bellingshausen Sea can Newfoundland conjugate margins (e.g., Sibuet et al., 2007) be roughly estimated to be about 50 Ma, using a few identi- and the Nova Scotia margin (e.g., Funck et al., 2003), which ï¬ed spreading anomalies (Eagles et al., 2004; Scheuer et al., exhibit widely extended, thinned, and block-faulted conti- 2006). Oceanic crustal age along the margin becomes pro- nental crust. Whether any updoming of lower continental gressively younger toward the northeast (Larter and Barker, crust is a possible process, as Gaina et al. (2007) suggest for 1991). Ultraslow subduction is still active in the segment of the Enderby Land margin, is difï¬cult to assess because of a the remaining Phoenix plate between the Hero FZ and the lack of drill information. Shackleton FZ along the South Shetland Trench. The only Approximately 70 percent of the rifted passive margins deep crustal seismic data along this converted margin south contain continental crust stretched over more than 50 km of the Hero FZ are from two proï¬les by Grad et al. (2002), oceanward of the shelf edge. In about two-thirds of these showing that the crust thins oceanward from about 37 km extended margins the COT has a width of more than 100 km, thickness beneath the Antarctic Peninsula to a thickness of 25 in many cases up to 300 km. The total area of extended con-
36 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD tinental crust on the continental shelf and beyond the shelf modelers still use relatively rudimentary crustal and litho- edge, including COTs with substantial syn-rift magmatic- spheric models of the Antarctic continent. Although the volcanic accretion, is estimated to be about 2.9 106 km2. tomographic inversion of seismological data has improved Crustal stretching factors still remain uncertain for a good the knowledge of the lithospheric structure beneath Ant- proportion of the extended margins due to a lack of crustal arctica and surrounding ocean basins (e.g., Morelli and thickness measurements in most sectors. However, assuming Danesi, 2004), its spatial resolution of the upper 60-70 km stretching factors between 1.5 close to the shelf edge and in of the relevant elastic lithosphere (Ivins and James, 2005), marginal rifts and increasing to 2 or 3 outboards in the deep and in particular of the boundary between continental and sea, the continental crust, which has to be added to the origi- oceanic lithosphere, is still extremely crude. Considering nal continent of normal crustal thickness, makes up about that ice sheets advanced to the shelf breaks of most Antarc- 1.5 106 km2. The quantiï¬cation of extended marginal crust tic continental margins during glacial maxima, the isostatic has implications for plate-kinematic reconstructions, paleo- response must be directly related to the width and depths of bathymetric models, and possibly for isostatic balancing of any extended continental crust and lithosphere oceanward of the Antarctic continent in glacial-interglacial cycles. the shelf, which would probably have an effect on estimates In almost all large-scale plate-kinematic reconstructions of sea-level change. To quantify this effect, good-quality in which Antarctica is a key component, plate motions are deep crustal and lithospheric data are needed to derive the calculated by applying rotation parameters derived from geometries and rheologies of the extended crust and litho- spreading anomalies and fracture zone directions, and conti- spheric mantle. nent-ocean boundaries are ï¬xed single-order discontinuities, in most cases identiï¬ed by the shelf edge or the associated CONCLUSIONS margin-parallel gravity anomaly gradient (e.g., Lawver and Gahagan, 2003; Cande and Stock, 2004). This has caused In a comprehensive compilation of circum-Antarctic conti- misï¬ts in terms of substantial overlaps or gaps when plates nental margin types, about 70 percent of the rifted passive are reconstructed to close ï¬t. For instance, a large misï¬t margins contain extended continental crust stretching more occurs when ï¬tting the southeastern Australian margin to than 50 km oceanward of the shelf edge. Most of these the eastern Wilkes Land margin in the area between 142Â°E extended margins have a continent-ocean transition with a and 160Â°E where extended continental crust reaches up width of more than 100 kmâin many cases up to 300 km. to 300 km off the Terre AdÃ©lie Coast (Stock and Cande, Only a quarter of the rifted margins seem to be of volcanic 2002; Cande and Stock, 2004). In their reconstruction of type. The total area of extended continental crust on the shelf the breakup processes of the Weddell Sea region, KÃ¶nig and oceanward of the shelf edge, including COTs with sub- and Jokat (2006) accounted for extended continental crust stantial syn-rift magmatic-volcanic accretion, is estimated to in the southern Weddell Sea and derived a reasonable ï¬t of be about 2.9 106 km2. This has implications for improved the conjugate margins of South America, Antarctica, and plate-kinematic and paleobathymetric reconstructions and Africa. An appropriate approach for reconstructing best ï¬ts provides new constraints for accurate calculations of isostatic of continents and continental fragments is to apply a crustal responses along the Antarctic margin. balancing technique by restoring crustal thickness in rift zones and plate margins (Grobys et al., 2008). This, however, ACKNOWLEDGMENTS requires detailed knowledge of pre- and postrift crustal thick- ness, crustal composition, and magmatic accretion. I gratefully acknowledge the masters, crews, and scientists Detailed delineations of the COT and COB are impor- of the many marine expeditions during which the geophysi- tant ingredients for paleobathymetric reconstructions. In the cal data used in this paper were acquired. Many thanks go reconstruction of the circum-Antarctic ocean gateways (e.g., to Nick Direen and an anonymous reviewer whose review Lawver and Gahagan, 2003; Brown et al., 2006; Eagles et al., comments substantially helped improve the paper. I also 2006), but also of the bathymetric features along nongateway thank the co-editor Howard Stagg for useful comments and continental margins, the differentiation between continental the editorial work. crust that was stretched and faulted and possibly intruded by magmatic material on one side and oceanic crust generated REFERENCES from spreading processes on the other side may make a sig- niï¬cant difference in estimating the widths and depths along Barker, P. F. 1982. The Cenozoic subduction history of the Paciï¬c margin of the Antarctic Peninsula: Ridge crest-trench interactions. Journal of and across pathways for paleo-ocean currents. the Geological Society of London 139:787-801. Parameters of crustal and lithospheric extensions, Brown, B., C. Gaina, and R. D. MÃ¼ller. 2006. Circum-Antarctic palaeo- depths, and viscoelastic properties are key boundary condi- bathymetry: Illustrated examples from Cenozoic to recent times. 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