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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. The Signiï¬cance of Antarctica for Studies of Global Geodynamics R. Sutherland1 ABSTRACT by subduction zones with highly uncertain relative motions between subducting and overriding plates (Figures 1 and Antarctica has geometric signiï¬cance for global plate kine- 2). A full understanding of geodynamics requires global matic studies, because it links seaï¬oor spreading systems of determinations of past relative plate motions and boundary the African hemisphere (Indian and Atlantic Oceans) with locations, and motions of plates relative to hotspots (e.g., those of the Paciï¬c. Inferences of plate motions back to 44 Lithgow-Bertelloni and Richards, 1998). Ma, around the onset of rapid spreading south of Australia Most previous calculations of global plate motions and formation of a new boundary through New Zealand, are assume that hotspots in the Paciï¬c hemisphere, speciï¬cally consistent with Antarctic rifting and formation of the Adare Hawaii and Louisville, have been ï¬xed relative to each other Basin during 44-26 Ma (i.e., no additional plate motions and to African-hemisphere hotspots during Cretaceous- are required in the South Paciï¬c). The time period 52-44 Cenozoic time (Engebretson et al., 1985; Gordon and Jurdy, Ma represents a profound global and South Paciï¬c tectonic 1986). However, paleomagnetic data from the Emperor change, and signiï¬cant details remain unresolved. For 74 Ma seamount chain in the North Paciï¬c are inconsistent with a signiï¬cant nonclosure of the South Paciï¬c plate-motion cir- this assumption, and mantle ï¬ow calculations predict sig- cuit is identiï¬ed if Antarctic motion is not included. Alternate niï¬cant advection of the rising mantle plume responsible inferences of motion through Antarctica during the interval for the Hawaii hotspot (Tarduno et al., 2003; Steinberger et 74-44 Ma imply signiï¬cantly different subduction volumes al., 2004). The only way to determine global relative plate and directions around the Paciï¬c, and imply different relative motions and, hence, test predictive hotspot models based motions between hotspots. on mantle ï¬ow calculations is to piece together the kine- matic evidence that was formed at plate boundaries; this INTRODUCTION includes seaï¬oor fracture zones and magnetic anomalies, and the records that are preserved within continents and The surface of Earth can be divided into hemispheres with their margins. distinct tectonic character. The African hemisphere con- Antarctica is signiï¬cant in the global relative plate- tains spreading ridges in the Indian and Atlantic Oceans motion circuit because it geometrically connects the African that allow relative plate motions to be determined, and the and Paciï¬c hemispheres along a path that can be directly motions are shown by studies of seamount chains to be well reconstructed at past times from seaï¬oor and continental approximated by a single hotspot (absolute) reference frame records (Figures 1 and 2). Therefore, quantification of (Muller et al., 1993). The Paciï¬c hemisphere is surrounded internal deformation of Antarctica is an essential part of any global relative-plate-motion model. Further, because internal deformation of plates is commonly characterized by local rotation poles (Gordon, 1998), small local displacements 1 GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand may be described by relatively large rotation angles, which (email@example.com). propagate (when incorporated into a plate-motion chain) into 115
116 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD -10000 -8000 -6000 -4000 -2000 0 2000 Topography (m) Africa USA Pacific FIGURE 1 Global bathymetry (from Antarctica Smith and Sandwell, 1997). Convergent plate boundaries shown in orange. Arrow indicates the pathway of kinematic con- nection between the African and Paciï¬c hemispheres that did not contain destruc- tive boundaries during the Cenozoic era. Oblique Mercator projection. -350 -150 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 150 350 Gravity anomalies (mGal) FIGURE 2 Gravity anomalies (from Sandwell and Smith, 1997). very large predicted plate displacements at greater distances boundary during the interval 74-50 Ma. It is accepted that (e.g., at equatorial latitudes). The validity of Antarctic recon- reï¬nement to this model will be necessary, to ï¬t observations structions must be consistent with other motions in the South of crustal strain and the timing of deformation in detail. The Paciï¬c, because Antarctica is part of a closed plate-motion hypothesis is presented and then tested against crustal geol- circuit that includes Australia and New Zealand. ogy of the Antarctic continent, the geometry of seaï¬oor in This paper presents a model for the block motion the South Paciï¬c plate-motion circuit, and the global motions of Marie Byrd Land relative to the East Antarctic craton of plates relative to hotspots. A plausible tectonic explana- since 74 Ma. The model is simplistic by design, because tion for why such a model makes physical sense is brieï¬y the primary purpose of this paper is to propose a new and discussed. Finally, global implications of the hypothesis are quantitative hypothesis for motion on an intra-Antarctic plate explored with regard to subduction budgets since the late
SUTHERLAND 117 Adare Emeraldfracturezones Basin nk Iselin n Ba Trough WilkesBasin eli Is 26 44 56 74 Ross 2000m Sea 150 FIGURE 3 Map of the hypothesized intra- Marie Subduction Antarctic plate boundary that stretches Byrd 26 Land 74-44Ma between the Ross Sea through the Byrd 44 Subglacial Basin to inboard of Thurston Is- 74 land. Signiï¬cant tectonic regions are labeled East Byrd Antarctic Subglacial and their margins are shown bold dashed. 120 craton Basin Arbitrary points within the plate boundary 74 zone show the motion of the Marie Byrd 56 26 Land plate relocated (pink dotted) relative to South 44 500km Pole Thurston the East Antarctic plate using the proposed 90 Island kinematic model (Table 1). 60E 0 60W 80South 70South Cretaceous, because this is of broad international interest for TABLE 1 Finite Rotations Describing Marie Byrd Land both geodynamics and continental margin geology. Relative to East Antarctica Age Latitude Longitude Angle (Ma) (Â°N) (Â°E) (Â°) MOTION OF MARIE BYRD LAND RELATIVE TO EAST ANTARCTICA 26.6 â18.2 â17.9 0.0 33.6 â18.2 â17.9 0.7 The plate-motion model (Figure 3) is divided into four 43.8 â18.2 â17.9 1.7 phases: (1) 74-56 Ma is a time of slow dextral extension in 56.0 â70.0 â30.0 4.0 73.6 â80.0 â70.0 8.5 the Ross Sea and dextral transpression near Thurston Island; (2) 56-44 Ma is a time of accelerated rifting in the Ross Sea and highly oblique dextral transpression near Thurston Island; (3) 44-26 Ma is the time of Adare Basin formation be revised to >400-450 km if crustal addition were accounted (Cande et al., 2000) and includes rifting inboard of Thurston for (Behrendt et al., 1991). Island; and (4) there has been no signiï¬cant motion since 26 The total motion in the Ross Sea that is implied by the Ma (Table 1). The oldest ï¬nite rotation corresponds to chron model proposed in this paper is ca. 300 km since 74 Ma 33y (Cande and Kent, 1995), which is the time of the oldest (Figure 3). Therefore, ca. 70 percent of the total thinning is magnetic anomaly that is widely preserved and recognized implied to have occurred after 74 Ma. This estimate could be in the South Paciï¬c. revised downward if some extension were distributed over a broader region. Test 1: Antarctic Geology and Geophysical Data The oldest strata in the Ross Sea that have been drilled are late Eocene and Oligocene in age (Hayes et al., 1975; Ross Sea Rift Barrett, 1989; Barrett et al., 1995), and these postdate nor- mal-faulted strata everywhere except the Victoria Land Basin It has been known for several decades that the Ross Sea has in the western Ross Sea, where faulting continued through thinned crust, rifted sedimentary basins, and a rift ï¬ank uplift Oligocene time (34-24 Ma) (Cooper et al., 1987; Henrys et called the Transantarctic Mountains, and that Cretaceous- al., 1998; Hamilton et al., 2001). Hence, a large component Cenozoic extensional tectonics were implicated (Davey of the extension is constrained to be Eocene or older. Models et al., 1982; Behrendt et al., 1991; ten Brink et al., 1993; of apatite ï¬ssion track data from a basement rock sample Fitzgerald, 1994; Cooper et al., 1995). Crustal thickness collected at DSDP site 270, which is sited on a rifted horst estimates imply 350-400 km of total extension, which could in the central Ross Sea, suggest exhumation was completed
118 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD at some time during the interval 90-50 Ma (Fitzgerald and Summary of Test 1 Baldwin, 1997). The kinematic hypothesis predicts that 70 percent of exten- Apatite ï¬ssion-track and (U-Th)/He thermochronology sion in the Ross Sea occurred after 74 Ma, and that dextral- results from the Transantarctic Mountains record cooling and oblique compression followed by extension occurred in the hence inferred exhumation during the time interval 80-40 Ma Thurston Island region. I contend that both predictions are (Fitzgerald, 1992, 1994; Stump and Fitzgerald, 1992; Lisker, consistent with local observations, but are not necessarily the 2002; Fitzgerald et al., 2006). Age-elevation correlations favored models of local workers. The initial phase is most have been used to suggest an episodic uplift model with an controversial, because there is so little geological evidence initial phase starting before 80 Ma and a second phase of for activity during this time. However, the syn-rift strata have more rapid exhumation starting at 55-45 Ma (Stump and not been sampled and if a heating model were proposed to Fitzgerald, 1992). This model has appeal, because the ï¬rst explain the rift ï¬ank uplift (ten Brink et al., 1993), burial phase corresponds to Gondwana breakup (local separation rather than exhumation could be expected during the early of New Zealand) and the second phase is a time of profound stages of rifting, which could explain the apatite ï¬ssion track global and South Paciï¬c tectonic change (below). However, results. close inspection of the data does not provide compelling evi- dence for a discrete regional (rather than local) event before 80 Ma. Instead, the ages are broadly distributed across the Test 2: South Paciï¬c Plate Motions time interval 80-45 Ma (or even older) and thermal model inversions produce results that are consistent with exhuma- 43-0 Ma tion histories during that interval. The regional increase in Rifted boundaries in the southeast Indian Ocean (East Ant- cooling rate within the Transantarctic Mountains at ca. 55-45 arctica-Australia) and south of New Zealand (ENZ-WNZ) Ma is compelling, and hence an increase in exhumation rate formed at about chron 20 (43 Ma) (Sutherland, 1995; Tikku is inferred. and Cande, 2000), after cessation of spreading in the Tasman Sea (Australia-WNZ) (Gaina et al., 1998). Paciï¬c-Marie Byrd Subglacial Basin Byrd Land spreading is constrained by numerous magnetic anomaly and fracture zone picks (Cande et al., 1995; Cande South of the Ross Sea the Byrd Subglacial Basin is a region of and Stock, 2004). The âmissing linkâ in this plate-motion bedrock elevations below sea level, with some regions being circuit (Figure 4) is rifting within Antarctica and formation deeper than 1000 m below sea level (Lythe et al., 2001). The of seaï¬oor in the Adare Basin (Cande et al., 2000). However, basin is interpreted to be of rift origin and has only a thin (<1 the location of the pole of rotation that describes Antarctic km) sedimentary record imaged beneath the ice, but gravity rifting is imprecisely constrained by this analysis, and a interpretations suggest localized narrow basins with up to 5 rotation pole nearer to Antarctica has been suggested (Davey km of sediment (Behrendt et al., 1991; Anandakrishnan et et al., 2006). This paper uses a magnetic anomaly 13 plate- al., 1998; Studinger et al., 2001). Fission track data from the motion circuit inversion and extrapolation to anomaly 20 southern Transantarctic Mountains (Scott Glacier) yield ages (Cande et al., 2000); the model (Table 1; Figure 5) produces in the range 120-60 Ma (Stump and Fitzgerald, 1992). The rifting of only slightly decreased magnitude toward the east Byrd Subglacial Basin has a similar width to the Ross Sea (Figure 3). (ca. 800 km) and is entirely ice-covered. 56-43 Ma Thurston Island Region This was a time of profound change throughout the Paciï¬c The region of Antarctica from Thurston Island to the Antarc- and Indian Oceans, and uncertainty surrounds the exact tic Peninsula has undergone a complex tectonic history since nature and timing of these changes. One of the least affected Jurassic time (Dalziel and Elliot, 1982). Pre-Gondwanaland boundaries was that between East New Zealand and Marie breakup reconstructions must account for extensional basins, Byrd Land, where seaï¬oor spreading continued with only such as the Byrd Subglacial Basin; dextral transpression minor changes in direction and rate (Cande et al., 1995). The along the paleosubduction continental margin; and continu- Tasman Sea stopped opening at chron 24 (52 Ma) (Gaina et ity of geological characteristics (Storey and Nell, 1988; Sto- al., 1998), but rapid divergence south of New Zealand (WNZ- rey, 1991; McCarron and Larter, 1998; Larter et al., 2002). ENZ) and Australia did not start until ca. 43 Ma (Sutherland, The relative magnitudes, timing, and spatial distribution of 1995; Wood et al., 1996; Tikku and Cande, 2000). Analysis dextral transpression and extension remain constrained by of plate closure (Figure 5) during this interval is hampered by only a small number of ï¬eld observations due to extreme the relatively short time interval; the possibility of intraplate remoteness and ice cover. deformation within New Zealand, which was very close to the instantaneous pole of ENZ-WNZ rotation after 43 Ma;
SUTHERLAND 119 Australia WNZ Pacific ENZ Southeast Indian Ocean FIGURE 4 The South Paciï¬c plate-motion Bellingshausen, circuit is the closed loop Australia-East MBL Phoenix, Aluk Antarctica-Marie Byrd Land (MBL)- East New Zealand (ENZ)-West New Zea- E Antarctica land (WNZ)-Australia. Bold lines show TI active plate boundaries. Fine blue lines show locations of signiï¬cant boundaries during the interval 74-0 Ma. TI = Thurston Island. and the difï¬culty of interpretation of crust in the southeast and Australia-Antarctic continental geology, are achieved Indian Ocean. with only a slightly larger plate movement and the Australia- Antarctica boundary is relatively long, so the implications for global plate motions are relatively small. During this 74-56 Ma time, oblique convergence occurred adjacent to Thurston This was a time of tectonic quiescence within New Zealand, Island and a subducting microplate, the Bellingshausen plate, with the only records of tectonic activity coming from the moved independently to Marie Byrd Land until ca. 60 Ma Taranaki Basin of central New Zealand, where very small (Stock and Molnar, 1987; Heinemann et al., 1999; Larter et amounts of rifting are implied (King and Thrasher, 1996). al., 2002). The tectonic model (Table 1; Figure 5) results in Therefore, the total motion through New Zealand is very sim- acceptable plate closure for this time interval. ilar at 56 Ma and 74 Ma, and is well approximated by ï¬tting rift boundaries (ENZ-WNZ) (Figure 4) south of New Zea- Test 3: Global Plate Motions Relative to Hotspots land (Sutherland, 1995). Tasman Sea spreading (Australia- WNZ) is quantiï¬ed by magnetic anomalies (Gaina et al., With a model for intra-Antarctic motion (Table 1), it is 1998), as is ENZ-MBL spreading (Cande et al., 1995). It possible to compute the relative motion of the Paciï¬c plate is not clear that magnetic lineations in the southeast Indian relative to those in the African hemisphere by following Ocean are isochrons, but detailed analyses and published a path Africa-East Antarctica-Marie Byrd Land-East New reconstructions exist (Tikku and Cande, 1999, 2000; Muller Zealand-Paciï¬c (Cande et al., 1995; Nankivell, 1997; Cande et al., 2000). It is notable that substantial overlaps between and Stock, 2004), assuming the motions of Table 1 and that Tasmania and Wilkes Land region have been predicted by East New Zealand was ï¬xed to the Paciï¬c during this time. some regional analyses (Tikku and Cande, 2000) and are Therefore, it is possible to rotate the African hotspot refer- inconsistent with local restoration of seaï¬oor (Royer and ence frame to Hawaii and account for predicted movement Rollet, 1997; Tikku and Cande, 2000); this suggests some of the Hawaii hotspot caused by mantle ï¬ow (Steinberger et internal deformation (extension) of either the Australian or al., 2004). The results (Figure 6) reveal that inclusion of the Antarctic plates, or the regional data are open to alternate Antarctic deformation model (Table 1) produces a good ï¬t interpretation (Whittaker et al., 2007). Reconstructions of between observations and predictions. seaï¬oor older than 74 Ma in the southeast Indian Ocean,
120 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD DISCUSSION OF THE ANTARCTIC PLATE-MOTION MODEL The kinematic model (Table 1) results in a much improved ï¬t to global observations of hotspots and it adequately closes the South Paciï¬c plate-motion circuit. I believe the model to lie within the constraints of local observations from Ant- arctica, but I accept that ï¬ne-tuning will be required as the hypothesis is tested in detail. Speciï¬cally, it is likely that the number of continental fragments will be increased and the precise locations of the poles of the relative rotations that describe their movements will be revised. South Paciï¬c reconstructions (Figure 5) place moder- ately strong constraints on predicted movements between 50N 4 3 2 40N 1 30N 80 Ma 40 20N 160E 180 Hawaii FIGURE 6 Model positions of Emperor-Hawaii seamounts for the time 0-74 Ma, based upon rotation of the African hotspot refer- ence frame (Muller et al., 1993) and predicted movements of the Hawaii hotspot (movement of the hotspot is shown in the Paciï¬c plate frame of reference; it is color-coded green-orange and tick marks are shown at 10 Myr increments; and the two movement lines were computed for models 2 and 3 below) (Steinberger et al., 2004). From east to west the model assumptions are: (1) (dotted), no relative hotspot motion and no intra-Antarctic motion; (2) (red), Adare Basin motion 43-26 Ma (Cande et al., 2000) and hotspot motion; (3) (yellow), hotspot motion and the intra-Antarctic issue avoided by following a path through Australia before 50 Ma; (4) (orange), hotspot motion and the Antarctic deformation model of this paper (Table 1). FIGURE 5 South Paciï¬c tectonic reconstructions. See discussion in text and animated version at http://www.gns.cri.nz/research/ tectonics.
SUTHERLAND 121 East Antarctica and Marie Byrd Land in the Ross Sea, and relative positions of Paciï¬c hemisphere oceanic plates with to a lesser extent on the angle of rotation between East Ant- overriding plates that border the African hemisphere: South arctica and Marie Byrd Land. Hotspot observations (ages and America (Muller et al., 1993); India (Muller et al., 1993); positions of seamounts) place a moderately strong constraint North America (Klitgord and Schouten, 1986; Muller et on the angle of rotation between East Antarctica and Marie al., 1990); and Eurasia (Lawver et al., 1990; Srivastava and Byrd Land, because most hotspot chains are at middle or Roest, 1996; Rosenbaum et al., 2002). Hence, it is possible low latitudes and the likely pole of intra-Antarctic rotation to compute subduction histories. is at high latitude. Since the time of the Emperor-Hawaii bend at ca. 50 Intra-Antarctic motion during the interval 74-56 Ma, Ma (Sharp and Clague, 2006), there is fairly good agreement which is the most controversial part of the model, is plau- within the published literature with estimates of relative and sible because it connects a complex zone of rifting near the absolute plate motions (Steinberger et al., 2004). However, Ross Sea (Figure 3) with a subduction boundary adjacent to there is substantial disagreement before that (Raymond et Thurston Island and the Antarctic Peninsula. Antarctica was al., 2000; Tarduno et al., 2003; Steinberger et al., 2004). not an isolated continent surrounded by spreading ridges at Consequently, I focus here on the time interval 74-50 Ma. that time. It is possible that features such as the Iselin Trough Speciï¬cally, the plate pairs examined are Paciï¬c-Eurasia, and Wilkes Basin also formed during this time interval Kula-North America, Farallon-North America, and Farallon- (Figure 3). South America (Figure 7). Two Antarctic models are used (Figure 8): the preferred model is given in Table 1; the second is the same, but assumes no intra-Antarctic motion during IMPLICATIONS FOR GLOBAL SUBDUCTION BUDGETS the interval 74-44 Ma. The relative motions of oceanic plates in the Paciï¬c hemi- It is clear from Figure 8 that the model of Antarctic sphere can be determined from magnetic anomalies within motion is signiï¬cant for the computation of global subduc- the Paciï¬c, based upon the assumption of symmetric spread- tion rates and directions for the interval 74-50 Ma. The ing because conjugate crust has mostly been subducted preferred model (yellow, Table 1) predicts a hinge point, (Engebretson et al., 1985). The motion of the Paciï¬c plate where Paciï¬c plate subduction rates were very low in south- relative to Africa can be computed from Table 1 and magnetic east Asia; this may have geodynamic signiï¬cance, because anomalies in the southwest Indian Ocean (Nankivell, 1997) it is close to the southern limit of the plate boundary and and South Paciï¬c (Cande et al., 1995; Larter et al., 2002; may represent a propagating system (perhaps similar to the Cande and Stock, 2004). Hence it is possible to compute the tectonic setting of the Scotia Sea today). Alternatively, it is Kula Pacific Farallon Aluk or Phoenix FIGURE 7 Simpliï¬ed global plate tectonic reconstruction for 55 Ma.
122 ANTARCTICA: A KEYSTONE IN A CHANGING WORLD Pacific Kula Farallon Pacific India Farallon Pacific Farallon FIGURE 8 Model subduction directions, relative to the overriding major plate, and magnitudes during the interval 74-50 Ma. Red arrows assume no intra-Antarctic deformation before 44 Ma and predict a more westward motion of oceanic plates than a model (yellow) with Antarctic motion during 74-44 Ma (Table 1). possible that a ridge has since been subducted and that a nected active tectonic zones of rifting near the Ross Sea remanent of the Izanagi plate, rather than Paciï¬c plate, was (Figure 3) with a subduction boundary adjacent to Thurston being subducted beneath Eurasia at that time (Whittaker et Island and the Antarctic Peninsula. al., 2007). Kula and Farallon subduction for the period 74- The key geographic connection that Antarctica pro- 50 Ma beneath North and South America, when computed vides between subducting oceanic plates of the Paciï¬c with using the preferred Antarctic model, is rotated clockwise by diverging plates of the African hemisphere means that an 10-15Â° to an orientation more closely orthogonal to the mar- understanding of Antarctic deformation has signiï¬cance for gin, and has a rate that is ca. 15 percent higher at equatorial both global geodynamics and for subduction-related geology latitudes. The increase in rate at equatorial latitudes results of the entire circum-Paciï¬c. from the additional intra-Antarctic rotation about a pole at high latitude. ACKNOWLEDGMENTS I thank Bryan Storey, Alan Cooper, and the ISAES 2007 CONCLUSIONS organizing committee for their editorial and facilitation An hypothesis for the motion of Marie Byrd Land relative roles, and Carol Finn and Joann Stock for their very helpful to East Antarctica is presented. The model is shown to be reviews. I used GMT software (Wessel and Smith, 1995) consistent with South Paciï¬c plate motions and it provides and Atlas software of Cambridge Paleomap Services. This an improved global reconciliation of relative versus hotspot- research was funded by the New Zealand Foundation for derived plate motions. While the proposed model is broadly Research, Science and Technology. consistent with available observations from Antarctica, it is inevitable that a more complex and precise model will REFERENCES emerge as the hypothesis is tested in the future. However, I conclude that South Paciï¬c and global data constrain the Anandakrishnan, S., D. D. Blankenship, R. B. Alley, and P. L. Stoffa. 1998. motion of Marie Byrd Land relative to East Antarctica to be Inï¬uence of subglacial geology on the position of a West Antarctic ice stream from seismic observations. Nature 394:62-65. described by a rotation pole with high latitude and rotation Barrett, P. J. E. 1989. Antarctic Cenozoic history from CIROS-1 drillhole, angles similar to those proposed. Such a model is plausible McMurdo Sound. DSIR Bulletin 245:1-254. because the proposed intracontinental plate boundary con-
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