Journal of the Geological Society, London, Vol. 155, 1998, pp. 255–268. Printed in Great Britain.
Late Cretaceous to early Tertiary subduction history of the Antarctic Peninsula
JOE J. McCARRON1,2 & ROBERT D. LARTER1 1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK (e-mail: [email protected]) 2Present address: Companias Shell de Colombia, c/o Expat Mail, Postbus 245, 2501 CE Den Haag, The Netherlands
Abstract: Quantitative estimates of the rates and azimuths of Phoenix plate convergence with the Antarctic Peninsula have been derived from plate rotation calculations for two periods in the Late Cretaceous and early Tertiary (83.0–67.7 and 61.1–53.4 Ma). Published marine magnetic anomaly identifications and ‘flow lines’ interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive Phoenix–Pacific stage rotations. These were combined with published Pacific–Antarctic rotation data to determine contemporaneous Phoenix–Antarctic stage rotations. The results indicate a significant change in azimuths of relative motion between the Late Cretaceous and early Tertiary. Late Cretaceous, and perhaps earlier, oblique subduction probably caused migration of fore-arc slivers along the margin, resulting in variations in width of the accretionary prism. Comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the Antarctic–Phoenix ridge during the early Tertiary establishes the time of a major decrease in spreading rate, and hence also in convergence rate, as chron C23r (52.3 Ma). The associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the George VI Sound region and initiated uplift of blueschist facies rocks now exposed on Smith Island. The calculated convergence history does not provide a simple explanation for the occurrence of high-Mg# andesite lavas on Alexander Island, which were erupted more than 20 Ma before the Antarctic–Phoenix ridge arrived at the margin. The existence of these lavas implies either earlier subduction of another ridge or slab break-up beneath Alexander Island.
Keywords: Antarctica, plate rotation, magnetic anomalies, subduction, andesites.
The effects of changes in plate motion on the overriding plate so that successive sections of the Pacific–Phoenix ridge were at a subduction zone have been inferred largely on the basis of replaced by paired Pacific–Antarctic and Antarctic–Phoenix statistical correlations between subduction parameters for ridge crest sections (Cande et al. 1982). As a consequence of modern subduction zones (e.g. Jarrard 1986). To examine this ridge propagation, a large area of ocean floor produced by the effects of actual changes in motion it is first necessary Pacific–Phoenix spreading was captured by the Antarctic plate to determine the history of such changes for particular sub- at chron C21 (47 Ma; Figs 1 & 2). The part of the Phoenix duction zones. For some subduction zones, past rates and plate that remained extant during the Cenozoic has also been azimuths of convergence can be estimated by combining plate referred to as the ‘Aluk’ (Herron & Tucholke 1976) or ‘Drake’ rotation data that describe the history of motion at several plate (Barker 1982), but in this paper we use the name divergent plate boundaries, linking the subducting and over- ‘Phoenix’ throughout for simplicity. Stock & Molnar (1987) riding plates via a ‘plate circuit’ (e.g. Pilger 1981). Here we use proposed that an additional plate, the ‘Bellingshausen’ this method to estimate Late Cretaceous and early Tertiary plate existed off eastern Marie Byrd Land during the Late convergence rates and azimuths for subduction of the Phoenix Cretaceous and early Tertiary (Figs 1 & 2). Cande et al. plate beneath the Antarctic Peninsula. (1995) found supporting evidence for the existence of the The Antarctic Peninsula was part of a now-fragmented Bellingshausen plate prior to anomaly C27 (61 Ma), at which western margin of Gondwana that extended from South time they suggest there was a plate reorganisation and America through the Antarctic Peninsula to Marie Byrd Land Bellingshausen–Antarctic motion ceased. The gravity field and New Zealand until the Late Cretaceous. Arc magmatism derived from ERS-1 satellite altimetry data (McAdoo & along the peninsula was active throughout the majority of the Laxon 1997) and new marine magnetic data from near the Mesozoic and Tertiary (Pankhurst 1982; Thomson et al. 1983; Marie Byrd Land margin (J. Stock pers. comm.) provide Leat et al. 1995). The arc formed in response to eastward further support for this interpretation. The boundaries of the subduction of proto-Pacific ocean floor at a trench formerly former Bellingshausen plate remain poorly defined, and it is situated along the western margin of the Antarctic Peninsula not clear whether or not it included any part of the West (Suárez 1976), but reliable estimates of convergence rate and Antarctic continental lithosphere. In this paper we assume that direction only exist for the mid- to late Cenozoic (Barker 1982; most of West Antarctica, including the Antarctic Peninsula, Larter & Barker 1991a). In the Jurassic and Early Cretaceous has behaved as a single rigid plate since the middle Cretaceous. the Pacific, Farallon and Phoenix plates probably formed a This interpretation is consistent with recently published simple three plate system (Fig. 1; Larson & Chase 1972). palaeomagnetic results (DiVenere et al. 1994; Luyendyk About 105 Ma ago a section of the Pacific–Phoenix ridge et al. 1996), although Luyendyk et al. (1996) interpret collided with a trench at the New Zealand part of the Antarctic discordance between palaeomagnetic poles from East margin (Bradshaw 1989) and at c. 85 Ma, New Zealand started Antarctica and Marie Byrd Land as evidence of Late to separate from West Antarctica along the new Pacific– Cretaceous clockwise rotations within or between West Antarctic ridge. Subsequently, this ridge propagated to the NE Antarctic microplates.
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Fig. 1. Reconstructions of the South Pacific in the Antarctic reference frame, assuming no relative motion between the component blocks of West Antarctica since 105 Ma, based on data from Cande et al. (1982), Mayes et al. (1990) and Cande et al. (1995). The position of the trench in the vicinity of Alexander Island is based on interpretation of the LeMay Group accretionary complex (McCarron 1995). The 62 Ma reconstruction approximately corresponds to the time that the Bellingshausen plate is thought to have been incorporated into the Antarctic plate (Cande et al. 1995). The Phoenix plate (Larson & Chase, 1972) has also been referred to as the ‘Aluk’ (Herron & Tucholke 1976) or ‘Drake’ plate (Barker 1982). Al I, Alexander Island; An I, Anvers Island; BELL, Bellingshauen plate; A, B and C, ridge segments on the Antarctic–Phoenix ridge, referred to in text; CR, Chatham Rise; NR, new ridge segment formed by propagation of the Pacific–Antarctic ridge at about 47 Ma.
During the Tertiary most of the Antarctic–Phoenix ridge southern Alexander Island to mid-Pliocene N of Anvers Island segments migrated into the trench at the Antarctic Peninsula (Larter & Barker 1991a; Larter et al. 1997). The unusual margin (Herron & Tucholke 1976; Barker 1982; Larter & tectonic situation, with the trailing flank of the ridge crest and Barker 1991a). The direction of the fracture zone offsets the overriding lithosphere at the trench both being part of resulted in a progressively later arrival of the ridge crest the Antarctic plate (Fig. 1), made continued subduction segments at the trench to the NE, from mid-Eocene off dynamically impossible, and trench basement topography was
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Fig. 2. Sketch map (Mercator projection) showing the regions of ocean floor in the South Pacific produced at different spreading centres, based on the data from Cande et al. (1982, 1995). Approximate positions of magnetic anomalies are shown on the two areas of ocean crust produced by Pacific–Phoenix spreading. BoxesA&Bshowtheareas covered by Figs 4a and 4b, respectively. FZ, fracture zone; T, trough (Henry/Hudson); PAC-ANT, Pacific–Antarctic; PAC-BEL, Pacific–Bellingshausen; PAC-PHO, Pacific–Phoenix; PAC-FAR, Pacific–Farallon; ANT-PHO, Antarctic–Phoenix. Areas shaded black represent both land and ice shelves on Antarctica.
eliminated with the arrival of ridge segments at the margin geology of Alexander Island and the Antarctic Peninsula are (Tucholke & Houtz 1976; Larter & Barker 1991b). discussed, and the merits of different tectonic scenarios that Subduction-related magmatic sequences on Alexander might explain the origin of high-Mg# andesite lavas on Island were described by Burn (1981). Recent detailed geo- Alexander Island are considered. chronological and geochemical studies of these rocks have shown them to be part of an anomalous fore-arc magmatic sequence containing high-Mg# andesite lavas. In the north of the island, high-Mg# andesite lavas were erupted at c. 53 Ma Plate rotation calculations and in the south lavas with many similar characteristics were Published marine magnetic anomaly interpretations and plate probably erupted at c. 75 Ma (McCarron & Millar 1997; rotation data were used to calculate Phoenix–Marie Byrd Storey et al. 1996), in each case more than 20 Ma before Land stage rotations for two intervals, one in the Late Antarctic–Phoenix ridge segments arrived at the parts of Cretaceous (chrons C34n–C31n), and one in the early Tertiary the margin offshore from these areas (Larter et al. 1997). (chrons C27n–C24n). According to the geomagnetic polarity McCarron (1997) presented a model to explain the temporal, time scale of Cande & Kent (1995), these intervals correspond spatial and chemical characteristics of these rocks which to 83.0–67.7 and 61.1–53.4 Ma, respectively. If the component involved the earlier subduction of another active spreading blocks of West Antarctica have not moved relative to one centre. another since the mid-Cretaceous, as suggested by recently The subduction history of the region is well constrained published palaeomagnetic data (DiVenere et al. 1994), these by marine magnetic anomaly data for the mid–late Cenozoic, rotations also define the motion of the Phoenix plate relative to but is less well known for the period prior to this. The aim the Antarctic Peninsula, providing estimates of convergence of this paper is to address this imbalance by presenting the rate and direction. first quantitative estimates of rates and azimuths of Phoenix An area of ocean floor produced by Late Cretaceous spread- plate convergence with the Antarctic Peninsula for the Late ing at the Pacific–Phoenix ridge is preserved in the SW Pacific Cretaceous, and improved estimates of these parameters Ocean, E of New Zealand (Fig. 2; Cande et al. 1982). The for the early Tertiary. These estimates are calculated using ocean floor produced on the opposite flank of the ridge during published marine magnetic anomaly interpretations (Cande the same period has been subducted beneath the Antarctic et al. 1982), free-air gravity maps derived from satellite Peninsula. However, assuming symmetric spreading, magnetic altimetry data (McAdoo & Marks 1992; Sandwell et al. 1995) anomaly identifications in the SW Pacific can be used to and plate rotation data (Mayes et al. 1990; Cande et al. 1995). calculate a Late Cretaceous Phoenix–Pacific stage rotation. A A 900 km long magnetic profile collected on USNS Eltanin stage rotation was calculated in this way and then combined cruise 42 allows an independent check on the early Tertiary with published Pacific–Marie Byrd Land rotation data Antarctic–Phoenix spreading and convergence rates. The (Table 1) to obtain a Phoenix–Marie Byrd Land stage rotation effects of changes in convergence obliquity and rate on the (Fig. 3).
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Table 1. Magnetic anomaly ages (Cande & Kent 1995) and finite rotations for Pacific plate to Marie Byrd Land
Age Angle Anomaly (Ma) Latitude Longitude ()) Reference
C34n 83.00* 64.94 "62.49 53.09 Mayes et al. (1990) C31n 67.67* 69.33 "53.44 51.05 Cande et al. (1995) C27n 61.10 71.38 "55.57 44.90 Cande et al. (1995) C24n 53.35† 73.62 "52.50 40.03 Cande et al. (1995) C21n 47.91† 74.52 "50.19 37.64 Cande et al. (1995)
*Young edge of normal interval. †Old edge of normal interval. If there has been no subsequent movement between the component blocks of West Antarctica, each rotation is also a finite rotation for the Pacific plate to West Antarctica as a whole. Conventions: north and east are positive, and a positive angle indicates that a counter-clockwise rotation describes the motion of the Pacific plate relative to Marie Byrd Land when reconstructing.
Phoenix–Pacific stage rotation, assuming symmetric spreading, then it was combined with published Pacific–Marie Byrd Land rotation data (Table 1) to obtain a Phoenix–Marie Byrd Land stage rotation (Fig. 3). In both of the areas mentioned above there are insufficient published magnetic anomaly identifications to determine rotation parameters using a least squares fitting algorithm (e.g. Hellinger 1981; Shaw & Cande 1990). Instead, the rotations required to bring together pairs of magnetic lineations were determined by a simple spherical geometry calculation, using just two points to define a great circle fitting each lineation. The method used determines the pole of rotation and angle necessary to bring together two great circles (lineations) and a specified point on each great circle. This method is very sensitive to the exact positions chosen for the two points that are to be brought together, as these will lie on a small circle around the calculated pole. The method could give erroneous results if the lineations used are disrupted by fracture zone offsets or if spreading was oblique to the ridge axis during the formation of either lineation. However, in both of the areas considered the ocean floor was formed at high spreading rates (Cande et al. 1982), and fast-spreading ridges are generally well Fig. 3. Schematic diagram showing how the addition of Phoenix organised, with long ridge segments that lie perpendicular to relative to Pacific plate motion and Pacific relative to Antarctic plate the spreading direction (e.g. the present East Pacific Rise). motion are used to calculate Phoenix relative to Antarctic plate The coordinates of the points used as input to the rotation motion. Much of the Antarctic plate ocean floor shown in this calculations are shown in Table 2. In the SW Pacific area, reconstruction is now thought to have formed as part of the magnetic anomaly identifications from Cande et al. (1982) Bellingshausen plate. However, this does not affect the calculations were used for two points on anomaly 31 and one point on because the Pacific–Antarctic (or, strictly, Pacific–Marie Byrd Land) anomaly 34. Marine magnetic profiles were examined to locate rotations used in this paper are derived from ocean floor formed at the part of the Pacific–Antarctic ridge to the west of this area the points on anomaly 31 precisely on the young edge of the (Mayes et al., 1990; Cande et al., 1995). positive anomaly. We were not able to obtain the magnetic profile on which Cande et al. (1982) identified anomaly 34 at approximately 36.2)S, 150.1)W, so these coordinates are measured directly from their fig. 3. We defined the anomaly 34 An area of ocean floor produced by early Tertiary spreading lineation by joining this point to a point where we identified at the Pacific–Phoenix ridge is preserved in the SE Pacific anomaly 34 on a marine magnetic profile from RV Conrad Ocean, W of the southern tip of South America (Fig. 2). This cruise 12 farther to the SW. The latter point is located on the ocean floor was originally produced on the Pacific flank of the SE edge of the positive anomaly. The free-air gravity field Pacific–Phoenix ridge and was captured by the Antarctic plate derived from satellite altimetry data in this area clearly shows as a result of NE propagation of the Pacific–Antarctic ridge at the gravity effect of the Heezen Fracture Zone, and also shows about chron C21 (Cande et al. 1982). Once again, the ocean a faint linear fabric, trending approximately parallel to the floor produced on the opposite flank of the Pacific–Phoenix fracture zone, on the ocean floor to its NE (Fig. 4a). Such a ridge during the same period has been subducted beneath the fabric could have been produced from small offsets in the ridge Antarctic Peninsula. Marine magnetic anomaly identifications crest, or along-ridge variations in magma supply. In either case in the SE Pacific were used to calculate an early Tertiary it probably represents the direction of Phoenix–Pacific motion.
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Table 2. Points used to calculate Phoenix–Pacific stage rotations
Point Anomaly Latitude Longitude Source
1 C34n "36.20 "150.10 Cande et al. (1982) 2 C34n "37.60 "156.50 See text and footnote 3 C31n "44.70 "145.02 USNS Eltanin 24 4 C31n "45.59 "147.70 RV R. Conrad 1212 5 C27n "55.32 "93.95 RV R. Conrad 1212 6 C27n "56.38 "94.20 See text and footnote 6A C27n "55.88 "94.08 See text and footnote 6B C27n "56.88 "94.32 See text and footnote 7 C24n "58.79 "83.82 RV R. Conrad 2107 8 C24n "59.27 "84.05 RV Umitaka Maru
Points 2 and 6 were selected by interpreting ‘flow lines’ of plate motion from the free-air gravity field derived from satellite altimetry data (Fig. 4), and tracing these from points 4 and 8 to their intersections with the anomaly 34 and anomaly 27 lineations, respectively. Points 6A and 6B are alternative positions for point 6, used in calculations to investigate the effect of different flow line interpretations.
Accordingly, the second point on anomaly 34 (point 2 in parallel to discontinuous linear trends in the free-air gravity Table 2) was chosen by tracing a line parallel to the field, interpreted as representing the direction of Phoenix– gravity fabric, from point 4 on anomaly 31 (Table 2), to the Pacific motion, from point 8 on anomaly 24 (Table 2) to the intersection with the anomaly 34 lineation. intersection with the anomaly 27 lineation. Recognizing that Assuming symmetric spreading, the rotation calculated to this ‘flow line’ is poorly constrained by the gravity data, we bring together anomalies 31 and 34, and points 2 and 4, is half also calculated alternative stage rotations by reconstructing of the Phoenix–Pacific stage rotation in the reference frame of point 8 to points on the anomaly 27 lineation 0.5)N and 0.5)S the Pacific plate (rotation 1 in Table 3). The rotation angle was of point 6 (points 6A and 6B in Table 2), and then repeated the doubled to give the full stage rotation (rotation 2 in Table 3). calculations described below using these alternative rotations The stage rotation pole was rotated by the Pacific–Marie Byrd to investigate the effect on the resulting Phoenix–Marie Byrd Land finite rotation for chron C31 (Table 1) to give the Land stage rotation. equivalent stage rotation in the reference frame of Marie Byrd Assuming symmetric spreading, the rotation calculated Land (rotation 3 in Table 3). The Pacific–Marie Byrd Land to bring together anomalies 24 and 27, and points 6 and 8 stage rotation for chrons C34–C31, also in the reference frame (Table 2), is half of the Phoenix–Pacific stage rotation in the of Marie Byrd Land, was calculated from the finite rotations local reference frame of the captured crust (rotation 6 in in Table 1 (rotation 4 in Table 3). This was added to the Table 3). The rotation angle was doubled to give the full stage Phoenix–Pacific stage rotation to give a Phoenix–Marie Byrd rotation (rotation 7 in Table 3). The captured crust was part of Land stage rotation for chrons C34–C31, in the reference the Pacific plate prior to chron C21, and since then it has been frame of Marie Byrd Land (rotation 5 in Table 3). part of the Antarctic plate (Fig. 1). At chron C24 the stage In the SE Pacific area, magnetic anomaly identifications rotations for the interval between chrons C24 and C27 in from Cande et al. (1982) were used for two points on anomaly Pacific and Antarctic reference frames were finite rotations and 24 and one on anomaly 27. Marine magnetic profiles were were therefore identical. The stage rotation pole calculated examined to locate these points precisely in the middle of the here has been displaced from the Antarctic reference frame by positive anomaly in the case of anomaly 27, and on the older Pacific–Antarctic motion between chrons C24 and C21. There- edge of the positive anomaly in the case of anomaly 24. The fore it must be rotated by the Pacific–Antarctic stage rotation free-air gravity field over the area of captured, formerly Pacific, for chrons C24–C21 to move it into the Antarctic reference crust is rather irregular (Fig. 4b), and there are only a few frame. A Pacific–Marie Byrd Land stage rotation for chrons discontinuous trends that may indicate the direction of C24–C21 was calculated from the finite rotations in Table 1 Phoenix–Pacific motion. The Humboldt Fracture Zone (rotation 8 in Table 3). Assuming that there has been no (Fig. 2), which forms the NE boundary of the captured crust, relative motion between the component blocks of West represents the trace of the Pacific–Farallon–Phoenix triple Antarctica since chron C24, this is also a Pacific–Antarctic junction (Cande et al. 1982) and therefore its orientation is not stage rotation. Hence the calculated Phoenix–Pacific stage indicative of the Phoenix–Pacific motion direction. The part of rotation pole (for chrons C27–C24) was rotated by this the Tula Fracture Zone along the SW boundary of the Pacific–Marie Byrd Land stage rotation (for chrons C24–C21) captured crust formed as a transform fault between Pacific to give the equivalent Phoenix–Pacific stage rotation in the crust generated at the Pacific–Phoenix ridge and Antarctic Marie Byrd Land (Antarctic) reference frame (rotation 9 in crust generated at the Pacific–Antarctic and Antarctic– Table 3). An alternative way of comprehending this procedure Phoenix ridges. Its orientation, which is not clearly defined by is to consider that rotating the Phoenix–Pacific stage rotation the free-air gravity field, therefore reflects Antarctic–Pacific pole by the Antarctic–Pacific finite rotation for chron C21 rather than Phoenix–Pacific motion. The complexity of the compensates for Antarctic–Pacific motion since chron C21 gravity field over the SW part of the captured crust suggests and puts the stage rotation into the Pacific reference frame. complex tectonic evolution of the Pacific–Antarctic–Phoenix Subsequent rotation of the Phoenix–Pacific stage rotation pole triple junction during this period. The second point on by the Pacific–Antarctic finite rotation (i.e. rotation in the anomaly 27 (point 6 in Table 2) was chosen by tracing a line opposite direction) for chron C24 transfers it to the Antarctic
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Table 3. Calculated stage rotations
Rotation Plate pair Reference frame Chrons Latitude Longitude Angle Comment
1 PHO–PAC PAC C34–C31 7.65 "107.78 "11.50 Half stage rotation 2 PHO–PAC PAC C34–C31 7.65 "107.78 "23.00 Full stage rotation 3 PHO–PAC MBL C34–C31 "1.33 "60.47 "23.00 Rot. 2 in MBL ref. frame 4 PAC–MBL MBL C34–C31 0.15 "69.77 5.33 Calculated from Table 1 5 PHO–MBL MBL C34–C31 "1.20 "57.65 "17.76 Sum of rotations 3 and 4 6 PHO–PAC Captured area C27–C24 "74.00 "129.82 16.11 Half stage rotation 7 PHO–PAC Captured area C27–C24 "74.00 "129.82 32.22 Full stage rotation 8 PAC–MBL MBL C24–C21 57.37 "59.85 2.50 Calculated from Table 1 9 PHO–PAC MBL C27–C24 "75.25 "125.88 32.22 Rot. 7 in MBL ref. frame 10 PAC–MBL MBL C27–C24 52.70 "54.77 5.18 Calculated from Table 1 11 PHO–MBL MBL C27–C24 "71.65 "103.43 28.71 Sum of rotations 9 and 10
Conventions: north and east are positive, and a positive angle indicates that a counter-clockwise rotation describes the motion of the first plate relative to the second plate in the ‘plate pair’ column when reconstructing, e.g. PHO-PAC means a rotation that reconstructs the Phoenix plate to the Pacific plate. PAC, Pacific plate; PHO, Phoenix plate; MBL, Marie Byrd Land.
Table 4. Results of plate rotation calculations; motion of the Phoenix Antarctica, the calculated Phoenix–Marie Byrd Land stage plate in the Antarctic (Marie Byrd Land) reference frame for various rotation for the Late Cretaceous (83.0–67.7 Ma) implies that positions in the SE Pacific the convergence rate between the Phoenix plate and the Antarctic Peninsula was high, between 110 and 121 mm a"1, Chrons C34–C31 Chrons C27–C24 increasing southward in the area of interest. The azimuth of (83.00–67.67 Ma) (61.10–53.35 Ma) convergence varied little with latitude, but changed signifi- Azimuth Rate Azimuth Rate cantly with longitude, from 104) at 65)S, 70)W to 125) at 65)S, Latitude Longitude (mm a"1) (mm a"1) 90)W. A calculated azimuth of 93) for 60)S, 60)W suggests highly oblique convergence at the NE tip of the peninsula. The calculations suggest that by the early Tertiary interval "60 "80 116 113 119 106 (61.1–53.4 Ma) the Phoenix–Marie Byrd Land pole of rotation "65 "80 115 118 132 77 "70 "80 114 121 157 56 had shifted from an equatorial position to a position on the "65 "70 104 116 137 98 Marie Byrd Land margin. A consequence of the proximity of "65 "90 125 119 121 59 the pole to the area of interest is that the calculated conver- "60 "60 93 110 128 145 gence rates increase quite markedly to the NE, from 56 mm a"1 at 70)S, 80)W to 145 mm a"1 at 60)S, 60)W. Perhaps more significantly, the calculated early Tertiary stage rotation reference frame. Carrying out these two finite rotations in indicates a clockwise change in convergence azimuths by more sequence has the same effect as rotating by the Pacific– than 30), compared to the Late Cretaceous interval, along the Antarctic stage rotation for chrons C24–C21. The Pacific– Antarctic Peninsula margin (Fig. 5). Marie Byrd Land stage rotation for chrons C27–C24, also in Calculations based on alternative positions for point 6 the Marie Byrd Land reference frame, was calculated from the (Table 2), carried out to investigate the consequences of the finite rotations in Table 1 (rotation 10 in Table 3). This was orientation of Phoenix–Pacific motion having been slightly added to the Phoenix–Pacific stage rotation to give a Phoenix– different from our preferred interpretation, result in early Marie Byrd Land (Phoenix–Antarctic) stage rotation for Tertiary Phoenix–Marie Byrd Land stage rotations with pole chrons C27–C24 in the reference frame of Marie Byrd Land positions that are within 250 km of the stage pole position (rotation 11 in Table 3). given in Table 3 (rotation 11). Using point 6A results in a Phoenix–Marie Byrd Land stage rotation with a pole at 70.64)S, 108.73)W and a rotation angle of 28.86), while using Results point 6B results in a stage rotation with a pole at 72.43)S, 97.36)W and a rotation angle of 28.74). Adopting either of Convergence rates and azimuths from plate rotation these alternative stage rotations would not affect our conclu- calculations sion that there was a significant clockwise change in the direction of convergence between the Phoenix plate and The results of the plate rotation calculations are shown in the Antarctic Peninsula in the latest Cretaceous or earliest Table 4 and Fig. 5. Assuming no deformation within West Tertiary.
Fig. 4. Free-air gravity field derived from satellite altimetry data over two areas of the Pacific Ocean containing ocean crust produced at the Pacific–Phoenix ridge (Sandwell et al., 1995). Data displayed with shaded relief, illuminated from NE. (a) Data from the SW Pacific (box A in Fig. 2), white squares mark positions on anomaly 34 and white diamonds mark positions on anomaly 31 (Table 2) used to calculate a Phoenix–Pacific stage rotation. The line of gravity highs extending southeastward from about 41)S, 165)W is caused by the Louisville Ridge seamount chain, which formed during Tertiary time (Watts et al., 1988). (b) Data from the SE Pacific (box B in Fig. 2), white squares mark positions on anomaly 27 and white diamonds mark positions on anomaly 24 (Table 2) used to calculate a Phoenix–Pacific stage rotation.
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Fig. 5. Map showing the azimuths of Phoenix plate motion relative to the Antarctic Peninsula, derived from plate rotation calculations (Table 4). Closed arrows indicate late Cretaceous motion (83.0–67.7 Ma) and open arrows represent early Tertiary motion (61.1–53.4 Ma), the lengths of the arrows are proportional to rate. Trench positions at different times, as interpreted by McCarron (1995), are indicated by dashed lines, the numbers on each line indicating approximate age in Ma. Al I, Alexander Island; G VI S, George VI Sound.
Antarctic–Phoenix spreading rates from a marine the Antarctic Peninsula were both part of the Antarctic plate, magnetic profile this was also the convergence rate. The plate rotation calcu- lations yielded a rate of only 59 mm a"1 for the interval A 900 km long magnetic profile collected NW of Alexander 61.1–53.4 Ma for the area in which the profile is located Island, between the Heezen and Tharp Fracture Zones (Fig. 2), (Table 4). However, as noted above, the proximity of the on USNS Eltanin cruise 42 (Herron & Tucholke 1976) allows calculated Phoenix–Marie Byrd Land early Tertiary stage an independent check on early Tertiary spreading rates at the rotation pole implies rates of motion that increased markedly Antarctic–Phoenix ridge. A synthetic magnetic profile that to the NE, so the rate calculated for any specific location in corresponds closely to the observed profile was generated this area is very sensitive to a small error in the rotation pole from a sea-floor spreading model that includes three decreases location. A likely explanation for this discrepancy is that in spreading rate between 65.0–41.9 Ma (Fig. 6), using the uncertainties in the data and rotations used have been geomagnetic polarity time scale of Cande & Kent (1995). compounded in the rotation calculations and that the early The only significant discrepancies between the observed Tertiary stage rotation pole was actually slightly farther away and synthetic profiles occur over the De Gerlache seamounts from the area than calculated. (Figs 2 and 6). A positive anomaly is associated with the larger The exact positions of the Tharp and Heezen Fracture of the two seamounts crossed, and a negative anomaly is Zones are not well constrained within c. 700 km of the margin. associated with the smaller of the two. The close correspon- The fact that a continuous sequence of magnetic anomalies dence between the synthetic and observed magnetic profiles in occurs on such a long profile confirms that it is probably Fig. 6 suggests that the half spreading rates on the trailing approximately parallel to the trends of the early Tertiary flank of the Antarctic–Phoenix ridge as it converged with the fracture zones. This provides supporting evidence for the peninsula were the same as those used in the model. Assuming generally NW–SE azimuth for early Tertiary Phoenix– symmetric spreading, this means that the full spreading rate for Antarctic motion derived from the plate rotation calcula- the interval between chrons C27n and C23r (61.1–52.3 Ma) tions. A NW–SE azimuth is also similar to that indicated was 102 mm a"1. As the trailing flank of the ridge crest and for mid–late Tertiary Phoenix–Antarctic motion by the
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Fig. 6. Observed bathymetric and magnetic profiles collected on USNS Eltanin cruise 42, and a synthetic magnetic anomaly profile calculated from the sea-floor spreading model shown. The observed profiles are resolved along 140). The model was constructed using the geomagnetic polarity time scale of Cande & Kent (1995), the oceanic age-depth relationship of Trehu (1975), a magnetic layer thickness of 0.5 km, and a remnant magnetisation vector with inclination= "77) and declination 0). The location of the De Gerlache Seamounts in relation to tectonic features in the S Pacific is shown in Fig. 2.
orientation of fracture zones farther NE (Larter & Barker quantify uncertainties in the ‘flow lines’ interpreted from 1991a). gravity field data and in the assumption of symmetric spread- ing. However, the early Palaeocene change in convergence azimuth indicated by the calculations appears consistent with Discussion geological observations on the Antarctic Peninsula (see below) and with evidence for a general plate reorganisation in the The results outlined above have important implications for the S Pacific at this time. Cande et al. (1995) reported that a geology of the Antarctic Peninsula region based on the three general plate reorganisation took place in the S Pacific at following observations. chron C27 time (61 Ma). Unless the change in Pacific– Antarctic motion at this time was entirely the result of a Early Palaeocene change in convergence azimuths. The results change in Pacific plate absolute motion, some change the presented in Table 4 and Fig. 5 suggest that the azimuth of absolute motion of the Antarctic plate is implied. A change in convergence offshore from Alexander Island changed between Antarctic plate absolute motion would result in a correspond- the Late Cretaceous (83.0–67.7 Ma) and the early Tertiary ing change in Phoenix–Antarctic relative motion, so it is very (61.1–53.4 Ma) from 114 to 157 . The limited areas of oceanic ) ) likely that a change in Phoenix–Antarctic relative motion also crust produced at the Pacific–Phoenix ridge that are preserved took place at C27 time. today make rigorous calculation of Phoenix–Pacific rotations impossible. We recognise that the confidence limits associated with the Phoenix–Pacific rotations presented in this paper must Early Eocene decrease in Antarctic–Phoenix spreading rates. be large, and that these uncertainties propagate through to the The sea-floor spreading model in Fig. 6 shows a c. 60% calculated Phoenix–Marie Byrd Land rotations. The simple decrease in half-spreading rates to 21 mm a"1 at chron C23r spherical geometry calculations we employed do not provide a (52.3 Ma). This implies a similarly abrupt decrease in conver- measure of uncertainty and it would also be difficult to gence rate, from 102 mm a"1 to 42 mm a"1 (assuming
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symmetric spreading), because of the unusual tectonic setting Implications for onshore geology of Alexander Island described earlier. In a statistical study of relations among and the Antarctic Peninsula subduction parameters from 39 modern subduction zones, The obliquity of subduction during the Late Cretaceous has Jarrard (1986) found a moderate correlation between conver- implications for onshore geology along the length of the gence rate and ‘strain class’, with faster convergence corre- Antarctic Peninsula. Hamilton (1995) argues that extension is sponding to more contractional environments in the overriding the dominant strain regime throughout the majority of arcs, plate. On this basis it seems likely, as suggested by previous based on earthquake first motion studies and other geological authors (e.g. Storey et al. 1996), that such a decrease in and geophysical data from active arcs. However, Hamilton convergence rate would have a profound effect on the stress (1995) recognises that shortening does occur in many fore-arc regime of the overriding plate, enhancing slab roll-back and regions, especially near the trench. The statistical analysis of therefore causing extension within the arc and fore-arc regions. Jarrard (1986) indicates that ‘strain class’ is probably deter- Storey et al. (1996) proposed that dextral transtensional mined by the linear combination of convergence rate, slab age deformation observed in the region of George VI Sound and shallow slab dip, with more contractional environments (Storey & Nell 1988) resulted from an Eocene decrease in corresponding to faster convergence, lower slab age and lower convergence rate, although the precise time and magnitude of angles of subduction. the decrease have not previously been determined. A similar During oblique subduction deformation is generally par- mechanism has been proposed to explain late Cenozoic exten- titioned into strike-slip and contractional components (Tikoff sion in Bransfield Strait, northern Antarctic Peninsula, follow- & Teyssier 1994), and above a critical angle of obliquity a ing slowing and eventual cessation of spreading on the strike-slip fault may develop, defining a fore-arc sliver (Platt Antarctic–Phoenix ridge (Barker & Dalziel 1983; Maldonado 1993). LePichon et al. (1994) described strain partitioning and et al. 1994). strike-slip faulting in the Nankai Trough accretionary wedge off SW Japan, and McCaffrey (1991) noted that fore-arc slivers Time of Antarctic–Phoenix ridge crest arrival at the trench. The SW of Sumatra deformed by arc-parallel stretching and did sea-floor spreading model based on the USNS Eltanin 42 not behave rigidly. Jarrard (1986) shows that strike-slip is magnetic data in Fig. 6 implies that the segment of the common in modern subduction zones 100–300 km from the Antarctic–Phoenix ridge between the Heezen and Tharp trench, near or within the arc, where the convergence azimuth Fracture Zones arrived at the trench later than previously is greater than c. 12) from normal to the trench, producing estimated (Larter & Barker 1991a; Larter et al. 1997). Previous generally trench-parallel faults. While it is difficult to be sure of estimates were made by using spreading rates measured on the exact position of the trench in an ancient subduction zone, ocean floor produced elsewhere along the Antarctic–Phoenix estimates have been made in Figs 1 and 5, based on available ridge to extrapolate from the location of the youngest onshore data interpreted by McCarron (1995). Farther N magnetic anomaly on published maps to the margin. By this along the margin, the position of the trench at the time of method Larter & Barker (1991a) estimated that the SW end arrival of the Antarctic–Phoenix ridge has been estimated from of the ridge segment entered the trench at 53.5 Ma, but seismic reflection profiles to be approximately at the current because of the obliquity of the ridge segment to the margin position of the 2500 m bathymetric contour (Larter & Barker its NE end did not reach the trench until 45 Ma. These ages 1991b). Based on these interpretations and our results it were derived using the geomagnetic polarity time scale of appears that convergence during the Late Cretaceous would LaBrecque et al. (1977). Eocene magnetic reversals are have been highly oblique along the Pacific margin of Ellsworth assigned ages which are about 3 Ma younger on the geo- Land and offshore from the South Shetland Islands, with the magnetic polarity time scale of Cande & Kent (1995). The likelihood of dextral strike-slip in the fore-arc region. Our age at which the SW end of the segment reached the trench results give no indication when the period of oblique sub- was calculated assuming that the Tharp Fracture Zone duction started, but it may have begun long before 83 Ma. reaches the margin at about 87 W, as shown by Barker ) Oblique subduction and the removal of fore-arc slivers along (1982). However, the position at which this fracture zone the margin of Ellsworth Land during the Cretaceous or earlier, reaches the margin is poorly constrained. In this paper we may have been an important factor in building the wide are particularly concerned with the timing of arrival of ridge accretionary complex (500 km) in the bight of the Antarctic segments at the margin directly offshore from Alexander Peninsula (Storey & Nell 1988; Nell & Storey 1991). Part of Island, i.e. E of 83 W. Irrespective of the exact position of ) this accretionary complex is now exposed on Alexander Island the Tharp Fracture Zone, the time required for the ridge– (Burn 1984). It may also explain the lack of obvious accretion- trench intersection to migrate along the part of the margin ary material exposed onshore along the coast of Ellsworth offshore from southern Alexander Island (from 83 Wtothe ) Land (although outcrop is very poor in this area). Oblique position at which the Heezen Fracture Zone reaches the subduction further N during the same period may also have margin), is about 4 Ma, and the USNS Eltanin profile crosses caused fore-arc slivers to migrate to the NE opposite the South the margin near the middle of this part of the margin. The Shetland Islands, resulting in a narrower, offshore accretionary identification of anomaly C20n (43.8–42.5 Ma) near the foot complex in that region (Maldonado et al. 1994). The change to of the continental slope on this profile (Fig. 6) implies that a more SE azimuth of convergence in the early Tertiary the ridge–trench intersection migrated along this part of the probably initiated a new phase of growth in the accretionary margin between 44–40 Ma. prism. An additional interesting observation from the USNS Eltanin magnetic profile is that recognisable magnetic anomalies can be traced to the foot of the continental slope, Subduction-related extension in the arc and fore-arc indicating that the zone of highly subdued magnetic anomalies regions that exists along much of the Antarctic Peninsula margin George VI Sound, a major curvilinear trough, 500 km long (Larter & Barker 1991a) is absent in this area. and 30–70 km wide (Maslanyj 1987) separates the Antarctic
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Peninsula magmatic arc from the fore-arc sequences on high (Drummond & Defant 1990; Hochstaedtar et al. 1994). Alexander Island and is located about 300–550 km from the High Mg# andesite lavas in northern Alexander Island were present continental margin (Fig. 5). Storey & Nell (1988) erupted at c. 53 Ma (McCarron & Millar 1997), 23 Ma before describe a set of mainly east-dipping (towards the sound) the Antarctic–Phoenix ridge segment between the Heezen and oblique-slip dextral/extensional faults on Alexander Island, Tula Fracture Zones reached the trench. McCarron & Smellie striking NNW–SSE and subparallel to the sound. A related set (this volume) also reports andesite lavas that have a high Mg# of late N–S dextral and NE–SW-trending strike-slip faults are from Staccato Peaks in southern Alexander Island. These lavas also seen cutting the fore-arc sediments. These faults are the have similar geochemical characteristics to the high Mg# most recent strike-slip features seen on Alexander Island, andesite lavas in northern Alexander Island, except that they formed during a period of Cenozoic dextral transtension that lack the elevated Cr and Ni contents. The andesite lavas at probably also resulted in the initial development of the sound Staccato Peaks are probably coeval with a nearby 75 Ma (Storey & Nell 1988). Storey & Nell (1988) considered that the pluton (McCarron & Smellie this volume, Storey et al. 1996), Cenozoic strike-slip tectonics of Alexander Island were related which means that they were erupted >30 Ma before the to the separation of New Zealand from Marie Byrd Land. An Antarctic–Phoenix ridge segment SW of the Heezen Fracture alternative model was proposed by Storey et al. (1996), where Zone reached the trench off southern Alexander Island. extension due to slab roll-back produced the transtensional The thermal effect on a fore-arc of conduction from a faults in Alexander Island and George VI Sound. The esti- hot, young, subducting slab was evaluated through numerical mates of early Tertiary convergence directions presented in this modelling by James et al. (1989). Their models indicate that the paper, and the change in convergence rate at 52.3 Ma, are main temperature-limiting parameter is the age of the sub- consistent with the model of Storey et al. (1996). ducted ocean crust, and that rate of convergence, angle of Smith Island, one of the South Shetland Islands (Fig. 5), is subduction, amount of radiogenic heating and values of located approximately 70 km from the last remaining part of thermal conductivity are all of secondary importance. Using the trench and is composed of blueschist metamorphic rocks reasonable reference values for the secondary parameters, that have undergone polyphase ductile deformation (Smellie & James et al. (1989) were only able to explain production of Clarkson 1975; Rivano & Cortes 1976). Grunow et al. (1992) felsic melts (not high Mg# andesites) at <35 km depth, requir- obtained an 40Ar/39Ar age on white mica from a phyllonitic ing temperatures >700)C, by steady-state subduction of ocean schist of 47&0.5 Ma, indicating that these blueschist rocks crust <0.5 Ma old. The models showed that similar tempera- were uplifted into the upper crust long before the arrival of a tures could also be achieved by transient heating following ridge segment at the nearby part of the margin during the abrupt cessation of subduction of 1.5 Ma old ocean crust. On Pliocene (Larter & Barker 1991a). Grunow et al. (1992) the basis of these results it seems highly improbable that the suggested that uplift of the Smith Island metamorphic rocks much higher temperatures required to produce high Mg# may have resulted from a reduction in the compressive stresses andesite melts at <50 km depth could have resulted from in the fore-arc as a consequence of slowing of subduction. prolonged subduction of young oceanic lithosphere beneath Alexander Island. If oceanic lithosphere sufficiently young to have such a heating effect was subducted, the spreading ridge producing it would need to have been very close to the trench. Implications of the subduction of young oceanic crust The constraints imposed by the regional tectonic setting in the and slab window formation Late Creaceous and early Tertiary (Fig. 1) make it virtually Subduction ceased off southern Alexander Island during the impossible for a spreading ridge to have approached the trench Eocene, as a segment of the Antarctic–Phoenix ridge migrated and then remained stationary close to it: any ridge that into the trench obliquely (segment B in Fig. 1). The ridge– approached close enough to the trench to enable very young trench intersection moved ENE offshore from southern oceanic lithosphere to be subducted would also have been Alexander Island between 44–40 Ma. Subduction ceased off subducted itself. northern Alexander Island when the ridge segment NE of In view of the geochemical characteristics of the high Mg# the Heezen Fracture Zone reached the margin (segment C in andesite lavas, and the modelling results discussed above, the Fig. 1). Larter & Barker (1991a), using the geomagnetic occurrence and age of such lavas on Alexander Island appears polarity time scale of LaBrecque et al. (1977), calculated that to imply direct contact between asthenosphere-temperature this segment reached the margin at 32&3 Ma. The equivalent mantle and the fore-arc mantle wedge (i.e. existence of a age on the geomagnetic polarity time scale of Cande & Kent ‘slab window’ (Thorkelson 1996)) long before the Antarctic– (1995) is 30&3Ma. Phoenix ridge arrived at the trench. Conduction of heat McCarron & Smellie (this volume) describes ‘Type 2’ low through a slab window could elevate temperatures in the Ca, high-Mg# andesite lavas (Crawford 1989) from the mag- mantle wedge sufficiently to cause melting of depleted hydrous matic fore-arc sequences in Alexander Island. High Mg# peridotite, producing high Mg# andesite magmas. andesite magmas are unusual melts that are associated with Prior to the plate reorganization at 61 Ma, more southerly anomalously hot fore-arc regions; experimental studies indi- segments of the Bellingshausen–Phoenix ridge than those cate that they are produced at temperatures between 1150)C shown in the 62 Ma reconstruction in Fig. 1 could have and 1200)C and at shallow levels (<50 km; Crawford 1989). subducted beneath parts of the margin as far NE as central The high Mg# andesite lavas in northern Alexander Island Alexander Island. Following the reorganization and change in typically have elevated Cr and Ni contents of 200–500 and Phoenix–Antarctic convergence direction, ridge segment B 150–280 ppm, respectively, which is consistent with melt for- (Fig. 1) arrived at the part of the margin off southern mation in equilibrium with mantle olivine (McCarron & Alexander Island. Thus much of Alexander Island may have Smellie this volume). Trace element data indicate that these been affected by arrival of different segments of the same high Mg# andesite lavas were not produced by slab-melting as ridge at different times. This scenario provides a potential the Sr/Y(<30) and Zr/Sm(<28) are too low and Y values too explanation for the c. 75 Ma magmatism in southern
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Alexander Island, but cannot explain the later magmatism in northern Alexander Island. The requirement for a slab window beneath northern Alexander Island at c. 53 Ma implies either earlier subduction of another ridge, as proposed by McCarron & Smellie (this volume), or break-up of the slab in the upper part of the subduction zone (van den Beukel 1990). Earlier subduction of another spreading ridge would imply that an additional plate existed between the Phoenix plate and the trench off Alexander Island during the early Tertiary. We propose that this hypo- thetical plate be known as the ‘Charcot’ plate, and that this name be used to describe any additional plate or microplate which may have existed between the Phoenix plate and the trench at any stage during the Late Cretaceous or early Tertiary. A difficulty with the Charcot plate hypothesis is that the Phoenix plate has had a long history (Larson & Chase 1972), and it is clear that several thousand kilometres of ocean floor were added to it by Mesozoic sea-floor spreading (Fig. 1). Therefore if a Charcot–Phoenix ridge did exist it must have originated by rupture of the Phoenix plate. Furthermore, the high rates we have derived for convergence between the Phoenix plate and the Antarctic Peninsula require that Charcot–Phoenix ridge segments, once formed, must have migrated rapidly into the trench. Taking account of these constraints we suggest two scenarios as plausible explanations for the origin of a Charcot–Phoenix spreading ridge. (1) A ridge segment such as segment A in the 62 Ma reconstruction in Fig. 1 may have been eliminated from the Antarctic–Phoenix (formerly Bellingshausen–Phoenix) ridge Fig. 7. Reconstruction of the South Pacific in the Antarctic by the reorientation of tranform faults required by the early reference frame for 58 Ma, schematically illustrating one possible Teriary plate reorganization. In this situation, the part of the origin for the postulated Charcot–Phoenix ridge (A). A Charcot Phoenix plate between the ridge segment and the trench could microplate (cross hatched) may have originated as a result of a have become detached from the rest of the plate, forming a change in the direction of Phoenix–Antarctic motion during the separate Charcot microplate (Fig. 7). The ridge segment would early Tertiary. Development of a slab window following arrival of then have continued to spread and would have migrated the Charcot–Phoenix ridge at the trench may have provided the heat rapidly to the margin off Alexander Island. source for generation of high-Mg# andesite lavas at c. 53 Ma in (2) As the spreading ridges generating Phoenix plate ocean northern Alexander Island. floor converged with the continental margin during the early Tertiary, the plate became long and thin (Fig. 1). In this up if it was subducted within a few million years of its situation it is quite likely to have ruptured in a similar manner formation. This result is interesting in relation to the Antarctic to the late Tertiary fragmentation of the Farallon plate into Peninsula because it implies that the slab beneath southern Nazca, Guadalupe (Cocos), Rivera and Juan de Fuca plates Alexander Island came close to meeting the criteria for slab (Atwater 1989). Separation of the southernmost part of the break-up during the early Tertiary, with half spreading rates Phoenix plate as a separate Charcot microplate would prob- on the Antarctic–Phoenix ridge >5 cm a"1 until the ridge was ably have produced a slow-spreading ridge at a high angle to <200 km from the trench, but such a situation never developed the margin, analogous to the Galapagos spreading centre. The again anywhere else along the Antarctic Peninsula. On this intersection of such a ridge with the trench would probably basis it seems unlikely that a slab window beneath northern have migrated relatively slowly along the margin, which would Alexander Island at c. 53 Ma could have formed by slab be consistent with the slow northward migration of fore-arc break-up. magmatism in Alexander Island described by McCarron & Several authors have considered spreading ridge subduction Millar (1997). If a spreading centre of this kind existed off as a possible cause of ophiolite obduction (e.g. Dewey 1976; Alexander Island in the early Tertiary, there should be a subtle van den Beukel 1990). Late Cretaceous or early Tertiary magnetic bight on the ocean floor to the NW of Alexander ophiolite obduction is a possible explanation for the large, island. Collection of additional magnetic data in this area positive magnetic anomaly reported by Maslanyj et al. (1991) would provide a means of testing this hypothesis. and Johnson & Ferris (1997) over the continental shelf NW of Thermal and mechanical models to examine the factors southern Alexander Island. The anomaly does not continue controlling slab window formation by the break-up of young farther NE along the shelf, and the emplacement or accretion oceanic lithosphere in the upper part of a subduction zone of the source body must post-date the youngest accreted were developed by van den Beukel (1990). The results from material on Alexander Island, which is younger than mid- these models indicated that the critical parameter determining Cretaceous (Holdsworth & Nell 1992). Following the early whether or not young oceanic lithosphere would break up Tertiary plate reorganization, young ocean floor on the trailing during subduction was the spreading rate at which it had flank of a ridge segment that had already migrated into formed. Oceanic lithosphere created at half spreading rates the trench could have been pushed against the margin and greater than about 3–4 cm a"1 was found to be likely to break obducted as a result of the change in Phoenix plate
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convergence direction. Among other possible explanations for spreading on a ridge segment eliminated from the Antarctic– this anomaly is the intrusion of plutons into the fore-arc as Phoenix ridge by the change in direction of Phoenix–Antarctic a result of spreading ridge subduction, slab break-up, or motion, or fragmentation of the Phoenix plate in a similar the opening of a slab window following the cessation of manner to the late Tertiary fragmentation of the Farallon subduction. plate. Numerical modelling results (van den Beukel 1990) suggest that the slab beneath southern Alexander Island came close to meeting the criteria for slab break-up in the early Tertiary, but that break-up is unlikely to have occurred Conclusions beneath northern Alexander Island, the area where high-Mg# Published interpretations of oceanic magnetic anomalies andesite lavas erupted at this time. indicate sustained fast spreading rates (>50 mm a"1 half-rate) Following the early Tertiary plate reorganisation in the at the Pacific–Phoenix and Antarctic–Phoenix ridges during S Pacific, young ocean floor on the trailing flank of a ridge the Late Cretaceous and early Tertiary. Plate rotation calcu- segment that had already migrated into the trench could have lations presented in this paper indicate that the convergence been pushed against the margin and obducted as a result of rates between the Phoenix plate and the Antarctic Peninsula the change in Phoenix plate convergence direction. This is a were also fast throughout this interval. However the Late plausible explanation for the large magnetic anomaly over the Cretaceous (and perhaps earlier) convergence direction was continental shelf NW of southern Alexander Island, although highly oblique along the Pacific margin of Ellsworth Land and other explanations are also possible. off the South Shetland Islands. This probably caused dextral strike-slip faulting in the fore-arc and removal of fore-arc We thank B. Storey, A. Vaughan and A. Johnson for useful slivers from these parts of the margin. Accumulation of such discussions during the preparation of this paper. We also thank slivers along parts of the margin where convergence was less John Smellie for his comments on an early version of the oblique may explain the origin of the extensive accretionary manuscript, and R. Livermore for checking some of the plate complex that now crops out on Alexander Island, as previously rotation calculations. The paper was improved by constructive reviews from J. Stock and M. Hole. J. Stock also provided a copy of suggested (Storey & Nell 1988). an unpublished manuscript and data which influenced our revision At some time between chrons C31 and C27 (68–61 Ma), of the paper. convergence of the Phoenix plate with the Antarctic Peninsula switched to a more NW–SE direction, which has prevailed through the rest of the Cenozoic. It seems most likely that this References change took place at chron C27, which has recently been A, T. 1989. Plate tectonic history of the northeast Pacific and western identified as the time of a general plate reorganisation in the North America. In:W, E.L., H,D.M.&D, R.W. S Pacific (Cande et al. 1995). (eds) Eastern Pacific Ocean and Hawaii. The Geology of North America, N, Magnetic anomalies on a long marine magnetic profile NW Geological Society of America, 21–72. of Alexander Island reveal an abrupt decrease in Antarctic– B, P.F. 1982. 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Received 27 August 1996; revised typescript accepted 21 August 1997. Scientific editing by Nick Rogers.
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