The Cenozoic Subduction History of the Pacific Margin of the Antarctic Peninsula: Ridge Crest-Trench Interactions

J. Red. Soc. London, Vol. 139, 1982. pp. 787-801, 7 figs. Printed in Northern Ireland

The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest-trench interactions

P. F. Barker

SUMMARY: New magnetic anomaly identifications W of the Shackleton Fracture Zone show 5 spreadingsections, separated by fracturezones. In the 2 most southerly,the ridge crest collided with a trench at the margin of the Antarctic Peninsula only 6.5 and 4 Ma ago, the latest of a series of collisions starting at the base of the peninsula SO Ma ago. Following each collision, spreadingand subductionboth stopped. Opposite the SouthShetland trench and actively extending Bransfield Strait, spreading also stopped 4 Ma ago, but before the spreading centre reached thetrench. The tendency of thesubducting plate to continue sinking probably initiated the opening of the Bransfield Strait. The parallelism between fracture zone orientation on the descending plate and convergence direction, previously thought responsible for the tectonic segmentation of the peninsula, was effectivefor only 10-30 Ma before collision. Before that,either the fracturezones did not extend to the trench, or the subducting ocean floor formed at the Farallon-Phoenix (Nazca- Aluk)boundary, with fracturezones oblique to thesubduction direction. The Aleutian Arc-Kula Ridge model, in which arc magmatism virtually ceases when ocean floor younger than 25-30 Ma occurs beneath the arc, fits the distribution of Antarctic Peninsula radiometric dates, explaining a 50-60 Ma gap between collisions and youngest ages in the S, and possibly the migration of the youngest activity towards the trench. Thus, gaps in the geologic record of arc magmatism need not imply cessation of subduction. The progressive steepening of the peninsular margin towards the 4 Ma collision site suggests tectonic erosion of the fore-arc as the ridge crest approached.

Virtually all the exposed geology of the Antarctic Shackleton Fracture Zone, and the definition in Fig. 1 Peninsula (Adie 1964) is related to the subduction of of Shetland and Drake micro-plates, are discussed by oceaniclithosphere at its Pacific margin,from well Forsyth (1975), by Barker & Dalziel (in press) and in before the break-up of Gondwana to the present day. some detail here. However,the ocean floor magneticrecord in the Some attempts have been made to interpret aspects Pacific only extendsback intothe Jurassic and, be- of the onshore geology in terms of the morerecent cause of more recent subduction, is unavoidably in- subduction history (e.g. Barker 1976; Barker & Dal- complete. Thus, in attempting a detailed explanation ziel, in press; Tarney et al., 1982; and more specifically of onshore geology in terms of specific episodes of the Hawkes 1981). However, a much more precise subduction history, it is more fruitful to consider the account of the subduction history is now possible, as a most recent events. result of the more extensive oceanic magnetic anomaly The interpretation of oceanic magnetic anomalies in coverage of western Drake Passage, described here, the SE Pacific (Herron 1971; Herron & Tucholke and a similar development in the area farther to theW 1976; Barker 1970, 1972; Barker & Burrell 1977; (Cande et al. 1982). In particular, the discovery of a Weissel et al. 1977) has shown thatthe most recent very recent ridge crest-trench collision off Anvers I. event along most of the margin of the Antarctic Penin- has prompted me to compare the effects of this pro- sula was the migration of a spreading centre into the cess along the Peninsula with those hypothesized else- subduction zone, whereuponspreading and subduc- where,notably for the mid-CenozoicAleutian Arc- tion both stopped. In the northern part (N of about Kula Ridge interaction (see, for example, DeLong et Brabant I.) spreading and subduction also stopped, or al. 1978). I have also assessed the length of time for almost stopped, but before the spreading centre had which fracture zones corresponding to those now seen reached thetrench. Activeplate boundaries today at the margin could havebeen subducted along the (Fig. 1; Barker & Dalziel, in press) are essentially same line, as a contribution to the discussion of tecto- remote from the margin of the peninsula: South nic segmentation, opened by Hawkes (1981). American(SAM-ANT) motion is slow, sinistral and E-W, and involves slow subductionat the South Americanmargin. The ShackletonFracture Zone (S Magnetic anomalies in in Fig. 1) is the locus of strike-slip motionat its western Drake Passage southernend, butprobably becomes obliquely subducting as it merges with the South American margin Magnetic profiles in western Drake Passage, W of the farther N. The scattering of earthquakes W of the ShackletonFracture Zone, have been plotted along 0016-764918211100-0787$02.00 @ 1982 The Geological Society

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FIG. 1. Shallow earthquake epicentres (top) and present plate boundaries, Scotia Sea region. SAM-ANT motion is 20-24 mm/year,sinistral, E-W; Drake(Aluk) and Shetlandmicroplates are discussed in text. S is Shackleton fracture zone. From Barker & Dalziel (in press).

ship tracks (Fig. 2). These profiles were acquired over mean track direction (mainly directions between 130" several seasons between 1968 and 1979, aboard RRS and ZOO") as shown by the lines joining tracks and Bransjield, HMS Endurance and RRS Shackleton. All profiles, and with positive anomalies to the NE and E. tracks were satellite-fixed, and are known everywhere In Fig. 3, three representative profiles (lettered in Fig. to better than 2 km. Bathymetric data were also ac- 2) arecompared with asynthetic magnetic profile, quired on almost all tracks, and gravity data on a few. generated using the reversal time scale of LaBrecque After removal of theInternational Geomagnetic et al. (1977). Fracture zones in Fig. 2 are located partly Reference Field (IAGA 1969, 1976), the residual whererequired to explain magnetic anomaly offset, magnetic anomalies in Fig. 2 were projected onto the but mainly on bathymetric evidence.

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NW 16 SE 23 Ma 9 6 44 6 9 16 23

FIG.3. Magnetic profiles AB, CD and EF (located in Fig. 2) resolvedalong 130" and compared with synthetic anomalyprofile generated using time scale of LaBrecque et al. (1977). Small circlesshow course alterations. Bathymetricprofiles of trench and margin at B, D and F showsteepest margin at collision site. Some of the anomaly identifications in Figs 2 and 3 continuation of spreading right to thecentre at the were published by Barker & Burrell (1977), but with- previousspreading rate.The abundant earthquake out the supporting evidence provided here. Barker & activity (Figs 1 and 2), although scattered, is to some Burrell also described the Shackleton Fracture Zone, extent coincident with the spreading centres and the which developed as the Scotia Sea opened over the youngest parts of the fracturezones, which suggests past 30 Ma or so, and which forms the NE margin of that some kind of very slow spreading may have the area being considered (Fig. 2). Before 30 Ma ago continued to the present day. the margins of southern SouthAmerica and the Antarctic Peninsulaprobably formed some kind of continuous, continental, subducting Pacific margin. Regional magnetic anomaly SW of the Shackleton Fracture Zone, 5 spreading data sections separated by 4 fracture zones have been identified, all apparently part of a simple two-plate system. In Fig. 4, the newly described magnetic lineations are The 3 northeasterly sections show both flanks of the combined with others from a broader area of the SE spreading centre, which was active from at least 21 Ma Pacific (from Cande et al. 1982) and from Drake ago until about 4 Maago. The two southwesterly Passage (Barker & Burrell 1977; Barker & Dalziel, in sections each show only a northwestern flank, with the press), the separate data sets being distinguished by a oldestanomaly 28 Ma old (Anomaly 8) and the slightly different ornament. The data sets cover diffe- youngest at the Antarctic peninsular margin about 6.5 rentareas and could be merged without conflict. A and 4 Ma old (Anomaly 4 and Anomaly 3). At the 3 possible exception, which cannot be resolved here, abandoned ridge crests, up to 20 km of ocean floor concerns the area around 56"S,77"W, off the South appears to have been formed since 4 Ma ago, but the American margin, and identifications of anomalies 6A central anomaly is positive, which argues against the and 6B. More importantly, it is clear from Fig. 4 that

magnetic data are sparse,particularly in the W, so that Ma rnmlyr considerableuncertainty attachesto the location of >65-50 50-70 (2110 mmiyear Pacific-Phoenix) some fracture zones and the extent and true orienta- 5&36 16 tion of some identified magnetic lineations. Neverthe- 3623 18 less, several useful conclusions may bedrawn about 2223-16 the history of subduction at the margins of the Antarc- 169 26 tic Peninsula and southern South America. 9- 6 32 Firstly, it is evident that the anomalies so far de- 6 4 20 scribed are part of a greater set, oriented NE-SW and 4- 0 2 possibly extendingW of theentire length of the Antarctic Peninsula.This setappears to joinasecond set, The more recent of these rates are deduced from Fig. oriented NNW-SSE and bordering the South Amer- 2 and used in Fig. 3, while the older data (>28 Ma) ican margin, although the details of the magnetic bight come from Herron & Tucholke (1976) and Cande et betweenthem are obscure.Both sets young toward al. (1982). The estimated arrival times range from the margins (except for the 3 sections off the South 50 Ma in the far S to 4 Ma directly S of the Hero Shetland Is, and anomalies E of the Shackleton Frac- Fracture Zone, and correspond reasonably wellwith ture Zone within the Scotia Sea), indicating a series of times of spreading rate changes; this would beex- collisions of the spreading centres with one or other pected, since each collision changed the balance of trench. Off South America, the locus of collision has forces acting on the plates. migrated northward,and now occurs atabout 46"s (Herron et ut. 1981). Subduction continued after each South Shetland trench and Bransfield collision, albeitat the much slower rate (20-24 mm/ year) of SAM-ANT convergence (Fig. 1). Reflection Strait profiles (Hayes & Ewing 1970) show oceanic base- At the 3 northernmost spreading sections, spreading ment dipping towards the trench, but overlain by up to stopped at the same time as this latest arrival, 4 Ma 2 km of trench turbidites which obliterate the trench ago, but before each section of spreading centre basement topography. A crude combination of spread- reached the trench. It is not clear why this final arrival ing rates and SAM-ANT convergence rates produces should have caused the stoppage, but the coincidence an estimatedtime of subduction of the ridge crest does seem to suggest it. Spreading had also stopped in between 50"s and 54"s of 13 Ma ago. South of 523, eastern Drake Passage, atabout 6 Ma (Barker & convergence is oblique, andprobably even slower, Burrell 1977), so this would have been the firstnew representing Scotia-ANT motion (Fig. 1). Its reality, arrival since that stoppage. It is possible thatthe however, is demonstrated by the extent to which quite resulting changes in forces at the margins of this small young ocean floor has been subducted near 56"s. plate remnant were sufficient to prevent further At the margin of the Antarctic Peninsula, the sub- spreading. duction history is fundamentally different. The age of In this connection, it has been pointed out previous- successive ridge crest-trench collisions again decreases ly (Barker 1970. 1976; Ashcroft 1972; Davey 1972; northwardbut, after each collision, subductionand Barker & Dalziel, in press) that the active extensional spreading bothstopped. Seismic reflection profiles trough of Bransfield Strait occupies the same length of with sonobuoy-determined velocity control (Herron & the Peninsula as does the surviving SouthShetland Tucholke 1976), near the TuIa Fracture Zone, show trench, opposite these 3 dead spreading sections (Fig. that the trench basement topography was eliminated, 4). We have suggested (Barker & Dalziel, in press) andthe margin became passive. The evidence is that Bransfield Strait opened because of the cessation sparse, but ocean floor on the surviving flank of the of spreading tothe N, asa result of thecontinued spreading centre apparently cooled and subsided ex- sinking and peeling back of the remnant plate at the actly as normal oceanic lithosphere, both before and trench (the 'trench suction' of Forsyth & Uyeda 1975). after the collision. The chronology of Bransfield Strait extension isin The age ranges of anomalysequences in adjacent accord with this: Roach (1978) deduced a 1.3 Ma age spreadingsections overlap sufficiently to show that for the start of back-arc spreading, but normal faulting only a two-plate system was involved, and to allow an and subsidence on either margin pre-date this, being estimate of the time of arrival of each section of of late Pliocene age in part (Barton 1965; Weaver et. spreading centreat the trench. The plategeometry af. 1979). The presenttectonic situation is far from was extremely simple, since the Antarctic plate pro- clear, however. There are no deep earthquakesassoci- vided both the over-riding plate at the trench and the ated with the South Shetland trench and only 2 of 10 NWflank of the spreading centre. Half spreading bathymetric profiles N of the trench show features rates, which were also ridge approach rates and resembling theouter rise (Walcott 1970; Watts & (assuming symmetric spreading) half subduction rates Talwani 1974), yet subductionshould continue if before collision, were approximately: Bransfield Strait extension persists.

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Onshore geology-Discussion (b) a regional low-grade thermal metamorphism and possible intrusion of the fore-arc caused by the passage beneath it of much hotter oceanic litho- Process and models sphere as the ridge crest arrives; In the case of Bransfield Strait extension, a reason- (c) the cessation or decrease of subduction-related ably convincing connection can beargued between magmatic activity as the ridge crestapproaches, subductionhistory andonshore geology. The resulting from the earlier escape of bound water apparently simple history and geometry of Cenozoic because of the higher temperature, less con- subduction along the margin of the Antarctic Peninsu- tinuoussediment cover and more fracturedand la suggests thatother connections might be equally permeablenature of the younger oceanic crust. clear and instructive. For exampleHawkes (1981), DeLong et al. (1978) suggested that the age of the drawing attention to the parallelism between oceanic ocean crust at 100 km depth could be an appropri- fracture zones and the direction of convergence at the ate cut-off indicator, and noted for the Aleutian margin, interpreted many aspects ofthe onshore arc an age of 25-30 Ma. Upward migration of hot geology in terms of a segmentation of the Peninsula, water at shallow depths could assist or extend the with segment boundaries corresponding to long-lived thermal metamorphism (b above). sites of fracture zone subduction. There is much intui- It is not my intention here to discuss extensively the tive support for this notion, for several reasons. First- many aspects of the geology of the Antarctic Peninsula ly, fracture zones separate oceanic regions of different interpreted in terms of fracture zone segmentation by age and thus depth, heat content and, probably, sedi- Hawkes (1981). In view of the improved understand- ment cover, all of which could exert some influence on ing of Pacific spreading history now available, howev- the margin during subduction. Secondly, fracture zone er, it would be useful to establish for how long the topography can exceed 2 km in amplitude; its passage parallelism between fracture zonesand subduction beneath the sameplace on a margin over a long period direction existed. This will help resolve which older, could well be significant. Thirdly, fracture zone apparently segmented features of the onshore geology topography could provide an effective barrier within could have been generated initially by this mechanism, the trench to lateral sediment transport and thus great- and which are more probably accidents of present-day ly influence the development of the accretionary differences in erosion level, produced by later uplift wedge (compare the northward transport of sediment (itself possibly resulting from ridge crest-trench from South Americaand the contrast in the Lesser interaction and thus similarly systematic). Antilles fore-arc between theBarbados Ridge and Of the predictions of the Kula Ridge- Aleutian Puerto Rico trench). model (DeLong et al. 1978), it is not clear at present A related phenomenon, of course, is the approach that onshore exposures are adequate to assess either andarrival of eachsection of the ridge crestat uplift or thermal metamorphism. A marine geophysic- thetrench. Ridgecrest-trench collision, although al survey of thebroad shelf area of the Antarctic probably a relatively common occurrence at the mar- Peninsula could well adduce a history of vertical move- gins of an ocean basin as old as the Pacific, has been ment, but does not exist. Adie (1964) noted a perva- little studied.Other recentexamples occur only off sive 'late-stage block faulting' which may result from Chile near 46"s (Herron et al. 1981), off Costa Rica, the ridge crest-trench collision, but the age of this Mexico, Indonesia,Japan and probably the South episode along the peninsula is very poorly known. The Scotia Ridge (Barker & Hill 1981). Many of these lack magmaticconsequences of themodel, however, can the simple parallelism between spreading and subduc- be assessed, by comparing the published radiometric tion found along the Antarctic Peninsula. Moreover, age data from the peninsula with the estimated sub- the principal efforts to assess the onshore geological duction history. In addition, I comment briefly below consequences stem not from these localities, but from onother aspects,notably the possibility of tectonic the mid-Cenozoicsubduction of the Kula Ridge be- erosion. Firstly, because of the emphasis on older neath the Aleutian arc (DeLong & Fox 1977; DeLong ocean crust in both the magmatic and fracture zone et al. 1978, 1979). In thislast example, subduction studies, it is necessary to estimate the regional tectonic continued after collision, but the derived model may environment some way back into the Mesozoic, using beapplied with little modification tothe Antarctic data from beyond the boundaries of Fig. 4. Peninsula,where subduction stopped.The essential characteristic of both regions is that the direction of convergence was essentially perpendicular toboth Pre-Cenozoic plate motions and boundaries trench and spreading centre.The predicted effects were: Fig. 5 is a schematic diagram of plate boundaries in (a) uplift of the margin as progressively shallower the South Pacific (a) 100 Ma and (b) 60 Ma ago. The ocean floor is subducted, followed by subsidence disposition of the Mesozoic M-anomalies in the west- after the collision; ern equatorial Pacific today (Larson 1976; Hilde et al.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/787/4887553/gsjgs.139.6.0787.pdf by guest on 02 October 2021 794 P. F. Barker 1977) shows that, by the latest Jurassic, a three-plate South American margin throughout, rather than ever system existed, involving the Pacific, Farallonand reaching the peninsula. It can be seen that the western Phoenix plates. The Phoenix and Hawaiian magnetic part of the Phoenix (Aluk) plate contained magnetic lineations have ENE-WSW and NW-SE orientations anomalies and fracture zones with the orientation of similar to those of the two Cenozoic anomaly sets in those seen now off the peninsula, but that the eastern Fig. 4, and the mapping of younger anomalies farther partcontained E-W anomaliesand N-S fracture S by Weissel et al. (1977) showed that the magnetic zones. The bight betweenthese two zones would bight between these anomalysets (and hence the migrate slowly north-eastward along the peninsular three-plate system)persisted essentially unchanged margin as subduction progressed. In the extreme east- into the late Cretaceous, through the Cretaceous nor- ern corner of the present Pacific margin, off Elephant mal magnetic polarity interval. Thus the Nazca plate is I., Anomaly 6 appears to bend around to the E (Fig. a relic of the Farallon plate, and the Aluk or Drake 2). This may bea relic of the otherwise-subducted plate the last surviving fragment of the Phoenix plate. bight between Antarctic-Aluk and Nazca-Aluk linea- Before subduction of the southernmost section of the tions. Pacific-Nazca spreading centreat the margin of S Fig. 5(b) shows a similar scheme for 60 Ma ago. At Chile (-54"s) about 13 Ma ago there would also have about 85 Ma ago (Weissel et al. 1977), the Campbell existed at the surface there thethird arm of this RRR Plateau margin of the New Zealand block began to triple junction-the Aluk-Nazca (Phoenix-Farallon) separatefrom the WesternEllsworth/Marie Byrd spreading centre,oriented roughly E-W with N-S Land block of West Antarctica. Whether or not the fracture zones. northern margin of the New Zealand block was the The analysis of intra-Gondwana magnetic anomalies site of active subductionbefore this, the boundary (Norton & Sclater 1979) has suggested that, before betweenthese two provinces (Tharp Fracture Zone, Drake Passage started to open about 30 Ma ago (Bar- Fig. 4) was the effective southernboundary of the ker & Burrell 1977), southern South America and the Antarctic Peninsula from then on, with subduction Antarctic Peninsula lay close together at the Pacific occurring tothe E but not tothe W. The West margin of Gondwana, and theSouth Atlantic Antarctic-New Zealand spreading centre became link- sedimentaryrecord (Barker, Dalziel et al. 1977) im- ed to the Pacific-Phoenix spreading centre and, just plies that they formed a possibly flexing but effectively before 65 Ma, the latter split into two slower centres, continuous continental Pacific margin, presumably a Pacific-Antarctic and an Antarctic-Phoenix (Aluk) continually subductingFarallon and Phoenix plates. ridge. Ocean floor produced onthe S flank of the The location of the Farallon-Phoenix spreading cen- former and N flank of the latter became attached to tre, also being subducted during this time, is quite the Antarctic plate, so that the Antarctic-Aluk ridge unknown, but the general southeastward migration of thenceforthmigrated towards the margin of the the triplejunction suggests that it remained atthe Antarctic Peninsula at exactly the spreading rate mea-

FIG. 5. Schematic plate motions and boundaries in the S Pacific (a) 100 Ma ago and (b) 60 Ma ago. Thin lines flanking spreading centresdelineate ocean floor formed in previous 5 Ma. T=Tharp Fracture Zone. Note that Phoenix-Antarctic (PHO-ANT) convergence direction parallels fracture zones formed by Pacific-Phoenix (PAC- PHO) or Antarctic-Phoenix (ANT-PHO) separation, butnot those formed by Farallon-Phoenix (FAR-PHO) separation.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/787/4887553/gsjgs.139.6.0787.pdf by guest on 02 October 2021 Cenozoicsubduction history of Pacific margin of Antarctic Peninsula 795 sured on the surviving anomaliesproduced on its N It seems most likely that, if the tectonic segmentation flank. The split propagated eastward with time, which he inferred is real, it results from relatively reaching the Tula Fracture Zone at about 64 Ma, and recent events at the margin and possibly in particular passing the triple junction at about 50 Ma, since when from the period between subduction of a ridge crest on it has propagatednorthward, splitting the Pacific- one side of the fracture zone, and on the other, when Nazca boundary (Weissel et al. 1977; Cande et al. contrasts at the trench and beneath the margin could 1982). beat their greatest.A real segmentation of much Almost all the ocean floor now seen off the peninsu- oldercomponents of the geology of the peninsula lar margin (Fig. 4), the colliding ridge crest sections could reasonably be attributed to Pacific margin pro- and the younger subducted ocean crust, was produced cesses in a general sense but cannot be explained in by Antarctic-Phoenix (Aluk) spreading, but itis im- similar detail. portant to remember that, during the early Cenozoic, ocean floor being subducted at the margin had been Ridge crest subduction and magma genesis produced at the much faster-spreading (110 mmiyear half rate) Pacific-Phoenix centre. A remnant of Pacific In Fig. 7A the locations of 125 published radiomet- plateproduced at this faster rate, transferred to the ric age determinations on volcanic and plutonic rocks Antarctic plate by the propagating ridge split at 50 Ma from the Antarctic Peninsula are marked. There are (Anomaly 21 time), may be seen at 603, 8&90°W, in 20 Rb-Sr determinations (circles) and one Pba deter- Fig. 4 (Cande et al. 1982). mination, and the remainder are K-Ar ages (square if The direction of subduction atthe margin of the calc-alkaline, triangle if alkaline). In Fig. 7B their ages Antarctic Peninsula is not known with certainty before are plotted against their position along the peninsula, 85 Ma.After the start of New Zealand-Antarctic together with thedetermined arrival times of ridge separation, however, the lengthening Tharp Fracture crest sections at the trench (solid line). Ages greater Zone (Fig. 5b) appearsto have provided an in- than 150 Ma are all plotted on that isochron. creasingly tight constraint onthe Phoenix (A1uk)- Gledhill et al. (1982) point to 15 Ma discrepancies Antarctic convergence direction, both before and after between K-Ar and Rb-Sr ages for samples from two the split mentioned above. plutons and suggest explanations in terms of either reheating or the delay between emplacement (Rb-Sr) and cooling/unroofing (K-Ar). If the latter explanation Fracture zones and tectonic segmentation were widely applicable then K-Ar ages on plutons (the It is now possible to establish details of the collision majority of the data in Fig. 7) would be 15 Ma too history at the margin of the Antarctic Peninsula, and young. However, at other sites (e.g. see Rex 1976) the to comment on the longevity of the subducted fracture discrepancies are within experimental error so that zones. Fig. 6 shows schematically the known periods unroofing may generally follow much more rapidly of existence and subduction of fracture zones,and upon emplacement. With the exception of these two times of ridge crest-trench collisions, together with the specific examples, therefore, Rb-Sr and K-Ar dates spreading rate history at the Antarctic-Aluk (Phoenix) are considered equally representative of emplacement plate boundary. Also drawn is the field of subduction ages. of ocean floor produced at the Farallon-Phoenix (Naz- The alkali basalts (triangles), mainly from the Seal ca-Aluk) plate boundary, the fracture zones of which Nunataks and James Ross I. areas, areneglected in this would have migrated along the trench as subduction analysis, as not being subduction-related in the same progressed, andthus not have contributed to any way as calc-alkaline rocks. With the exception of these tectonic segmentation. It is reasonably certain from an alkaline samples, the measured dates are very much examination of older anomalies in the equatorial and olderthan the ridge crest arrival times. As in the SW Pacific (Larsen 1976; Hilde et al. 1977; Weissel et assessment of fracture zone longevity, above, a com- al. 1977), that this province did not extend very far W parison of these older dateswith the ages of subducted at the Pacific margin of the peninsula, even in the Late ocean floor is subject to many uncertainties. For exam- Cretaceous, because of the relatively low Pacific-Faral- ple,the youngest measuredradiometric age in the lon spreading rates.However, because of a lack of segmentbetween theTharp and Heezenfracture data directly Nand NE of New Zealand, and the zones is about 100 Ma. At the time of the ridge crest intervention of the Cretaceous normal polarity interval arrival atthe trench (5G39 Ma ago) ocean crust at (8&108 Ma) when no magnetic lineations were pro- 100 km depth (assuming a 45" dip of the subduction duced, it is impossible to say whether the Heezen and zone) was at most about 6 Ma older. At 65 Ma, the Tula fracture zones existed at all before the ridge split Cretaceous-Tertiary boundary,the ridge crest was -65 Ma ago (Fig. 5). Their certain subduction history 100&1100 km away from the trench but, because until is correspondingly short (c30 Ma),and that of the then ocean floor on its flanks had been produced at the Anvers FractureZone, which Hawkes (1981) consi- single, very fast Pacific-Phoenix spreading centre, the dered of major importance, is shorter still (S25 Ma). age of ocean floor at 100 km depth in the subduction

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FIG. 6. Schematic diagram of estimated fracture zone longevity and behaviour off Antarctic Peninsula. For each fracture zone (located in Fig. 4) thin line shows period within which fracture zone was generated (dashed where less certain). Thick line shows period within which fracture zone was being subducted. Line joining fracture zones shows time of collision of intervening ridge crest with trench (assuming no pivoting of the ridge crest-Menard 1978). Dots show when subduction was taking place on one side of the fracture zone only. In the field above the hachured line, ocean floor being subducted had been produced at the Phoenix-Farallon (Aluk-Nazca) plate boundary, so that fracture zonesmigrated along the trench as they were subducted. Antarctic-Aluk half spreadingrates (ridge crest-trench approach rates before subduction, and half subduction rates, assuming symmetric spreading) are shown in mm/yr at top. zone was probably only about 11 Ma older. If these but 28 Ma ago was about 21 Ma old. However, it then spreading rates extend well back into the Cretaceous became younger as the subduction of slowly formed normal polarity interval (now represented only by the ocean floor began, and never became 25-30 Ma old. poorly understood area NE and N of New Zealand) Within the Heezen-B segments lie two radiometric the ridge would need to havebeen about 2600km ages of about 75 Ma which, in the circumstances, from the trench for ocean floor within the subduction probably reflect the sparse nature of activity under zone to have been 25 Ma older. It is not possible to conditions of subduction of young ocean floor, rather estimate when this might have occurred, but clearly, in than any serious disagreement with the Kula-Aleutian the case of this segment of the Peninsula, subduction model. One might expect other isolated younger ages of aconsiderable quantity of young oceaniclitho- to be found, in these and other segments. Given the sphere (possibly more than 5000 km) took place over heterogeneity of the uppermost layers of the oceanic 50 Ma without (as yet) detectablearc magmatic crust and its sediment cover, a simple and ubiquitous activity. dependence of dehydration depth upon crustal ages These conditions apply also to the Heezen-Tula and seems unlikely, even under the ideal geometric condi- other segmentsas far as midway in the B-Anvers tions pertaining here. segment, where additional uncertainties of the magne- For the more northeasterly segments, it is possible tic bight intrude. The fast generation of ocean floor at tocompute reasonablycertain estimates of age at the Pacific-Phoenix (Aluk) spreading centre continued 100 km depth only for more recent times,after the here to 50 Ma and such ocean floor would have con- migration away of the Nazca-Aluk (Farallon-Phoenix) tinued to be subducted until 30 Ma ago. However, the anomaly province. Thus, about 20 Ma ago, oceanic spreading rate (and thus subduction rate) slowed con- lithosphereat 100 km depth in the Anvers-C and siderably 50 Ma ago, to such an extentthat, until C-Hero segments was about 23 and 25 Ma old, respec- 25-30 Maago, the age of oceaniclithosphere at tively. With the present degree of understanding of SE 100 km depth in the subduction zone actually steadily Pacific magnetic anomalies, it does not seem fruitful to increased,even as the ridge crestapproached the attempt to compute Nazca-Aluk spreading rates and trench. For example, in the A-B segment, ocean floor fracturezone offsets.We may speculate, however, at 100 km depth about 50 Ma ago was only 11 Ma old, that spreading rates were lower than Antarctic-Aluk

I I I I I B

Hero 2'; **

'. \

*. ** ieezen -4 4 ** i

Tharp

8 i;9

FIG.7. Radiometric ages from theAntarctic Peninsula compared with collision history. (a) Locations of dated rocks, from Scott (1965), Grikurov et al. (1970). Halpern (1972). Rex (1972, 1976), Dalziel ef al. (1973). Rex & Baker (1973), Valencio et al. (1979), Pankhurst et al. (1980), Farrar et al. (1982), Gledhill et al. (1982) and Watts (1982). (b)Ages compared with collision history along the peninsula (solid line): numbers are estimated ages of oceanic lithosphere at 100 km depth in subduction zone at that time. Age determinations on calc-alkaline rocks marked by diamonds if K-Ar or Pba and solid circles if Rb-Sr. Determinationson alkaline rocks marked by triangles.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/787/4887553/gsjgs.139.6.0787.pdf by guest on 02 October 2021 798 P. F. Barker or Pacific-Phoenix rates and that fracture zone offsets again SE of the 4 Ma collision site. A similar maximal were generally sinistral, which would provide the older slope is found directly at the ridge crest subduction site oceaniclithosphere atthe subductionzone thatthe near 46"s off southern Chile (Herron et al. 1981). more extensivemagmatic activity implies. Off the The most plausible way that a subducting margin SouthShetland Is, the observednorthward offset of might steepen as a ridge crest approaches is by tecto- anomalies atthe fracture zones (Figs 2 and 4) also nic erosion. Off the Antarctic Peninsula generally, as contributes to this effect; when subductionstopped off southern Chile, there isno well-developed mid- 4 Ma ago, the ocean floor ages at 100 km depth be- slope break, unless it be in fact the shelf break. Thus, neaththe South Shetlandtrench were 17, 24 and we are considering most probably the tectonic erosion 30 Ma (from SW to NE), which makes a 9.5 Ma age of an accretionary wedge of the lower fore-arc. It is for a granodiorite from Cornwallis I.(Rex & Baker almost superfluous to define tectonicerosion as an 1973; Fig. 7) consistent with the model. excess of erosion over sedimentation in the fore-arc, Thus,the concept evolved in studying Aleutian yet there may be no more to this effect than a relative Arc-Kula Ridge interaction, of a diminution or cessa- dearth of sedimententering the trench. Certainly tion of arc magmatic activity when young oceanic young ocean floor is usually not thickly sedimented, lithosphere is being subducted, appears to explain in and tectonic erosion may neither be confined to nor general terms the dearth of young radiometric dates in invariably accompany the subduction of young ocean the southern partof the Antarctic Peninsula, and to be floor. Nevertheless, there may be other contributory at least compatible with the more extensive age range characteristics of young ocean floor in this particular found farther N. This correlation would fall, of course, subduction configuration. For example, the dominant if large numbers of younger ages were to emerge from asymmetric half-graben character of ocean crust futuredeterminations. The only foreseeable way in topography will not be obscured by sediment near to which the present data set could be unrepresentative is the ridge crest and, as subduction proceeds, will pre- in their distribution perpendicular to the length of the sent protected pockets for existing fore-arc sediments Peninsula. There is ageneral tendency for younger to descend into and be carried down the subduction plutons to occur closer tothe Pacific margin (Rex zone. The general dearth and uneven distribution of 1976); south of 65"S, there are no published ages on sediments will also prevent the establishment of a calc-alkaline rocks fromthe westernmostexposures. stableoverpressured decollement and, with thein- Of course, such migration of arc magmatic activity creasing buoyancy of the descending oceanfloor, toward the trench may also be explained on the Aleu- greatly increase the shear stresses on the base of the tian-Kula model. There was a general younging of the existing accretionary wedge. As the ridge crest itself slab all along the margin of the Antarctic Peninsula arrives, intrusion of the accretionary wedge may also during the Cenozoic (with certain short-lived excep- aid its subsequent subduction. Thus, the passage of a tions), and possibly also during the Late Cretaceous, ridge crest maywell be accompanied by a low or although the details are obscure. If, as a result, bound mediumgrade regional thermal metamorphism, as water was driven off at progressively shallower depths, DeLong et al. (1978) suggested, but also,compared then the sites of arc magmatic activity could have with adjacent regions, the thermally metamorphosed moved trenchward before activity ceased completely. terrane should perhaps be anomalously short of com- Flexural models of slab geometry (Furlong et al. 1982) ponents representing a young accretionary wedge. In also predict this effect. this respect, it would be interesting to examine more closely the nature of the well-known 'Middle Jurassic Tectonic erosion Unconformity' (Adie 1964; Dalziel 1982)which is widespread in the fore-arcprovinces of southern South Fig. 3 shows 3 bathymetric sections at the Pacific America and the Peninsula and is apparently characte- margin of the peninsula, AB and CD off the South rized by uplift and erosion, a metamorphic 'event' and Shetland 1s where until 4 Ma ago the ridge crest was subsequently renewed calc-alkaline activity. Could this still approaching thetrench, and EF directly at the have been a ridge crest-trench collision also? 4 Ma collision site (for locations, see Fig. 2). Ocean floor agesat the trench are c. 23, 13 and4 Ma, respectively. Together with interveningbathymetric profiles onthe other tracks in Fig. 2, these profiles Summary and conclusions showa progressive steepening of the margin as the ridge crest approaches. As already noted, after colli- A large body of marine magnetic data in western sion the surviving flank of the ridge subsides exactly as Drake Passage, mostly hitherto unpublished, shows 5 normaloceanic lithosphere, and it is reasonable to sections of a late Cenozoic spreading centre. In com- expect that the margin would behave like a passive, bination with another recently re-interpreted and ex- extensionalmargin, cooling and subsiding also. Cer- tended data set from farther W, these new magnetic tainly, the slope of the margin becomes more gradual data permita much more precise assessment of the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/6/787/4887553/gsjgs.139.6.0787.pdf by guest on 02 October 2021 Cenozoicsubduction history of PaciJic margin of Antarctic Peninsula 799 Cenozoic subduction history at the Pacific margin of DeLong et al. (1978) noted that arc magmatism was the Antarctic Peninsula. effectively cut off while ocean floor younger than 1. The most recent event along most of the margin 25-30 Ma was being subducted. A comparison of ex- was the subduction, atsuccessive fracture zone-defined isting radiometric age determinations with the subduc- segments of the margin, of lengths of the Antarctic- tion history shows, in the S of the Peninsula, a delay of Aluk ridge crest, at times ranging from 50 Ma ago at 5&60 Ma between the youngest radiometric age on a the southern endof the Peninsula to only 4 Ma ago off calc-alkaline sample and the time of collision. This is Anvers I. After each ridge section reached the trench, explained underthe Aleutian-Kula model by both spreading and subduction stopped. the very young age of ocean floor being subducted 2. Farther N spreading stopped before the ridge through much of this time, which resultedfrom the crest sections reached the South Shetland trench, also very fast spreading rates through the late Cretaceous 4 Ma ago, suggesting a causal link between the stop- andearly Tertiary atthe Pacific-Phoenix spreading page and the final collision. As a further consequence centre. The distribution of ages farther N along the of this stoppage Bransfield Strait opened, probably in Peninsula is also compatible with this model, but does response to a tendency of the already-subducted ocean not provide such a stringent test of its applicability. floor to continue sinking. Current shallow earthquake 5. In general terms the model’s mechanism, in which activity atthese surviving spreading centre sections bound wateris driven off the subducting slab at progres- and their intervening transform faults makes the pre- sively shallower depths as the slab youngs, may also sent tectonic situation obscure. explain the observed migration of arc magmatic activ- 3. Subduction parallel to the presently mapped frac- ity towards the trench. ture zones priorto ridge crest-trench collision was 6. The extremely steep margin at the site of the invoked by Hawkes (1981) to explain an apparent most recent ridge crest-trench collision suggests that tectonic segmentation of the onshore geology of the tectonicerosion of the fore-arcaccompanied the Peninsula. It has proved possible to estimate for how approach of the ridge crest. A hiatus in the geological long such parallelism hasexisted. Only theTharp record in a fore-arc terrane, particularly if accompa- Fracture Zone, which marks the southern boundary of nied by low or medium grade thermal metamorphism subduction, and thus also probably of the peninsula in of the surviving sediments, may thus mark the subduc- the broad tectonic sense, can be shown to have been tion of a ridge crest. It may bespeculated thatthe active for more than 30 Ma before collision. The sub- Middle JurassicUnconformity known from southern duction life of most of the fracture zones is closer to South America and the Peninsula (e.g. Dalziel, 1982) 15 Ma.Thus any elements of theonshore geology had this origin. which are older than the onset of parallel subduction may owe their apparent segmentation to differences in ACKNOWLEDGMENTS.I am grateful to the many people erosion level, produced during the more recent tecto- from Birmingham and Research Vessel Services, Barry, and nically segmented history of vertical movement. to the ship’s companies of RRS Shackleton, RRS Bransfield 4. The most detailedstudy of onshore geological and HMS Endurance who,over the years and often in consequences of ridge crest subduction is that associ- adverse conditions, have made possible the accumulation of the marine magnetic and bathymetric data interpreted here. I ated with the interaction of the Aleutian arc and Kula owe particular thanks to Steve Cande, for giving me access to Ridgein the mid-Cenozoic. Of themajor effects his paper on the Lamont-Doherty data from the SE Pacific in hypothesized there, the most amenable to investiga- advance of publication, and to Don Hawkes for pointing out tion along the peninsula is the dependence of arc the potential interest of the area off northern Anvers I. This magmatic activity on the age of subducted ocean floor. work has been supported by a NERC research grant.

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Received 29 October 1981; revised typescript received 17 May 1982. P. F. BARKER, Department of Geological Sciences, University of Birmingham, Birmingham, B15 2TT.

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