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Late Cretaceous to Early Tertiary Subduction History of the Antarctic Peninsula

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Late Cretaceous to Early Tertiary Subduction History of the Antarctic Peninsula 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. 255 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/2/255/4886485/gsjgs.155.2.0255.pdf by guest on 25 September 2021 256 J. J. McCARRON & R. D. LARTER 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/2/255/4886485/gsjgs.155.2.0255.pdf by guest on 25 September 2021 ANTARCTIC PENINSULA SUBDUCTION HISTORY 257 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.
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