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High-Resolution Animated Tectonic Reconstruction of the South Pacific and West Antarctic Margin

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High-Resolution Animated Tectonic Reconstruction of the South Pacific and West Antarctic Margin Article Geochemistry 3 Volume 5, Number 7 Geophysics 10 July 2004 Q07002, doi:10.1029/2003GC000657 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society High-resolution animated tectonic reconstruction of the South Pacific and West Antarctic Margin Graeme Eagles and Karsten Gohl Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, D-27515 Bremerhaven, Germany ([email protected]) Robert D. Larter British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK [1] An animated reconstruction shows South Pacific plate kinematics between 90 and 45 Ma, using the satellite-derived gravity anomaly field, interpolated isochrons and plate rotation parameters from both published and new work on marine geophysical data. The Great South Basin and Bounty Trough, New Zealand, are shown as the earliest Pacific–Antarctic plate boundary that opened before 83 Ma. The earliest true Pacific–Antarctic seafloor formed within the eastern parts of this boundary, but later and farther west, seafloor formed within its Antarctic flank. After 80 Ma, the Bellingshausen plate converged with an oceanic part of the Antarctic plate to its east, while its motion simultaneously caused rifting in continental Antarctica to the south. The Pacific–Bellingshausen spreading center developed a set of long offset transform faults that the Pacific–Antarctic plate boundary inherited around chron C27 when the Bellingshausen plate ceased to move independently as part of a Pacific- wide plate tectonic reorganization event. Southwest of these transforms the Pacific–Antarctic Ridge saw an increase in transform-fault segmentation by 58 Ma. One of the long offset Pacific– Bellingshausen transforms, referred to as ‘‘V,’’ was modified during the C27 reorganization event when a Pacific–Antarctic–Phoenix triple junction initiated on its southern edge. Eastern parts of ‘‘V’’ started to operate in the Pacific–Phoenix spreading system, lengthening it even more, while its western parts operated in the Pacific–Antarctic system. This complicated feature was by-passed and deactivated by ridge axis propagation to its northwest at 47 Ma. We interpret our animation to highlight possible connections between these events. Components: 11,268 words, 11 figures, 1 table, 1 animation. Keywords: plate tectonics; plate reconstructions; Pacific Ocean; Antarctica. Index Terms: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 8157 Tectonophysics: Plate motions—past (3040); 9310 Information Related to Geographic Region: Antarctica. Received 30 October 2003; Revised 23 March 2004; Accepted 5 May 2004; Published 10 July 2004. Eagles, G., K. Gohl, and R. D. Larter (2004), High-resolution animated tectonic reconstruction of the South Pacific and West Antarctic Margin, Geochem. Geophys. Geosyst., 5, Q07002, doi:10.1029/2003GC000657. 1. Introduction describe the tectonic history of an important region for global tectonics: the South Pacific [2] In this paper, we present an animated and its West Antarctic margin. The animation reconstruction of gridded, satellite-derived displays events that are described in the liter- free-air gravity data, and use it as a tool to ature, as well as some new interpretations, and Copyright 2004 by the American Geophysical Union 1 of 21 Geochemistry 3 eagles et al.: animated tectonic reconstruction Geophysics 10.1029/2003GC000657 Geosystems G it gives a self-consistent view of the region’s ent animated reconstructions of large volumes of tectonics. vector data, many digitized from grids, that successfully illustrate global and more detailed regional tectonic models. Gaina et al. [1998a, 1.1. Gridded and Animated 1998b] (http://www.es.usyd.edu.au/geology/ Reconstructions people/staff/dietmar/Movies/tasman.html) present an animated reconstruction of gridded satellite- [3] Detailed data sets of free-air gravity anoma- lies, derived from satellite altimetry over the derived gravity data for the Tasman Sea region. world’s oceans [e.g., McAdoo and Laxon, 1997; Vector animations have the advantage of lower Sandwell and Smith, 1997] (Figure 1), illustrate data volumes, and therefore lower data transfer in detail the plate tectonic fabric of the ocean rates are required to view them, but they suffer floor, including the fracture zones (FZs) formed some of the same drawbacks as static line at transform faults (TFs), and the traces of triple drawing reconstructions. In particular, the work junction (TJ) migration. Tectonic histories associated with digitizing large numbers of fea- derived from such gridded data are most directly tures is time consuming and unavoidably and meaningfully illustrated by reconstruction involves interpretations and selections that can of the data themselves. Such ‘‘state of the art’’ never fully be documented by the human digi- reconstructions have already been produced tizer. With ongoing increases in computer speed, for the South Pacific [Marks and Stock, 1997; reconstruction and animation of gridded data McAdoo and Laxon, 1997]. Figure 2 shows that becomes both feasible and desirable, and can a static reconstruction using gridded gravity data be achieved with a similar or smaller amount can have the following advantages over a static of human effort. line drawing (vector) reconstruction: (1) there is a massive increase in the number of reconstructed 1.2. Tectonic Introduction features, (2) the reconstruction of gridded data is [6] The West Antarctic continent (Figure 1) hosts largely free of selection pressure because an important example of a continental margin that the choice of ‘‘significant’’ features is confined has changed from an active to a passive setting, to the identification of often already well- and is a crucial yet poorly known link in the Late constrained ancient plate boundaries within Cretaceous and Tertiary global plate circuit. The which all data are reconstructed, (3) gridded early to mid-Cretaceous proto-Pacific margin of reconstructions explicitly show areas of underlap Gondwana was a subduction zone, until Chatham (compare the Phoenix plate in the two parts of Rise started to separate from it late within the Figure 2; the gray underlap area in the gridded Cretaceous Normal Polarity Superchron (CNS; reconstruction clearly shows its destruction at the 118–83 Ma). Subsequently, Campbell Plateau West Antarctic margin subduction zone, which is also separated from the Antarctic margin during not obvious from the vector reconstruction), and the reversed part of chron C33 (83–79.1 Ma), (4) reconstructed gridded data are amenable leaving a rifted continental margin bordering the for further quantitative analysis, already in Amundsen and westernmost Bellingshausen seas paleocoordinates. (Figure 1) [Larter et al., 2002]. A spreading center, that was in morphological continuity with the [4] Animated reconstructions are powerful and valuable tools that give viewers a rapid Pacific–Antarctic ridge in the eastern Amundsen grasp of a tectonic history with which they Sea, separated a small plate, known as the Belling- may be unfamiliar, and a full appreciation of shausen plate [Stock and Molnar, 1987; Stock et al., the continuity of plate motion models. They 1996], from the Pacific plate until its incorporation also permit an appreciation of long time into the Antarctic plate around chron C27 (61 Ma) series geological data in their paleogeograph- [Cande et al., 1995]. The ‘‘Bellingshausen’’ parts of ical context. Like snapshot reconstructions, the ridge are thought to have been the first to form, animated reconstructions can use vector or before chron C34y (83 Ma), although the Belling- gridded data. shausen plate probably did not come into existence until shortly afterward [Eagles et al., 2004]. The [5] The PLATES group (http://www.ig.utexas. Bellingshausen plate had slow convergent and di- edu/research/projects/plates/plates.htm#recons), vergent margins with Antarctica [Heinemann et al., Reeves and Sahu [1999] (http://kartoweb.itc.nl/ 1999; Cunningham et al., 2002; Larter et al., 2002]. gondwana/), and Hall [2002], for example, pres- Further east, the plate facing the West Antarctic 2of21 Geochemistry 3 eagles et al.: animated tectonic reconstruction Geophysics 10.1029/2003GC000657 Geosystems G margin was the Phoenix (also called Aluk, or Drake) McCarron and Larter, 1998; Larter and Barker, plate. The Phoenix plate’s divergent boundaries with 1991], but its southeastern boundary was continu- the Pacific, Bellingshausen and Antarctic plates ously a subduction zone where it was being over- have changed over time [Cande et al., 1982, 1995; ridden by the Antarctic Peninsula. The Phoenix Figure 1 3of21 Geochemistry 3 eagles et al.: animated tectonic reconstruction Geophysics 10.1029/2003GC000657 Geosystems G Figure 2. (a) Snapshot line reconstruction of the West Antarctic margin at chron C34y, from Mayes et al. [1990]. The label for an Aluk plate has been changed to PHO (Phoenix plate) and that for the Bellingshausen plate has been removed to avoid confusion. (b) Equivalent snapshot gridded free-air gravity reconstruction, with data from the BEDMAP compilation [Lythe et al., 2000] included onshore Antarctica. Graticule interval: 10°. Mercator projection. plate’s later history has seen most of it lost as seg- plate [Larter and Barker, 1991; Livermore et al., ments of the Antarctic–Phoenix ridge successively 2000]. converged with the Antarctic Peninsula from south- west to northeast. This continued until around chron
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