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Weddell Sea Tectonics and Gondwana Break-Up: an Introduction

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Weddell Sea Tectonics and Gondwana Break-Up: an Introduction Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Weddell Sea tectonics and Gondwana break-up: an introduction E. C. KING, R. A. LIVERMORE & B. C. STOREY British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK The Weddell Sea, part of the circumpolar Aim of the volume Southern Ocean, is probably the most remote, least known and least accessible sea in the world. Many of the major controversies concerned with Difficult ice conditions have limited the acqui- 're-uniting Gondwana', such as the original sition of ship data, although this has been partly position of Madagascar relative to East Africa, offset in recent years by access to satellite radar and that of India relative to Antarctica have now altimetry data. The Weddell Sea was originally been resolved to the satisfaction of most Earth defined by the Admiralty Hydrographic Depart- scientists. The major exception is the question of ment in 1932 and redefined by the Antarctic how the continental fragments surrounding the Place Names Committee in 1976 (Hattersley- Weddell Sea should be reconstructed so as to Smith 1991). It is bounded on the western side by avoid overlaps and provide a plausible break-up the east coast of the Antarctic Peninsula, on the history. The major overlap is that of the southern side by the Ronne and Filchner ice Antarctic Peninsula with the Falkland Plateau, fronts, and on the southeastern side by the which was highlighted by the earliest attempts to Dronning Maud Land and Coats Land coasts of quantify the fit of the southern continents (Smith East Antarctica (Fig. 1). The South Scotia Ridge & Hallam 1970). separates the Weddell Sea from the Scotia Sea to Far from providing a simple explanation for the north and a line joining Southern Thule in this overlap, new data have identified a series of the South Sandwich Islands and Kapp Norvegia enigmatic physical features and tended towards in Dronning Maud Land, separates it from the more complicated models, involving the move- South Atlantic Ocean to the NE. ment of several microplates. Its remoteness and Within this volume, papers relate to the harsh climatic conditions have protected the Weddell Sea as defined above, together with Weddell Sea from the type of comprehensive part of the adjoining South Atlantic Ocean up to investigations carried out in other key oceanic 50°E, and to the geology of the once neighbour- regions, so that progress towards a generally ing continents of Gondwana. The term Weddell acceptible solution is slow and characterised by Sea embayment is also used informally through- controversy. Such a solution depends upon out this volume to include the embayment area knowledge of the structure and age of the to the south of the Weddell Sea now covered by oceanic crust to the north, the stretched con- the Ronne and Filchner ice shelves, including tinental or oceanic crust to the south, and the Berkner Island, and the continental shelf north nature of the continent-ocean boundaries sur- of the Ronne and Filchner ice fronts (Figs 1 & 2). rounding the Weddell Sea. The Weddell Sea was first penetrated by The identification of marine magnetic anoma- James Weddell in the sealing brig Jane to c. lies and fracture zones provides the basis for 74°15'S, 34°17'W in 1823 and labelled Sea of most plate kinematic models, at least for the George the Fourth or King George the Fourth's major plates. Surprisingly, motions between the Sea on his charts but he later settled on the name South American, African and Antarctic plates Weddell Meer (Hattersley-Smith 1991) and that are still poorly known for times earlier than name, in its Anglicized form, now has common chron C34 (83 Ma). Improved models of plate currency. It is perhaps most well known as the separation, based upon correlation of pre- place where Sir Ernest Shackleton's ship Endur- break-up features, rifting history and marine ance was crushed in the pack ice and sank in studies, should permit the application of a test 1915. for closure of the three-plate system, to deter- From Storey, B. C., King, E. C. & Livermore, R. A. (eds), 1996, WeddellSea Tectonics and Gondwana Break-up, Geological Society Special Publication No. 108, pp. 1-10. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 2 E.C. KING ET AL. + North Scotia Ridge PL, 50~N 3o~ ~RICE Anta/c~ Ridge EV,nNG BANK i '-.... '''' ""............... .........................../ii ............. / Fig. 1. Place names around the Weddell Sea. Abbreviations are BI, Berkner Island; ST, Southern Thule; KN, Kapp Norvegia. Inset shows plates and plate boundaries in the region. mine whether the Weddell Sea has formed mainly Tectonic setting as a consequence of seafloor spreading between South America and Antarctica, or whether Weddell Sea motions of other fragments are recorded. The Weddell Sea forms part of the present-day Wider questions related to supercontinent Antarctic plate (Fig. 1, inset), most or all of the break-up are also addressed. The Weddell Sea oceanic crust within it having formed on the region was clearly affected by major magmatic southern flank of a spreading system which is events associated with, and following, rifting of now represented by the South American - southwest Gondwana (White & McKenzie 1989; Antarctic Ridge (Lawver& Dick 1983; Barker Cox 1992). Correlation of igneous events in the &Lawver 1988). Most of the lithosphere created Weddell Sea, and the regions which surrounded on the northern, South American, flank of this it prior to and during break-up, with the system has been subducted beneath the Scotia kinematic history, potentially provides an in- Sea, a process which terminated when the ridge sight into the causes of continental separation. crest interacted with the former South Scotia Of these topics this volume addresses the trench, as at Jane Bank (Fig. 2a); (Barker et al. following: 1984). The South Scotia Ridge has since evolved (1) history of seafloor spreading in the Weddell into a predominantly sinistral strike-slip bound- Sea; ary, separating the Weddell Sea from the Scotia (2) origin and nature of the Weddell Sea Sea, and forming the boundary between the embayment area: continental or oceanic? Antarctic and Scotia plates. (3) origin and nature of continent--ocean boundaries and the prominent escarpments bounding the Weddell Sea and embayment area; Geology of surrounding regions (4) kinematic history of microplates based on palaeomagnetic data; The Weddell Sea and embayment regions are (5) kinematic history of crustal shear zones bounded by three of the five main crustal blocks related to Weddell Sea evolution; or Mesozoic microplates within West Antarctica (6) Gondwana break-up processes and the (Fig. 2b); (Dalziel & Elliot 1982). These include: initial rifting history in the Weddell Sea area. on the western side, the Antarctic Peninsula, on Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 INTRODUCTION 3 _ I --4-- f..... L 'i .... ,\ ...~.-. H J ! ! i ~ Anomaly-T Andsnes Fig. 2. (a) Structural features in the Weddell Sea related to Gondwana break-up and early seafloor spreading. Fracture zone trends derived from satellite altimetry form the herring-bone pattern in the northern Weddell Sea. Polar stereographic projection. (b) Microplates of the Weddell Sea region. West Antarctic blocks, shown in white lettering, are: ANP, Antarctic Peninsula; THI, Thurston Island block; MBL, Marie Byrd Land; EWM, Ellsworth-Whitmore Mountains (block includes Haag Nunataks, HN). South American blocks, shown in black lettering, are: FIB, Falkland Islands block; FPB, Falkland Plateau Basin; MEB, Maurice Ewing Bank. The regions in black show bedrock above sea level (expect for East Antarctica where the interior basins are not differentiated). Medium grey fill extends to the 2000 m bathymetric contour, light grey fill extends to the 3000 m bathymetric contour off South America only. the southwestern side, Haag Nunataks and the ary complex on the fore-arc side (Storey & Ellsworth-Whitmore mountains, and on the Garrett 1985), and thick back- arc basin se- southeastern side, part of the East Antarctic quences on its eastern Weddell Sea side. The craton (Tingey 1991). latter include the Middle and Upper Jurassic infill of the Latady Basin to the SE (Rowley etal. Antarctic Peninsula. The Antarctic Peninsula is 1983) and Cretaceous and Tertiary sedimentary predominantly a Mesozoic magmatic arc (Pank- rocks of the Larsen Basin to the NE (Macdonald hurst 1982; Leat et al. 1995) that formed along et al. 1988; Crame et al. 1991). The peninsula was the active margin of Gondwana and West deformed by wide ductile shear zones which may Antarctica by subduction of Pacific and proto- relate to the history of the Weddell Sea region Pacific ocean floor. It contains a wide accretion- (Storey et al.). Subduction ceased by a series of Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 4 E.C. KING ETAL. collisions of the Pacific-Phoenix spreading ridge Microplates within Gondwana and break- with the trench during Tertiary times, except for up models a small currently active segment off the South Shetland Islands (Barker 1982). Gondwana reconstructions Although the fit of the major continents within Haag Nunataks. This small crustal block, situ- Gondwana is well established, there is consider- ated between the base of the Antarctic Peninsula able debate over the number and palaeoposition and the Ellsworth Mountains (Fig. 2b), is of microplates within the Weddell Sea region. A formed of Proterozoic basement possessing precise Gondwana reconstruction is an import- Grenvillian Rb-Sr whole-rock ages between ant starting point in trying to understand 1176 and 1003Ma (Millar & Pankhurst 1987). break-up mechanisms and processes, and the Aeromagnetic surveys have revealed the extent history of the Weddell Sea region.
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