18
Journal of Geodynamics 53 (2012) 43–60 Contents lists available at ScienceDirect Journal of Geodynamics j ourna l ho me page: http://www.elsevier.com/locate/jog Geodynamic reconstructions of the South America–Antarctica plate system Christian Vérard , Kennet Flores, Gérard Stampfli Institut de Géologie et Paléontologie, Université de Lausanne, Quartier Dorigny, 1015 Lausanne, Switzerland a r t i c l e i n f o Article history: Received 23 March 2011 Received in revised form 20 July 2011 Accepted 27 July 2011 Available online 13 September 2011 Keywords: Scotia Sea Sandwich Island Sea Patagonia Antarctic Peninsula Powell Basin Drake Passage Plate tectonics Geodynamic reconstructions Slab window a b s t r a c t The South America–Antarctica plate system shows many oceanic accretionary systems and subduction zones that initiated and then stopped. To better apprehend the evolution of the system, geodynamic reconstructions (global) have been created from Jurassic (165 Ma) to present, following the techniques used at the University of Lausanne. However, additional synthetic magnetic anomalies were used to refine the geodynamics between 33 Ma and present. The reconstructions show the break up of Gondwana with oceanisation between South America (SAM) and Antarctica (ANT), together with the break off of ‘Andean’ geodynamical units (GDUs). We propose that oceanisation occurs also east and south of the Scotian GDUs. Andean GDUs collide with other GDUs crossing the Pacific. The west coast of SAM and ANT undergo a subsequent collision with all those GDUs between 103 Ma and 84 Ma, and the Antarctic Peninsula also collides with Tierra del Fuego. The SAM–ANT plate boundary experienced a series of extension and shortening with large strike-slip component, culmi- nating with intra-oceanic subduction leading to the presence of the ‘V-’ and ‘T-’ anomalies in the Weddell Sea. From 84 Ma, a transpressive collision takes place in the Scotia region, with active margin to the east. As subduction propagates northwards into an old and dense oceanic crust, slab roll-back initiates, giving rise to the western Scotia Sea and the Powell Basin opening. The Drake Passage opens. As the Scotian GDUs migrate eastwards, there is enough space for them to spread and allow a north–south divergence with a spreading axis acting simultaneously with the western Scotia ridge. Discovery Bank stops the migration of South Orkney and ‘collides with’ the SAM–ANT spreading axis, while the northern Scotian GDUs are blocked against the Falkland Plateau and the North-East Georgia Rise. The western and central Scotia and the Powell Basin spreading axes must cease, and the ridge jumps to create the South Sandwich Islands Sea. The Tierra del Fuego–Patagonia region has always experienced mid-oceanic ridge subduction since 84 Ma. Slab window location is also presented (57–0 Ma), because of its important implication for heat flux and magmatism. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction The South America–Antarctica plate system (Fig. 1) shows many peculiar features in terms of plate tectonics. Several excellent works have been carried out in the region (e.g. Hill and Barker, 1980; Larter and Barker, 1991; Storey et al., 1996, and references therein; Coren et al., 1997; Maldonado et al., 1998; Barker, 2001; Ghidella et al., 2002; Eagles and Livermore, 2002; Eagles, 2003, 2010a,b; Livermore et al., 2005; Eagles et al., 2005, 2006; Lagabrielle et al., 2009; Breitsprecher and Thorkelson, 2009), but in general for rel- atively specific zones and limited time intervals. At larger scale, the Scotia Sea realm is often left with many question marks or not even shown in figures. The present paper aims to integrate and syn- thesise all data of geodynamical interest, and to replace them in a general framework, where plate driving forces are derived from our Corresponding author. Tel.: +41 21 692 4360; fax: +41 21 692 4305. E-mail address: [email protected] (C. Vérard). full global geodynamical model (see below). Several key questions, in particular, remain open: Has subduction under the Scotia Sea realm initiated while a buoyant spreading ridge was subducting as seems to be the case from the present-day geography of the South Sandwich Islands Arc? Why two spreading systems (one NW–SE and the second N–S) are active together before ceasing and jump- ing to open the South Sandwich Islands Sea? Why has the Powell Basin opened and then stopped? Can we say more on the expected nature of the Weddell Sea crust, and in particular on the pres- ence of the ‘Weddell Sea Magnetically Quiet Zone’? In the Pacific Ocean side, can we reconstruct more in details the subduction of the Nasca–Antarctica–Phoenix triple junction? What does it imply in terms of subducting slab and slab window location? How and why the Antarctica active margin turned into a passive margin? In other words, how an ongoing subduction zone can be stopped? For this purpose, we propose a series of geodynamical recon- structions depicting the evolution of plate tectonics related to the South America (SAM)–Antarctica (ANT) plate system from the Gondwana break-up (165 Ma) to the present-day, with emphasis 0264-3707/$ see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2011.07.007

Geodynamic reconstructions of the South America–Antarctica plate system

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Page 1: Geodynamic reconstructions of the South America–Antarctica plate system

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Journal of Geodynamics 53 (2012) 43– 60

Contents lists available at ScienceDirect

Journal of Geodynamics

j ourna l ho me page: ht tp : / /www.e lsev ier .com/ locate / jog

eodynamic reconstructions of the South America–Antarctica plate system

hristian Vérard ∗, Kennet Flores, Gérard Stampflinstitut de Géologie et Paléontologie, Université de Lausanne, Quartier Dorigny, 1015 Lausanne, Switzerland

r t i c l e i n f o

rticle history:eceived 23 March 2011eceived in revised form 20 July 2011ccepted 27 July 2011vailable online 13 September 2011

eywords:cotia Seaandwich Island Seaatagoniantarctic Peninsulaowell Basinrake Passagelate tectonicseodynamic reconstructionslab window

a b s t r a c t

The South America–Antarctica plate system shows many oceanic accretionary systems and subductionzones that initiated and then stopped. To better apprehend the evolution of the system, geodynamicreconstructions (global) have been created from Jurassic (165 Ma) to present, following the techniquesused at the University of Lausanne. However, additional synthetic magnetic anomalies were used to refinethe geodynamics between 33 Ma and present.

The reconstructions show the break up of Gondwana with oceanisation between South America (SAM)and Antarctica (ANT), together with the break off of ‘Andean’ geodynamical units (GDUs). We proposethat oceanisation occurs also east and south of the Scotian GDUs. Andean GDUs collide with other GDUscrossing the Pacific. The west coast of SAM and ANT undergo a subsequent collision with all those GDUsbetween 103 Ma and 84 Ma, and the Antarctic Peninsula also collides with Tierra del Fuego. The SAM–ANTplate boundary experienced a series of extension and shortening with large strike-slip component, culmi-nating with intra-oceanic subduction leading to the presence of the ‘V-’ and ‘T-’ anomalies in the WeddellSea. From 84 Ma, a transpressive collision takes place in the Scotia region, with active margin to the east.As subduction propagates northwards into an old and dense oceanic crust, slab roll-back initiates, givingrise to the western Scotia Sea and the Powell Basin opening. The Drake Passage opens. As the Scotian GDUsmigrate eastwards, there is enough space for them to spread and allow a north–south divergence with aspreading axis acting simultaneously with the western Scotia ridge. Discovery Bank stops the migration

of South Orkney and ‘collides with’ the SAM–ANT spreading axis, while the northern Scotian GDUs areblocked against the Falkland Plateau and the North-East Georgia Rise. The western and central Scotia andthe Powell Basin spreading axes must cease, and the ridge jumps to create the South Sandwich IslandsSea. The Tierra del Fuego–Patagonia region has always experienced mid-oceanic ridge subduction since84 Ma. Slab window location is also presented (57–0 Ma), because of its important implication for heatflux and magmatism.

. Introduction

The South America–Antarctica plate system (Fig. 1) shows manyeculiar features in terms of plate tectonics. Several excellent worksave been carried out in the region (e.g. Hill and Barker, 1980;arter and Barker, 1991; Storey et al., 1996, and references therein;oren et al., 1997; Maldonado et al., 1998; Barker, 2001; Ghidellat al., 2002; Eagles and Livermore, 2002; Eagles, 2003, 2010a,b;ivermore et al., 2005; Eagles et al., 2005, 2006; Lagabrielle et al.,009; Breitsprecher and Thorkelson, 2009), but in general for rel-tively specific zones and limited time intervals. At larger scale,he Scotia Sea realm is often left with many question marks or not

ven shown in figures. The present paper aims to integrate and syn-hesise all data of geodynamical interest, and to replace them in aeneral framework, where plate driving forces are derived from our

∗ Corresponding author. Tel.: +41 21 692 4360; fax: +41 21 692 4305.E-mail address: [email protected] (C. Vérard).

264-3707/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.jog.2011.07.007

© 2011 Elsevier Ltd. All rights reserved.

full global geodynamical model (see below). Several key questions,in particular, remain open: Has subduction under the Scotia Searealm initiated while a buoyant spreading ridge was subducting asseems to be the case from the present-day geography of the SouthSandwich Islands Arc? Why two spreading systems (one NW–SEand the second N–S) are active together before ceasing and jump-ing to open the South Sandwich Islands Sea? Why has the PowellBasin opened and then stopped? Can we say more on the expectednature of the Weddell Sea crust, and in particular on the pres-ence of the ‘Weddell Sea Magnetically Quiet Zone’? In the PacificOcean side, can we reconstruct more in details the subduction ofthe Nasca–Antarctica–Phoenix triple junction? What does it implyin terms of subducting slab and slab window location? How andwhy the Antarctica active margin turned into a passive margin? Inother words, how an ongoing subduction zone can be stopped?

For this purpose, we propose a series of geodynamical recon-structions depicting the evolution of plate tectonics related tothe South America (SAM)–Antarctica (ANT) plate system from theGondwana break-up (165 Ma) to the present-day, with emphasis

Page 2: Geodynamic reconstructions of the South America–Antarctica plate system

44 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

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ig. 1. Age of the ocean floor for the South America–Antarctica plate system from MOCO., Coco; NAZ., Nazca; ANT., Antarctica (PH., Phoenix, now belonging to the Anta

rom 33 Ma to 0 Ma. The model presents reconstructions integrat-ng data from the entire globe and, in particular, from the Atlanticnd the Pacific realms. Note that the model is actually global over00 Ma, and the present paper focuses on the ‘Scotia realm’; theeodynamical evolution of the Pacific and Andean terranes, in par-icular, is shown for the general framework, and will be furtheriscussed elsewhere (Flores et al., in preparation).

. Method

The techniques and definitions used to create the model wereartly presented in Stampfli and Borel (2002, 2004) and Hochard2008), and will be soon further detailed in Hochard et al. (in prepa-ation). However, key criteria are:

1) All tectonic plates must have closed boundaries, following thedynamic plate boundaries approach (Stampfli and Borel, 2002).Synthetic ocean spreading axes, in particular, are reconstructedfor time tA, and used as isochrons (or ‘synthetic magneticanomalies’) at time tB. Where available, tectonic plates are posi-tioned relative to one another using magnetic anomalies asdefined by Müller et al. (1997, 2008). Isochrons (synthetic ornot) are important features of the reconstructions since theyconstrain plate geometries, and therefore, plate kinematics.Plate velocity is defined as ‘acceptable’ up to 22 cm/yr (whichcorrespond to the present-day rotation rate of the Rivera plate;1.9781◦/Ma or equatorial velocity of 21.99 cm/yr after DeMetset al., 1994).

2) Buoyant crustal fragments are defined as not deformable geo-dynamical units (GDUs). Note that GDUs are defined in thepresent-day configuration. Consequently, geological bending,stretching or shortening is not corrected within a GDU, but tight(untight) fits are used not to underestimate crustal extension(shortening).

3) Differences between continental and oceanic region are relatedto the nature of the crust and not shore lines associated withsea-level. Present-day coast lines, of course, are shown as geo-

graphical references only.

4) Reconstructions are global, and are made in a Europe fixedreference frame, using all possible data of geodynamical inter-est, which have been compiled in the PaleoDyn Database (see

et al. (1997); poster cut off after Sloss (1996). Intervening plates are: PAC., Pacific; plate); SCO., Scotia; SSI., South Sandwich Islands; SAM., South America; AF., Africa.

Hochard, 2008). The model comprises 48 reconstructions over600 Ma every 5–20 Ma.

However, Müller et al.’s dataset includes anomalies of 10 Ma,20 Ma and 33 Ma ages. At 33 Ma, the Scotia Sea was not opened, andat 10 Ma, the spreading axis system has already jumped in the SouthSandwich Islands Sea (Fig. 2). In other words, the time resolution istoo low to satisfactorily apprehend the evolution of the region.

Therefore, new synthetic magnetic anomalies have been definedin order to create 10 additional reconstructions. These are ‘new’ inthe sense they are drawn as “continuous” isochrons at global scale(not just sparse segments). The technique employed is differentfrom that of Müller et al. (1997, 2008; based on the Hellinger’s code;Hellinger, 1981) and uses additional data compiled world wide bythe IHS company (IHS Energy Consulting, personal communication,2008). The IHS dataset consists of segments described in the liter-ature, most of which have assigned ages but not all, in particular inthe Scotia Sea (Fig. 3).

To illustrate the technique employed, let us define an arbitrary15 Ma old magnetic anomaly (An-15) (see sketches in Fig. 4). Mülleret al.’s magnetic anomalies An-10 and An-20 (10 Ma and 20 Ma,respectively) are connected to define a “polygon” (drawn on asphere) (Fig. 4a and b). An-20 is rotated many times until the areabetween An-10 and An-20 is minimum (Fig. 4c). The Euler pole andangle are then computed. Where no additional datum is available,the new anomaly An-15 is drawn as the average line between therotated An-10 and rotated An-20, with the same Euler pole and, inthis example, half the angle. Everywhere else, the line is fitted to theIHS segments (Fig. 4d). The same work is carried out on the plate onthe other flank of the ridge, and a second An-15 is defined. Becauseof the additional IHS data, the two new An-15 (say, on plate A andplate B) are different (not the same length, not the same shape, andnot the same number of nodes). Therefore, the algorithm deter-mining the minimum area between the two An-15 is used againto obtain the final Euler pole and angle. The remaining area maybe viewed as the “goodness-to-fit” and documents the uncertaintyabout the definition of the Euler pole (all calculation done on asphere).

Thereby, the following isochrons were defined at global scale:the present-day configuration, configuration at chron C2 with anage taken as 2 Ma, C2A (3 Ma), C3 (5 Ma), C3A (6 Ma), C4 (8 Ma),C5 (10 Ma) = Müller et al.’s An-10, C5A (13 Ma), C5C (16 Ma), C6

Page 3: Geodynamic reconstructions of the South America–Antarctica plate system

C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 45

Fig. 2. Present-day reconstruction with geodynamical units (GDUs) with magneticanomalies of Müller et al. (2008). Written in grey are the names of most interveningGDUs. However, the paper focuses on GDUs with the following abbreviations: AB,Aurora Bank; BB, Bruce Bank; BwB, Burdwood Bank; DB, Discovery Bank; DvB, DavisBank; DvBs, Dove Basin; HB, Herdman Bank; JBk, Jane Bank; JBs, Jane Basin; PB,Pirie Bank; PtBs, Protector Basin; PwBs, Powell Basin; SG, South Georgia; SO, SouthOrkney; SR, Shag Rocks; SSI, South Sandwich Islands; TR, Terror Rise. Also, An.V andAn.T: ‘V-’ and ‘T-’magnetic anomalies in the Weddel Sea; Flk Is, Falkland Islands; FlkPt, Falkland Plateau; MFft, Magallanes–Fagnano Fault; IOR, Islas Orcadas Rise; NEGR,North-East Georgia Rise; ESS, East Scotia Sea; ESSR, East Scotia Sea Rise; WSS, WestScotia Sea. Written in green are names of cratons. Orthogonal projection centredol

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Fig. 3. Purple lines correspond to magnetic anomalies (isochrons) as defined byMüller et al. (1997, 2008). All other segments correspond to magnetic anomalies(chrons) from the IHS dataset (personal communication) compiled worldwide fromthe literature. In grey, non-dated chrons; in orange, dated chrons not used in thisstudy (>33 Ma); in red, used dated chrons (≤33 Ma). Orthogonal projection, centredon the Falkland Islands. (For interpretation of the references to color in this figure

n the Falkland Islands. (For interpretation of the references to color in this figureegend, the reader is referred to the web version of the article.)

20 Ma) = Müller et al.’s An-20, C7 (24 Ma), C8 (26 Ma), C9 (27 Ma),10 (29 Ma) and C12–13 (33 Ma) = Müller et al.’s An-33.

The proposed reconstructions (below) are presented in twoarts: (1) reconstructions from 165 Ma to 33 Ma, every 10–20 Ma

see Hochard, 2008; Flores, 2009); and (2) reconstructions from3 Ma to present, every 2–3 Ma, which includes world wide IHSdditional magnetic anomalies.

legend, the reader is referred to the web version of the article.)

3. Geological and geophysical inputs

One difficulty of creating geodynamical reconstructions is thedelimitation of GDUs. For South America and Antarctica, GDUs arelargely delimited on the basis of geological information and, inparticular, the occurrence of ophiolites and high pressure meta-morphic rocks. For the Scotia Sea region, topographic/bathymetric(ETOPO1; Amante and Eakins, 2009), gravity (GRACE data; Tapleyet al., 2005) and magnetic (EMAG2; Maus et al., 2009) data wereused. However, the very nature of some GDUs – oceanic or conti-nental – remains unclear. Following in particular Barker (2001),Maldonado et al. (2006) and Galindo-Zaldívar et al. (2006), weassume (see Fig. 2) continental basement for GDUs in the Northand South Scotia ridges (Burdwood Bank, Davies Bank, Aurora Bank,Shag Rocks, South Georgia, and Clarence Islands, South OrkneyIslands) and also for Terror Rise, Pirie Bank, Bruce Bank, and Dis-covery Bank. Those last are separated by oceanised Protector Basin,Dove Basin and Scan Basin. Oceanised crust, however, is provedfor the Protector Basin (Galindo-Zaldívar et al., 2006; Eagles et al.,2006), Dove Basin (Eagles et al., 2006), Jane Basin (Maldonado et al.,2006) and Powell Basin (Coren et al., 1997; Eagles and Livermore,2002), in addition to the East, Central and West Scotia seas.

Although the presence of continental fragments cannot be ruledout, we consider Jane Bank, Herdman Bank and East Scotia Sea Ridgeas remnant of intra-oceanic volcanic arc, and the South SandwichIslands as the active intra-oceanic volcanic arc, built on basementsof oceanic nature.

Key geological information for the long term evolution(165–033 Ma) is compiled in a database (the PaleoDyn Database;see Hochard (2008) – now belonging to Neftex Petroleum Con-sulting Ltd.), and consists largely on the location and timing of

magmatism and sedimentation associated with rifting, subduction,obduction, collision and post-collision processes.
Page 4: Geodynamic reconstructions of the South America–Antarctica plate system

46 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

Fig. 4. Sketches showing the technique employed in this study to define new synthetic magnetic anomaly. The example shows the definition of an arbitrary anomaly A-15f averaw n’, in p

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rom anomalies A-10 and A-20 as given by Müller et al. (2008). Anomaly A-15 is anhere possible. Note that all calculations are carried out on the sphere (the ‘polygo

. Reconstructions

.1. First part (165 Ma–033 Ma)

.1.1. 165 Ma (Fig. 5a)An initial break occurs within Gondwana. Contrary to our pre-

ious reconstructions (e.g. Stampfli and Borel, 2002), a branch ofhe break is represented south of the “Scotian” GDUs. According to

agmatic events (in particular, peraluminous granitoids dated at64.1 ± 1.7 Ma after Mukasa and Dalziel, 1996), a rifting phase is

nitiated along the western coast of South America (e.g. Stern andeWitt, 2003; see also Flores et al., in preparation, for details). The

egion west of Gondwana is, herein, left blank.

.1.2. 155 Ma (Fig. 5b)Gondwana breaks up. The Scotian GDUs are attached to West-

rn Gondwana. Andean GDUs (namely Darwin, North and Southuegan, West Chilenia, Constitucion, and Arequipa; Flores et al., inreparation) break off and a spreading ridge is initiated, creatinghe Rocas Verdes Ocean (e.g. Stern and DeWitt, 2003, and refer-nces therein). However, we assume that the ophiolites found inouth Georgia (e.g. Storey et al., 1977; Mukasa and Dalziel, 1996)re not directly linked to the Rocas Verdes Ocean, but to the earlypening of the South America–Antarctica Ocean (see below).

.1.3. 142 Ma (Fig. 5c)The Andean GDUs drift away from Western Gondwana, and

ceanic accretion occurs NW and SW of Madagascar. The riftingf India has started.

The dextral movement between Eastern and Western Gond-ana is accommodated by a pair of transform faults located north

nd south of the Scotian GDUs. Such ‘decoupling’ allows to movehe Scotian GDUs from a ‘Gondwana fit’ to a ‘pre-Scotia Sea open-ng fit’, deduced from magnetic anomalies in the Scotia Sea (seeeconstruction at 33 Ma, below). As a result, oceanisation occurred

ast and south of the Scotian GDUs as evidenced by the Southeorgia ophiolites dated at 150 ± 1 Ma (Mukasa and Dalziel, 1996).ote that the Scotian GDUs are left against the Falkland Plateau toccount for geological continuity, the absence of arc-related rocks

ge line of A-10 and A-20 where no other data is available, and fit the IHS segmentsarticular, is actually a ‘spherical polygon’).

along the Falkland Plateau, and for geometrical (space) considera-tion (see discussion in Eagles, 2010a,b, and references therein foralternative point-of-view).

4.1.4. 131 Ma (Fig. 5d)The Andean GDUs keep on drifting away whereas other GDUs

crossing the Pacific (namely Chonos, Gonzalo, Diego de Almagro,and Smith-Elephant, South Scotia Ridge, Alexander, Bellinghausen,and Maher, among others; Flores et al., in preparation) migrate tothe east as a result of intra-oceanic subduction. Those GDUs mustoriginate from the ‘Pacific magnetic triangular zone’ (see Mülleret al., 2008; Flores et al., in preparation). Since part of the cross-Pacific GDUs have reached a velocity close to the limit that wedefine as ‘acceptable’ in the model (∼20 cm/yr), the collision (seebelow) has to occur off-shore SAM.

Oceanic accretion keeps on acting between Eastern and WesternGondwana. Rifting takes place between SAM and Africa, with therise of the Paraná–Etendeka trapps (e.g. Peate, 1997) and furthermagmatic activity along the future margins of the two continents.South American rift basins are probably coeval with this extension.We tentatively represent at 131 Ma the N–S to NW–SE orientedbasin (namely the Austral, Malvinas, Malvinas Plateau, Canadon-Asfalto, Peninsula de Valdes, Rawson, and Claromeco basins), andthe more E–W oriented ones at 121 Ma (San Jorge, Colorado, andSaldano basins) (see in particular Milani and Thomaz-Filho, 2000).

4.1.5. 121 Ma (Fig. 5e)The cross-Pacific GDUs and Andean GDUs collide (e.g. Thomson

and Hervé, 2002; Hervé and Fanning, 2003; Willner et al., 2004;Flores et al., in preparation). The width of the Rocas Verdes Oceanis governed by the fact that the cross-Pacific GDUs have reachedtheir maximum drifting speed. Thus, the width of the ocean is upto 3000 km, id est maximum between the Darwin and PatagoniaGDUs.

The collision is diachronous from north to south. Consequently,

the eastern passive margin of the Andean GDUs inverts and theRocas Verdes Ocean now subducts. The sense of subduction is con-strained by arc magmatism in the Andean GDUs, and the fact it hasnever been documented on the South American flank at that age.
Page 5: Geodynamic reconstructions of the South America–Antarctica plate system

f Geod

tihb

Ff

C. Vérard et al. / Journal o

The plate boundary between SAM and ANT is localised south of

he Scotian GDUs and is transtensive, meaning that the spread-ng ridge shows many transform segments. Such geometry willave important implications later (see the reconstruction at 84 Ma,elow).

ig. 5. (a–d) Geodynamical reconstructions from 165 Ma to 131 Ma. Grey: continental partrom 121 to 095 Ma. (i–l) Geodynamical reconstructions from 084 to 048 Ma. (m and n) G

ynamics 53 (2012) 43– 60 47

4.1.6. 112 Ma (Fig. 5f)

India rifts off Madagascar. The spreading system between Africa

and Madagascar ceased, as oceanic accretion localised betweenMadagascar and India, probably in relation with the Kerguelen hotspot volcanism.

s; darker grey: cratons. Orthogonal projection. (e–h) Geodynamical reconstructionseodynamical reconstructions from 040 to 033 Ma (abbreviations as per Fig. 2).

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48 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

Fig. 5. (Continued )

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C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 49

Fig. 5. (Continued )

Page 8: Geodynamic reconstructions of the South America–Antarctica plate system

50 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

(Cont

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Fig. 5.

The plate boundary between SAM and ANT is still transten-ive. Shortening occurs between the south and north Rocas Verdescean, triggering transpressive intra-oceanic subduction.

.1.7. 103 Ma (Fig. 5g)The 103 Ma and the following 95 Ma old reconstructions are

nterpolated between the 121 and 84 Ma old reconstructions,ecause of the Cretaceous Normal Superchron.

The eastward migration of some GDUs leads to a collision form-ng the Antarctic Peninsula. The north Rocas Verdes Ocean is stillpened since evidence for collision in the South Andes are youngere.g. Kellogg and Rowley, 1991; Trouw et al., 1998, 2000). The col-ision provokes the formation of a new ridge and new subductionone along Antarctica, re-delimiting the Phoenix plate. SAM andNT diverge with a strong strike-slip component, and plume headseach the surface and form the North Georgia Rise, and, at least, partf the Astrid Ridge, Explora Wedge and Maud Rise. Note that we

otentially link proposed older ages (e.g. Jokat et al., 2003, amongthers) with reminiscence of the Karoo–Ferrar magmatism; lavaows, however, cannot be older than the age of the oceanic crusthat supports them (i.e. around 103 Ma in our reconstruction).

inued ).

4.1.8. 095 Ma (Fig. 5h)South America is clearly separated from both Antarctica and

Africa. The Phoenix plate subducts under west ANT, while AndeanGDUs start colliding from north to south against SAM.

4.1.9. 084 Ma (Fig. 5i)The Andean GDUs have now collided with SAM, and GDUs in

the region of Tierra del Fuego collide with those from the north-ern Antarctic Peninsula, implying rotation, potentially enhanced atlocal scale (e.g. Rapalini et al., 2001, 2008; Grunow, 1993) but notresolved at the scale of our model. The movement remains trans-pressive. The collision might also explain why the Scotia region willbreak “into crumbs” during opening (see reconstructions at 33 Maand younger, below). The collision, which can easily bring base-ment rocks at surface, is coherent with detrital-zircon ages recentlyobtained in Tierra del Fuego (Barbeau et al., 2009). Note that thePhoenix–Penas spreading ridge subducts under Tierra del Fuego,

and has remained situated in the region until the present-day. Anisland arc migrates thanks to intra-oceanic subduction (up-left cor-ner of Fig. 5i) from the “Pacific magnetic triangular zone” (NWPacific) towards the south-east (Flores et al., in preparation).
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C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 51

Fig. 6. (a–d) Geodynamical reconstructions from 033 Ma to 026 Ma. Orthogonal projection. (e–h) Geodynamical reconstructions from 020 to 016 Ma. f2 is a 3D sketch at20 Ma aiming to clarify how driving forces act on the upper plate and lead to the formation of a double spreading ridge system. Grey dashed curves on the South America(SAM) plate are merely drawn to highlight the shape of the plate, and in particular, its curved downgoing slab. (h–k) Geodynamical reconstructions from 013 to 006 Ma. (1–o)Geodynamical reconstructions from 005 Ma to 000 Ma (abbreviations as per Fig. 2).

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52 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

Fig. 6. (Continued )

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C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 53

Fig. 6. (Continued )

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54 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

(Cont

bsdptb

Fig. 6.

Anomaly C34 (Müller et al., 2008) shows that shortening occursetween SAM and ANT. We propose, therefore, that intra-oceanicubduction zone initiates within the Antarctica plate (in the Wed-

ell Sea), along a weak zone formed by the transform faultsreviously mentioned. However, although the two plates converge,he ANT–SAM spreading axis is still acting, thanks to a ridge jump,ecause anomaly C34 is identified in the Antarctic Ocean (Müller

inued ).

et al., 2008). The jump can be identified south of Madagascar andIndia.

In the model, ‘Weddell Sea subduction’ stops because the

spreading axis is parallel to the subduction zone. When the spread-ing ridge ‘collides’ with the nascent arc, the subducting plate candetached, before (or while) the ridge jumps. However, the buoy-ant Astrid, Explora Wedge, and Maud oceanic plateaus may have
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C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 55

F enix–r ree std ear, al

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4

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trPaRGbAo

ig. 7. Method for defining the position and extension of slab windows for the Phoepresented arbitrarily after 57 Ma (i.e. after the arc–ridge collision; see text). (A) Thipping slab (B1: cross-section; B2: map view). (C) Possible configuration of a slab t

layed an important role in stopping the subduction. As alreadyroposed by McAdoo and Laxon (1996) for instance, we regardhe ‘V’ and ‘T’-anomalies in the Weddell Sea as remnants of thepreading axis, and therefore as an abandoned plate boundary. Theontinent-ocean boundary (COB) fits with the Orion, Ardenes, andxplora Escarpments (e.g. Livermore and Hunter, 1996; Jokat et al.,996).

.1.10. 070 Ma (Fig. 5j)The migrating Pacific island-arc adopts a more north–south dis-

lacement so that the model predicts the existence of a transformargin along SAM and potentially a pause in the magmatic activ-

ty in the South American cordillera. The Phoenix plate keeps onubducting under ANT, while SAM moves towards the west rela-ive to ANT, sustaining the transpressive collision between Tierrael Fuego and the Antarctic Peninsula, and active margin east ofhe South Orkney GDU. The development of this active margin isegarded as the triggering event for the future Scotia Sea to opens a back-arc.

.1.11. 057 Ma (Fig. 5k)In the Pacific Ocean, the intra-oceanic subduction stops because

he island arc “collides with” the mid-oceanic spreading axis. Theidge jumps southwards, probably assisted by the arrival of theeter I Island hot-spot, and divides the Phoenix plate from the Far-llon plate. Another hot-spot, the Islas Orcadas Rise and the Shonaidge is equally linked a ridge jump in the South Atlantic Ocean. The

DU, corresponding to the Coronation Islands, changes of plates,ecause of the ongoing transpressive collision between SAM andNT. Consequently, the subduction zone propagates north, towardsld and dense oceanic crust.

Farallon, Phoenix–Antarctica, and Nazca–Antarctica plate limit. Slab windows areeps (t0, t1 and t2) showing the delineation of a slab window. (B) Projection of a 60◦

though slabs are considered instantaneously broken in the model.

4.1.12. 048 Ma, 040 Ma and 033 Ma (Fig. 5l–n)Strike-slip movements between SAM and ANT lead to the obduc-

tion of the South Georgia ophiolites. The subduction along theScotian GDUs reaches the old oceanic crust (at least 150 ± 1 Ma afterMukasa and Dalziel, 1996), and slab roll-back generates a transformfault, corresponding nowadays to the Magallanes–Fagnano Fault.Slab roll-back also induces extension in the upper plate, and rift-ing processes initiate at some time between 48 Ma and 33 Ma (e.g.Eagles et al., 2006). The Phoenix–Farallon spreading axis remainslocated under Tierra del Fuego, which may have significant impli-cations in terms of heat flux and magmatism in the region.

4.2. Second part (033 Ma–000 Ma)

4.2.1. 033 Ma – Chron C12–13 (Fig. 6a) or A33 from Müller et al.(2008)

With slab roll-back, almost all Scotian GDUs move relative toone another (Fig. 6a). Rifting is definitely initiated in the futureScotia Sea and the future Powell Basin. In the Pacific Ocean, the ridgejumps again, so that the Phoenix–Farallon plate limit is a transformfault, which will turn into a transtensive spreading axis.

4.2.2. 029 Ma – Chron C10 (Fig. 6b)According to Coren et al. (1997), magnetic anomalies in the Pow-

ell Basin are identified from chron C8 (26 Ma), and the crust withages spanning 27–30 Ma is transitional in nature. However, theirmagnetic profile indicates anomalies up to ∼30 Ma and a spread-ing axis is, therefore, represented in the model. Accordingly, Eaglesand Livermore (2002) have proposed the end of rifting at 29.7 Ma.

4.2.3. 027 Ma – Chron C9 (Fig. 6c)The rifting in the future Scotia Sea is not complete and the Pow-

ell Basin keeps on opening. Segments of oceanic accretion at the

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5 f Geod

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6 C. Vérard et al. / Journal o

hoenix–Antarctica spreading ridge are drawn parallel to the sub-uction zone. Such configuration, already proposed by Larter andarker (1991), gives a solution as to why the Antarctic passive mar-in supersedes the active margin. Thus, the subduction stops stepy step (segment by segment).

.2.4. 026 Ma – Chron C8 (Fig. 6d)The older magnetic anomaly identified in the Scotia Sea is inter-

reted as chron C8 (Barker and Burell, 1977; Maldonado et al.,998). However, the different Scotian GDUs, in particular Clarenceidge, Terror Rise, Pirie Bank, Bruce Bank and Discovery Bank, mustrift away from one another. Accretion style and spreading axis

ocation remain unclear all around these GDUs (e.g. Barker, 2001),lthough the works of Maldonado et al. (2006) and Galindo-Zaldívart al. (2006), in particular, well constraint the opening of the Pro-ector Basin (between the Terror Rise and Pirie Bank).

.2.5. 024 Ma – Chron C7 (Fig. 6e)The northern Scotian GDUs (Burdwood Bank, Davis Bank, Aurora

ank) slide along the Magallanes–Fagnano Fault. However, Southeorgia progresses faster to the east than Shag Rocks, and Shag

ig. 8. (a–f) Geodynamical reconstructions from 057 Ma to 027 Ma, showing subducted

ubducted isochrons are left in dashed grey). Slabs have been deformed to take into acd = 52.5◦; cf. Lallemand et al., 2005). White dashed lines are 100 km, 200 km, 300 km andnder 600 km whatever depth is reached). Orthogonal projection. (g–l) Geodynamical recm–r) Geodynamical reconstructions from 008 Ma to present-day, showing subducted slaegend, the reader is referred to the web version of the article.)

ynamics 53 (2012) 43– 60

Rocks faster than Aurora Bank, because the passive margin alongthe Falkland Plateau must tear down before the oceanic lithospherecan be consumed by roll-back processes. We chose this time toinitiate the opening of the Jane Basin and individualise the JaneBank as island arc.

4.2.6. 020 Ma – Chron C6 (Fig. 6f) or A20 from Müller et al. (2008)On the one hand, the eastward movement of Aurora Bank

is somewhat hampered, so that South Georgia and Shag Rockshave enough space to spread north-eastward. On the other hand,Discovery Bank moved further east than South Orkney, and canspread south-eastward. Such configuration results in a north–southdivergence that leads to the formation of an east–west trendingspreading axis in the central Scotia Sea. The double spreading sys-tem is related to strain partitioning associated with the curvatureof the plunging slab (Fig. 6f2). The curvature is itself related to thebuoyancy of the subducting ridge to the south (creating a half-slab

window), and slab tear to the north. The ages of the central ScotiaSea magnetic anomalies remain indecisive (see “Oligocene againstMiocene scenario” discussed in Eagles, 2010a,b), but spreading timespan has been satisfactorily refined using stratigraphic correlation

slabs under South America and Antarctica (red slab with green-yellow slab limit;count slab dip (we arbitrarily assume a slab curvature of R = 300 km and slab dip

600 km isobaths, respectively (note that the style of projection remains the sameonstructions from 026 Ma to 010 Ma, showing subducted slabs under SAM and ANT.bs under SAM and ANT. (For interpretation of the references to color in this figure

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C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60 57

(Cont

odt

4(

tmwtSeGsto

aibcA

Fig. 8.

f seismic units (Maldonado et al., 2006; see below). Slab roll-backrives Jane Bank towards the ANT–SAM spreading axis and widenshe Jane Basin (Maldonado et al., 1998).

.2.7. 016 Ma, 013 Ma and 010 Ma – Chron C5C, C5A and C5

Fig. 6g–i) – A10 from Müller et al. (2008)According to Coren et al. (1997), Eagles and Livermore (2002),

he Powell Basin spreading axis ceased 18 Ma ago. Consistently, theodel indicates that the subduction zone of the Jane Bank “collidesith” the ANT–SAM spreading axis, and that Discovery Bank stops

he migration of South Orkney. Since our model links the Centralcotia Sea spreading system with the cessation of that of the Pow-ll Basin through the intervening South Orkney and Discovery BankDUs, the age of cessation (18 Ma) is viewed as an independent con-traint on the timing of the Central Scotia Sea opening, and favourshe “Miocene scenario” suggested by Maldonado et al. (2006) andthers.

The western and the central Scotia Sea spreading axes keep oncting simultaneously. Galindo-Zaldívar et al. (2006) date the open-

ng of the Protector Basin (between Terror Rise and Pirie Bank)etween chrons C5Dn (∼17–18 Ma) and chron C5ACn (∼14 Ma). Athron C5C (016 Ma), according to the model, the triple junctionntarctica–Nasca–Phoenix plate is about to subduct.

inued )

4.2.8. 008 Ma – Chron C4 (Fig. 6j)On the north-east side, South Georgia is blocked against North-

East Georgia Rise (oceanic plateau). On the south side, Jane Bankbecomes inactive as the subduction zone “collides with” themid-oceanic ridge. Therefore, the western and central Scotia Seaspreading axes are abandoned, and the plate limit jumps into theSouth Sandwich Island volcanic arc, which starts splitting.

4.2.9. 006 Ma and 005 Ma – Chron C3A and C3 (Fig. 6k and l)The older magnetic anomaly determined in the Sandwich Island

Sea is chron C3A (Maldonado et al., 1998, 2006). Bathymetric datareveals a topographic high between South Georgia and DiscoveryBank. As Barker (2001) mentioned, it would not be surprising to findrocks of continental nature in the Scotia Sea, but the topographichigh is likely to be the remnant arc of the South Sandwich Islands.

4.2.10. 003 Ma, 002 Ma and 000 Ma – Chron C2A and C2 andpresent-day (Fig. 6m–o)

Magnetic anomalies indicate that the Phoenix–Antarctica

spreading axis ceased after chron C3. The age coincides with theinitial break of the South Shetland Island and the opening of theBrandsfield Strait (Barker et al., 2003, among many others). Weinterpret this contemporaneity as follows: as the initial break
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58 C. Vérard et al. / Journal of Geodynamics 53 (2012) 43– 60

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5

dw

pmsHFWiLSsNc

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Fig. 8.

ccurs, slab roll-back takes place, and horizontal stresses that pullhe Phoenix plate under the Antarctic plate reduce enough (as Southhetland Islands moves north-westwards) to inhibit divergenceetween the Antarctic and Phoenix plates.

. Slab windows

Although it is not the main purpose of the present study, geo-ynamical reconstructions allow, in addition, the definition of slabindows in space and time.

As schematically illustrated in Fig. 7, we first define subductedlate boundaries as linear segments between rotated end-points ofagnetic anomalies. Such geometry gives us the maximum exten-

ion of the slab – as if it was horizontal – under the overriding plate.owever, subducting slabs are inclined, and we represent fromig. 8a–r, a projection of slabs with inclination as shown in Fig. 7b2.e use slab curvatures (radius ≈ 300 km) asymptotically reach-

ng the mean plunging value under continental plate (52.5◦ afterallemand et al., 2005), close to the present-day geometry underouth America (Fig. 7b2). Moreover, for sake of clarity, we only showlab windows for the Phoenix–Farallon, Phoenix–Antarctica, and

azca–Antarctica plate limits from 57 Ma (i.e. after the arc–ridgeollision) to present.

After 57 Ma, the model suggests two times for ridge jumpsn the Pacific Ocean: 33 Ma and 10 Ma. In our slab window

inued ).

representations, we assume new plate boundaries instantaneouslysplit subducting slabs as well. However, such may not be the case.We think, in particular, that slab tearing may propagate throughtime, as illustrated in Fig. 7c. Diachronous magmatism in Patago-nia (e.g. Guivel et al., 2006; Lagabrielle et al., 2009; Breitsprecherand Thorkelson, 2009; Boutonnet et al., 2010) might be the resultof such configuration.

6. Conclusions

Global geodynamic reconstructions give solutions (possibly notunique) to our main issues: (1) The Scotia Sea has not openedagainst a young and buoyant spreading ridge, but on the con-trary, over an old, cold and dense (>150 Ma) oceanic crust, whichresults from Gondwana break-up. (2) Slab roll-back allowed ‘mid-dle’ Scotian GDUs (in particular South Georgia and Discovery Bank)to migrate faster than ‘northern’ and ‘southern’ Scotian GDUs, sothat a north–south divergence was possible. Two spreading sys-tems (one NW–SE and the second N–S) were active simultaneously,until the ‘southern’ Scotian GDUs “collide with” the SAM–ANTspreading axis, and the ‘northern’ Scotian GDUs collide with the

Falkland Plateau and the North-East Georgia Rise (oceanic plateau).(3) The Powell Basin is a back-arc basin, which stops to opendue to the migration of the Discovery Bank in front of the SouthOrkney GDU, constraining the timing of the Central Scotia Sea
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pening. (4) Anomalies “V” and “T” (e.g. Livermore and Hunter,996; McAdoo and Laxon, 1996) in the Weddell Sea are remnantsf a plate boundary corresponding to an arc–ridge collision before4 Ma. Intra-oceanic subduction developed in favour of a weakone in the crust (numerous transform faults), in response to theonvergence between SAM and ANT. A crust of oceanic natureprobably older than ∼100–130 Ma) remains south of the “V-” andT-” anomalies, since the COB fits the Ardenes and Explora Escarp-ents (King et al., 1996, and references therein). The subsequent

<84 Ma) development of the Antarctic oceanic crust correspondsell to the flow-lines synthetised by Livermore and Hunter (1996),aldonado et al. (1998), or Livermore et al. (2005). (5) Additionalagnetic anomalies compared to those of Müller et al. (2008) well

onstrain the localisation of the Nasca–Antarctica–Phoenix tripleunction, and suggest its subduction at around 16 Ma, i.e. slightlyounger than the 18 Ma of Breitsprecher and Thorkelson (2009) andquivalent to the age suggested by Eagles et al. (2009). The Tierra deluego–South Andes area underwent spreading ridge subductionince ∼108 yrs, which may have significant implications in terms ofeat flux and magmatism in the region. (6) The model suggests thathe best way to stop an ongoing subduction is a configuration where

spreading ridge, parallel to the trench, enters the subduction zone.he arc–spreading axis ‘collision’ leads to slab detachment, anday trigger ridge jump. Concerning the West Antarctica margin,

t seems that the active margin turned into passive margin stepy step, in agreement with Larter and Barker’s (1991) conclusions.ubduction ceased when spreading segments ‘collide’ with the arc,hile the other segments, separated by transform faults, were still

ctive (divergent).Regarding circum-polar oceanic current, the link between the

acific and Atlantic Oceans is cut between the early Upper Creta-eous (95–84 Ma) and chrons C12–13 (33 Ma) or C10–C9 (29–27 Ma),hen the Drake Passage opens. The model does not predict a notice-

ble constriction of the Drake Passage as mentioned, in particular,y Lagabrielle et al. (2009).

The model corresponds well to others drawn more specificallyor the Pacific realm (Breitsprecher and Thorkelson, 2009; Eaglest al., 2009), the Weddell Sea region (Livermore and Hunter, 1996;ivermore et al., 2005), the Powell Basin (Maldonado et al., 1998;agles and Livermore, 2002), or the Scotia Sea (Barker, 2001), butiffers from other interpretation (Ghidella et al., 2002; Eagles,010a).

cknowledgements

We kindly acknowledge Petroconsultants IHS Global S.A.Geneva) and Shell (U.K.) for access to their database. We grate-ully thank Cyril Hochard and Laurent Thum for their preciouselp with the reconstructions. The Swiss National Fund is equallycknowledged for funding the present work. The geodynamicalodel now belongs to NEFTEX Petroleum Consultant Ltd., with

icence to UNIL; “Nous remercions NEFTEX pour sa très grâcieuseutorisation de divulguer dans le cadre de cette publication des élé-ents de reconstruction géodynamique issus de notre travail”.

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