Opening of the Scotia Sea

GLOBAL-02142; No of Pages 22 Global and Planetary Change xxx (2014) xxx–xxx

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Global and Planetary Change

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A model of oceanic development by ridge jumping: Opening of the Scotia Sea

Andrés Maldonado a,⁎, Fernando Bohoyo b, Jesús Galindo-Zaldívar a,c,Fo. Javier Hernández-Molina d, Francisco J. Lobo a, Emanuele Lodolo e, Yasmina M. Martos a,LaraF.Péreza, Anatoly A. Schreider f, Luis Somoza b a Instituto Andaluz de Ciencias de la Tierra, Campus de Fuentenueva, CSIC-UGR, Granada, Spain b Instituto Geológico y Minero de España, Madrid, Spain c Departamento de Geodinámica, Universidad de Granada, Granada, Spain d Dept. Earth Sciences, Royal Holloway Univ. London, Egham, UK e Istituto Nazionale di Oceanograﬁa e di Geoﬁsica Sperimentale (OGS), Trieste, Italy f P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia article info abstract

Article history: Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial Received 23 October 2013 for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust Received in revised form 24 June 2014 of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed Accepted 30 June 2014 to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has signif- Available online xxxx icant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and

Keywords: western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected fl fi Ocean gateways during the last two decades is analyzed here. The data include multichannel seismic re ection pro les, and mag- Scotia Arc netic and gravimetric data. Drake Passage The oceanic basement is highly deformed by normal, reverse and transcurrent faults, as well as affected by deep Oceanic spreading intrusions from the mantle. The initial extension and continental thinning, with subsequent oceanic spreading, Back-arc basins were followed by compression and thrusting. Several elongated troughs, bounded by faults, depict a thick se- Cenozoic paleoceanography and climate quence of depositional units in the basin. Eight seismic units are identified in a deep trough of the eastern Ona Basin. The deposits reach a thickness of 5 km, a consistent value not previously reported from the Scotia Sea. A body of chaotic seismic facies is also observed above the thinned continental crust of the Ona High. Magnetic seafloor anomalies older than C10 (~28.5 Ma) may be present in the region. The anomalies could include up to chron C12r (~32 Ma), although their identification is difficult, since the amplitude is subdued and the original oceanic crust was highly deformed by later faulting and thrusting. The magnetic anomaly distribution is not congruent with seafloor spreading from a single ridge. The basin plain is tilted and subducted southwestward below the South Shetland Islands Block, particularly in the western part, where an accretionary prism is identified. Such tectonics, locally affecting up to the most recent deposits, imply that a portion of the primitive oceanic crust is absent. Based on the stratigraphy of the deposits and the magnetic anomalies, an age of 44 Ma is postulated for the initiation of oceanic spreading in the eastern Ona basin, while spreading in the western Ona Basin would have occurred during the early Oligocene. The tectonics, depositional units and the age of the oceanic crust provide additional evidence regarding the Eo- cene opening of Drake Passage. The initial tectonic fragmentation of the South America–Antarctic Bridge, followed by oceanic spreading, was characterized by jumping of the spreading centers. An Eocene spreading center in the eastern Ona Basin was the precursor of the Scotia Sea. A model comprising four tectonic evolutionary phases is proposed: Phase I, Pacific subduction — Paleocene to middle Eocene; Phase II, eastern Ona back-arc spreading — middle to late Eocene; Phase III, ridge jumping and western Ona back-arc spreading — early Oligocene; and Phase IV, ridge jumping and West Scotia Ridge spreading — early Oligocene to late Miocene. The development of shallow gateways allowed for an initial connection between the Pacific and Atlantic oceans and, hence, initiated the thermal isolation of Antarctica during the middle and late Eocene. Deep gateways that enhanced the full isolation of Antarctica developed in Drake Passage from the Eocene/Oligocene transition

⁎ Corresponding author. B E-mail addresses: [email protected] (A. Maldonado), [email protected] (F. Bohoyo), [email protected] (J. Galindo-Zaldívar), [email protected] (F. J. Hernández-Molina), [email protected] (F.J. Lobo), [email protected] (E. Lodolo), [email protected] (Y.M. Martos), [email protected] (L.F. Pérez), [email protected] (A.A. Schreider), [email protected] (L. Somoza).

http://dx.doi.org/10.1016/j.gloplacha.2014.06.010 0921-8181/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

onward. A signiﬁcant correlation is observed between the tectonics, stratigraphic units and major climate events, thereby indicating the inﬂuence of the local tectonic and paleoceanographic events of the Southern Ocean on global evolution. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction tectonic reconstructions proposed a Drake Passage opening during the early Oligocene (Lawver and Gahagan, 2003). Other kinematic models The opening of the Scotia Sea and Drake Passage, located between account for a progressive opening of Drake Passage since about 50 Ma, South America and the Antarctic Peninsula, is a subject of considerable through continental stretching and oceanic spreading (Eagles et al., controversy. Conflicting ages have been proposed, spanning a time 2005; Livermore et al., 2005; Eagles et al., 2006; Livermore et al., 2007; frame of more than 20 million years (Ma), from early Eocene to early Eagles and Jokat, 2014). On the basis of geological, geophysical and bio- Oligocene (Lawver and Gahagan, 1998; Barker, 2001; Lawver and logical arguments, several studies claim that the opening of Drake Pas- Gahagan, 2003; Eagles et al., 2005; Livermore et al., 2005; Eagles et al., sage was earlier than the Eocene/Oligocene boundary (cf., Thomson, 2006; Lodolo et al., 2006; Livermore et al., 2007; Dalziel et al., 2013a; 2004; Brown et al., 2006; Scher and Martin, 2006; Lagabrielle et al., Eagles and Jokat, 2014). Drake Passage and the Scotia Sea are widely 2009; Katz et al., 2011; Le Roux, 2012, and references therein). recognized as a key gateway that controlled Cenozoic water-mass circu- Evidence of oceanic crust formation or the occurrence of deep-water lation and climate in the Southern Hemisphere (Scher and Martin, marine deposits that would attest to the development of a gateway in 2006; Lagabrielle et al., 2009; and references therein). The gateway per- Drake Passage prior to the Oligocene is lacking, however. In this contri- mitted the gradual instauration of the Antarctic Circumpolar Current bution, we try to decipher the earliest evolutionary stages of Drake Pas- (ACC), which isolated Antarctica from the influx of warm currents sage and the Scotia Sea, as well as describe the impact of this evolution from the north and intensified its glaciation (Kennett, 1977). The atmo- on the global climate and the development of Antarctic ice sheets. To spheric systems were, in turn, affected by an equator-ward migration this end, a dense geophysical data set collected in the southwestern that influenced ocean currents as well as the climate patterns of conti- Scotia Sea is analyzed, from a sector named (herein) Ona Basin nental areas. (Fig. 1). We present evidence of oceanic crust that is older than previ- The Eocene/Oligocene (33.5 Ma) transition is characterized by a ously reported, and the sediment distribution provides insight as to de- major shift of the oxygen isotope records derived from foraminifera, positional processes and basin growth patterns. Additionally, we reflecting a significant decrease of global temperatures that was also ac- describe the development of a proto island arc where ridge jumping companied by a major sea level drop (Zachos et al., 2001; Miller et al., was active, creating gateways across Drake Passage. The tectonic 2005; Kominz et al., 2008; Zachos et al., 2008; Cramer et al., 2009). Sev- models are improved by information regarding the nature of the crust eral hypotheses aim to explain this global change and the onset of a new and the distribution of deposits, which altogether serve for a robust re- oceanographic circulation pattern, characterized by the production of construction of the early opening of the Scotia Sea and the breakup of cold-water masses in high latitudes that flow toward the deep ocean ba- the South America–Antarctic Peninsula Bridge during the Eocene. sins. An initial interpretation attributed the climatic change and development of the large Antarctic ice sheets to the opening of Drake 2. Geological and oceanographic setting Passage gateway (Kennett, 1977), the last barrier to full circum- Antarctic circulation. This idea was contested for a long time, however, 2.1. Geological framework since the Tectonic Map of the Scotia Sea shows that the oldest seafloor-spreading magnetic anomaly is C10 (~28.5 Ma) off Tierra del The Scotia Arc is composed by the South Sandwich forearc, the North Fuego, indicating an Oligocene age for the opening of Drake Passage Scotia Ridge (NSR), the South Scotia Ridge (SSR), the South Shetland (British Antarctic Survey-BAS, 1985; Barker, 2001). Islands Block (SSIB) and the Shackleton Fracture Zone (SFZ), which sur- This time lag between the initiation of the Antarctic glaciations and round the basin plain of the Scotia Sea (Fig. 1). The Scotia Sea contains the tectonic development of a gateway in Drake Passage argued in favor several spreading ridges, some of which were simultaneously active and of other driving mechanisms, rather than tectonic and paleoceanographic resulted in the opening of Drake Passage (British Antarctic Survey — processes. It has moreover remained unclear at what time and stage the BAS, 1985; Barker et al., 1991; Barker, 2001). The SSR is formed by a com- transition from a shallow to an intermediate and then a deep water pas- plex array of continental blocks that show evidence of significant tectonic sage took place, leading to the onset and development of a significant activity at present (Galindo-Zaldivar et al., 1996, 2002; Bohoyo et al., deep current (Barker, 2001; Dalziel et al., 2013b). A further hypothesis at- 2007; Civile et al., 2012). The contact between the northern margins of tributed the Eocene/Oligocene climate change and the subsequent Ceno- the SSIB and the oceanic basin plain of the southwestern Scotia Sea is lo- zoic glaciation of Antarctica to variations in the concentrations of cally a reverse fault, which may be sealed by the uppermost sedimentary greenhouse gases such as CO2. The ice sheet model, with invariant cover (Aldaya and Maldonado, 1996; Civile et al., 2012). ocean general circulation and different CO2 levels, points to significant Drake Passage is entirely crossed by the SFZ, an intra-oceanic ridge changes in the Antarctic ice sheet thickness (DeConto and Pollard, several hundreds to thousands of meters above the surrounding sea- 2003). The ice sheets became widespread with CO2 concentrations similar floor (Maldonado et al., 2000). The SFZ separates the western Scotia to pre-industrial levels (280 ppm), but tended to disappear with CO2 con- Plate of the Scotia Sea to the east and the former Phoenix and Antarctic centrations eight times higher than pre-industrial levels. A model that Plates to the west, in the Pacific Ocean (Fig. 1). The SFZ is bounded east- comprises a general ocean circulation model – coupled to a simplified at- ward by an active, transpressional fault that accommodates, in conjunc- mospheric model and a dynamic–thermodynamic sea ice model – tion with the NSR and SSR, the relative motion between the Scotia and suggests, moreover, that changes in ocean gateways in Drake Passage Antarctic plates (Barker et al., 1991; Galindo-Zaldivar et al., 1996; could have contributed to the glaciation of Antarctica (Sijp and England, Klepeis and Lawver, 1996; Maldonado et al., 2000; Galindo-Zaldivar 2004; Sijp et al., 2009). et al., 2002; Livermore et al., 2004; Geletti et al., 2005; Martos et al., Recent studies of seafloor magnetic anomalies in the Scotia Sea reveal 2013). The SFZ intersects two extinct spreading centers – the West Sco- the occurrence of anomaly C10 (~28.5 Ma) in the southwestern Scotia tia Ridge (WSR) and the Phoenix–Antarctic Ridge – between which the Sea, suggesting an age for the opening of Drake Passage still younger SFZ acted as a ridge-to-ridge transform fault when both spreading cen- than the Eocene/Oligocene climatic change (Lodolo et al., 2006). Plate ters were active (Maldonado et al., 2000). The Phoenix Plate underwent

Fig. 1. Simplified bathymetry map derived from the GEOSAT gravimetric anomaly of the Scotia Sea (Smith and Sandwell, 1997) with the geological setting of the Scotia Sea and the distribution of main oceanographic flows in the region. Geological elements legend: BB, Bruce Bank; DB, Discovery Bank; HB, Herman Bank; JB, Jane Basin; JBk, Jane Bank; PB, Powell Basin; PiB, Pirie Bank; SGI, South Georgia Island; SSIB, South Shetland Islands Block; SOM, South Orkney Microcontinent; TR, Terror Rise. Oceanographic currents legend: ACC, Antarctic Circum- polar Current; CDW, Circumpolar Deep Water; WG, Weddell Gyre; WSDW, Weddell Sea Deep Water; Dots with circle, Boreholes. subduction below the Antarctic Plate during the late Mesozoic and Ce- The sedimentary deposits of the region attest to the importance of nozoic (Dalziel, 1984; Barker et al., 1991; Larter and Barker, 1991). bottom-current processes along the seafloor. Numerous types of The spreading axis of the plate was extinct at chron C2A (2.6–3.6 Ma), contouritic drifts and erosional features have been identified in the Pacif- and became part of the Antarctic Plate thereafter (Livermore et al., ic continental margin of the Antarctic Peninsula, the Scotia Sea and the 2000). Subduction continued at the South Shetland Trench due to roll- northern Weddell Sea (Howe and Pudsey, 1999; Maldonado et al., back processes at the hinge of subduction and active spreading in the 2003; Hernández-Molina et al., 2006; Maldonado et al., 2006; Bransfield Strait (Larter and Barker, 1991; Maldonado et al., 1994; Hillenbrand et al., 2008; Martos et al., 2013; among others). These de- Catalán et al., 2013 and references therein). posits are organized into depositional units recognized regionally in mul- The high relief and uplift of the SFZ is a recent feature of Drake Pas- tichannel seismic reflection (MCS) profiles (Maldonado et al., 2006). The sage. The most likely time for crustal uplift of the SFZ was during a peri- older units identified below a characteristic reflection named Reflector c od of regional Scotia Sea compression after 17 Ma due to the migration are distinctive in each oceanic basin, whereas the three younger units of the pole of rotation, which created transpression during the Miocene in different regions of the Scotia Sea are similar (Maldonado et al., (Livermore et al., 2004). Compression eventually led to the extinction of 2006; Lindeque et al., 2013). the WSR after chron C3A (6.4 Ma) (Maldonado et al., 2000). The SFZ is thrusted in the southern part by the SSIB, where a triple junction is de- 2.2. Oceanographic setting veloped (Aldaya and Maldonado, 1996). Kinematic reconstructions of South America–Antarctica motions Drake Passage is the oceanic gateway between the Pacific and Atlan- during the Cenozoic are mostly based on the analysis of magnetic sea- tic that influences the circulation of the main water masses in the South- floor spreading anomalies in the Scotia Sea and surrounding oceanic ern Ocean (Fig. 1). The Circum-Antarctic Low Pressure Belt instigates a areas. They depict a progressive opening of Drake Passage through con- westward-flowing current along the continental margin, known as the tinental stretching and oceanic spreading (Lawver and Gahagan, 2003; Sub-polar Current or East Wind Drift (Le Roux, 2012). North of the Eagles et al., 2005; Livermore et al., 2005; Eagles et al., 2006; Lodolo Circum-Antarctic Low Pressure Belt, the Antarctic Circumpolar Current et al., 2006; Livermore et al., 2007; Vérard et al., 2012; Eagles and (ACC) flows clockwise around Antarctica (Fig. 1). The ACC controls the Jokat, 2014). During the initial stages of spreading the Scotia Sea transport of heat, salt, and nutrients around the Southern Ocean and is contained several small oceanic basins, particularly in the southern re- also the principal contributor to the boundary currents of the South gion (Maldonado et al., 1998; Eagles et al., 2006; Galindo-Zaldívar Atlantic, South Pacific and Indian oceans (Nowlin and Klinck, 1986; et al., 2006; Maldonado et al., 2006). The basins developed as a result Foldvik and Gammelrsød, 1988; Naveira Garabato et al., 2002). The of subduction of the Weddell Sea and the southwestern South ACC moreover plays an important role in maintaining the thermal isola- American Plate below several fragments of an eastward and northward tion of the continent, keeping northern warm waters away from migrating trench (Barker et al., 1991; Barker, 2001). Ona Basin is the Antarctica. The ACC is intersected by the NSR and splits into a shallow, westernmost of these basins in the southern Scotia Sea (Fig. 1). warm branch flowing to the north and a deeper, cold branch moving

Please cite this article as: Maldonado, A., et al., A model of oceanic development by ridge jumping: Opening of the Scotia Sea, Glob. Planet. Change (2014), http://dx.doi.org/10.1016/j.gloplacha.2014.06.010 4 A. Maldonado et al. / Global and Planetary Change xxx (2014) xxx–xxx farther to the east and then turning northwards, forming the Falkland– Three representative depth-converted MCS profiles and the GEOSAT Malvinas Current, which in turn flows along the Atlantic margin at less satellite-derived free-air anomaly data (Sandwell and Smith, 2009) than 1.5 km water depth up to the La Plata River, later being deflected were used to produce 2.5 D gravity models by means of Gravmag® soft- eastward (Piola and Matano, 2001; Le Roux, 2012). The Circumpolar ware (Pedley et al., 1993). The morphological and geometric constraints Deep Water (CDW) constitutes the deeper components of the ACC, of the model are based on the bathymetry of the seafloor and the sedi- comprising two fractions, the lower and upper, respectively named ment thickness derived from the MCS profiles (Fig. 3). The sparse seis- LCDW and UCDA (Fig. 1). The CDW is located above the South Pacific mic reflections locally observed at depth in the MCS profiles and Deep Water (SPDW), which flows east (Naveira-Garavato et al., 2002; attributed to the Mohorovicic (MOHO) discontinuity reveal the crust- Hillenbrand et al., 2008). A deeper and denser water mass in the south- mantle boundary (Fig. 3-1, 3-2; cf., Maldonado et al., 2000). Acoustic ve- ern Scotia Sea and crossing the southern Drake Passage is the westward locity data of the nearest seismic refraction profile, located in Powell directed Weddell Sea Deep Water (WSDW), derived from the Weddell Basin, were used to generate the time-to-depth conversion. Acoustic Gyre (WG), flowing across passages of the SSR into the Scotia Sea velocities of 1500 m/s, 2200 m/s and 5541 m/s were respectively ap- (Orsi et al., 1999; Naveira Garabato et al., 2002; Palmer et al., 2012; plied for the seawater, sediments and basement (cf., King et al., 1997). Pérez et al., in this issue). The values of density used to model the water and the sedimentary de- The morphological influence exerted by the SFZ and other barriers posits were 1.03 g/cm3 and 2.4 g/cm3, respectively, while a density of upon bottom-current circulation has been considered by several studies. 2.85 g/cm3 was used to characterize the igneous basement of Ona It is suggested that deep-water flows were inhibited in Drake Passage by Basin, according to the typical densities of the oceanic crust in the re- overlapping continental slivers along the SFZ until the late Oligocene gion (Galindo-Zaldivar et al., 2002, 2006; Larter et al., 2003; Lodolo (Barker, 2001), or by remnant fragments of a volcanic arc in the central et al., 2006). The density of the continental crust for the model was Scotia Sea until the late Miocene (Dalziel et al., 2013b). Yet other studies 2.67 g/cm3. A density of 2.7 g/cm3 was applied for the chaotic wedge ob- suggest that the SFZ is a rather recent feature that acted as a barrier only served in the central part of profile HESANT92/93-31 (Fig. 3-3). A den- after 17 Ma (Livermore et al., 2004). The uplift of the barrier induced the sity of 2.75 g/cm3 was used for the intrusion of deep materials from the northward displacement of the bottom water masses since the Miocene mantle observed in Profile SA500-003 (Fig. 2A), and a density of (Hernández-Molina et al., 2006; Martos et al., 2013). 3.25 g/cm3 represents the mantle. The main target of the gravity model- ing was to assess the density distribution and lateral variations of the 3. Regional data and methods basement. We used a methodology applied in previous studies of the region in The geophysical data were obtained during five oceanographic order to assess the age of the depositional units. We first applied the in- cruises onboard the BIO HESPERIDES and RV EXPLORA along several terval velocities of the migrated MCS profiles for the time-to-depth con- profiles in the southwestern Scotia Sea (Fig. 2A; Table 1). We collected version of the selected sites. The age of the base of the deposits is MCS profiles, high-resolution sub-bottom profiles (TOPAS: Konsberg constrained by the age of the oceanic crust derived from the magnetic Simrad PS018), swath bathymetric (SIMRAD EM 12) and total intensity anomalies. Then, estimated sedimentation rates are applied to the de- magnetic field data (Geometrics G-876 proton precession magnetome- posits, based on the results of shallow and deep cores of the region. ter). Marine gravimeter data (Bell Aerospace TEXTRON BGM-3) were Site 697B in Jane Basin (Fig. 1) is the only borehole in the proximity of also available from the SCAN2004 cruise. The data set was supplement- the study region and recovered fine-grained sediments representing ed with a set of MCS profiles for the region made available by the deposits of Pliocene to Recent age (Barker et al., 1988). This site was cor- Antarctic Seismic Data Library System for Cooperative Research (see related with the units of Ona Basin by cross-basin seismic transects (for website http://sdls.ogs.trieste.it/). MCS profiles of twelve oceanographic regional correlation see also: Maldonado et al., 2006, their Fig. 2B; cruises from institutions of Spain, Italy, Russia, Japan and Brazil were an- Lindeque et al., 2013, their Fig. 2). The method provides an estimated alyzed (Fig. 2; Table 1). Details on the acquisition and processing of the age for each depositional unit, although it is tentative since sedimenta- MCS profiles are provided in Table 1. The commercial software package tion rates may show wide variability and significant hiatuses may be Kingdom Suite® was used to compile MCS profile data. present. Total intensity magnetic field data were obtained from the The tectonic deformations and the distribution of the sedimentary HESANT92/93 (profiles M-05, M-25, M-26 and M-31) and SCAN2004 infill disclose the growth patterns of the basin. The magnetic anomaly cruises (profile M-17) aboard the BIO HESPERIDES and from the data contribute new clues regarding the age of the oceanic crust and im- IT89AW (profile IT-41) and IT95AP (profile IT-153) RV EXPLORA Italian prove previous interpretations. The gravity models provide relevant in- Antarctic expeditions (Fig. 2A). The magnetic data were acquired every formation about the regional nature and structure of the crust. 12.5 m on average along the ship track lines. Data were filtered in order to eliminate spikes and smoothed using a running mean to obtain representative values. The magnetic anomalies were calculated taking 4. Structure and deposits into account the Definitive Geomagnetic Reference Field (DGRF). In addition, diurnal corrections obtained by the Spanish magnetic observato- The Scotia Sea basin developed through a process of oceanic spread- ry located on Livingston Island (South Shetland Islands) were applied to ing during the Cenozoic (Barker et al., 1991; Barker, 2001; Dalziel et al., the SCAN2004 data. The top of the oceanic basement, attributed to oce- 2013a). The southwestern region of this oceanic basin is referred to as anic Layer 2 in the basin plain (cf., Maldonado et al., 2000), was consid- Ona Basin; it includes two sub-basins — the western and eastern Ona ered the upper boundary of the magnetized layer (Fig. 3). The BIO basins — separated by a structural high formerly named the Ona Plat- HESPERIDES and RV EXPLORA profiles were integrated with magnetic form (Civile et al., 2012)thatwecallhereintheOnaHigh(Fig. 2). Ona data taken from the National Geophysical Data Center at NOOA (profiles Basin is bounded by Terror Rise to the east, the SFZ to the west, and NBP-93-1 and NBP-93-2), and from the Deep Sea Drilling Program the SSIB to the south, whereas its opens to the north into the western dataset (profile DSDP-Leg36). Scotia Sea basin plain (Figs. 1, 2). The Ona basin plain is thrusted The identification of magnetic chrons was based on the magnetic time below the SSIB (Aldaya and Maldonado, 1996; Lodolo et al., 1997; scale of Gradstein et al. (2004). The parameters adopted for the synthetic Civile et al., 2012). The orientation of the reverse faults at the foot of model are: dip: −68°; declination: 10°; body thickness: 0.5 km; depth of the slope of the SSIB is roughly WSW-ENE (Fig. 2B). top of the ridge: 3.0 km; susceptibilities: 0.070 (at 7.4 Ma), and 0.008 A thick sedimentary cover is observed above the basement, with an (at 29 Ma); spreading rates: 1.6 cm/yr from 7.4 to 25.0 Ma; 1.0 cm/yr average thickness of about 2.0 s two-way travel time (TWTT), locally from 25.0 to 27.0 Ma; and 0.2 cm/yr from 27.0 to 29.0 Ma. reaching up to 3.4 s TWTT (Figs. 2B, 3). The sediments are generally

Fig. 2. A — Bathymetry predicted from satellite-derived free-air anomaly field of the Scotia Sea (Smith and Sandwell, 1997), with the location of the several cruises mentioned in the text. Thick red lines represent the location of the MCS profiles represented in Fig, 3, where the yellow lines correspond to the profiles discussed in detail with the magnetic and gravimetric models. B — Tectonic sketch of the southwestern Scotia Sea showing the main cortical elements of the region. The distribution of the basin depocenters is illustrated (contours in seconds — TWTT). Legend: 1, syncline (depocenter); 2, reverse fault; 3, basal detachment; 4, northern boundary of Ona Basin; 5, boundary of the Ona High; SSIB, South Shetland Island Block; SFZ, Shackleton Fracture Zone. Bathymetry predicted from satellite-derived free-air anomaly field of the Scotia Sea (Smith and Sandwell, 1997).

thin or absent above the Ona High, the SFZ and the slopes of the SSIB, except in some restricted areas (Figs. 2B, 3).

4.1. The basement Standard time migration 2400 m 96 channels 1 array of 5 airguns BOLT (29.6 l)

The basement top in the basin is characterized by a high-amplitude reflector horizon, which shows a clear and irregular contact with the overlying deposits (Fig. 3). A layer with high-amplitude reflections and lateral continuity is located below this top basement reflection, Standard time migration 1200 m 96 channels 1 array 6 airguns BOLT (15.26 l) which is underlined by high amplitude and discontinuous reflections. Beneath, irregular diffractions are identified over disperse and weak reflections, underlain by a thicker layer with sparse, normally inclined reflections (Fig. 3). This basement is attributed to Layer 2 of the oceanic crust, which is interpreted as the igneous crust in the region fi Standard time migration 1200 m 96 channels 1 array 7 airguns BOLT (16.3 l) (cf., Maldonado et al., 2000, 2006). The pro les locally show a discontinuous and high-amplitude reflection at about 2 s (TWTT) below the basement top, which may be laterally traced. This reflection is attributed to the MOHO discontinuity (Fig. 3-1, 3-2). The basement is tilted to the southwest. It is thrusted below the SSIB Standard no migration 72 channels 8 airguns (8.8 l) to the south and below the SFZ to the west (Fig. 3-1, 3-2, 3-6). The basement also shows an irregular and fragmented relief, as it is affected by NNW–SSE to WNW–ESE trending structures, such as normal, reverse and transcurrent faults and deep-rooted intrusions (Figs. 2B, 3-1, 3-4). The faults bound several deep basement depressions containing thick Standard no migration 1 array GI gun (65.55 l) deposits. A WSW–ENE oriented depression is also observed in the central area near the contact with the slope of the SSIB, and two NNW–SSE depressions are observed in the eastern Ona Basin (Fig. 2). The depressions bounded by normal faults suggest important extensional tecton- fi Standard no migration 192 channels 240 channels 1800 m 1 array GI gun (9.83 l) ics. The reverse faults observed in the pro les reveal, however, a more recent phase of compression. Most of the basement faults do not cut across the deposits, except in the proximity of the SFZ and the southwestern basin plain (Fig. 3-1, 3-2). The contact between the basin and the slope of the SSIB is marked by Standard no migration 600 m 24 channels 2 water guns (13.1 l) a reverse fault that in places shows several thrust faults, with transcurrent flower structures above the relatively undisturbed basin sediment fill. The older deposits of the basin are thrusted along the SSIB and reveal some amount of subduction (Fig. 3-2, 3-5, 3-6). The re- 11 l) les analyzed in this study. 4 airguns – fi – verse fault at the foot of the slope shows evidence of recent activity in Standard no migration 775 m 32 channels (9 1 array 3 the western region, while the slope is sealed by the younger units in the eastern region (Fig. 3-6). The morphology of the SSIB slope mimics the structure of the basement of Ona Basin (Fig. 2B), with deep valleys incising the slope of the margin where deep basement depressions are thrusted. This may indicate tectonic erosion and compaction of the Standard no migration 1500 m 120 channels 2 GI guns (6.7 l) overridden slab due to the mechanical properties of the thick subducted sediment pile. The contact between the SFZ and the basin plain is also delineated by faults (Fig. 3-1). The slope of the SFZ is cut by transtensive and transpressive faults, whereas a major thrust is observed at depth. The Standard no migration 3000 m 120 channels 2 arrays 15 air-guns each (71.96 l) SFZ is subducted to the south below the SSIB, generating a prominent margin bulge (Aldaya and Maldonado, 1996; Martos et al., 2013). The Ona High is a morphological and structural high occupying the central sector of the basin with a very irregular and eroded relief that ssing methods of the multichannel seismic (MCS) pro is attached, southwards, to the SSIB (Figs. 2, 3). The MCS profiles reveal an irregular and eroded sedimentary cover on top of a basement with Standard time migration 3000 m 120 channels 2 arrays 15 air-guns each (45.16 l) short and discontinuous internal reflections (Fig. 3-3). Reverse and normal faults are observed over the margins and within the structural block. The seismic facies of the basement resemble those of other banks of the southern Scotia Sea, interpreted as continent-like or transi- tional continental crusts (Galindo-Zaldivar et al., 2006; Bohoyo et al., 2007; Civile et al., 2012; Galindo-Zaldivar et al., in this issue). Standard time migration 3000 m 120channels IT_89 IT_9115 air-guns each (45.16 l) IT_92 IT_95 RAE TH87/88 TH96 TH97 SA500 ANT92 HESANT92/93 SCAN2004

4.2. The depositional units

Five major seismic sedimentary units (Units 5 to 1, from bottom to sequence streamer top) have been previously described in the Scotia Sea (Maldonado Shot intervalSampling rate 50Processing m 4 ms 50 m 4 ms 50 m 4 ms 25 m 50 m 2 ms 50 m 4 ms 50 m 25/50 m 50 m 50 m 4 ms 50 m 2 ms 50 m 2 ms 2 ms Marine Geophysical cruise CountryYearSource Italy 1989/1990 2 arrays Italy 1990/1991 1991/1992 Italy 1994/1995 1994 Italy 1988 Russia 1996/1997 Japan 1997/1998 1987/1988 Japan 1992 Japan 1992/1993 2004 Brazil Spain Spain Spain

Table 1 Oceanographic cruises, acquisition and post-proce et al., 2006). An older Unit 6 is also reported in regional studies of the

Please cite this article as: Maldonado, A., et al., A model of oceanic development by ridge jumping: Opening of the Scotia Sea, Glob. Planet. Change (2014), http://dx.doi.org/10.1016/j.gloplacha.2014.06.010 A. Maldonado et al. / Global and Planetary Change xxx (2014) xxx–xxx 7 southwestern Scotia Sea (Maldonado et al., 2010; Martos et al., 2013). amplitude reflections (Fig. 3-5, 3-6). Several minor discontinuities can Unit 5 is a lenticular infill of basement depressions with a tentative Ol- be identified within the unit. It exhibits a wedge to sheet-like external igocene to early Miocene age. Unit 4 is a sheet-like deposit, considered shape with a maximum thickness of about 900 m (Fig. 4). The upper to be early to middle Miocene in age. Unit 3 exhibits both mounded part of Unit 4 onlaps and buries the chaotic body in the eastern Ona and sheet-like geometries with a suggested middle to late Miocene Basin (Fig. 3-3). age (Maldonado et al., 2003, 2006). Mounded and sheet-like shapes are dominant in Unit 2, which is attributed a late Miocene to early Plio- 4.2.2. Youngest seismic units cene age. Unit 1 is described as a relatively thin early Pliocene to Recent The three youngest seismic units have abundant internal erosional unit. In this study, we identified a very deep depression of limited extent features and significant thickness variations. Unit 3 is characterized by in the eastern Ona Basin, which allows for the distinction of eight main a stratified laminar configuration, with very high-amplitude and lateral- seismic units above the basement. These units are numbered 8 to 1, ly continuous reflections, especially in the lower part of the unit (Fig. 3- from bottom to top (Figs. 3-3, 4). Units 5 to 1 are equivalent to those 6). Its internal reflections can show gentle downlap terminations, al- previously described in the southwestern Scotia Sea (cf., Maldonado though the unit may also onlap against basement highs. The unit depicts et al., 2006). a predominant sheet-like external geometry. Minor discontinuities are The seismic units are bounded by moderate to high-amplitude con- frequently observed within the unit. The thickness is variable, but is tinuous reflections designated g to a — from bottom to top — that con- normally less than 600 m (Figs. 3-5, 3-6, 4). Unit 2 is generally conform- stitute major stratigraphic discontinuities in the sedimentary record. able over the underlying deposits in the central part of the basin plain, They are erosive unconformities, which may evolve laterally into the whereas it shows onlap terminations toward the basin margins (Fig. basin plain to concordant surfaces. Reflector c is a widespread, very 3-5, 3-6). The unit exhibits sub-parallel, high-amplitude reflections high-amplitude reflection that locally depicts remarkable erosive fea- with high lateral continuity, and locally fills underlying depressions. tures (Maldonado et al., 2006; Lindeque et al., 2013). This discontinuity Small amplitude sediment waves are locally identified toward the top can be traced along the southwestern Scotia Sea and other adjacent ba- of Unit 1, which is incised by channels and moats in many places. The sins, and serves to divide the sedimentary record into the oldest units average thickness of Units 2 and 1 is 300 to 500 m, but they reveal sig- (8 to 4) and youngest units (3 to 1). The oldest units are more deformed nificant thickness variations (Fig. 3). and their distribution is controlled by the basement structures, although the youngest units are also locally deformed. In addition, small wedge- 5. Magnetic anomalies shaped deposits with internal chaotic character, weak acoustic response and irregular top boundaries, attributed to mass transport deposits, are The oceanic basement of Ona Basin is thrusted beneath the conti- identified within the sedimentary record, especially over the margins of nental blocks of the SSIB, as shown by the MCS profiles (Figs. 2B, 3). the banks. The continent/ocean boundary (COB), marked by a set of reversal faults, A thick, irregular seismic body is recognized in the eastern Ona Basin is depicted by a sharp increase of the magnetic field, which reaches up over the margins of the Ona High (Figs. 2, 3-3). This body displays a len- to 120 nT in profile M-05 (Figs. 3-6, 5, 6). This strong anomaly is associ- ticular shape with internal chaotic facies and shows abundant hyperbol- ated with the Pacific Margin Anomaly (PMA), which is interpreted to be ic reflections, a sharp basal contact with the underlying basement, and a caused by a linear magmatic complex that follows the Antarctic Penin- strong top reflection. The chaotic facies appear to be equivalent to sula and continues into the SSIB and the SSR (Garrett, 1990; Suriñach deposits emplaced by tectonics or mass transport processes, such as et al., 1997; Martos et al., in this issue). the olistostrome of the Gulf of Cádiz in the central eastern Atlantic Taking into account previous studies of the Scotia Sea (Eagles et al., (cf., Medialdea et al., 2004, among others). This chaotic seismic body 2005; Lodolo et al., 1997, 2010; Maldonado et al., 2010) based on mag- overlies a high-amplitude reflection that dips toward the WSW, being netic profiles recorded by oceanographic surveys, we adopt the same observed up to 9 s (TWTT) beneath the referred body (Fig. 3-3). approach for the interpretation of spreading magnetic anomalies in Ona Basin. Although aeromagnetic data are available in the study region 4.2.1. Oldest seismic units (Ghidella et al., 2002; Martos et al., in this issue), profiles from marine The oldest units are thicker in the basement depressions (Figs. 2B, 3). surveys are more appropriate to model spreading magnetic anomalies Seismic units 8 and 7 are only identified in the eastern Ona Basin, along in complex areas, as they are recorded at sea level and most of them po- the axis of a deep trough (Figs. 2B, 3-3, 4). Unit 6 is more widespread, sitioned according to the spreading corridors. The Ona basin plain but it is best developed in the southern part of the western Ona Basin, shows the presence of some magnetic anomalies that can be associated where the basement is deeper and tilted toward the SSIB (Fig. 3-5, 3-6). with oceanic spreading, but because of their subdued amplitude, these Unit 8 is characterized by massive to discontinuous, high-amplitude anomalies are clearly recognizable only in a few cases. It is important chaotic facies with no coherent internal layering and hyperbolic reflec- to note, moreover, that the sparse magnetic coverage of the study area tions (Fig. 3-3). It is a wedge-shaped unit with a maximum thickness of does not allow for a precise and confident identification of the entire se- about 800 m (Fig. 4). Unit 7 shows an aggradational configuration with quence of anomalies, and in some cases this fact precludes an accurate reflections onlapping the basement and reflection amplitudes increas- assignation of specific magnetic chrons. Spreading-related anomalies ing from bottom to top. The thickness is about 350 m (Fig. 4). Seismic are, however, tentatively identified in profiles M-26 and M-31, as well Unit 6 contains weak discontinuous reflections, which increase their as the northern part of M-25, M-17, IT-153 and IT-41 (Fig. 5). amplitude and continuity upward. It has a maximum thickness of The old magnetic profile Shack 71, running along the southern flank about 800 m (Figs. 3-5, 3-6, 4). Unit 6 overlaps the chaotic body in the of the western spreading corridor of the WSR, has been interpreted in eastern Ona Basin (Fig. 3-3); it is well developed near the contact of several studies (Eagles et al., 2005; Lodolo et al., 2006), because it repre- the abyssal plain with the SSIB in the western Ona Basin (Fig. 3-5, 3- sented one of the few available profiles pertaining to a single spreading 6). Unit 5 shows a more widespread distribution than the underlying corridor. It crosses the entire sequence of anomalies from the aban- deposits (Fig. 3). This unit exhibits high amplitude and continuous re- doned spreading segment of the WSR to the northern margin of the flections, mostly in its lower part (Fig. 3-5, 3-6). The external geometry SSIB. Profile NBP-93-1, its southern part shown in Fig. 5, parallels the is wedge-shaped to lenticular, with a maximum thickness of 500 m Shack 71 profile, and here is taken as the preferable reference because (Fig. 4). Unit 5 downlaps the underlying deposits and progrades west- it is DGPS-positioned. Yet both profiles show the same sequence of wards, reflecting a progressive uplift of the basement in the eastern anomalies and a similar value of the corresponding amplitudes. The part of Ona Basin (Fig. 3-3). Unit 4 shows parallel, continuous, high- youngest anomaly identified in this profile falls within the normal po- amplitude reflections that laterally become discontinuous, low- larity parts of chron C3A (6.6–5.9 Ma), as interpreted by Maldonado

Fig. 3. Representative multichannel seismic (MCS) profiles and line drawing interpretations of the same profiles: 3-1, Profile ANT92-18A of the western Ona Basin crossing the basin plain and the SFZ; 3-2, Profile TH96-35 of the western Ona Basin crossing the basin plain and the SSIB; 3-3, Profile HESANT92/93-31 between the western and eastern Ona Basin crossing the Ona High. 3-4, Profile HESANT92/93-04 of the western Ona Basin crossing the basin plain and the SSIB; 3-5, Profile SA500-019 of the western Ona Basin crossing the basin plain and the SSIB; 3- 6, Profile HESANT92/93-05 of the western Ona Basin crossing the basin plain and the SSIB; The main reflections identified in the study (Reflectors a to g) are shown in the profiles. See Fig. 2A for location. Discussion in the text.

Fig. 3 (continued).

body is a regular layer 0.5 km thick. The angular parameters of the mag- 6. Gravity anomalies netization vector were consistently taken according to the parameters of the symmetrical dipole of the earth's field axis, considering that lati- The satellite-derived free-air anomaly values vary from about +100 tude during oceanic spreading was roughly close to the present day lat- to −100 mGal in the region (Fig. 5). The SSIB depicts a WSW–ENE elon- itude. The parameters of the present day magnetic field coincide with gated trend, usually with positive values of about +30 to +120 mGal. the parameters of the DGRF field of the time of the survey. The best fit An elongated feature with lower values (up to −150 mGal) is observed of the model with the observed anomalies was obtained considering a southwards of the eastern part of the SSIB; it is attributed to the Hespér- magnetization of 2 A/m (Fig. 6). The magnetic seafloor anomalies ides Deep and indicates stretched continental crust (Acosta and Uchupi, shown in our model of Profile M-05 are tentatively identified consider- 1996; Galindo-Zaldivar et al., 1996; Bohoyo et al., 2007; Civile et al., ing a process of regular seafloor spreading. We suggest a spreading cen- 2012). Most of the basin shows low positive and negative values ter in the southern region of the western Ona Basin, spanning chrons (Fig. 5). The NW–SE-trending SFZ, in the western region, displays high C11r to C12r, whereas in the northern region of the basin the pattern positive values (Martos et al., 2014a). The South Shetland Trench is ob- of magnetic anomalies would correspond to the southern flank of the served to the west of the intersection between the SFZ and the SSIB, WSR, which may include chron C11n.1n (Figs. 5, 6). A mean full spread- with negative values of around −100 mGal. Ona Basin exhibits two ing rate of 8.4 km/Ma is envisaged during this early 3 Ma period, where areas with slightly lower gravity anomalies: (a) a NW–SE elongated the dissymmetry of the magnetic anomalies between the two flanks anomaly parallel to the SFZ in the western Ona Basin with a decreasing could be attributed to the normal faults affecting the northern flank value near the SFZ, and (b) a WSW–ENE elongated anomaly parallel to (Figs. 3-6, 6). The regional compilation of profiles does not allow a linear the SSIB (Fig. 5). The western Ona Basin fabric depicts a rough WNW– correlation of the anomalies, however (Fig. 5). Tentative correlations ESE trend in the central sector. The Ona High shows positive anomalies could be made with the anomalies on the nearest profile to the west, that protrude seaward from the SSIB, while it is bounded by negative NBP-93-2 (~30 km away), but the anomalies are not consistent with anomalies in the basin (Fig. 5). profile IT-41, which intersects it at an acute angle. Furthermore, the dis- The gravity models orthogonal to the SSIB in the western Ona Basin tribution pattern of anomalies is not coherent with a single spreading indicate an igneous oceanic crust with significant thickness variations, center (Figs. 5, 6). A chaotic, preliminary episode of opening associated from about 2 km to more than 8 km in thickness (Fig. 7A, B). The oceanic with the early stages of continental fragmentation of the South crust is thinner in the southern part and it is thrusted by the continental America–Antarctic Peninsula Bridge was suggested to explain the na- crust of the SSIB margin. The sediments of the basin, in contrast, thicken ture of the magnetic anomalies in the SW Scotia Sea (Barker and southward, from about 1 km to more than 2 km at the base of slope. The Burrell, 1977). igneous crust is affected by normal faults and the sediment distribution

Fig. 3 (continued).

Fig. 3 (continued). is influenced by the structure of the crust (Figs. 3-4, 3-5, 7A, B). The con- the early stages of development of the Scotia Sea. We propose a model tinental crust of the SSIB is about 10 to 12 km thick near the slope, and of development for Ona Basin that involves ridge jumping. becomes more than 15 km thick to the south. Gravity profile ANT92/93- M31, which crosses the Ona High, reveals small discrepancies between 7.1. Age of the oceanic crust the data and the model that can be attributed to the complex structure and morphology of the area, and also to the fact that the model does not On the basis of identification of the southwestern Scotia Sea magnet- take into account lateral variations. We tested several hypotheses based ic anomalies, three tectonic scenarios for the creation of oceanic crust in on the information derived from the MCS profiles; the best fit congruent Ona Basin may be put forth: (a) the present day westernmost spreading with the geology of the region is shown in Fig. 7C. The Ona High is corridors of the southwestern Scotia Sea (to the east of the SFZ) were interpreted as a thinned continental crustal block, with a maximum created by the WSR oceanic accretion; in this scenario, Ona Basin thickness of 10 km, but highly thinned by faults (Figs. 3-3, 7C). The oce- would be part of the southernmost flank of the western spreading cor- anic crust is slightly thicker than 6 km in the western Ona Basin. In the ridor; (b) several spreading centers were active in different periods in eastern Ona Basin the crust shows major thickness variations, from the Ona Basin region, and a remnant of the original ridge is present in about 6 km thick to only a few hundred meters thick or absent in the the oceanic crust of the area; and (c) part of the initial oceanic crust central sector (Figs. 3-3, 7C). The sediments thicken in the central de- was subducted below the SSIB, probably including the original spread- pression of the eastern Ona Basin and the chaotic wedge is depicted ing center. by an irregular body on top of the oceanic crust, over the eastern margin The analyzed profiles show oceanic magnetic anomalies in the west- of the Ona High. ern Ona Basin, although their precise recognition is elusive (Figs. 5, 6). The existence of a single spreading center in the southwestern Scotia 7. Discussion Sea is not supported by the sequence of anomalies, since continuous spreading of the WSR is only reliable until anomaly C10n. The profiles Analysis of the extensive geophysical data set pertaining to Ona reveal cycles of magnetic reversals in the southern region of Ona Basin Basin highlights the relevance of the region for an understanding of between C11n and the northern SSIB slope, although an organized

Fig. 4. A, Segment of multichannel seismic reflection profile HESANT92/93-31 of the eastern Ona Basin with the line drawing interpretation showing the seismic units and reflectors identified in this study. Depth in seconds of two-way travel time (TWTT). B, Seismic units and reflectors identified in this study, showing sedimentation rates and thickness (km), according the method described by Maldonado et al. (2013) (details in: Section 3). See Figs. 2Aand3-3 for location of the profile. Further discussion in the text. pattern of older reversal anomalies is not well recognized from the the western Ona Basin (Fig. 6). The mean full spreading rate during this available profiles (Fig. 6). These older anomalies are not documented 3 Ma period was of 8.4 km/Ma, which is high in comparison to the in the conjugated margin off Tierra del Fuego (Lodolo et al., 2006). mean half-spreading rate of the southern Scotia Sea of 2.5 km/Ma, ob- The older anomalies in the western Ona Basin are attributed to a for- served between anomalies C5 and C10 (Livermore et al., 2005). The mer spreading center, where we tentatively suggest chrons C11n and spreading center jumped northward to develop the WSR after chron C12n (Fig. 6). The nature of the anomalies does not allow a definitive C11r (30.5 Ma) (Fig. 5). These anomalies appear to be offset in the west- identification, however, and the anomalies may be even older than ern Ona Basin, probably due to the strike-slip motion by transform faults interpreted. Seafloor spreading processes probably started during chron and intra-plate deformation during subsequent tectonic phases (Figs. 2B, C12r (33.3 Ma) or earlier and ceased during chron C11r (30.5 Ma) in 3, 6). Based on the available data, we conclude that it is still impossible to

7.2. Tectonics and seismic stratigraphy of the Ona Basin

The Ona High is interpreted as a continental block fragment, similar to other isolated banks occurring along the southern Scotia Sea (Bohoyo et al., 2007; Civile et al., 2012; Galindo-Zaldívar et al., in this issue). The Fig. 6. Magnetic anomaly data and model of proﬁle M-05 (HESANT92/93) of the western crust of the continent-like block is thin according to the gravity models, Ona Basin crossing the basin plain and the SSIB. See Figs. 2Aand3-6 for location and the suggesting the existence of magmatic intrusions. However, the present MCS proﬁle interpretation, respectively. Discussion in the text.

Fig. 5. Position map of the magnetic profiles used in this study, superposed on the GEOSAT satellite-derived gravity map (Sandwell and Smith, 2009). Seafloor linear magnetic anomalies plotted along tracks and superposed on the free-air gravity map are shown. Different track colors indicate the different sources: black, Spanish profiles; red, Italian profiles; white, National Geophysical Data Centre (NGDC) profiles at NOOA; yellow, DSDP profile at DSDP database. Magnetic data, acquired in different years by different Institutions, were homogenized to allow for uniform representation of the anomalies. In general, these vary from −400 to 400 nT along most of the presented magnetic anomaly profiles. The sequence of the high-amplitude anomalies found along the southern part of profile M-05 depicts the occurrence of the well-known Pacific Margin Anomaly (PMA) (Garrett, 1990). Discussion in the text.

Fig. 7. Representative gravity model profiles to illustrate the geometry and nature of the crust in Ona Basin. Legend of profiles SA500-003, HESANT92/93-05 and HESANT92/93-31: 1, water (1.03 g/cm3); 2, sediments (2.4 g/cm3); 2′, chaotic body (2.7 g/cm3); 3, oceanic crust (2.85 g/cm3); 4, mantle (3.25 g/cm3); 5, deep mantle intrusion (2.75 g/cm3); 6, thinned continental crust (2.67 g/cm3); 7, continental crust (2.67 g/cm3). Detailed discussion in the text. See Fig. 2A for location of the profiles.

Fig. 8 (continued).

Fig. 9. Sketches of geological cross section reconstructions during four tectonic phases identiﬁed in this study, showing the main crustal elements of the region. See Fig. 8 for the plate tectonic reconstructions.

day structure is dominated by reverse faults (Figs. 3-3, 7C). The Ona High deposits reveal four elongated depocenters, generally bounded by nor- is bounded on both margins by oceanic crust, although the COB is not mal faults trending WNW–ESE in the NE part of the basin, to almost well imaged and it is probably affected by reverse faulting. Rock samples NW–SE in the proximity of the SFZ (Fig. 2B). The deposits display a recently dredged from the Ona High during the SCAN2013 cruise reveal southward thickening from less than 2 s (TWTT) to more than 2.5 s a large variety of plutonic, metamorphic and volcanic rocks that attest to (TWTT) in the proximity of the SSIB, where Unit 6 is observed (Fig. 3- the continental nature of the block (Maldonado et al., 2013). 5, 3-6). Although the igneous basement shows signiﬁcant thickness var- The basement of the western Ona Basin is tilted to the south, from 5 s iations in the gravity models, it is considered to be equivalent to other (TWTT) in the northern sector to about 6.5 s (TWTT) in the southern oceanic provinces of the western Scotia Sea (cf., Maldonado et al., 2000). sector, and it is thrusted by the SSIB (Figs. 3-2, 7A, B). In the proximity The eastern Ona Basin contains a basement with different structural of the intersection between the SFZ and the SSIB, the basement is largely trends (Fig. 2B). The igneous crust is very thin and thrusted to the east deformed by reversal faults while also thrusted by the SFZ (Fig. 3-1). The along the axis of the basin (Figs. 3-3, 7C). The depositional units

Fig. 8. Plate tectonic reconstruction of the region during the four tectonic phases identiﬁed in this study. The relative positions of the South American and Antarctic Peninsula crustal blocks for the tectonic reconstructions at selected time slices were provided by the PLATES Project data compilation (Lawrence Lawver, pers. comm.). See Fig. 9 for the geological cross sections of the successive tectonic phases.

Please cite this article as: Maldonado, A., et al., A model of oceanic development by ridge jumping: Opening of the Scotia Sea, Glob. Planet. Change (2014), http://dx.doi.org/10.1016/j.gloplacha.2014.06.010 18 laect hsatcea:Mloao . ta. oe foencdvlpetb ig upn:Oeigo h ctaSa lb lnt Change Planet. Glob. Sea, Scotia the of Opening jumping: ridge by development oceanic of model A (2014), al., et A., Maldonado, as: article this cite Please http://dx.doi.org/10.1016/j.gloplacha.2014.06.010 .Mloaoe l lbladPaeayCag x 21)xxx (2014) xxx Change Planetary and Global / al. et Maldonado A. – xxx

Fig. 10. Synthetic chart summarizing the major global changes during the Cenozoic, including climatic changes and tectonic events, and correlation with the regional stratigraphy in the Scotia Sea. Three types of global records were taken into account: sea level (in meters related to the present sea level), deep-sea oxygen isotope (in ‰) and atmospheric CO2 (proxied by partial pressure of CO2 in parts per million by volume (ppmv)). Global sea level records: (1) global sea level for the interval 0 to 7MaafterMiller et al. (2005); (2) global sea level for the interval 7 to 65 Ma after Miller et al. (2005); (3) global sea level for the interval 7 to 65 Ma after Kominz et al. (2008). Oxygen isotopic records: (4) oxygen isotopic synthesis after Cramer et al. (2009); (5) stacked deep-sea bethic foraminiferal oxygen-isotope curve after Zachos et al. (2008). Atmospheric CO2 records: (6): boron proxy after Zachos et al. (2008);(7)alkenoneproxyafterZachos et al. (2008). Major climatic changes and events synthetized from Zachos et al. (2001, 2008). Major tectonic events extracted from Maldonado et al. (2006) and this study. Legend: NHIS: Northern Hemisphere Ice Sheets; PETM (ETM1): Eocene Thermal Maximum 1; ETM2: Eocene Thermal Max- imum 2; SFZ: Shackleton Fracture Zone; TWTT (s): two-way travel time (seconds). A. Maldonado et al. / Global and Planetary Change xxx (2014) xxx–xxx 19 identified in the eastern Ona Basin provide additional insight into the this phase (cf., Livermore et al., 2005) gave rise to a process of continen- age of the initial spreading (Fig. 4). Two large NNW–SSE-trending tal thinning, although there is no evidence of back-arc spreading in depocenters are observed: one near the axis of the basin and the other Drake Passage. We suggest asymmetrical rifting, resembling that of on the eastern margin of the Ona High (Fig. 2B). The deposits reach a other Atlantic-type conjugated margins, as illustrated by the continental maximum thickness of 3.4 s (TWTT) along the axis of the depocenter, margin off Labrador (Dickie et al., 2011). Evidence of continental where the older units of Ona Basin are identified (Figs. 3-3, 4). In view stretching during the latest Paleocene and early Eocene in Tierra de of the interval velocities obtained from the MCS profiles, this thickness Fuego has been reported (Ghiglione et al., 2008), while the deposits of represents about 5 km of deposits. The chaotic body observed over the the passive Atlantic margin contain lower slope units (Olivero and eastern margin of the Ona High is characterized by high density in the Malumián, 1999, 2008; Torres Carbonell and Olivero, 2012). An early- gravity models (2.7 g/cm3), which would indicate a volcano-clastic ori- middle Eocene pulse of Andean uplift is recognized by the erosional un- gin, largely emplaced by mass transport processes and partially conformities in Tierra del Fuego (Olivero et al., 2003) and in the tectonized (Figs. 3-3, 7C). Unit 4 fossilizes the chaotic body, whereas Malvinas Basin (Baristeas et al., 2012). This event may have coincided the underlying units are syntectonic with its emplacement (Fig. 3-3). with the end of Phase I and preceded the onset of back-arc spreading. The occurrence of this thick depositional wedge, not previously recog- Phase II included back-arc spreading processes in the eastern Ona nized in the Scotia Sea, supports the hypothesis that this region was Basin, which created a NNW–SSE trending rift resulting from the south- the initial site of seafloor spreading. Assuming an age of 33 Ma for the eastward migration of a proto Scotia Arc system prograding over the base of Unit 6, which is located on the western Ona Basin atop a base- Atlantic Ocean and the Weddell Sea (Figs. 8B, 9B). The larger block of ment probably older than chron C12n, the estimated age of the base of continental crust was located in the eastern flank, while a smaller Unit 8 is middle Eocene (about 44 Ma) (Figs. 3-3, 4, 6). This timing sug- block was probably located in the western flank, overriding the Phoenix gests that the initial spreading of the eastern Ona Basin occurred several Plate. An abrupt change from N–StoWNW–ESE of the direction of sep- million years earlier than previously postulated. We speculate that, ac- aration of South America and Antarctica near anomaly C21 (47 Ma) has cording to the deposit distribution, the initial phase of oceanic spreading been put forth, accompanied by an increase of the spreading rate from occurred in the eastern Ona Basin, which originated from a N–Soriented about 3 to 24 km/Ma (Livermore et al., 2005); this would agree with spreading center. These observations would indicate that the oldest the estimated spreading age of 44 Ma for the eastern Ona Basin. Most basin of the Scotia Arc region is the eastern Ona Basin, whereas oceanic of the continental blocks now dispersed around the Scotia Arc were pos- spreading in the western Ona Basin occurred later, during the early sibly located during Phase II to the east of the eastern Ona spreading Oligocene. center, in turn located near the tip of the Antarctic Peninsula (Fig. 9B). The initial phases of extension and continental thinning, followed by Phase III was characterized by a westward jumping of the spreading oceanic spreading, were characterized by jumping of the spreading cen- center, which became WNW–ESE oriented in the western Ona Basin ters, initially from the eastern Ona Basin to the southern part of the (Figs. 8C, 9C). A model of magma-poor rifted margin could be evoked western Ona Basin, and subsequently to the north to generate the to explain the elusive nature of the magnetic anomalies (Bronner WSR. These initial phases of spreading are related to the expanding et al., 2011). The deformation of the region during successive tectonic forearc system of the Scotia Arc toward the Atlantic Ocean and the events makes it difficult to determine the age and distribution of the Weddell Sea (Barker, 2001; Eagles et al., 2005; Livermore et al., 2005; original oceanic crust (Figs. 2B, 3). The primitive continental bridge mi- Eagles and Jokat, 2014). Yet we propose a novel model that accounts grated eastward, whereas the subduction of the Weddell Sea propagat- for the early development of the Scotia Arc, and provide ground evi- ed northward and eastward to the Atlantic Ocean. We speculate that dence for the timing of the opening of Drake Passage (Figs. 8, 9). Follow- during this phase the Phoenix Plate subduction below the South ing subduction of the Pacific plates below the South America–Antarctic America–Antarctic Peninsula continental bridge ended, and the bound- Bridge, the initial phases of the tectonic evolution of Scotia Sea were ary between the developing Scotia Plate and the Phoenix Plate became characterized by oceanic back-arc spreading and ridge jumping, while the SFZ intra-oceanic transform fault zone. The Scotia Arc developed the forearc migrated from the Pacific to the Atlantic margin of the bridge progressively as an active margin over the Atlantic Ocean (Figs. 8C, (Figs. 8, 9). After seafloor spreading ceased in Ona Basin, the entire re- 9C). The new orientation of the rift facilitated the fragmentation of the gion underwent tectonic inversion and thrusting of the recently created continental blocks located near the eastern tip of the Antarctic Peninsu- oceanic crust, which also became subducted below the SSIB (Fig. 2B). la, which first became isolated in the continental bridge and were later dispersed eastward with the overriding slab. Continuous stretching and 7.3. The opening of the Scotia Sea: a model of ridge jumping thinning produced delamination of the crust and facilitated the exhu- mation of the mantle. The Atlantic and Weddell plates were affected A model for the opening of the Scotia Sea is proposed in four by subductions under the expanding Scotia Arc. The high WNW–ESE phases: Phase I, Pacific subduction and initial continental fragmenta- separation rate (24 km/Ma) of South America–Antarctica also reported tion, Paleocene–middle Eocene; Phase II, eastern Ona Basin back-arc during this phase (Livermore et al., 2005), facilitated back-arc processes spreading, middle–late Eocene; Phase III, ridge jumping and western and the stretching of the primitive continental bridge. Ona Basin back-arc spreading, early Oligocene; and Phase IV, ridge Phase IV was triggered by a new ridge jumping and the initiation of jumping, WSR spreading and continental block dispersal, early Oli- the WSW–ENE trending WSR after chron C11n (30 Ma). The intra- gocene–late Miocene. The plate tectonic reconstruction of the region oceanic ridge of the SFZ became a transform fault and marked the during the successive phases is depicted in Fig. 8, whereas the main boundary between the Phoenix and Scotia plates. Back-arc extension crustal elements are illustrated in the cross sections of Fig. 9. The rel- with eastward migration of the subduction zone was accompanied by ative positions of the South American and Antarctic Peninsula crustal extension and rifting of the continental fragments. New basins such as blocks for the tectonic reconstructions at selected time slices were Scan, Protector, Pirie and Dove basins (Galindo-Zaldivar et al., 2006, in providedbythePLATESProjectdatacompilation(Lawver and this issue; Pérez et al., in press, in this issue) were created along the Gahagan, 2003; Laurence Lawver, pers. comm.). northern margin of the SSR as the continental fragments were dispersed Phase I was characterized by the subduction of the Phoenix Plate eastward during the Oligocene and Miocene (Figs. 8D, 9D). We suggest below the South America–Antarctic continental bridge (Figs. 8A, 9A). that the opening of Powell and Scan basins may have occurred during A short segment of the margin experiencing subduction in the north- the initiation of this tectonic phase, although conflicting ages have western Weddell Sea is postulated in some plate tectonic reconstruction been proposed (cf., Eagles and Livermore, 2002; Pérez et al., in this models, however (Vérard et al., 2012; Eagles and Jokat, 2014). The ex- issue). The present configuration and geodynamic setting of the Scotia tremely slow N–S separation of Antarctica and South America during Sea was progressively achieved during the late Miocene, although

7.4. Implications for paleoceanography and paleoclimate: initial gateways cant drop in atmospheric CO2 is observed in the upper part of the of Drake Passage early Oligocene, about 3 Ma after the development of Drake Passage gateways according to our model, in coincidence with the beginning The model of opening of Ona Basin, together with oceanic spreading of tectonic Phase IV (Fig. 10). influenced by ridge jumping, configured a changing scenario of shallow During tectonic Phase IV there were significant fluctuations in cli- and deep gateways during the initial Eocene to Oligocene tectonic mate, sea level and atmospheric CO2 (Fig. 10). The ACC became fully de- phases, across a tectonic arc between the Pacific and Atlantic oceans veloped in the central Scotia Sea, whereas during the middle Miocene (Figs. 8, 9). To assess how the development of new gateways influenced the first incursions of the WSDW were imprinted in the depositional the deposits and the paleoceanography of the region, we compiled a units, when new gateways in the SSR allowed the connection between synthetic chart including available long-term information on sea level, the Scotia and Weddell seas (Maldonado et al., 2003, 2006). The uplift oxygen isotopic records and atmospheric CO2 records (Fig. 10). The of the SFZ since the middle Miocene (about 12 Ma) imposed new re- major tectonic and depositional events identified in the Scotia Sea are strictions on the flows of the ACC into the Scotia Sea and initiated the also analyzed within the context of global changes, so as to investigate present deep-water circulation regime (Hernandez-Molina et al., the possible cause/effect correlation between the local tectonic evolu- 2006; Martos et al., 2013). Some of the regional seismic reflections tion and the global changes. that we recognize in the sedimentary record, not discussed here in de- During the Paleocene to middle Eocene — Phase I — subduction of tail, likewise reveal a significant correlation between local tectonic the Phoenix Plate along the active Pacific margin of South America events in the Scotia Sea, ice sheet dynamics and/or global climatic and and the Antarctic Peninsula obstructed a Pacific–Atlantic connection tectonic events (see Maldonado et al., 2006). It is particularly important across the original continental bridge. This remained as an effective bar- to analyze the causes behind the development of Reflector c, which was rier to water-mass flow due to the slow N–S separation of South influenced by the opening of new gateways and the dynamics of ice America and Antarctica. Sea levels were relatively high and the δ18Ore- sheets in West Antarctica (Lindeque et al., 2013; Martos et al., 2013). cord shows low values (Fig. 10). A climatic optimum is reported in the early Eocene (Zachos et al., 2008; Cramer et al., 2009). Tectonic process- 8. Conclusions es identified in the southeastern Pacific include high convergence rates between the South American and the Farallon Plates (Molnar et al., 1. Ona Basin is the westernmost of the southern Scotia Sea small ocean- 1975; Pardo-Casas and Molnar, 1987; Vérard et al., 2012), along with ic basins developed behind fragments of a tectonic arc migrating to the Andean uplift of Tierra del Fuego (Olivero et al., 2003), which may the east. This basin includes two sub-basins, the western and eastern have influenced climatic change through a shifting of the climatic Ona basins, separated by the Ona High. The structural evolution and belts and bottom currents. Nonetheless, we fail to observe any signifi- the geometry and distribution of deposits reveal Ona Basin to be cant tectonic changes in the Drake Passage region that would be corre- older than previously postulated. lated to climate events. 2. The basement of the basin plain is southwestward tilted and thrusted The middle Eocene shows another climatic optimum during tectonic below the SSIB and the SFZ. The igneous basement is fragmented by Phase II (Fig. 10). The trend of the δ18O displayed a general increase, par- depressions and incised by deep intrusions probably rooted in the allel to a sea level lowering (Fig. 10). This steady decrease in tempera- mantle. An accretionary prism adjacent to the foot of the SSIB is ture of the deep-sea record was also accompanied by an overall well developed in the westernmost region of the overriding block. decrease in atmospheric CO2, although there were significant fluctua- The Ona High is a thinned continental block attached to the SSIB. tions (Zachos et al., 2008; Cramer et al., 2009). The continental bridge 3. A total of eight seismic depositional units above the basement are of South America and the Antarctic Peninsula was fragmenting; shallow identified in a narrow north–south trending depression of the east- gateways consequently led to a connection between the Pacificand ern Ona Basin. The deposits reach a thickness of about 5 km (3.4 s Atlantic oceans, initiating the thermal isolation of Antarctica during TWTT), a consistent value not previously reported from the entire themiddleandlateEocene(Figs. 8B, 9B). The onset of the flow between Scotia Sea. A body of chaotic seismic facies is also observed directly the Pacific and Atlantic oceans by the Argentinian continental margin above the basement in the eastern Ona Basin. has been related to unconformity AR4, which was initially attributed 4. Linear magnetic seafloor spreading anomalies older than previously to the late Eocene, and represents the base of a buried giant drift reported are tentatively recognized in Ona Basin, including anomaly (Hinz et al., 1999). In the Malvinas Basin, the occurrence of thick glauco- C11 and probably up to chron C12r. The distribution of magnetic nitic Paleocene–Eocene sandy deposits signals the occurrence of power- anomalies, moreover, is not consistent with a single spreading ful bottom flows (Baristeas et al., 2012). The fragments of continental center. crust of the original bridge within Drake Passage, including the Ona 5.Thegrowthpatternsoftheregionareestablishedfromthenatureof High and the deep oceanic trough of the eastern Ona Basin, produced the crust, the tectonic evolution and the distribution of deposits, a paleogeographic configuration that facilitated these flows (Figs. 8B, which altogether provide for a robust reconstruction of the early open- 9B). The early opening of Drake Passage is postulated in several papers, ing of the Scotia Sea and the breakup of the South America–Antarctic although we present the first structural and sedimentary evidence for Peninsula Bridge during the Eocene.

6. The initial phases of continental thinning and extension, followed by Bronner, A., Sauter, D., Manatschal, G., Péron-Pinvidic, G., Munschy, M., 2011. Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted margins. oceanic spreading, were characterized by jumping of the spreading Nat. Geosci. 4 (8), 549–553. centers. Two extinct spreading centers are identified in Ona Basin: Brown, B., Gaina, C., Müller, R.D., 2006. Circum-Antarctic palaeobathymetry: illustrated the eastern Ona spreading center is the precursor of the Scotia Arc examples from Cenozoic to recent times. Palaeogeogr. Palaeoclimatol. Palaeoecol. 231 (1–2), 158–168. and it was active during the Eocene; the western Ona spreading cen- Catalán, M., Galindo-Zaldivar, J., Davila, J.M., Martos, Y.M., Maldonado, A., Gambôa, L., ter is attributed to the early Oligocene. These spreading centers were Schreider, A.A., 2013. Initial stages of oceanic spreading in the Bransfield Rift from active prior to the beginning of the WSR. magnetic and gravity data analysis. Tectonophysics 585, 102–112. – 7. A model with four tectonic phases is proposed for the development Civile, D., Lodolo, E., Vuan, A., Loreto, M.F., 2012. Tectonics of the Scotia Antarctica plate boundary constrained from seismic and seismological data. Tectonophysics of the western Scotia Sea: Phase I, Pacific subduction; Phase II, east- 550–553, 17–34. ern Ona Basin back-arc spreading; Phase III, ridge jumping and west- Cramer, B.S., Toggweiler, J.R., Wright, J.D., Katz, M.E., Miller, K.G., 2009. Ocean overturning ern Ona Basin back-arc spreading; and Phase IV, ridge jumping and since the late Cretaceous: inferences from a new benthic foraminiferal isotope compilation. Paleoceanography 24 (PA4216), 1–14. WSR spreading. Based on the stratigraphy of the deposits and the Dalziel, I.W.D., 1984. The Scotia Arc: an international geological laboratory. Episodes 7, recognition of oceanic spreading magnetic anomalies, a middle Eo- 7–13. cene age is derived for the onset of spreading in the eastern Ona Dalziel, I.W.D., Lawver, L.A., Norton, I.O., Gahagan, L.M., 2013a. The Scotia Arc: genesis, evolution, global significance. Annu. Rev. Earth Planet. Sci. 41, 767–793. http://dx. basin, while spreading in the western Ona Basin occurred during doi.org/10.1146/annurev-earth-050212-124155. the early Oligocene. Dalziel, I.W.D., Lawver, L.A., Pearce, J.A., Barker, P.F., Hastie, A.R., Barfod, D.N., Schenke, H. 8. Shallow gateways in Drake Passage are postulated to have been ac- W., Davis, M.B., 2013b. A potential barrier to deep Antarctic circumpolar flow until the late Miocene? Geology 41, 947–950. http://dx.doi.org/10.1130/G34352.1. tive at the beginning of tectonic Phase II (44 Ma), whereas deep gate- DeConto, R.M., Pollard, D., 2003. Rapid Cenozoic glaciation of Antarctica induced by de- – ways become fully developed during Phase III (33 30 Ma). A clining atmospheric CO2. Nature 421 (6920), 245–249. significant correlation is observed between the tectonics, strati- Dickie, K., Keen, C.E., Williams, G.L., Dehler, S.A., 2011. Tectonostratigraphic evolution of fl the Labrador margin, Atlantic Canada. Mar. Pet. Geol. 28 (9), 1663–1675. graphic units and major climate events, indicating the in uence of Eagles, G., Livermore, R.A., 2002. Opening history of Powell Basin, Antarctic Peninsula. local tectonic and paleoceanographic events of the Southern Ocean Mar. Geol. 185 (3–4), 195–205. upon the global evolution. Eagles, G., Livermore, R.A., Fairhead, J.D., Morris, P., 2005. Tectonic evolution of the West Scotia Sea. J. Geophys. Res. B Solid Earth 110 (2), 1–19. Eagles, G., Livermore, R., Morris, P., 2006. Small basins in the Scotia Sea: the Eocene Drake Acknowledgments Passage gateway. Earth Planet. Sci. Lett. 242 (3–4), 343–353. Eagles, G., Jokat, W., 2014. Tectonic reconstructions for paleobathymetry in Drake Passage. Tectonophysics 611, 28–50. http://dx.doi.org/10.1016/j.tecto.2013.11.021. Funding was provided by Spain's Antarctic Research Program, Foldvik, A., Gammelrsød, T.P.P.P., 1988. Notes on Southern Ocean hydrography, presently under the management of the “Ministerio de Economía y sea-ice and bottom water formation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 67, – Competitividad”, to the SCAN (SCotia-ANtarctic) projects, active in the 3 17. Galindo-Zaldivar, J., Balanyá, J.C., Bohoyo, F., Jabaloy, A., Maldonado, A., Martínez- region since 1992. We thank the Commander, officers and crew of the Martínez, J.M., Rodríguez-Fernández, J., Suriñach, E., 2002. Active crustal fragmenta- BIO HESPERIDES for their support in obtaining the data, sometimes tion along the Scotia–Antarctic plate boundary east of the South Orkney – – under severe sea conditions. Particularly, this work was undertaken Microcontinent (Antarctica). Earth Planet. Sci. Lett. 204 (1 2), 33 46. Galindo-Zaldivar, J., Bohoyo, F., Maldonado, A., Schreider, A., Suriñach, E., Vázquez, J.T., within the objectives of projects PN-MICINN-CTM2008-06386-CO2- 2006. Propagating rift during the opening of a small oceanic basin: the Protector 01/02 and CTM2011-30241-C02-01/02, also supported by the FPI and Basin (Scotia Arc, Antarctica). Earth Planet. Sci. Lett. 241 (3–4), 398–412. JAE-Predoc programs. Additional funds were provided by the Italian Galindo-Zaldivar, J., Jabaloy, A., Maldonado, A., Sanz de Galdeano, C., 1996. Continental fragmentation along the South Scotia Ridge transcurrent plate boundary (NE Antarc- PNRA (Programma Nazionale di Ricerche in Antartide). This research tic Peninsula). Tectonophysics 258 (1–4), 275–301. is also related with the CTM 2012-39599-C03, IGCP-619 and INQUA Galindo‐Zaldívar, J., Puga, E., Bohoyo, F., González, F.J., Maldonado, A., Martos, Y.M., Pérez, 1204 Projects. We acknowledge the database obtained from the Antarc- L.F., Ruano, P., Schreider, A.A., Somoza, L., Suriñach, E., Díaz de Federico, A., 2014. The magmatism, structure and age of the Dove Basin (Antarctica): a key to understanding tic Seismic Data Library System. LawrenceLawver provided the conti- the South Scotia Arc development. Glob. Planet. Chang. in this issue. nental reconstructions used in the plate tectonics shown in Fig. 8.We Garrett, S.W., 1990. Interpretation of reconnaissance gravity and aeromagnetic surveys of also thank A. Caballero for his instrumental help in preparing the fig- the Antarctic Peninsula. J. Geophys. Res. 95 (B5), 6759–6777. ures. 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