Earth and Planetary Science Letters 414 (2015) 156–163

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Earth and Planetary Science Letters

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The Cretaceous opening of the South Atlantic Ocean

a, b Roi Granot ∗, Jérôme Dyment a Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel b Institut de Physique du Globe de Paris, CNRS UMR 7154, Sorbonne Paris Cité, Université Paris Diderot, 75005 Paris, France a r t i c l e i n f o a b s t r a c t

Article history: The separation of South America from Africa during the Cretaceous is poorly understood due to the Received 2 October 2014 long period of stable polarity of the geomagnetic field, the Cretaceous Normal Superchron (CNS, lasted Received in revised form 8 January 2015 between 121 and 83.6 Myr ago). We present a new identification of magnetic anomalies located Accepted 13 January 2015 ∼ within the southern South Atlantic magnetic quiet zones that have arisen due to past variations in the Available online xxxx strength of the dipolar geomagnetic field. Using these anomalies, together with fracture zone locations, Editor: P. Shearer we calculate the first set of magnetic anomalies-based finite rotation parameters for South America and Keywords: Africa during that period. The kinematic solutions are generally consistent with fracture zone traces plate motions and magnetic anomalies outside the area used to construct them. The rotations indicate that seafloor South Atlantic spreading rates increased steadily throughout most of the Cretaceous and decreased sharply at around magnetic anomalies 80 Myr ago. A change in plate motion took place in the middle of the superchron, roughly 100 Myr ago, quiet zone around the time of the final breakup (i.e., separation of continental–oceanic boundary in the Equatorial Atlantic). Prominent misfit between the calculated synthetic flowlines (older than Anomaly Q1) and the fracture zones straddling the African Plate in the central South Atlantic could only be explained by a combination of seafloor asymmetry and internal dextral motion (<100 km) within South America, west of the Rio Grande fracture zone. This process has lasted until 92 Myr ago after which both Africa and ∼ South America (south of the equator) behaved rigidly. The clearing of the continental–oceanic boundaries within the Equatorial Atlantic Gateway was probably completed by 95 Myr ago. The clearing was ∼ followed by a progressive widening and deepening of the passageway, leading to the emergence of north– south flow of intermediate and deep-water which might have triggered the global cooling of bottom water and the end for the Cretaceous greenhouse period. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The available kinematic models for the opening of the South Atlantic during the CNS (Eagles, 2007; Heine et al., 2013; Jones Seafloor spreading between South America and Africa began et al., 1995; Moulin et al., 2010; Nürnberg and Müller, 1991) around Anomaly M13 (134 Myr ago, Channell et al., 1995) in the are based on interpolation between the rotation parameters of southernmost part of the South Atlantic Ocean and then propa- M0y and C34y, predicting an opening age of 110–104 Myr ago ∼ gated northward toward the Equatorial Atlantic Ocean. The ini- (clearing of continental–oceanic boundaries, COBs) for the Equa- tial breakup probably resulted from the inception of the Tristan torial Atlantic Gateway, even though equatorial sedimentary fa- hotspot that formed the Paraná–Etendeka large igneous province cies and subsidence rates (Pletsch et al., 2001)and global com- (Renne et al., 1992). Despite their geodynamic and paleooceano- pilation of benthic foraminifera stable isotopes (Friedrich et al., graphic importance, the relative motions between the two plates 2012)indicate that the establishment of deep-water circulation during most of the breakup period are still poorly resolved due to between the South and Central Atlantic Basins started roughly the lack of reversal-related magnetic anomalies on crust of age 121 10–15 Myr later. Recent studies have tried to overcome this con- (Anomaly M0y) to 83.6 (Anomaly C34y) Myr old (Malinverno et al., tradiction by using either the seaward-edge of the salt basins 2012; Ogg, 2012). This crust, formed during the Cretaceous Normal (Torsvik et al., 2009)or gravity scars located within the South Superchron (CNS), is termed the magnetic “quiet zone” and is the Atlantic quiet zones (Pérez-Díaz and Eagles, 2014)to constrain focus of the present study. the rotation parameters during the superchron. These works had to assume that these markers were stable since their formation * Corresponding author. Tel.: +972 8 6477509. (i.e., isochrones-like features) and assign ages, which are poorly E-mail address: [email protected] (R. Granot). constrained. http://dx.doi.org/10.1016/j.epsl.2015.01.015 0012-821X/© 2015 Elsevier B.V. All rights reserved. R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163 157

Fig. 1. Tectonic map of the South and Equatorial Atlantic Oceans superimposed on a satellite-derived free-air gravity field (Sandwell et al., 2014). Magnetic picks and FZs crossings are shown with empty symbols for the six anomalies used (rounded ages are shown in Myr). Filled symbols show the rotated picks (rotation parameters from Table 1). The gray flowlines are calculated on the basis of the rotation parameters of Cande et al. (1988), covering the last 79 Myr (Anomaly C33o to present), and the red lines are flowlines calculated on the basis of the new stage rotations (Fig. 3b, Anomalies M2o to C33o, marked with filled black circles). Red stars denote the initiation points of the displayed flowlines. For uncertainties on selected flowline locations see Fig. 4. Dashed red flowlines delineate the expected location of FZs using interpolation between the rotation parameters of Anomalies M0y and C34y. Blue lines mark the oldest conjugate anomalies formed along the fossil spreading axes (black dashed lines) (Cande et al., 1988; Cande and Rabinowitz, 1978). Arrows show the expected position of the flowlines based on the gravity data. Gray shaded area demarcates a sliver of transferred crust (Sandwell et al., 2014).

Although geomagnetic reversal-related anomalies are the most M0y, C34y and C33o. From these finite rotations we derive stage prominent magnetic signature of the oceanic crust, past changes in rotations that constrain the relative motion of South America and the strength of the dipolar geomagnetic field are also recorded, re- Africa at 5 to 15 Myr intervals between 124 and 79 Myr ago. We sulting in globally correlatable wiggles within geomagnetic chrons then discuss the evolution of seafloor spreading in the South At- (Bouligand et al., 2006; Cande and Kent, 1992; Granot et al., 2012; lantic and conclude with the geodynamic and paleooceanographic Pouliquen et al., 2001). Two globally traceable magnetic anoma- implications of our results. lies from the CNS (Granot et al., 2012)are used here to define internal ages within the South Atlantic quiet zones: the oldest is 2. Data and methods a long-wavelength wiggle (Anomaly Q2, 108 1 Myr ago) and the ± youngest (Anomaly Q1, 92 1Myrago) is a short-wavelength wig- We constrain South America–Africa motion by using magnetic ± gle, which is followed by subdued magnetic anomalies toward the anomaly picks and FZ traces found south of the Tristan da Cunha younger end of the quiet zones. The ages (and uncertainties) of FZ, where both quiet zones are fully preserved (Figs. 1, 2 and these anomalies are based on the ages of the oldest sediments that S1). Locations of FZs were selected from the satellite gravity were drilled in the Central Atlantic Ocean (Granot et al., 2012). map (version 23) of Sandwell et al. (2014). Magnetic anomaly We have picked the location of these CNS magnetic anomalies and data were obtained from the National Geophysical Data Center combined them with fracture zone (FZ) locations to compute inter- (NGDC) and contain both shipboard and aeromagnetic (Project nal rotation parameters for the superchron period. We complement MAGNET) profiles. The locations of the reversal-related anomalies these two rotations with a new set of picks for Anomalies M2o, were picked based on forward modeling whereas the location of 158 R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163

Fig. 2. Magnetic anomaly profiles used in this study. Location of Anomalies Q2 and Q1 are shown with arrows. The profiles are located between FZs to prevent tectonic complications at segment ends. the geomagnetic intensity-related anomalies were determined by describe the evolution of relative motions between the two plates comparison against profiles from other oceanic basins (Granot et throughout most of the Cretaceous. al., 2012). We identified Anomalies Q1 and Q2 within each quiet The ages of the Cenozoic anomalies follow the geomagnetic po- zone by (1) tracing the general, long wavelength shape of the mag- larity timescale of Ogg (2012). The ages of the Mesozoic anomalies netic profiles and (2) picking their exact locations by correlation (M-sequence) are less certain. We adopt 121 Myr old as the age of the short wavelength signal. We note that tectonic-related fea- for Anomaly M0y and 124 Myr old as the age for Anomaly M2o tures (e.g., buried seamounts, variability in the basalt iron content, (He et al., 2008; Malinverno et al., 2012). We note that the choice undetected small offsets of the spreading centers, etc.) may alter of the Mesozoic timescale affects only the calculated spreading the shape of the anomalies enough to locally mask the intensity- rates and tectonic inferences for the period bounded between related signal. We therefore assigned conservative data uncertain- Anomalies M2o and Q2, since Anomalies Q2 and Q1 were inde- ties of 4 km and 5 km to the magnetic and FZ picks, respectively, pendently dated (Granot et al., 2012). to account for these difficulties while computing the finite rotation parameters. 3. Finite rotations Finite rotation parameters were calculated by implementing the statistical routines of Kirkwood et al. (1999) and Royer and Chang (1991). We define the location of six anomalies (M2o, M0y, Q2, The six finite rotations and their uncertainty ellipses are shown Q1, C34y and C33o, Supplementary Table S1), and together with in Fig. 3. The rotations and their covariance matrices are presented FZ crossings we calculate a set of finite rotation parameters (poles in Table 1. Generally, the locations of the poles are well-defined and angles), with their 95% uncertainty intervals (Fig. 3a and Ta- and their positions form a clear trend. The values of the statistical ble 1). From the finite rotations we compute five stage poles that parameter κ (Royer and Chang, 1991)are near one (Table 1) in- ˆ R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163 159

Fig. 3. Kinematics of the South Atlantic Ocean. a, Finite rotation poles with their 95% confidence limits. Dashed red line corresponds to the 95% confidence limits for the solution calculated on the basis of data obtained south of the Rio Grande FZ. b, Stage poles for South America–Africa displacements. Poles are shown for South America (red) and Africa (green). c, Spreading rates between South America and Africa along the Rio Grande FZ synthetic flowlines (blue). Gray bars show 95% confidence intervals. Bars along the abscissa show chrons. Gray line shows spreading rates that are based on Mesozoic (Nürnberg and Müller, 1991)and Cenozoic (Cande et al., 1988)kinematic solutions, with interpolated spreading rates for the CNS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 abc Finite rotations and covariance matrices for the relative motion of Africa (fixed) and South America. The covariance matrix is given by the formula 1 bde 10 7 rad2. κ ∗ ∗ − ˆ ! ce f" Age Mag. ano. Lat. Long. Angle κ ab cdefPoints Segs. ˆ (Ma) (◦N) (◦E) (◦) 79.9C33r 62.47 326.08 30.94 0.59 7.73 4.31 4.83 4.77 3.69 4.26 63 7 − − 83.6C34 61.98 325.32 33.48 1.19 6.45 3.61 4.01 5.48 3.77 3.83 58 7 − − 92.0Q1 60.84 323.50 38.58 0.69 18.93 14.70 15.72 14.84 14.23 16.17 33 7 − − 108.0Q2 62.17 319.83 47.35 1.41 491.62 531.78 653.66 579.8709.15 874.31 26 4 − − 120.6M0y53.55 324.36 52.75 0.87 51.66 58.28 64.08 74.76 75.82 83.10 28 4 − − 124.0M2o 52.16 324.90 53.86 2.33 38.17 45.92 48.62 60.63 60.91 64.60 31 4 − − dicating that the uncertainty assigned to the data points used to relative motion between the plates for the middle part of the su- calculate the solutions were reasonable. perchron. The identification of anomalies within the quiet zones is not straightforward and requires special care. We validate the kine- 4. Stage rotations matic solution of Anomaly Q1 by re-calculating the rotation pa- rameters by including magnetic picks from an additional spreading To evaluate the predictive quality of the new finite rotations we segment located between the Bode Verde and Rio de Janeiro FZs, calculate stage rotations (Fig. 3b) and generate synthetic flowlines for which there are sufficient data on both flanks (Fig. 1). A re- (Figs. 1 and 4) that, ideally, should trace the actual position of FZs. Indeed, the southern flowlines (north of the Falkland-Agulhas and cent update of the satellite-derived world gravity field (Sandwell Gough FZs) trace their positions within uncertainties (Fig. 4). Sim- et al., 2014)unraveled conjugate tectonic features of pseudofaults ilarly, north of the Ascension FZ, in the northern part of the South and a sliver of transferred crust within this spreading segment Atlantic Ocean and in the Equatorial Atlantic Ocean (Figs. 1 and 4), (Fig. 1). This suggests that an episode of northward ridge propa- the synthetic flowlines trace the actual location of FZs. Altogether, gation possibly related to the Tristan hot spot activity has affected these results indicate that the kinematic solutions presented here the crust located 100 km west of the (soon to be failed) spread- ∼ are robust, even for the Cretaceous quiet zones. ing axis. To avoid the possibility of a biased kinematic solution, we In contrast to the southern and northern sections of the spread- have limited our identifications of Anomaly Q1 to the region lo- ing system, there is a growing misfit between the FZs and the cated south of the tectonically-affected crust. The new solution is synthetic flowlines on the African flank between (and including) identical to the one constrained solely by the southern spreading the Rio Grande and Ascension FZs. The misfit starts at Anomaly Q1 segments, but it is better defined (Fig. 3a), confirming the Q1 iden- and grows to about 100 km towards Anomaly Q2 (Figs. 1 and 4). tifications. The identifications and kinematic solution of Anomaly This one-sided misfit could have resulted from an asymmetric Q2 are less certain (Table 1) due to its long wavelength nature spreading episode that would have taken place between Anoma- and the general tendency for strong and varied wiggles within lies Q2 (or possibly before) and Q1, whereby eastward migration of the middle part of the quiet zones (Granot et al., 2012). Although the ridge there would have led to more lithosphere on the South less reliable, the Q2 solution provides a useful constraint on the American flank. Alternatively, the misfit might have formed due to 160 R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163

Fig. 4. Oblique Mercator projections (Anomaly C34y pole, 325◦N, 62◦E) of selected FZs. Synthetic flowlines (red) are based on new stage poles (Fig. 3b). Dashed red lines show interpolated flowlines; green lines are flowlines computed assuming internal motion within Africa; purple lines are flowlines assuming internal motion within South America; and light blue lines are flowlines assuming asymmetric spreading (70 percent of lithosphere created on the South American plate between Q2 and Q1). Details of the parameters used are given in the text. Confidence intervals for the locations of Q1 and Q2 are shown. Arrows show the expected positions of the flowlines based on the gravity data. a missing plate boundary that straddled the Rio Grande FZ or a considered to be a deformed zone that might have accommodated nearby FZ, cutting through either of the two continents. some (tens of kilometers) strike slip motion in the Cretaceous Kinematic simulations (Fig. 4) enable us to choose the most (Eagles, 2007; Moulin et al., 2010; Torsvik et al., 2009). suitable of the three possible tectonic scenarios. First, for the flow- Alternatively, asymmetry in seafloor spreading may have been lines to fit the FZs of the African flank due to continental defor- fueled by the proximity to the active Tristan da Cunha hotspot mation within the African plate, it is necessary to have an inter- (Rohde et al., 2013), resulting in an eastward migration of the nal African rift that could be described by a 1.3◦ rotation over a ridge axis. We modeled asymmetric seafloor spreading by assign- 345◦E/15◦S pole. In such a scenario, the misfit between the syn- ing roughly 70 percent crustal accretion on the South American thetic flowlines and the FZs of the African flank is indeed reduced, flank between Anomalies Q2 and Q1. We assigned 60 percent but it introduces an unacceptable misfit to their conjugate FZs asymmetry for the spreading segment confined between the Bode located on the South American flank (Fig. 4, green lines). Further- Verde and Rio de Janeiro FZs due to the ridge propagator and cap- more, this scenario would require an unrealistic opening motion ture of transferred crust there (Sandwell et al., 2014). Asymmetric within Africa ( 110 km of N–S opening near the East African seafloor spreading helps to fit (yet not perfectly) the FZs of the ∼ Rift). This prediction is not supported by geological observations African flank without affecting the South American fit while still (Heine et al., 2013; Moulin et al., 2010)and we thus consider it complying with the location of the anomalies (Fig. 4, blue lines). unlikely. Two Cretaceous fossil spreading axes dated by magnetic anomalies Introducing 106 km of dextral strike slip motion along the to be active prior (Cande and Rabinowitz, 1978)and after (Cande ∼ Rio Grande FZ and further west within the South American Plate et al., 1988)the CNS are indeed found on the current South Amer- ( 1 rotation over a 306 E/79 N pole) provides a better fit to the ican flank (Fig. 1), supporting the scenario that seafloor spreading − ◦ ◦ ◦ flowlines of the African flank while not affecting the fit to the asymmetry must have started before and continued after the CNS, South American FZs (Fig. 4, purple lines). Importantly, the region with a preferentially eastward motion of the spreading axis, i.e., where the Rio Grande FZ meets the South American continent is more lithosphere accreted to the South American flank. Moreover, R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163 161

Fig. 5. Plate reconstructions and global bottom water temperatures. Plate reconstructions are shown at Anomalies Q2, 108 1Myrago (a), Q1, 92 1Myrago (b), and ± ± C34y, 83.6 Myr ago (c). South America is fixed. Contours correspond to 1000- and 2000-m isobaths. The geometry of the mid-ocean ridges are constrained only where we have magnetic anomaly picks (Fig. 1) and interpolated elsewhere. d, Benthic foraminifera oxygen isotopic data from Friedrich et al. (2012). Arrow delineates the initiation of cooling of global deep-water masses. gravity scars located within the South American quiet zone may by 92 Myr ago, in agreement with geological observations from preserve evidence for the abandoned spreading center that was left Africa and South America (Heine et al., 2013; Moulin et al., 2010; behind when the ridge migrated eastward (Pérez-Díaz and Eagles, Torsvik et al., 2009). 2014). To conclude, our new rotations suggest that up to 100 km of 5. Geodynamic implications ∼ dextral motion has been accommodated within the South Amer- ican Plate, west of Rio Grande FZ, after 121 Myr ago (Anomaly The new kinematic solutions presented here provide impor- M0y). Additionally, the central part of the South Atlantic was prob- tant constraints on the relative motions between Africa and South ably formed with some degree of asymmetric seafloor spread- America during their final stages of separation. The close prox- ing resulting in preferential accumulation of crust to the South imity of the early rotation poles (Fig. 3b) to the Equatorial At- American Plate. The exact amount of internal dextral deforma- lantic implies that a slow rotational motion governed the equa- tion within South America depends on the magnitude of asym- torial area while the continents were still partly attached (Fig. 5). metric seafloor spreading and therefore still remains uncertain. Applying linear interpolation between the different rotations, our Finally, the excellent fit of our flowlines to the actual position plate reconstructions indicate that the transition from rift to drift of FZs on crust younger than Anomaly Q1 (Figs. 1 and 4) im- (breakup) took place at around 99 1Myr, assuming rigidity of ± plies that internal continental deformation must have stopped both plates. Independent palaeomagnetic data indicate that a vir- 162 R. Granot, J. Dyment / Earth and Planetary Science Letters 414 (2015) 156–163 tual geomagnetic pole standstill of South America ended at around major reorganization of oceanic circulation patterns emerged, lead- 100 Myr ago and was followed by a westward drift of the con- ing to circulation system similar to that of the modern Atlantic tinent (Somoza and Zaffarana, 2008). This finding supports the Ocean, where north–south flow of intermediate and deep-water 100 Myr timing for the final breakup between Africa and South dictates water-mass and heat exchanges (Frank and Arthur, 1999). ∼ America. Rapid northward migration of the stage rotation poles af- Oxygen isotopic signatures of benthic foraminifera from different ter Anomaly Q2 (Fig. 3b) reveals a major plate reorganization of oceanic basins (Friedrich et al., 2012)(Fig. 5d) show that a global the South Atlantic spreading system. This change in plate motion cooling of bottom water started at about 92 Myr and continued has previously been linked to a supposedly global plate reorgani- progressively until 80 Myr. zation event at around 105–100 Myr, possibly triggered by the dy- ∼ namics of subduction and mantle plumes (Matthews et al., 2012). 6. Conclusions Our new reconstructions provide an alternative explanation sug- gesting that the plate motion reorganization in the South Atlantic Magnetic anomalies and FZ locations from the southern and coincided with the final separation of South America and Africa. central South Atlantic are used to construct a set of six rotation Seafloor spreading rates increased steadily from slow rates (full parameters that provide new constraints on the relative plate mo- spreading rate of 20 mm/yr along the Rio Grande FZ) before the tion between South America and Africa during the Cretaceous. The CNS and peaked at intermediate rates (77 mm/yr, Fig. 3c) immedi- rotations are consistent with the pattern of magnetic anomalies ately after the superchron. Motion has decreased to intermediate found in other parts of the basin (for Anomaly Q1) and fracture spreading rates (40 mm/yr) towards 60 Myr ago (Cande et al., zone locations. 1988). Interestingly, spreading rate has peaked some 20 Myr af- The rotations presented here indicate that Lower Cretaceous ter the completion of separation of the plates. A similar tectonic slow spreading rates have gradually increased and peaked at in- pattern, i.e. reorientation of relative plate motion direction and an termediate spreading rates shortly after the CNS, followed by a increase followed by a delayed decrease in spreading rates, oc- decrease in spreading rates. A change in the direction of relative curred in the Northeast Atlantic during and after the breakup of motions between the two plates took place in the middle of the Greenland from Europe (Gaina et al., 2009). These two examples CNS, at around 100 Ma. This event generally coincided with the suggest that the development of plate motions during (and up to time of final separation between the plates, and hence might be 20 Myr after) the transition from rift to drift might be influenced linked with the final transition into complete drift motion. Misfit by the mechanical setting of the rift zone and not governed solely of synthetic flowlines in the central South Atlantic indicates that by far-field (i.e., subduction-related) forces. internal strike-slip deformation within the South American Plate Our kinematic model has tectonics implications for the South (at around the Rio Grande FZ) has lasted until roughly 92 Myr and Equatorial Atlantic Basins. The new stage poles suggest ago. This process was probably active in conjugation with seafloor that, until the middle of the CNS, the relative motions between asymmetry. Finally, the results indicate, without taking a-priori as- South America and Africa were slower than previously expected sumptions, that the separation of the plates was later than previ- (Nürnberg and Müller, 1991)(Fig. 3c). Since the apparent overall ously thought. This in-turn seems to agree with the global deep- overlap between the continents in the Equatorial Atlantic, as is ev- water isotopic signature that suggest an age of roughly 95–90 Myr ident from the rotation parameters of Anomalies M2o and M0y, is for the establishment of deep water exchange across the Equatorial similar to that suggested by previous studies (Moulin et al., 2010; gateway. Nürnberg and Müller, 1991), our results imply that internal de- formation lasted longer than previously expected, probably until Acknowledgements roughly 100 Myr ago and no later than 92 Myr ago. This age stands in general agreement with geological observations from the rift This work was supported by the European Union Seventh zones of Africa (Fairhead et al., 2013; Heine et al., 2013). Framework Programme (FP7/2007–2013) under grant agreement The seaward edge of the Aptian salt basins found along the n◦ 320496 (GEOPLATE). We thank Nicolas Flament, an anonymous Brazilian and Angola–Congo–Gabon margins have been used by reviewer and the Editor for their constructive comments. Chris Torsvik et al. (2009) as fixed conjugate isochrons for 112 Myr Heine is thanked for his comments on the early version of this time. Linear interpolation between our M0y and Q2 finite rotations manuscript. This is IPGP contribution 3606. brings these basins to match at 116 Myr ago. We note that al- though it seems that the age of breakup of the basins is somewhat Appendix A. Supplementary material older than previously assumed, our model is not well constrained for that period of time. Supplementary material related to this article can be found on- Finally, the opening of the Equatorial Atlantic Gateway had a line at http://dx.doi.org/10.1016/j.epsl.2015.01.015. profound effect on the Cretaceous climate (Frank and Arthur, 1999; MacLeod et al., 2011), yet current magnetic anomaly-based kine- References matic models have failed to match the timing of the opening to ge- ological observations. Our new reconstruction model suggests that Bouligand, C., Dyment, J., Gallet, Y., Hulot, G., 2006. Geomagnetic field variations the final clearing of the continent–ocean boundaries in the Equato- between chrons 33r and 19r (83–41Ma) from sea-surface magnetic anomaly pro- rial Atlantic Ocean was completed by 95 1Myr, followed by the files. Earth Planet. Sci. Lett. 250, 541–560. ± opening of the gateway (Fig. 5). The initial deep-water circulation Cande, S.C., Kent, D.V., 1992. 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