A little spin in the Indian Ocean plate circuit

Accepted Manuscript A little spin in the Indian Ocean plate circuit

Graeme Eagles PII: DOI: Reference:

S0040-1951(19)30038-1 https://doi.org/10.1016/j.tecto.2019.01.015 TECTO 128033

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

1 November 2018 23 January 2019 31 January 2019

Please cite this article as: G. Eagles, A little spin in the Indian Ocean plate circuit, Tectonophysics, https://doi.org/10.1016/j.tecto.2019.01.015

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ACCEPTED MANUSCRIPT A little spin in the Indian Ocean plate circuit Graeme Eagles

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Alfred Wegener Institut, Helmholtz Zentrum für Polar und Meeresforschung Am Alten Hafen 26 27568 Bremerhaven Germany

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COMPASS Research Group Department of Earth Sciences Royal Holloway University of London Egham Surrey TW20 0EX UK


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Email: [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT

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Divergence of the Australian and East Antarctic plates is well understood from the late Jurassic onset of half graben development on the Australian continental shelf, and from post mid-Eocene (chron 20; 45 Ma) seafloor spreading isochrons further offshore. Relative plate motion between these times is less confidently interpretable from magnetic reversal anomalies landwards of isochron 20 and localised evidence for mid-to-late Cretaceous subsidence and growth strata from the continental shelf and rise south of Australia. A new test of this history examines it within the post-34y (84 Ma) Indian Ocean plate circuit, built using seafloor spreading data from the Wharton and central Indian Ocean basins. The Australian-Antarctic Jurassiconset rift system is interpreted to have been abandoned before 34y, because motion in the circuit is inconsistent with an active plate boundary during 34y26y (84-58 Ma). Starting 26y, the model depicts plate divergence by distances and angles that, after 25y (57 Ma), can be independently confirmed by reassessment of the pre-chron 20 magnetic reversal anomaly pattern in terms of seafloor spreading. Previous studies have identified evidence for mantle exhumation and focused magmatism in basement older than this spreading. These processes are not directly or confidently dated, but mantle exhumation is inconsistent with the circuit model’s fast plate divergence at 26y-25y. Hence, plate motion during the immediate build-up to post-57 Ma seafloor spreading may have been accommodated by focused magmatism, whilst mantle exhumation may mark the conclusion of the Jurassic-onset rift phase during a slower pre-84 Ma period of plate divergence. Using the new model to make tectonic reconstructions results in a large overlap between Tasmania and Victoria Land that can be explained with reference to Eocene strike-slip faulting and transtension in recently-discovered subglacial basins of Wilkes and George V lands and Terre Adélie.

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HIGHLIGHTS  Current Australian-Antarctic divergence phase started ~58 Ma, not late Cretaceous  First accommodation by magmatic intrusion, soon followed by seafloor spreading  Accommodation reactivated an abandoned Jurassic-onset basin  Abandoned basin was partly floored by exhumed mantle  Overlaps suggest transtensional deformation of George V Land, Antarctica

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ACCEPTED MANUSCRIPT 1.

INTRODUCTION

The history of Australian-East Antarctic plate divergence (Figure 1) plays an important role in numerous geoscientific concepts and hypotheses. It is cited to illustrate how rifts may weaken as they widen, allowing plate divergence to accelerate and cause global plate kinematic reorganizations (Brune et al., 2016), and to illustrate the global response to subduction initiation in the NW Pacific at 50-53 Ma (Whittaker et al., 2007). It forms the context within which Scher et al. (2015)

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modelled breaching of the final barrier to establishment of Antarctic circumpolar ocean circulation around 33 Ma, an important step in Earth’s transition to its

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Neogene icehouse climate state. In addition, the model plate divergence history

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impacts understanding and exploitation of the hydrocarbon province on the southern Australian extended margin.

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For times after around 45 Ma (magnetic reversal chron 20), organized seafloor spreading between diverging East Antarctic and Australian plates is confidently

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interpretable from well-formed coherent magnetic reversal anomalies west of the George V Fracture Zone (e.g. Weissel and Hayes, 1972; Cande and Mutter, 1982; Tikku and Cande, 1999; Whittaker et al., 2007). The oldest isochrons east of the

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fracture zone, chrons 18 and 17 (38-37 Ma), are slightly younger (Royer and Rollet,

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1997). From much earlier times, widespread growth strata in basement-floored basins bounded by normal faults all along the southern Australian continental shelf also clearly demonstrate plate divergence that started earlier in the west (Jurassic;

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~165-145 Ma) than in the east (early Cretaceous; 145-130 Ma) (Totterdell et al., 2000;

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Bradshaw et al., 2003; Direen et al., 2011; Blevin and Cathro, 2008; Ball et al., 2013). Evidence for plate divergence at 130-45 Ma is more difficult to understand selfconsistently or unequivocally. Magnetic data (Figure 1a) from the Great Australian Bight and its Antarctic conjugate reveal a short sequence of between three and five reversal anomalies. Pioneering workers interpreted them to show either isochrons 22-20 (Weissel and Hayes, 1972) or, extremely condensed, 34-20 (Cande and Mutter, 1982). The landward end of the magnetic anomaly sequence, an anomaly edge picked as an isochron and termed Quiet Zone Boundary (QZB; Tikku and Cande, 1999) was shown to correspond to the landward limit of a zone of complex and relatively strong gravity anomalies to a smoother and weaker field (e.g. Whittaker et

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ACCEPTED MANUSCRIPT al., 2007). Seismic data show that this zone corresponds to complex basement topography and structure, which have been attributed to a diversity of breakup processes (Tikku and Direen, 2008; Sayers et al., 2001; Close et al., 2009; Colwell et al., 2006; Direen et al., 2007; 2011; 2013; Ball et al., 2013). The ages of these processes have been studied using dredged rocks and ties of ‘breakup unconformities’ interpreted in seismic reflections to exploration wells. This work suggests breakup may have propagated from west to east in the period ~93-51 Ma (Chatin et al., 1998; Beslier et

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al., 2004; Krassay et al., 2004; Halpin et al., 2008; Tikku and Direen, 2008; Ball et al., 2013). However, the well ties are made from the shelf to the deep sea over very long

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distances, often involving jump correlations. Illustrative of this, the interpretation of

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the seawards edge of Cretaceous strata overlying Paleogene basement off the Ceduna Sub-basin (Fig. 1b; Totterdell and Bradshaw, 2004) shows that the seismic stratigraphy should be treated with considerable caution. In doing so for their study

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dating from seismic reflections only.

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of the distal parts of the margins, Gillard et al. (2015) restricted their study to relative

Tectonic extension has also been interpreted for various stretches of Cretaceous time from steep segments of subsidence curves calculated using exploration well data

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from the southern Australian shelf (Figure 1c; Falvey and Mutter, 1981; Totterdell et

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al., 2000; Brown et al., 2001; DiCaprio et al., 2009). In some but not all sub-basins, this subsidence segues into the 93-51 Ma breakup period as might be expected to be observed for the final stages of continental extension. Similarly, and despite the good

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reconnaissance-scale network of seismic reflection data, there is no close or consistent relationship between the development of growth strata in the sub-basins and the

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timing of first seafloor spreading (Figure 1c). Where growth is demonstrated, furthermore, it is never unequivocally or solely attributable to continental extension in Australian-East Antarctic plate divergence. Blevin and Cathro (2008) relate, for example, how mild early Cretaceous growth evident in the hanging walls of three basement-reaching faults in the Bremer Sub-basin may also be related to regional stresses developed during the onset of Indian-Australian plate divergence further west. The same authors state that Cretaceous growth strata at the eastern end of the margin can be related to Australia-Zealandia breakup-related stress (Blevin and Cathro, 2008). Late Cretaceous growth in the Ceduna Sub-basin, finally, is related to

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ACCEPTED MANUSCRIPT gravitational sliding in the thick sedimentary pile of the Ceduna delta and not to tectonic extension (Espurt et al., 2009; 2012). Evidently, therefore, the data sets from the Australian and East Antarctic conjugate margins do not contribute to a clear and self-consistent understanding of when and where plate divergence was accommodated prior to 45 Ma. For the period after the development of Jurassic-early Cretaceous basins on the Australian shelf, a loose

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consensus (described for example by Ball et al., 2013 and Gillard et al, 2015) views the margins as products of very slow plate divergence that was at first

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accommodated by continental extension and hyperextension, followed by

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exhumation of continental mantle rocks and/or localised magmatic intrusion, and finally by the strongly-diachronous onset of very slow seafloor spreading. A series of studies (Direen et al., 2011; Ball et al., 2013; Gillard et al., 2015; 2016) has observed

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similarities between the conjugate extended continental margins of Australia and Antarctica and those of Iberia and Newfoundland, whose breakup is also thought to

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have followed an early stage of slight continental extension and a later ~40 Myr long stage of extension, hyperextension, and mantle exhumation (Tucholke et al., 2007).

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Here, I test the idea that a long period of slow Australian-East Antarctic plate

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divergence gave way to seafloor spreading at 93-51 Ma by building a model of relative plate motions that is not led by interpretations of seismic stratigraphy, sparsely-distributed dredge samples, crustal affinity, or magnetic isochrons at the

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Australian and Antarctic conjugate continental margins, nor by analogy to their Iberia-Newfoundland counterparts. To do this, I sum rotations that describe relative

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motions of pairs of the plates in the circuit in the eastern Indian ocean: the Australian, Indian, Capricorn, and East Antarctic plates. Rotations to describe these motions exist in the literature, but they cannot be combined at high resolution or for quantitative uncertainty estimates because they have been generated using varying techniques, for short periods of overlapping time (e.g. Cande et al., 2010; Cande and Patriat, 2015; Jacob et al., 2014), or at low temporal resolution (Royer and Sandwell, 1989). In the face of this, the two following sections describe the generation of new sets of closely spaced rotations to describe motions in the circuit since late Cretaceous times at higher resolution. A fourth section describes how these rotations are combined for interpretation in terms of Australian-East Antarctic plate motions.

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ACCEPTED MANUSCRIPT Finally, I go on to briefly discuss the implications of these motions for understanding continental breakup and the development of the Australian and Antarctic conjugate margins.

2. INVERSE MODEL FOR WHARTON BASIN Wharton Basin opened between the Indian and Australian plates, starting in

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Cretaceous times and ending at ~38 Ma (during chron 18). Prominent north-trending fracture zones and fossil medium-length transform offsets, marking the location of a

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fossil mid-ocean ridge, characterize the basin floor in satellite-derived gravity anomaly data (Figure 2; Sandwell et al., 2014). The basin’s western margin is the

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Ninetyeast Ridge. Its eastern margin is the Java Trench. Subduction at this trench has seen the loss of much of the seafloor that had accreted to the Indian Plate at the fossil

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ridge. The basin has experienced regional shortening deformation since ~20-15 Ma (Miocene; DeMets et al., 2005; Krishna et al., 2009; Bull et al., 2010).

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Royer and Sandwell (1989) and Jacob et al. (2014) all modelled Wharton Basin’s opening history using Hellinger’s (1981) technique. The technique calculates rotation

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parameters by minimizing the misfits of reunited conjugate plate boundary markers from pairs of plates. Jacob et al.’s (2014) two-plate model is of high resolution,

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featuring fourteen rotations for the 59-38 Ma period. The two-plate Hellinger technique is not workable for times before this period, because of the loss of Indianplate data to subduction at the Java Trench. At lower resolution, Royer and Sandwell

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(1989) took the alternative approach of visual fitting of non-conjugate magnetic isochrons (four rotations). In contrast, Jacob et al. (2014) built a three-plate model of

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the Australia-East Antarctica-India plate circuit using an extension of Hellinger’s (1981) technique. The three plate model comprises seven rotations, of which five predate the same authors’ two-plate model. Notably, the three-plate model omitted magnetic isochron constraints from Wharton Basin with the aim of investigating the distribution and magnitude of shortening deformation in the basin. In contrast to these studies, I model Wharton Basin for the period 84-38 Ma by simultaneously fitting entire fracture zone shapes and rotated isochrons. The shapes of oceanic fracture zones can be observed at consistent and high resolution in satellite-derived global gravity data sets (Sandwell et al., 2014). They constitute a

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ACCEPTED MANUSCRIPT complementary yet more widespread and more consistently-interpretable source of constraint on plate kinematics than magnetic isochron interpretations from nearsurface trackline data sets. Optimal use of fracture zone data can lead to substantial improvements in the stability (e.g. Shaw and Cande, 1990) and range of applicability (e.g. Eagles, 2007; Pérez-Díaz and Eagles, 2014) of plate kinematic models. To enjoy these advantages, I implement an inversion scheme that was developed by Nankivell (1997) and is described by Eagles (2004) and Livermore et al. (2005). It differs from

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Hellinger’s (1981) technique not only by its optimal use of fracture zone location data, but also by the option to fit magnetic isochron picks to non-conjugate as well as

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conjugate targets. When applied with large numbers of non-conjugate isochron fits,

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the technique is capable of returning results of at-least comparable resolution to those obtained by summation in a plate circuit (e.g. Nankivell, 1997; Eagles, 2016). Supplementary section S1 provides further information on the inversion scheme

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used, as well as an idea of the relative stability of results achieved using sparser data

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sets that are wholly or partially characterized by non-conjugate isochron fits. 2.1. Isochron Data

Seafloor spreading in Wharton Basin happened on a set of stable mid-ocean ridge

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crest segments between well-defined transform faults. The relatively smooth gravity

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field between these features illustrates that spreading rates were for the most part intermediate or fast (70-100 mm/yr). Rates like this are between one and two orders of magnitude faster than those interpreted from the Australian-East Antarctic

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magnetic anomalies introduced above. These circumstances result in the formation of a far less condensed and far more coherent set of magnetic anomalies in Wharton

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Basin than those at the Australian and East Antarctic conjugate margins. The magnetic reversal isochrons in Wharton Basin are therefore identifiable with much greater confidence. Accordingly, there are strong similarities between all published sets of isochron identifications in the basin (Sclater and Fisher, 1974; Liu et al., 1983; Royer et al., 1991; Krishna et al., 1995; Jacob et al., 2014). Figure 2 shows the locations of magnetic isochron picks chosen for modelling. The choices closely follow those of Jacob et al. (2014), with only slight alterations close to some of the fracture zones. 2.2. Fracture Zone data

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ACCEPTED MANUSCRIPT Gravity anomalies over the basin’s fracture zones present as N-S trending steps and troughs whose map traces are slightly concave to east. In Figure 2, these traces are picked at 10 km intervals from the vertical gradient of gravity (Sandwell et al., 2014). 2.3. Inversion and Results The inversion starts with the rotations of Jacob et al. (2014), linearly extrapolated into late Cretaceous times. A stable solution is reached after around 30 iterations. By this

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point, the median misfit of magnetic isochron picks to conjugate targets in the new model is 4.9 km. Including outliers and non-conjugate fitting, the populations show

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mean values of -0.9 and -0.2 km (for magnetic data and fracture zone data) and

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standard deviations of 20.2 and 7.7 km. Figure 3 summarizes these misfits visually. Values like these can be used as estimates of locational errors for estimating 95% uncertainties in the rotation parameters (e.g. Shaw and Cande, 1990; Nankivell; 1997;

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Eagles, 2004). Supplementary section S1 shows however that this could lead to underestimated uncertainties in the Wharton Basin model for reasons related to its

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extensive fitting of non-conjugate isochrons. Because of this, locational errors for pre26y data in Wharton Basin are manually assigned larger values than calculated from the misfit populations. This ensures the resulting 95% confidence estimates, shown in

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Figure 4, are conservative. Table 1 lists the rotation parameters and their covariances,

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via which the confidence regions are calculated. The larger errors are evenly distributed throughout the basin suggesting their cause, whether it is dominated by navigational or process-related uncertainty (for instance owing to shortening of the

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basin since the Miocene), is not geographically correlatable at the basin scale and that

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the confidence estimates are appropriate. The inset to Figure 3 allows comparisons between the new model and earlier ones. The closeness of same-aged flowpoints in the new model and Jacob et al.’s (2014) two-plate model shows that the two are greatly similar over their shared post-26y time period. The new model’s flowlines are longer and smoother, however, reflecting how the inversion technique used here benefits both from fuller use of fracture zone data and non-conjugate magnetic isochron fitting for times prior to chron 26y. Over this earlier period, the new model produces a much better fit to the fracture zone shapes than Royer and Sandwell’s (1989) model. This is largely attributable to the fact that Royer and Sandwell (1989) interpreted their fracture zones in underway

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ACCEPTED MANUSCRIPT bathymetry and magnetic data, with results that differ significantly from those based on satellite-derived gravity data. The difference between fracture zone shapes in the new model and Jacob et al.’s (2014) three-plate model, which uses partial fracture zone orientations from satellite altimetry in Wharton Basin, is correspondingly smaller. In more detail, Jacob et al.’s (2014) non-use of magnetic isochron data constraints in

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their three-plate model leads to some useful observations. The inset to Figure 3 shows how, with the exception of chron 21y, dated flow points along synthetic

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fracture zones in Jacob et al.’s (2014) three-plate model are systematically mislocated

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by as much as 100 km ridgewards of same-aged magnetic isochron picks in the basin and same-aged flowpoints calculated from Table 1 or Jacob et al.’s two-plate model. Amongst these, the mislocations for chron 24o are most illustrative because they can

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be observed on both flanks of the paleo-ridge, ruling out spreading asymmetry as a cause. The chron 24o-aged rotation in Jacob et al.’s (2014) three-plate model under-

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rotates the 24o flow points by around 50 km, compared to its two-plate counterparts. This under-rotation cannot be attributed to Miocene and later shortening of Wharton Basin, which should have resulted in the two-plate models’ isochrons lying closer

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together, not further apart, than those in the three-plate model. I will show in section

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4 that a more plausible explanation for the systematic under-rotation of isochron picks in Jacob et al.’s (2014) three-plate model may be misidentification of pre-21y

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magnetic isochrons off the conjugate Australian and East Antarctic margins.

3. INVERSE MODEL FOR CENTRAL INDIAN BASIN

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Starting in early Cretaceous times, the Central Indian Basin opened by divergence of the Indian and East Antarctic plates and, since ~20-15 Ma, the Capricorn and East Antarctic plates. The plates diverged across a set of discrete mid-ocean ridge segments between north- and, after chron 20, NE-trending transform faults. Seafloor spreading rates were intermediate or fast. In the SE of the basin, a small plate, the Mammerickx Plate, developed and rotated independently of the Indian and East Antarctic plates during chron 21 (Matthews et al., 2015). In the NW of the basin, Miocene and later Capricorn-Indian plate motion has been extremely slow and accommodated regionally, leading to slight shortening deformation distributed over

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ACCEPTED MANUSCRIPT a large area that is contiguous with that in Wharton Basin (Krishna et al., 2009; Bull et al., 2010). 3.1. Isochron Data For the most part, seafloor spreading in the Central Indian Basin has been fast and stable enough to produce magnetic anomalies whose interpretations as isochrons are uncontroversial. Sparse sampling on the East Antarctic Plate leaves some ambiguity

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only regarding the locations of the oldest isochrons. Studies by Cande and Patriat (2015) and Cande et al. (2010) demonstrate that these data can be interpreted together

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with data from the flanks of the Southwest Indian and Carlsberg ridges in such a

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way to close the India/Capricorn-Africa-East Antarctica plate circuit within the level of uncertainty typical of navigational errors. On this basis, and because the SW Indian and Carlsberg ridge isochron pick sets are not subjects of controversy, the

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central Indian identifications can be considered as reliable and their interpretational uncertainty small. Where available, the picks used for modelling here are made along

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wiggle traces plotted from NCEI data holdings

(https://www.ngdc.noaa.gov/mgg/geodas/trackline.html), following the identifications of Desa et al. (2006; 2009), Cande and Patriat (2015) and Cande et al. (2010), which

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themselves are based on earlier identifications by Sclater et al. (1997). A small

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number of the picks used by Cande and Patriat (2015), for which the trackline data are not widely available, were taken from the GSFML website (Seton et al., 2014; http://www.soest.hawaii.edu/PT/GSFML/ML/index.html). No picks were taken from conjugates.

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isochrons within the extinct Mammerickx Plate or their Antarctic and Indian

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3.2. Fracture Zone data

Gravity anomalies over the Central Indian Basin’s fracture zones present as NE- and north-trending steps and troughs. The picks in Figure 2 were made at 10 km intervals in the vertical gradient of gravity (Sandwell et al., 2014). As with the magnetic isochron data, these picks avoid regions affected by independent rotation of the Mammerickx Plate. 3.3. Inversion The inversion starts with the rotations of Cande and Patriat (2015). It is slightly slower to achieve stability than the Wharton Basin model. Including outliers, the

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ACCEPTED MANUSCRIPT mean misfits are -4.0 and 0.0 km (for magnetic data and fracture zone data) and the standard deviations of the misfit populations are 19.5 and 6.0 km. These values are likely to be appropriate as estimated errors for the uncertainty analysis because the model uses conjugate isochron fitting for all of its rotations. Figure 5 summarizes the fits visually, and Figure 6 shows the locations of the finite poles and the 95% confidence regions calculated from the covariances of the rotation parameters, all of which are tabulated in Table 2. With data density and quality approximately similar

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throughout much of this model, the steady increase in rotation angle with age explains the tendency for the confidence regions to become geographically smaller

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with age. As before, the geographical distribution of misfits suggests that the 95%

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confidence estimates are appropriate given the data set used.

Figure 5 also allows a visual comparison to the models of Royer and Sandwell (1989)

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and Cande and Patriat (2015). The fits of magnetic isochrons to their conjugates in the new model are tight (median misfit 7.5 km), as in the model of Cande and Patriat

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(2015). The new model and that of Royer and Sandwell (1989) show smoother progressions of plate motion and a closer resemblance to fracture zones than that of Cande and Patriat (2015). The smoothness in Royer and Sandwell’s (1989) case is

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largely a consequence of interpolation through their model’s coarse temporal

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resolution of six stages. The smooth progression in the new model, in contrast, is maintained over fifteen stages. In summary, the new model features a combination of close resemblance to magnetic isochron locations and fracture zones and a finer

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temporal resolution than previously published ones for the Central Indian Basin. It is therefore likely to be a more reliable and higher-resolution approximation to plate

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motions than previous models.

4. INTERROGATION OF THE CIRCUIT For models that assume internally rigid tectonic plates, the sum of rotations describing relative motions in a circuit beginning and ending with the same plate must be zero. Royer and Sandwell (1989) took pains to ensure their rotations for past divergent motions in the circuit of Australian, Indian and East Antarctic plates close in this way. Another consequence of the assumption of plate rigidity is that the sum of rotations describing relative motions between n-1 of the pairs of n plates is a rotation that describes the relative motion of the nth pair without the need for

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ACCEPTED MANUSCRIPT detailed knowledge of that pair’s shared boundary. Jacob et al. (2014) made use of this when interpreting their three-plate model in terms of Miocene and later deformation of Wharton Basin related to the motion of a fourth plate, Capricorn, whose action was recognised subsequent to Royer and Sandwell’s (1989) study. To do so, they assumed that their model of the three-plate circuit would close were it not for the action of the Capricorn Plate. Section 2.3 notes how this assumption fails a coarse applicability test because the sense of the circuit’s failure to close in Wharton

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Basin implies lithospheric extension there, rather than the independently-observed shortening. An alternative explanation for this is that our current understanding of

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past Australian-East Antarctic plate divergence is based on misinterpretations of the

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available data sets from the Australian and East Antarctic conjugate margins. In this section, I aim to test this notion by analysing the plate circuit with the highresolution rotations of Tables 1 and 2. My objective is to sum rotations from the

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Wharton and Central Indian basins for a set of rotations that describe relative motions of the Australian and East Antarctic plates in the period since chron 34y.

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Table 4 sets out the summed rotations. Figure 8a locates the rotation poles, revealing their close proximity in space, but not in time, to those of Australia-East Antarctica models built using data from the SE Indian Ocean (Royer and Sandwell, 1989; Tikku

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and Cande, 1999; 2000; Whittaker et al, 2007). The likeness between Tikku and

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Cande’s (1999; 2000) post-34y rotations and the circuit-derived rotations for chron 25y and later times is particularly strong.

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4.1. Sources of uncertainty in the summed rotations To examine the circuit with confidence, a number of possible sources of error need to

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be understood and assessed in terms of the uncertainty they imply in the summed rotations. This section describes these sources of error and how I assess their contribution to uncertainty in the summed Australian-East Antarctic rotations. Firstly, a range of geological and plate kinematic observations shows that the plate circuit in the eastern Indian Ocean has not consisted of just three plates since Miocene times (Figure 7; Royer and Gordon, 1997; DeMets et al., 2005; Krishna et al., 2009; Bull et al., 2010). The observations are accounted for with the concept of the socalled Capricorn component plate. Slow relative Capricorn-Australia-India plate motions have led to the accommodation of up to 150 km of displacement over broad

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ACCEPTED MANUSCRIPT (3000-4000 km) zones of distributed convergent seafloor deformation in the Wharton and Central Indian basins (Figure 2; Figure 7). It is thought that deformation occurs in this way when relative plate motion is too slight to allow for strain focussing onto a narrow plate boundary (Zatman et al., 2001). The two previous studies of the circuit did not account for this deformation, either because it was unknown at the time (Royer and Sandwell, 1989), or after concluding it to be small (Jacob et al., 2014). I consider it necessary, however, in view of the fact that even small rotations

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describing motion of the Capricorn Plate will be magnified when combined with the larger ones in the rest of the circuit. To do this, I use a Capricorn-India rotation that is

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designed to correct the circuit in such a way as to visually reconstruct the well-

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characterised pre-Capricorn isochron 20y of the SE Indian Ridge (Table 3; Figure 9). The full rotation is applied to the period after chron 27y for which the majority of data north of the ridge in the Central Indian Basin lie within the Capricorn Plate. For

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times between chrons 27y and 34y, I close the circuit using progressively smaller rotation angles around the Capricorn-India pole, in order to take some account of the

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fact that those isochrons lie progressively further towards the Indian side of the diffuse Capricorn-India plate boundary zone (Figs. 2, 7) and likely therefore record

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progressively less Capricorn-India motion.

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The second consideration I make is of locational error in the data from which the Wharton and Central Indian basin models are built. Covariances in Tables 1 and 2 describe the finite rotation uncertainties due to these errors. I combine the two sets of

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covariances in order to calculate uncertainties in the summed Australian-East Antarctic rotations according to a procedure described by Chang et al. (1990). These

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covariances, and the 95% confidence regions determined from them for Figure 8a, should be viewed as incomplete because I have made no attempt to determine uncertainties for the rotation in Table 3 and incorporate them into the circuit. Owing to its small angle, the uncertainty in this rotation would be likely to have a relatively small effect to enlarge the 95% confidence ellipses, and this enlargement would affect all of the ellipses in a similar way because the rotation post-dates them all. The third source of error lies in the Wharton Basin model, because it is not built using conjugate pairs of isochron data for times before chron 26y. This means the rotation angles about pre-26y rotation poles in Table 1 might not fully capture the

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ACCEPTED MANUSCRIPT effects of asymmetrical spreading, which can affect the spacing of non-conjugate pairs of isochrons. Similarly, this spacing might be locally affected by heterogeneities in Miocene and later north-south directed shortening of the Australia-India diffuse plate boundary zone. I deal with these uncertainties firstly by considering the post26y part of the circuit model, which uses conjugate data and hence is not prone to asymmetry-related error, separately. For the pre-26y parts of the model, I describe in a later section two separate quantitative estimates of the possible effects of

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asymmetry and heterogeneous regional shortening based on the considerations given in Supplement S1, on Müller et al.’s (2008) observation-based assessments of

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the degree of asymmetry, and on the observation that the direction of Miocene to

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present shortening in Wharton Basin is approximately parallel to the direction of earlier spreading there.

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Finally, data location errors from narrow basins permit a greater range of possible ridge geometries than in wider ones. Because of this, a narrow basin’s estimated

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ridge geometry might be more significantly different than the true geometry if locational errors are under- or overestimated. In particular the likelihood of error underestimation in Wharton Basin might be expected to be amplified as a

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consequence of kilometres-scale heterogeneity, if present, in Miocene and later

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regional shortening strain. Whilst it is difficult to quantify the uncertainty due to this effect, we can expect it to manifest itself as high-frequency instabilities in stage rotations calculated from the finite rotations. This is because the uncertainty

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ultimately stems from data locations that are assumed to be randomly distributed within their estimated errors. If a geometrical effect is suspected on this basis, its

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magnitude and importance can be assessed by comparing a two-plate model of the suspect basin to the corresponding leg of a three-or-more-plate model built using independent data from neighbouring basins. Cande and Patriat (2015) modelled the western Indian Ocean in this way, demonstrating in the process that their CapricornEast Antarctic rotations derived from data in the Central Indian Basin are not meaningfully affected by geometrical uncertainty. In section 4.4, I outline a similar test for the Wharton Basin model. 4.2. Interpretation of Australian-East Antarctic rotations: post-26y

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ACCEPTED MANUSCRIPT Figure 9 illustrates the post-26y parts of the Australian-East Antarctic relative plate motion model. It uses synthetic flowlines that describe paths followed by points on the model Australian-East Antarctic plate boundary. This part of the model is unaffected by undetected spreading asymmetry in Wharton Basin because of the availability of conjugate isochron pairs. The post-26y flowlines show the plate boundary moving away from the plate

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interior. They illustrate a phase of 320-540 km model plate divergence, comprising six individual stages. Divergence is confidently resolved between 95% confidence

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regions around the end points of all but the shortest of the six stages, and at all

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distances along the southern Australian margin. Confidence regions around their East Antarctic counterparts are larger because of their greater present-day distance from the rotation poles, but still confidently resolve divergence for many individual

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stages and nearly all successive pairs of stages. The 25y-20y parts of these flowlines smoothly trace out the north-south widths of the deep seafloor regions characterized

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by pre-20y linear magnetic reversal anomalies. Together with the strong resemblance between the circuit-derived post-25y rotations and Tikku and Cande’s (1999; 2000) post-34y rotations (Figure 8a), the flowlines thus strongly support the idea that some

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or all of the magnetic isochrons in the sequence 21y-34y of Cande and Mutter (1982)

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are in fact misinterpretations of anomalies that really portray the isochron sequence 21y-25y.

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Consistent with this, Figure 10 shows magnetic reversal models for the 21-25 sequence and compares them to magnetic anomalies along ship profiles. In detail, the

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reversal model sequence is formed by plate divergence, initially at intermediate but, after chron 24, slow spreading half-rates (10-20 km/Myr). The slow section produces four closely-spaced peaks (21-24) landward of isochron 20. In comparison, although the detailed waveforms of the ship-track anomalies are not strongly coherent over multiple profiles, the majority of them do present between three and five closelyspaced peaks and intervening troughs (Figure 10). Where profiles are densely spaced, these features clearly organise into long linear magnetic reversal anomalies (e.g. Golynsky et al., 2018). These anomalies are confidently interpretable in terms of seafloor spreading. Further landwards, the reversal models feature a broad low, the reversed part of isochron 24, and a final peak representing the normal polarity part

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ACCEPTED MANUSCRIPT of chron 25. Many of the ship-track profiles present comparable lows and peaks. Coincident seismic data however show these anomalies often to be formed over basement ridges and other complex topography (e.g. Ball et al., 2013), questioning the idea that they too might be simply related to a magnetic polarity reversal recorded during seafloor spreading. Early interpretations regarded these features as volcanic constructs in deformed oceanic crust (e.g. Tikku and Cande, 1999). More recently, seismic reflection images and gravity anomaly modelling have been used to

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interpret how they may have formed by exhumation of mantle rocks along low angle normal fault zones and/or by magmatic intrusion in discrete elongated extinct

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volcanic centres along a very mature continental rift zone (Close et al., 2009; Direen et

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al., 2011; Ball et al., 2013; Gillard et al., 2015). Rather than seafloor spreading, therefore, the magnetic anomalies landward of the reverse-polarity part of chron 24 are likely at least in part to represent mechanical and/or volcanic processes in pre-

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existing lithosphere. Whilst the circuit model allows for the possibility that these processes may all date from 26y-25y, the attendant magnetic anomaly interpretation

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does not strictly require them to do so.

4.3. Interpretation of Australian-East Antarctic rotations: pre-26y

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Figure 11 uses single flowlines from each of the flanks of the model SE Indian Ridge

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to illustrate the model circuit’s Australian-East Antarctic motions prior to chron 26y. It is possible that undetected spreading asymmetry and/or shortening strain within Wharton Basin affect these parts of the model by having displaced its magnetic

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isochron picks by more than the range of error that might otherwise be assigned to them. Supplementary section S1 described an attempt to capture the effects of this

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possibility in the covariances of Table 1 by careful a priori assignment of data location errors. Derived from these covariances and those of Table 2, the 95% confidence regions calculated from Table 4 are larger than their younger counterparts. As an alternative to this analysis, the inset to Figure 11 shows flowlines built assuming worst-case scenarios for undetected isochron asymmetry in Wharton Basin. This is done by adjusting the angles of pre-26y stage rotations calculated from Table 1 by ±30%. This value allows to visualize the possible effects on isochron spacing of (i) the largest local spreading asymmetry values estimated by Müller et al. (2008) on the basis of corridor-to-corridor variability in modelled spreading rates, or (ii) extreme heterogeneity of Miocene to present shortening in Wharton Basin in which the entire

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ACCEPTED MANUSCRIPT (150 km maximum) of strain implied by studies of relative motions of the Capricorn, Indian and Antarctic plates is localized into any 500 km-wide sub-region of pre-26y seafloor in the basin. The model plate boundary converges with the plate interior at 30o-26y (68-58 Ma) within the resolution of both the 95% confidence regions and the Wharton Basin asymmetry analysis. This convergence is thus robust, both as a feature of the circuit

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and in the face of undetected spreading asymmetry or extreme heterogeneity in the accommodation of shortening strain in Wharton Basin since Miocene times. Whilst

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robust, it is clearly not interpretable in terms of real Australian-East Antarctic plate

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convergence. There is no geological evidence for such convergence (e.g. Blevin and Cathro, 2008). A more appropriate interpretation is that the underlying assumption of an Australian-East Antarctic plate boundary having existed at 30o-26y is false. By

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rejecting this assumption, the calculated rotations for that period do not describe real

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plate motions anywhere in the Indian Ocean.

Prior to 30o, Figure 11 shows the model plates diverging in the period 84-68 Ma. The very large estimated uncertainties for 34y ensure that this divergence is nearly

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indistinguishable from other interpretations, locally including zero motion, at 95%

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confidence level (Figure 11). The determination of divergence is more robust in the face of the effects of possible spreading asymmetry or heterogeneous shortening in Wharton Basin, which allow for divergence by a minimum of 270 km and a

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maximum of 600 km. This divergence, if it occurred, would have been accommodated in the deep, magnetically-quiet basins bordering the continental rises.

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These basins, at around 150 km in width each, cannot have accommodated a total of more than 300 km of plate divergence by any mechanism. They would be required to have extended by factors of greater than 10 to accommodate values in the small feasible part (i.e. divergence of more than 270 km but not more than 300 km) of the estimated range. Extension factors this large are difficult to reconcile with the absence of unequivocal geological evidence for significant margin-wide extension at 84-68 Ma (Figure 1c). This, coupled with the previous interpretation for the 30o-26y period, means the safest interpretation of the Australian-East Antarctic plate boundary in the period 34y-30o seems to be that it was not yet active.

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ACCEPTED MANUSCRIPT 4.4. Sensitivity of the circuit to the Wharton Basin model In section 4.1, I described the possibility that errors in estimating the geometry of the short lengths of paleo-ridge crest in Wharton Basin might give rise to uncertainty that should manifest itself as instabilities in stage pole locations. Figure 8b shows that the circuit’s set of six post-26y stage poles, describing plate divergence during the time interpreted above to have featured an active Australian-East Antarctic plate boundary, are as stable and clustered as those derived from published two plate

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models that use pre-chron 20y data from that boundary. It seems unlikely therefore that geometrical effects in the Wharton Basin model adversely affect the circuit in the

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post-26y period.

In contrast, Figure 8c shows that the progression of pre-26y stage poles is far less stable. If this instability indeed stems from poorly-estimated ridge geometries, then it

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is important to ask whether the plate circuit might be so sensitive to such poor estimates that it could tolerate solutions like that in Figure 11 simultaneously with

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solutions like those currently described in the literature to feature slow late Cretaceous Australian-East Antarctic divergence. This is difficult to test directly in the Wharton Basin model because the putative geometrical uncertainty cannot easily

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be isolated for analysis. An alternative test can be carried out by closing the circuit

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with rotations describing slow late Cretaceous Australian-East Antarctic divergence to generate an alternative model of Wharton Basin that is certainly not affected by geometrical uncertainty. If this circuit-derived model of Wharton Basin differs only

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slightly, that is by amounts that small adjustments to estimated data location errors might permit, from that of Table 1 and Figure 3, then we can conclude that

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geometrical uncertainty could be a significant barrier to confidence in the interpretation of Figure 11. Figure 12 shows a test of that kind. The Australia-East Antarctica-Capricorn-India plate circuit is closed using rotations from Tables 2 and 3 alongside those from either of two models of late Cretaceous to Paleocene East Antarctic-Australian divergence (Tikku and Cande, 1999; 2000; Whittaker et al., 2007). The divergence azimuths in those two models represent extremes in the spectrum of published estimates of Australian-East Antarctic divergence (cf. inset to Figure 9), and thus extremes of the possible roles that those estimates might play in the circuit. Figure 12 compares

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ACCEPTED MANUSCRIPT flowlines produced in those models to Wharton Basin’s seafloor fracture zone fabric and to flowlines modelled from Table 1, which locate dated points along the fracture zones with high accuracy (Figure 3). Neither of the new sets of flowlines closely resembles the shapes of the gravity anomalies over the fracture zones or produces dated points that fall within or near the corresponding 95% confidence regions for flow points based on Table 1. Of the two, those derived using Tikku and Cande’s (1999; 2000) rotations are a better match to the fracture zones, albeit with the wrong

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sense of curvature. The figure also shows a second set of estimated 95% confidence regions for the circuit-derived model with Tikku and Cande’s (1999; 2000) rotations.

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These confidence regions are not subject to any of the uncertainties that relate to the

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Wharton Basin data set. In all but one instances, these confidence regions fail to overlap with those for the locations of equivalent-aged markers derived from Table 1. From these observations, I conclude that locational errors estimated for the

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Wharton Basin and Tikku and Cande’s (1999; 2000) or Whittaker et al.’s (2007) data sets are not reasonably likely to combine in any way that permits a model of the

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Indian Ocean plate circuit to resolve slow Australian-East Antarctic plate divergence in Cretaceous and Paleocene times. Finally, whilst this analysis does not rule out that pre-26y parts of the model are affected by small-scale geometrical uncertainty related

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to the short Wharton Basin paleo-plate boundaries, I note an alternative

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interpretation of the changing stability of stage pole locations before and after chron 26y. In this interpretation, the onset of stage pole stability at 26y reflects the establishment of stabilizing Australian-East Antarctic boundary torques in the circuit

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around the time that boundary first came into existence.

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5. DISCUSSION AND OUTLOOK 5.1. Australian-East Antarctic continental extension and breakup Previous work has shown that the history of continental breakup between Australia and East Antarctica started with Jurassic extension and graben formation that, by mid- or late Cretaceous times, had given way to further extension and hyperextension and, by the late Cretaceous, to faulting, mantle exhumation and/or magmatic intrusion in the run-up to seafloor spreading (e.g. Totterdell et al., 2000; Ball et al., 2013; Gillard et al., 2015). Comparisons with the magma-poor margins of Iberia and Newfoundland (e.g. Tucholke et al., 2007) are taken to suggest that these processes might have taken place over a period of several tens of millions of years.

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In stark contrast, section 4 interprets a history in which the Jurassic-onset extension was complete by some time prior to 84 Ma. It was followed by a second phase of plate divergence starting no earlier than chron 26y (58 Ma), which led to seafloor spreading by some time during the period of reverse magnetic polarity after chron 25y (54-57 Ma). Prior to this, basement structures landwards of isochron 25y have been taken as evidence for mantle exhumation and magmatic intrusion affecting pre-

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existing lithosphere (e.g. Ball et al., 2013; Gillard et al., 2015). The uncertainty of longdistance seismic correlations means, however, that these processes might conceivably

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be assigned to (i) the latter stages of the Jurassic-onset extensional episode, or (ii) the

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initial stages (26y-25y) of Paleogene plate divergence, or (iii) some combination of the two. Current understanding of the kinematics of magma-poor extended continental margins sees mantle exhumation as a consequence of suppressed decompression

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melting in normal or refractory mantle that ascends slowly between slowlydiverging plates (Larsen et al., 2018). In this context, the intermediate rate of plate

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divergence at 26y-25y (Figure 10) is difficult to reconcile with the presence of exhumed mantle. One interpretation of these conditions might therefore be that the conclusion of the Jurassic-onset rift phase saw slow plate divergence accommodated

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by mantle exhumation, and that renewed plate divergence in Paleogene times was

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accommodated by renewed faulting and magmatic intrusion. This or alternative scenarios can be tested once the difficulties in long-distance

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seismic correlation at the Australian-East Antarctic conjugate margins are overcome, for example following future deep-water drilling. In the nearer term, they might

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prompt geodynamic model experiments to investigate the possible roles that preexisting regional crustal and lithospheric structures might play during continental breakup.

5.2. Reconstruction overlaps and accommodation of breakup stress Figure 13 shows three reconstructions of the Australian and Antarctic margins built using the circuit-derived plate divergence parameters of Table 4. In the west, the reconstructions feature overlap between areas of rough basement topography in the so-called Diamantina Zone on the Australian plate and Labuan Basin on the East Antarctic Plate. This is a repeat observation of an overlap that has long been

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ACCEPTED MANUSCRIPT understood in terms of crustal extension. Tikku and Cande (1999; 2000) noted that their chron 20-34y rotations can be used to explain the width and presence of the rough topography in terms of a ~42 Myr long history of extension in thick crust of the Kerguelen Plateau-Broken Ridge large igneous province. The overlap in Figure 13 invites a similar interpretation, but its chron 20-26y/25y date requires a briefer (~15 Myr long) bout of extension.

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Over the central segments of those margins, the reconstructions illustrate the conclusions of section 4. The reconstruction for chron 22o tightly reunites picks

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(made from the data in Figure 10) of those isochrons, consistent with the

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interpretation of their formation by lithospheric creation at a focused mid-ocean ridge. In contrast, the older rotations produce reconstruction overlaps that conceivably constrain the accommodation of plate divergence by pre-seafloor

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spreading processes. As noted above, these processes are less likely to have included mantle exhumation than magmatic intrusion, but the difficulties of long-distance

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seismic correlation mean it is not possible to rule out additional or alternative processes such as normal faulting. In estimating the amount of plate divergence that might have been accommodated, the 25y rotation causes the magnetic isochrons

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picked as 34y by Tikku and Cande (1999) to overlap by around 25 km, similar to the

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widths of large intrusive bodies interpreted for the pre-spreading phase by Ball et al. (2013). In a maximum estimate, the full 26y rotation brings Tikku and Cande’s (1999) magnetic anomaly QZB, near the boundary between Gillard et al’s (2015) zone of

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exhumed mantle and older continental basement, into overlap by a further 75 km, implying significant Paleogene extension (stretching factor 1.33) of the 300 km-wide

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Jurassic-onset rift. Because such extension is not currently known (e.g. Figure 1c; Blevin and Cathro, 2008), it seems more likely that the onset of Paleogene plate divergence lay closer in time to 25y than 26y. The much wider overlap east of the George V Fracture Zone is also a repeat observation, having previously been presented and discussed by Tikku and Cande (2000). Over the last decade, a number of studies (e.g. Whittaker et al., 2007; Williams et al., 2011; Whittaker et al., 2013) have concluded the overlap to be a product of erroneous Australian-East Antarctic two-plate models that place the Australian continental margin too far west. This, in turn, has prompted wide-ranging work on

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ACCEPTED MANUSCRIPT full-fit or palinspastic reconstructions and investigations of pre-rift geological correlations between Australia and East Antarctica (e.g. Aitken et al., 2014; White et al., 2013; Williams et al., 2011; 2019). The repeat observation in Figure 13, however, is based on data sets that are entirely independent of those used for Australian-East Antarctic two-plate modelling, and so does not sustain an interpretation of the overlap as a modelling artefact. As such, it commends rotation parameters like those in Table 4 or, after appropriate adjustment of the dates assigned to the individual

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rotations, Tikku and Cande (1999; 2000) as the starting point for correlation-based and/or palinspastic work on the pre-stretching locations of Australia and East

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Antarctica.

The western overlap requires geological, rather than methodological, interpretation. Typically, such interpretations have involved independent motion of a third or

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further plates, all small and confined to the seafloor around Tasmania (e.g. Royer and Rollet, 1997; Tikku and Cande, 1999; 2000; Cande et al., 2004; Cande and Stock, 2008).

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Repeated studies have concluded that there is little geological corroboration for such plate motions on- or near shore Australia (see White et al. (2013) and Williams et al. (2019) for reviews). Given this, an alternative class of interpretations for the western

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overlap envisages an Australian-East Antarctic plate boundary that accommodates

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part of the relative motion between the two plates elsewhere than in what is now the Australian-plate east of the George V Fracture Zone. One example of such an interpretation, in Figure 14, suggests these short-lived segments of the boundary

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were located in the Antarctic continental interior, and that they operated in leftlateral strike-slip and transtension during Paleogene times. Here, recent years have

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seen the acquisition of ice-penetrating radar data in Wilkes Land, Terre Adélie and George V Land that reveal the presence of a number of deep steep-sided bedrock depressions, for example in the Concordia, Astrolabe and Adventure subglacial trenches (Figure 15; Ferraccioli et al., 2001; Tabacco et al., 2006; Cianfarra and Salvini, 2014; 2016; Maggi et al., 2016; Yildiz et al., 2016). Those studies show that, morphologically, the subglacial basins seem to be of tectonic, in many cases transtensional, origin. Using a range of indirect considerations, they suggest that some of the basins were most recently active in Cenozoic times. Figure 15 shows that the Concordia, Astrolabe and Adventure trenches all strike slightly anticlockwise of small circles drawn about the 25y-24o stage pole for Australian-East Antarctic

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ACCEPTED MANUSCRIPT relative motion, as would be expected for them to have worked in left-lateral transtension on those plates’ shared boundary. At a broader scale, gravity and magnetic anomaly data reveal areas of linear fabric running parallel and sub-parallel to the small circles, consistent with the possibility that Australian-East Antarctic strain was accommodated in a distributed sense. In this view, the subglacial trench basins may be seen as local consequences of strain focusing on favourably-oriented

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pre-existing crustal structures in a broad plate boundary zone. Figures 13 and 14 show that the eastern overlap is only produced in circuit

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reconstructions for times before chron 22o, which unites the margins at and east of

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the Adélie rift block of Colwell et al. (2006). A coarse conclusion from these observations is thus that transtensional basins and strike-slip faults like those that can be interpreted from the data in Figure 15 may have been active along or within a

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plate boundary zone running through Wilkes and George V lands and Terre Adélie in the period 58-50 Ma. The overall trend of this interpreted boundary suggests that

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it may have terminated at a triple junction somewhere along what is now the West Antarctic Rift System. Unlike the large-scale Cretaceous plate convergence suggested by Jacob and Dyment (2014), this forms a plausible plate kinematic context for the

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variety of mechanisms suggested for Paleogene uplift of the Transantarctic

6. CONCLUSIONS

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Mountains, none of which involves crustal thickening (Wannamaker et al., 2017).

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A new plate circuit-based model of the history of Australian-East Antarctic plate divergence avoids reliance on ambiguous data from the southern Australian margin

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and its Antarctic conjugate. The model results indicate the presence of a divergent boundary between the two plates in the period since 58 Ma at the earliest, but speak against the existence of such a boundary in the 84-58 Ma period. The circuit-derived rotations for Australian-East Antarctic plate divergence independently reproduce previously-observed overlapping relationships in reconstructions of the extended continental margins. The western overlaps are interpretable in terms of lithospheric extension prior to the onset of seafloor spreading. The eastern overlap can be interpreted in terms of a different early position of the plate boundary, or the motion of a further plate or plates in the

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ACCEPTED MANUSCRIPT Tasmania-Victoria Land sector. Either may have been accommodated in Eocene times by the action of a set of transtensional basins and strike-slip faults in East Antarctica. The revised dating of Australian-East Antarctic plate divergence is likely to be of value in future understanding of the global plate circuit, the feedbacks between paleogeography and paleoclimate, the role of rifts in plate divergence and

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geodynamics, the south Australian margin’s hydrocarbon systems, and the

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formation of continent-ocean transition zones.

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ACKNOWLEDGEMENTS

I thank the COMPASS consortium for travel funding. The magnetic anomaly data in Figure 10 were provided by Geoscience Australia and by Jo Whittaker. Peter Burgess,

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Dietmar Müller, and Jennie Totterdell helped understanding some of the issues surrounding Cretaceous subsidence of the south Australian shelf. Steve Cande,

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Morgane Gillard, Isabel Sauermilch, Lloyd White, Jo Whittaker and two anonymous reviewers commented extensively on the manuscript. The magnetic isochron pick data used for modelling are available via the GSFML website

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Figure 1. Setting and current understanding of Australian-East Antarctic plate divergence. (a) free air gravity anomalies from Scheinert et al. (2016) and Sandwell et al. (2014). Red lines: Bird’s (2002) present-day plate boundaries. Orange squares: isochron 20 picks from Tikku and Cande (1999) and Whittaker et al. (2007). Green squares: isochron 34 picks from the same sources. Black lines: magnetic anomaly profiles between the picks. (b) Plate-divergence related features from Australia’s southern margin. Grey dashed lines: Magnetic anomaly lineations and their interpretations as reversal anomaly isochrons by Cande and Mutter (1982) and numerous publications since, and (in brackets) Weissel and Hayes (1972). Light green: basement ridge. Blue: grabens with late Jurassic-early Cretaceous growth strata. Red crosshairs: selected wells from Brown et al. (2001). Green solid line: seaward limit of Cretaceous sediments interpreted by well-tie in seismic reflection data (Bradshaw et al, 2003). Green dashed line: seaward limit of directly-tied Cretaceous strata of the Ceduna delta. CWB: Cape Wickham Basin; GAB: Great Australian Bight, ST: Shipwreck Trough; Tor.: Torquay sub-basin. (c) Tectonostratigraphic chart of features from which plate divergence has been interpreted (growth on basement-linked faults from Blevin and Cathro (2008); rapid subsidence from Brown et al. (2001); magnetic reversal isochrons from Cande and Mutter (1982) and Royer and Rollet (1997)).

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Figure 2. Location and picks of magnetic anomaly isochrons and fracture zones in the Wharton and Central Indian basins. Background: vertical gradient of free-air gravity (Sandwell et al., 2014). Ivory transparency: areas of seafloor affected by relative motions at margins of Capricorn Plate. Pink transparency: body of Mammerickx Plate (Matthews et al., 2015). Thick mauve line: extinct axis of Wharton Basin. Thick red line: active ridge crest in Central Indian Basin and thin red lines: other active plate boundaries (all from Bird (2002)). Top left: key for magnetic isochron picks. Yellow lines: chains of fracture zone picks. AFR: African Plate; ANT: East Antarctic Plate; AUS: Australian Plate; CAP: Capricorn Plate; CB: Central Indian Basin; IND: Indian Plate; WB: Wharton Basin.

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Figure 3. Inversion fits in Wharton Basin. Blue lines and black rings: synthetic flowlines and flowpoints of model in Table 1. Small black triangles: fracture zone picks as in Figure 2. Other coloured symbols: Magnetic anomaly isochron picks as in Figure 2 and key. Unfilled symbols: isochron picks after rotation to conjugate (only isochrons 26y and younger) and non-conjugate (all isochrons) targets. Inset uses a long fracture zone on the Indian side of the ridge to compare the new model to those of Jacob et al. (2014) (“J14”) with two or three plates, and of Royer and Sandwell (1989). To aid comparison the gravity-picked flowline picks are highlighted with a thick grey line.

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Figure 4. Rotation poles and their 95% confidence ellipses for Wharton Basin reconstruction. Coloured 95% confidence ellipses: this study/Table 1. Poles connected by solid grey line: Jacob et al.’s (2014) 2 plate model. Poles connected by grey dashed line: Royer and Sandwell (1989).

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Figure 5. Inversion fits in the Central Indian Basin. Blue lines and black rings: synthetic flowlines and flowpoints of model in Table 1. Green lines: synthetic flowlines from Royer and Sandwell (1989). Orange lines: synthetic flowlines from Cande and Patriat (2015). All other symbols as in Figures 2, 3, and key.

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Figure 6. Rotation poles and their 95% confidence ellipses for Central Indian Basin reconstruction. Coloured 95% confidence ellipses: this study/Table 2. Poles connected by solid grey line: Cande and Patriat (2015). Poles connected by grey dashed line: Royer and Sandwell (1989).

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Figure 7. Sketch of the Indian Ocean plate circuit at the present day. ANT: East Antarctic Plate, AUS: Australian Plate, CAP: Capricorn Plate; IND: Indian Plate. The sketch illustrates how, for example, a rotation describing motion of the Australian Plate with respect to the Antarctic Plate (blue arrow) might be calculated by the addition of rotations describing motions (red arrows) of the Australian Plate with respect to the Indian Plate, the Indian Plate with respect to the Capricorn Plate, and the Capricorn Plate with respect to the Antarctic Plate. Red lines: boundaries of the plates in the circuit (Bird, 2002), except for diffuse boundary zones and diffuse triple junction zone (DTJ) of the Capricorn Plate with its neighbours (red dashed lines, Royer and Chang, 1991; Royer and Gordon, 1997). Dark grey lines: other plate boundaries (Bird, 2002).

Figure 8. a) Finite rotation poles for Australian-East Antarctic plate motion and 95% confidence regions. Colours as in Figures 4 and 6. Grey finite pole progressions from studies by Royer and Sandwell (1989) (italic labels), Tikku and Cande (1999; 2000) (underlined labels) and Whittaker et al. (2007) (plain font labels). b) Stage poles for Australian-East Antarctic plate motion prior to isochron 20. Bold line, discs, and labels: this study. Previous studies as in part (a). RTJ: Rodriguez triple junction; SEIR: SE Indian Ridge; SWIR: SW Indian Ridge. c) Full set of stage pole locations calculated from finite rotations in Table 4 (only the age of the old end of each stage is labelled, for clarity).

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Figure 9. Synthetic ridge crest flowlines for the post-26y period with respect to (top) Australian Plate and (bottom) East Antarctic Plate according to Australian-East Antarctic rotation parameters derived within the circuit (Table 4). Coloured ellipses: 95% confidence regions for points on selected flowlines (colours as in Figures 4 and 6). Disks: picks of isochron 13 from the Australian and East Antarctic plates (pink) and same, but rotated from the East Antarctic to the Australian Plate (white). Triangles: isochron 20y picks on the Australian (pink) and rotated from the East Antarctic (white) plates. Green symbols: picks of Cretaceous isochrons from Tikku and Cande (1999). Inset a: Comparison of post-26y model to existing models of Australian-East Antarctic divergence since chron 34y; JBC: Jacob et al.’s (2014) Bullard contour fit; J3p: Jacob et al.’s (2014) 3-plate model; P: Powell et al., 1988; RS: Royer and Sandwell (1989); TC: Tikku and Cande (1999; 2000); W07: Whittaker et al., 2007; W11: Williams et al., 2011; W13: Whittaker et al., 2013. Inset b: previous interpretations (in brackets) and suggested reinterpretations of these isochrons.

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Figure 10. Black: Models of magnetic reversal isochrons formed on the Australian and East Antarctic plates at spreading rates based on the plate divergence parameters in Table 4. Source is a flat layer 0.5 km thick, at 5.6 km depth. Anomalies are modelled for their present-day latitudes and strikes assuming Paleocene latitude of 60°S and E-W strike. Blue: recorded anomalies along the profile tracks of Figure 1. Green: segments of Australian-side profiles associated with landward basement ridges.

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Figure 11. Representative synthetic ridge crest flowlines with respect to the Australian Plate (top) and East Antarctic Plate (bottom), according to Australian-East Antarctic rotation parameters derived within the circuit (Table 4). The flowline segments for the period 26y-30o is shown in green and that for 30o-34y in blue. The green segment shows the synthetic ridge crest moving towards the plate interior and so implies a convergent plate boundary. The flowlines have been chosen to illustrate extremes of the range of variability in size of 95% confidence ellipses throughout the system. The ellipses for 30o and 26y illustrate that geologically-untestified convergence is a robust prediction of the rotations. Inset: effect of unnoticed spreading asymmetry or heterogeneous Miocene and later shortening in Wharton Basin on the flowline segments; the divergent-then-convergent character of model Australian-East Antarctic motion would be resolved even in the presence of ±30% unnoticed spreading asymmetry throughout the whole basin, or alternatively in the presence of the 30% localised shortening (e.g. by 150 km affecting any 500 km wide sub-segment of the basin).

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Figure 12. Comparison of seafloor spreading markers in Wharton Basin modelled according to Table 1 (plain black lines) and according to the predictions of the plate circuit when built using models of slow late Cretaceous and Paleocene divergence between Australia and East Antarctic by Tikku and Cande (1999; 2000), red, and Whittaker et al. (2007), blue. Background: vertical gradient of free-air gravity (Sandwell et al., 2014). 95% confidence ellipses are calculated from the covariances in Table 1 and by combination of the covariances in Table 2 with those of Tikku and Cande’s model.

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Figure 13. Three reconstructions (to chrons 22o, 25y and 26y) of the AustraliaAntarctica plate system using rotations summed in the eastern Indian Ocean plate circuit (Table 4). Orange fill: eastern overlap (darker where involving unstretched continental crust on either or both plates). Pink fill: western overlap between outer edges of rough topography in Diamantina Zone and Labuan Basin at 22o, and between their inner edges at 25y and 26y. Black lines: features on Australian Plate, coloured lines: features on Antarctic Plate rotated as labelled on each map. Short dashed lines: seaward edges of extended crust by the end of the Jurassic-Cretaceous plate divergence episode. Long dashed lines: fracture zones associated with the same episode. ARB: Adélie Rift Block; STR: South Tasman Rise; Tas: Tasmania.

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Figure 14. A scheme for explaining the eastern overlap as a consequence of transtension in subglacial basins of East Antarctica (AsT and AT: Astrolabe and Adventure subglacial trenches). Portions of East Antarctica west of the basins start to move with the rest of the East Antarctic Plate at the times shown. The reconstructions are made with the Australian Plate fixed using rotations interpolated for the starts of the time periods shown within the sequence of Table 4. As such, the plate on the eastern side of the active basin (pink fill) at any given time is the Australian Plate. Alternative reconstructions are possible by incorporating more or different subglacial basins.

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Figure 15. Radio echo sounding (BEDMAP2; Fretwell et al., 2013), free-air gravity (AntGG; Scheinert et al., 2016) and total field magnetic anomaly (ADMAP2; Golynsky et al., 2018) images of the subglacial interface in and around George V Land. Subglacial trenches interpreted with Cenozoic transtensional origins: As: Astrolabe, Ad: Adventure, Co: Concordia. Thick white dashed lines: small circle segments about the 25y-24o stage pole for Australian plate motion with respect to East Antarctica. Thin white dashed lines: block interfaces used in Figure 14. TAM: Transantarctic Mountains, WARS: Ross Sea sector of the West Antarctic Rift System.

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TABLES Variances and covariances , , ,

,

,

,

Chron

0.1845 0.1895 0.0855 0.0230 0.0071

0.7963 0.6050 0.2735 0.0753 0.0226

-0.0051 -0.0225 -0.0138 -0.0047 -0.0024

6.6941 3.5683 1.9905 0.5194 0.1462

-0.0428 -0.1263 -0.0848 -0.0265 -0.0146

0.0015 0.0075 0.0087 0.0081 0.0055

C20y C21y C21o C22o C24o

159.23 154.47

-6.78 -7.01

16.29 18.99

0.0091 0.0074

0.0296 0.0403

-0.0042 -0.0076

0.1818 0.4685

-0.0268 -0.0884

0.0073 0.0201

C25y C26y

158.41 162.75 167.28 169.28 168.37 168.81 166.69 167.07 163.32

-6.57 -5.76 -5.04 -4.36 -4.44 -4.26 -4.56 -3.66 -3.77

22.51 25.86 27.98 32.13 33.37 35.21 38.28 42.48 45.40

0.0643 0.0709 0.0561 0.0548 0.0588 0.0459 0.0303 0.1211 0.1957

0.3701 0.3858 0.2630 0.2365 0.2106 0.1805 0.1202 0.5743 1.0538

-0.0737 -0.0699 -0.0399 -0.0357 -0.0369 -0.0467 -0.0320 -0.1979 -0.4622

3.0866 2.7372 1.6497 1.3657 1.2143 1.0154 0.6751 3.0865 7.4688

-0.6031 -0.4837 -0.2461 -0.2046 -0.2068 -0.2568 -0.1879 -1.0856 -3.2727

0.1259 0.0932 0.0436 0.0407 0.0457 0.0857 0.0720 0.4093 1.4625

C27y C28o C29o C30o C31o C32y C33y C33o C34y

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Rotation parameters Ang Long () Lat () ( ) 168.79 -5.29 1.15 157.53 -5.87 3.64 155.89 -6.86 5.05 158.87 -6.69 7.15 159.70 -6.93 12.58

Variances and covariances , , , 1.7235 1.3205 0.1013 1.1348 1.1370 0.1149 0.3223 0.2956 0.0686 0.1527 0.1307 0.0349 0.1688 0.1336 0.0564 0.2362 0.2090 0.0701 0.2666 0.2385 0.0822 0.5459 0.4660 0.1712 0.0477 0.0442 0.0170 0.0211 0.0207 0.0091 0.0191 0.0219 0.0065 0.0068 0.0080 0.0029 0.0031 0.0039 0.0013 0.0029 0.0035 0.0016 0.0023 0.0029 0.0013 0.0022 0.0029 0.0010 0.0024 0.0032 0.0009 0.0016 0.0022 0.0006 0.0012 0.0015 0.0004 0.0013 0.0015 0.0005

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CE

PT E

Rotation parameters Long () Lat () Ang () -151.98 -22.43 5.77 -156.79 -24.41 10.49 -150.65 -18.01 14.49 -149.96 -17.33 19.94 -147.03 -14.19 24.51 -150.52 -16.14 24.76 -153.30 -15.77 26.60 -153.66 -14.04 27.95 -155.57 -13.81 29.60 -159.15 -12.39 34.16 -161.02 -11.58 37.94 -160.55 -9.98 39.67 -163.01 -10.04 43.04 -164.30 -9.69 45.82 -165.24 -9.55 48.02 -166.21 -9.26 51.09 -166.58 -9.23 52.25 -167.27 -9.19 54.39 -167.89 -9.31 56.18 -168.99 -9.43 60.46

D

Table 1. Finite rotations and covariances modelled from Wharton Basin for reconstruction of Australian Plate with respect to Indian Plate. All rotations righthanded.

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,

,

,

1.0471 1.1543 0.2770 0.1157 0.1102 0.1972 0.2268 0.4140 0.0460 0.0239 0.0292 0.0123 0.0065 0.0069 0.0051 0.0052 0.0057 0.0041 0.0026 0.0025

0.0777 0.1137 0.0633 0.0306 0.0451 0.0613 0.0728 0.1485 0.0152 0.0074 0.0070 0.0027 0.0013 -0.0002 0.0010 0.0010 0.0012 0.0007 0.0003 0.0004

0.0095 0.0207 0.0276 0.0102 0.0221 0.0232 0.0279 0.0567 0.0077 0.0063 0.0042 0.0029 0.0019 0.0054 0.0031 0.0023 0.0019 0.0017 0.0016 0.0024

Chron C5y C6o C8o C13o C18o C20y C21y C21o C22o C24o C25y C26y C27y C28o C29o C30o C31o C32y C33y C33o

ACCEPTED MANUSCRIPT -169.73

-9.34

63.64

0.0010 0.0015 0.0007 0.0037 0.0004

0.0050 C34y

Table 2. Finite rotations and covariances modelled from the Central Indian Basin for reconstruction of Capricorn Plate with respect to East Antarctic Plate. All rotations right-handed.

latitude 55.12

angle 0.34

visual fit isochron 20y

applicable times Pre-18

plate pair India-Capricorn

PT

longitude 178.59

Variances and covariances , , ,

-149.02 -146.99 -145.08 -144.23 -141.37 -139.88 -135.44 -136.21 -136.04 -137.66 -136.21 -135.46 -136.03 -132.74 -132.15 -130.77

35436.75 5097.16 6256.37 1190.78 1028.32 2271.21 185459.16 17959.02 10423.25 13126120.85 54346.31 9021.33 4575.82 494.54 861.48 1104.63

NU

-3032.84 -387.90 -377.98 -65.12 -33.99 -46.34 368.57 72.51 48.39 -1621.20 100.75 48.41 27.64 7.38 18.45 56.72

MA

4253.51 611.76 639.96 117.13 69.63 100.69 -500.23 -131.51 -133.90 5384.20 -380.53 -149.13 -74.39 -19.72 -2.69 23.26

D

23.99 24.45 25.07 25.15 26.01 27.39 29.26 28.84 27.72 26.44 25.52 26.03 26.27 26.89 27.50 30.56

PT E

-15.39 -14.15 -11.51 -10.78 -6.98 -4.38 0.38 0.85 1.18 -0.03 0.55 1.49 1.57 4.13 3.91 7.32

,

,

,

Chron

510.94 73.91 65.95 11.77 4.92 4.85 2.47 2.40 3.55 3.57 4.23 4.01 2.68 1.83 4.76 3.95

-363.99 -46.47 -38.56 -6.35 -2.26 -1.96 -0.64 -0.14 -0.19 -0.41 -0.43 -0.52 -0.19 -0.07 0.92 2.06

259.57 29.54 22.86 3.57 1.13 0.97 0.85 0.40 0.33 0.25 0.24 0.31 0.21 0.16 0.60 3.13

C20y C21y C21o C22o C24o C25y C26y C27y C28o C29o C30o C31o C32y C33y C33o C34y

SC

Rotation parameters Long Lat Ang () () ( )

RI

Table 3. India-Capricorn rotation derived from visual fitting of conjugate East Antarctic and Australian magnetic isochrons 20y. See Figure 9 for visual fits.

AC

CE

Table 4. Model Australian-Antarctic rotations and covariances by completion of the circuits East Antarctica-Capricorn-India-Australia (pre-chron 18) and East Antarctica-Capricorn-Australia (post-chron 18). Covariances are combined from the Australian-Indian and Capricorn-East Antarctic arms of the circuit only.

45