The Skytrain plate and tectonic evolution of southwest Gondwana since Jurassic times

Abstract

Uncertainty about the structure of the Falkland Plateau Basin has long hindered understanding of tectonic evolution in southwest Gondwana. New aeromagnetic data from the basin reveal Jurassic-onset seafloor spreading by motion of a single newly-recognized plate, Skytrain, which also governed continental extension in the Weddell Sea Embayment and possibly further afield in Antarctica. The Skytrain plate resolves a nearly century-old controversy by requiring a South American setting for the Falkland Islands in Gondwana. The Skytrain plate’s later motion provides a unifying context for post-Cambrian wide-angle paleomagnetic rotation, Cretaceous uplift, and post-Permian oblique collision in the Ellsworth Mountains of Antarctica. Further north, the Skytrain plate’s margins built a continuous conjugate ocean to the Weddell Sea in the Falkland Plateau Basin and central Scotia Sea. This ocean rules out venerable correlation-based interpretations for a Pacific margin location and subsequent long-distance translation of the South Georgia microcontinent as the Drake Passage gateway opened.

South Georgia microcontinent: Displaced fragment of the southernmost Andes

“…Despite a consensus for this correlation developed by a century of geological fieldwork, there is a current tendency to dismiss such evidence if it does not conveniently fit with regional plate modelling of the large geophysical databases that are now available. As one recent example of that approach (but by no means the only one), we would draw attention to Eagles and Eisermann (2020). These authors, amongst other controversial proposals, reject -with slight disdain -the Andean correlation of South Georgia because their regional model derived from an interpretation of geomagnetic data "rules out venerable correlation-based interpretations for a Pacific margin location and subsequent long-distance translation of the South Georgia microcontinent" (op.…”

Paleomagnetic Constraints From South Georgia on the Tectonic Reconstruction of the Early Cretaceous Rocas Verdes Marginal Basin System of Southernmost South America

“…There is thus a close correlation with the rocks of the Fuegian Andes (Dalziel et al., 2021). Moreover, the provenance of the turbidites on South Georgia can be closely matched with sources in Tierra del Fuego that are absent in the island's present location and differ markedly from those to be expected in the more easterly original location implied by the plate reconstruction models of Eagles (2010a) and Eagles and Eisermann (2020) (Carter et al., 2014; Dalziel et al., 1975; see further discussion in Dalziel et al., 2021).…”

“…The results are directly comparable to those obtained from samples from Tierra del Fuego and therefore support the geological evidence that the microcontinent originated as a continuation of the Andean Cordillera and was displaced eastward relative to South America during the opening of the South Atlantic Ocean basin. position immediately east of Cape Horn and due south of Isla de los Estados and Burdwood Bank, which we take to be our null hypothesis; however, other plate reconstruction models resulting from study of geophysical data from the South Atlantic Ocean, the Scotia Sea and the Weddell Sea would place the SGM off Maurice Ewing Bank much farther to the east (Eagles, 2010b;Eagles & Eisermann, 2020). Constraining the initial position of the SGM is important for understanding the paleogeography of this southern margin of the South American continent as it evolved during the late Mesozoic and Cenozoic, playing a critical role in the onset and development of the Antarctic Circumpolar Current (Carter et al, 2014;Dalziel, Lawver, Norton, et al, 2013;Dalziel, Lawver, Pearce, et al, 2013) and hence influencing global climate.…”

“…Both the significant counterclockwise rotation (−27.2 ± 11.2°) and/tilting (−10.5° ± 4.5°) implied by the SGM stable secondary magnetization are thus not inconsistent with the tectonic history inferred from the Fuegian Andes. It should be acknowledged that the paleomagnetic data do not in themselves preclude a paleoposition of the SGM much farther to the east of Cape Horn, adjacent to Maurice Ewing Bank (Figure 1), as argued by different plate kinematic scenarios for the Scotia Sea (Eagles, 2010b(Eagles, , 2016Eagles & Eisermann, 2020); however, the correspondence of tectonostratiographic frameworks, termination of the Rocas Verdes basin at the Cape Horn escarpment and provenance of the Cretaceous sedimentary strata of the SGM, combined with the general similarities in the paleomagnetic signatures are strongly supportive of the traditional restored position of the SGM adjacent to the Fuegian Andes east of Cape Horn and south of Burdwood Bank (Dalziel & Elliot, 1971;Dalziel et al, 1975Dalziel et al, , 2021; Figure 8). The SGM, as part of the Fuegian Andes, apparently rotated ∼30° counterclockwise with variable local northward tilting in the Late Cretaceous before the SGM rifted off and somehow was transported over 1,700 km along the North Scotia Ridge to its current position in the Scotia Sea, possibly in an "orogenic stream" (Redfield et al, 2007) escaping eastward during closure of the Rocas Verdes marginal basin (Dalziel et al, 2021).…”

Footprints of palaeocurrents in sedimentary sequences of the Cenozoic across the Maurice Ewing Bank

“…Three prevailing schemes exist for the paleogeography of the Falkland/ Malvinas Islands microplate prior to the Gondwana breakup. The first explains the present location with a clockwise rotation from its origin southeast of South Africa and drift along the transform faults (e.g., Eagles and Eisermann, 2020;Marshall, 1994;Stone et al, 2009;Taylor and Shaw, 1989;Thomson, 1998). The second suggests a paleo-location between South America and South Africa (Du Toit, 1927; Du Toit, (Smith and Sandwell, 1997).…”