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

The study addresses the long-standing lack of a self-consistent plate kinematic framework for southwest Gondwana during Jurassic–Cretaceous times. Disagreements over the structure and origin of the Falkland Plateau Basin (FPB), the positions and motions of mobile continental blocks (South Georgia, Falkland Islands, Ellsworth Mountains), and interpretations of large paleomagnetic rotations have hindered understanding of supercontinent breakup mechanisms, the role of Toarcian magmatism and basin anoxia, and the development of the Drake Passage oceanic gateway. The research aims to clarify the FPB’s crustal nature, timing, and plate-tectonic context using new aeromagnetic data, and to test correlation-based models that invoke large block rotations and translations. The central hypothesis is that a previously unrecognized plate (Skytrain) governed Jurassic-onset divergence and margin development across the FPB–Weddell–Scotia system, providing a unified explanation for regional geological and geophysical observations.

Classic regional correlations linked South Georgia to Tierra del Fuego in a late Jurassic back-arc setting and proposed 1600 km of Cenozoic strike-slip translation. Alternative views reinterpreted South Georgia’s tectonostratigraphy as an intra-Gondwanan Jurassic extensional margin and the central Scotia Sea as a Jurassic–Early Cretaceous oceanic fragment. The Falkland Islands were correlated either with South Africa’s Cape Fold Belt (requiring ~120° clockwise rotation and ~1000 km westward drift) or with Patagonian intracontinental basins; paleomagnetic data are robust mainly in a single dyke population and may reflect syn-intrusion deformation near present position. The Ellsworth Mountains show strong, multi-lithology Cambrian paleomagnetic rotations (~90° anticlockwise) but lack a pre-Gondwanide deformation phase seen elsewhere, leaving their original position debated. The FPB interior crustal type was ambiguous (extended continental crust with underplate vs thick oceanic crust) until new seismic refraction data indicated 10–12 km-thick two-layer igneous crust bounded by magma-poor continent–ocean transitions. Long-standing models invoked large rotations, synchronous opening with the Weddell Sea Embayment, and major strike-slip along the putative Gastre Fault in Patagonia—features not demonstrated in the field. High-quality magnetic anomaly coverage had been lacking to resolve spreading fabric and age in the FPB.

The AIRLAFONIA aerogeophysical survey (Nov 2017 and Nov 2018) targeted the interior of the Falkland Plateau Basin. Platform: AWI Basler BT-67 aircraft (Polar 6; converted Douglas C‑47 Skytrain airframe). Instruments: Scintrex Cs-3 caesium vapor magnetometer in a tail stinger; three-component fluxgate magnetometer in the tail fin for real-time aircraft compensation; Gravimetric Technology GT2A gravimeter. Gravity data were tied to the International Standard Gravity Network using an absolute station in Stanley, Falkland Islands, with a portable LaCoste & Romberg meter. Flight plan: 25,185 line-km over FPB along east–west and NE–SW tracks, predominantly level flight at 2000 ft a.s.l. (with segments at 1000–3000 ft for icing avoidance); ~12 km line spacing for EW tracks. Additional 39,000 km of legacy marine and helicopter magnetic data were leveled to AIRLAFONIA to extend coverage to the basin’s eastern and northern margins. Processing: spike removal and diurnal correction using a temporary base magnetometer on East Falkland; standard leveling and gridding in Geosoft Oasis Montaj; given small vertical separation between marine and airborne datasets, no vertical continuation was applied. Interpretation integrated the new total field magnetic anomaly grid with satellite-derived free-air gravity anomalies and published seismic reflection/refraction constraints. Synthetic magnetic modeling was used to correlate linear anomalies with Jurassic magnetic reversal isochrons and to estimate spreading rates and ages. Plate-kinematic reconstruction used visual-fit rotation poles to co-fit conjugate margin segments, isochrons, fracture-zone/flowline orientations, and spacing of M-chrons in the central Scotia Sea (adjusted for Drake Passage opening). Circuit closure without Skytrain motion was enforced by chron C34o (~126 Ma).

- New magnetic anomalies define seafloor spreading fabric in the FPB: in Area 1a (eastern FPB), five narrow NW-trending linear anomalies over a broad gravity high indicate a mid-ocean ridge recording geomagnetic reversals; anomalies terminate westward against a NE-trending feature likely a fracture zone/transform. - Area 1b marks a magma-poor continent–ocean transition (COTZ) along southern Maurice Ewing Bank, without seaward-dipping reflectors or high-velocity lower crust, and intersected by the Cretaceous Falkland Escarpment transform. - Western FPB interior (Areas 2a, 2b) shows broader NW- and NE-trending magnetic anomalies coincident with flat-lying basement reflections interpreted as extensive sills/flows forming the upper igneous crustal layer, implying a shallow to subaerial divergent plate boundary with along-strike segmentation; Area 2c shows similar characteristics further east. - Area 3 is a ~100 km-wide zone of disorganized, low-amplitude anomalies with eastward-thinning oldest sediment fill, interpreted as an eastward-plunging divergent segment formed beneath a sediment wedge where hydrothermal alteration suppressed magnetization. - Area 4 (to the North Scotia Ridge) shows subdued broad anomalies consistent with basement deepening and flexure from Miocene near-orthogonal convergence and later slight left-lateral shear; critically, no additional evidence of the Eocene–younger E–W strike-slip fault system required for 1600 km South Georgia translation is seen in the FPB trough. - Area 5 (SW-striking FPB continental margin at the Falkland Islands) exhibits strong narrow positive magnetic anomalies and a narrow (~80 km) necking zone, consistent with a volcanically active, steep transform that accommodated right-lateral shear transforming FPB divergence. Onshore Falkland dykes show structures and microtextures consistent with syn-cooling right-lateral transpression; dyke intrusion dated at ~182 Ma (Toarcian), providing an upper bound for transform motion and FPB divergence onset. - Age of FPB oceanic crust: Correlation and synthetic modeling support Kimmeridgian magnetic reversal sequence M25–M22A (156–152 Ma), with fast half-spreading rates up to ~60 km/Myr; an alternative Toarcian (178–171 Ma) model is possible but less consistent with basin subsidence stratigraphy. Pre‑156 Ma oceanic crust may be present NE of modeled anomalies. - Identification of a previously unrecognized Skytrain plate: FPB half-rates exceed contemporaneous East–West Gondwana ridges (Riiser-Larsen, Mozambique/Somali Basins), requiring a third plate south of FPB; right-lateral shear at the Falkland margin places this plate in the Weddell Sea. Conjugate Kimmeridgian lineations are recognized in the SW Weddell Sea; alignment of conjugate margins links the East Falkland sheared margin to the Charcot Anomaly and the FPB extended margin to the shelf edge/N stretch of the Eastern Palmer Land Shear Zone, interpreted as an inverted pre-Kimmeridgian extended margin on Skytrain. - Plate kinematics: A visual-fit model simultaneously aligns conjugate margins and isochrons, fracture-zone/flowline orientations, and central Scotia Sea M-chrons. Independent Skytrain motion ceased by at least ~126 Ma (C34o) and possibly earlier (~84 Ma circuit closure). - Regional tectonics: Skytrain motion initiated ~182–177 Ma, coeval with Falkland transform dykes, Latady Basin back-arc basalts, and early Jurassic granitoids (Mount Sullivan region). By ~156 Ma, seafloor spreading was established in FPB and southern Weddell Sea; a contemporaneous Rocas Verdes Basin opening may represent an abandoned western Skytrain–WGON boundary. Post‑M25 convergence between Skytrain and East Gondwana south and west of Coats Land explains Ellsworth Mountains’ late Jurassic–Early Cretaceous uplift/denudation and oblique collision features, and a broad zone of subdued magnetics west of the range. By ~126 Ma, Skytrain boundaries were being abandoned, the central Scotia Sea oceanic fragment became part of West Gondwana/South America, and the Weddell Rift was formed parallel to Coats Land. - South Georgia: Reconstructions show the FPB and central Scotia Sea formed a continuous late Jurassic–Early Cretaceous oceanic barrier along South Georgia’s proposed translation path from Tierra del Fuego, ruling out the classic long-distance back-arc translation scenario. Detrital zircon age peaks in South Georgia can be explained by local margin magmatism (mid-Jurassic) and post-breakup volcanism (late Albian) and by sediment sources consistent with its extended-margin position adjacent to the Karoo hinterland. - Paleomagnetism: Up to ~90% of the Ellsworth Mountains’ post-Cambrian rotation (85 ± 16°) can be explained by Skytrain Jurassic–Early Cretaceous motion (~55° anticlockwise about a nearby Euler pole). The ~120° apparent rotation recorded by ~182 Ma Falkland dykes is better attributed to local, syn-intrusion right-lateral shear/rotation in a transform zone rather than whole-plate rotation, supporting a South American (Patagonian) setting for Falklands in Gondwana.

The new aeromagnetic grid demonstrates that the FPB contains Jurassic oceanic crust with clear reversal isochrons and diagnostic margin architecture. When these geophysical constraints are integrated with gravity and seismic data, they necessitate a kinematic model in which a separate Skytrain plate existed south of West Gondwana. This framework resolves multiple long-standing controversies: it obviates the need for large, prescribed rotations/translations of the Falkland Islands and Ellsworth Mountains tied to a continuous Gondwanide orogen; it explains the Ellsworth Mountains’ paleomagnetic rotation, late Jurassic–Early Cretaceous uplift, and post-Permian oblique collision within a single, time-bounded collisional episode between Skytrain and East Gondwana; and it removes the requirement for a long-lived, large-offset Gastre Fault in Patagonia. The contiguous Jurassic–Early Cretaceous oceanic lithosphere reconstructed between the FPB and central Scotia Sea contradicts models placing South Georgia immediately east of Tierra del Fuego in a back-arc setting with subsequent ~1600 km translation, for which neither the necessary strike-slip fault system nor subduction-transform edge propagation structures are observed in FPB/Scotia data. In the Weddell sector, aligning conjugate margins/isochrons links the East Falkland sheared margin to the Charcot Anomaly and identifies the Eastern Palmer Land Shear Zone as an inverted Skytrain extended margin, tying regional deformation and magmatism to the same plate boundary system. The plate-circuit closure by C34o (~126 Ma) brackets Skytrain’s independent motion to Jurassic–earliest Cretaceous times and yields a coherent set of stage rotations that reproduce observed fabric and isochron spacing, despite acknowledged uncertainties around M22A–M19 tied to pre-Drake central Scotia Sea geometry. Overall, the Skytrain plate model provides a unified, internally consistent tectonic narrative for southwest Gondwana’s breakup and early Scotia–Weddell evolution.

This study introduces and substantiates the Skytrain plate, whose Jurassic–earliest Cretaceous motion governed the opening and architecture of the Falkland Plateau Basin and southern Weddell Sea, linked to the central Scotia Sea. High-resolution aeromagnetic data reveal Kimmeridgian reversal isochrons (M25–M22A), magma-poor COTZs, and transform/sheared margin segments, enabling conjugate fits to Weddell Sea anomalies and Antarctic margin features (Charcot Anomaly, Eastern Palmer Land Shear Zone). The Skytrain framework reconciles disparate observations—Ellsworth Mountains paleomagnetic rotations, Cretaceous uplift, and oblique collision; Falkland dyke kinematics; and the absence of required large-offset strike-slip systems—while invalidating the long-distance translation model for South Georgia. By closing the regional plate circuit by ~126 Ma, the model constrains Skytrain’s lifespan and integrates South Atlantic, Weddell, and Scotia records into a single kinematic solution. Future work should: (i) acquire denser, better-oriented magnetic and seismic datasets in the southern Weddell Sea to refine conjugate isochron correlations; (ii) re-examine Jurassic granite paleomagnetic poles (e.g., Pagano Nunatak, Nash Hills) with improved paleo-horizontal control; (iii) further quantify pre‑Drake Passage geometry of the central Scotia Sea to reduce uncertainties in M22A–M19 stage rotations; and (iv) test the timing and extent of inversion along the Eastern Palmer Land Shear Zone with additional structural and geochronological studies.

Key limitations include sparse, variably oriented aeromagnetic coverage and greater depth to magnetic sources in the southern Weddell Sea, preventing detailed, high-confidence conjugate isochron picking relative to the FPB. The largest kinematic uncertainty lies between chrons M22A and M19, where constraints transition from two-sided FPB–Weddell isochrons to a one-sided central Scotia Sea record whose pre‑Drake Passage position and orientation are only coarsely estimated. Alternative (Toarcian) age correlations for FPB anomalies are possible but less favored; precise timing of Skytrain circuit closure could be earlier than 126 Ma but is unresolved at higher resolution. Some alignments (e.g., conjugate margin fits) are visual and therefore subject to subjective uncertainty. The absence of observed E–W strike-slip systems in FPB/Scotia restricts but cannot absolutely preclude deeply buried structures beneath the North Scotia Ridge accretionary complex.