Junction Jigsaw
Authors:   , van de Lagemaat Suzanna Henderijne Aleide 
Published: September 2023

Reconstructing Earth's past tectonic plate motion is vital for understanding its geological history, with implications for geodynamics, paleogeography, paleoclimatology, and resource exploration. However, many challenges exist in the reconstruction of subducted plates. While existing ocean basins can be reconstructed using marine magnetic anomalies and fracture zone data, the reconstruction of subducted plates lacked a clear framework due to varying interpretations of geological and geochemical data. Recently, however, a reconstruction protocol was developed to limit input data of a reconstruction to quantitative geological constraints. This approach avoids geodynamic interpretations, and yields transparent, reproducible, and adaptable reconstructions. In this thesis, this reconstruction protocol is applied to the southwest and west Panthalassa realm, resulting in kinematic reconstructions spanning from Patagonia to Japan, culminating in the complex reconstruction of the Junction Region between the Panthalassa and Tethys realms.
The new reconstructions presented in this thesis have implications for both regional and global tectonics and geodynamics. The Cenozoic reconstruction of the SW Pacific region in a mantle reference frame shows that since subduction initiation, the Tonga-Kermadec slab was dragged laterally through the mantle for over 1200 km, including its lower-mantle portion. The most important finding of the Mesozoic SW Pacific reconstruction is that subduction along the East Gondwana margin continued until at least 90 Ma, and possibly until 79 Ma, which is 10 to 25 Ma longer than the generally accepted 100-105 Ma age for the end subduction there. In the southeast of the Panthalassa domain, the reconstruction of the Scotia Sea region shows that the South Sandwich subduction zone originates from Late Cretaceous (~80 Ma) subduction initiation below South Orkney continental crust, which is part of Antarctica. Subsequently, this subduction zone propagated northwards by delamination of South American lithosphere, transferring the crustal lithosphere to the upper plate. The opening of the Drake Passage started at c. 50 Ma when South America started moving towards the west and initial opening was thus related to upper plate retreat. Oceanic spreading centers formed in the Scotia Sea ~35–25 Ma, reducing dynamic coupling between the down-going slab and the escaping overriding plate, allowing slab roll-back to contribute to back-arc extension.
In the northwest Panthalassa domain, new paleomagnetic, biostratigraphic, geochemical and geochronological data were obtained from Sabah, Borneo. These data suggest that the plate that subducted below Borneo in the Late Cretaceous was a newly recognized plate (the Pontus Plate) that was separated from the paleo-Pacific plates by a subduction zone. Moreover, these data suggest that Mesozoic subduction cessation in the Proto-South China Sea region possibly resulted from an obstructed trench, likely due to the arrival thickened, buoyant oceanic lithosphere.
The reconstruction of the Junction Region, in between the Panthalassa and Tethys realms, suggests that the preserved latest Jurassic oceanic crust in the Philippines originated from the northern Australian Plate margin, where continental margin subduction was active in the Permian and Triassic. In addition, the reconstruction shows there is no requirement for spontaneous subduction initiation at the Izu-Bonin Mariana trench, which initiated along the pre-existing Mesozoic subduction zone.