Reconstructing motions of subducted oceanic plates - Lydian Boschman - 2021 MagIC Workshop (1/20/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-22

" Reconstructing plate motions of subducted oceanic plates using paleomagnetic data from subduction complexes - with examples from Japan, New Zealand, Mexico and Costa Rica" by Lydian Boschman, Department of Environmental System Sciences, ETH Zürich

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
In this presentation, I will focus on the plate kinematic history of the Mesozoic Panthalassa Ocean. The Panthalassa Ocean, covering approximately half of the globe during the culmination of Pangea, has decreased in size since then due to breakup of the supercontinent and associated seafloor spreading in the Atlantic and Indian oceans. The present-day descendant of the Panthalassa, the Pacific Ocean, is mainly underlain by the Pacific Plate, which started forming during the Jurassic (Larson & Chase, 1972; Larson & Pitman, 1972; Boschman & van Hinsbergen, 2016). The degree of post-Jurassic subduction in the circum-Pacific region is emphasized by the loss of Mesozoic oceanic lithosphere that formerly underlay the Panthalassa Ocean, combined with the extent of post-Jurassic lithosphere that formed at new spreading centers in the Pacific Ocean, involving the Pacific Plate and its conjugates. 

Even though the former Panthalassa plates are no longer at the surface, their formation and demise has left an imprint upon the Earth and their plate motions may be reconstructed from indirect sources of data. First, marine magnetic anomalies and fracture zones preserved on the Pacific Plate provide constraints on the past motions of plates conjugate to the Pacific Plate (e.g. the conceptual Izanagi, Farallon and Phoenix plates) back to ~190 Ma (Engebretson et al., 1985). Second, remnants of oceanic ridges, rises, seamounts, and intra-oceanic arcs, scraped off the downgoing plates during subduction and presently exposed in circum-Pacific accretionary complexes, enables the acquisition of geological and paleomagnetic data from the rock record representative of the former Panthalassa plates. Such accreted materials range from fragments to entire sequences of ‘Ocean Plate Stratigraphy’ (OPS), a concept developed by Isozaki et al. (1990); Isozaki (2014). OPS sequences ideally consist from bottom to top of magmatic basement (MORB, arc, plateau or seamount), cherts and/or limestones, hemipelagic sediments, and tuffaceous and turbiditic trench-fill deposits, representing rocks of an oceanic plate that erupted and accumulated during its journey from the spreading ridge to a trench. The age difference between the base and top of an ideal OPS sequence reflects the age of the oceanic crust at the time of subduction, thereby providing a valuable geological source of information on the nature and drift of lost oceanic plates. Moreover, paleomagnetic data from OPS rocks may provide paleolatitudes of the subducted oceanic plate, thereby constraining latitudinal plate motion. 

Here, we illustrate plate kinematic reconstruction of the Panthalassa Ocean using new paleomagnetic data from accreted Permian to Cretaceous OPS rocks exposed in the accretionary complexes and tectonic mélanges of Cedros Island (Baja California, Mexico), the Santa Elena Peninsula (Costa Rica), the North Island of New Zealand, and Japan, yielding plate motions of the Farallon, Phoenix and Izanagi plates, respectively. We supplement this dataset with previously published paleomagnetic data from the same regions (Ando et al., 2001; Kirschvink et al., 2015; Kodama et al., 2007; Oda & Suzuki, 2000; Shibuya & Sasajima, 1986). We develop a reconstruction of Izanagi, Farallon and Phoenix plate motions since 260 Ma that satisfies the available paleomagnetic data and is consistent with constraints from upper plate deformation and accretion in the circum-Pacific continental margins. This work explores the possibility of developing reconstructions of subducted oceanic plates using paleomagnetic data, and develops a method that may be applied to any other ancient lost ocean, as long as associated accretionary complexes are identified exposing off-scraped remnants of the former oceanic plates.