Western North American Tectonics from Unsubducting and Re-Subducting Lost Oceanic Lithosphere: Paired Inverse and Forward Tectonic Modeling

Abstract:

Plate tectonic reconstructions are vital to our conceptualization of the geologic past and are key inputs to many global geodynamic, climate, and biological models. However, the past tectonic configurations of convergent margins are obscured by subduction (i.e., the destruction of oceanic lithosphere) and constraints have been limited to upper plate geologic remnants only. Recent advances in mantle tomographic imaging have revealed the present configuration of subducted slabs in Earth’s mantle. We highlight a recent plate reconstruction workflow that utilizes structural restoration to ‘unsubduct’ these newly imaged slabs to constrain plate tectonic models. These restored slabs are then input into forward mantle flow models to ‘re-subduct’ them, allowing for a comparison of the predicted and observed mantle structure. We present a case study of this method in the form of a new Late Jurassic to present plate tectonic reconstruction of western North America. Our structural restoration of upper mantle slabs below the Pacific Northwest and Alaska accounts for Pacific-Kula subduction since ~60 Ma below the Aleutian Islands and at least ~75 Ma of Farallon subduction below southern California. These results constrain: (1) the presence of a lost tectonic plate, previously called “Resurrection”, offshore Canada and Alaska during Paleocene to Eocene time and (2) a major slab breakoff event below southern California during Late Cretaceous time that we interpret to represent collision of the Wrangellia composite terrane (WCT) island arc with North America. Mantle flow models support this interpretation by demonstrating that plate tectonic reconstructions that include a Late Cretaceous WCT collision reproduce an enigmatic lower mantle slab anomaly observed below North America, while models that include an earlier WCT collision do not. Many opportunities exist for future applications of similar workflows to many modern and ancient convergent margins.