The evolution of basal mantle structure in response to supercontinent aggregation and dispersal

Abstract

Seismic studies have revealed two Large Low-Shear Velocity Provinces (LLSVPs) in the lowermost mantle. Whether these structures remain stable over time or evolve through supercontinent cycles is debated. Here we analyze a recently published mantle flow model constrained by a synthetic plate motion model extending back to one billion years ago, to investigate how the mantle evolves in response to changing plate configurations. Our model predicts that sinking slabs segment the basal thermochemical structure below an assembling supercontinent, and that this structure eventually becomes unified due to slab push from circum-supercontinental subduction. In contrast, the basal thermochemical structure below the superocean is generally coherent due to the persistence of a superocean in our imposed plate reconstruction. The two antipodal basal thermochemical structures exchange material several times when part of one of the structures is carved out and merged with the other one, similarly to “exotic” tectonic terranes. Plumes mostly rise from thick basal thermochemical structures and in some instances migrate from the edges towards the interior of basal thermochemical structures due to slab push. Our results suggest that the topography of basal structures and distribution of plumes change over time due to the changing subduction network over supercontinent cycles.

A tectonic-rules-based mantle reference frame since 1 billion years ago – implications for supercontinent cycles and plate–mantle system evolution

“…The roots of individual plumes are not stationary but migrate in response to the deformation of basal structures, as found previously (e.g. Cao et al, 2021b;Hassan et al, 2016;Arnould et al, 2019). Plume tilt mostly forms in response to relatively fast plate motion and induced sub-horizontal lower mantle flow (Fig.…”

Early Cambrian renewal of the geodynamo and the origin of inner core structure

“…In particular, prior ideas on when the deep mantle degree-2 pattern originated vary from ≥540 to <330 Ma and stem from the debate on whether the Atlantic and Pacific large low shear velocity provinces (LLSVPs) formed from post-Pangea subduction 50 or are much older 51 . Our analyses, together with inferences from recent plate reconstructions 52 , suggest that the present-day degree-2 structure formed before the assembly of Pangea, but that the African LLSVP is no older than ~450 Ma. The seismic anisotropy of the innermost inner core likely records dominance of subduction in the paleo-Pacific hemisphere (degree-1) 52 prior to 450 Ma.…”

“…Our analyses, together with inferences from recent plate reconstructions 52 , suggest that the present-day degree-2 structure formed before the assembly of Pangea, but that the African LLSVP is no older than ~450 Ma. The seismic anisotropy of the innermost inner core likely records dominance of subduction in the paleo-Pacific hemisphere (degree-1) 52 prior to 450 Ma.…”

Slab control on the mega-sized North Pacific ultra-low velocity zone

“…6c). Numerical models show flow in the deep lower mantle under the North Pacific from 100-50 Ma due to long-lasting subduction history [48][49][50][51] . Moreover, the Pacific LLVP could have been stable prior to Pangea at 330 Ma 17 .…”