Make subductions diverse again
Résumé
In the original plate-tectonic centric framework for Earth evolution as proposed in the 1960s, the term 'subduction' was initially applied to the down thrusting of oceanic lithosphere below a continental or oceanic upper plate, delineated by a Wadati-Benioff zone of earthquake foci. Over time, the use of the term has broadened and its meaning weakened by its application to the diversity of mechanisms accommodating lithospheric convergence, foundering and recycling. This has led to complex and sterile debates regarding the tectonic processes in orogens, the initiation of (modern) plate tectonics, or the tectonic regime on other planets, hampering a clear and concise discussion of those problems. We discuss three instances where the use of the ill-defined term "subduction" or even "proto-subduction", through biases and polysemism, has hampered scientific discourse, namely (i) the Cenozoic subduction of the Western Tethys leading to the formation of the European Alps; (ii) the initiation of (modern) "subduction" in the (eo)Archean; and, (iii) lithospheric recycling on Venus. We highlight that Benioff-type subduction is only one of a spectrum of mechanisms that have operated through time (Archean to present) for the foundering and recycling of lithosphere into the mantle. We propose a framework that summarizes the various end-member modes of foundering by focussing on two parameters (i) the type of lithosphere being foundered into the mantle and (ii) the mechanical driver of the foundering process, which might predominantly control convergence and recycling.
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