Paradigms, new and old, for ultrahigh-pressure tectonism

Abstract

Regional ultrahigh-pressure (UHP) metamorphic terranes exhibit a spectrum of lithological, structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them. At least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths: i) continental margin subduction, ii) microcontinent subduction, iii) sediment subduction, iv) intracontinental subduction, v) subduction erosion, and vi) foundering of a crustal root. Most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of UHP rocks. At least six geodynamic processes can be envisioned to have exhumed UHP continental rocks: i) eduction, ii) microplate rotation, iii) crustal stacking, iv) slab rollback, v) channel flow, and vi) trans-mantle diapirs. Most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one UHP terrane. More-detailed and systematic field investigations are warranted to assess the predictions of numerical models, and more-sophisticated and realistic numerical models are required to replicate and explain the petrological, structural, and chronological data obtained from UHP terranes.