"Subduction-zone initiation (SZI)" is the onset of downward plate motion forming a new slab, which later evolves into a self-sustaining subduction zone. - Crameri et al. (under review)
"Ocean-plate tectonics (OPT)" is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process. - Crameri et al. (2019)
Crameri, F., C.P. Conrad, L. Montési, and C.R. Lithgow-Bertelloni (2019), The life of an oceanic plate, Tectonophysics, 760, 107-135, doi:10.1016/j.tecto.2018.03.016
Lower lavas (FAB)
“Lower Lavas” or “FAB” are MORB-like lavas, which are suggested to be the very first lavas erupted during subduction initiation. They underlie boninites and arc-like lavas. In the Izu-Bonin Mariana system, these are defined as Forearc Basalts (FAB) due to their presence in drill cores in the forearc (Reagan et al., 2010). These lavas differ from typical MORBs due to low Ti/V ratios (high V at similar Ti contents). They do not yet have any signals from slab fluids and may be transitional between MORB and boninites/arc-like lavas.
Reagan, M. K., Ishizuka, O., Stern, R. J., Kelley, K. A., Ohara, Y., Blichert‐Toft, J., ... & Hickey‐Vargas, R. (2010). Fore‐arc basalts and subduction initiation in the Izu‐Bonin‐Mariana system. Geochemistry, Geophysics, Geosystems, 11(3).
“Boninites” are primitive andesitic magma with a chemical composition >54 wt% SiO2, <0.5 wt% TiO2 and >8 wt% MgO (Mg# >0.6). High MgO contents and presence of clinopyroxene suggest high temperatures and water contents in the mantle wedge with a highly depleted harzburgite residue. Boninites can be subdivided into Low-Ca and high Ca suites (see Crawford et al., 1989, for full definition). Boninites are the first magmas in which the influence of the slab is geochemically observed.
Crawford, A. J., Falloon, T. J., & Green, D. H. (1989). Classification, petrogenesis and tectonic setting of boninites.
“SZI-Ophiolites” (in contrast to e.g., BA or ridge-subduction ophiolites) are generally referred to as SSZ-ophiolites in the literature and are remnants of oceanic crust that formed during spreading of the overriding plate close to the trench at the early stages of subduction. These pieces of oceanic crust and mantle are then tectonically emplaced above sea level (e.g., Dilek and Furnes, 2014). Not all ophiolites are indicating SZI.
Dilek, Y., & Furnes, H. (2014). Ophiolites and their origins. Elements, 10(2), 93-100.
“Metamorphic soles” are thin (<500 m thick), fault-bounded sheets of highly strained meta-volcanic and -sedimentary rocks showing an inverse pressure- and temperature field gradient, and that structurally underlie many ophiolite complexes. Metamorphic soles are interpreted to derive from the top of a nascent oceanic slab that accreted to the base of the still hot overriding plate in the nascent stages of intra-oceanic thrusting (Casey and Dewey, 1984; Hacker, 1990; Wakabayashi and Dilek, 2000; Dewey and Casey, 2011).
Casey, J. F., & Dewey, J. F. (1984). Initiation of subduction zones along transform and accreting plate boundaries, triple-junction evolution, and forearc spreading centres—implications for ophiolitic geology and obduction. Geological Society, London, Special Publications, 13(1), 269-290.
Dewey, J. F., & Casey, J. F. (2011). The origin of obducted large-slab ophiolite complexes. In Arc-Continent Collision (pp. 431-444). Springer, Berlin, Heidelberg.
Hacker, B. R. (1990). Simulation of the metamorphic and deformational history of the metamorphic sole of the Oman ophiolite. Journal of Geophysical Research: Solid Earth, 95(B4), 4895-4907.
Wakabayashi, J., & Dilek, Y. (2000). Spatial and temporal relationships between ophiolites and their metamorphic soles: a test of models of forearc ophiolite genesis. SPECIAL PAPERS-GEOLOGICAL SOCIETY OF AMERICA, 53-64.