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Glossary

Subduction zone initiation

Definition

"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

Definition

"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

Early basalts

Explanation

Early basalts (a.k.a. “Fore-arc basalts (FAB)” or “Lower lavas”) are MORB-like volcanic rocks, which are suggested to be the products of the very first lavas erupted during SZI. They underlie Boninites and arc-like volcanic rocks. These basalts 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 precursors to Boninites/arc-like volcanic rocks. Even though this type of basalt was initially observed in the fore-arc setting of the Izu-Bonin Mariana system (hence the term “Fore-arc basalt”; Reagan et al., 2010), it is not clear where it is actually emplaced during SZI. The clearer term “Early basalts” avoids misconceptions about its emplacement and is therefore recommended.

 

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

Explanation

“Boninites” are primitive andesitic extrusive rocks with a chemical composition of >54 wt% SiO2, <0.5 wt% TiO2 and >8 wt% MgO (Mg# >0.6). High MgO contents and the 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-Si and high-Si suites (see Pearce and Reagan (2019) for full definition). Whereas low-Si Boninites are not only associated to SZI and can be found in other environment, the high-Si ones appear to be uniquely associated with SZI. If associated to SZI, Boninites are the first melt products 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

Explanation

“SZI-Ophiolites” (in contrast to other ophiolites with a different geochemical signature like back-arc or mid-ocean ridge ophiolites) are generally referred to as supra-subduction-zone or SSZ ophiolites in the literature and are remnants of oceanic crust that formed during spreading of the overriding plate in a fore-arc setting at the early stages of subduction. As ophiolites, these pieces of oceanic crust and mantle have been tectonically emplaced above sea level (e.g., Dilek and Furnes, 2014).

 

Dilek, Y., & Furnes, H. (2014). Ophiolites and their origins. Elements, 10(2), 93-100.

Metamorphic soles

Explanation

“Metamorphic soles” are thin (<500 m thick), fault-bounded sheets of highly deformed meta-volcanic and meta-sedimentary rocks showing an inverted pressure- and temperature gradient that structurally underlie many SZI-ophiolite complexes. Metamorphic soles are interpreted to be derived 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.

 
 
 
 
 
 
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