Plate interface rheological switches during subduction infancy: Control on slab penetration and metamorphic sole formation

Abstract

International audienceSubduction infancy corresponds to the first few million years following subduction initiation, when slabs start their descent into the mantle. It coincides with the transient (yet systematic) transfer of material from the top of the slab to the upper plate, as witnessed by metamorphic soles welded beneath obducted ophiolites. Combining structure–lithology–pressure–temperature–time data from metamorphic soles with flow laws derived from experimental rock mechanics, this study highlights two main successive rheological switches across the subduction interface (mantle wedge vs. basalts, then mantle wedge vs. sediments; at ∼800 °C and ∼600 °C, respectively), during which interplate mechanical coupling is maximized by the existence of transiently similar rheologies across the plate contact. We propose that these rheological switches hinder slab penetration and are responsible for slicing the top of the slab and welding crustal pieces (high- then low-temperature metamorphic soles) to the base of the mantle wedge during subduction infancy. This mechanism has implications for the rheological properties of the crust and mantle (and for transient episodes of accretion/exhumation of HP-LT rocks in mature subduction systems) and highlights the role of fluids in enabling subduction to overcome the early resistance to slab penetration

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)

“…Metamorphic soles are therefore direct witnesses of deep processes over the first few million years of the existence of subduction zones, and provide key constraints for numerical modelling of young subduction zones (Duretz et al., ; Gerya, ; Gurnis, Hall, & Lavier, ; Hacker, ). Their emplacement, by accretion at the base of the ophiolitic mantle and transfer across the subduction plate interface, testifies to mechanical coupling during that period, with implications for crustal and mantle rheologies (Agard et al., ). Surprisingly, although very thin (≤500 m) compared to the thick (≤10–20 km) fragments of oceanic lithosphere (Coleman, , ; Nicolas, ), metamorphic soles are found welded and distributed below ophiolites across equally vast distances (i.e.…”

“…They are interpreted as forming during the first few million years of intraoceanic subduction (Figure a; Boudier, Ceuleneer, & Nicolas, ; Dewey, ; Dewey & Casey, ; Hacker, ; Spray, Bebien, Rex, & Roddick, ), when the crust of the subducted plate is heated by the overlying mantle, scrapped off and accreted to the upper plate (the future ophiolite). In this contribution, “subduction initiation” or “subduction infancy” (Stern & Bloomer, ) is defined as the period immediately following intraoceanic subduction nucleation, when a newly formed subducting slab starts to penetrate into a young and hot mantle (Agard et al., ; Hacker & Gnos, ; Plunder, Agard, Chopin, Soret, & Okay, ) until reaching a steady‐state (Peacock & Wang, ; Syracuse, van Keken, & Abers, ). Metamorphic soles are therefore direct witnesses of deep processes over the first few million years of the existence of subduction zones, and provide key constraints for numerical modelling of young subduction zones (Duretz et al., ; Gerya, ; Gurnis, Hall, & Lavier, ; Hacker, ).…”

“…The later geodynamic evolution leads to continental subduction and obduction s.s. (b) Simplified geological map of the Semail ophiolite, highlighting metamorphic sole outcrops (modified after Nicolas et al., ). (c) Review of radiometric ages for metamorphic sole formation along the Oman ophiolite (Hb: hornblende; Ms: white mica; Zr: zircon; see Agard et al., , for references). (d) Overview of the basal ophiolitic complex, with the mylonitic peridotite overthrust onto the HT and LT metamorphic soles which overlie unmetamorphosed continental sediment of the Hawasina complex…”

“…Large uncertainties remain, however, on the detailed structure and P–T conditions of metamorphic soles (Gnos, ; Hacker, ; Hacker & Mosenfelder, ; Searle, ), or on the amount of associated melting and genetic link with localized magmatic intrusions in the ophiolite (Boudier et al., ; Hacker & Gnos, ; Ishikawa, Fujisawa, Nagaishi, & Masuda, ; Searle & Cox, ; Soret et al., ), due to the fact that analytical and modelling methods with contrasting accuracy have been used over the last 30 years. Previous studies have shown that metamorphic soles show a temperature decrease structurally downward yet with large uncertainties, while pressure variations remain poorly constrained (see Agard et al., and references therein). This directly impedes solving some major issues such as the “metamorphic sole conundrum” (Hacker & Gnos, ; i.e.…”

“…This directly impedes solving some major issues such as the “metamorphic sole conundrum” (Hacker & Gnos, ; i.e. much greater inferred depths/lithostatic pressure estimates than the ophiolite thickness above) or the enigmatic emplacement mechanisms of metamorphic soles (Agard et al., ; Dewey & Casey, ; Plunder et al., ). The latter requires understanding how thin metamorphic soles are welded along distances >100 km (despite having formed at ~30 km depth in a short‐lived subduction zone), whether they have been significantly thinned, if and how they can have similar P–T conditions at the front or rear of the ophiolite thrust (i.e.…”