Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy

Abstract

Abstract. This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions, and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and plastered below the upper plate mantle wedge during the first million years of intra-oceanic subduction, when the subduction interface is still warm. Their formation and intense deformation (i.e. shear strain ≥ 5) attest to a systematic and transient coupling between the plates over a restricted time span of ~1 My and specific rheological conditions. Combining micro-structural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deformation, water activity, mineral change, grain size reduction and phase mixing. Results indicate, in particular, that increasing pressure-temperature conditions and slab dehydration (from amphibolite to granulite facies) lead to the crystallization of mechanically strong phases (garnet, clinopyroxene and high-grade amphibole) and rock hardening. In contrast, during early exhumation and cooling (from ~850 down to ~700 °C – 0.7 GPa), the garnet-clinopyroxene-bearing amphibolite experiences pervasive retrogression (and fluid ingression) and significant strain weakening essentially accommodated by dissolution-precipitation and grain boundary sliding processes. Observations also indicate cyclic brittle deformation near peak conditions and throughout the early exhumation, which contributed to fluid channelization within the amphibolites, and possibly strain localization accompanying detachment from the slab. These mechanical transitions, coeval with detachment and early exhumation of the HT metamorphic soles, controlled mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the peridotites above. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling between mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascapedia), in lower continental crust shear zones and oceanic detachments.

Slabitization: Mechanisms controlling subduction development and viscous coupling

“…Our group has studied these tectonic environments across a variety of locations (Oman, Turkey, New Caledonia;Canada;e.g., respectively, Agard et al, 2016;Plunder et al, 2016;Soret et al, 2016;Dubacq et al, 2019). The present study combines information from both sides of the plate interface, focusing in part on the works of M. Soret and C. Prigent in Oman-UAE (Soret et al, 2017(Soret et al, , 2019Prigent et al, 2018a,b,c). Major results and new findings are used to refine the evolution of this complex system during its first few million years of existence (~1-10 Ma?)…”

“…8b) in shear bands, pressure shadows or fractures, and by thermodynamic modelling ( Fig. 8c; Soret et al, 2017Soret et al, , 2019).…”

“…Several important features of this well-preserved geological set-up need to be underlined ( Fig. 6): (i) while the juxtaposition of HTa to the (proto)mylonitic peridotites (Prigent et al, 2018a) represents a genuine snapshot of what is happening at the exact subduction plate interface, their later deformation and early exhumation takes place somewhere along the interface and this overprint (e.g., mylonitic to ultramylonitic structures in banded peridotites; Prigent et al, 2018a; see below) must be disentangled from features formed at peak burial; early exhumation of HTa resulted in particular in the strong thinning of this unit, by a factor of 5-10 (as discussed in Soret et al, 2017Soret et al, , 2019; (ii) the peak metamorphic conditions experienced by HTb, and its accretion to the HTa-banded peridotite pair, represent another snapshot in time to study the mechanical behaviour at the plate interface; (iii) mineral transformations and deformation mechanisms witnessed by HTb may inform on the prograde evolution of HTa prior to its accretion, largely erased since.…”

“…Metamorphic sole accretion can be viewed as the transient period during which the newborn slab faces resistance of the upper plate mantle acting as a buttress (Agard et al, 2016;Soret et al, 2019;Figs. 1a,b).…”

“…Which deformation mechanisms physically govern the convergence of mantle and crust viscosities at mineral scale and over which distances they remain similar, is however unknown. This evolution in both strain localization and slab penetration is such an intricate yet generic geodynamic process, as recalled in introduction, that deformation must be studied down to the grain scale (Bercovici et al, 2012;Bercovici and Ricard, 2014;Billen and Arredondo, 2018;Prigent et al, 2018c;Soret et al 2019).…”