Structure and metamorphism of a subducted seamount (Zagros suture, Southern Iran)

Abstract

Millions of seamounts on modern and past seafloor end up being subducted, and only small pieces are recovered in suture zones. How they are metamorphosed and deformed is, however, critical to understand how seamount subduction can impact subduction zone geometry, fluid circulation or seismogenic conditions, and more generally to trace physical conditions along the subduction boundary. Since geophysical studies mostly reach the shallowest subducted seamounts and miss internal structures due to low resolution, there is a high need for fossil seamount exposures. We herein report on a fully exposed, 3D example of seamount that we discovered in the Siah Kuh massif, Southern Iran. Through a series of sections across the whole massif and the combination of magmatic-metamorphic-sedimentary petrological data, we document several distinct stages associated with seamount build-up on the seafloor and with subduction. In particular, we constrain different stages of metamorphism and associated mineralogy, with precise conditions for subduction-related metamorphism around 250 °C and 0.7 GPa, in the middle of the seismogenic zone. Extensive examination of the seismogenic potential of the Siah Kuh seamount reveals that it was not a large earthquake asperity (despite the report of a rare example of cm-scale, high-pressure pseudotachylyte in this study), and that it possibly behaved as a barrier to earthquake propagation. Finally, we discuss the nature of high-pressure fluid circulation preserved in this seamount.

Structure of the Offshore Obducted Oman Margin: Emplacement of Semail Ophiolite and Role of Tectonic Inheritance

“…If its magmatic nature is confirmed, this late Cretaceous seamount must have formed on the Semail oceanic lithosphere following subduction initiation, and possibly represents incipient arc formation (MacLeod et al., 2013; Rioux et al., 2016). The discovery, north of the Hormuz strait, of a ∼87 Ma seamount with arc chemistry, now squeezed in the Zagros suture zone (Bonnet et al., 2019, 2020), supports this possibility. To the east, our correlations indicate that the Hatat and Tiwi basins lack deposits older than the late Cretaceous and the U0 unit is absent. Systematic mapping using seismic data suggests that the ophiolitic basement observed offshore Sohar extends further to the east, across fault F2 (Figure 6).…”

“…If its magmatic nature is confirmed, this late Cretaceous seamount must have formed on the Semail oceanic lithosphere following subduction initiation, and possibly represents incipient arc formation (MacLeod et al., 2013; Rioux et al., 2016). The discovery, north of the Hormuz strait, of a ∼87 Ma seamount with arc chemistry, now squeezed in the Zagros suture zone (Bonnet et al., 2019, 2020), supports this possibility.…”

“…The discovery, north of the Hormuz strait, of a ∼87 Ma seamount with arc chemistry, now squeezed in the Zagros suture zone (Bonnet et al, 2019(Bonnet et al, , 2020, supports this possibility.…”

“… Simplified structural map of the Arabian plate, modified from Breton et al., (2004). Major Mesozoic ophiolites: (1) Masirah: Gnos et al., 1997; (2) Semail: R. G. Coleman, 1981; Nicolas, 1989; (3) Siah Kuh seamount: Bonnet et al., 2019; (4) Neyriz: Ricou, 1974; Lanphere & Pamić, 1983; (5) Kermanshah: Braud, 1987; Whitechurch et al., 2013; Ao et al., 2016; (6): Troodos: Moores and Vines, 1971; Maffione et al., 2017; (7) Makarem High. Note the presence of a former Panafrican suture (Western deformation front) and of Infra‐Cambrian salt basins south of the study area.…”

Subduction of trench-fill sediments beneath an accretionary wedge: insights from sandbox analogue experiments

“…As shown in this study (experiment B), the strength of the subducting seamount is crucial to maintain the subduction channel for sediment transfer to the deep interior. Although common isolated seamounts composed of volcaniclastic rocks and hyaloclastite with various degreed of alteration can be shred off during the shallow stage of subduction, some of them can be subducted into the HP environment (Ueda, 2005;John et al, 2010;Bonnet et al, 2019aBonnet et al, , 2019b. Alternatively, a large-scale aseismic ridge and horst Research Paper composed of oceanic crust may have sufficient mechanical strength against the overriding plate and open the subduction channel for sediments.…”