Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

Abstract

Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore-arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or offridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

Sedimentary and tectonic evolution of the southern Qiangtang basin: Implications for the Lhasa‐Qiangtang collision timing

“…Furthermore, the metamorphic sole of the ophiolite yielded much older ages of 180–175 Ma [ Zhou et al , ], which would confine ophiolite displacement timing to late Early Jurassic time. This timing is several million years earlier than the obduction to continental margin if metamorphic sole is inferred to have been associated to intraoceanic subduction initiation [ van Hinsbergen et al , ]. Therefore, if the ophiolite obduction event is assumed to have resulted from the Lhasa‐Qiangtang collision [ Girardeau et al , ], then collision should have been older than the depositional age of the Dongqiao Formation (Late Jurassic to Early Cretaceous) and younger than the age of the metamorphic sole (180–175 Ma).…”

Kinematics of Late Cretaceous subduction initiation in the Neo‐Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria

“…The ophiolites investigated in this study all have a suprasubduction zone geochemical signature [e.g., Robertson , , ]. Formation of SSZ ophiolites and associated metamorphic soles is widely viewed as intrinsically related to subduction initiation [e.g., Stern and Bloomer , ; Dilek and Furnes , ; Stern et al , , Maffione et al , , van Hinsbergen et al , ]. As a consequence, the occurrence of a major Late Cretaceous subduction initiation event within the eastern Mediterranean Neo‐Tethys Ocean is now widely accepted [e.g., Robertson , , ].…”

“…Previous studies have—logically—argued that intraoceanic subduction zones are likely to initiate at preexisting active intraoceanic plate boundaries: transform faults and (detachment faults along) spreading ridges [e.g., Gurnis et al , ; Maffione et al , ; van Hinsbergen et al , ]. Our new results, however, seem to point out to a different kinematics incompatible with these widely accepted models.…”

“…These ophiolites have a SSZ geochemical signature and crustal ages that are consistently clustered around 95–90 Ma (see reviews in, e.g., Robertson [], Moix et al [], and van Hinsbergen et al []). These ophiolites are underlain by thin and disrupted metamorphic soles with Ar/Ar cooling ages that are comparable to those of the ophiolitic crust [ van Hinsbergen et al , , ].…”

The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

“…Metamorphic soles are widespread below the West Vardar Ophiolites and show K–Ar and 40 Ar– 39 Ar hornblende closure temperature ages of ~170 Ma (Dimo‐Lahitte et al, ; Roddick et al, ; Spray & Roddick, ; Spray et al, ; Sostarić et al, ; Figures and ). These isotopic closure temperature ages in metamorphic soles probably slightly postdate the actual inception of the subduction (van Hinsbergen et al, ), indicating that NE dipping subduction in the Vardar Ocean initiated in the Middle Jurassic, sometime between 175 and 170 Ma (Maffione & van Hinsbergen, ). The West Vardar Ophiolites also show a distinct west to east change of geochemical composition and internal structure.…”