Thermo-mechanical modeling of the obduction process based on the Oman Ophiolite case

Abstract

Obduction emplaces regional-scale fragments of oceanic lithosphere (ophiolites) over continental lithosphere margins of much lower density. For this reason, the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics. We present two-dimensional (2D) thermo-mechanical models of obduction and investigate possible dynamics and physical controls of this process. Model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the Oman (Semail) Ophiolite. Our model reproduces the stages of oceanic subduction initiation away from the Arabian margin, the emplacement of the Oman Ophiolite on top of it, and the domal exhumation of the metamorphosed margin through the ophiolitic nappe. A systematic study indicates that 350-400 km of bulk shortening provides the best fit for both the maximum Pressure-Temperature conditions of the metamorphosed margin (1.5-2.5 GPa / 450-600°C) and the dimension of the ophiolitic nappe (~170 km width). Our results confirm that a thermal anomaly located close to the Arabian margin (~100 km) is needed to initiate obduction. We further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement. Highlights We have set up thermo-mechanical numerical models of obduction Model allow to reproduce first order geological features of the Oman Ophiolite Crustal rheology and locus of obduction initiation are first order parameters

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

“…The entire sequence shows therefore an inverted temperature gradient, with isograds sub‐parallel to the basal peridotite foliation, grading steeply from the granulite/amphibolite facies to greenschist facies (≥1,000°C/km; Hacker & Mosenfelder, ). This gradient is two orders of magnitude higher than those inferred at the slab/mantle wedge interface in most numerical models (Duretz et al., ; Gerya, ; Maffione et al., ) and 10 times steeper than any inverted thermal gradient preserved at palaeo‐subduction interface (Peacock, ).…”

“…The new model proposed here accounts for the formation and evolution of discontinuous metamorphic soles during subduction infancy, via multiple stages of deformation and plate interface coupling, and further constrains existing rheological (Agard et al., ) and numerical models (Duretz et al., ). The evidence of successive episodes of slicing of the slab sheds light on the thickness, scale and evolution of mechanical coupling at the plate interface during early subduction dynamics.…”

“…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, ). 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., ).…”

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

“…Such a configuration is unusual because, in general, the subducted domain is the oceanic one. However, a comparison comes to mind with the Oman belt where tectonic windows exhibiting HP-LT metamorphism affecting continental crust support the non-metamorphic Semail ophiolite (see Duretz et al, 2016;Goffé et al, 1988;Rioux et al, 2016).…”

Mesozoic magmatic activity and tectonic evolution in the southern East China Sea Continental Shelf Basin: Thermo‐mechanical modelling

“…In the model domain, there is an average 10 km of the 'sticky air' layer, which is used to make a free crustal surface between the surface and the model top. The contact surface between sticky air layer and the upper crust simulates the surface topography, which includes approximate erosion and sedimentation (see Duretz et al, 2016). Region, the depth of magma generation is generally between 70 and 90 km (Zhang, Shi, & Wu, 2003).…”