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Obduction: Why, How and Where See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/260794877 Obduction: Why, how and where. Clues from analog models Article in Earth and Planetary Science Letters · May 2014 DOI: 10.1016/j.epsl.2014.02.021 CITATIONS READS 24 287 6 authors, including: Xuran Zuo Francesca Funiciello The University of Hong Kong Università Degli Studi Roma Tre 7 PUBLICATIONS 24 CITATIONS 110 PUBLICATIONS 2,712 CITATIONS SEE PROFILE SEE PROFILE Claudio Faccenna Dimitri Savva Università Degli Studi Roma Tre Beicip-Franlab 290 PUBLICATIONS 8,986 CITATIONS 15 PUBLICATIONS 139 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Landscape evolution of the Acque Albule Basin and its relationships with the human presence from Prehistory to the Present time View project Crustal deformation of the Tien Shan belt (Central Asia) View project All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Xuran Zuo letting you access and read them immediately. Retrieved on: 05 October 2016 Earth and Planetary Science Letters 393 (2014) 132–145 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Obduction: Why, how and where. Clues from analog models ∗ P. Agard a,b, ,X.Zuoa, F. Funiciello c, N. Bellahsen a, C. Faccenna c,D.Savvaa a Sorbonne Universités, UPMC Univ Paris 06, UMR 7193 CNRS-UPMC, Institut des Sciences de la Terre Paris (ISTeP), F-75005 Paris, France b Institut Universitaire de France, F-75005, Paris, France c Dipartimento di Scienze Geologiche, Universita degli Studi “Roma TRE”, Rome, Italy article info abstract Article history: Obduction is an odd geodynamic process characterized by the emplacement of dense oceanic “ophiolites” Received 17 October 2013 atop light continental plates in convergent settings. We herein present analog models specifically Received in revised form 5 February 2014 designed to explore the conditions (i.e., sharp increase of plate velocities — herein coined as ‘acceleration’, Accepted 6 February 2014 slab interaction with the 660 km discontinuity, ridge subduction) under which obduction may develop as Available online xxxx a result of subduction initiation. Editor: Y. Ricard The experimental setup comprises an upper mantle modeled as a low-viscosity transparent Newtonian Keywords: glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by obduction pushing a piston with plate tectonics like velocities (1–10 cm/yr) onto a model comprising a continental subduction margin, a weakness zone with variable resistance and dip (W ), an oceanic plate (with or without a slab dynamics spreading ridge), a preexisting subduction zone (S) dipping away from the piston and an upper active ophiolites continental margin, below which the oceanic plate is being subducted at the start of the model (as for mechanical coupling the Neotethyan natural example). Several configurations were tested over thirty-five parametric models, with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them. Measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation. Models consistently demonstrate that once conditions to initiate subduction are reached, obduction may develop further depending on the effective strength of W .Results(1)constrainthe range of physical conditions required for obduction to develop/nucleate and (2) underline the key role of such perturbations for triggering obduction, particularly plate ‘acceleration’. They provide an explanation to the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ∼50–10 Myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. They also demonstrate that the emplacement of dense, oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large-scale, normal (oceanic then continental) subduction processes. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Why, how and where obduction develops is still poorly con- strained. Whether obduction nucleates on preexisting discontinu- Within the frame of plate convergence, obduction (Coleman, ities such as mid-ocean ridges, transform faults or ocean-continent 1971) is an apparent geodynamic anomaly, whereby fragments transition zones is unclear (see below). Whether obduction is of dense oceanic lithosphere — “ophiolites”, are emplaced onto driven by additional horizontal forces, tied to mantle-scale su- light, deeply buried continental margins over distances of several perplumes (Vaughan and Scarrow, 2003) or to lithospheric-scale, hundred kilometers (e.g., Oman, Turkey, Balkans, Newfoundland, abrupt changes of plate velocities (coined as plate ‘acceleration’; New Caledonia, Papua). Obduction appears as a transient (i.e., < Agard et al., 2007; Fig. 1a) is also an open question. Constrain- ∼10–15 My) yet recurring geodynamic process through time (Ab- ing which physical properties drive obduction is thus important batte et al., 1985; Nicolas, 1989; Vaughan and Scarrow, 2003), with not only to regional geodynamics but also to our understanding of recent Mio-Pliocene examples in SE Asia (Linthout et al., 1997; plate tectonics. Pubellier et al., 2004). Previous interpretations on how obduction forms (see also Michard et al., 1985; Moores et al., 2000) comprise two main types: (1) thrusting of ophiolite (Coleman, 1971)via“flaketec- * Corresponding author. tonics” (Oxburgh, 1972; Vaughan and Scarrow, 2003), or (2) con- http://dx.doi.org/10.1016/j.epsl.2014.02.021 0012-821X/© 2014 Elsevier B.V. All rights reserved. P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 133 Fig. 1. a. Simplified reconstructed geodynamic situation at the inception of intra-oceanic subduction for the Oman (Neotethyan) case study. The high temperature (HT) metamorphic sole marks the onset of a newly formed subduction at ∼95 Ma, while the preexisting one is jammed, as testified by the exceptional, transient exhumation of deeply seated blueschist facies (BS) rocks along the subduction plane. This subduction initiation will ultimately lead to the emplacement of the Neotethys ophiolite onto the Arabian continent (to the left). Subduction initiation coincides with a period of plate reorganization and increased convergence velocities (×2–3) after 115 Ma. b. Model set-up comprising a preexisting subduction zone (S)andaweakdiscontinuity(W ). To avoid edge effects the plates are inserted in the middle of the tank. See text for details. c. Top-view of the model set-up, showing the position of the measurements systematically performed on photographs taken every 30 or 60 s (as well in side-views). d. Experiments performed are here given as a function of the type of initial weakness introduced. Bold larger font size letters: models detailed in the manuscript. Others: normal font: runs for which obduction was successfully reproduced; between brackets: unsuccessful obduction. tinental subduction beneath an oceanic upper plate (Mattauer et is beyond the scope of this study, all of them are characterized by al., 1981; Boudier et al., 1988; Agard et al., 2007; Fig. 1a). In the ophiolite relative displacements >100’s of km within 10–15 My, latter interpretation, ophiolite emplacement is mostly viewed as by the presence of an extensive HT metamorphic sole at the base ‘passive’ and is essentially a consequence of subduction initiation, of the metamorphosed, several km thick ophiolite, and of HP con- one of the frequent outcomes of plate reorganization on a regional- tinental rocks beneath. Major unresolved issues for all of them scale (i.e., on the order of several hundreds of km; Dewey and Bird, comprise uncertainties as to their exact geochemical affinity and 1971; Gurnis et al., 2004; Stern, 2004). geodynamic setting (i.e., MORB, back-arc or fore-arc type; Dewey, We herein explore, through analog models, the conditions un- 1976; see below for Oman), as to their thermal structure at the der which obduction may develop as a result of subduction initia- time of emplacement, or why ophiolite thickness and the pressure tion, with the following aims: recorded by the metamorphic sole welded at its base generally (1) study if and how obduction can develop and which pa- do not match (e.g., the ophiolite “conundrum”; Hacker and Gnos, rameters are most favorable (i.e., lithospheric strength, buoyancy 1997; Moores et al., 2000). We thus focus in the following on the contrasts, etc.); best known example, namely the Oman obduction, taken here as (2) understand which perturbations (among which plate ‘accel- a generic representative case study and which has been preserved eration’, ridge subduction and/or interaction with the 660 discon- from later collision. tinuity; hereafter referred to as A, R and D, respectively) may trigger subduction initiation and obduction, and the nature and 2.1. Reference geological setting range of forces required; (3) constrain the partitioning and force balance between two Our model configuration is directly inspired from the regional interacting subduction zones, which has so far never been mod- context of obduction of the Oman ophiolite, which took place eled. on the southern side of the Neotethyan ocean (Fig. 1a). As for other Peri-Arabic obducted
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