Obduction: Why, How and Where

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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

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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 constrained. 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 ophiolites from Turkey to Oman (Ricou, 2. Model set-up 1971), a stage of subduction initiation is documented at 95 Ma by the formation of extensive metamorphic soles (Thuizat et al., 1981; Several reasonably well-preserved, large-scale obducted ophio- Hacker, 1994; Wakabayashi and Dilek, 2003). This event coin- lites worldwide are available for study (e.g., Oman: Coleman 1971, cides with blueschist exhumation on a comparable scale on the 1981; Newfoundland: Dewey and Bird, 1971; Suhr and Cawood, northern side of the Neotethys (Monié and Agard, 2009; Fig. 1a) 1993;Turkey:Çelik et al., 2011; Balkans: Pamic´ et al., 2002;New and follows a major, regional-scale geodynamic reorganization at Caledonia: Cluzel et al., 2001, Lagabrielle et al. (2013) Papua: Lus ∼115 Ma marked by a rapid increase in the rate of shortening of et al., 2004). Although a review of their detailed geological settings the Neotethyan ocean (Agard et al., 2007, 2006). 134 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145

The obducted Oman ophiolite, most likely the best preserved velocities and can be viewed as corresponding to ridge push or on earth (it was selected as the classic ophiolite type by the additional horizontal forces (Bott, 1993). Such forces, in the case Penrose conference, in 1972), has been the target of numerous of Oman, were likely provided by the late Cretaceous Pacific super- structural, petrological and geochemical studies, largely devoted swell and opening of the Southern-Central Atlantic (Vaughan and to processes of oceanic lithosphere formation (e.g., Nicolas, 1989; Scarrow, 2003; Agard et al., 2007). Godard et al., 2000, 2003; Nicolas et al., 2000; Shervais, 2001; The choice of placing an initial weakness W in the model (as Python et al., 2008). imposed by Chemenda et al., 1996, 1995)isdictatedbythelackof It is generally agreed that the Oman ophiolite once formed possible localization due to the quasi-Newtonian behavior of sili- in the Neotethyan realm (which formed from the Permo-Triassic cone material over laboratory timescales (see below; as opposed onwards; Lippard et al., 1986; Chauvet et al., 2009) and was em- to Faccenna et al., 1999). This is also justified by the existence of placed onto Arabia as a result of a NE-dipping subduction zone ubiquitous, inherited weak tectonic discontinuities in nature (i.e., initiated at 95 Ma. Stable sedimentation along the north Arabian ancient suture zones, rift-related normal faults, subsiding basins, platform indeed abruptly ended at the end of the Cenomanian, transform faults, etc.; Hacker et al., 1996; Chauvet et al., 2009; 90 Ma ago (Béchennec, 1987), when the passive margin was flexed Lévy and Jaupart, 2012). Such discontinuities are in fact systemat- down, turned into a foreland basin (Glennie et al., 1973, 1974)and ically introduced for nucleating subduction initiation in numerical subducted shortly afterwards as shown by HP metamorphic con- models (e.g., Toth and Gurnis, 1998; Funiciello et al., 2003; Gurnis ditions (i.e., between ∼90–85 Ma and ∼75 Ma; Searle and Cox, et al., 2004; Yamato et al., 2007a; Lévy and Jaupart, 2012; Duretz 1999; El-Shazly et al., 2001; Warren et al., 2003, 2005; Yamato et and Gerya, 2013). However, by contrast to Chemenda et al. (1995, al., 2007b). 1996) or Boutelier et al. (2003, 2004), there is no predefined sub- However, despite numerous petrological and geochemical stud- duction in our models (at W ) where obduction may nucleate. ies, the exact nature and original setting of the Oman ophio- The absence of net extension in the lithospheric plates of our lite is still debated, with evidence for both a MORB-type sig- models means that the amount of slab-pull driven oceanic sub- nature (Boudier et al., 1988; Ceuleneer et al., 1988; Nicolas et duction never exceeds the amount of overall shortening imposed al., 2000; Godard et al., 2000, 2003; Le Mée et al., 2004) and a by the piston. The geodynamic situation modeled here thus cor- supra-subduction, back-arc geochemical imprint (Shervais, 2001; responds to a case such as in the Indian Ocean, where intrao- Searle et al., 2004; McLeod et al., 2013). ceanic compressive deformation has long been documented (Bull Irrespective of its exact geodynamic setting, age constraints and Scrutton, 1990). for the ophiolite crustal sequence (∼110–95 Ma, with mainly 96.5–95.5 Ma ages; Ceuleneer, 1986, p. 149 and references therein; 3. Methods Rioux et al., 2012, 2013), for syn-obduction cross-cutting pla- giogranite dykes derived from the metamorphic sole 3.1. Materials and similarity criteria (∼95 Ma: Tilton et al., 1981) and for the metamorphic sole it- self (∼95–94 Ma; Hacker, 1994), indicate that the Oman ophio- Lithospheric plates are treated as thin viscous sheets on top of lite corresponds to a young portion of oceanic lithosphere (i.e., glucose syrup and arranged in a rectangular Plexiglas tank (110 cm <1–10 My) formed close to the Arabian continental margin. long, 60 cm wide and 20 cm high; Fig. 1b) much larger than the Given the short time span between subduction initiation plate so as to minimize lateral friction. Following previous analog (95 Ma) and peak burial of the edge of the Arabian continental studies (Bellahsen et al., 2003 and references therein), the rheology lithosphere (∼80 Ma; Warren et al., 2003, 2005), any hetero- of the lithosphere is approximated by a visco-elastic material (sil- geneity localizing deformation and thereby subduction initiation icone putty; Table 1) behaving viscously at laboratory strain rates, would indeed have had to be close to Arabia (i.e., <200–300 km; as the experimental time scale is higher than the Maxwell relax- Searle et al., 2004). However, whether subduction initiated near ation time (∼1 s). This type of material has been shown to repro- the continent-ocean transition (COT; e.g., Ishikawa et al., 2005; duce the most essential features of subduction processes (Becker et Lévy and Jaupart, 2012) or at some other tectonic discontinu- al., 1999; Funiciello et al., 2003; Schmeling et al., 2008). The upper ity, such as a ridge (e.g., Nicolas and Le Pichon, 1980; Boudier mantle is simulated by a low-viscosity and high-density Newtonian et al., 1988) or transform fault (Hacker et al., 1996; Rioux et al., fluid (glucose syrup). 2013), remains an open question. Whether an active Neotethyan Physical parameters used to scale our laboratory models, in- ridge even existed at the time is also unknown (Ricou, 1994; cluding densities, were set through similarity criteria by Bellahsen Dercourt et al., 2000). et al. (2003; Table 1). This experimental setting is properly scaled for normal gravity field to simulate the competition between act- 2.2. Experimental set-up ing gravitational and viscous resistive forces stored within the mantle and the lithosphere adopting adimensional number such The model set-up (Fig. 1b) corresponds to the Neotethyan case as buoyancy number (B, Table 1; e.g., Weijermars and Schmeling, study during the late Cretaceous (Fig. 1a): convergence is simu- 1986; Davy and Cobbold, 1991; Becker et al., 1999). The viscosity lated by a piston pushing onto a continental passive margin (with and density ratio between the lithosphere and the mantle is set variable continent to ocean transition structure; COT), an oceanic to 1.02 and 400, respectively to properly simulate the range ob- plate and an active upper plate continental margin, under which served in nature (Mitrovica and Forte, 1997). The scaling factor is − a pre-existing subduction zone is set. An accretion ridge (R), char- 1.6×10 7: 1 cm in the model corresponds to 60 km, and the characterized by a lighter and weaker silicone putty, is inserted with acteristic time is such that 1 min equals 1 Myr. The convergence the oceanic plate in some models. In Fig. 1b, labels S (subduction rate, which is controlled by the velocity of the piston, varies ac- zone) and W (Weakness zone; see Fig. 1d) respectively denotate cordingly between 3.5 and 13.5 cm/yr. Intertia is negligible in the plate boundaries where subduction takes place and where subduc- experiment as in nature. In some experiment, the velocity of con- tion initiation and later obduction are expected to take place. vergence (i.e., piston velocity) increase sharply by a factor of 2.5 or The S boundary behaves as a proxy for existing subduction 1.75 (Table 3). zones, either adjacent (as was the case in New Caledonia or SE The bottom of the Plexiglas tank corresponds to the upper– Asia; Cluzel et al., 2001; Stern, 2004) or more distant. The push lower mantle boundary at ∼660 km, treated here as an imper- from the piston was set so as to produce adequate convergence meable barrier as direct penetration of the slab through this layer P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 135

Table 1 List of physical parameters of the model and their corresponding natural values. Piston velocities were varied between 0.58 and 2.26 cm/mn (i.e., 13 and 50 Hz), which scales back to kinematic velocities of 3.48 to 13.56 cm/yr.

Parameters Units Model Nature − g Gravitational acceleration m s 2 9.81 9.81

Thickness ho Oceanic lithosphere m 0.012 70 000 hc Continental lithosphere 0.016 100 000 z Upper mantle 0.11 660 000 Ridge 0.012 100–30 000 Thickness ratio (ho/z) 0.11 0.10 Thickness ratio (hc/z) 0.145 0.15 − Length scale factor = 1.6 × 10 7 (model/nature)

− Density kg m 3 ρo Oceanic lithosphere 1449 3300 ρc Continental lithosphere 1394 3000 ρm Upper mantle 1420 3220 ρr Ridge 1070 2850–3100 Density ratio (ρo/ρm) 1.02 1.02 Density contrasts (ρ = ρo − ρm)2980

Viscosity Pa s 5 23 μo Oceanic lithosphere 2 × 10 4 × 10 5 23 μc Continental lithosphere 1.4 × 10 10 21 μm Upper mantle 460 4 × 10 μr Ridge Viscosity ratio (μo/μm) 435 100–500 Viscosity ratio (μo/μc)1.44

Characteristic time 13 t (tnature/tmodel(μnρm gmlm/μmρn gnln))s 60(1mn)3× 10 (1 Myr)

Buoyancy number 2 B (B = ho gρ/μm v) 0.50 0.54

Convergence rate v (piston) cm/mn 0.58–2.26 3.48–13.56 (cm/yr)

is inhibited during the timescale of our experiments (a few tens al., 2000), which will depend on ridge density, size and orien- of millions years). Note that the viscosity increases at the upper– tation, lower mantle boundary is at least one order of magnitude (Davies, – interaction with the upper–lower mantle discontinuity (D), 1995; Guillou-Frottier et al., 1995; Christensen, 1996; Funiciello which has been shown to be a major barrier for some slabs et al. 2003). No thermal gradient exists across the model. The (e.g., Houseman and Gubbins, 1997), with time scales of slab thermal consequence of the aging oceanic lithosphere and phase penetration on the order of a few Myrs. transformations increasing crustal density are implicitly taken into account by the choice of a negatively buoyant oceanic plate. The initial strength of W was qualitatively controlled using var- The influence of these obvious simplifications with respect to ious types of weaknesses (Fig. 1d), both in terms of location and nature (i.e., viscous rheology, lack of temperature gradient, ab- resistance, from very weak (W 0; throughgoing fault) to very strong sence of density changes within the slab, exaggerated density (W max). The weakest types (W 0andW 1) are characterized by a contrast across the upper/lower mantle interface) are discussed COT shape allowing for an initial overlap and lubrication of the later. W interface (using Vaseline and glucose syrup, as indicated by the bold line in Fig. 1c). Models with weakness zones as small putty bridges, either lubricated or not (W 3, W 3 ), were also tested. Lu- 3.2. Parametric study and measurements brication was not used for models W 4toW max. The location of W was chosen so as to initiate obduction close to the continen- Ten preliminary tests and twenty-six parametric models were tal margin, either within the ocean or at the COT, and the density performed using variable combinations of material, strain rates and contrast across W was modified accordingly (e.g., #19). boundary conditions (see supplementary material). Eleven mod- The experimental procedure is as follows: (1) subduction is ini- els were performed with an oceanic ridge, twenty-one included tiated by forcing the plate tip inside the syrup down to a depth a sharp increase in convergence velocity (i.e., ‘acceleration’) and where gravitational forces overcome resisting forces (i.e., 2.5 cm the slab reached the bottom of the tank in twenty of them. The ∼150 km in nature) and subduction evolves to a self-sustaining parametric study was not only meant at successfully modeling ob- process (McKenzie, 1977), (2) the upper plate, lubrificated beneath duction as a result of subduction initiation but also at assessing its forearc to enhance decoupling between the two plates and pro- the respective influence of the following triggers: mote subduction, is then added and (3) the overall system is short- ened by means of the piston (i.e., velocity boundary conditions). – sharp changes in plate velocity (‘acceleration’, A), as hypothe- Experiments lasted ∼30–40 min and were monitored through top- sized by Agard et al. (2007), and side-view photographs every 30 or 60 s. Eleven characteristic – entrance of a buoyant oceanic ridge (R), as for the Californian lengths (Fig. 1c; Table 2) were measured at each time step using or Taitao ridges (Page and Engebretson, 1984; Lagabrielle et regularly-spaced parallel lines drawn atop the plates to calculate 136 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145

Table 2 Variables and units used in the manuscript. Abbreviations: n.d.: dimensionless parameter; n subscript refers to values normalized to the same amountoftotalmodel ◦ ◦ shortening; i and f to initial and ﬁnal values; w : value of w at time t . Measured values (in cm; see Fig. 1c): w, s1, s2, u, im, io, iu, lm, ir, lo, lu, t, F .Othervariables: ∗ c = u − iu; i = im + io + iu; l = (lm + lo + lu)/3; p = w + s2 + i; s = s2 − c; SU = u lu.Notethat%w + %s + %c + %i adds up to 100%.

Variable Type Unit

Dw = wi − wt amount of convergence across W cm %w = Dw/Dp fraction of overall convergence taken up by W n.d. ◦ ◦ t time at which models are stable (for calculation of w )mn ∗ t = t − tacceleration time referenced with respect to ‘acceleration’ (Fig. 5b) mn ◦ ◦ ◦ D%w = %w − %w variations in %w with respect to time t (i.e., %w ) n.d. Dwf,%wf, D%wf ﬁnal values of Dw,%w,%Dw ◦ eW n = (Dwf/Dw )n deformation ratio across W n.d. ∗ −1 w = (wt − wt+dt )/dt instantaneous deformation rate across W cm mn F fraction of effective obduction on overall convergence across W (= Sw/Dw; Fig. 2b) n.d. O = %wf.F fraction of obduction with respect to convergence across W and S n.d. ∗ Ds,%s, D%s, eSn, s subduction across S as for w ∗ Dp, eP, p along strike model shortening as for w %DPn = (pi − p)/(pi − pf)n fraction or progress of overall model shortening (in %; normalized for all runs) — see supplementary material n.d. RWq calibrated resistance of W (Fig. 1d) qualitative RW = (lot+dt − lot )/(wt − wt+dt ) resistance of W (estim. before perturbation) n.d. RS = (lut+dt − lut )/(st − st+dt ) resistance of S (estim. after perturbation) n.d. C = (lut − lut+dt )/(s2t − s2t+dt ) · 100 fraction of fore-arc deformation with respect to convergence across S n.d.

Table 3 Summary of the characteristics and main results for each model. RWq corresponds to the arbitrary resistance value derived from the weakness type (i.e., W 1toW 5 for models considered here) which is plotted in Fig. 5d. ‡ superscript denotate fractions including along strike (w, s, i) and out of plane deformation (l).

#Experiment 111213151718 18b19 Weakness type W 5 W 1 W 2 W 3 W 1 W 1 W 1 W 1 ◦ t 888885 5 7 t∗ 31 18 12 11 12 9 9 10 ‘Acceleration’ (A) ×2.5 ×2.5 ×2.5 ×2.5 ×2.5 ×1.75 ×1.75 ×1.75 Ridge (R) ++–––– – – Slab at 660 (D)1312 – 17 – 12 12 11.5

Effective obduction – +++++ + + Impact\A + – ++ (+) ++ + (+)– Impact\R (+) ++ Impact\D –(+) ++ (+) + –

eWn 33.48 59.54 67.79 52.82 65.07 81.10 75.87 47.20 eSn 76.39 69.07 57.49 62.03 60.82 57.73 60.56 77.93 eP 45.19 51.11 48.53 57.20 51.23 39.74 39.74 45.45 %wf‡ 14.02 23.03 32.70 22.36 23.98 26.23 26.76 17.27 %sf ‡ 56.03 54.34 46.04 49.19 58.03 51.21 51.57 64.40 %lf ‡ 11.53 8.14 8.04 14.76 7.45 9.14 8.78 7.05 %if ‡ 18.42 14.48 13.22 13.68 10.54 13.41 12.89 11.29 ◦ %w 9.85 17.76 8.76 10.04 14.71 19.21 14.27 24.96 D%wf 4.86 5.77 23.52 11.42 9.02 18.20 16.46 −5.82 %W = %wf/(%wf + %sf) 20.01 29.77 41.52 31.25 29.24 41.36 34.16 21.14 %S = %sf/(%wf + %sf) 79.99 70.23 58.48 68.75 70.76 58.64 65.84 78.86

F 0.00 91.00 71.00 52.00 94.00 70.00 64.00 80.00 %O 0.00 27.09 29.48 16.25 27.49 28.96 21.86 16.91

RWq 10.00 1.50 2.00 3.50 1.00 1.50 1.50 1.50 RW 0.83 0.32 0.32 0.59 0.31 0.29 0.38 0.36 RS 0.47 0.33 0.69 0.87 0.21 1.40 0.75 0.15

finite and cumulative deformation (with Image-J software). These videdthroughrun#18,whichwasmeasuredacrosstwodifferent measurements allow defining a number of (mainly dimensionless) transects. variables such as the respective contributions through time of convergence across W or S (see Table 2). Side-view measurements 4. Results (though slightly blurred by the Plexiglass wall and less precise than top-views) are used to evaluate the amount of effective new sub- One reference model (#13; Fig. 2) and six other representative duction (or “obduction”). Obduction efficiency at W is defined as ones (#11, 12, 15, 17, 18, 19) are presented below. Sketches of the amount of effective underthrusting (or subduction) over the to- their evolution and systematic measurements are shown in Figs. 3 tal amount of convergence (F ; Table 2;seeFig. 2b), and is used to and 4, respectively. estimate the integrated contribution of obduction to overall convergence (%O ; Table 2). 4.1. Reference model (#13) Although models evidence mainly in-plane deformation and non-cylindrical set-ups have been avoided (except in #25, with the This model (weakness type: W 2; Fig. 1d) is devoid of intra- presence of a transform fault), minor lateral contrasts exist in most oceanic ridge and the piston was accelerated after 12 My (i.e., models. An estimate of the extent of such variations is herein pro- 12 min), before the slab could touch the 660 km discontinuity. P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 137

Fig. 2. Detailed evolution of model #13. (a) Top views; (b) side-views (see also Fig. 3). Obduction eﬃciency (F ; Table 2) is deﬁned as the amount of effective underthrusting at W ,i.e.F = Sw/DW; (c) evolution of characteristic lengths and surfaces (see Table 2); (d) comparison of internal and lateral deformation in the three parts of the model set-up (M, O , U : Fig. 1c); (e) comparison of deformation velocities across S and W . Inset: evolution of the fraction of convergence taken up by S as a function of the amount of convergence on W .

Four sequential top (Fig. 2a) and lateral views (Fig. 2b) are shown conspicuous 4–5 min after the ‘acceleration’ (Fig. 2c). From this after 10–15–17–21 min, together with the evolution of some key moment onward, obduction becomes effective (see side-view at variables (Figs. 2c–e; Table 2). t = 17 min; Fig. 2b) and the fraction of convergence across S de- Fig. 2a shows that deformation is mainly in-plane: there is creases (Fig. 2c). Subduction gets progressively jammed, as testified very little rotation of the model and insignificant lateral escape of by the increase in fore-arc deformation (Dc) and the decrease of the oceanic plate, as for most models. Subduction initiation fol- the upper plate surface (SU). lowed by obduction is conspicuous from Fig. 2b. Measurements The progressive evolution of instantaneous velocities best de- allow for a good precision, as testified by the close agreement be- picts the mechanical behavior of the system and how deformation tween the overall shortening rate deduced from measurements of is partitioned between W and S (Fig. 2e): shortly after the ‘accel- the total length of the model and the along strike shortening of eration’ both W and S increase their shortening rates, but after the model deduced from marker measurements (Fig. 2c: %Sh,%p; 5 min the upsurge in velocity ceases for S, whereas the contrary see Table 1 for the extensive description of all the adopted pa- is observed for W . The proportion of the overall shortening taken rameters). Both also fit the calibrated speed of the piston within up by S-subduction, shown as a function of the amount of con- error. Similarly, the amount of subduction is very consistent for vergence across W (Dw:inset,Fig. 2e), points to competing plate both types of measurements, namely Ds1 and Ds2(Fig. 2c). Fig. 2c boundaries and W progressively catching up with S. Finally, about also indicates that forearc deformation (Dc) is only small during 9 min after the ‘acceleration’ (i.e., after 21 min), the contribution most of the experiment (as also shown by Ds being close to Ds2). of W to model convergence further increases (Figs. 2c, e). Inter- Fig. 2c gives the cumulative contribution of deformation across W nal deformation of the margin (Dim) no longer increases (Fig. 2d), to along strike shortening of the model at a given time (%w). Inter- which indicates that all the deformation between the piston and nal deformation, both in terms of width (Dlm, Dlo, Dlu) and length W is absorbed by obduction. changes (Dim, Dio, Diu) evolves smoothly and similarly (Fig. 2d; except Dim after ‘acceleration’: see below). 4.2. Results from the parametric study The model evolves steadily after 2–3 min, once the resorp- tion of all the small spaces left by the initial juxtaposition of the Six representative models (#11, 12, 15, 17, 18b, 19) are shown plates is over. Convergence across W (%w) initially amounts to in Figs. 3 and 4 and compared to the reference one (#13; Figs. 2c, only ∼10% of the overall convergence. A minor increase across W 4). For the sake of comparison, linedrawings of side-views at takes place at the time of acceleration, yet a sharper increase is the start, during ‘acceleration’, ridge subduction, encounter of the 138 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145

Fig. 3. Sketches of slab shapes in side-view at critical time intervals: at the start of the experiment, during ‘acceleration’, ridge subduction, slab at 660 km discontinuity (i.e., potential triggers A, R or D, respectively) and at the end of the experiment. Amounts of shortening on W (eWn) and effective obduction (%O )areindicated(seealso Table 2).Inset:comparisonforslabshapesatthetimeof‘acceleration’.Theblackdotmarksthetransitionbetweenthetop,bendedpartoftheslabandtheportion beneath. upper-mantle discontinuity (i.e., potential triggers A, R or D,re- all these cases, once subduction initiates it is followed by obduc- spectively) and at the end of the experiment are provided in tion; Fig. 3. The main characteristics of each model, including the over- (b) those for which there is no obduction (e.g., #11). The overall all amounts of model shortening, shortening across W and S, and shortening (eWn = 33.5%; Fig. 3) and slight increase in the fraction their respective contributions are given in Table 3. Fig. 4 provides of convergence across W in this run (D%wf = 4.86%; Table 3)is a continuous record of surface deformation, where the timing of thus not accommodated through obduction. The same holds true A–R–D events is indicated together with other remarkable obser- for all models with W 3 (except #15). vations (small arrows). The comparison of slab shapes at the time of ‘acceleration’ (inset, Fig. 3) evidences distinct bending curvatures. A black dot high- lights the inflexion point, which marks the transition between the 4.2.1. Evolution of slab shapes top, bended part of the slab and the portion beneath. The contrast- Comparison of the final shapes of subducted lithosphere shows ing lengths of the slabs are due to the different durations of these that all but #13 and #17 rest on the 660 km discontinuity at the experiments prior to ‘acceleration’ (e.g., much longer for #11). end of the experiment. Amounts of shortening on W (eWn)differ from 33.5 to 75.9%. The amount of effective obduction, which de- 4.2.2. Evolution of deformation and shortening pends on the amount of overall W -convergence weighted by the Along strike shortening (Fig. 4) is shown as the contributions = fraction of effective underthrusting (%O %wf.F ; Table 2), varies from S (%s), W (%w), internal deformation (%i) and forearc defor- between 16 and 29% (Fig. 3; Table 3; F varies from 0 to 94%). Two mation (%c), the latter reflecting changes in mechanical coupling main types of behavior are readily observed in the models, which across the subduction interface. After the first few minutes, dur- are typified by the examples selected here: ing which plates get in contact, oscillations are only minor (#13, (a) those for which obduction effectively develops (#15, 17, 18, 17) or unnoticeable (#12, 15). Initial partitioning is such that the ◦ 18b, 19, as for #13), with a new slab clearly visible in side-views initial contribution of W to shortening (%w ) ranges between ∼9 (Fig. 3). Obduction is relatively minor for models #15 and #19. In and 25%. Initial amounts >10% are only observed for types W 1 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 139

Fig. 4. Detailed evolution of the contributions from S (%s), W (%w), internal deformation (%i) and forearc deformation for some selected models. Arrows indicate the timing of ‘acceleration’ (A; long black arrows), ridge subduction (R; thin arrows), interaction with the 660 discontinuity (D; thick grey arrows) or some other event (smallest arrows) discussed in the text.

or W 1 , but are otherwise only loosely related to weakness types. for #13 (Fig. 5a). By contrast, D%wf 5–6% for runs in which The eight evolutions display variable final fractions of convergence W -convergence does not develop well (i.e., with low eWn values; ∼ ‡ across W , in the range 14–33% (%wf , Table 3). #11, 19; Table 3) or only transiently (#12). The greatest differ- Convergence across W and S are clearly anticorrelated for all ences are only observed for models with rather weak types of models but #19, for which a sharp increase in internal deformation W (W 1 , W 2: 13, 18, 18b), yet the difference for #17 (W 1type) (%i) is observed. Increases in forearc deformation (smallest arrows is less than for #15 (W 3 type). Fig. 5b also emphasizes the rate in Fig. 4) are observed during the ‘acceleration’ of the piston (all at which W -convergence may or not develop (see supplementary but #12), the subduction of the ridge (#11; but not #12), the inter- material for a similar plot as a function of normalized shortening). action with the 660 discontinuity (#15, 18b) or may result from a No systematic relationship is observed between this rate and the slightly later adjustment (e.g., #18b). Increases in %w (Fig. 4) coin- ‘acceleration’ rate (×1.75 or 2.5; Table 3). cide with ‘acceleration’ (models #13, 17, 18), and to a minor extent Models #18 and #18b (Fig. 5a) illustrate some of the lateral, (#15, 18b), ridge subduction (#11, 12) or the 660 km discontinuity along strike changes observed in cases. Initially, the contribution (#15, 18b). of W -convergence is slightly larger for #18 than for #18b and The influence of ‘acceleration’ on all models but #12 and #19 is confirmed by the evolution of the respective contribution of develops more rapidly after ‘acceleration’, probably because of a W -convergence through time (D%w; Table 2) as a function of the somewhat stronger coupling across the S subduction. Subduction ∗ timing of ‘acceleration’ (i.e., t ; Fig. 5a; see also inset Fig. 3 for then gets choked laterally in #18b, as revealed by the sharp in- a comparison of slab shapes at that time). The value of D%wf is crease in %c and coeval decrease in SU after 15–16 min (Fig. 4e). on the order of 10–15% for all runs where W -convergence is at The increase in W -convergence is sharper from then onwards and least partially triggered by ‘acceleration’ and significantly higher final %wf values are almost similar for #18b and #18. 140 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145

∗ ◦ Fig. 5. (a) Net difference of the fraction of convergence taken up by W at t (time referenced with respect to ‘acceleration’; Table 2) with respect to time that at t (once model evolution is stabilized). (b) Evolution of the amount of convergence across S (Ds)andW (Dw) as a function of along-strike deformation of their respective hanging wall (lu, lo; Fig. 1c). These evolutions are used to define the resistance across S (RS)andacrossW (W ). See small cartoons above figure and text for details. (c) Comparison of the overall amount of obduction (%O ) with the respective contributions from S, W , internal and lateral deformation. (d) Relationship between efficiency (F ; Table 2)and the estimated resistance across W (RW; Fig. 5b).

TheresponseofS and W to overall shortening can be mon- and #19 (Fig. 5b; and only moderate for #17). Lesser changes are itored (at least in first approximation) through the evolution of observed in the evolution of Dw (except for #15). along-strike footwall deformation at S and W (Ds and Dw; Fig. 5b) versus across-strike hanging-wall deformation (lu and lo, 5. Discussion which are very precisely measured and very sensitive to deformation increments; e.g., Fig. 2d). Roughly linear correlations are 5.1. Obduction: how and how much evidenced for both. Ds shows consistently greater values than Dw, indicating that the subduction process across S remains effective Obduction ensuing from subduction initiation is successfully re- throughout the whole experiment. The evolution of Ds/lu is char- produced in a number of models (15 out of 26) over timescales of acterized by a marked slope decrease (i.e., an increase in fore-arc a few Myrs, as continental lithosphere gets underthrust beneath deformation with respect to subduction) for all models but #11 oceanic lithosphere (Fig. 3). In order to better understand what P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 141 controls subduction initiation and, in turn, obduction, the amount 5.2. Deformation partitioning and force balance between subduction of obduction (%O = %wf.F ) is compared, for all runs, to the net and obduction contributions from W - and S-convergence and from internal and lateral deformation (Fig. 5c; Table 3). Fig. 5c shows that lateral Fig. 6a shows how the ratio of deformation rates (W over S; ∗ ∗ deformation is negligible in first approximation (%lf < 10% for all w /s ) evolves as a function of the fraction of shortening taken up models except #15 and #11), suggesting that along-strike defor- by fore-arc deformation (c; Table 2). Deformation rates across W mation dominates and that plate width was scaled correctly with reach 1.5 to 2 times that of S for models #12, 15, 17, 18b and are respect to the piston push. Internal deformation (%i )isalsosignif- roughly an order of magnitude larger for #13 and #18. By contrast f ∗ ∗ icantly smaller than deformation across W and S,exceptfor#11. w /s ratios remain low for #11 and #19. Obduction starts devel- This predominance of S and W explains why their respective con- oping for values of c < 5%. Once deformation starts localizing at W tributions are broadly anticorrelated (Fig. 5c; except for #17). there is very little increase in the amount of fore-arc deformation Differences in the time evolution of the various models are in- for models #12, #13 and #18b. terpreted as follows (Figs.3,4,5and Table 3): These results suggest that once the resistance across S reaches (i) Models #13 and #18 are the most efficient ones: both were a certain threshold (i.e., when the subduction cannot fully adjust designed with a fairly low initial W resistance (W 2 and W 1 , to the change in boundary conditions), deformation starts being respectively) and concentrate much deformation on W (net short- transferred to W and subduction/obduction initiate after deforma- ening: eWn ∼ 70–80%; Table 3) while showing the smallest short- tion localized there. Convergence then partitions between the two ening across S of all models (eSn ∼ 57%). plate boundaries, thereby controlling the amount of obduction. Estimating the new resistance across S (RS) after the pertur- (ii) Despite a significantly lower %wf value, model #17 is nearly as efficient as #13 and #18, probably due to its weaker initial bation is not straightforward since the resistance across S and W resistance (W 1). Time fluctuations in the obduction process are cannot be easily evaluated independently after loading. A reason- nevertheless noticeable (Fig. 4d). Similar obduction amounts are able proxy for the resistance across S, however, is the dimension- = obtained for #12, for which time fluctuations are strongly corre- less ratio RS lu/Ds (Fig. 5b), that is the ratio of upper plate lated with ridge subduction. deformation over effective amount of subduction. As for RW,the (iii) Obduction amounts are significantly lower for #15 and #19 greater the value of RS the more deformation is transferred to the adjacent overthrusting plate. Fig. 6b shows that models can (%O ∼ 16–17%), yet similar despite different weakness types (W 3 be classified satisfactorily as a function of the resistance between and W 1 , respectively). This low amount is due, for #15, to the re- S and W . Shortening across W (eWn) increases relatively regu- sistance across W and the importance of lateral deformation. For larly with decreasing values of RW and increasing values of RS,as #19, this due to a particularly efficient subduction (eSn ∼ 78%) and expected intuitively. Obduction amounts (%O ) anticorrelate mainly to the existence of a lighter, continental-type COT (see supplemen- with RW. tary material), which partly increased the resistance of the W 1 The force balance at S, prior to subduction initiation at W , weakness with respect to other similar runs. As a result model #19 is controlled by (Conrad and Hager, 1999; Funiciello et al., 2003; preferentially accommodates deformation through the buckling of Gurnis et al., 2004): the oceanic lithosphere rather than through W -convergence. – driving forces: (1) the force applied by the piston (i.e., ridge (iv) Finally, no subduction initiation (nor obduction) takes place push)and(2)slabpullforces. in #11: internal deformation is larger than for other models – resisting forces: (1) the resistance to the bending of the plate (Fig. 5c), shortening across W is the smallest of all while short- (≈ v.h3.μ/r3, where v, h, μ and r respectively denotate veloc- ening across S is large and similar to #19 (eWn = 33% and eSn = ity, plate thickness, viscosity and radius of curvature; Conrad and 76%; Table 3). When subduction gets jammed (as shown by the Hager, 1999) and (2) viscous resistance forces, which comprise vis- increase in %c; Fig. 4a), internal deformation takes most of the cous coupling along the subduction plane down to lithospheric deformation and W -convergence does not take place through un- depths, and slab repulsion and slab anchor forces beyond (e.g., derthrusting but through folding and horizontal shortening. Scholz and Campos, 1995; Heuret and Lallemand, 2005). Note that These results suggest that obduction efficiency closely reflects viscous mechanical coupling across S is minimum during the early the initial weakness type, though only qualitatively calibrated stages of model evolution since subduction initially proceeds well. (Fig. 1c): the resistance of W is consistently greater for #15, which Any increase in driving forces will be balanced by an increase had a fairly stiff initial interface (W 3 ), and even greater for the in resisting forces, yet with a different partitioning between bend- stiffest of all models (#11; W 5). This is readily apparent when ing and viscous resistance in each model. Increasing bending forces using the initial lo/Dw ratio (i.e., before the onset of obduction; will induce a decrease in the radius of slab curvature (i.e., a smaller Fig. 5b) as an independent, quantitative estimate of the resistance value of r), slab steepening, advancing trench behavior (at least across W (RW). The larger this ratio, the more deformation is in- during the first stages), hence more frontal convergence and cou- deed expected be transferred to the adjacent overthrusting plate. pling at S. Fig. 5d shows that the efficiency and RW (and initial weakness Comparison of slab shapes at the time of ‘acceleration’ (inset, type) are indeed largely anticorrelated. Those for which obduction Fig. 3) with the rate of obduction development (Fig. 5a) shows that effectively develops (all but #11) plot in the area where RW is the runs most sensitive to ‘acceleration’ (#13 and #17) correspond small, with a threshold around 0.6 (for #15 and all other runs with to those with the shallowest inflexion points (i.e., largest r;inset, W 3 ) allowing to map out models which failed to reproduce sub- Fig. 3). This is consistent with the fact that, despite the increase duction initiation and, in turn, obduction. in bending, their dip is still less than in other models: as a conse- These experiments thus qualitatively but consistently explore a quence, the lithospheric horizontal shear component of the push variety of situations and of mechanical coupling across W .Their is maximum and most efficiently transferred to the upper plate, reliability is ensured by the similar behaviors observed for all inducing more mechanical coupling and less efficient subduction of them: negligible rotation, across-strike deformation or model at S. thickening, comparable amounts of deformation and shortening Later model evolution depends on the resistance of W (as em- across W and S, and sensitivity to weakness type (see below). phasized by Fig. 6b): if W is weak enough then a new subduction Their relevance (in particular as to the set-up, choice of materi- initiates and a significant part of the piston push is absorbed there. als, . . . ) is addressed in a later Section 5.3. One can anticipate that, as slab pull forces and slab dip increase, 142 P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145

Fig. 6. (a) Evolution of the ratio of deformation velocities (W over S) as a function of fore-arc deformation. (b) Values of the amount of shortening and obduction for all models (Fig. 3; Table 3) as a function of the resistance across W (RW)and(RS), as estimated in Fig. 5b. (c) Sketch summarizing the feasibility of subduction initiation and obduction with respect to the resistance at S and W . (d) Idealized 3D conﬁguration illustrating the balance of forces and deformation partitioning between S and W . See text (5.2 through 5.4)fordetails.

− net viscous coupling will decrease and S will recover partly. Ob- and ridge push forces (i.e., 1013 Nm 2; Bott, 1993). Deformation − − duction will in any case depend on the partitioning of forces at rates are 10 11 s 1 since time scales of subduction initiation both W and S and may be quite complex (and will also depend are ∼105 years (in any case <1 My, as shown by metamorphic on toroidal and poloidal flows; e.g., Piromallo et al., 2006). sole clusters). Natural, integrated viscosities at W (which can be viewed as a proxy to the resistance of W ) are therefore ∼1023 Pa s, 5.3. Scaling and respective influence of key parameters triggering which is the order of magnitude of dry to only slightly hydrated obduction mantle viscosities and the magnitude chosen for our experimental lithosphere (Table 1). Although weaknesses in these analog mod- Analog models demonstrate that obduction can be triggered by els cannot be assessed more precisely than in nature, they are a modification of the force balance at a preexisting subduction probably reasonable since subduction initiate for some but not all zone, as captured in Fig. 6c. Prior to extrapolating these results configurations. to nature, the following key questions need to be addressed: (1) Is Despite inherent limitations, our models provide a means to the weakness at W realistic? (2) Can we infer which of the three compare the respective influence of ‘acceleration’, ridge subduc- external perturbations (A, R, D) has the greatest influence? (3) Is tion and encounter of the 660 km discontinuity on the triggering the coupling threshold at S applicable to nature? of obduction. Obduction is either triggered by ‘acceleration’ (#13, Deformation in nature must in any case start localizing at some 17, 18; see also #4, 8, 21), by ‘acceleration’ with a later contribu- discontinuity such as W in order to nucleate (or initiate) sub- tion from the 660 km discontinuity (#15), by the 660 km rather duction. The type of this weakness in nature (e.g., existing fault than ‘acceleration’ (#18b) or by ridge subduction (#12). Given the ∗ ∗ weaknesses integrated over the COT, presence of a sedimentary similar trends observed for w /s (ridge only: #12; ‘acceleration’ basin; Shemenda, 1992) and its strength are completely unknown. only: #13; Fig. 6a), ridge subduction and ‘acceleration’ are the most Qualitative brackets can be put forward: it should not be too weak influential triggers and probably comparable. The impact of the since S continues and not too strong since subduction initiation 660 km discontinuity is minor (Fig. 6a; though essential in #15 (and obduction) happens repeatedly through time (see also Hall and 18b), particularly when considering that the lower boundary et al., 2003; Gurnis et al., 2004). One can thus confidently postu- of the model is a (exaggeratedly) stiff barrier compared to the nat- late that they must be of the same order of magnitude as slab pull ural prototype. P. Agard et al. / Earth and Planetary Science Letters 393 (2014) 132–145 143

More models are required to better assess the respective im- Warren et al., 2008; Bialas et al., 2011; Duretz and Gerya, 2013). pact of A–R–D in nature, in particular to overcome difficulties in Several authors partly circumvented the former problem by fo- scaling the physical properties of the ridge and choosing adequate cusing on the ability of their models to return some continental rheologies. Although probably exaggeratedly weak and buoyant in material, by introducing a sharp decollement layer between the models #1–11, ridge properties are also difficult to assess in na- crust and mantle (Chemenda et al., 1995, 1996; Bialas et al., 2011; ture, due to the influence of ridge push, accretion type and thermal which is impossible with silicone only) or extensional boundary structure on both parameters. Convergence velocities are slightly forces (Duretz and Gerya, 2013). exaggerated with respect to our natural reference case study, but still of the same magnitude (i.e., 5–10 cm/yr versus ∼2–3 cm/yr 6. Implications and conclusions for the Neotethys). On the other hand, this sharp increase in convergence velocities (‘acceleration’) would be more influential with Despite inherent rheological simplifications, our analog models rheologies promoting localization (i.e., unlike the quasi-Newtonian provide useful insights on obduction processes and regional-scale silicone used here). The influence of these perturbations also de- geodynamics: pends on the extent of slab pull at the time: for comparable slab (1) Obduction is herein reproduced for the first time at conver- shapes, slab pull is stronger for #18b than #13 at the time of gence rates (3–13 cm/yr), ‘acceleration’ rates (×2.5) and timescales ‘acceleration’ (which is consistently less influential for the former of ∼10 Myr, which are all comparable to nature. model). Note that slab pull forces at S are both underestimated, (2) Models show that obduction does not require extraordinary considering the lack of phase transformations and densification in boundary conditions and provide clues to the following questions: the model, and overestimated due to the lack of thermal gradient – Why and when? Obduction is a consequence of modifying the (promoting slab weakening at depth; e.g., Houseman and Gubbins, force balance at preexisting (adjacent) subduction zones. Obduc- 1997). tion proceeds when adjacent subduction zones cannot fully adjust to changing boundary conditions and get mechanically locked. This 5.4. Implications for regional-scale geodynamics coupling threshold is reached in models when forearc deformation exceeds ∼5% of the subduction rate. Keeping in mind the above limitations and necessary approx- – How and where? Obduction may simply proceed as a result imations, our models demonstrate that subduction may initiate of subduction initiation. The extent of obduction will depend on without pre-defining a subduction plane (by contrast to Chemenda (1) the subduction force balance between W and S and (2) the et al., 1996) and may be a viable mechanism for obduction devel- resistance in the weak zones (i.e., obduction may develop if W opment at realistic timescales (i.e., ∼10 My). We also note that no is sufficiently weak). Further scaling is critical to evaluate natural slab pull is needed to initiate the obduction-driven subduction of coupling thresholds across S and W . the light continental lithosphere at W , although negative buoyancy (3) These models support the feasibility of the scenario envi- (which is lacking in #19) can nevertheless be helpful. sioned by Agard et al. (2007) for the Neotethys, in which regional- Our models support the scenario envisioned by Agard et al. scale geodynamic reorganization in response to plate ‘accelera- (2007) for the Neotethys, in which the blocking of the subduction’ increased mechanical coupling in the subduction zone to the tion zone beneath Eurasia, attested by widespread blueschist ex- north of the Neotethys and triggered the onset of obduction in the humation coeval with plate ‘acceleration’ and regional-scale geo- south. Although further analog and numerical modeling models dynamic reorganization (Monié and Agard, 2009), led to defor- are needed, including detailed 3D analysis with tracking of surface mation partitioning across the Neotethys and to the triggering of evolution, the obduction models presented here represent a useful obduction. Such long-term changes of mechanical coupling may be scenario for further field studies in well-constrained geodynamic compared to the short-term changes in coupling observed during settings (i.e., large-scale obducted ophiolites from Newfoundland, megaearthquakes in present-day subduction zones (e.g., Moreno et Timor, New Caledonia, Balkans, western Turkey). al., 2010). The models may also apply to natural examples of the (4) We finally underline that the present models represent the Zenisu ridge (Mazzotti et al., 2002) and to the intraplate deforma- first attempt to model the dynamic interaction between two sub- tion evidenced by the large-scale folds in the Indian Ocean (Bull duction zones. and Scrutton, 1990; Chamot-Rooke et al., 1993). Fig. 6d provides a 3D sketch emphasizing how subduction ini- Acknowledgements tiation (then obduction) may develop once the S subduction gets jammed after an increase in mechanical coupling of a few per- This work was essentially funded through the ONLAP project cent. The effective development of obduction is probably complex (ANR blanche, SIMI6; 2010 BLAN 615 01). Additional support was in 3D. Since subduction initiation and obduction can develop over provided by IUF, INSU and the ISTeP laboratory. thousands of km along strike (as shown by the examples of the Neotethys, during both the Late Jurassic and Late Cretaceous, or Appendix A. Supplementary material Papua), one may wonder if the blocking threshold at S needs to be integrated over the whole length of the incipient subduction Supplementary material related to this article can be found on- zone, or if a local instability may be sufficient for the lateral prop- line at http://dx.doi.org/10.1016/j.epsl.2014.02.021. agation of subduction initiation. The applicability of our models is restricted to the onset and References early evolution of obduction since the continent, forced by the piston push, goes in without ultimately blocking the obduction Abbatte, E., Bortolotti, V., Passerini, P., Principi, G., 1985. The rhythm of Phanerozoic process, contrary to what happens in nature (e.g., Oman; Searle ophiolites. Ofioliti 10, 109–138. et al., 2004; Agard et al., 2010). Plate tectonic forces, in addi- Agard, P., Monie, P., Gerber, W., Omrani, J., Molinaro, M., Meyer, B., Labrousse, tion, are balanced in nature on the whole system of plate bound- L., Vrielynck, B., Jolivet, L., Yamato, P., 2006. Transient, synobduction exhuma- aries and not solely on two boundaries (S and W )asmodeled tion of Zagros blueschists inferred from P–T, deformation, time and kinematic constraints: Implications for Neotethyan wedge dynamics. J. Geophys. Res. 11, here. These shortcomings are common, however, to analog or nu- B11401. http://dx.doi.org/10.1029/2005JB004103. merical models developed so far for continental subduction (e.g., Agard, P., Jolivet, L., Vrielynck, B., Burov, E., Monie, P., 2007. Plate acceleration: The Chemenda et al., 1995; Boutelier et al., 2004; Yamato et al., 2008; obduction trigger?. Earth Planet. Sci. Lett. 258, 428–441. 144 P. 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