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TOMO 1 - Geodinámica Andina EFFECTS OF RIDGE SUBDUCTION ON UPPER PLATE DEFORMATIONS: THE JUAN FERNANDEZ RIDGE CASE, CHILE AND ARGENTINA. INSIGHTS FROM 3-D ANALOGUE MODELLING Nicolas Espurt, Joseph Martinod, Vincent Regard, Laboratoire des Mécanismes et Transferts en Géologie, Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France ; [email protected], [email protected], [email protected]. Francesca Funiciello, Claudio Faccenna, Dip. Scienze Geologiche, Università degli Studi Roma TRE, Roma, Italy ; [email protected], [email protected]. INTRODUCTION The subduction of an oceanic ridge can have important consequences on upper plate deformations (Cloos, 1992; Scholtz and Small, 1997, Dominguez et al., 1998; Hampel, 2004). For example, the effects of the subduction of the Juan Fernandez ridge are correlated far from the trench, in the back-arc basin (e.g., Ramos et al., 2002, Giambiagi and Ramos, 2002, Charrier et al., 2002). TTTTo understand the effects of ridge subduction on the overriding plate tectonics, we built analogue experiments closed to that used in Funiciello et al. (2003) and Martinod et al. (2005), to simulate the subduction of an oceanic lithosphere including an oceanic ridge, perpendicular to the trench, below a continental plate. Lithospheric plates are modelled using silicone putty initially floating above a low viscosity glucose syrup that represents the upper mantle asthenosphere. Plate convergence is imposed by a piston advancing at constant velocity. We observe the evolution of slab geometry and the upper plate deformations during experiments above the ridge and the ocean. ANALOGUE EXPERIMENT RESULTS We observe that the advance of the overriding plate forces the subduction of the oceanic ridge (Fig. 1A). At the beginning, the buoyancy of the ridge is compensated by the negative buoyancy of the dense oceanic lithosphere. After few minutes, the buoyancy of the ridge perturbs the process of the subduction, the dip of the slab decreases on the center of the plate and a horizontal subduction occurs below the upper plate only after a large amount of ridge has been subducted (Espurt et al., in review). " 197 XI CONGRESO GEOLOGICO CHILENO Figure 1 : (A): Top views of the experiment at different time. (B): Strain-rate in the overriding. The principal axes of strain rate tensors are presented. After 10 mn, three zones with different strain-rates appear in the upper plate: close to the trench, the shortening is small, then, a zone where the shortening rate is maximum (dark grey area) and, close to the piston, the shortening is moderate. We note the curvature of the different zones correlated with the flat-slab segment. (C): Topographic profiles along the center line of the overriding plate (grey profile) in comparison with along the oceanic subduction part (dark profile) at different times. (D): Overriding plate shortening and dip of the slab, in the center line (dashed black line) and on the edges of the upper plate (grey line), vs. time. Continental shortening parallel to the trench begins few minutes after the beginning of the experiment. During the subduction of the dense ocean, the shortening of the overriding upper plate is homogenous in the whole of the plate (Fig. 1B). The subduction of the oceanic ridge involves an increase of the shortening rate above the ride during only some minutes (Fig. 1D). After, we note that the shortening rate is the same above the flat segment and above the steep slab. The shortening is constant till the end of the experiment, despite the dip of the upper part of the slab decreases from 30° to zero (Fig. 1D). However, this constant shortening is divided in three zones bearing different patterns of deformation: (i) Extension parallel to the plate convergence direction appears close to the 198 " TOMO 1 - Geodinámica Andina trench, in an area that progressively spreads out within the upper plate; (ii) A second domain appears behind the stretched area, in which the shortening rate is strong. This domain progressively moves away from the trench; (iii) Far from the trench, the strain rate remains moderate. We show that these three zones are curved towards the back-arc basin and mould the flat subducted part of the ridge (Fig. 1B). Sections perpendicular of the trench obtained by laser data (Fig. 1C) show the evolution of the continental relief during the experiment above the oceanic part and above the ridge. We note that the uplift of the chain starts rapidly at the beginning of ridge subduction in comparison with the oceanic part. In fact, the process of horizontalisation of the slab involves vertical motions from the trench with the growth of the chain towards the back-arc basin and the migration of this basin. DISCUSSION OF THE JUAN FERNANDEZ RIDGE CASE The Juan Fernandez ridge corresponds to a small, narrow (less than 100 km) and discontinuous topographical anomaly in the present-day Pacific Ocean bathymetry (Fig. 2). The precise reconstruction of the ridge geometry below South America shows it coincides with the flatter and longer segment of the Central Chile flat-slab (Yañez et al., 2001). The present-day horizontal subduction is clearly illustrated by seismicity at 31°S (Pardo et al., 2002). It provides evidence for the friction now existing between plates along the flat-slab segment. Along the profile A of the figure 2, the uplift of the Sierras Pampeanas, located 750 km east of the trench, can be correlated with the eastern part of the presentday flat-slab segment. More to the South where the subduction is steep, the profile B shows that the thrust front is located at 550 km east of the trench. Giambiagi and Ramos (2002) show that it exists a good correlation between variation in dip of the slab and amount of shortening in the foreland. At the scale of the chain, experiment suggests that the shortening linked to the subduction of an oceanic ridge does not involve a real increase of the global shortening. The buoyancy of the Juan Fernandez segment controls the geometry of the Nazca slab at depth generating horizontal subduction beneath South America. This process only plays a part on the migration of the shortening far from the trench, above the eastern boundary of the horizontal slabs, and generates vertical motions in the back-arc basin with geometries associated to inversion tectonics and thin-skinned tectonics (Ramos et al., " 199 XI CONGRESO GEOLOGICO CHILENO 2002, Giambiagi and Ramos, 2002, Charrier et al., 2002). Thus, analyses of the shortening distinctions between flat and steep subduction require the establishment of regional sections on the both segments of subduction. -Figure 2: Dem of the studied area. Topographic profiles are located. The profile A is situated above the central Chile flatslab and the profile B above a normal subduction. The areas with actual and major shortening are shown (cross). REFERENCES Charrier, R.; Baeza, O.; Elgueta, S.; Flynn, J.J.; Gans, P.; Kay, S.M.; Muñoz, N.; Wyss, A.R.; Zurita, E. 2002. Evidence for Cenozoic extensional basin development and tectonic inversion south of the flat-slab segment, southern Central Andes, Chile (33°-36°S.L.). J. South Am. Earth Sci., vol. 15: 117-139. Cloos, M. 1992. Thrust-type subduction-zone earthquake and seamount asperities: A physical model for seismic rupture. Geology, vol. 20: 601-604. Dominguez, S.; Lallemand, S.E.; Malavielle, J.; von Huene, R. 1998. Upper plate deformation associated with seamount subduction. Tectonophysics, vol. 293: 207-224. Espurt, N.; Funiciello, F.; Martinod, J.; Faccenna, C.; Regard, V. in review. What makes subduction flatten? What consequences on upper plate shortening? Insights from 3-D analogue modelling. Earth Planet. Sci. Lett. Funiciello, F.; Faccenna, C.; Giardini, D.; Regenauer-Lieb, K. 2003. Dynamics of retreating slabs: 2. Insights from three-dimensional laboratory experiments. J. Geophys. Res., vol. 108, 2207, doi: 10.1029/2001JB000896. Giambiagi, L.B.; Ramos, V.A. 2002. Structural evolution of the Andes in a transitional zone between flat and normal subduction (33°30’-33°45’S), Argentina and Chile. J. South Am. Earth Sci., vol. 16: 101-116. Hampel, A.; Adam, J.; Kukowshi, N. 2004. Response of the tectonically erosion south Peruvian forearc to subduction of the Nazca Ridge: Analysis of the three-dimensional analogue experiments. Tectonics, vol. 23, TC5003, doi:10.1029/2003TC001585. Martinod, J.; Funiciello, F.; Faccenna, C.; Labanieh, S.; Regard, V. 2005. Dynamical effects of subducting ridges: Insights from 3-D laboratory models. Geophys. J. Int., vol. 163, 1137-1150. Pardo, M.; Comte, D.; Monfret, T. 2002. Seismotectonic and stress distribution in the central Chile subduction zone. J. South Am. Earth Sci., vol. 15, 11-22. Ramos, V.A.; Cristallini, E.O.; Pérez, D.J. 2002. The Pampean flat-slab of the Central Andes. J. South Am. Earth Sci., vol. 15: 59-78. Scholtz, C.H.; Small, C. 1997. The effects of seamount subduction on seismic coupling. Geology, vol. 25, 487-490. Yañez, G.A.; Ranero, C.R.; von Huene, R.; Díaz, J. 2001. Magnetic anomaly interpretation across the southern central Andes (32°-34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin. J. Geophys. Res., vol. 116, 6325-6345. 200 "