Download effects of ridge subduction on upper plate deformations: the juan

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).
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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
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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.,
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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).
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