Strain partitioning in an accretionary wedge
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摘要

In subduction zones undergoing oblique convergence, strain partitioning is often expressed by an important deformation inducing strike-slip faulting. In accretionary wedges, parameters such as obliquity of the convergence and friction at the bottom of the wedge play an important role in the strain partitioning. The impact of these parameters is studied using sandbox experiments. Two backstop geometries have been designed to account for different geological settings. These experiments show that the wedge taper remains constant and close to αcoulomb for variable obliquities. Measurements of critical tapers on the models suggest that the Coulomb wedge theory cannot be simply applied to determine parameters on wedges developed under oblique convergence. Parameters deduced from this theory are valid only when measured in the direction parallel to the convergence. In addition, the partition degree increases with the obliquity of the convergence, and strain partitioning occurs independently of the basal friction. We remark that the model morphology changes when an obliquity value, mainly, is exceeded. A transcurrent structure develops. The models show that oblique structures located above the velocity discontinuity are associated with strike-slip faults. Similar structures have been observed within the Hikurangi accretionary wedge (New Zealand).

Introduction. – In subduction zones, for high values of the convergence obliquity γ, transcurrent faults are observed, parallel to the trench, on the continental plate. Such structures are present in Sumatra and the Philippines along the volcanic arc [Bellier and Sébrier, 1995 ; Malod et al., 1993 ; Barrier et al. 1991] or at the rear of the accretionary wedge for the South Ryukyu (Taiwan). These strike-slip faults are a consequence of strain partitioning. Partition is controlled by obliquity of convergence, basal friction and probably geometry of backstop. Using analog models, the influence of each parameter on strain partitioning, on the occurrence of transcurrent faults and its morphology are analyzed.

The strain partitioning can be quantified by its degree

Formula(1)

It requires the knowledge of the slip vector, which is situated between the azimuth of the convergence and the perpendicular to the trench.

Experimental procedure. – The aim of our experiments is to better understand the effects of several parameters which determine the strain partitioning in accretionary wedges undergoing oblique convergence. For this purpose, a large-size table (140 cm × 250 cm) covered with a plastic film moving towards the backstop is used (fig. 1). The basal friction of the wedge can change according to the different textures of the plastic films used. Various angles of convergence (0o, 20o, 40o and 60o) are studied (fig. 1).

Two types of backstop geometries are designed. In the first case, the plastic sheet moves under a thin PVC plate resting at the base of the material, in order to localize the strain above the velocity discontinuity, at the tip of the plate. In the second case, the backstop presents a 4 cm high, low-friction vertical wall. The whole device is covered by 2 cm of dry sand. The sand satisfies a Coulomb failure envelope, with an internal friction angle of about 30o and a very low cohesion. Colored markers perpendicular and parallel to the convergence direction, and perpendicular to the backstop, are spread on the surface of the sand. They allow recording of kinematics during the experiments.

During these experiments, the plastic film converges towards the backstop with a constant velocity. Pictures of the deforming model are taken perpendicularly to the surface, every 5 cm of shortening. Intersections of the colored lines are followed and recorded at every stage of shortening, in order to describe the deformation and determine the velocity field. Vs corresponds to the displacement-vector of these points. The knowledge of the kinematics allows to deduce (using geometric construction) the slip-vector Vg and its obliquity ψ relative to the normal to the trench.

The partition degree Kv is a function of the obliquity of convergence vector and of the obliquity of slip vector (fig. 2). The critical taper is measured at the end of each experiment, perpendicularly to the backstop and parallely to the direction of convergence.

Results and discussion. – The morphology of the wedge suggests different structural domains : (a) a stable domain, (b) a zone of wrenching, (c) a wedge of imbricated thrusts. Above a value of the obliquity of convergence, the three domains become more distinct. Oblique structures develop in relation with the strike-slip fault. Their orientation compared with the orientation of the strike-slip fault is controlled by the geometry of the backstop (fig. 3). In the first type, these structures seem to be perpendicular to the convergence, in contrast with the second type where they tend to be parallel.

Physical models of accretionary wedges show that the critical taper increases with increasing basal friction [e.g., Malavieille et al., 1992]. Furthermore, the sand used in the experiments obeys Coulomb failure criterion. Davis et al. [1983] apply this criterion to convergent thrust-wedges. They determine a relationship between critical taper wedge (αcoulomb), dip (β) of the subducting plate and basal friction (μb) :

Formula(2)

In our experiments, for both geometries of backstop, the slope measured perpendicularly to the trench increases with the obliquity of the convergence, whereas the slope measured parallel to the direction of the convergence remains constant. Its value is close to the slope deduced from relation 1. The slope measured perpendicularly to the trench increases with increasing basal friction (fig. 4).

The partition degree Kv depends on the relation between the convergence vector Vc and the slip vector Vg. Each of them is defined by their obliquities which are respectively γ and ψ. The partition degree Kv can be deduced from the obliquity of the convergence vector relative to the slip vector, Liu et al. [1995]. See equation (1).

For both backstop geometries (fig. 5), Kv increases with the obliquity of the convergence γ. Moreover, the strain partitioning develops for γ superior or equal to 20o. In the second type of experiments, the partition degree is homogeneous in the whole structure. It moves like a single block along a strike-slip fault located above the velocity discontinuity (fig. 6).

Studies on the critical taper allow Davis et al. [1983], to determine a relationship applied to a frontal convergence. In our experiments, the measurements of the critical taper of the accretionary wedge is equal to αcoulomb and remains constant in the direction of the convergence. The critical taper equation (2) seems to apply in the direction of convergence. So the measures taken in the direction of the convergence permit to obtain some correct parameters.

Using sand-silicone analogue models, Pinet and Cobbold [1992] show that a minimum angle of oblique convergence (30o) is required before strain partitioning occurs. Our models involving dry sand clearly reveal that strain partitioning :

From physical models, Chemenda et al. [2000] proposed that the strain partitioning at lithospheric scale may occur for a high interplate friction only. Thus, the rheology of the material probably has a strong influence on the critical value of the convergence obliquity beyond which strain partitioning may appear.

Comparison with natural examples. – To the south of the Ryukyu Trench, northeast Taiwan, bathymetric data highlight a dextral strike-slip fault at the rear of the accretionary wedge [Lallemand et al., 1999]. Seismic profiles analysis crossing the same area gives constraints on the shape of the Ryukyu arc basement. It ends with a several kilometers high subvertical basement wall [Font et al., 2001]. This vertical backstop favors strain partitioning and locates strike slip faulting. Our models also clearly outline the impact of backstop geometry on strain partitioning and on strain location.

In New-Zealand, along the Hikurangi margin, the Pacific plate meets the Australian plate at a very oblique convergence (60o). Between the Cook canyon (175o00E and 176o45E), three crustal faults, having the same direction, are observed. The Palliser-Kaiwhata fault which outlines the edge of the continental margin is interpreted as a subvertical strike-slip fault. Close to the trench, two other faults are interpreted as reverse faults which may correspond to a strike slip oblique component. Sonar data reveals a lineament associated only with the Palliser-Kaiwhata fault trend [Barnes et al., 1998]. In our experiments, oblique structures associated to strike-slip faulting developed (backstop geometry 2, fig. 3d). These structures seem to be linked with strike-slip displacement of faults. Knowing the boundary conditions of the models and about the structural surface of the Hikurangi wedge, a similar configuration to our models could exist, with the possible presence of a velocity discontinuity above the Palliser-Kaiwhata fault.

The present study points out several results :

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