Journal of Conference Abstracts

Deformation Patterns of Oceanic Second-Order Discontinuities: Analog Modelling

Olivier Dauteuil Geosciences Rennes, U.P.R.-CNRS 4661, Campus Beaulieu, 35042 Rennes Cédex, France

dauteuil@univ-rennes1.fr

Introduction

Slow-spreading ridges, such as the Mid-Atlantic Ridge, are affected by different kinds of discontinuities (Macdonald et al., 1988; Grindlay et al., 1992), which segment the oceanic rift at different scales, from one to several hundreds of kilometers. Recent studies indicate that the ridge segments limited by second-order discontinuities represent a key scale in the understanding of tectonic and magmatic processes. Indeed, each segment has its own independent evolution that is controlled by the presence of the magma chamber (Gente et al., 1995).

These discontinuities display various morphologies corresponding to different geometries. The analysis of Mid-Atlantic discontinuities suggests four types of geometries: 1) without lateral offsets (MARK area), 2) with simple lateral offset (transform A - FAMOUS area), 3) with overlap of segment extremities (transform B - FAMOUS area), 4) with oblique offset (transform C - FAMOUS area). These zones represent particular areas in the deformation pattern of the oceanic rift system. Indeed, they accommodate the transfer of extension from one segment to another. Furthermore, at the intersection between discontinuity and ridge segment, mantle rocks often crop out by means of complex fault systems. It should be noted that the deformation pattern associated with these structures is complex. To better constrain the controlling parameters of the deformation pattern, analog modelling has been performed in small-scale experiments.

Method and apparatus

The rheological structure of the oceanic lithosphere can be simplified as an elastic layer overlaying a ductile layer. The thickness of the elastic layer depends on the thermal structure of the lithosphere (Tapponnier and Francheteau, 1978; Chen and Morgan, 1990): it increases with distance from the axial ridge. Shaw (1992) has described the thermal structure of a segment; he considers that the brittle/ductile boundary deepens towards the extremities of the segment because of the presence of the magma chamber beneath the center. It is possible to model the upper brittle layer using sand that is a Mohr-Coulomb material with an internal friction angle close to 30°, and the deep viscous layer using a silicon putty that is a Newtonian fluid (Davy, 1986; Dauteuil and Brun, 1993).

The experimental apparatus is made up of two plastic sheets, placed side by side, whose shape imposes two diverging boundaries and one strike-slip boundary which offsets the other two by 5 cm. These three boundaries are perpendicular. At diverging boundaries, the plastic sheets are fed through thin elongate slits made in the table on which the apparatus is set up. Stips of silicon are placed only along the diverging boundaries. All experiments were performed with the same stretching rate (3 cm/h).

To test different patterns of discontinuities, various configurations of silicon strips were placed along the boundries: 1) without silicon strips; 2) with a narrow silicon strip just along the diverging boundaries; 3) with a narrow silicon strip along divergent boundaries and across the offset; 4) with a wide silicon strip largely overlying the divergent boundaries and across the offset. The model was then entirely covered by sand whose thickness did not exceed 2.5 cm. The silicon band thickness is 1.5 cm. A passive grid of white sand was drawn on the model to analyze the displacement and strain patterns and photographs of the surface were taken every 5 minutes. Four experiments corresponding to the different silicon strip configurations are presented.

Fig. 1: Small-scale model of a second order discontinuity without silicon putty (surface view). The black arrows indicate the pulling direction. The deformation is extemely localized inside the discontinuity, on the narrow deformed zone that joins the two inner corners.

Fig. 2: Small-scale model of a second order discontinuity with a wide strip of silicon putty (surface view). The black arrows indicate the pulling direction. The shape of the deformed zone is similar to the geometry of the ductile band. Inside the discontinuity, the deformation is distributed on largely oblique faults gathered into a wide band.

Results

For each experiment, the fault pattern is organized into two domains corresponding to the two kinds of boundary. The fault pattern above the diverging boundaries is similar regardless of the silicon strip configuration: it is formed by normal faults with parallel and linear trends, dipping towards the center of the deformed zone (Figure 1). The fault spacing decreases through time forming narrower blocks. The size of these blocks is strongly controlled by the sand thickness which decreases during stretching. The width of the deformed band is also controlled by the width of the underlying silicon strip: an increase of silicon strip widens the surficial deformed zone (Figure 2).

Above the offset, the fault pattern is strongly influenced by the presence and the shape of the silicon strip. The shape of the surficial deformed zone follows the shape of the deep ductile layer. By increasing the silicon strip width, the deformed zone widens. For the experiment without a silicon strip (Figure 1), the deformation is accommodated along a few strike-slip faults making a 30° angle with the strike-slip discontinuity. The displacement is highly localized on one fault that joins the two inner corners of the rifted zone. The faults display a curved end close to the rifted zone. In the case of the wide silicon strip (Figure 2), the deformed zone inside the strike-slip discontinuity is formed by numerous linear faults gathered into a band roughly parallel to the offset. This band crosses the entire deformed zone. The faults trend at 50-60° to the strike-slip discontinuity. They correspond to normal faults with a weak strike-slip component. The deformation is distributed within the whole deformed band, contrary to the case without a silicon strip where it is extremely localized.

In the case of the wide silicon strip, the fault pattern displays two trends more or less connected or in interference, at the intersection between rifted zone and strike-slip zone (Figure 2). In the cases of a narrow silicon strip or without silicon, the deformation is very localized, and a depression is generated at the intersections. The bounding walls are formed by faults accommodating a large vertical throw.

Conclusion

This analog modelling of second order discontinuities points out two end-members of deformation partitioning controlled by the shape of the silicon strip. The deformation can be localized along one fault or a highly deformed zone jointing the two inner corners. This corresponds to cases where the ductile layer is absent under the discontinuity, i.e. the ductile layer is disconnected between the two segments. This case is similar to another study realized by Mauduit and Dauteuil (1996) focused on the transform zones.

When a ductile layer is present under the discontinuity, the deformation is more diffuse and displays a complex fault pattern. Two faults sets are present: (i) faults largely oblique to the discontinuity trend, and (ii) faults parallel to the rift trend.

By considering these extreme end members that fit to two different thermal stages of the segment, it is possible to explain the complexity of the second-order discontinuity geometry. This modelling reveals the sensitivity of the deformation pattern to the rheology of the oceanic lithosphere.

References

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