Mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex

Abstract

Sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads. The impact of heterogeneous sedimentary input on wedge mechanics, evolution and mass transfer was investigated. We report the first experimentally documented growth of basal duplexes. These occurred for high basal friction conditions, with restricted output of the lower section. The upper and lower sections were completely decoupled due to the intervening layer of glass beads, with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes. IMERSE multichannel seismic reflection data from the Western Mediterranean Ridge (WMR) image Tertiary clastics beneath a thick section of Messinian evaporites. The base of the evaporites is identified as the primary décollement for deformation in the frontal part of the accretionary complex. Constriction of the channel of subducting Tertiary sediments, as well as internal deformation observed as arcward-dipping reflectors argue for basal underplating and/or two different active décollements. We propose an evolution of the WMR in accordance with the sandbox experimental results. A weak mid-level detachment (base of evaporites) combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward-verging duplexes.

Introduction

The growth and evolution of submarine accretionary wedges at convergent margins can be described in terms of the critical taper model (Davis et al., 1983, Dahlen, 1984, Dahlen, 1990, Lallemand et al., 1994). This concept assumes that marine sediments entering a trench and then being underthrust or scraped off from the lower plate and accreted against the upper plate may be described as a frictional Coulomb material analogous to a sand wedge in front of a moving bulldozer. For the submarine case, the mechanical state is a function of internal and basal (i.e. at the décollement) friction angles, pore pressures, and cohesion. In accordance with the Mohr–Coulomb approach, the mechanical properties of the décollement, which marks the plate boundary (Moore, 1989), govern the mass transfer modes present in the whole accretionary complex controlling the style of deformation and the relative amount of accretion, subduction and underplating (Kukowski et al., 1994, Gutscher et al., 1998a).

Many insights into the systematics of growth and deformation of accretionary wedges have been gained from carefully designed analogue sandbox modelling (Malavieille, 1984, Mulugeta, 1988, Byrne et al., 1993, Kukowski et al., 1994, Gutscher et al., 1998a). In accretionary wedges the backstop against which ongoing accretion occurs may be an older accretionary wedge (e.g. off Alaska, von Huene et al., 1998) or a faulted probably continental crystalline body (e.g. off Peru, Byrne et al., 1993, Kukowski et al., 1994). In either case the backstop is stronger than the sediments forming the modern accretionary wedge. Consequently in analogue modelling, backstops made of wet sand or rock powder, both of which are stronger and more cohesive than dry quartz sand, have been found to realistically simulate different types of convergent margins backstops (Byrne et al., 1993, Kukowski et al., 1994, Gutscher et al., 1998b).

In some accretionary systems it has been shown through seismic images (e.g. Moore et al., 1991, von Huene et al., 1996, von Huene et al., 1998) that sediments are subducted beneath the entire backstop and through studies of the isotopic composition of arc magmas (Morris et al., 1990, Kita et al., 1993) that sediments are subducted even into the mantle. Therefore, the removal of material from the experimental apparatus has to be considered when simulating mass transfer in the frontal part of a convergent margin. This has been done by introducing a ‘subduction window’, an opening at the base of the hinterland side of the apparatus, in studies of the Peruvian (Kukowski et al., 1994) and Alaskan (Gutscher et al., 1998b) margins.

A systematic series of experiments with different boundary conditions (Gutscher et al., 1998a) has revealed that the different styles of mass transfer are a function of basal friction and the ratio of input to output. However, in all of these previous experiments, the incoming section was homogeneous, whereas in nature the sediments entering a subduction zone may have quite variable mechanical properties. This can dramatically affect the style of deformation. For instance, in convergent margins (e.g. Nankai or Barbados), the level of the décollement is controlled by the varying mechanical properties of the different lithologies within the incoming sediment pile (Taira et al., 1992, Moore et al., 1998); variations in the mechanical properties along the layer can thus lead to changes in the level of the décollement. In the case of the Mediterranean Ridge for example, it has been argued that near the deformation front the décollement occurs at the base of the Messinian evaporites, but that further arcward the décollement is deeper at the level of the Aptian shales (Ryan et al., 1982, Reston et al., 2002a).

In this paper we use the results of sandbox modelling to investigate the effect of two décollement levels on the geometric evolution of the accretionary system. To simulate the shallower décollement, we incorporate a thin layer of micro glass beads in the incoming section; the deeper décollement is controlled by the height of the subduction window. The results have implications for geodynamic models for the evolution of the Mediterranean Ridge.