A Glimpse into the Subduction Zone Plate Interface: 3D structural and mechanical mapping of the Chrystalls Beach mélange, New Zealand

Subduction zone plate boundary interfaces are some of the largest shear zones on our planet and are host to the largest earthquakes, plus other diverse seismic and aseismic slip phenomena. These zones are often highly heterogenous mélanges. Accreted and exhumed subduction interface mélanges therefore provide a ‘window’ into the conditions and processes within these otherwise inaccessible environments. The geometries of blocks, the proportion of blocks to matrix, and the relative mechanical properties between different block populations and between blocks and matrix have been demonstrated to control the physical behaviour of these mélange zones, including their propensity towards seismicity. Here we report a detailed multi-scale 3D characterisation of the material properties, block geometries and fracture networks within the Chrystalls Beach mélange, New Zealand.

3D structural analysis utilised a tiled photogrammetric model constructed from ca. 12,500 images and consists of detailed and systematic analysis of the mélange fabric, block geometries, and distribution and orientation of faults, fractures and veins. In-situ rock mechanics tests were performed using a Schmidt rebound hammer with measurement sites located to cm-accuracy in the field and on the 3D model. Samples were collected from these same sites for point-load strength tests and laboratory-based triaxial shear experiments. Through this approach, we aim to identify systematic relationships between measurable physical properties of the exhumed rock and the inferred original rheological behaviour of this mélange.

The Chrystalls Beach mélange consists of centimetre – decametre-scale blocks of sandstone, chert, and siltstone with minor altered basalt within a pelitic matrix and has been deformed within the shallow portion of the subduction zone. In-situ strength measurements show that the strength of blocks vary from up to twice as strong as the matrix to similar to — or in places below — the strength of the surrounding matrix. The matrix is also heterogenous in its material properties with two distinct matrix types defined on the basis of matrix lithology, included block populations, and material properties.

Patterns of fractures and brecciation of the blocks provide a structural indication of the comparative rheology of each of the block populations during deformation, with each lithology exhibiting distinct behaviour. Blocks in the mélange are either high-aspect-ratio, boudinaged, dismembered beds or variably rounded brecciated fragments, with stronger lithologies forming more angular, higher-sphericity, and less aligned fragments. This mélange is pervasively cut by several centimetre-thick veins which form an anastomosing network, often at the boundaries of the chert and sandstone blocks which they are deflected around.

This preliminary analysis has revealed varied deformation styles operate between blocks of different mechanical properties and that this deformation style depends both on the rheologies of the individual components and also on the difference in rheology between the blocks and the matrix. The patterns of the thick veins reveal the locations of the greatest slip localisation throughout the mélange and show that veins localise at the margins of blocks with the greatest rheological contrast. This analysis therefore provides the material and geometrical input parameters and end results which provide real-world constraints for future simulations of deforming mélange zones.