Heterogeneous input in a subduction system & its effect on plate boundary processes: Structural analysis of the Osa Mélange, SW Costa Rica

The Middle America Subduction Zone is the type example of an erosive margin; i.e. material is removed from the forearc and incorporated into the subduction channel. The Osa Mélange (SW Costa Rica) occupies the forearc of this subduction zone and is currently being tectonically eroded. This mélange consists of blocks of altered basalt, chert, and carbonate within a volcanoclastic pelitic matrix. Using field study, microstructural analysis, rock physics, and numerical modelling, we have characterised the geological history of this mélange and determined the pattern of its future deformation.

This mélange originated on the incoming plate by mass wasting from a seamount chain into its flexural moat. Flexural moats are major sites of oceanic sedimentation and may contain up to 3 km of sediment. Sediment thickness at the trench is a fundamental control on whether the subduction zone is erosive or accretionary, therefore subduction of a flexural moat may promote localised accretion, even if the subduction zone is otherwise erosive. The Osa Mélange is the first recognised example of an accreted seamount moat.

Mélanges are commonly envisaged as strong, stiff blocks within a weak, compliant matrix; however triaxial deformation experiments on an altered basalt megablock and its surrounding volcanoclastic matrix show that the matrix of the Osa Mélange is now stronger and stiffer than the blocks. Compliant blocks in stiffer matrix make the rock unit more prone to fracture, therefore low block-to-matrix ratios do not always promote aseismic creep. Whether fractures form in blocks or matrix depends on the difference in strength and difference in Young’s modulus. Failure in one block promotes failure in its neighbours and surrounding matrix, priming the subduction interface for through-going rupture.

As the mélange is tectonically eroded by hydrofracturing of the hanging wall, the style of incorporation is controlled by its fabric and rheology. Where aspect ratios are low, weaker material is comminuted into the plate boundary interface, whereas high aspect ratios, reactivate pre-existing weaknesses to form blocks in the subduction channel composed of both lithologies.