Seamount Chain – Subduction Zone Interactions: Localised subduction accretion and erosion in response to subduction of high relief seafloor

Abstract

The Osa Mélange, together with the Osa Igneous Complex, comprise an accreted igneous terrane located in southwest Costa Rica that forms the forearc of the erosive Middle America Subduction Zone at the depth of seismic nucleation. The tectonic setting in which this mélange formed remains unconstrained with both the ocean trench (DiMarco et al., 1995; Buchs et al., 2009) and seamount flanks (Vannucchi et al., 2006) considered.

This mélange is highly heterogeneous and consists predominantly of deformed debris avalanche deposits interlayered with pelagic sediment, and up to 800 metre thick blocks of brecciated basalt with Galapagos seamount affinity (Hauff et al. 1997, 2000; Hoernle et al. 2002). No material derived from the Central American arc or forearc was found. Detailed microstructural, petrological and geomechanical analysis has been conducted on this material in order to characterise its physical properties and constrain its model of formation.

Both the brecciated basalts and clastic sedimentary rocks exhibit considerable brecciation and cataclasis not confined to discreet shear zones. This results in the large blocks themselves possessing a pervasive block-in-matrix texture, with comminuted fracture-fill as the mechanical matrix. Extensive hydrothermal alteration of the basalts has also contributed to the deformation of this material.

The high sediment volumes and lack of blocks exotic to the Farallon Plate indicate deposition in an oceanic setting, adjacent to a seamount train, and away from the continental margin. We therefore propose that the Osa Mélange was formed in the flexural moat of a Galapagos hotspot-derived seamount complex.

Subduction erosion — a process whereby material is removed from the hanging wall of the subduction zone as it migrates upwards — leads to the active entrainment of Osa Mélange material into the plate boundary interface of the Middle America Subduction Zone. Characterising the inputs into this subduction zone is necessary to understand the processes operating at the depth of seismic nucleation — a major objective of the IODP CRISP project. Our improved understanding of the parameters of seismic and aseismic slip within the Middle America Subduction Zone may be extrapolated to other erosive subduction zones around the world.