Seamount Chain – Seismogenic Zone Input: The Upper Plate Contribution in Costa Rica. Structural Analysis of the Osa Mélange

Abstract

The Middle America subduction zone is a textbook example of the subduction erosion tectonic process—accounting for half of modern subduction zones—which causes material from the forearc to be entrained into the plate boundary interface as it migrates upwards (Vannucchi et al., 2012). The Osa Peninsula, Costa Rica, lies above the seismogenic portion of this subduction zone and exposes material from the forearc basement—the Osa Mélange—which forms the hanging wall at depth. Offshore Osa Peninsula is uniquely within reach of modern drilling technologies at the depth of seismic nucleation and is the target of the CRISP expedition (Kinoshita et al., 2006). The forearc consists of the Osa Mélange and overlying slope sediments.

The mélange is a highly deformed terrane of accreted seamounts and is predominantly composed of variably deformed basalt, mudstone and carbonates with minor gabbro, chert and clastic sediments, which in turn display a range of structural styles, from ductile flow and injection into cracks in the surrounding lithologies to extensive brittle fracturing and brecciation. This material varies from being largely undeformed, through moderately fractured, to intensely comminuted displaying grain-by-grain disaggregation. This deformation is observed to occur through increasing the proportion of fracture fill, likely through a combination of chemical and physical processes. This comminuted material forms a mechanical matrix for the rheologically stronger material and is observed to flow around these blocks. In this situation, basalt—usually considered to be the mechanically stronger lithology—is in places deformed so that it is weaker than the surrounding sedimentary rocks. As such, the response of this material to the conditions likely to be found at the plate boundary interface cannot be predicted a priori.

Microstructural analysis has been conducted to characterise the complex fabric and rock deformation laboratory experiments on the basalt are planned to define the physical properties, the deformation style and the failure mode envelopes in conditions close to those at the plate boundary interface. Experimental conditions will span the full range of temperatures and pressures present in the mélange. This will be used to define the effect of this prior fabric on subsequent deformation within the plate boundary interface with a focus on whether the deformation is brittle or ductile and how its localisation evolves.

The results of this research will inform the planned drilling project to sample this subduction zone at depth. By characterising this plate boundary we can infer the attributes of earthquake nucleation and propagation in pre-deformed rocks and can extrapolate this to other erosive subduction zones around the world.