Deformation and alteration during seamount subduction: Insights from an exhumed intra-oceanic accretionary complex

Current studies on seamount subduction propose contrasting effects on megathrust behavior. Some suggest that subducting seamounts increase normal stress and promote large earthquakes, while others argue that seamounts fracture the upper plate, enhancing microseismicity and aseismic creep that may inhibit major ruptures. Field- and microscale observations from the Azuero Accretionary Complex, an exhumed intra-oceanic accretionary complex in Panama, provide new constraints on the deformation processes associated with seamount subduction.

Coastal exposures on the Azuero Peninsula expose the contact between the autochthonous Azuero Plateau and the allochthonous Azuero Accretionary Complex. The Azuero Plateau forms part of the Caribbean Large Igneous Province and consists mainly of massive to pillowed oceanic plateau basalts with minor Upper Cretaceous chert. In contrast, the accretionary complex comprises massive to pillowed basalts and volcanic breccias with ocean island affinity, locally interbedded with Paleogene carbonates. A ~3 km wide deformation zone, the Azuero Mélange, separates these units, and is inferred to be a deformed portion of the plateau based on new field observations and geochemical data.

The accreted seamount lithologies show pervasive faulting, cataclasites, abundant zeolite veins, and a chlorite-rich shear zone located ~60 m below the mélange. These rocks lack evidence for large displacement through-going faults. In contrast, plateau-derived rocks record both brittle faulting and ductile deformation. Ductile strain localized within the Azuero Mélange, where clay-rich cataclasites accommodated deformation through cataclastic flow and dissolution–precipitation creep. At the structural base of the mélange, a ~10 m thick shear zone composed of foliated cataclasites with basalt and limestone clasts within a clay-rich matrix is observed, interpreted to be sheared seamount lithologies.  

Fluids and alteration played a crucial role in localizing strain within the upper plate and the décollement, enhancing mechanical weakening and diffusive mass transfer. Cataclasis increased permeability, enabling fluid infiltration and the formation of mechanically weak phases such as clays and chlorite. These processes promoted strain localization and facilitated deformation by cataclastic flow and dissolution-precipitation creep. The interplay between these alteration and deformation processes likely favored aseismic creep during seamount subduction.