Deformation processes during seamount dismemberment and underplating along the shallow subduction interface: a case study from the Chugach Complex, Alaska

Topographic features such as seamounts can influence the buoyancy of the slab and the short- and long-timescale mechanical properties of the subduction interface. How seamounts in the trench interact with the upper plate accretionary wedge during subduction— their stress field, their potential for ‘decapitation’, and their ability to host large megathrust earthquakes— is not fully understood. We utilize exhumed rocks to investigate seamount–upper plate interactions at shallow subduction interface conditions. 

We focus on a 250-m-thick cross-section of deformed, weakly metamorphosed basalt, limestone, chert, argillite and greywacke exposed in the inboard part of the Chugach accretionary complex near Grewingk glacier, southern Alaska. Temperatures from Raman spectroscopy on graphite yield ~260°C, suggesting deformation and metamorphism down to ~15-20 km depth. Detrital zircon data from greywacke lenses within and outside the shear zone overlap within error suggesting emplacement over less than ~1 m.y. at ~167 Ma. 

Basalts in the shear zone are dismembered into ~3 slices up to 35 m thick, all of which contain limestone patches suggesting the basalt is derived from the seamount’s very top (limited decapitation). The basalt slices are bounded by high-strain melange-like shear zones up to 25 m thick, interpreted to represent décollements along which the seamount slices were underplated. These mélange belts exhibit a block-and-matrix texture with a macroscopically ductile argillite and chert matrix, and pervasively disaggregated and brittlely deformed greywacke and basalt lenses. Both the matrix and the blocks show several generations of dilational and shear veins, suggesting high fluid pressures and low differential stresses. Features suggesting deformation at fast (potentially seismic) strain rates include fluidized cataclasites, but these do not extend along strike for more than 0.25 m and do not occur within the larger (m-to-dm-scale) basalt lenses, suggesting that large-magnitude earthquakes were limited during seamount underplating. Instead, the observed mix of brittle and macroscopically ductile deformation at high fluid pressures is more consistent with a potential record of shallow tremor and slow slip.

Our findings support geophysical observations and numerical models that suggest relatively weak mechanical and seismic coupling between seamounts and the overriding plate, and are consistent with recent suggestions (e.g. for the Hikurangi margin) that sediment envelopes around subducting seamounts are conducive to slow slip and tremor.