Strain localisation on the shallow subduction interface

Abstract:

The shallow plate interface in subduction zones accommodates both slow slip events and megathrust earthquakes as a function of pressure-temperature conditions, compositional and fluid-pressure heterogeneities, and deformation mechanisms. These heterogeneities control where, and how, strain localises as rocks are entrained and deformed on the plate interface. It is not well understood how mafic volcanic rocks and oceanic sedimentary rocks, which variably occupy the plate interface during subduction, weaken or deform with respect to one another. Due to the contrasting composition and rheology of these rocks, they are likely responsible for a variety of seismic behaviour.

We present results from field, microstructural, and experimental studies of volcanic and mixed volcanic-sedimentary tectonic mélange from the Chugach Complex of southern Alaska. In the Chugach, underplated slices of volcanic and sedimentary rocks were subducted to depths of 2 to 10 km and reached peak metamorphic conditions of around 250°C during the Jurassic to Early Cretaceous. These slices of the shallow subduction interface shear zone are composed of either purely seafloor volcanic rocks or a tectonic melange of mixed sedimentary and volcanic rocks and are defined by sharp strain gradients in the field.

We document multiple structures in the Chugach that capture progressive deformation of seafloor basalts with increasing strain. Several generations of fluid injection drive micro-brecciation of pillow basalt, followed by the development of a spaced tectonic foliation (S planes). Ongoing deformation leads to the formation of shear bands (C + C’ planes) that isolate and wrap lenses of relict brecciated basalt and eventually localise into narrow zones of ultracataclasite. Where sediments (chert, mudstone, blocks of greywacke) directly overlie pillow basalt, deformation is more distributed and forms a mixed basalt-sediment clast-in-matrix mélange. Stronger, resistant fragments of chert and greywacke form angular, fractured clasts and altered basalt forms elongated, folded clasts; the clasts are wrapped by a sheared, cataclastic matrix that is derived from altered basalt and mudstone. We interpret these results in terms of the relative and absolute strength of the basalt and basalt-derived mélange and possible styles of seismicity.