Structure and evolution of the oceanic Juan de Fuca plate from accretion
to subduction

Mladen Nedimovic, IGPP May 5, 2006

Abstract

The region comprising the northeast Pacific Ocean and the Cascadia margin
is a natural laboratory for studies of various tectonic and geologic
processes of global importance. Seafloor spreading, slab bending and
hydration, oceanic subduction and interplate coupling, and slab
dehydration and intraslab seismicity are some of the many processes that
occur within a distance of only several hundred kilometers. I will first
briefly describe the study area, the general crustal structure, and the
operating tectonic and geologic processes. Then, I will focus on
presenting and interpreting three specific observations that can be made
by analyzing seismic reflection images. In the direction of increasing
crustal age these are: Reflections from localized thick Moho transition
zones, normal faulting seaward from the trench, and variable megathrust
reflectivity. Imaged thick Moho transition zones appear to provide support
for the existence of complex magmatic plumbing systems at mid-ocean ridges
and for the crustal generation hypothesis from multiple magma bodies.
Extensional regimes seaward of subduction trenches and due to slab bending
are likely the key for hydrating the approaching oceanic slabs but at the
Cascadia margin this process appears to be mostly limited to the crust,
possibly resulting in reduced occurrence rate and magnitude of intraslab
seismicity. Variations in the character and thickness of the megathrust
reflection package seem to provide a high-resolution method for mapping at
least the landward end of maximum co-seismic slip during subduction
earthquakes.