LINKING PLATE INTERFACE PROCESSES WITH THE SURFACE EXPRESSION OF SUBDUCTION (Invited Presentation)

The expression of subduction along the Cascadia forearc varies significantly despite relatively low variability in downgoing plate properties. Clear correlations are observed between geological and geophysical structure and processes, as expressed in topography, overriding crustal composition and architecture, uplift and exhumation rates, potential field measurements, non-volcanic tremor density and slow slip recurrence intervals, seismicity in both the downgoing and overriding plates, seismic velocity structure, and slab curvature and dip. However, linking this diverse array of observations through causative associations has proven difficult, as hypotheses that primarily attribute variations to either overriding or downgoing plate properties can only adequately explain a subset of these observations.

In this study, we attempt to link these correlative observations in a mechanistic way. We create a new model of seismic discontinuity structure along the Cascadia margin that capitalizes on both long-term and temporary seismic deployments over 30 years. We interpret this model in the context of a previously published 3D shear-wave velocity model to gain insight into how the seismic structure of the Cascadia forearc relates to lateral variations in the manifestation of subduction. Our models indicate that the forearc crust is the thickest at the mantle wedge corner, which we interpret as the result of the basal accretion of material beneath the forearc. In the northern and southern Cascadia, this “subcreted” material is characterized by ~10 km thick zones of anomalously slow shear-wave velocities and relative Bouguer gravity lows, indicating that these zones may be dominantly composed of sedimentary material. These regions also correlate with relatively shallow slab dip angles, increased amounts of non-volcanic tremor and slow slip events, and elevated topography and uplift rates. This indicates that subcretionary processes may drive the long-term surficial expression of subduction in Cascadia, and may be applicable to other margins as well.